In an oscillation heat pipe (OHP), when two-phase flow oscillates to the condensation region, saturated vapor bubbles/slugs are subjected to a sudden temperature reduction or immediate subcooling. Rapid condensation ruptures vapor bubbles and generates cavitation erosions on the tube interior surface. In this article, a thorough study is performed to understand discrepancy of variation tendency between acoustic radiation and OHP temperature difference that both are operating temperature-dependent. On this basis, three temperature zones were identified: (1) low operating temperature zone with strong cavitation collapse and acoustic radiations, (2) optimal temperature zone with the minimum temperature difference and weakening cavitation collapses, and, (3) high-temperature zone where dryout and oscillation failures develop. At the optimal operating temperature, high-frequent oscillations reduce subcooling of two-phase flow, alleviating the impact of cavitation collapses and ceasing acoustic radiations. At high operating temperature, liquid surface tension dramatically reduces and dynamic contact angle significantly increases. Both the factors tend to lower the critical diameter necessary to maintain pressure difference and oscillating two-phase slug flow. When the critical diameter reduces to be less than the OHP tube diameter, liquid slugs are not able to seal the OHP capillary tubes, leading to dryout or insufficient heat and mass transfer.

When vapor bubbles are subjected to suddenly reduced pressure or immediate subcooling, it may rapidly condense, rupture, and generate cavitation corrosions. This phenomenon often occurs behind the blade of a rapidly rotating propeller or on any surface vibrating in liquid with sufficient amplitude and acceleration. In this article, we reported cavitation and its corrosive collapse occurring in capillary tubes, called pulsating heat pipes. Visualization images of cut and opened tubes show that internal copper surface was seriously etched after a certain period of operation. Sub-millimeter etching pits are observed on the tube internal surface. Copper particles in size of a few hundred micrometers are also found in the reclaimed operating fluid. Starting from this finding, the temperature effect of performance is analyzed to understand the cavitation occurrence and collapse. Pulsating heat pipe requires a certain temperature difference between the evaporator and condenser sections, typically > 10°C, to generate continuous two-phase oscillating movements. However, during the transient startup period, this temperature difference could reach as high as 50°C. Large saturation temperature difference, associated with highly turbulent two-phase flow, drives the saturated vapor bubbles from the hot evaporation region to the subcooled environment in less than 100ms. During the rapid condensation, the accelerated shrinking vapor bubbles create interface instability, followed by forming a strong impingement jet to etch the solid pipe wall. The collapse of cavitation is associated with the generation of acoustically tinkling signals that are often heard during most of the operating pulsating heat pipe.

Surface etches caused by cavitation often occur behind the blade of a rapidly rotating propeller or on a vibrating surface where liquid is subjected to suddenly reduced or oscillating saturation pressure. This phenomenon has never been reported in a continuous capillary pipe in which flow pressure varied gently without injection of external radiation energy. On the other aspect, acoustically tinkling signals were recognized decades ago during operation of the oscillation capillary heat pipe, but lacking of vigorous scientific understanding of the root causes. In this article, we report cavitation and its surface etches in meandering capillary tubes with the inner diameter of 1.8 mm. Numerous etching pits were observed on the interior face at the heat rejection region after over 200 hours operation. Irregular copper debris, with sizes ranging from 20 to 500 μm, is found in the reclaimed operating fluid. Analysis of temperature and acoustic data indicates that, driven by spiking temperature difference, highly turbulent two-phase flow carries the saturated vapor bubbles from the evaporator to the subcooled condensation region in a very short time of oscillation, turning vapor phase into oversaturated status. Rapid condensation accelerated shrinkage of the vapor bubbles causing strong micro jet impingement to damage the pipe wall, radiating acoustical signals.

A heat pipe utilizes liquid–vapor phase change mechanism to efficiently transfer heat. Among different heat pipes, loop heat pipe (LHP) and pulsating heat pipe (PHP) are known to be capable of high heat flux/high load heat transfer. In this article, LHP and PHP heat transfer systems are combined to achieve passive, reliable, and remote/long-distance heat transfer for thermal management of modern avionics systems. Aiming at this goal, a 2 m long LHP is developed to transport heat from the avionics chassis to the remote heat rejection site. To reduce inner saturation pressure and ensure structural safety at high operating temperature, water is used as the operating fluid in LHP. Within the avionics chassis, conduction heat transfer is enhanced by sandwiching a PHP with two printed circuit boards (PCBs) and solder-bonding them. Each PHP/PCB assembly is 20 cm long and 12.5 cm wide, with electrical heaters mounted on both sides to mimic electronic heat dissipation. Heat transfer demonstration of the LHP and PHP combo system is conducted in a lab environment with input power varying from 100 to 400 W. For all the three PHP/PCB assemblies set in the avionic chassis, heat source temperature is maintained below the required 150 °C even when heat dissipation is twice as high as the state-of-the-art (and coolant temperature is 50 °C). This combo heat transfer system reduces power consumption and increases reliability, enabling the avionic system operation in harsh environments.

Water droplets on bio-mimicked hierarchical roughness exhibit superhydrophobic properties, such as large contact angles, minor dynamic hysteresis, and high mobility. Vapor condensation on such superhydrophobic surface enables rapid condensate removal and surface cleaning, thereby significantly enhancing the heat transfer coefficient. In this paper, research attention is given to dropwise condensation on/in specially designed one-tier and hierarchical roughness structures. Utilizing a normal optical tomographic system composed of a Sensi-Cam and a Nikon microscope, close-up visualization is conducted to characterize small condensate droplets, in size of a few micrometers, between structural units of roughness. Experimental snapshots show that, within the one-tier roughness, condensate droplets tend to stick to surrounding structures. Low mobility of these droplets extends their residence time, and therefore increases their average diameter. In comparison, surface energy of the hierarchical structure is significantly reduced. As a result, small condensate droplets behave nonsticky to their surroundings, which enable rapid drain of the droplets and accomplish self-cleaning of the structure. Because of high mobility, the droplet average diameter in the two-tier structure is smaller than those in the one-tire roughness. Condensation sites reach the maximum in the middle of the structure where dew point of moisture is reached. Less condensation droplets on both the top and bottom of the roughness are blamed to the unsaturated moisture and the reduced humidity, respectively.

Droplets on Micro/nano hierarchical structures exhibit extiaordiriary hydrophobic properties, such as large contact,angles, low dynamic liyiteresis, and high :mobility. Vapor condensation on such,the surface may potentially, achieve: rapid, condensate removal and surface cleaning, therefore: significantly enhanchig the heat transfer coefficient. This article reports novel conical hierarchical structures (CHS) and their mechanisms for enhancing Vapor/Moisture condensation. Through a normal optical tomography, visualization images show, in spite of ultrahigh surface structure, condensate droplets are able to rapidly precipitate under capillary forces and Maintain at the stable Cassie state in a dynamic condensation environment. Within CHS, the major condensation is advanced at where the incident moisture is cooled to its dew point. Cori Tared with the traditiOnal dropwise condensation, CHS reduces the mass transport resistance when moisture must diffuse through the entire nonconderisable gas (NCG) layer. The stable Cassie state in dyriamic condebsation environment, as well as the CHS structural toleriuce to NCG, enables high efficient vapor/inoisttire condensation in a complicated industrial environment.

To mitigate temperature overshoot and dissipate highly concentrated heat from high-power electronic components, it is important to develop an ultrathin vapor chamber/heat spreader to fit in a compact 3D electronic system. As a semiconductor material, silicon is highly thermal conductive, micromachinable and process-compatible with microelectronic manufactures. Thus, a silicon based vapor chamber (SVC) can be directly integrated with microelectronic devices to achieve hot spot cooling, without introducing an additional thermal interface. This article reports the development of SVC, stating from analysis of structural safety, followed by numerical simulations of the liquid and vapor flows. Advanced multiscale wick structures are implemented to balance the heat and mass transports of high heat flux under a gravitational force. On these bases, SVC with structural reinforcement of a 13 x 8 pillar array is developed through a triple bonding approach. The successful development of the SVCs results in a large scale (50 mm x 70 mm) and ultrathin (1 mm thick) phase change heat transfer device, with the effective density less than 1.5 x 10(3) kg/m(3). Using water as the operating fluid, we experimentally demonstrate a high effective thermal conductivity over 10,000 W/m.K in both 1D and 2D heat transfer modes. (C) 2016 Elsevier Ltd. All rights reserved.

A heat pipe can efficiently transfer heat through it liquid-vapor phase change mechanism. Among different heat pipes, loop heat pipe (LHP) and pulsating heat pipe (PHP) are known to be capable of high heat flux/high load heat transfer. In this article, LHP and PHP heat transfer systems are combined to achieve passive, reliable and long-distance heat transfer for successful thermal management of modern avionic systems. Aiming at this development goal, a 2m long LHP is designed to transport heat from the avionics chassis to the remote heat rejection site. To reduce saturation pressure and ensure structural safety at higher operating temperature, water is used as the operating fluid of LHP. Within the avionic chassis, conduction heat transfer is enhanced by sandwiching and solder-bonding a PHP between two PCBs (printed circuit boards). Each PHP/PCB assembly is 20cm long and 12.5cm wide, with electrical heaters mounted to mimic electronic heat dissipation. Heat transfer demonstration of the LHP and PHP combo system is conducted in a lab environment with input power varying from 100 to 400W. For all three PHP/PCB assemblies set in the avionic chassis, the maximum heat source temperature is less than 130°C, even when input power is twice as much as the state of the art. The enhanced heat transfer consumes less power, increases system reliability, and enables avionic system operation in harsher environments.

Thermal management of power electronics is an extremely challenging problem in the harsh environment of military hybrid vehicles, where the local air and liquid coolant's temperature exceed 100 degrees C under regular operating conditions. This paper presents the development work of a high heat flux, jet impingement-cooled heat exchanger for a 600-V/50-A silicon carbide (SiC) power module (rated at 175 degrees C device junction temperature), used for bidirectional power conversion between a 28-V battery and a 300-V dc bus. A total of 50 volume% mixture of water-ethylene glycol (WEG) coolant at 100 degrees C inlet temperature is the only available coolant. An array of WEG coolant microjets impinges on the base plate of the SiC module. The jet impingement cooling system has been optimized by experimental studies on a surrogate module, along with a high-fidelity computational model, to accurately estimate the SiC device junction temperature in relevant operating conditions. Results indicate that at the design heat load of 151 W (worst-case scenario), the SiC device junction temperature is reduced from 290 degrees C with commercial-off-the-shelf (COTS) cold plate cooling and 215 degrees C with COTS microchannel heat exchanger cooling, to 169 degrees C with a jet impingement-cooled heat exchanger, using the same flow rate.

Electroplating has been implemented in the fabrication of radio frequency circuits for many years. Combined with a photolithography process, this technology can be employed to precisely define both the particle size and distributions of copper wick structures in a phase-change heat transfer system. In this paper, two electroplated copper wick structures are developed to investigate characteristics of phase-change heat transfer at the early phase-change stage when applied heat flux is below ∼250  W/cm2. Using subcooled operating fluid, both visualization and heat transfer characterization indicate that the appearance of the onset of nucleation boiling plays a critical role in the transitions of the heat transfer model. Before the onset of nucleation boiling is triggered, heat transfer is dictated by the heat conduction of the wick, as well as evaporation on the meniscus interface. After the onset of nucleation boiling, the heat transfer coefficient is rapidly enhanced by boiling and evaporation within the ...

In a heat pipe, operating fluid saturates wick structures system and establishes a capillary-driven circulation loop for heat transfer. Thus, the thermophysical properties of the operating fluid inevitably impact the transitions of phase-change mode and the capability of heat transfer, which determine both the design and development of the associated heat pipe systems. This article investigates the effect of liquid properties on phase-change heat transfer. Two different copper wick structures, cubic and cylindrical in cross section, 340 μm in height and 150 μm in diameter or width, are fabricated using an electroplating technique. The phase-change phenomena inside these wick structures are observed at various heat fluxes. The corresponding heat transfer characteristics are measured for three different working liquids: water, ethanol, and Novec 7200. Three distinct modes of the phase-change process are identified: (1) evaporation on liquid–vapor interface, (2) nucleate boiling with interfacial evaporation, and (3) boiling enhanced interface evaporation. Transitions between the three modes depend on heat flux and liquid properties. In addition to the mode transition, liquid properties also dictate the maximum heat flux and the heat transfer coefficient. A quantitative characterization shows that the maximum heat flux scales with Merit number, a dimensionless number connecting liquid density, surface tension, latent heat of vaporization, and viscosity. The heat transfer coefficient, on the other hand, is dictated by the thermal conductivity of the liquid. A complex interaction between the mode transition and liquid properties is reflected in Novec 7200. In spite of having the lowest thermal conductivity among the three liquids, an early transition to the mode of the boiling enhanced interface evaporation leads to a higher heat transfer coefficient at low heat flux.

Liquid-to-vapor phase change on porous wick structures has been proven efficient and capable of high density heat removal. This superior thermal performance is attributed to thin film evaporation on liquid–vapor interfaces, capillary enhanced liquid return mechanism, as well as rapid vapor ventilation. In this complicated physical process, wick geometrical parameters, such as wick thickness, particle diameter/gap size and heating area dimension, of the wick structures play critical roles in determining the maximum phase change capability. This article presents an analytical model to extract the wick geometrical effects on heat and mass transport limits, based on mono porous wick structures composed of cylindrical pillars. The model considers two extreme cases: (a) thin wick structure with large heating area; and, (b) thick wick structure with small heating area. Two dimensionless geometrical numbers are derived based on liquid and vapor phase flow resistances. For a thin wick structure with large heating area, wherein the liquid phase is dominant in flow resistance, the maximum heat flux is proportional to liquid phase geometrical number, defined as a product of the pillar diameter and the wick thickness divided by square of the heating area dimension. In contrast, phase change capability of a thick wick structure with small heating area is attributed to the vapor phase geometrical number, written as the ratio of the pillar diameter over the wick thickness. The analytical model is validated through experimental results by characterizing phase change performance of the silicon mono wick structures. A ratio between the heating area width and the wick thickness is presented to justify the broad applicability of the analytical model. The two analytic cases of thin wick structure with large heating area and thick wick structure with small heating area correspond to the ratios being >1.0 and <0.1, respectively.

De-rating of power electronics is a common practice in harsh environment operating conditions. Advanced cooling solutions are needed to overcome the thermal limitations and prevent the de-rating of power components. This study investigates a silicon carbide power module for 28 V DC to 300 V DC power conversion (or reverse) in military hybrid vehicle applications. A combined experimental and numerical modeling effort is reported, to accurately predict the silicon carbide switch (device) junction temperature. The results show that jet impingement cooling at the module base plate increases the module heat dissipation capability by 2.5X and 1.5X, compared to the commercial-off-the-shelf cold plate cooling and micro-channel cooling solutions respectively.

Electroplating has been implemented in the fabrication of RF circuits for many years. Combined with a photolithography process, this technology can be employed to precisely define both the particle size and geometry of copper wick structures in a phase change heat transfer system. In this article, two electroplated copper wick structures are developed to investigate characteristics of phase change heat transfer at the early stage when applied heat flux is below 250W/cm. Using subcooled operating fluid, both visualization and heat transfer characterization indicate that the appearance of the onset of nucleation boiling (ONB) plays a critical role in determining the heat transfer model. Before the ONB is triggered, heat transfer is dictated by the heat conduction of the wick, as well as evaporation on the meniscus interface. After the ONB, the heat transfer coefficient is rapidly enhanced by phase change within the wick structures. On the characterization curves of heat flux versus the substrate temperature, the wick properties, such as porosity, directly affect the position of the ONB. Submicron porous structures on electroplated copper pillars accelerate the ONB and enhance the heat transfer coefficients of the phase change.

Silicon is the primary material of integrated circuit (IC) manufacturing in microelectronic industry. It has high thermal conductivity and superior thermomechanical properties compatible to most semiconductors. These characteristics make it an ideal material for fabricating micro/mini heat pipes and their wick structures. In this article, silicon wick structures, composed of cylindrical pillars 320 μm in height and 30–100 μm in diameter, are developed for studies of phase change capability. Fabrication of the silicon wick structures utilizes the standard microelectromechanical systems (MEMS) approach, which allows the precise definition on the wick dimensions, as well as the heated wick area. On these bases, experimental characterizations of temperature variations versus input heat fluxes, associated with simultaneous visualization on the liquid transport and the dryout, are performed to investigate the wick dimensional effects on the maximum phase change capability. On the wick structure with the pillar diameter/pores of 100 μm and a heated wick area of 2 mm × 2 mm, the phase change reached a maximum heat flux of 1130 W/cm2. Despite of the liquid bottom-feed approach, interactions between liquid and vapor phases enables the heated wick structure absorb liquid from its surrounding wick area, including from its top side with a longer liquid transport path. In contrast, a wick structure with fine pillars (10 μm in diameter) inhibited the generation of nucleate boiling. Evaporation on the meniscus interface becomes the major phase change mechanism. A large heated wick area (4 mm × 4 mm) increases the viscous loss in transporting liquid to wet the entire wick, advancing the dryout at 135 W/cm2. Mass transfer analysis, as well as discussion of the experimental results, indicates that a dimensional ratio r/l (pillar diameter/characteristic length of the heated wick area) is a key parameter in determining the maximum phase change capability. A low r/l ratio enhances heat and mass transport capability, as well as heat transfer coefficient.