Patent Application
20120097373
This technology generally relates to methods and device for improving pool boiling and, more particularly, methods for at least one of improving heat transfer and increasing critical heat flux in pool boiling and apparatuses thereof.
In a cooling system with a network of multiple flow passages a fluid used for cooling is introduced. The fluid may be single-phase liquid, gas or a two-phase liquid-vapor mixture. As the fluid flows through the network, heat transfer is by convection from the heated walls. The heat transfer rate to the fluid from the heated walls is characterized by the heat transfer coefficient. Higher heat transfer coefficients are desired for higher heat dissipation rates. Additionally, providing smaller channel internal dimensions leads to higher single phase heat transfer performance.
Employing liquid as the introduced fluid results in a higher heat transfer rate than with gas for the same flow conditions due to the higher thermal conductivity of liquids as compared to gases. To further improve this heat transfer rate and take advantage of the large latent heat of vaporizations compared to the sensible heat transfer with a few degrees temperature change, flow boiling can be employed. Heat transfer by flow boiling occurs when the liquid is forced to flow in the passages and boiling of the liquid occurs. This flow requires an external mechanism, such as a pump, to drive the liquid and vapor mixture through the passages. Due to the confined nature of the flow boiling system, sometimes backflow occurs in one or more channels causing the liquid to flow in a backward direction. This condition can lead to a critical heat flux condition at relatively low heat fluxes.
Pressure drop through a cooling system with flow boiling is also often a concern. As a result, efforts are made to reduce the pressure drop and/or external pumping power to achieve a desired cooling performance. Pressure drop also affects the saturation temperature of the liquid as it flows through the cooling system. Short passage lengths are desirable to reduce the pressure drop in a flow boiling system. However, reducing the passage length requires large number of inlets and outlets. As a result, the header design for flow boiling cooling systems can become quite complex.
In contrast, heat transfer by pool boiling occurs without any external pumping when a heated surface, which presents no enclosed channels to contain the liquid, is cooled by the liquid and boiling of the liquid occurs. When the bulk of the liquid is at its saturation temperature corresponding to the existing pressure in the liquid and boiling occurs on the heated surface, heat transfer is by saturated pool boiling mode. When the bulk of the liquid is at a temperature below the saturation temperature corresponding to the existing pressure in the liquid and boiling occurs over the heated surface, heat transfer is by subcooled pool boiling. Pool boiling covers both subcooled and saturated pool boiling. Boiling covers both pool and flow boiling.
Pool boiling can occur when nucleating bubbles are generated over the heated surface in a liquid environment when the liquid superheat exceeds the nucleation criterion. Another method of generating nucleating bubbles is to provide localized microheaters in conjunction with a natural or artificial nucleation cavity. The heating of liquid around the cavity above the liquid saturation temperature leads to bubble nucleation when the nucleation criterion for the cavity is satisfied.
In addition to a natural convection mechanism over the portion of the heater surface that is unaffected by the nucleation activity, heat transfer in pool boiling generally occurs as a result of three mechanisms: microconvection caused by convection currents induced by a bubble; transient conduction caused by the transient heat transfer to the fresh liquid that displaces the heated liquid over the heated surface in the region of nucleating bubbles; and microlayer evaporation caused by the evaporation of a thin liquid layer that appears underneath the nucleating bubble. A significant portion of the heat transfer during pool boiling occurs due to microconvection and transient conduction modes. The heat transfer by all these mechanisms aid in transferring heat from the heater surface and evaporating liquid into the growing vapor bubbles.
Another method of heat transfer involves introducing gas bubbles (not resulting from boiling) that grow and depart in the liquid in the vicinity of a heated surface and create motion at the liquid-gas interface. However, evaporation is not the primary mechanism in this case as the temperatures are generally below the saturation temperature of the liquid at the system pressure. The absence of evaporation in these systems with introduced gas bubbles results in considerably lower heat transfer rates as compared to pool boiling. Nevertheless, the heat transfer rate in such systems is still higher than that in systems with stagnant liquids.
To enhance pool boiling, surface features protruding from a base, such as pin fins of various cross sections, offset strip fins with rectangular pin fins arranged in staggered fashion, and other fin configurations, can be employed to enhance pool boiling. Additionally, to enhance pool boiling heat transfer fins, porous surfaces and active nucleation sites formed on the heated surface can be employed.
The maximum heat that can be dissipated with boiling without causing excessive temperature rise is limited by the Critical Heat Flux (CHF). It is desirable to increase the CHF limit during boiling. This limit is also an important consideration in the design of a boiling system.
The CHF limit can be increased by changing the contact angle of the liquid-vapor interface of a growing bubble. Increasing wettability of a surface by reducing the contact angle leads to enhancement of CHF. Reducing the wettability leads to a decrease in CHF.
A method for pool boiling includes introducing a liquid into a chamber of a housing which has one or more protruding features. One or more diverters extend at least partially across the one or more protruding features in the chamber. One or more bubbles are formed in the liquid in the chamber as a result of bubble nucleation. One or more of the bubbles resulting from nucleation are diverted with the one or more diverters to generate additional localized motion of the liquid along at least one of the one or more protruding features and other surfaces in the chamber of the housing to at least one of transfer additional heat to the liquid and increase the critical heat flux limit. The motion of liquid and vapor created by the one or more diverters may increase the critical heat flux limit by allowing removal of vapor and access of liquid to regions previously occupied by vapor.
A pool boiling apparatus includes a housing with a chamber, one or more protruding features in the chamber of the housing, and one or more diverters extending at least partially across the one or more protruding features in the chamber. The chamber of the housing with the one or more protruding features and the one or more diverters is configured to form one or more bubbles as a result of boiling to transfer heat. Additionally, the chamber of the housing is configured to divert one or more of the bubbles as a result of bubble nucleation with the one or more diverters to generate additional localized motion of the liquid along at least one of the one or more protruding features and other surfaces in the chamber of the housing to at least one of transfer additional heat to the liquid and increase the critical heat flux limit. The motion of liquid and vapor created by the one or more diverters can increase the critical heat flux limit by allowing removal of vapor and access of liquid to regions previously occupied by vapor.
This technology provides more efficient and effective methods and apparatuses for at least one of improving heat transfer performance and increase critical heat flux in pool boiling. With this technology, heat can be removed more effectively from heated surfaces than with prior pool boiling systems. Additionally, this technology is superior to prior flow boiling cooling techniques because it does not require an external pumping device or a complicated input and/or exit header design to remove heat from the heat transfer surfaces. Instead, this technology utilizes nucleating bubbles and one or multiple cover element devices to control and divert the localized motion of the bubbles and liquid through the passageways formed by the surface features for effective heat transfer in the region affected by the nucleating bubbles and in a more compact and simpler heat transfer apparatus. The localized motion of liquid and vapor created by the diverters can also improve the critical heat flux limit.
This technology incorporates one or multiple diverters positioned over a chamber and features to divert liquid around one or more nucleating bubbles over the surfaces of the chamber and/or features to provide enhanced heat transfer. With this technology, fresh liquid for additional heat transfer is introduced in the regions or passageways where the diversion occurred with little resistance as a result of the diverted fluid. The diverters are designed to introduce very little resistance to fluid flow in the regions or passageways which helps in bringing the liquid into the regions or passageways especially at high heat fluxes, thereby improving Critical Heat Flux. In addition to facilitating fresh liquid entering the regions or passageways with little resistance, this technology ensures the surfaces of the one or more features and other surfaces in the chamber of the housing do not dry out or remain under dry conditions for extended time, and increase the critical heat flux. The neighboring diverters can be designed to interact with each other in directing liquid and vapor in specific directions to allow for more efficient flow of fluids through the passageways, vapor out of the passageways and liquid into the passageways. The diverters could also be designed to control vapor and liquid motion in all three dimensions by providing different shapes and profiles.
With this technology, the diverted growth and/or motion of one or more bubbles also causes enhanced microconvection over the one or more and other surfaces in the chamber of the housing and/or other features. This enhanced microconvection over the one or more and other surfaces in the chamber of the housing and/or other features leads to enhanced heat transfer. The enhanced microconvection may lead to increase of the heat transfer by other modes of heat transfer during boiling.
An exemplary pool boiling assembly 12(1) is illustrated in
Referring more specifically to
The plurality of strip fins 16(1) are located in the chamber 14(1) of the pool boiling assembly 12(1), although the chamber of the pool boiling assembly could have other numbers and types of features. (For ease of illustration only one of the plurality of strip fins in
The surfaces of the chamber 14(1) of the pool boiling assembly 12(1) and the plurality of strip fins 16(1) are formed with natural and/or artificial cavities to promote nucleation to start bubble formation, although other manners for promoting bubble formation can be used. The bubbles resulting from this nucleation induce localized movement of a liquid in the chamber 14(1) of the pool boiling assembly 12(1) without an external pumping device, although other manners for promoting pool boiling bubble formation can be used.
Six diverters 32(1) are spaced apart and extend across the chamber 14(1) of the pool boiling assembly 12(1), although other types and numbers of diverters can be used. Each end of the six diverters 32(1) is secured to the pool boiling assembly 12(1), although other manners for securing the diverters can be used. In this example, each of the diverters 32(1) has a rectangular cross-sectional shape, although the diverters could have other types of shapes and configurations as illustrated with exemplary diverters 32(4)-32(12) in
Additionally, three optional fasteners 34(1) are spaced apart, extend at least partially across, and are secured to each of the diverters 32(1) to secure the position of each of the diverters, although other types and numbers of fastening mechanisms could be used. Openings to the chamber 14(1) are defined between the diverters 32(1) and fasteners 34(1), although other types of arrangements could be used. Although not illustrated, the pool boiling assembly 12(1) could also have a containment cover spaced from and seated over the chamber 14(1) and the diverters 32(1) and fasteners 34(1) to retain the cooling liquid, in particular the vaporized liquid, in the pool boiling assembly 12(1). Additionally and also not illustrated, the pool boiling assembly 12(1) could include a condensation system to capture, condense and return any vaporized liquid to the regions 18(1) in the chamber 14(1). Additionally and also not illustrated, the pool boiling assembly 12(1) could include a means to circulate the cooling liquid into and out of the volume formed by the containment cover and the chamber 14(1). The loop could include an external heat exchanger to remove heat from the cooling fluid and to condense any vapor that leaves the volume. As discussed earlier, the cooling fluid may be single-phase liquid, gas or a two-phase liquid-vapor mixture, although other types of fluids could be used.
Referring to
A plurality of strip fins 16(2) are located in the chamber 14(2) of the pool boiling assembly 12(2), although the chamber of the pool boiling assembly could have other numbers and types of features. (For ease of illustration only one of the plurality of strip fins 16(2) in
The surfaces of the chamber 14(2) of the pool boiling assembly 12(2) and the plurality of strip fins 16(2) are formed with natural and/or artificial cavities to promote nucleation to start bubble formation, although other manners for promoting bubble formation can be used. The bubbles resulting from this nucleation induce localized movement of a liquid in the chamber 14(2) of the pool boiling assembly 12(2) without an external pumping mechanism, although other manners for promoting bubble formation can be used.
Six diverters 32(2) are spaced apart and extend across the chamber 14(2) of the pool boiling assembly 12(2), although other types and numbers of diverters can be used. Each end of the six diverters 32(2) is secured to the pool boiling assembly 12(2), although other manners for securing the diverters can be used. In this example, each of the diverters 32(2) has a rectangular cross-sectional shape, although the diverters could have other types of shapes and configurations as illustrated with exemplary diverters 32(4)-32(12) in
Additionally, three optional fasteners 34(2) are spaced apart, extend at least partially across, and are secured to each of the diverters 32(2) to secure the position of each of the diverters, although other types and numbers of fastening mechanisms could be used. Openings to the chamber 14(2) are defined between the diverters 32(2) and fasteners 34(2), although other types of arrangements could be used. Although not illustrated, the pool boiling assembly 12(2) could also have a containment cover spaced from and seated over the chamber 14(2) and the diverters 32(2) and fasteners 34(2) to retain the cooling liquid, in particular the vaporized liquid, in the pool boiling assembly 12(2). Additionally and also not illustrated, the pool boiling assembly 12(2) could include a condensation system to capture, condense and return any vaporized liquid to the regions 18(2) in the chamber 14(2). Additionally and also not illustrated, the pool boiling assembly 12(2) could include a means to circulate the cooling liquid into and out of the volume formed by the containment cover and the chamber 14(2). The loop could include an external heat exchanger to remove heat from the cooling fluid and to condense any vapor that leaves the volume.
Referring to
A plurality of pins 16(3) are located in the chamber 14(3) of the pool boiling assembly 12(3), although the chamber of the pool boiling assembly could have other numbers and types of features. The fin shown is circular in cross section, although fins could be of any constant or variable cross sections. (For ease of illustration only one of the plurality of pins in
The surfaces of the chamber 14(3) of the pool boiling assembly 12(3) and the plurality of pins 16(3) are formed with natural and/or artificial cavities to promote nucleation to start bubble formation, although other manners for promoting bubble formation can be used. The bubbles resulting from this nucleation induce localized movement of a liquid in the chamber 14(3) of the pool boiling assembly 12(3) without an external pumping device, although other manners for promoting pool boiling bubble formation can be used.
Four diverters 32(3) are spaced apart and extend across the chamber 14(3) of the pool boiling assembly 12(3), although other types and numbers of diverters can be used. Each end of the four diverters 32(3) is secured to the pool boiling assembly 12(3), although other manners for securing the diverters can be used. In this example, each of the diverters 32(3) has a rectangular cross-sectional shape, although the diverters could have other types of shapes and configurations as illustrated with exemplary diverters 32(4)-32(12) in
Additionally, one optional fastener 34(3) extends at least partially across and is secured to each of the diverters 32(3) to secure the position of each of the diverters 32(3), although other types and numbers of fastening mechanisms could be used. Openings to the chamber 14(3) are defined between the diverters 32(3) and fastener 34(3), although other types of arrangements could be used. Although not illustrated, the pool boiling assembly 12(3) could also have a containment cover spaced from and seated over the chamber 14(3) and the diverters 32(3) and fastener 34(3) to retain the cooling liquid, in particular the vaporized liquid, in the pool boiling assembly 12(3). Additionally and also not illustrated, the pool boiling assembly 12(3) could include a condensation system to capture, condense and return any vaporized liquid to the regions 18(3) in the chamber 14(3). Additionally and also not illustrated, the pool boiling assembly 12(2) could include a means to circulate the cooling liquid into and out of the volume formed by the containment cover and the chamber 14(2). The loop could include an external heat exchanger to remove heat from the cooling fluid and to condense any vapor that leaves the volume.
A method for transferring heat with pool boiling assembly 12(1) will now be described with reference to
A liquid or liquid vapor mixture is initially introduced into regions 18(1) of the chamber 14(1) of the pool boiling assembly 12(1). The liquid contacts surfaces of the plurality of strip fins 16(1) and other surfaces of the chamber 14(1) to transfer heat from the pool boiling assembly 12(1). At least portions of the surfaces of the plurality of strip fins 16(1) and/or the chamber 14(1) of the pool boiling assembly 12(1) are formed with natural and/or artificial cavities to promote nucleation. The heated surfaces of the chamber 14(1) and/or plurality of strip fins 16(1) along with the cavities trigger nucleation to start the formation of bubbles to induce localized movement of the liquid in the chamber 14(1) of the pool boiling assembly 12(1).
For example, as the introduced liquid engages with natural and/or artificial cavities in a heated surface of the pool boiling assembly 12(1) and/or the plurality of strip fins nucleation may be triggered. When nucleation is triggered, one or more bubbles, such as a bubble B shown in
As the bubble B grows as shown in
As shown in
Another method for transferring heat with pool boiling assembly 12(1) with asymmetric diverters 32(12) will now be described with reference to
When nucleation is triggered, one or more bubbles as shown in
As described earlier, this localized movement of the liquid causes more interaction and heat transfer between the liquid and surfaces of the pool boiling assembly 12(1) and/or the plurality of strip fins 16(1). In this example, heat transfer from this boiling occurs as a result of microconvection, transient conduction, and microlayer evaporation.
Accordingly, as illustrated and described with reference to the examples herein, this technology provides a more efficient and effective method and apparatus for transferring heat with pool boiling from a heated surface to an introduced fluid. With this technology, heat can be removed more effectively from heated surfaces than with prior pool boiling systems. Additionally, this technology is superior to prior flow boiling cooling techniques because it does not require an external fluid pumping device or complicated fluid input header designs. Instead, this technology utilizes nucleating bubbles and one or multiple cover element devices to control and divert the localized motion of the bubbles, liquid-vapor interfaces and liquid through the passageways for effective heat transfer and in a more compact and simpler heat transfer apparatus. The efficient movement of vapor and liquid allows for dissipating larger heat fluxes and enhances the heat transfer rate for a given wall superheat and also increases the critical heat flux as compared to prior pool boiling and flow boiling systems.
Having thus described the basic concept of the invention, it will be rather apparent to those skilled in the art that the foregoing detailed disclosure is intended to be presented by way of example only, and is not limiting. Various alterations, improvements, and modifications will occur and are intended to those skilled in the art, though not expressly stated herein. These alterations, improvements, and modifications are intended to be suggested hereby, and are within the spirit and scope of the invention. Additionally, the recited order of processing elements or sequences, or the use of numbers, letters, or other designations therefore, is not intended to limit the claimed processes to any order except as may be specified in the claims. Accordingly, the invention is limited only by the following claims and equivalents thereto.
