TECHNICAL FIELD
The present disclosure is related generally to chemical etching and more particularly to a scalable process to prepare etched surfaces having microscale roughness features and microcavities.
BACKGROUND
Flow boiling is ubiquitous to a variety of industrial sectors such as purification, distillation, chemical synthesis, desalination, thermoelectric power generation, refrigeration, and cryogenics since it offers the dual advantage of near-isothermal operation, and ultra-efficient energy transfer. Although the flow boiling heat transfer coefficient, a characteristic measure of the efficiency of heat transfer, is higher when compared to other modes of thermal exchange such as single phase flow, many researchers and engineers have looked for methods to further enhance the heat transfer coefficient via surface macro, micro, and nanostructuring or decreased channel diameter (e.g. microchannels). For example, the transition of wide band gap semiconductor devices made from gallium nitride and silicon carbide from lab-scale to industrial platforms, coupled with thermal limitations placed on silicon processor densification, has recently renewed the push to develop technologies that are able to safely and reliably transfer ever-higher heat transfer rates. To achieve enhancement, past work has developed silicon nanowires and silicon micropillars as a platform technology capable of increasing boiling heat transfer in microchannels with water as the working fluid. While structured microchannels can significantly improve heat transfer coefficients, the requirement of flow splitting between multiple small-diameter channels to maintain reasonable pressure drops increases the complexity and introduces the potential for flow maldistribution. Furthermore, the majority of surface structuring techniques developed over the past decade to enhance flow boiling are difficult or impossible to scale, not characterized in terms of durability, difficult to manufacture on typical heat exchanger materials, or have been typically studied with fluids such as water or dielectric fluids at ambient pressure. Limited research and methods exist aimed at developing ultra-scalable technologies to augment two-phase flow heat transfer in conventional millimetric-scale channels relevant to the majority of energy systems.
BRIEF SUMMARY
Described in this disclosure are a surface-modified component for enhanced heat transfer during cooling, a method of modifying a surface of a component, and a method of achieving high heat transfer during cooling.
The method of achieving high heat transfer during cooling includes providing an aluminum body having an inner surface enclosing a channel, where the inner surface comprises microscale roughness features and microcavities configured to enhance nucleation site density during flow boiling. A refrigerant is transported through the channel. During the transport, the refrigerant absorbs heat from a thermal load and undergoes flow boiling. The heat is transferred to the refrigerant at an average heat transfer coefficient of at least about 10 kW/(m2·K) at a mass flux of about 300 kg/(m2·s).
The surface-modified component comprises an aluminum body having an inner surface enclosing a channel, where the inner surface comprises microscale roughness features of about 1 micron to about 15 microns in height and microcavities of about 2 microns to about 30 microns in linear size. The aluminum body does not include an interface between the inner surface and a sub-surface region of the aluminum body. The inner surface includes aluminum and native aluminum oxide.
The method of modifying a surface of a component for enhanced heat transfer during cooling includes providing an aluminum body having an inner surface enclosing a channel; cleaning the inner surface with an organic solvent and/or deionized water; after the cleaning, exposing the inner surface to a hydrochloric acid (HCl) solution comprising a HCl concentration of 2 M to 5 M; and, after the exposing, rinsing the inner surface with deionized water and then drying. Thus, a surface-modified component comprising the aluminum body is obtained, where the inner surface comprises microscale roughness features and microcavities.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A shows a schematic of a surface-modified component comprising an aluminum body (e.g., tube) having an inner surface enclosing a channel and comprising microscale roughness features and microcavities to promote enhanced heat transfer when a refrigerant is transported through the channel.
FIG. 1B shows scanning electron microscopy (SEM) images of an etched or structured aluminum surface, such as the inner surface of FIG. 1A.
FIGS. 2A-2C show experimental average heat transfer coefficient (h) as a function of heat flux (q″) for flow boiling of R134a at mass flux (G) of (A) 102 kg/(m2·s), (B) 203 kg/(m2·s), and (C) 306 kg/(m2·s), where the saturation temperatures are Tsat=30° C. Error for h is represented by the shaded region; error bars for plain and boehmite (aluminum oxide hydroxide nanostructures) tubes are smaller than the symbols and the uncertainty is represented by the shaded region between adjacent data-points.
FIG. 3 shows measured heat transfer coefficient (h) as a function of time over a period of 28 days, showing negligible change and indicating structural durability; insets showing similar SEM images before and after flow boiling tests confirm the structural durability.
FIG. 4A shows a pressure drop comparison between a microstructured etched Al tube, nanostructured boehmite tube and a plain tube at a mass flux G=306 kg/(m2·s).
FIG. 4B shows a plot of enhancement factor (Øe.f.=(hstructured/hplain)/(ΔPstructured/ΔPplain)) versus heat flux (q″) for an etched Al tube.
FIG. 5A is a schematic showing part of a method of producing the structured inner surface on an aluminum body (e.g., an aluminum tube), which is shown in the schematic immersed in a hydrochloric acid solution.
FIG. 5B shows SEM images of four aluminum grades, Al 1100, Al 3003, Al 5052 and Al 7075, after application of the surface structuring procedure.
FIG. 6A-6B are schematics showing fabrication procedures developed for preparing internal surfaces comprising (A) microstructured etched Al and (B) nanostructured boehmite.
FIG. 7A shows time lapse optical photography of water spreading in an etched aluminum tube demonstrating high wickability, and the right side image shows a transverse cut-out of the etched aluminum tube along with the corresponding SEM image.
FIG. 7B shows water spreading behavior within a boehmite structured tube with water as the working fluid.
FIGS. 8A-8D show SEM images of (A) etched Al and (B) boehmite, along with the corresponding focused ion beam (FIB) milling images identifying the height of each roughness feature as (C) ˜5 μm for etched Al and (D) ˜200 nm for boehmite.
FIGS. 8E-8F show roughness profiles for (E) etched Al determined using confocal microscopy, and (F) boehmite using atomic force microscopy (AFM).
FIGS. 8G-8H show X-ray photoelectron spectroscopy (XPS) spectra, which confirm material composition for the (G) etched Al and (H) boehmite.
FIG. 9 shows a schematic of a custom flow boiling experimental facility.
FIG. 10 shows (top) a schematic of a test-section showing the location of the first thermocouple, and (bottom) experimentally measured local (near the test section entrance) heat transfer coefficients (hloc) as a function of vapor quality (x) at different mass flux values: (I) G=102 kg/(m2·s), (II) G=203 kg/(m2·s), and (III) G=306 kg/(m2·s).
FIG. 11 shows heat flux (q″) of the microstructured etched Al tube and plain Al tube as a function of wall superheat (Tw−Tsat) at mass flux G=102 kg/(m2·s).
FIG. 12 shows schematics (not to scale) of the enhanced nucleation phenomena attributed to increased nucleation site density.
FIG. 13 shows experimentally measured local (near the test section exit) heat transfer coefficient (hloc) as a function of vapor quality (x) at a mass flux of (I) G=102 kg/(m2·s), (II) G=203 kg/(m2·s), and (III) G=306 kg/(m2·s), where the error for hloc is represented by the shaded regions; in the case of the plain and boehmite tubes, the shaded region between adjacent data-points represents the uncertainty since the error is smaller than the symbols.
FIG. 14 is a flow regime visualization recorded at test-section exit displaying lower liquid film thickness for microstructured etched Al tube in the stratified wavy regime (I) at q″=9 kW/m2 and annular flow regime at q″=43 kW/m2 (II); scale bar inset is same for all images.
FIG. 15A shows a zoomed-in view of a sectioned 9.5 mm diameter etched extruded Al grooved tube.
FIGS. 15B-15E show an optical isometric-view image of the tube, a peak image, peak-valley-peak image, and valley image showing conformality of the microscale roughness features.
FIG. 16 show experimental average heat transfer coefficient (h) as a function of heat flux (q″) for flow boiling of R515B at mass flux (G) of 150 kg/(m2·s) for etched and plain aluminum tubes with an internal diameter of 4.6 mm. Error for h is represented by the shaded region; error bars for the plain tube are smaller than the symbols and the uncertainty is represented by the shaded region between adjacent data-points.
DETAILED DESCRIPTION
A method of achieving high heat transfer during cooling, e.g., in a refrigeration or heating, ventilation and air-conditioning (HVAC) system, is described in this disclosure. The method is enabled by the development of an ultra-scalable, cost-effective and simple surface structuring technique for aluminum substrates based on hydrochloric acid (HCl) etching. In particular, inner surfaces of aluminum tubes and other heat transfer components used to transport refrigerants may undergo surface structuring. The resulting superhydrophilic etched surfaces may include highly durable microscale roughness features and microcavities that can facilitate enhanced thermal performance during flow boiling of a refrigerant. Described in this disclosure are the ultra-scalable surface structuring method and the surface-modified component that results, along with a method of achieving high heat transfer during cooling that exploits these technologies.
FIG. 1A shows a surface-modified component 100 comprising an aluminum body 102 having an inner surface 104 enclosing a channel 106, where the inner surface comprises microscale roughness features 108 and microcavities 110 configured to enhance nucleation site density for flow boiling when a refrigerant 112 passes through the channel 106. The aluminum body 102 may comprise an aluminum tube. The microscale roughness features 108 and microcavities 110 are visible in the scanning electron microscopy (SEM) images of FIG. 1B, and further description of features of the surface-modified component 100 is provided below.
A method of achieving high heat transfer during cooling entails providing such a surface-modified component 100 for a flow boiling process to effect cooling. Referring again to FIG. 1A, the method includes providing an aluminum body 102 having an inner surface 104 enclosing a channel 106, where the inner surface 104 comprises microscale roughness features 108 and microcavities 110. A refrigerant 112 is transported through the channel 106 and undergoes flow boiling as it absorbs heat from a thermal load. The refrigerant 112 may comprise, for example, a hydrochlorofluorocarbon (HCFC), a hydrofluoro-olefin (HFO), a hydrofluorocarbon (HFC), such as 1,1,1,2-tetrafluoroethane (R134a), a zeotropic refrigerant blend, and/or another environmentally-friendly working fluid. An example is R515B, which is intended to replace R134a and may be described as an azeotropic blend of R1234ze (91.1%) and R227ea (8.9%). Advantageously, heat is transferred to the refrigerant 112 during transport through the channel 106 at an average heat transfer coefficient of at least about 10 kW/(m2·K), e.g., when heat flux is 15 kW/m2 and/or mass flux is about 300 kg/(m2·s) for an internal diameter of 3 mm. As shown by the data of FIGS. 2A-2C, which are discussed in detail below, substantial increases in average heat transfer coefficient (h) are obtained for aluminum tubes 102 having a structured inner surface 104 as described in this disclosure in comparison with plain (unetched) aluminum tubes and boehmitized aluminum tubes. It is noted that terms such as “inner surface,” “structured inner surface,” “structured surface,” and “microstructured surface,” which may be found in this disclosure, may refer to the etched surface of the aluminum body 102.
Referring to FIG. 1B, the microcavities 110 of the structured inner surface 104 may be about 2 microns to about 30 microns in linear size, with a majority of the microcavities 110 having a linear size (e.g., width or diameter) in the range from 2 microns to 15 microns. As explained below, a high density of microcavities 110 in this size range may be associated with a high nucleation site density and consequently with high heat transfer coefficients. The microscale roughness features 108 may range in height from about 1 micron to about 15 microns. The inner surface 104 of the aluminum body 102 and in particular the microscale roughness features 108 are resilient and able to withstand external forces, such as shear and abrasion, in contrast to fragile nanostructures employed in other heat transfer approaches. Accordingly, the average heat transfer coefficient may be stable (e.g., within +/−5%) for at least 28 days, as established by data described below and shown in FIG. 3. It is anticipated that the average heat transfer coefficient may remain stable for much longer time periods, such as several months or years, given the structural strength and durability of the microscale roughness features.
The aluminum body 102 may comprise an aluminum alloy, which may have an alloy designation in the 1000 through 7000 series. For example, in specific experiments described in this disclosure, the aluminum alloy may comprise a 1100, 3003, 5052, or 6061 alloy. The method has been shown to be effective with a wide range of aluminum alloys. The inner surface 104 of the aluminum body 102 may comprise both aluminum and aluminum oxide, as revealed by the data of FIG. 8G. This is in contrast to traditional surface modification approaches, such as boehmitization, as described below, which may lead to an aluminum oxide surface coating and/or aluminum oxide surface features on an aluminum substrate with an interface in between that may be prone to delamination. In this work, the aluminum body 102 preferably does not include an interface between the inner surface 104 and a sub-surface region (that is, the underlying unetched region) of the aluminum body 102; thus, interfacial stresses and failure modes such as delamination, interfacial cracking, and blistering may be avoided.
Pressure drop may be an important factor to consider since it provides a measure of the pumping cost and changes in the saturation temperature associated with the observed heat transfer enhancements. While many prior studies focused on flow boiling have not reported pressure drop or have measured pressure drop penalties of up to 20%, a significant pressure drop is not found in this work. FIG. 4A shows a comparison of pressure drop across a test section for aluminum tubes containing a structured inner surface, boehmitized aluminum tubes, and plain aluminum tubes. The pressure drops across all the tubes increased with heat flux because of the higher vapor qualities present. As expected, the two-phase pressure drop for the aluminum tube with the structured inner surface is close to 10% higher due to the largest features and highest roughness amongst tested surfaces. To quantify the advantageous nature of the inventive structured inner surface, an enhancement factor, which incorporates the average heat transfer enhancement and total pressure drop, is used, and defined as: Øe.f.=(hstructured/hplain)/(ΔPstructured/ΔPplain) The enhancement factor is at least about 2 at the mass flux of about 300 kg/(m2·s), as revealed by the data of FIG. 4B, and peaks at Øe.f.=3.15, indicating that the detrimental impact of a pressure drop increase is offset by a higher increase in heat transfer coefficients.
The method of producing the structured inner surface 104 is scalable and is thus amenable to coating large-size components, such as aluminum tubes 102 having a diameter of about 3 mm or greater and lengths of 1 m or more. Such surface-modified components 100 may be suitable for refrigeration, air-conditioning, power generation, distillation and purification, electronics cooling, and/or other applications. The method may include a first step of cleaning the inner surface with an organic solvent and/or deionized water. The organic solvent may include acetone, ethanol, and/or isopropanol. The cleaning may comprise immersing the aluminum body in the organic solvent and/or the deionized water or otherwise exposing the inner surface to the organic solvent and/or the deionized water. In one example, the cleaning comprises immersing the aluminum body in acetone, ethanol, isopropanol, and the deionized water in succession. After cleaning, the inner surface is exposed to a hydrochloric acid (HCl) solution comprising a HCl concentration of 2 M to 5 M. Typically, the exposure entails immersing the aluminum body into the HCl solution, and the exposure may occur for a time duration from about 7 min to about 30 min, or from about 15 min to about 25 min. In the exemplary schematic of FIG. 5A, the aluminum body (tube) is immersed in the HCl solution in a stainless-steel container. An outer surface of the aluminum body may be protected or covered during the exposure to the HCl solution such that only the inner surface is etched. For example, some or all of the outer surface may be covered by a photoresist layer or a chemically-resistant tape (e.g., polyimide (“Kapton”) tape). After the exposure, the inner surface may be rinsed with deionized water and/or isopropanol and then dried (e.g., in air), thereby obtaining the inner surface comprising microscale roughness features. Additionally, any photoresist or tape may be removed. Typically, the method is carried out at room temperature (e.g., 18° C.-25° C.).
Since there is no specialized equipment requirement for the surface structuring, the preparation cost is mainly related to the chemical reagent cost. The estimated lab-scale manufacturing cost for etched aluminum tubes reduces to as low as $7/m2, which is lower than the majority of alternate surface modification methods described in the literature, such as sintering, nanowire growth and nanoparticle deposition. FIG. 5B shows SEM images after application of the surface structuring procedure for four additional aluminum grades—Al 1100, Al 3003, Al 5052 and Al 7075. The formation of microstructures similar to Al 6061, which is shown in FIG. 1B, demonstrates the applicability of the process to a wide variety of commercially available aluminum grades.
In some examples, the inner surface 104 of the aluminum body 102 may comprise grooves, micro-fins, or other structural features, as shown in FIGS. 16A and 16B, and the above-described method may conformally apply the microscale roughness features 108 to the structural features (e.g., see FIGS. 15C-15E), as further discussed below.
Surface structuring of aluminum substrates using hydrochloric acid (HCl) etching is described below in regard to various experiments. To examine the role of structure length scale on the flow boiling performance, the structured or etched surfaces formed via the etching method are compared to nanoscale hydrophilic structures (in particular, boehmite and copper oxide). The flow boiling performance is evaluated using a custom-built experimental facility with a hydrofluorocarbon (HFC) refrigerant, 1,1,1,2-tetrafluoroethane (R134a), as the working fluid. The results presented below demonstrate enhanced flow boiling thermal performance in surface structured aluminum tubes of about 1 m in length. Nanostructures (formed on the boehmite and copper oxide surfaces) are conclusively shown to have a negligible effect on the flow boiling performance. Experiments carried out over a range of heat flux and mass flux conditions reveal heat transfer coefficient enhancement up to 270% for etched surfaces when compared to plain aluminum tubes with similar pressure drop characteristics, as mentioned above. To demonstrate the scalability of this approach and the ability to coat complex internal structural features with ease, conformal etching of commercially available 9.5 mm (⅜″) diameter low-fin aluminum tubing is carried out. A continual 28-day flow boiling test mentioned above shows a negligible change in heat transfer performance over the entire test duration, which suggests that the microscale roughness features are highly durable.
The etching technique, which may be described as crystallographic etching, to develop structured surfaces has three main advantages. The structure length scale for etched Al is larger than the majority of considered structures in the past, enabling greater structure strength and resilience to external forces such as shear and abrasion when compared to fragile nanostructures. Second, crystallographic etching of Al results in a structured surface that may include aluminum and native aluminum oxide and which is integrally connected to the Al substrate with no additional oxide layer or interface. In other words, the structured or modified surface of the component does not constitute a separate coating or layer (e.g., Al2O3) which may be susceptible to delamination. This results in elimination of interfacial stresses and failure modes such as delamination, interfacial cracking, and blistering. Lastly, common oxidation methods to enhance heat transfer are sensitive to the chemistry of the working fluid. Slight acidity in the working fluid can result in reduction of the structures and failure of the enhancement.
The baseline tubes in these experiments are commercially available ¼″ plain Al tubes with outer diameters of Dout=6.35 mm, inner diameters of Din=3.048 mm, and lengths of L=90±0.1 cm. The test-section length was chosen for two reasons: (1) to demonstrate the scalability of the technique for microstructure fabrication on the internal surfaces of long tubes, and (2) to enable the transition from fully single-phase liquid flow to fully single-phase vapor flow inside a single test section. While numerous definitions exist for the classification of channels as micro/mini/conventional, the tubes employed here are classified as conventional, with two distinct surface or roughness feature length scales: microscale aluminum (up to or about 15 μm), and nanoscale aluminum (˜200 nm).
The cleaning procedure used for fabricating all tube samples was identical. The tube was first dipped in acetone for 5 minutes to remove organic materials, and then was cleaned with ethanol, isopropanol and deionized (DI) water in succession. The HCl-etching process on the internal surface of the tube is illustrated in FIG. 6A. A one-inch diameter stainless-steel (SS) tube was used as a container with one end closed with a rubber stopper. The stainless-steel tube was filled with 500 ml of 2 M HCl. The Al tube was wrapped with 1-mm thick Kapton tape to maintain a smooth outer surface and was then placed in the HCl solution and left for 15 minutes. The inner diameter of the etched tube changes by ˜0.13 mm and this tube was finally cleaned with deionized water and isopropanol to remove any residue, and the internal surface was dried in air. Referring now to FIG. 6B, to create internal boehmite nanostructures, the stainless-steel tube is first filled with DI water. This solution is heated by a rope heater wound around the tube and the solution temperature was monitored with a T-type thermocouple. Once the temperature reaches 90° C., the Kapton-wrapped aluminum tube is immersed in the de-ionized water for about 60 minutes. The heater is controlled to keep the solution temperature between 90° C. and 100° C. for the duration of the soaking process. The resulting needle-like nanostructured aluminum tube is then removed and cleaned with a nitrogen stream.
With regards to the wetting characteristics of the fabricated structures, the greatly enhanced wicking ability of the superhydrophilic microstructured aluminum surface is visible in FIG. 7A with water as the working fluid. While superhydrophilic boehmite also demonstrates wicking, it is of a lower magnitude than that of etched aluminum, as shown in FIG. 7B.
FIGS. 8A-8H show scanning electron microscopy (SEM), focused ion beam milling (FIB), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) analysis of the two fabricated micro/nanostructured surfaces. The etched surface displays two length scales: 1) a ˜5 μm structure length observed through FIB and 2) a ˜40 μm peak to valley height observed through 3-D optical profilometry. The larger length scale is a result of etching along the aluminum grain boundaries. The etched structures also exhibited the largest cavity sizes (˜5 μm) while the boehmite structures have much smaller pores (˜45 nm) due to the dense interconnected layout as deduced through SEM images.
For the Al samples, FIGS. 8G and 8H show the wide-band XPS spectra of Al 2p and O 1s with the expected binding energy revealing the presence of Al and O on the sample surface. Insets of FIGS. 8G and 8H illustrate the high resolution XPS spectra showing the presence of only aluminum oxide on the boehmite surface and both aluminum oxide (native) and Al metal on the etched Al surface.
In order to further explore liquid spreading and increased wicking, both of which have been shown to contribute to improved boiling performance, wicking tests were performed with the refrigerant FC-72. Here, FC-72 was used for two reasons: (1) its ability to exist in liquid form at room temperature enables comparison with water wickability tests, and (2) the low surface tension of FC-72 makes it a good candidate for qualitative comparison with refrigerants. While the boehmite surfaces exhibit negligible wicking, the etched aluminum surface was able to wick FC-72, though less effectively than water due to a reduction in capillarity associated with low surface tension fluids. Wicking occurs when the apparent contact angle is smaller than the critical contact angle θc=cos−1[(1−ϕ)/(r−ϕ)] where ϕ is the solid fraction of the developed structures and r is the roughness factor. The increased roughness of the etched aluminum surface leads to an increase of the critical contact angle, which in turn leads to increased wetting behavior, even with low surface tension fluids. The difference in the wetting characteristics and structure length scales indicates possible differences in flow boiling characteristics amongst these micro/nanostructures, as explored below.
After testing the wickability and conducting surface characterization of the surface-modified structures, heat transfer performance is quantified by measuring the heat transfer coefficient across the test section of interest (using the structured tubes and a plain tube as a control), in a custom flow boiling experimental facility, as shown schematically in FIG. 9A variable frequency gear pump is used for circulating the refrigerant through the flow loop, regulating the mass flux in the test section. The mass flow rate is measured by with a Coriolis flowmeter, placed at the exit of the pump. The flow then passes through a pre-heater to control the inlet conditions into the test section, which is maintained 1 to 2° C. below the saturation temperature. The preheater consists of a 500 W rope heater wound around a 1.8 m long copper tube with a 6.023 mm inner diameter. The heat added to the flow in the preheater is controlled with a variac power supply. The refrigerant is then routed to the test section, and then the saturated refrigerant passes through a brazed plate heat exchanger coupled to an ethylene-glycol chiller and is fully condensed so it can safely be returned to the pump. The refrigerant temperature and pressure are measured in a variety of locations throughout the flow loop to define thermodynamic states. Resistance temperature detectors (RTDs) are used to measure the test section inlet and outlet temperatures and calibrated T-type thermocouples are used for all other measurements in the loop. The system pressure is regulated with a piston cylinder and continually measured with piezoresistive pressure transducers. The entire facility is insulated to minimize heat loss to the ambient. All data are recorded at steady state using a National Instruments Data Acquisition System with LabVIEW.
While the majority of past flow boiling work on structured surfaces has focused on water, refrigerant boiling is characterized because of its prevalence in a wide variety of applications where water is inappropriate to use. The primary refrigerant used for testing is R134a, which is a non-toxic, non-corrosive and non-flammable alternative to chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) widely used in the automotive, aerospace, pharmaceutical and manufacturing industry. Prior to flow boiling experiments, the facility was vacuumed to avoid contamination of the working fluid with air.
During the experiments, the mass flux was varied between 100 and 300 kg/(m2·s) and the heat flux was adjusted to cover two-phase vapor qualities, the ratio of the mass of vapor to total mass of saturated mixture, from 0 to 1. The mass flux range studied made it possible to traverse multiple flow regimes which is critical to develop an understanding of the heat transfer performance for both nucleate and convective flow boiling regimes. The local wall temperature was measured at six locations along the test section by one surface mounted thermocouple attached to the top and another thermocouple attached at the bottom of the tube at each location. These measurements were used to determine the local heat transfer coefficients and the mean of these values was reported as the average heat transfer coefficient across the test-section. A needle valve was used to mitigate flow boiling instabilities across the test-section while the pressure drop across the test section was recorded using a differential pressure transducer to quantify changes in the required pumping power. Finally, the refrigerant was routed through a glass tube placed 3.8 cm beyond the exit of the test section and the flow regimes were recorded with a Phantom High-Speed Camera at 5,000 frames per second.
Nucleate boiling and convective boiling are the two major mechanisms that exist during flow boiling. In the nucleate boiling regime, the heat transfer coefficient is primarily function of the heat flux, with an increasing number of nucleation sites being activated on an increase in supplied heat. On the other hand, the heat transfer coefficient is nearly independent of the heat flux in the convective boiling regime. FIGS. 2A-2C show the measured heat transfer coefficients across the length of the test section (h) as a function of wall heat flux (q″). For all tested mass fluxes, flow boiling heat transfer enhancement was observed for the porous etched Al tube, with the significant increase in h as a function of q″ pointing towards nucleate boiling dominance across the test-section. A maximum increase of 270% for average heat transfer coefficient was attained at the highest mass flux case of G=300 kg/(m2·s), as shown in FIG. 2C. A strong correlation between heat flux and heat transfer coefficients at a specified mass flux along with a negligible effect of mass flux on heat transfer coefficients at a specified heat flux implies nucleate boiling dominance for the microstructured etched Al surface. A similar negligible mass flux effect is observed for the plain Al and boehmite tubes; however, the effect of heat flux is at a significantly lower scale when compared to the etched tube. Thus, while nucleate boiling dominance is exhibited through flow boiling tests with the three surfaces, the microstructured etched Al surface demonstrates the greatest nucleate boiling effect.
To gain an understanding of the enhancement mechanisms, local properties were analyzed near the entrance and exit of the test section. The plots of FIG. 10 show the local heat transfer coefficients (hloc) at the location closest to the tube inlet as a function of the local vapor quality (x), which was varied by increasing the applied heat flux at a constant mass flux G. Significantly higher heat transfer coefficients are demonstrated for the microstructured Al etched tubes at all tested mass flux values. This is attributed to a dominance of nucleate boiling, increase in the number of active nucleation sites, as well as an increase in bubble departure frequency.
To model the nucleation site density, a formulation based on parameters such as degree of subcooling, wall superheat, and apparent contact angle was employed. The analysis revealed that the maximum length scale of active nucleation sites is close to 12 μm. Experimentally measured local (near the test-section inlet) heat transfer coefficients (hloc) as a function of vapor quality (x) at mass flux (I) G=102 kg/(m2·s), (II) G=203 kg/(m2·s), and (III) G=306 kg/(m2·s) are shown by the data of FIG. 10. Measurement uncertainty is represented by the shaded region. Error bars for plain and boehmite tubes are smaller than the symbols; however, the shaded region between two data-points represents the uncertainty. The increase in local heat transfer by 10 times for the etched Al tube at the highest mass flux (bottom plot III of FIG. 10) is attributed to the higher density of microcavities in the desired range (˜5 μm diameter), with boiling being sustained at lower wall superheats. The same mechanism of enhancement applies to the lower mass fluxes as well. An increase in heat flux leads to the activation of more nucleation sites, resulting in the higher heat transfer coefficients as observed for all examined surfaces and for all mass flux values. When compared to the plain Al tube, nanostructured boehmite surfaces also showed an improvement due to the increase in nucleation sites, but the enhancement was well below the enhancement obtained with the microstructured etched Al surface. The lack of substantial enhancement is attributed to the smaller cavity size of the boehmite surface (˜45 nm) in comparison to the etched Al surface. The cavity size may be optimized for the working fluid of interest as there may be a maximum cavity diameter for heterogenous nucleation, beyond which nucleation may be suppressed. The nucleation suppression can occur due to subcooled liquid flooding on surfaces with cavities that are too large, which may lead to the suppression of vapor entrapment in the pores. The effect of flooding may be even more detrimental for low surface tension fluids such as R134a, the working fluid under consideration here, due to the smaller volume of gas entrapment when compared with high surface tension fluids such as water. The cavity depth also may play an important role in nucleation. The deeper cavities on the etched Al surface in comparison to the boehmite surface (e.g., ˜15 μm versus ˜200 nm) may enable greater vapor entrapment in the pores, further enhancing heat transfer. Therefore, the combined effects of increased cavity diameter and depth of the microstructured Al etched surface lead to significantly enhanced flow boiling near the test-section inlet.
The dominance of nucleate boiling near the entrance of the etched tubes can be seen via analysis of the boiling curve in FIG. 11, which shows heat flux (q″) of the microstructured etched Al tube and plain Al tube as a function of wall superheat (Tw−Tsat) at mass flux G=102 kg/(m2·s). It is noted that the error for heat flux (q″) is smaller than the symbol and is therefore not shown. Beginning at about 0° C., the wall superheat varies linearly with heat flux for the plain tube, implying convective boiling dominance due to the independence of heat transfer coefficient on heat flux. On the other hand, a non-linear trend is observed for the microstructured etched surface, implying a transition from the convective to the nucleate boiling regime. Similar non-linear trends for the microstructured Al surface hold for high mass fluxes.
Nucleation and the resulting bubbles formed drive thermal performance in the region of high heat transfer enhancement at low vapor qualities (x<0.1). Concentrating on the bubble characteristics, as illustrated in FIG. 12, an increase in the rate at which bubbles depart from the surface, known as the bubble departure frequency (f), contributes to an increase in heat transfer during nucleate boiling. The duration of bubble growth is dependent on the time required for the bubble to reach departure diameter size, and therefore the departure frequency is inversely proportional to bubble departure diameter (Dd). The departure diameter is in turn related to the wettability of the surface and the magnitude of drag force assisting bubble departure during flow. The bubble departure diameter Dd of the high roughness Al etched surface decreases with contact angle (θ˜0°) since Dd˜θ[σ/σ/g(ρ1−ρv)]1/2, where θ is the apparent receding contact angle as measured between the liquid-vapor interface, and solid-liquid interface, σ is the working fluid liquid-vapor surface tension, g is the gravitational constant, and ρ1 and ρv are the working fluid liquid and vapor densities, respectively. In addition, the drag and inertia forces during flow balances the capillary force (Fc), which is the force keeping the bubble attached to the wall. The capillary force (Fc=πDd sin θ) reduces with increased wetting associated with etched surfaces and therefore enables bubbles to depart the surface at smaller diameters and results in higher heat transfer coefficients. With regards to the average heat transfer coefficient, plain and boehmite surfaces showed similar behavior with the exception the highest mass flux case (G=300 kg/(m2·s)), where the boehmite tube exhibited a slightly better performance, showing a 17% increase in h, as indicated in FIG. 2C.
While the greater performance enhancement towards the beginning of the test section is attributed to enhanced nucleation characteristics, a large portion of the test section is in the coalescing bubble/annular flow regime. Due to suppression of nucleate boiling at higher vapor qualities, the degree of heat transfer enhancement towards the end of the test-section (top of FIG. 13) for the microstructured etched Al surface is of a significantly lower magnitude when compared to that observed towards the beginning of the test-section. As an example, the local heat transfer coefficient is 3 times higher than that for the plain surface at the highest mass flux case, as shown in plot III of FIG. 13. To better understand the mechanism of heat transfer in this regime, the heat transfer and flow regime characteristics were examined near the test section exit. At low mass flux (G=100 kg/(m2·s)) where phase velocities are low, stratified flow was observed at the exit of the test section where both liquid and vapor are separated into two distinct regions with the liquid occupying the bottom of the tube due to gravity, as indicated in FIG. 14 (top). At higher mass flux (G>200 kg/(m2·s), an increase in the vapor flow rate results in annular flow where liquid wets the tube around the periphery of the tube wall with a central vapor core, as indicated in FIG. 14 (bottom).
High speed video comparison of annular flow regimes for plain and microstructured surfaces indicate an increase in turbulence for the etched surface, attributed to an increase in cavity distribution. Enhanced turbulence results in improved vapor removal from the heated surface as well as increased mixing of cold liquid from the bulk towards the heated surface, both of which may contribute to improved heat transfer. In addition, the flow regime characteristics also depend on the non-dimensional Capillary number, which represents the ratio of the viscous force to surface tension force. The plain tube has a higher Capillary number than the structured tubes by approximately 6%, so the plain tube is expected to have a thicker film. This was confirmed for the stratified flow regimes using the recorded visualization images. Thinner films lead to a lower conduction resistance across the liquid, resulting in higher heat transfer coefficients for the etched tube towards the end of the test section. Thus, while the heat transfer enhancement is lower near the end of the test section because nucleate boiling is suppressed in the stratified and annular regimes, the etched tube still shows a benefit due to the reduced film thickness in stratified flow and increased turbulence in annular flow.
Due to the sudden increase in wall temperature associated with high vapor qualities, dry-out is another important parameter to consider. Partial dry-out is defined as the region in which intermittent wetting and re-wetting of fluid in the periphery of the tube wall occurs, and is found to occur at a vapor quality of approximately 0.9. This value increases with increasing mass flux for both structured and unstructured tubes due to an increase in the amount of heat that needs to be supplied to reach the specified vapor quality. While prior studies have reported enhancements in dry-out completion with water as the working fluid, no significant improvement in partial dry-out was observed in this study, primarily due to the low liquid-vapor surface tension working fluid being considered (σ=8 mN/m for R134a when compared to σ=72 mN/m for water at room temperature).
Prolonged sustainability of previously designed micro/nanostructures for flow boiling enhancement remains a concern. To demonstrate the applicability of the microstructured etched Al surface for commercial flow boiling applications, a preliminary durability study of the etched surface was carried out by conducting daily 8-hour long two-phase heat transfer experiments for a total of 28 days. As described above, the structured surface is formed through chemical etching of inner surface of an aluminum tube. Due to the absence of a significant oxide layer, shear stress is minimized at the metal-oxide interface when subjected to flow. This in turn results in highly stable structures, as confirmed through steady experimental heat transfer coefficient results over time, as shown in FIG. 3. The proven structural integrity of the microstructured etched surface points to their possible use for longer time periods typically encountered in industrial applications. In contrast, the presence of an oxide layer in the fabricated boehmite and CuO structures can make these surfaces prone to wear during flow and thermal cycling, primarily due to the thermal expansion coefficient mismatch between the oxide layer and base metal. Thus, in addition to advantages highlighted earlier with regards to thermal performance, the chemically etched aluminum surfaces can also lead to favorable mechanical properties during two-phase flow.
Improvement in thermal performance attributed to an increase in surface area and earlier flow regime transitions have been previously demonstrated in extruded axial- and helical-grooved tubes. It is expected that microstructuring of these tubes can lead to further improved heat transfer performance. To examine the applicability of the above-described method to such tubes as well as to demonstrate the highly conformal nature of aluminum etching, axial-grooved tubes are etched as set forth above. Referring to FIGS. 15A-15E, scanning electron microscopy (SEM) analysis demonstrates that surface structuring occurs at both the valleys and peaks of the grooves, thereby demonstrating conformal formation of the microscale roughness features on more expensive and intricate structures.
To demonstrate applicability of the microstructured etched Al technique to a variety of refrigerants/diameters, flow boiling studies were performed in a 4.6 mm internal diameter tube with a new low-GWP (global warming potential) refrigerant, R515B. FIG. 16 demonstrates a 270% heat transfer coefficient enhancement for the etched Al surface when compared with a plain Al surface.
Although the present invention has been described in considerable detail with reference to certain embodiments thereof, other embodiments are possible without departing from the present invention. The spirit and scope of the appended claims should not be limited, therefore, to the description of the preferred embodiments contained herein. All embodiments that come within the meaning of the claims, either literally or by equivalence, are intended to be embraced therein.
Furthermore, the advantages described above are not necessarily the only advantages of the invention, and it is not necessarily expected that all of the described advantages will be achieved with every embodiment of the invention.
Patent Application
20220373271
