SURFACE-MODIFIED COMPONENT AND METHOD OF ACHIEVING HIGH HEAT TRANSFER DURING COOLING
A 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 present patent document claims the benefit of priority under 35 U.S.C. 119(e) to U.S. Provisional Patent Application No. 63/189,776, which was filed on May 18, 2021, and is hereby incorporated by reference in its entirety.
FEDERALLY FUNDED RESEARCH AND DEVELOPMENTThis invention was made with government support under N00014-21-1-2089 awarded by the Office of Naval Research. The government has certain rights in the invention.
TECHNICAL FIELDThe 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.
BACKGROUNDFlow 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 SUMMARYDescribed 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.
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.
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
Referring to
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
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.
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
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.
In some examples, the inner surface 104 of the aluminum body 102 may comprise grooves, micro-fins, or other structural features, as shown in
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
With regards to the wetting characteristics of the fabricated structures, the greatly enhanced wicking ability of the superhydrophilic microstructured aluminum surface is visible in
For the Al samples,
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
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.
To gain an understanding of the enhancement mechanisms, local properties were analyzed near the entrance and exit of the test section. The plots of
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
The dominance of nucleate boiling near the entrance of the etched tubes can be seen via analysis of the boiling curve in
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
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
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
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
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.
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.
Claims
1. A method of achieving high heat transfer during cooling, the method comprising:
- providing an aluminum body having an inner surface enclosing a channel, the inner surface comprising microscale roughness features and microcavities configured to enhance nucleation site density during flow boiling;
- transporting a refrigerant through the channel, the refrigerant absorbing heat from a thermal load and undergoing flow boiling,
- wherein 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).
2. The method of claim 1, wherein the microcavities have a linear size in a range from about 2 microns to about 30 microns.
3. The method of claim 1, wherein the microscale roughness features have a height in a range from about 1 microns to about 15 microns.
4. The method of claim 1, wherein the average heat transfer coefficient is stable within +/−5% for at least 28 days.
5. The method of claim 1, wherein the channel has a diameter of at least about 3 mm and/or a length of at least about 1 m.
6. The method of claim 1, wherein the refrigerant comprises a hydrochlorofluorocarbon, a hydrofluoro-olefin, a hydrofluorocarbon, and/or a zeotropic refrigerant blend.
7. The method of claim 1, wherein the aluminum body comprises an enhancement factor Øe.f. of at least about 2 at the mass flux of about 300 kg/(m2·s), where Øe.f.=(hstructured/hplain)/(ΔPstructured/ΔPplain).
8. The method of claim 1, further comprising, prior to providing the aluminum body, forming the inner surface comprising the microscale roughness features and microcavities, the forming comprising:
- 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, thereby obtaining the inner surface comprising the microscale roughness features and microcavities.
9. A surface-modified component for enhanced heat transfer during cooling, the surface-modified component comprising:
- an aluminum body having an inner surface enclosing a channel, the inner surface comprising microscale roughness features of about 1 microns to about 15 microns in height and microcavities of about 2 microns to about 30 microns in linear size,
- wherein the inner surface comprises aluminum and native aluminum oxide, and wherein the aluminum body does not include an interface between the inner surface and a sub-surface region of the aluminum body.
10. The surface-modified component of claim 9, wherein, during flow boiling of a refrigerant through the channel, an average heat transfer coefficient of at least about 10 kW/(m2·K) is achieved at a mass flux of about 300 kg/(m2·s).
11. The surface-modified component of claim 9, wherein the aluminum body comprises an aluminum tube, and/or
- wherein the aluminum body comprises an aluminum alloy having an alloy designation in the 1000 through 7000 series.
12. The surface-modified component of claim 9, wherein the channel has a diameter of at least about 3 mm and/or a length of at least about 1 m.
13. The surface-modified component of claim 9, wherein the inner surface of the aluminum body further comprises micro-fins or grooves comprising the microscale roughness features.
14. A method of modifying a surface of a component for enhanced heat transfer during cooling, the method comprising:
- 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, thereby obtaining a surface-modified component comprising the aluminum body, wherein the inner surface comprises microscale roughness features and microcavities.
15. The method of claim 14, wherein the exposure to the HCl solution takes place for a time duration from about 7 min to about 30 min.
16. The method of claim 14, wherein the cleaning comprises immersing the aluminum body in acetone, ethanol, isopropanol, and the deionized water in succession.
17. The method of claim 14, wherein the rinsing further comprises exposing the inner surface to isopropanol.
18. The method of claim 14, wherein the microcavities have a linear size in a range from about 2 microns to about 30 microns.
19. The method of claim 14, wherein the microscale roughness features have a height in a range from about 1 microns to about 15 microns.
20. The method of claim 14, wherein the channel has a diameter of at least about 3 mm and/or a length of at least about 1 m.
Type: Application
Filed: Apr 27, 2022
Publication Date: Nov 24, 2022
Inventors: Nenad Miljkovic (Urbana, IL), Nithin Vinod Upot (Urbana, IL), Kazi Fazle Rabbi (Urbana, IL), Anthony M. Jacobi (Mahomet, IL), Allison J. Mahvi (Lakewood, CO)
Application Number: 17/730,502