Phase-change Heat Transfer Devices Research Articles

Performance of a novel loop heat pipe coupled with micro vapor-driven jet injector – An experimental and numerical study

Loop heat pipe (LHP) is an efficient phase-change heat transfer device which has been widely applied in cooling high-power electronics on ground and in spacecraft. In previous work, a novel LHP coupled with vapor-driven jet injector (VDJI) has been proved to be feasible and shows excellent performance, namely LHPI. However, due to the lack of pressure data during LHP operation, the coupling mechanisms of VDJI and LHP remain unknown. Especially, the heat-mass transfer between high-speed vapor and subcooled liquid inside VDJI, which has great influence on the LHPI, need to be investigated to guide the design of the LHPI. In this paper, the start-up process, steady-state operation characteristic of LHPI and internal flow field of VDJI were investigated experimentally and numerically. By using the pore-forming sintering and integrated sintering method, the bi-porous wick was prepared and embedded on the base plate surface of evaporator, which brought a significant improvement in its performance. The results indicated that the LHPI could operate under a power of 600 W (50.0 W/cm2) without dry-out. Maintaining the heating wall temperature within 85 °C, the minimum thermal resistance of the LHPI with bi-porous and integrated sintered wick was below 0.18, and the maximum heating power can reach up to 404 W, which are 60% lower and 52.5% higher than mono-porous wick, respectively. In addition, four operation modes of VDJI were classified as low-efficiency mode, normal-injection mode, restricted expansion mode and superheated mode. The operation modes of VDJI had great influence on the performance of LHPI. In normal-injection mode, the entrainment ratio of VDJI exceeded 100, which resulted in the lowest thermal resistance of LHPI. For long-term operation of LHPI, the VDJI is suggested keep operating in normal-injection mode. Moreover, by using 3D numerical simulation method, the internal flow structure of the VDJI was obtained to gives an insight into mechanisms of these four operation modes. The numerical results showed the profile of compression waves and confirmed the existence of condensation shock wave in VDJI.

Tailoring Substrate Topography to Enhance Wicking Performance of Electrodeposited Cu Porous Films

Porous metallic films have received emerging interests in the fields of catalysts, fuel cells, lithium batteries, and phase-change heat transfer devices. Substrate topography is known to have a strong impact on adhesion and morphology of porous coating layers prepared by electrodeposition. In this study, we have tailored the topography of a Cu substrate by wet etching in a sodium persulfate/sulfuric acid (SPS) solution prior to electrodeposition of porous Cu films. An analytic approach is proposed to quantitatively analyze nucleation and growth of dendritic Cu deposits on the SPS-treated Cu substrates. Such a dendritic porous Cu film can serve as a capillary wick to complete recirculation of condensed liquid in the phase-change heat transfer devices. The maximum heat transfer rate of a heat pipe or vapor chamber indeed is proportional to the capillary performance (K/Reff ) of the wick structure, where K and Reff stand for liquid permeability and effective pore radius of the wick, respectively. A large K/Reff value of 0.975 µm has been attained for the electrodeposited dendritic Cu film on a Cu substrate with an optimal SPS etching treatment. The influence of dendritic deposits on the wicking performance of the electrodeposited Cu film is investigated. Figure 1

Key features and highly effective prediction of complete startup from frozen state for high-temperature heat pipe in heat pipe reactor

High-temperature heat pipe is an efficient phase change heat transfer device, widely applied in the space nuclear reactors, the China Fusion Engineering Test Reactor, solar collectors, and the thermal protection of hypersonic vehicles. In order to understand the startup characteristics of the heat pipe and to predict its transient behavior with high accuracy, this paper first experimentally investigated the startup performance of a sodium heat pipe from the frozen state under different heating powers from 534 W to 1897 W. Then, a new compressible flow model for startup from the frozen state was developed and verified, solved the independence problems of mesh number and time-step used in the numerical treatments of most traditional heat pipe codes. Finally, based on the assessed model and experimental data, the startup procedure and the influences of heating power on the working temperature, kick-in time, and temperature difference were analyzed respectively. The comparisons indicated that the new model is extremely informative and effective, and the results provided knowledge for understanding the safe and optimum working regime of a high-temperature heat pipe using in heat pipe reactor.

Biomimetic Micropillar Wick for Enhanced Thin-Film Evaporation

Sustainable liquid cooling solutions are recognized as the future of thermal management in the chip industry. Among them, phase change heat transfer devices such as heat pipes and vapor chambers have shown tremendous potential. These devices rely on the physics of capillary-driven thin-film evaporation, which is inherently coupled with the design and optimization of the evaporator wicks used in these devices. Here, we introduce a biomimetic evaporator wick design inspired by the peristome of the Nepenthes alata that can achieve significantly enhanced evaporative cooling. It consists of an array of micropillars with multiple wedges along the sidewall of each micropillar. The efficacy of the wedged micropillar is evaluated based on a validated numerical model on the metrics of dryout heat flux and effective heat transfer coefficient. The wedge angle is chosen such that wedged micropillars cause liquid filaments to rise along the micropillar vertical walls. This results in a significant increase in thin-film area for evaporation. Additionally, the large mean curvature of the liquid meniscus produces strong capillary pumping pressure and simultaneously, the wedges increase the overall permeability of the wick. Consequently, our model predicts that the wedged micropillar wick can attain ∼234% enhancement of dryout heat flux compared to a conventional cylindrical micropillar wick of similar geometrical dimensions. Moreover, the wedged micropillars can also attain a higher effective heat transfer coefficient under dryout conditions, thus outperforming the cylindrical micropillar in terms of heat transfer efficiency. Our study provides insight into the design and capability of the biomimetic wedged micropillars as an efficient evaporator wick for various thin-film evaporation applications.

High-performance electrodeposited copper wicks for heat-spreading vapor chambers

A capillary wick structure is a key component for producing the capillary pressure that drives two-phase circulation in phase-change heat transfer devices such as heat pipes and vapor chambers. This study examines the capillary performance of a porous Cu wick structure fabricated using a new electrodeposition process. This Cu wick exhibits large channels generated by hydrogen bubbles and small pores in the dendritic Cu deposits resulting from high-current electrodeposition. The capillary wick performance can be effectively enhanced by adjusting the morphology of the dendritic Cu deposits. Dendritic Cu wicks with different porous features are prepared using different electrodeposition current densities and Cu sulfate concentrations in the electrolyte. A capillary rate-of-rise method is used to assess the liquid permeability (K), effective pore radius (Reff), and capillary performance (K/Reff) of dendritic Cu wicks with deionized wateras the working fluid. The Cu wick is then thermally treated at 700 °C in ambient N2 for 90 min to improve its structural integrity. Despite partial welding occurring between the fine Cu dendrites, the Cu wick shows a superior K/Reff of 1.5 ± 0.06 μm and excellent structural stability. The high capillary performance can enhance the mass transport rate and effective transport distance of the working liquid in vapor chambers for high-density and large-area heat dissipation applications. A vapor chamber integrated with an electrodeposited Cu wick demonstrates excellent heat-spreading characteristics, with a vapor thermal conductivity of up to 6500 W/m⋅K.

Fabrication and capillary characterization of multi-scale microgroove wicks for ultrathin phase-change heat transfer devices

Multi-scale microgroove wicks (MWs) with the features of micro fins, micropillars, and reentrant cavities were fabricated for the application in ultrathin phase-change heat transfer devices. Wire electrical discharge machining (WEDM), electrical discharge shape machining (EDSM), micro-milling, and grinding were used to fabricate MWs with the same design parameters. The structural features of the MWs were characterized by SEM and their capillary performances were evaluated through the capillary rise test. The machining methods affected the structure characteristics of MWs greatly, i.e., MW fabricated by the combination of WEDM and EDSM (MW-WE) maintained the best forming shape, while MW fabricated by micro-milling deformed severely and some micropillars even disappeared. The capillary rise tests were conducted with ethanol and acetone as the working fluid, respectively. MWs showed similar capillary performances under both working liquids. MW-WE maintained the best capillary performance while MW fabricated by micro-milling performed worst. The wicking height and capillary parameter of MW-WE achieved 60.4 mm and 1.57 μm, respectively, which were about twice and 4.62 times that of MW fabricated by micro-milling. The superior capillary performance of MW-WE was attributed to the fluent microgrooves, capillary value effect induced by reentrant cavities, and rough surfaces.

Deployment of vapour chambers for electronic heat dissipation: state-of-the-art

Miniaturization and high performance are the two principal requirements of modern electronics. All commercial applications demand reliable performance at high operational efficiency for prolonged service life. In order to meet all these requirements, the cooling system should be able to handle large heat fluxes packed in compact spaces and take away the heat as soon as possible to avoid overheating of the chip. Phase change heat transfer devices can handle large heat fluxes on account of latent heating. Lately, vapour chambers (VCs) are found to be promising heat spreaders as they are passive two-phase heat transfer devices. They effectively eliminate the hotspots, transferring high heat fluxes with better efficiencies and offer lower thermal resistance compared to the conventional cooling techniques. A detailed review of the latest research and advancements in the field of VCs is presented in this paper, including the effect of different thermo-physical parameters on their performance. Finally, all the major findings are summarized, including the future scope of development in a systematic manner.

Effective passive phase-change light-emitting diode cooling system using graphene nanoplatelets coatings

An effective passive two-phase light-emitting diode (LED) cooling system using thermally cured graphene nanoplatelets (GNPs) coatings is introduced. The GNPs-coated surfaces demonstrate significant reduction in the static contact angle as compared to the uncoated aluminium surface. The ultrafast water permeation property of the micro/nano-capillaries-structured GNPs coatings engenders significant cooling performance enhancement of the LED. The water is absorbed by the GNPs coatings and permeating upwards against the gravitation force. The water molecules are propelled through the nanocapillaries formed by the carbon walls and the capillary force dominates over the gravitational force. This induces the filmwise evaporation on the GNPs-coated surfaces and significantly enhances the LED cooling. As compared to the uncoated case, a maximum heat transfer coefficient enhancement of 73% is achieved. The GNPs coatings also significantly lower the operating light emitting surface temperature up to 10 °C reduction, effectively removes potential hotspots which in turn optimise the LED functionality and durability. This study paves the way for the promising employment of thermally cured GNPs coatings in developing a passive phase-change heat transfer device for LED cooling.

Effect of wick oxidation on the thermal performance of a copper-acetone loop heat pipe

Loop Heat Pipe (LHP) is a passive phase-change heat transfer device. Among other factors, its performance largely depends on the heat leak from the evaporator to the compensation chamber (CC). For copper wick LHP, a high heat leak creates a problem in successful LHP start-up and can adversely affect its operation. The present study focuses on reducing the heat leak through manipulation of the copper wick properties. The effect of fluid charging, overall pressure drop, and wick oxidation on thermal performance is investigated. A cylindrical copper-acetone LHP is tested in favorable orientation for two cases with (1) oxygen-free, or pure copper wick and (2) oxidized copper wick in its evaporator. For Case #1, LHP is filled with acetone at different Charging Ratios (CR) of 50%, 60%, and 70%. For Case #2, the charging ratio is kept constant at 50%. For the pure copper wick, a charging ratio of 50% provides the best thermal performance, and it decreases with an increase in charging ratios. At CR = 50%, LHP can transfer a heat load of 90 W (hEvp = ∼900 W/m2K) and 180 W (hEvp = ∼2500 W/m2K) at the evaporator temperature below 100 °C for the pure copper and oxidized wick, respectively. The significant improvement in LHP thermal performance of the latter is attributed to a decrease in heat leak because of the low thermal conductivity of the oxidized porous wick. The heat leak for pure and oxidized wick is found ∼18% and ∼7% of input heat loads, respectively. This can be an effective methodology to pave the way for the usage of commercial copper-based LHPs for the thermal management of terrestrial devices.

Performance of a flat grooved heat pipe with a localized heat load

Heat pipes are phase change heat transfer devices used in wide range of heat transport applications due to their high thermal transport capacities with low temperature differences. Heat pipes are especially preferred for electronic cooling applications and aerospace avionics to satisfy high heat transfer rate requirements. In this study, heat transfer and phase change mechanisms of working fluid are investigated and modeled using a 3-D thermal resistance network for multichannel flat grooved heat pipes. First, heat transfer and fluid flow are modeled in half of a single grooved structure due to symmetry, and is subjected to uniform heat flux. Radius of meniscus curvature and temperature distribution along the groove are calculated. Results are compared with experiments in the literature and show good agreement. The validated heat transfer and fluid flow models are extended to a multichannel model to observe performance of grooved heat pipes with localized heat sources, not covering the entire width, a vital feature for realistic simulation of operational devices. Predictions of the temperature distribution along the multichannel of the heat pipe are provided and the effect of the distribution of heat sources on the heat pipe is discussed.

Inverse-thermocapillary evaporation in a thin liquid film of self-rewetting fluid

PurposeThe present study aims to investigate the inverse-thermocapillary effect in an evaporating thin liquid film of self-rewetting fluid, which is a dilute aqueous solution (DAS) of long-chain alcohol.Design/methodology/approachA long-wave evolution model modified for self-rewetting fluids is used to study the inverse thermocapillary characteristics of an evaporating thin liquid film. The flow attributed to the inverse thermocapillary action is manifested through the streamline plots and the evaporative heat transfer characteristics are quantified and analyzed.FindingsThe thermocapillary flow induced by the negative surface tension gradient drives the liquid from a low-surface-tension (high temperature) region to a high-surface-tension (low temperature) region, retarding the liquid circulation and the evaporation strength. The positive surface tension gradients of self-rewetting fluids induce inverse-thermocapillary flow. The results of different working fluids, namely, water, heptanol and DAS of heptanol, are examined and compared. The thermocapillary characteristic of a working fluid is significantly affected by the sign of the surface tension gradient and the inverse effect is profound at a high excess temperature. The inverse thermocapillary effect significantly enhances evaporation rates.Originality/valueThe current investigation on the inverse thermocapillary effect in a self-rewetting evaporating thin film liquid has not been attempted previously. This study provides insights on the hydrodynamic and thermal characteristics of thermocapillary evaporation of self-rewetting liquid, which give rise to significant thermal enhancement of the microscale phase-change heat transfer devices.

Hierarchically 3D-textured copper surfaces with enhanced wicking properties for high-power cooling

Phase change heat transfer devices (PCHTDs) have been considered as an ideal choice for the application of high-power cooling. Wick, as the core of PCHTD, has received extensive attention. In this study, hierarchically 3D-textured copper surfaces (TSs) were fabricated as the potential wicks to meet the needs of high-power cooling in PCHTDs. Laser machining, as an efficient special processing means, was employed to complete the fabrication of TSs with different structural parameters. The capillary performance of the TSs, including capillary rise height and velocity, was evaluated by the infrared thermal imaging method with ethanol as working fluid. In particular, the maximum volumetric flow rate of the textured surfaces due to the capillary wicking was found to dominate the two-phase heat transfer capacity through saturated pool boiling tests. The best two-phase heat transfer performance was achieved by TS75 with the critical heat flux of 173.9 W/cm2 and the maximum heat transfer coefficient of 34.6 W/cm2K, which are 1.54 and 5.71 times higher than those of the plain surface. Comparative studies with other capillary structures indicated that TSs have comparable or even better enhanced heat transfer performance, making it a promising wick for the application of high-power cooling devices.

U-drill embedded with phase change heat transfer device for machining applications

Metal machining is always associated with the generation of heat. Though heat generated in metal cutting is carried away by the chips, continuous engagement of tool with the workpiece increase the temperature at the tool–work interface and spreads to its neighborhood. In this article, a novel method of removing heat from the tool-work interface is implemented with the use of phase change heat transfer device (heat pipes) and applied for drilling operation carried out by U-drills. A custom built U-drill is designed with a heat pipe placed axially. The temperature of the tool-work interface is estimated analytically using the experimental values of cutting force components. Experiments were conducted at various speeds and feed to measure the performance of the U-drill. It is found that the proposed method of heat pipe (HP) embedded drill bit is able to remove heat and operates at a lower temperature at the tool-chip interface. Results show that a considerable reduction in the cutting force generated at various machining conditions. The heat pipe embedded U-drill will be useful in drilling components made of composite materials where coolants are not widely employed. It was evident from the experimental works that the inclusion of heat pipe in U-drill has brought forth a reduction of cutting temperature of about 30%. This also brought forth a better cutting parameter, and a significant reduction in tool wear increased the tool life which invariably decreases the production cost and time.

Molecular diffusion replaces capillary pumping in phase-change-driven nanopumps

Nano-scale fluid transport has vast applications spanning from water desalination to biotechnology [1,2]. It is possible to pump fluids in nano-conduits using pressure gradients [3], thermal methods [4], electric [5,6] and magnetic fields [7], and with manipulations of surface chemistry and electric fields [8-10]. Inspired by the capillary-driven phase change heat transfer devices, we present a phase-change driven nanopump operating almost isothermally. Meticulous computational experiments on different sized nanopumps revealed efficient operation of the pump despite the reduction in system size that extinguishes capillary pumping by annihilating the liquid meniscus structures. Measuring the density distribution of liquid in cross sections near to the evaporating and condensing liquid-vapor interfaces, we discovered that phase change induced molecular scale mass diffusion mechanism replaces the capillary pumping in the absence of meniscus structures. Therefore, proposed pumps can serve as a part of both nanoelectromechanical (NEMS) and microelectromechanical systems (MEMS) with similar working efficiencies, and can be used for continuous gas separation applications.

A numerical investigation on the conjugate heat transfer of thin liquid film of water in closed microcavity

The understanding of the conjugate heat transfer in an evaporating thin-film and an intrinsic meniscus is important for enhancing the thermal performance of phase-change heat transfer devices. In this paper, we numerically investigated the conjugate heat transfer in the thin film and the intrinsic meniscus under their own vapor in closed microcavities with different geometries. The results show that, with decreasing microcavity size, the percentage contribution of the thin liquid film to the overall heat transfer increases obviously. However, in order to optimally utilize the thin-film heat transfer, both the microcavity size and the length-height ratio need to be concerned for their effects on the thin-film contribution to the net heat transfer. With increasing superheats that are smaller than 5 K, the contribution of the thin film to the net heat transfer rate becomes somewhat smaller, although the variation seems to be less sensitive to the superheat than the microcavity size. Meanwhile, when the wettability improves, the contribution of the thin film to the net heat transfer rate increases, but the variation of the thin-film contribution is relatively less sensitive to the wettability than the microcavity size and the superheat.

Investigation of the Use of an Inorganic Aqueous Solution in Copper-Made Phase-Change Heat Transfer Devices

A novel inorganic aqueous solution (IAS) is shown to have a better heat transfer performance than water when used as the working fluid in copper-made phase-change heat transfer devices. First, the physical properties of IAS are measured and compared to those of water. Another, a chemical analysis is performed, and the chemical reactions involved between IAS and the copper surface are listed and categorized by their contributions to the heat transfer performance. In addition, a capillary rise test is performed to show how each chemical contributes to the improvement of the surface wettability. Last, using IAS in copper-made phase-change heat transfer devices is discussed, and the main focus is how IAS improves the heat transfer performance by a smaller thermal resistance and a larger critical heat flux. The conclusion is validated by thermo-siphon tests at different inclination angles.

Investigation of the Use of an Inorganic Aqueous Solution in Aluminum-Made Phase-Change Heat Transfer Devices

An inorganic aqueous solution, known as IAS, has shown its compatibility with aluminum phase-change heat transfer devices. When using IAS in aluminum devices, aluminum prefers to react with the two oxidizers, permanganate and chromate, rather than water to generate a thin and compact layer of aluminum oxide, which protects the aluminum surface and prevents further reactions. In addition, an electrochemical theory of aluminum passivation is introduced, and the existence of an electrochemical cycle is demonstrated by an aluminum thermosiphon test. The electrochemistry cycle, built up by liquid back flow and tube wall, allows the oxidizers to passivate the aluminum surface inside the device without being directly in contact with it. However, failure was detected while using IAS in thermosiphons with air natural convection cooling. The importance of a continuous liquid back flow to aluminum passivation in phase-change heat transfer devices is pointed out, and a vertical thermosiphon test with natural convection cooling is used to demonstrate that a discontinuous liquid back flow is the main reason of the failures.

Dielectric liquid pumping flow in optimally operated micro heat pipes

Micro heat pipe is a micro-scale capillary-driven two-phase heat transfer device of which thermal performance is governed by the strength of evaporation and the circulation effectiveness of condensate from the condenser to the evaporator. By employing a mathematical model based on the conservation laws, this study demonstrates the application of dielectric pumping flow in enhancing the circulation effectiveness of condensate and hence the thermal performance of micro heat pipes. Through the application of a non-uniform electric field, the Maxwell pressure gradient is induced to drive the condensate flowing towards the evaporator. Two different dielectric pump configurations are compared and the micro heat pipe using planar electrodes is found performing better than that with the pin electrodes. The performance enhancement of different dielectric pump lengths where the total amount of electrical energy of the pump is conserved is analysed. The dielectric pump performs the best when it covers the entire length of micro heat pipe. Compared to the case without dielectric liquid pumping flow, significant enhancement in the heat transport capacity can be obtained where the maximum enhancement exceeds 220%. Even with a significant performance enhancement, the use of dielectric pump renders a sufficiently small solid wall temperature drop of a micro heat pipe, justifying the typical characteristic of a phase-change heat transfer device.

Dryout analysis of overloaded microscale capillary-driven two-phase heat transfer devices

Dryout occurrence at high heat input is one of the detrimental factors that limit the thermal efficiency of a phase-change heat transfer device. In this work, we demonstrate that by employing visualization method, the dryout occurrence of an elongated liquid droplet in a transparent evacuated microscale two-phase flow device can be scrutinized. The circulation of liquid from the condenser to the evaporator is driven by the capillary action which is the primary limitation that governs the maximum heat transport capability of the device. When the evaporation rate exceeds the circulation rate of condensate, dryout will take place in the evaporator end. The propagation of dryout lengths can be accurately determined directly from visualization and a more accurate evaluation of the dryout length compared to the conventional method by measuring the axial temperatures has been developed. By quantifying the performance indicators of the cooling device over a wide range of operating conditions, including the underloaded and overloaded operations, the observation of dryout occurrence in this study correlates highly with the anticipated heat transfer characteristics of a phase-change heat transfer device. This study provides essential insights, particularly on the overloaded conditions, to the design of a microscale two-phase heat transfer device.

A new method for evaluating heat pipe fluid compatibility

While heat pipes are a widely used and highly efficient means of spreading heat or transferring it from one location to another, there are some fluid-casing combinations which cannot be used due to material incompatibilities. These incompatibilities generally result in the generation of non-condensable gases such as the hydrogen gas produced when water is used with aluminum devices. Some attempts at using inhibitors in aqueous solution are found in literature but a method for screening which inhibitors to use that can also predict and assess the performance of different concentrations is not available. Using a thermodynamic approach, principles of corrosion engineering have been applied to the specific application of phase change heat transfer devices. Experimental data from previous tests has been used to show the E–pH diagrams match the reported NCG generation observed experimentally. Using thermodynamic equilibrium diagrams and measurements of heat pipe fluid E and pH can be a valuable tool when investigating passivating fluids for previously incompatible fluid-casing combinations.