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Thermal management is the current bottleneck in advancement of high-power integrated circuits (ICs), and phase change heat sinks are a promising solution. With a unique structural configuration consisting of a membrane positioned above the heater surface, membrane-based heat sinks (MHSs) have thus far attained heat fluxes of up to 2 kW/cm2 and heat transfer coefficient (HTC) of up to 1.8 MW/ $\text{m}^{2}~\cdot $ K using water as the working fluid. This work reports the latest progress and performance evaluation of MHS for high flux thermal management. MHS is implemented in conjunction with a low surface tension liquid to rapidly expel bubbles from the heated surface and reach a critical heat flux (CHF) of 340 W/cm2 and a HTC of 120 kW/ $\text{m}^{2}~\cdot $ K. A parametric comparison shows that thermal efficiency, defined as the ratio of cooling capacity and pumping power consumption, of the prototypical devices exceeds values reported hitherto in literature by more than two orders of magnitude. Our results indicate that coupled with the surfaces of higher thermal conductivity and membranes of higher permeability, the MHS devices could be a promising solution to thermal management needs of high-power electronics and lasers. [ABSTRACT FROM AUTHOR]

In this study, six benchmark experiments are conducted on bubbles at different growth stages to evaluate the assumptions of the existing microchannel flow boiling heat transfer models/hypothesis. The results show that the bubble ebullition process triggers a spike in the local surface heat flux due to the thin film evaporation and transient conduction heat transfer mechanisms. This enhancement in the surface heat flux is limited to a very small area at the bubble–surface contact region at the nucleation site limiting the overall heat transfer contribution of the bubble ebullition process. The contribution of these two mechanisms of heat transfer increases as the bubble–surface contact area becomes larger. As the bubbles length increases, the time period of activation of the microlayer evaporation mechanism substantially increases while that of the transient conduction mechanism remains relatively unchanged. When the microchannel is mostly occupied by bubbles, the thin film evaporation mechanism becomes the dominant heat transfer mode. The results clearly indicate that single-phase heat transfer mechanism active at surface regions not covered by bubbles is governed by the laminar flow theory (for the test conditions presented here). In essence, a measureable enhancement effect in the liquid phase due to bubbles growth and flow has not been observed. A comparison with the existing microchannel flow boiling models suggests that the three-zone flow boiling model can qualitatively describe the heat transfer events observed in this experiment but fails to accurately predict the magnitude of the heat transfer mechanisms. [ABSTRACT FROM AUTHOR]

An introduction to the journal is presented in which the editor discusses various articles published within the issue, including one on advances in natural refrigerants, another on advances in refrigerant-to-air heat exchanger modeling, and another one on advances in heat and mass transfer processes in air-conditioning equipment.,

In this study, the conjugate heat and mass transfer process taking place during the absorption of a refrigerant vapor into a falling liquid film is analyzed using the Laplace transform method. The Laplace transform method has been previously utilized to model the process but under a uniform velocity profile assumption. Here, a more realistic linear velocity profile is employed. The results suggest that the uniform velocity profile assumption overestimates the refrigerant concentration across the liquid film, and underestimates the temperature profile and the vapor absorption rate. Overall, the uniform velocity profile assumption can result in up to 30% error in calculating the absorption rate. Using the new model, the effect of different flow parameters on the absorption rate has been studied. The efficacy of the model is demonstrated through comparison with various experimental and numerical results reported in the literature. The analytical solution developed in the present study provides a simple, fast and accurate approach for calculating the absorption rate at different operating conditions and fluid properties. [ABSTRACT FROM AUTHOR]

For nearly two decades, the microchannel flow boiling heat transfer process has been the subject of numerous studies. A plethora of experimental studies have been conducted to decipher the underlying physics of the process, and different hypotheses have been presented to describe its microscopic details. Despite these efforts, the underlying assumptions of the existing hypothesis have remained largely unexamined. Here, using data at the microscopic level provided by a unique measurement approach, we deconstruct the boiling heat transfer process into a set of basic mechanisms and explain their role in the overall surface heat transfer. We then show how this knowledge allows to relate the bubble growth and flow dynamics to the surface heat flux. [ABSTRACT FROM AUTHOR]

This study examines the microscale physics of heat transfer events in flow boiling of FC-72 in a microchannel. Experimental results presented here provide new physical insight on the nature of heat transfer processes during bubbles growth and flow through the microchannel. The study is enabled through development of a device with a composite heated wall that consists of a high thermal conductivity substrate coated by a thin layer of a low thermal conductivity material with embedded temperature sensors. This novel arrangement enables calculation of the local heat flux with a spatial resolution of 40–65 μm and a temporal resolution of 50 μs. The device generates isolated bubbles from a 300 nm in diameter artificial cavity fabricated at the center of a pulsed function micro-heater. Analysis of the temperature and heat flux data along with synchronized images of bubbles show that four mechanisms of heat transfer are active as a bubble grows and flows through the channel. These mechanisms of heat transfer are (1) microlayer evaporation, (2) interline evaporation, (3) transient conduction, and (4) micro-convection. Details of these mechanisms including their time period of activation and corresponding surface heat flux and heat transfer coefficient are extensively discussed. [ABSTRACT FROM AUTHOR]

In this opinion piece, we discuss recent advances in experimental methods for characterizing phase change heat transfer. We begin with a survey of techniques for high-resolution measurements of temperature and heat flux at the solid surface and in the working fluid. Next, we focus on diagnostic tools for boiling heat transfer and describe techniques for visualizing the temperature and velocity fields, as well as measurements at the single bubble level. Finally, we discuss techniques to probe the kinetics of vapor formation within a few molecular layers of the interface. We conclude with our outlook for future progress in experimental methods for phase change heat transfer. [ABSTRACT FROM PUBLISHER]

Abstract: This paper is the first of a two-part study concerning the dynamics of heat transfer during nucleation process of saturated FC-72 liquid. Experimental results discussed in this paper provide new physical insight on the nature of heat transfer events at the nucleation site during the nucleate boiling process. The thermal field underneath a bubble during the boiling of FC-72 was measured with a spatial resolution of 22--40μm. The time period of activation, area of influence, and magnitude of three different mechanisms of heat transfer active at the nucleation site were determined. These mechanisms consisted of: (1) microlayer evaporation following the rapid bubble expansion, (2) transient conduction due to rewetting of the surface during bubble departure, and (3) microconvection in the region external to the bubble/surface contact area. The area of influence of the transient conduction mechanism was found to be limited to the bubble/surface contact area, with most of the heat transfer occurring prior to the bubble detachment from the surface. The microconvection heat transfer mechanism was localized primarily outside the contact area and was found to be steady in nature. All three mechanisms of heat transfer were found to make significant contributions to the total surface heat transfer. The second part of this study provides the theoretical analysis of the results. [Copyright &y& Elsevier]

Abstract: This paper is the second part of a two-part study concerning the dynamics of heat transfer during the nucleation process of FC-72 liquid. The experimental findings on the nature of different heat transfer mechanisms involved in the nucleation process were discussed in part I. In this paper, the experimental results are compared with the existing boiling models. The boiling models based on dominance of a single mechanism of heat transfer did not match the experimental results. However, the Rohsenow model was found to closely predict the heat transfer through the microconvection mechanism that is primarily active outside the bubble/surface contact area. An existing transient conduction model was modified to predict the surface heat transfer during the rewetting process (i.e. transient conduction mechanism). This model takes into account the gradual rewetting of the surface during the transient conduction process rather than a simple sudden surface coverage assumption commonly used in the boiling literature. The initial superheat energy of the microlayer (i.e. microlayer sensible energy) was accurately calculated and found to significantly contribute in microlayer evaporation. This even exceeded the direct wall heat transfer to microlayer at high surface superheat temperatures. A composite model was introduced that closely matches our experimental results. It incorporates models for three mechanisms of heat transfer including microlayer evaporation, transient conduction, microconvection, as well as their influence area and activation time. The significance of this development is that, for the first time, all submodels of the composite correlation were independently verified using experimental results. [Copyright &y& Elsevier]

A novel microelectromechanical device has been developed to study the details of the heat transfer mechanisms involved at the nucleation site for the nucleate boiling process. This device enables quantifying the magnitude, time period of activation, and specific areas of influence of different mechanisms of heat transfer from the surface with a resolution several times greater than previously reported. This is achieved through the use of an array of embedded temperature sensors within a carefully designed dual-layer (silicon and benzocyclobutene) wall which allows for the accurate calculation of local heat flux, circumventing difficulties encountered when using existing methods. The sensors are radially distributed around the nucleation site. Heat is supplied to the wall by a thin film heater fabricated on the outer nonwetted surface. Single bubbles are generated at the center of the array while the temperatures and the bubble images are recorded with a sampling frequency of 8 kHz. The temperature data provided the necessary thermal boundary conditions to numerically calculate the surface heat flux with an unprecedented radial resolution of 22-40 μm. Fabrication, characterization, and the ability of the developed device to elucidate the heat transfer aspects of the nucleation process are demonstrated. [ABSTRACT FROM AUTHOR]

Corona discharge utilizes the effect of electrically induced secondary motions to enhance the air-side heat transfer coefficients between high-density fins in a fin structure. The objective of this study was to measure the velocity associated with the air jet generated by corona discharge for parametric ranges of interest to air-side heat transfer enhancement in compact heat exchangers. This is the first study of its kind where the nonintrusive laser Doppler velocimetry (LDV) technique was used to measure the velocity, with careful attention paid to correction for the electric field effects on the particles and the resulting slip velocity. Experiments were conducted with a prototypical wire-to-plate geometry (positively charged wire and grounded plate). In two series of experiments, the flow was seeded with 2.5 μm glass microspheres and 0.5 μm polystyrene nanospheres. The corrected velocity measurements were used to calculate the kinetic energy flux to the fluid and the resulting efficiency of the electric-to-kinetic energy conversion. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • A distribution transformer was 3-D simulated for pure oil and nanofluid. • The heat transport and electromagnetic field were simultaneously simulated. • The presence of TiO 2 nanoparticles in the transformer oil improved Nusselt number. • Maximum temperature of hot spot points decreased using nanofluid as a coolant. Abstract A transformer is an electrical device that transfers electrical energy between windings by electromagnetic induction while producing a considerable amount of heat in circuits. The heat produced in windings is removed by an appropriate heat transfer fluid such as liquid dielectrics. The cooling and insulating of a liquid dielectric depend on the properties of the oil filling the transformer. One of the approaches to enhance the thermal and dielectric properties of transformer oil is employing an appropriate nanoparticle in a transformer. In this paper, a three-phase distribution transformer is simulated three-dimensionally in order to study the heat transfer efficiency for pure oil (single phase) and nanofluid (TiO 2 nanoparticles- transformer oil). For both models, the electromagnetic field in solid sections and heat transfer in fluid and solid sections of the transformer are simultaneously investigated. The simulation results show that the presence of TiO 2 nanoparticles in the transformer oil increases the heat transfer coefficient, i.e. adding 1% (vol/vol) of TiO 2 nanoparticles to the transformer oil increases the Nusselt number from 2.17 to 2.49, while the maximum temperature of transformer components decreases from 47.20 °C to 43.05 °C. [ABSTRACT FROM AUTHOR]

• Underlying physics of critical heat flux (CHF) is explained. • Hydrodynamic and evaporation momentum limits have vastly different values. • Evaporation momentum limit (CHF EM) is the ultimate limit of CHF. • CHF EM is observed for l h / l c < 1 and hydrodynamic limit for l h / l c > 1. • CHF progressively increases beyond the zuber limit with decreasing l h / l c. The upper limit of heat transfer from a heated surface to a surrounding boiling liquid has been the subject of numerous studies ever since Nukiyama (1934) discovered this limit. The underlying physics governing this phenomenon, universally known as the critical heat flux (CHF) limit, has been extensively debated for nearly a century. Two prevailing hypotheses have emerged including hydrodynamic instability and evaporation momentum force thresholds proposed by Kutateladze (1948) and Steinchen and Sefiane (1996), respectively. Zuber (1959) and Kandlikar (2001) developed correlations based on these hypotheses that predict roughly similar CHF values i.e., ∼100 W/cm2 for water at 1 atm on a copper surface. Here, we present experimental and analytical studies conducted on liquids with a wide range of thermophysical properties on planar heater surfaces of different size (to stabilize the flow hydrodynamics) that show the evaporation momentum limit (CHF EM) is roughly 4 times the Zuber's limit (CHF Zuber). We show that CHF EM can only be observed when hydrodynamics of liquid and vapor above the surface is stabilized, delineating an ultimate limit governed by a force balance at the surface-fluid interface rather than the instability of liquid and vapor interface away from the surface. We find that CHF/CHF EM varies from 0.2 to 1 for all fluids, saturation temperatures, and heater geometries tested, representing 48 conditions. [ABSTRACT FROM AUTHOR]

In this study, a new heat sink architecture is introduced that operates at two different phase change heat transfer modes. At low wall superheat temperatures, the heat sink operates under the thin film evaporation heat transfer mode and then transitions to the boiling heat transfer mode when the wall superheat temperature increases. This unique function is enabled through constraining the liquid and vapor phases into separate domains using a capillary-controlled meniscus formed within a hierarchical 3D structure. The structure is designed to form thin liquid layers between vertically oriented menisci across which the liquid is evaporated into neighboring vapor channels. The entire structure is then capped by a hydrophobic vapor-permeable membrane to fully confine the liquid layers while allowing vapor to pass through the membrane. At low wall superheats, the liquid layers directly evaporate into the vapor channels. In this operation mode, the heat flux was linearly increased to a maximum of 54 W/cm 2 at approximately 8 °C wall superheat temperature, corresponding to a heat transfer coefficient of approximately 62 kW/m 2 K. The heat transfer coefficient only slightly declined with increasing the wall superheat temperature but substantially improved as the liquid supply pressure was increased. Increasing the superheat temperature beyond 7–9 °C resulted in transition to the boiling heat transfer mode with a pronounced increase in surface temperature fluctuations. This transition to boiling results in a decline in the heat transfer coefficient because the meniscus formed between the liquid and vapor spaces breaks down. However, the heat removal capacity is significantly increased, and a critical heat flux of about 300 W/cm 2 is reached at <30 °C wall superheat temperature, corresponding to a heat transfer coefficient of approximately 100 kW/m 2 K. [ABSTRACT FROM AUTHOR]