Enhanced Pool Boiling System and Method

Abstract

A boiling heat exchange system includes a heat exchanger including a chamber configured to hold a heat exchange fluid including a heat exchange vapor and a pool of heat exchange liquid and an evaporator tube having an outer surface and a thermally conductive open-cell porous material disposed on the outer surface and comprising a plurality of pores. The evaporator tube can be immersed in the heat exchange liquid held in the chamber and the heat exchange liquid will enter the pores. The open-cells of the open-cell porous material will heat the heat exchange liquid to cause the heat exchange liquid to boil to a heat exchange vapor and exchange heat with a source fluid flowing through the evaporator tube or with a component in thermal contact with the evaporator. A method of performing heat exchange is also disclosed.

Description

FIELD OF THE INVENTION

The present invention relates to heat exchange, and more particularly to pool boiling and flooded evaporator systems and methods.

BACKGROUND OF THE INVENTION

Flooded evaporator or pool boiling heat transfer is one of the most effective heat transfer methods and crucial for several energy conversion processes such as electronic cooling, steam generation, water purification and thermal management. Enhancing boiling heat transfer around tubes enables higher heat flux, which can improve efficiency and lead to compact heat exchangers. Delaying the onset of critical heat flux (CHF) allows for higher CHFs and protects equipment during anomalous transient conditions.

Boiling is a critical phenomenon for several energy conversion applications and for electronics cooling processes. During the boiling process, extremely high heat transfer rates can be attained at relatively low temperature differences. Enhancing boiling heat transfer has been an active area of research in recent years. Deploying passive techniques, particularly porous structures, has been extensively investigated to enhance boiling heat transfer. Significant advantages of such deployments include lightweight and compact heat exchangers. The enhancement is often attributed to the large surface area per unit volume, potential wicking phenomena to avoid dry-out, and higher nucleation sites density.

Another type of surface enhancement that is under increasing research is a porous cellular structure, such as porous metal foam. Such foams are described in US Patent Publication 2021/0389061 “HEAT EXCHANGE APPARATUS AND METHOD”, the disclosure of which is hereby incorporated fully by reference. These porous structures not only provide a larger surface area for heat transfer to take place, but the many interconnected cells in the mesh help provide fluid channels to prevent dry-out, which should delay the onset of CHF. Porous structures on flat surfaces in water has shown that heat transfer can be increased as much as three times over the value for a plain flat surface, and CHF can be doubled over the plain flat surface. It was also found that pore size and the thickness of the foam layer both had influence on the overall heat transfer enhancement. In addition to water, pool boiling experiments in Novec 7100 on flat copper surfaces enhanced with commercially available copper foam were reported in literature. In those studies, it was found that the copper foam increased the heat transfer through both increased bubble nucleation and enhanced convection, and that increased wickability of the foams improved high heat flux performance.

As previously seen, most of the literature focuses on flat surfaces, and research into using open-porous structures to enhance the pool boiling heat transfer of tubes appears somewhat limited. As reported in literature, pool boiling studies were performed in R134a on horizontal tubes covered in brazed on open cell copper foam. They found that heat transfer enhancement was greatest below approximately 30 kW/m², performing better than some commercially available enhanced tubes. They also found that, at low to medium heat fluxes, thinner foams with higher porosity performed better than thicker, denser foams. At high heat fluxes, smaller pores once again showed increased resistance to bubble departure.

SUMMARY OF THE INVENTION

A boiling heat exchange system includes a heat exchanger having a chamber and at least one evaporator tube. The chamber is configured to hold a heat exchange fluid including a heat exchange vapor and a pool of heat exchange liquid. The evaporator tube has a wall with an inner surface and an outer surface, and the evaporator tube has an input end and an opposite, output end. A thermally conductive open-cell porous material is disposed on the outer surface and includes a plurality of pores. At least a portion of the evaporator tube can be immersed in the heat exchange liquid held in the chamber and heat exchange liquid will enter the pores and open-cells of the open-cell porous material. The evaporator tube is configured to receive, at the input end, a source fluid to be cooled from a first temperature to a second temperature each higher than a boiling temperature of the heat exchange liquid, and guide the source fluid from the input end through the evaporator tube to the output end. The evaporator tube is immersed in the pool of heat exchange liquid and source fluid is moved through the evaporator tube, and heat from the source fluid will pass through the wall and the thermally conductive open-cell porous material to cause the heat exchange liquid within the open-cells to boil to a heat exchange vapor. The heat exchange vapor will move through the open cells of the thermally conductive porous material and will be replaced in the open-cells by more heat exchange liquid.

The system can further include a condensing system for condensing the heat exchange vapor to a heat exchange liquid and returning the heat exchange liquid to the pool of refrigerant liquid. The chamber can have a heat exchange vapor outlet to release heat exchange vapor and a heat exchange liquid inlet to receive heat exchange liquid. The condensing system comprises a condensing heat exchanger to receive the heat exchange vapor from the chamber, condense the heat exchange vapor to the heat exchange liquid, and return the heat exchange liquid to the pool of heat exchange liquid in the chamber.

The thermally conductive open-cell porous material can be a foam of a conductive material. The thermally conductive porous material can include at least one selected from the group consisting of metal, graphite, or carbon foams. The metal foam can include at least one selected from the group consisting of Cu, Al, or Fe, and alloys thereof.

The open cells of the thermally conductive open-cell porous material can have pore openings between cells. The pore size of the pore openings can be from 0.1 μm to 100 mm. The cell diameter of the open cells can increase from a first size proximate to the evaporator tube to a second size greater than the first size distal to the evaporator. The thermally conductive open-cell porous material can have a porosity in a range of 40%-99%. The thermally conductive porous material can have a pore density in a range of 5-100 pores per inch (PPI). The open cells of the thermally conductive open-cell porous material can have a cell diameter of from 1 μm to 10 mm.

The thermally conductive open-cell porous material can be provided as a layer surrounding the evaporator tube. The layer of thermally conductive porous material can have a thickness in a range of 10%-100% of an outer radius of the evaporator tube.

The system can further include a plurality of evaporator tubes embedded in a matrix of the thermally conductive open cell porous material. The thermally conductive open-cell porous material can include a coating to change the surface morphology on the layer of thermally conductive porous material.

The heat exchange fluid can be any suitable such fluid. IN one aspect, the heat exchange fluid comprises water.

A method of conducting heat exchange can include the step of providing a heat exchanger that includes a chamber and an evaporator tube. The chamber can be configured to hold heat exchange fluid including heat exchange vapor and a pool of heat exchange liquid. The evaporator tube has a wall with an inner surface and an outer surface, and an input end and an opposite, output end, with a layer of thermally conductive open-cell porous material disposed on the outer surface and comprising a plurality of pores. At least a portion of the evaporator tube can be immersed in the heat exchange liquid held in the chamber. The evaporator tube is configured to receive, at the input end, a source fluid to be cooled from a first temperature to a second temperature each higher than a boiling temperature of the heat exchange liquid, and guide the source fluid from the input end through the evaporator tube to the output end. A pool of heat exchange liquid is provided in the chamber such that when the evaporator tube is immersed in the heat exchange liquid, heat exchange liquid will enter the pores and the open-cells of the thermally conductive open-cell porous material.

Source fluid at the first temperature is directed through the evaporator. The source fluid exits the evaporator tube at the second temperature. The source fluid exchanges heat with the evaporator tube, the thermally conductive porous material, and thereby with the heat exchange liquid within the pores of the thermally conductive open-cell porous material. The heat exchange liquid will change state to a heat exchange vapor and the heat exchange vapor will move through the open-cells of the thermally conductive open-cell porous material and will be replaced by heat exchange liquid. The heat exchange vapor can contact and release heat to a condensing system, and will be transformed from heat exchange vapor to heat exchange liquid and will return to the pool of heat exchange liquid.

A method of heating a fluid can include the step of providing a heat exchange tube having a heat exchange wall with an inner surface and an outer surface for separating a first heat exchange fluid from a second heat exchange fluid. The first heat exchange fluid moves in a flow direction relative to the inner surface of the wall. The heat exchange tube includes a layer of thermally conductive open-cell porous metal foam having a plurality of pores. The thermally conductive open-cell porous metal foam is disposed on the outer surface of the heat exchange tube.

The first heat exchange fluid flows through the tube while the second heat exchange fluid penetrates the pores of the thermally conductive open-cell porous metal foam, such that the first heat exchange fluid exchanges heat with the second heat exchange fluid.

The thermally conductive open-cell porous metal foam includes open cells having a cell diameter. The cell diameter of the open cells increases from a first size proximate to the wall to a second size greater than the first size distal to the wall. The porous material can include open cells having a cell diameter, and the cell diameter of the open cells can increase from a first size at an upstream location relative to the flow direction to a second size less than the first size downstream relative to the flow direction.

A component heat exchange system can include a heat exchanger comprising a chamber and an evaporator tube. The chamber is configured to hold a heat exchange fluid including a heat exchange vapor and a pool of heat exchange liquid. The evaporator tube has an outer surface and includes a thermally conductive open-cell porous material disposed on the outer surface and having a plurality of pores. At least a portion of the evaporator tube is immersed in the heat exchange liquid held in the chamber such that heat exchange liquid will enter the pores and open-cells of the thermally conductive open-cell porous material.

The evaporator tube is thermally connected by a thermal connection to the component such that heat is transferred from the component to the evaporator tube. The temperature of the evaporator tube is higher than a boiling temperature of the heat exchange liquid, wherein heat from the component will pass through the wall and the thermally conductive open-cell porous material to cause the heat exchange liquid within the thermally conductive open-cells to boil to a heat exchange vapor, and the heat exchange vapor will move through the open cells of the thermally conductive open-cell porous material and will be replaced in the open cells by more heat exchange liquid which will then also evaporate.

The component can be an electrical component such as a processor, a mechanical component such as an internal combustion engine, or a power generation component such as a nuclear reactor. The component can include at least one selected from the group consisting of an electrical component, a mechanical component, a chemical reactor component, and a nuclear reactor component. The electrical component can include a processor. The mechanical component can include an internal combustion engine.

BRIEF DESCRIPTION OF THE DRAWINGS

There are shown in the drawings embodiments that are presently preferred it being understood that the invention is not limited to the arrangements and instrumentalities shown, wherein:

FIG. 1 is a cross-section of a heat exchange tube according to the invention for pool boiling heat exchange.

FIG. 2 is the cross-section of FIG. 1 showing the heat exchange tube in a pool boiling operation.

FIG. 3 is an expanded view of area FIG. 3 in FIG. 2.

FIG. 4 is a schematic diagram of a pool boiling heat exchanger with an external heat exchange fluid condenser.

FIG. 5 is a schematic diagram of a pool boiling heat exchanger with an internal heat exchange fluid condenser.

FIG. 6 is a schematic diagram of an open-loop pool boiling test system.

FIG. 7 is a schematic diagram of component cooling using the heat transfer system of the invention.

FIG. 8 is a schematic diagram of a high-pressure heat exchange system.

FIG. 9 is a schematic diagram, partially broken away, of a shell and tube pool boiling apparatus.

FIG. 10 is an expanded view of area FIG. 10 in FIG. 9.

FIG. 11 is cross-section of a heat exchange apparatus with heat exchange tubes in a thermally conductive porous open-cell material matrix.

FIG. 12 is a cross-section of a heat exchange tube with a thermally conductive porous open-cell material having a radial gradient cell size.

FIG. 13 is plot of heat transfer coefficient HTC (kWm⁻²K⁻¹) vs. heat flux q (kWm⁻²).

FIG. 14 is a plot of heat flux q (kWm⁻²) vs. excess temperature ΔT=T_wall−T_sat (° C).

FIG. 15 is a plot of heat transfer coefficient kWm⁻²K⁻¹ vs. heat flux kWm⁻².

FIG. 16 is a plot of heat transfer coefficient kWm⁻²K⁻¹ vs. heat flux kWm⁻².

FIG. 17 is a plot of heat transfer coefficient (kWm⁻²K⁻¹) vs. heat flux kWm⁻².

FIG. 18 is a plot of heat transfer coefficient (kWm⁻²K⁻¹) vs. heat flux (kWm⁻²).

DETAILED DESCRIPTION OF THE INVENTION

A boiling heat exchange system includes a heat exchanger with a chamber and an evaporator. The chamber is configured to hold a heat exchange fluid including a heat exchange vapor and a pool of heat exchange liquid. The evaporator tube has a wall with an inner surface and an outer surface, an input end and an opposite, output end. A layer of thermally conductive open-cell porous material is disposed on the outer surface and has a plurality of pores. At least a portion of the evaporator tube can be immersed in the heat exchange liquid held in the chamber and heat exchange liquid will enter the pores and open-cells of the thermally conductive open-cell porous material. The thermally conductive open-cell porous material can extend around the entire perimeter of the evaporator tube, so that when immersed in the heat exchange liquid heat exchange with the thermally conductive open-cell porous material is possible 360° about the perimeter of the evaporator tube.

The evaporator tube can be configured to receive, at the input end, a source fluid to be cooled from a first temperature to a second temperature each higher than a boiling temperature of the heat exchange liquid. The evaporator tube guides the source fluid from the input end to the output end. The evaporator tube is immersed in the pool of heat exchange liquid. As the source fluid is moved through the evaporator tube, heat from the source fluid will pass through the wall and the thermally conductive open-cell porous material to cause the heat exchange liquid within the thermally conductive open-cells to boil to a heat exchange vapor, and the heat exchange vapor will move through the open cells of the thermally conductive porous material and will be replaced in the open cells by more heat exchange liquid which will then also evaporate.

The system can be open-loop in which the heat exchange fluid is used only once and then vented or transferred to storage or another process. The system can also be closed-loop, in which the refrigerant is condensed and returned to the pool of heat exchange liquid for reuse. The system can include a condensing system for condensing the heat exchange vapor to a heat exchange liquid and returning the heat exchange liquid to the pool of heat exchange liquid. The chamber can include a heat exchange vapor outlet to release heat exchange vapor and a heat exchange liquid inlet to receive heat exchange liquid. The condensing system can include a condensing heat exchanger to receive the heat exchange vapor from the chamber, condense the heat exchange vapor to the heat exchange liquid, and return the heat exchange liquid to the pool of heat exchange liquid in the chamber. In an alternative embodiment, the condensing heat exchanger can be located in the vapor space within the chamber.

The thermally conductive open-cell porous material can be a foam of a conductive material. The conductive material can be at least one selected from the group consisting of metal, graphite, or carbon foams. The metal foam can include at least one selected from the group consisting of Cu, Al, or Fe, and alloys thereof. Other thermally conductive open-cell porous materials are possible.

The open cells have pore openings between cells. The pore size of the pore openings can vary. The pore size of the pore openings can be from 0.1 μm to 100 mm. The pore size of the pore openings can be 0.1 μm, 1 μm, 100 μm, 200 μm, 300 μm, 400 μm, 500 μm, 600 μm, 700 μm, 800 μm, 900 μm, 1 mm, 5 mm, 10 mm, 15 mm, 20 mm, 25 mm, 30 mm, 35 mm, 40 mm, 45 mm, 50 mm, 55 mm, 60 mm, 65 mm, 70 mm, 75 mm, 80 mm, 85 mm, 90 mm, 95 mm, or 100 mm, and can be within a range of any high value and low value selected from these values.

The cell diameter and/or the pore size of the open cells can increase from a first size proximate to the evaporator tube outer wall to a second size greater than the first size distal to the evaporator tube wall.

The thermally conductive open-cell porous material can have a porosity in a range of 40%-99%. The porosity of the foam can be 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 96, 97, 98, or 99%, and can be within a range of any high value and low value selected from these values.

The thermally conductive open-cell porous material can have a pore density in a range of 5-100 pores per inch (PPI). The pore density of the thermally conductive open-cell porous material can be 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, or 100 pores per inch, and can be within a range of any high value and low value selected from these values.

The open cells of the thermally conductive open-cell porous material can have any suitable cell diameter. The open cells have a cell diameter of from 1 μm to 10 mm. The cell diameter of the open cells can be 1 μm, 10 μm, 100 μm, 200 μm, 300 μm, 400 μm, 500 μm, 600 μm, 700 μm, 800 μm, 900 μm, 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, or 10 mm, and can be within a range of any high value and low value selected from these values.

The thickness of the thermally conductive porous material can vary. The layer of thermally conductive open-cell porous material can have a thickness in a range of 10%-100% of an outer radius of the evaporator tube. The layer of thermally conductive porous material can have a thickness of 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 or 100% of an outer radius of the evaporator tube, and the thickness can be in a range of any high value and low value selected from these values.

The invention can take different forms. A plurality of evaporator tubes can be embedded in a matrix of the thermally conductive open cell porous material. The evaporator tubes and matrix of the thermally conductive open cell porous material can be positioned within an outer housing.

The porous material can have a coating to change the surface morphology on the layer of thermally conductive porous material. This can be to alter the hydrophobic or hydrophilic characteristics of the porous material to suit a particular heat exchange fluid.

The system can be used with many different heat exchange fluids fluids. The heat exchange fluid should be capable of a phase transition, evaporation or boiling, in the temperature and pressure range that is required. The heat exchange fluid in one example can comprises water. Other heat exchange fluids can be refrigerants. The refrigerant can be pure or can include contaminants such as oil.

The evaporator tube can have any suitable shape. The evaporator tube will commonly be tubular, but as used herein the term encompasses other shapes such as square, triangular, pentagonal, hexagonal, octagonal, and oval shapes. Still other cross-sectional shapes are possible.

A method of conducting heat exchange can include the step of providing a heat exchanger with a chamber and an evaporator tube. The chamber is configured to hold a heat exchange fluid including a heat exchange vapor and a pool of heat exchange liquid. The evaporator tube has a wall with an inner surface and an outer surface, an input end and an opposite, output end. A layer of thermally conductive open-cell porous material is disposed on the outer surface and has a plurality of pores. At least a portion of the evaporator tube can be immersed in the heat exchange liquid held in the chamber and heat exchange liquid will enter the pores and open-cells of the thermally conductive open-cell porous material.

The evaporator tube is configured to receive, at the input end, a source fluid to be cooled from a first temperature to a second temperature each higher than a boiling temperature of the heat exchange liquid. The evaporator tube guides the source fluid from the input end to the output end. The evaporator tube is immersed in the pool of heat exchange liquid. As the source fluid is moved through the evaporator tube, heat from the source fluid will pass through the wall and the thermally conductive open-cell porous material to cause the heat exchange liquid within the thermally conductive open-cells to boil to a heat exchange vapor, and the heat exchange vapor will move through the open cells of the thermally conductive porous material and will be replaced in the open cells by more heat exchange liquid which will then also evaporate.

The method includes the step of providing a pool of heat exchange liquid in the chamber such that the evaporator tube is immersed at least partially in the heat exchange liquid, and the heat exchange liquid will enter the pores and the open-cells of the thermally conductive porous material.

The source fluid at the first temperature is directed through the evaporator. The source fluid exits the evaporator tube at the second temperature. The source fluid exchanges heat with the evaporator tube, the thermally conductive porous material, and thereby with the heat exchange liquid within the pores of the thermally conductive porous material. The heat exchange liquid changes state to a heat exchange vapor and the heat exchange vapor moves through the open-cells of the thermally conductive porous material and is be replaced by more heat exchange liquid moving into the open-cells.

The method can further include the step of providing a condensing system and condensing the heat exchange vapor. The heat exchange vapor will contact and release heat to the condensing system, and is transformed from heat exchange vapor to heat exchange liquid. The condensed heat exchange liquid is then returned to the pool of heat exchange liquid.

A component heat exchange system can include a chamber and an evaporator in the chamber. The chamber is configured to hold a heat exchange fluid including a heat exchange vapor and a pool of heat exchange liquid. A layer of thermally conductive open-cell porous material is disposed on the outer surface of the evaporator tube and has a plurality of pores. At least a portion of the evaporator tube can be immersed in the heat exchange liquid held in the chamber and heat exchange liquid will enter the pores and open-cells of the thermally conductive open-cell porous material.

The evaporator tube is thermally connected by a thermal connection to the component such that heat is transferred from the component to the evaporator tube. The evaporator tube does not in this embodiment flow a source fluid through it, but rather can be a solid material or can be filled with a thermally conductive material such that when heat is transferred from the component to the evaporator tube the temperature of the evaporator tube will rise. The temperature of the evaporator tube is higher than a boiling temperature of the heat exchange liquid. The evaporator tube is immersed in the pool of heat exchange liquid within the chamber. Heat from the component will pass to the evaporator tube and through the wall of the evaporator tube to the thermally conductive open-cell porous material to cause the heat exchange liquid within the thermally conductive open-cells to boil to a heat exchange vapor. The heat exchange vapor will move through the open cells of the thermally conductive porous material and will be replaced in the open cells by more heat exchange liquid which will then also evaporate.

The component that is cooled can any of several different kinds of possible components. The component can be an electrical component such as a processor, a mechanical component such as an internal combustion engine, or a power generation component such as a nuclear reactor.

There is shown in FIGS. 1-3 an evaporator tube 10 according to the invention. The evaporator tube 10 includes a tube 14 with an open interior 18 and an outer layer of a thermally conductive open-cell porous material 22. As shown in FIG. 2, the evaporator tube 10 can be immersed in a heat exchange liquid 34. A source fluid 30 flows through the tube 14. The source fluid 30 is at a temperature higher than the boiling temperature of the heat exchange liquid 34. Heat is transferred from the source fluid 30 to the wall 14 and thereby to the thermally conductive porous material 22. The heat exchange liquid 34 enters the pores of the porous material 22 and exchanges heat from the walls of the open cells making up the porous material 22. The heat exchange liquid 34 within the pores of the porous material 22 encounters an outer surface 16 (FIG. 3) of the tube 14 and absorbs heat sufficient to change state to heat exchange vapor bubbles 38. The vapor bubbles 38 progress away from the tube outer surface 16 and can join with other vapor bubbles to form larger vapor bubbles 40 which then leaves the porous material 22 and float to the top of the pool of heat exchange liquid 34. Additionally, heat exchange vapor bubbles 40 can form some distance from the tube outer surface 16 owing to the thermal conductivity of the porous material 22. Heat will be exchanged from the walls of the open cells of the porous material 22 directly to heat exchange liquid 34 within the open cells to vaporize the heat exchange liquid 34 into heat exchange vapor bubbles 40.

There is shown in FIG. 4 a heat exchange system 100 according to the invention having a chamber 104 defining a interior space 110 for the heat exchange fluid. An evaporator tube 112 is mounted within the interior space 110. The evaporator tube 112 has a tube 113 with an open interior flow passage 116 and a layer of thermally conductive open cell porous material 120 on an outer surface of the tube 113. The evaporator tube 112 is immersed in a pool of heat exchange liquid 124 within the chamber 104. The chamber contains both heat exchange liquid 124 and heat exchange vapor 128.

Source fluid flows through the open interior 116 of the evaporator tube 112, entering at a temperature T₁, which is higher than the boiling point of the heat exchange liquid 124 at the operating pressure of the chamber 104. Heat exchange vapor bubbles 132 form at the surface of the tube 113 and within the layer of porous material 120, and heat exchange vapor bubbles 136 rise in the direction shown by arrow 140. The source fluid exits at temperature T₂ that is less than T₁ but above the boiling point of the heat exchange liquid 124.

The vapor bubbles 136 progress into the heat exchange vapor 128 above the heat exchange liquid 124 and some heat exchange vapor 148 progresses in the direction of arrow 152 into a heat exchange vapor outlet 156. The heat exchange vapor 148 is directed to a condenser 160 having a condensing coil 176 which is contacted by cooling fluid flowing into the condenser 160 through an inlet 164 and through an outlet 172 and can be controlled by a suitable valve 178. The heat exchange vapor 148 is condensed back into a heat exchange liquid, which is returned through condenser outlet 180 back to the chamber 104 and falls as heat exchange liquid droplets 184 in the direction shown by arrow 188 back to the pool of heat exchange liquid 124.

There is shown in FIG. 5 a heat exchange system 200 having a chamber 204 defining an interior space 210 for the heat exchange fluid. An evaporator tube 212 is mounted within the interior space 210. The evaporator tube 212 has tube core 213 with an open interior flow passage 216 and a layer of thermally conductive open cell porous material 220 on an outer surface of the tube core 212. The evaporator tube 212 is immersed in a pool of heat exchange liquid 224 within the chamber 204. The chamber 204 has both heat exchange liquid 224 and heat exchange vapor 228.

Source fluid flows through the open interior 216 of the tube 213, entering at a temperature T₁, which is higher than the boiling point of the heat exchange liquid 224 at the operating pressure of the chamber 204. Heat exchange vapor bubbles 232 form at the layer of porous material 220, and heat exchange vapor bubbles 236 rise in the direction shown by arrow 240. The source fluid exits at temperature T₂ that is less than T₁ but above the boiling point of the heat exchange liquid 224.

The vapor bubbles 236 progress into the heat exchange vapor 228 above the heat exchange liquid 224 and some heat exchange vapor 248 progresses upward in the direction of arrow 252 where the heat exchange vapor contacts a heat exchange vapor condenser 260 within the chamber 204. The condenser 260 is part of a refrigeration cycle which removes the heat that is contained by the heat exchange vapor 248.

The refrigeration cycle can be closed-loop and utilize a refrigerant. The refrigeration cycle can include a compressor 272 which receives refrigerant gas from the heat exchange vapor condenser 260 through a connection 268. The compressed refrigerant gas is passed through a connection 276 to an external heat exchanger 280, where it is cooled by suitable heat exchanging apparatus such as cooling coils 286. The heat exchanger 280 receives cooling fluid through inlet 282 and exhausts cooling fluid through exhaust outlet 284. The refrigerant is passed through a connection 290 and a valve 292 and through the refrigerant inlet 264 to the heat exchange vapor condenser 260. The heat exchange vapor is condensed by the heat exchange vapor condenser 260 back into a heat exchange liquid, which falls as heat exchange liquid droplets 294 in the direction shown by arrow 296 back to the pool of heat exchange liquid 224.

There is shown in FIG. 6 system 300 for pool boiling and particularly for testing pool boiling performance of the system 300 and heat exchange tubes according to the invention. The system 300 includes a chamber 304 having an open interior 310. A heat exchange liquid 324 and heat exchange vapor 328 are provided in the interior 310. A plurality of evaporator tubes 312 are provided in the open interior 310. An outer layer of thermally conductive porous material 316 is provided on an outer surface of the evaporator tube 312. The evaporator tubes 312 in this embodiment comprise resistive heating elements for heating the thermally conductive porous material 316. The evaporator tubes 312 can be mounted on suitable supports 326 and 328.

Evaporator tubes 312 are connected by suitable electrical connections 330, 332 to respective electrical buses 331, 333. These connect to power supply lead lines 336 and 337. Power is applied to the resistive heating elements of the evaporator tubes 312 and thereby to the thermally conductive porous material 316 to heat the thermally conductive open-cell porous material to a temperature that is above the boiling temperature of the heat exchange liquid 324.

The heat exchange liquid 324 is heated by contact with the heated thermally conductive open-cell porous material 316 and evaporates to form heat exchange vapor bubbles 340. A heat exchange vapor condenser 350 is provided in the interior 310. The heat exchange vapor condenser 350 condenses the heat exchange vapor back to heat exchange liquid droplets 344, which fall back into the pool of heat exchange liquid 324. The heat exchange vapor condenser 350 receives cooling fluid from connection 354 which flows in the direction of arrow 356. The cooling fluid absorbs heat as it flows through the heat exchange vapor condenser 350 and exits through heated cooling fluid exhaust connection 360 in the direction shown by arrow 362. A suitable thermocouple 368 can have a sensor 370 extending into the heat exchange liquid pool 324. Another thermocouple 372 can have a sensor 374 to monitor temperatures of the heat exchange vapor 328. A pressure sensor 373 can also be provided.

There is shown in FIG. 7 a system 400 for cooling various components such as electronic devices. The system includes a chamber 404 having a heat exchange liquid 412 and a heat exchange vapor 408 in an open interior 410 of the chamber 404. A plurality of evaporator tubes 416 having a thermally conductive core 418 are provided in the open interior 410. An outer layer of thermally of thermally conductive open-cell porous material 422 is provided on an outer surface of the core 418. Electrical devices 430-432 or other devices in need of heat management are connected by thermally conductive connections 435-437 to the evaporator tubes 416. The thermally conductive connections 435-437 can be any suitable thermal conductor. The evaporator tubes 416 can be supported in the heat exchange liquid 412 by suitable supports 450, 452. In some cases it may be desirable to place the components that are being cooled directly within the chamber 404 as shown in phantom by electrical devices 440-442.

The heat generated by the electrical components 430-432 (or electrical components 440-442) is transmitted to the evaporator tubes 418 and this heat is transferred to the thermally conductive open-cell porous material 422. Heat is transferred to the heat exchange liquid 412, generating heat exchange vapor bubbles 460. These heat exchange vapor bubbles 460 travel upward and contact a heat exchange vapor condenser 470. Heat exchange vapor condenser 470 receives cooling liquid through a cooling fluid inlet connection 474 which flows in the direction of arrow 475 and absorbs heat from the heat exchange vapor and exits the heat exchange vapor condenser 470 as cooled heat exchange cooling fluid through a cooling fluid exhaust connection 476 in a direction shown by arrow 477. The is condensed into heat exchange liquid droplets 478 which drop back into the pool of heat exchange liquid 412. A thermocouple 482 can have a sensor 483 extending into the heat exchange vapor 408. A thermocouple 486 can have a sensor 487 which extends into the pool of heat exchange liquid 412. A pressure sensor 480 can also be provided for sensing the pressure within the chamber 404.

There is shown in FIG. 8 a heat exchange system 500 for operation at elevated pressures. The system 500 includes a pressure chamber 504 having an open interior 506 with heat exchange liquid 507 and a heat exchange vapor 508. A plurality of evaporator tubes 510 having a core 514 comprising resistive heating elements are provided in interior 506. An outer layer of thermally of thermally conductive open-cell porous material 518 is provided on an outer surface of the core 514. Electrical buses 512, 516 receive electrical power from power source connections 520, 524 and provide this power to the resistive heater elements 514 through suitable electrical connections 513, 517. The evaporator tubes can be mounted on suitable supports 528, 529 so as to be immersed or partially immersed in the heat exchange liquid 507.

Heat is transmitted to the thermally conductive open-cell porous material 518 which heats the heat exchange liquid 507 and generates heat exchange vapor bubbles 534. The exchange vapor bubbles 534 enter a heat exchange vapor outlet connection 536 as shown by arrow 537. A thermocouple 538 senses temperature in the line 536. The vapor enters an external condenser 540 such as a plate condenser and leaves as condensed liquid through return heat exchange liquid connection 544 as shown by arrow 545. A thermocouple 546 can be provided to monitor the temperature of the heat exchange liquid in the heat exchange liquid connection 544. The external condenser 540 receives cooling fluid through a cooling fluid inlet connection 550 as indicated by arrow 551 and the temperature of the cooling fluid can be monitored by thermocouple 552. The cooling fluid is exhausted from the external condenser 540 through a cooling fluid exhaust connection 560 as indicated by arrow 561. A thermocouple 562 can be provided to sense the temperature of the exhausted cooling fluid, and a valve 564 can be provided to control the flow of the cooling fluid.

A heat exchange fluid charge and vacuum connection 570 can be provided and controlled by valve 572. A pressures sensor 574 and temperature sensor 576 can be provided to sense the pressure and temperature in the charge or vacuum connection. A thermocouple 578 with sensor 579 can extend into the vapor 508, and a thermocouple 580 with sensor 581 can extend into the liquid 507. A pressure gauge 584 can be provided and can have a pressure sensor element 585 extending into the pressure chamber 504.

There is shown in FIGS. 9-10 a shell and tube type heat exchange system 600. The system 600 includes a chamber 604 with a plurality of evaporator tubes 610 mounted therein having a tube core 611 having an open interior 613 and an outer layer of thermally conductive open-cell porous material 612 on an outer surface of the tube core 611. The chamber receives a source fluid through a port 614 as shown by arrows 615, flows through the tubes 611 as shown by arrows and exhausts the heat exchange fluid through a port 618 as indicated by arrow 619. Within the chamber 604 is heat exchange liquid 630 and heat exchange vapor 632. The heat exchange liquid 630 flows into the chamber 604 through a port 624 as indicated by arrow 625. Some of the heat exchange vapor 638 rises and exits the chamber 604 through a port 628 as indicated by arrow 629. The exhausted vapor 640 exiting the port 628 can be condensed and recycled, or in the case of environmentally acceptable materials such as water can be vented directly to the atmosphere. As shown particularly in FIG. 10, the flow 617 of source fluid heats the tubes 611 and the thermally conductive porous material 612, which causes the formation of heat exchange vapor bubbles 634 which progress into the vapor space 632 and exits as flowing vapor 638 which exits through the port 628.

There is shown in FIG. 11 an alternative embodiment of a heat exchange system 700. A chamber 704 includes an open interior 706 in which there is a matrix 710 of a thermally conductive open-cell porous material which substantially fills the interior 706 of the chamber 704 and can be secured to the interior wall 708 of the chamber 704. A vapor space 716 between the top of the matrix 710 and the top of chamber 704 can be provided. A plurality of heat exchange tubes 714 is provided within the matrix 710. The heat exchange tubes 714 can have open interiors 715 which allow for the passage of a source fluid to exchange heat with a heat exchange fluid within the chamber 704. The heat exchange fluid enters through an inlet port 720 as indicated by arrow 721 and exits through an exit port 724 is indicated by arrow 725.

There is shown in FIG. 12 an embodiment of an evaporator tube 800 with a tube core 804, A thermally conductive open-cell porous material 802 is provided on an outer surface 806 of the tube core 804, and has a graduated porosity from a point near the tube outer surface 806 to an outer surface 808 of the thermally conductive open-cell porous material 802. The evaporator tube 800 has an innermost layer 810 of the thermally conductive open-cell porous material adjoining outer surface 806 of the tube 804 with a smaller porosity that outer layers. The thermally conductive open-cell porous material 802 has a graduated porosity, such that the porosity of a middle layer 812 is greater than the porosity of the innermost layer 810 and less than the porosity of an outermost layer 816. A source fluid flows in the direction of arrow 830 through open interior 820 of the tube 804 and exhausts as shown by arrow 834. The graduated porosity allows more space for more vapor which accumulates as the vapor progresses radially outward from the tube surface 806 given that some vapor forms at the outer surface tube 804 and still more vapor is formed by heat transfer with the walls of the open cell porous material some distance from the outer surface 806 of the tube 804. Although shown with porosity increasing radially outward, the porosity of the thermally conductive open-cell porous material can also or alternatively be increased longitudinally relative to the flow direction of the source fluid.

One of the applications for the invention is the thermal management of lithium-ion batteries with the two-phase immersion cooling with dielectric fluid. In order to evaluate the potential performance improvement of the proposed enhanced tubes in dielectric fluid, an open-loop pool boiling test rig was built. The apparatus consists of a clear rectangular container, an immersed test section, and a condensing unit, as illustrated in FIG. 6. The clear container was made of polycarbonate, and the test section was a horizontally mounted round tube immersed in a dielectric liquid pool, which served as the heat exchange liquid. The tube bundle contained four aluminum tubes placed in a staggered tube arrangement, with one of the tubes not being visible in FIG. 6. For each individual tested tube, four thermocouple holes were drilled alongside the tube wall, Type-T thermocouples were inserted for measuring the wall temperatures. The heat flux for the tube was supplied by a cartridge heater installed in the tube. Power to the tube heater was controlled using pulse width modulation, allowing for different heater outputs to be approximated over a time span of 1 second, and current transformers were used to measure the amp draw of the heater at a specific power setting.

The heat transfer coefficient of the externally enhanced tube (single tube) at various heat fluxes is compared with that of the bare aluminum tube, as shown in FIG. 13. The effective heat transfer coefficient of the enhanced tube, determined based on the outer diameter of the round tube, is 2.1˜4.8 times higher than the bare aluminum tube, and the metal foam had a better heat transfer enhancement at lower heat fluxes. FIG. 14 shows the comparison of the pool boiling curves for bare copper, bare aluminum, and metal foam enhanced aluminum tubes. For the bare copper and bare aluminum tubes, the critical heat fluxes occur at 135 kW/m² and 131 kW/m², respectively. However, the metal foam enhanced tube does not reach the critical heat flux during the whole test conditions, where the maximum heating power of the current test facility is 228 kW/m². This indicates that the metal foam around the tube sample delays the critical heat flux and maintains a certain level of heat transfer capacity at higher heat fluxes.

Three enhanced tube bundles with uncompressed, 2×compressed, and 3×compressed metal foams were experimentally evaluated, and the corresponding porosities are 81%, 75%, and 62%, respectively. FIG. 15 shows the comparison of their pool boiling performances in HFE-7000 against that of the bare tube bundle. The metal foam tubes with an 81% porosity showed a 100-166% enhancement in heat transfer coefficient compared to the bare tubes. As the porosity decreases, the heat transfer coefficient increases even more. For example, the metal foam tubes with a 75% porosity showed about a 120-212% improvement in heat transfer coefficient over the bare tubes. The enhanced heat transfer coefficient of the metal foam tubes, compared with the bare tubes, is mainly attributed to the increased surface area and nucleation sites. Experiments with another dielectric fluid, HFE 7100, were also carried out in order to better understand the behavior of metal foam tubes. Similar to the HFE 7000, the HFE 7100 working fluid also showed enhanced heat transfer performance for metal foam tubes, as shown in FIG. 16. The metal foam tubes with 81% porosity offered about 100-166% enhancement for the HFE 7000, whereas the enhancement percentage has been reduced to 70-90% for the HFE 7100.

Another application for the invention is refrigerant pool boiling in a flooded evaporator. Another apparatus was developed to evaluate the performance of the tube bundle for high-pressure refrigerants. The schematic of the refrigerant boiling set up is shown in FIG. 8. The main components of the this system are the high pressure chamber 504, a bundle of the tubes 510, the plate heat exchanger (condenser) 540, and a cooling unit. The tube bundle experiment for this application was conducted in the high-pressure chamber 504. Upon placing the tube bundle, the chamber 504 was vacuumed and charged with refrigerant 507. The heaters 514 were powered to begin the boiling experiments, and then the heating power was gradually increased to understand the boiling behavior at different heat fluxes. The vapor 508 produced by the boiling process was directed to the plate heat exchanger 540 and was condensed back to liquid by exchanging heat with cooling water flow indicated by arrow 551. The temperature and the flow rate of the cooling water were controlled to maintain the constant system pressure. The plate heat exchanger 540 was supplied with cooling water from a chiller (not shown) that provided the required cooling capacity to condense the vapor. The condensed refrigerant was passed to the high-pressure chamber 504 through the liquid return line 544. The temperature of the liquid refrigerant 506 and refrigerant vapor 508 was monitored at the top and bottom of the vessel using T-type thermocouples.

The average heat transfer coefficients of refrigerant, R134a, over the metal foam tube bundles with different porosities are compared against the plain aluminum tube bundle, as shown in FIG. 17. Overall, all of the tested metal foam tube bundles showed higher heat transfer coefficients compared to the plain aluminum tube bundle in the refrigerant pool. Among the metal foam tube bundles, 62% porosity exhibited a higher heat transfer coefficient than that of a plain tube, followed by 75% and 81% porosities. The metal foam tubes with 62% improved the heat transfer coefficient by 102-153% and 162-291% over the plain tubes at high (>39 KW m−2) and low (<39 KW m−2) heat fluxes, respectively. Similarly, compared with the plain tube bundle, the metal foam tubes with 75% porosity offered an enhancement of 95-127% and 133-202% at high (>39 KW m−2) and low (<39 KW m−2) heat fluxes, respectively. The metal foam tubes with 81% porosity, exhibited an enhancement of heat transfer coefficient that remained identical at 80-90% over the entire heat flux range. In addition to R134a (GWP=1300), another two low GWP refrigerants R-1234yf (GWP=4) and R-1234ze(E) (GWP=7) were tested. When the heat flux is less than ˜50 KW m−2, the HTCs of R-1234yf and R1234ze(E) are quite similar to those of R-134a. However, when heat flux is more than ˜50 KW m−2, the HTCs of R-1234yf are nearly 10% higher than those of R-134a for both plain and metal foam tubes, while a 5% reduction in heat transfer coefficient was observed for R-1234ze(E) compared with R-134a.

The metal foam enhanced tube bundles were able to improve the pool boiling performance for both dielectric fluids and refrigerants due to the larger surface area of the enhanced tube and a greater number of nucleation sites. For the dielectric fluids, the heat transfer coefficient of the metal foam tube bundle offered a 100-212% improvement in heat transfer coefficient compared with the bare tube bundle. The critical heat flux of the enhanced tube is at least double compared to the bare aluminum tube. For the refrigerants, the metal foam enhanced tube bundle provides an 80-291% enhancement in heat transfer coefficient under the current test conditions.

The invention as shown in the drawings and described in detail herein disclose arrangements of elements of particular construction and configuration for illustrating preferred embodiments of structure and method of operation of the present invention. It is to be understood however, that elements of different construction and configuration and other arrangements thereof, other than those illustrated and described may be employed in accordance with the spirit of the invention, and such changes, alternations and modifications as would occur to those skilled in the art are considered to be within the scope of this invention as broadly defined in the appended claims. In addition, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.

Claims

1. A boiling heat exchange system, comprising: a heat exchanger comprising: i) a chamber configured to hold a heat exchange fluid including a heat exchange vapor and a pool of heat exchange liquid; and,ii) an evaporator tube having a wall with an inner surface and an outer surface, wherein the evaporator tube has an input end and an opposite, output end and a thermally conductive open-cell porous material disposed on the outer surface and comprising a plurality of pores, wherein at least a portion of the evaporator tube can be immersed in the heat exchange liquid held in the chamber and heat exchange liquid will enter the pores and open-cells of the open-cell porous material, and wherein the evaporator tube is configured to receive, at the input end, a source fluid to be cooled from a first temperature to a second temperature each higher than a boiling temperature of the heat exchange liquid, guide the source fluid from the input end through the evaporator tube to the output end, wherein when the evaporator tube is immersed in the pool of heat exchange liquid and source fluid is moved through the evaporator tube, heat from the source fluid will pass through the wall and the thermally conductive open-cell porous material to cause the heat exchange liquid within the open-cells to boil to a heat exchange vapor, and the heat exchange vapor will move through the open cells of the thermally conductive porous material and will be replaced in the open-cells by more heat exchange liquid.

2. The system of claim 1, further comprising a condensing system for condensing the heat exchange vapor to a heat exchange liquid and returning the heat exchange liquid to the pool of refrigerant liquid.

3. The system of claim 2, wherein the chamber comprises a heat exchange vapor outlet to release heat exchange vapor and a heat exchange liquid inlet to receive heat exchange liquid; and the condensing system comprises a condensing heat exchanger to receive the heat exchange vapor from the chamber, condense the heat exchange vapor to the heat exchange liquid, and return the heat exchange liquid to the pool of heat exchange liquid in the chamber.

4. The system of claim 1, wherein the thermally conductive open-cell porous material comprises a foam of a conductive material.

5. The system of claim 4, wherein the thermally conductive open-cell porous material comprises at least one selected from the group consisting of metal, graphite, or carbon foams.

6. The system of claim 5, wherein the metal foam comprises at least one selected from the group consisting of Cu, Al, or Fe, and alloys thereof.

7. The system of claim 4, wherein the open cells have pore openings between cells, the pore size of the pore openings being from 0.1 μm to 100 mm.

8. The system of claim 7, wherein the cell diameter of the open cells increases from a first size proximate to the evaporator tube to a second size greater than the first size distal to the evaporator.

9. The system of claim 5, wherein the foam has a porosity in a range of 40%-99%.

10. The system of claim 5, wherein the foam has a pore density in a range of 5-100 pores per inch (PPI).

11. The system of claim 4, wherein the open cells of the thermally conductive open-cell porous material have a cell diameter of from 1 μm to 10 mm.

12. The system of claim 1, wherein the thermally conductive open cell porous material is provided as a layer surrounding the evaporator tube.

13. The system of claim 12, wherein the layer of thermally conductive porous material has a thickness in a range of 10%-100% of an outer radius of the evaporator tube.

14. The system of claim 1, further comprising a plurality of evaporator tubes embedded in a matrix of the thermally conductive open-cell porous material.

15. The system of claim 1, further comprising a coating to change the surface morphology on the layer of thermally conductive porous material.

16. The system of claim 1, wherein the heat exchange fluid comprises water.

17. A method of conducting heat exchange, comprising the steps of: providing a heat exchanger comprising: i) a chamber configured to hold heat exchange fluid including heat exchange vapor and a pool of heat exchange liquid;iii) An evaporator tube having a wall with an inner surface and an outer surface, wherein the evaporator tube has an input end and an opposite, output end, and a layer of thermally conductive open-cell porous material disposed on the outer surface and comprising a plurality of pores, wherein at least a portion of the evaporator tube can be immersed in the heat exchange liquid held in the chamber, and wherein the evaporator tube is configured to receive, at the input end, a source fluid to be cooled from a first temperature to a second temperature each higher than a boiling temperature of the heat exchange liquid, guide the source fluid from the input end through the evaporator tube to the output end;providing a pool of heat exchange liquid in the chamber such that the evaporator tube is immersed in the heat exchange liquid, and heat exchange liquid will enter the pores and the open-cells of the thermally conductive porous material;directing the source fluid at the first temperature through the evaporator, the source fluid exiting the evaporator tube at the second temperature, the source fluid exchanging heat with the evaporator tube, the thermally conductive open-cell porous material, and thereby with the heat exchange liquid within the pores of the thermally conductive open-cell porous material, whereby the heat exchange liquid will change state to a heat exchange vapor and the heat exchange vapor will move through the open-cells of the thermally conductive open-cell porous material and will be replaced by heat exchange liquid.

18. The method of claim 17, wherein the heat exchange vapor contacts and releases heat to a condensing system, and is transformed from heat exchange vapor to heat exchange liquid and the heat exchange liquid is returned to the pool of heat exchange liquid.

19. A method of heating a fluid, comprising the steps of: providing a heat exchange tube, comprising a heat exchange wall with an inner surface and an outer surface for separating a first heat exchange fluid from a second heat exchange fluid, the first heat exchange fluid moving in a flow direction relative to the inner surface of the wall, the heat exchange tube comprising a layer of thermally conductive open-cell porous metal foam having a plurality of pores, the thermally conductive porous metal foam being disposed on the outer surface of the tube; and,flowing the first heat exchange fluid through the heat exchange tube while permitting the second heat exchange fluid to penetrate the pores of the thermally conductive open-cell porous metal foam, wherein the first heat exchange fluid exchanges heat with the second heat exchange fluid.

20. The method of claim 19, wherein the thermally conductive open-cell porous metal foam comprises open cells having a cell diameter, and wherein the cell diameter of the open cells increases from a first size proximate to the wall to a second size greater than the first size distal to the wall.

21. The method of claim 19, wherein the thermally conductive open-cell porous material comprises open cells having a cell diameter, and the cell diameter of the open cells increases from a first size at an upstream location relative to the flow direction to a second size less than the first size downstream relative to the flow direction.

22. A component heat exchange system, comprising: a heat exchanger comprising a chamber and an evaporator tube, the chamber being configured to hold a heat exchange fluid including a heat exchange vapor and a pool of heat exchange liquid;the evaporator tube having an outer surface and comprising thermally conductive open-cell porous material disposed on the outer surface and having a plurality of pores;at least a portion of the evaporator tube being immersed in the heat exchange liquid held in the chamber such that heat exchange liquid will enter the pores and open-cells of the thermally conductive open-cell porous material;wherein the evaporator tube is thermally connected by a thermal connection to the component such that heat is transferred from the component to the evaporator tube; the temperature of the evaporator tube being higher than a boiling temperature of the heat exchange liquid, wherein heat from the component will pass through the wall and the thermally conductive open-cell porous material to cause the heat exchange liquid within the thermally conductive open-cells to boil to a heat exchange vapor, and the heat exchange vapor will move through the open cells of the thermally conductive porous material and will be replaced in the open cells by more heat exchange liquid which will then also evaporate.

23. The component heat exchange system of claim 22, wherein the component comprises at least one selected from the group consisting of an electrical component, a mechanical component, a chemical reactor component, and a nuclear reactor component.

24. The component heat exchange system of claim 23, wherein the electrical component comprises a processor.

25. The component heat exchange system of claim 23, wherein the mechanical component comprises an internal combustion engine.