Low Wall Superheat Research Articles

Effect of graphene coating on flow boiling in a minichannel at sub-atmospheric pressures

Flow boiling in a mini-channel with graphene-coated sintered porous copper surface was investigated at sub-atmospheric pressures by analyzing bubble motion and flow patterns. A method of surface modification based on graphene coating was presented for thermal management systems operating at lower temperatures. The experiments were carried out at mass fluxes of 60 kg.m–2.s–1 and 120 kg.m–2.s–1 and sub-atmospheric pressures of 48 kPa and 68 kPa in a mini-channel with dimensions of 50 mm × 15 mm × 2 mm using HFE-7100 coolant as the working fluid. At sub-atmospheric pressures, the bubble nucleation site density decreases, and the bubble diameter increases while the bubble release frequency decreases. The existence of porous surfaces and HFE-7100 coolant with lower surface tension leads to the inception of boiling at lower wall superheats. The graphene coating promotes microlayer evaporation and consequently increases the nucleation site density by enhancing the thermal conductivity and wicking rate. Additionally, it provides stable annular flow at high heat fluxes, whereas intermittent-annular flow is observed for the untreated surface under the same condition. This enhancement results in a more pronounced increase in the boiling heat transfer for the graphene-coated surface. As a result, this study provides the high potential of graphene coatings for phase change heat transfer enhancement at sub-atmospheric pressures.

Boiling heat transfer enhancement by a pair of elastic plates

Boiling heat transfer enhancement is essential for energy utilization. Traditional passive boiling enhancement methods aiming at bubble departure promotion usually depend on the modulation of surface tension force by surface modification. However, the limited driven force which benefits bubble departure cannot help the further improvement of heat transfer performance facing the increased heat dissipation amounts. In this paper, a novel boiling enhancement method using a pair of elastic plates (i.e., flexible parallel plates made of stainless steel) which promotes bubble departure by its swing effect for boiling on Pt wire in confined space is proposed and investigated. Compared to boiling with a pair of rigid plates (the thickness of 0.5 mm and the length of 12 mm), the heat transfer coefficient (HTC) is improved by 55.8% of those with elastic plates (the thickness of 0.01 mm and the length of 12 mm).Periodic expansion and recovery process of plates coupling bubble dynamics is observed and studied. Experimental results indicate that elastic plates enhance boiling heat transfer performance significantly by smaller bubble departure diameter and lower wall superheat at the same wall heat flux. A correlation for bubble departure diameter prediction in elastic channel is proposed which matches well with experimental results. It is found that the stronger pumping effect induced by higher swing rate of plates enhance HTC significantly. The mechanism where expansion process of plates promotes bubble departure while its subsequent recovery process benefits the supply of the cold liquid is verified by simulation. This study proposes a new approach for boiling heat transfer enhancement by inducing the energy exchange between bubbles and elastic plates, and provides theoretical and technical support for boiling study based on flexible materials.

Study of an asymmetric dual loop pulsating heat pipe: Visualization and parametric analysis

This study explores the two-phase flow visualization and thermal performance of an asymmetric dual-loop pulsating heat pipe (adPHP) and an identical conventional dual-loop pulsating heat pipe (cdPHP) using a borosilicate glass tube setup. The asymmetric PHP, features a repetitive pattern of larger–smaller diameters (2.5 mm and 1.8 mm), incorporating diametrical asymmetry within the adiabatic section, while the conventional PHP maintains a uniform ID throughout. The asymmetric design aims to optimize two-phase flow start-up and maintain a unidirectional fluid circulation. The experimental setup was subjected to gradual heat-loading conditions. Observations revealed diverse flow behaviors at various locations within the PHP, influenced by factors such as inclination angles, filling ratios, working fluid and heat inputs. The research has investigated the impact of the initial fluid distribution within the PHP on the system’s start-up behavior, subsequently influencing the system’s pressure dynamics. The findings of this study indicate that the bubble nucleation and vapor plug expansion in the evaporator are two key mechanisms responsible for initiating the start-up process. The velocity head of the working fluid affects the condenser pressure, particularly during the start-up phase of the PHP. Alternate asymmetry promotes the start-up at low wall super-heat, and initial circulation inside the PHP at low heat loads. Enhanced thermal performance is observed with water as the working fluid at a 60% filling ratio, while acetone exhibits greater sensitivity to inclination angles, achieving optimal performance at a 40% filling ratio, indicative of a relatively less stable operational behavior.

Numerical study of flow boiling heat transfer in a mini-channel under hyper-gravity

Gravity plays a crucial role in influencing bubble behavior and heat transfer in flow boiling, and its impact can vary significantly in aerospace settings. Therefore, this study aims to numerically investigate flow boiling heat transfer in a rectangular mini-channel under hyper-gravity conditions, specifically at 12 times the normal gravity. To accomplish this, a coupled volume-of-fluid and level set method is employed, taking into account fluid-solid conjugated heat transfer as well as a nucleus site density model derived from experimental data. By reproducing the flow pattern and heat transfer characteristics under different heat flux and flow rate conditions, the study unveils the effects of hyper-gravity on flow boiling heat transfer. When the flow rate is lower under hyper-gravity conditions, a notable phenomenon occurs wherein numerous bubbles detach from the heating wall and coalesce into a vapor film at the top of the mini-channel due to increased buoyancy. In contrast, under normal gravity, bubbles merge and slide on the heating wall, leading to the formation of a dry patch below. Consequently, hyper-gravity results in a lower wall superheat, and the disparity in average wall superheat between normal and high gravities escalates as the added heat flux rises. Notably, in the hyper-gravity environment, the frequent detachment of bubbles in the middle and downstream sections of the mini-channel leads to an initial increase in wall superheating, followed by a plateau along the flow direction. As the flow rate increases, the inertial force intensifies. However, intriguingly, the discrepancy in flow boiling heat transfer between normal and high gravities does not exhibit a monotonic decrease with the increasing flow rate. This behavior can be attributed to the pressing of more bubbles onto the heating wall under normal gravity, resulting in the formation of dry patches at high velocities.

Bubble behaviour and critical heat flux investigations on electrodeposited metallic tubes during pool boiling

There is colossal research scope in achieving greater functionality and durability in microelectronic devices as conventional air cooling systems cannot handle these devices' cooling requirements which may be due to their low heat transfer performance. In such instances, phase change heat transfer mechanisms are appropriate for heat extraction since they utilise both sensible and latent heat. The ability to remove huge amount of heat at low wall superheats and the lack of moving parts make pool boiling appealing. This experimental study emphasises the analysis of bubble behaviour and critical heat flux (CHF) on electrodeposited tubes developed via in-house built set-up. Copper has been deposited over SS 304 tubes of a length of 100 mm and diameter of 2.5–4.5 mm. Their surface morphologies have been investigated using various characterization techniques such as SEM, EDS, AFM, and XRD. The surface roughness measurement confirms that electrodeposition increases the roughness of the tube. The contact angle analysis confirms that the coated surfaces are hydrophobic in nature. The CHF visualization is carried out under uniform heat flux conditions. It is observed that with an increase in diameter, the magnitude of CHF reduces irrespective of the coated and uncoated surface. The hydrophobic surface exhibits a lesser CHF magnitude than the uncoated surface. The reduction in CHF magnitude is within a range of 4.27%–8.93%. At lower heat flux magnitude, the bubble nucleation is rigorous in coated tubes, making it efficient for phase change heat transfer applications.

Numerical Investigation on Pool Boiling Heat Transfer of Silica and Alumina Nanofluids

Surface modification through pool boiling of nanofluid is an efficient technique in delaying critical heat flux and attaining high heat flux at lower wall superheat temperature. The method of involving phase interaction, bubble evolving and dynamics phenomenon, and distribution of vapor void fraction altogether is the motivation behind current topic. The present work will contribute to the recovery and reuse of the wastewater and incinerator heat, cooling of high heat flux generating micro-electronics chips, and cooling of fast breeder test reactors. In the present study, the nucleate pool boiling of SiO2/water and Al2O3/water nanofluid are studied numerically and the effect of wall superheat temperature and heat flux on active nucleation site density, bubble departure diameter, and bubble waiting time are studied. The effect of various heater surface material and nanofluid concentration on wall superheat temperature and heat transfer coefficient are also investigated. A two-phase Eulerian approach, heat flux wall-partitioning model, and Rensselaer Polytechnic Institute boiling model are considered to simulate the current model. The pool boiling of nanofluids resulted in the formation of nano-porous layer on heater surface because of microlayer evaporation process that enhanced the heat transfer coefficient and reduced the wall superheat temperature.

Boiling heat transfer study of Micro-Diamond coated surface using underwater explosion shock wave method

Pool boiling heat transfer experiments are performed on micro-diamond particles coated heating surface (DPC heating surface) and no-coated smooth surface on copper substrates. Here DPC heating surface is fabricated with the aid of underwater explosion shock wave. Experiments are conducted on the substrate surface area of 15 × 15 mm2 using distilled water as the working fluid. The substrate is fixed tightly on the heater block, which is embedded with ceramic cartridge heater of 1500 W. The influence of pool boiling heat transfer is experimentally studied over a range of surface roughness values with particles coated heating surface at atmospheric pressure. The obtained results of DPC surfaces are compared with that of the smooth bare copper surface. A remarkable enhancement of the critical heat flux (CHF) at lower wall superheat is observed on the DPC surfaces. The DPC surface is found to be effective in increasing the nucleate boiling and CHF

Experimental study of heat transfer in a microchannel with pin fins and sintered coatings

ABSTRACT Increasing processing capacity without modifying the size of electronic devices has made thermal management important in the electronic industry. Commercialized thermal management, such as in a conventional air cooling system, is insufficient for electronic devices with high heat flux dissipation. Pool boiling is a better method for heat transfer because it can dissipate a substantial amount of heat at low wall superheats. This study focused on heat transfer enhancement using passive approaches, including nanostructures and microporous sintered surfaces over open microchannel surfaces and microchannels with pin-fins. In the present work, seven structures were studied in pool boiling, wherein experiments elucidated the effects of microchannels, sintered, and pin fins (micropillar) on boiling heat transfer from a copper chip in a pool of degassed water. Boiling performance is ascertained via critical heat flux (CHF) and heat transfer coefficient (HTC). The best heat transfer performance showed a heat flux of 243.75W/cm2 at 15.46°C on the pin-fins chip, which was 1.9 times the heat flux of the plain chip. The highest HTC was 181.03 kW/(m2 oC) at a heat flux of 172.61 W/cm2 for the microchannel with single pin-fins. The HTC enhancement was 2.8 times greater than the plain surface. It was found experimentally that HTC and CHF improved on all modified surfaces compared to the plain copper chip baseline.

Experimental investigation on the effects of swirling flow on flow boiling heat transfer and instability in a minichannel heat sink

Flow boiling in micro/mini-channel heat sinks provides an effective thermal management solution for high-heat-flux cooling applications. However, the plain micro/mini-channel heat sink has some shortcomings, such as severe boiling instability and insufficiency for more stringent heat dissipation requirements. To overcome these shortcomings, this work designs a minichannel heat sink (MCHS) with twisted tape inducing swirling flow. The cross-section of minichannel is square, as well as the width, height and length of minichannel are 2 mm, 2 mm and 150 mm, respectively. The twisted tape length (Ltt = 50, 100 and 150 mm) and twist ratio (R = 3, 4 and 5) are selected as designed parameters to generate different swirling flow intensities. The flow boiling heat transfer and instability in such MCHS are experimentally examined under two mass fluxes (120.7 and 210.5 kg/(m2·s)), two inlet temperatures (70 and 80°C) and a range of heat flux (42.3–263.0 kW/m2). Results show that the swirling flow can mitigate the rapid growth for bubbles and suppress the coalescence of small bubbles effectively, as well as cause the bubbles to generate helical motion behavior. Especially, the micro-corners between the twisted tape and minichannel wall provide more effective nucleation sites to produce more small bubbles. Thus, the swirling flow induced by twisted tape can not only trigger onset of nucleate boiling (ONB) at low wall superheat and improve heat transfer performance, but also suppress the flow boiling instability. Smaller twist ratio or longer length of twisted tape results in stronger swirling flow, thus bringing greater heat transfer enhancement effect. Also, longer twisted tape presents better stable flow boiling, but the twist ratio has an insignificant effect on boiling instability. Compared to plain MCHS, the MCHS having twisted tape with Ltt = 150 mm and R = 3 achieves a 34.4% decrease in wall superheat of ONB and a maximum of 80.9% increase in heat transfer coefficient. Furthermore, this MCHS reduces the relative standard deviations of inlet pressure by 44.4% at heat flux of 100.5 kW/m2 and mass flux of 120.7 kg/(m2·s). These findings suggest that using twisted tape insert to induce swirling flow can improve overall performance for minichannel heat sinks effectively.

Modelling of pool boiling on the structured surfaces using Lattice Boltzmann method

The process of boiling on spatially structured surfaces is simulated by a hybrid model based on Lattice Boltzmann Method and heat transfer equation. The model permits to study heat transfer at boiling in a wide range of surface superheats for different surface structural characteristics. The regimes of natural convection, nucleate boiling, and transition to film boiling are studied. The boiling curves for the surfaces with different structural and wetting properties are obtained. It was shown that the onset of nucleate boiling occurs at lower wall superheat on the structured surfaces than that on the smooth surface. However, at high wall superheats the heat flux and the critical heat flux at modified surfaces are lower. It was obtained that special modification of both structural and wetting properties of heat exchange surface permits to obtain higher removal heat flux as well as higher critical heat flux.

Lattice Boltzmann study of bubble dynamic behaviors and heat transfer performance during flow boiling in a serpentine microchannel

• Mesoscale study of microchannel flow boiling in U bend was conducted. • Detailed bubble dynamics and heat transfer characteristics were presented. • Effect of buoyancy on flow boiling process depends on flow orientation. • The onset of nucleate boiling is delayed as the Re number increases. Understanding the flow boiling process in a serpentine microchannel with U-bends is very important to its design and application in practice. In this study, a hybrid thermal multiphase model consisting of the pseudopotential multiphase lattice Boltzmann model and the finite difference method is employed to investigate the flow boiling heat transfer in a serpentine microchannel. Effects of curvature ratio, flow orientation, heat flux and Reynolds ( Re ) number on the bubble dynamic behaviors and heat transfer performance during flow boiling process are comprehensively evaluated. Bubble behaviors including bubble nucleation, growth, coalescence, departure, recontact, and migration are well captured, and typical flow patterns of bubbly flow and intermittent slug flow are identified. The simulation results show that increasing curvature ratio does not affect the heat transfer performance much, but generates elongated bubbles at the U-bend. Depending on the flow orientation, the buoyancy induced by gravity acceleration has both favorable and unfavorable effects on the bubble dynamic behaviors and local heat transfer characteristics in the serpentine microchannel. In addition, the vapor volume fraction is calculated under different Re numbers and heat fluxes. The lower vapor volume fraction exhibits lower wall superheat and better heat transfer performance.

Hook-shaped structures to improve pool boiling heat transfer

Boiling performance while using raised hook-shaped metal features, manufactured from copper by a skiving process, was investigated and compared with the boiling performance of a bare copper surface. Deionized water at the saturated conditions at 1 atm was used as the working medium. These fins can potentially improve the performance of immersion cooling spreaders in power electronics applications. The advantages of this technology compared with other methods presented in the literature include its scalability, low cost, resistance to erosion and clogging, and the industrialization of the manufacturing method. Four GRIPMetal hooked surfaces were investigated, each with different profiles (i.e., metal features, heights, and spacing) induced by the manufacturing process. The results show that the surface with hooks had lower wall superheat at the onset of nucleate boiling (ONB) compared with the bare surface. This appears to be due to the surface cavities—i.e., the negative potions of the raised hooks created by the skiving process, which helped initiate bubbles earlier. Moreover, the heat transfer coefficient (HTC) was superior for all surfaces with hooks at low heat fluxes because of increased surface area and enhanced bubble dynamics offered by the arrangement of the hooks. The hooked surface achieved a maximum HTC of 8.9 W/cm2K, which represents a 96 % enhancement ratio over the bare surface. The surface area increase (At/Abare) offered by the proposed hooks and their corresponding grooves was measured using the photogrammetry technique and was found to range from 1.35 to 1.8. Lastly, the critical heat flux (CHF) was significantly enhanced, reaching a 67 % increase over the bare surface; this is attributed to the available bare inter-fin area that can supply the liquid to the nucleation sites and the assisted suction flow through the front of the grooves.

Depletable micro-layer for nucleate boiling simulations in micro-gravity conditions: A new approach

Nucleate boiling constitutes high heat flux with considerably low wall-superheat. During nucleate boiling, the evaporation of a thin liquid layer trapped between the vapor bubble and the wall is a significant contributor to the bubble growth rate and heat-flux. Direct simulation of the thin liquid layer is challenging because its thickness is only a few micrometers. In this paper we proposed a new approach to simulate the liquid micro layer. In this approach we do not directly model the thin liquid layer but instead we calculate its contribution to vapor bubble growth by estimating the micro layer thickness at each time step and then integrating the temperature gradient across the thin triangular region to calculate the heat flux and the generated vapor volume. In addition, our approach models the thin layer depletion with an added evaporation term in the equation. The added term is able to model the reduction in the thickness of the microlayer. Results for nucleate boiling simulations under microgravity conditions are reported using the proposed micro layer approach in comparison with experiments performed on the International Space Station using perfluoro-n-hexane as the test liquid. Results for bubble growth rate, bubble shape and heat-flux are in good agreement with experiments and are verified with two different time-instants in the bubble life cycle.

Influence of surface roughness and Wettability of novel surface on nucleate boiling performance in deionised water at atmospheric pressure

Pool boiling is one of the very suitable techniques for an efficient thermal management system dealing with two-phases. The present work deals with the experimental exploration of critical heat flux for safety concern and heat transfer coefficient related to the performance point of view in nucleate boiling regime of pool boiling system. The copper substrate was coated with porous copper nanoparticles by sputtering technique to the thicknesses of 250 nm, 500 nm, and 750 nm. The surface characteristics of the copper nanocoated surfaces have been analysed as a result of wettability, surface roughness, and micro-structure. The contact angle goniometer, stylus profilometer, XRD, and SEM have been employed to analyze the surface structure. The maximum augmentation of critical heat flux was 59% for the thickness of 750 nm as compared to plain copper substrate. A 99% increase in the heat transfer coefficient was achieved for 750 nm thickness surface in comparison with the plain copper surface. The tremendous augmentation in critical heat flux and heat transfer coefficient was achieved due to wetting and rewetting properties of the deionized water on the copper nanocoated surfaces. The capillary action on the copper nanostructure improves the fluid supply to the test surface and removes the heat at low wall superheat than the plain copper surface. The average roughness of the copper nanocoated surface augments the heat transfer area which tends to enhance the performance factor significantly.

Microflow-Enhanced Bubble Dynamics Along With Gradient Porous Surfaces

Abstract Boiling heat transfer has been a popular topic for decades because of its ability to remove a significant amount of thermal energy while maintaining a low wall superheat during the liquid phase change. Such boiling mechanisms can be tailored by engineering new boiling substrates through surface wettability modification and/or microscale feature installation. Here, we create new types of heterogeneous boiling surfaces that integrate vertical gradient micropores on macroscale fins by using a template-free electrodeposition method. The gradient morphology and corresponding gradient wettability simultaneously enable bubble nucleation on the top pores and capillary wicking through the bottom pores. With these unique wetting characteristics, we find that the gradient pores installed at the trench bottom demonstrate the most significant boiling enhancement in critical heat flux and heat transfer coefficients by 160% and 600%, respectively. This enhancement can be attributed to the microflow-enhanced nature of bubble departures around the fins while isolating bubble nucleation and liquid supply through gradient pores. These results provide fundamental insights into boiling mechanisms using porous media and the potential for future works that can optimize the design of multidimensional heterogeneous surfaces to engineer flow patterns and boiling mechanisms accordingly.

Development of novel wet sublimation cascade refrigeration system with binary mixtures of R744/R32 and R744/R290

The use of carbon dioxide (R744) in refrigeration systems is usually limited by its triple point temperature of −56.5 °C; an objective of this study was to extend the range of this natural refrigerant to include low-temperature applications. A novel wet sublimation cascade refrigeration system was developed using low-GWP mixtures of R744 with hydrocarbons (HCs) and hydrofluorocarbons (HFCs) as low-temperature refrigerants. With propane (HC-290) and difluoromethane (HFC-32) serving as solvents for solid carbon dioxide, temperatures as low as −72 °C were obtained in a closed cycle system with mixtures containing 67% CO2 by mass. Visualisation experiments were performed to illustrate the crystallisation of a supersaturated carbon dioxide solution. The excess solute was visible as clouding or crystals, depending on the saturation level of the solution. High heat transfer rates, up to 3465 W m–2K−1, were obtained in the sublimator/evaporator section, resulting in low wall superheat values, indicating potential industrial applicability of the carbon dioxide-based slurry-like fluid. The pressure–mole fraction and temperature–mole fraction diagrams were constructed to study the system parameters working with binary mixtures. Moreover, vapour pressures of the binary mixtures of R744/R290 and R744/R32 at the temperature of –72.5 °C were experimentally obtained.

Experimental investigation of saturated nucleate pool boiling heat transfer characteristics of R245fa on a copper foam covered surface

This paper presents an experimental investigation of saturated nucleate pool boiling heat transfer characteristics of copper foam covered surfaces and plain copper surfaces. Six copper foam covered surfaces with different pore sizes and thicknesses and two plain copper surfaces with different roughnesses were used as the boiling test surfaces, all oriented horizontally. The refrigerant R245fa, which has potential to be used as an electronic coolant, was selected as the test fluid. The saturation temperature for the boiling experiments was about 18.5°C ±1°C. The results showed that the copper foam coating on the plain copper surface effectively increased the boiling heat transfer coefficient by up to 381.7% and the CHF by 196.7 %, while the increased roughness of the plain copper surfaces only slightly improved the heat transfer coefficient. The copper foam covered surface triggered nucleation at lower wall superheats with the ONB (Onset of nucleate boiling) point as low as about 2°C for R245fa on the 0.5 mm thick copper foam covered surface with a pore size of 400 PPI. The increased copper foam layer thicknesses did not further improve the pool boiling heat transfer performance, which closely depended on the relationship between the size of the copper foam pores and the active nucleation sites.

Numerical simulation of bubble dynamics and heat transfer in the 2D saturated pool boiling from a circular surface

In this paper, the saturated pool boiling heat transfer from a heated circular surface is investigated numerically based on the popular lattice Boltzmann phase-change method. The crucial interfacial phenomena, including the bubble growth, departure, coalescence, and rising under different wall superheats, wettabilities, and heater sizes are studied, and the corresponding heat transfer characteristics within these bubble dynamics are also revealed. The numerical experiments indicate that at low wall superheats the bubble nucleation occurs only at the top of the heated circle. With the increase in the wall superheat, the sides and the bottom of the heated circle also become nucleate sites. As the wall superheats continue to increase, more and more nucleate sites are activated so that the heated circle is wrapped by the vapor film early in the boiling process. And the average heat flux varies periodically with time corresponding to bubble dynamics in nucleation boiling regime. It reaches a steady state of film boiling regime after the stable vapor film is generated from the heated circular. It is also found that hydrophobic surface is conducive to the onset of boiling. However, it leads to a low critical heat flux and incurs film boiling at a lower wall superheat, which is also observed in both experimental and numerical studies. On the other hand, the boiling regimes can undergo the transition from nucleate boiling regime to film boiling regime at the same wall superheat with the increase in the heater size. In addition, under various wall superheats and wettabilities, the bubble departure diameters from the circular surface are smaller than those from the flat surface, while the bubble departure periods from the circular surface are less than those from the flat surface one. Finally, the saturated boiling curves from the circular surface for different wall wettabilities, heater sizes, liquid–vapor density ratios, and heating modes are also achieved.

Experimental study of post-CHF heat transfer in a vertical tubular test section

In this paper, post-CHF (critical heat flux) experiments are performed in a vertical tubular test section with water as the working fluid to study the heat transfer characteristics during the process of formation, growth, and propagation of dry patches. The formation, growth, and propagation of the dry patches are indicated from the measured wall temperatures of the test section. The so-called dry temperature and quench temperature, corresponding to the formation and departure of dry patches, are obtained by analyzing the first-order derivative of the measured wall temperatures. The temperature gradient along the axial direction of the test section wall is also calculated, up to 15 °C/mm. Large temperature gradients result in axial heat conduction as well as thermal stresses in the test section. The convective heat transfer coefficient for the dry patches is calculated based on the saturation temperature of the fluid and compared with three heat transfer correlations in the literature. The computed convective heat transfer coefficient for the dry patches in our experiments is larger than the calculated results of the correlations, especially for conditions of relatively low wall superheats.

Capillary hydrodynamics and heat transfer in two-phase gas-liquid microscale systems

This paper aims at discussing the hydrodynamics and heat transfer in gas-liquid microscale systems. A new approach for the determination of the flow pattern, based on measuring the statistical characteristics of gas-liquid flow is proposed that can be used for flow pattern map design. The characteristics of heat transfer during upflow boiling in microchannel heat exchangers that operate at low wall superheat are discussed. A novel method for prediction of the flow boiling heat transfer which accounts the heat transfer enhancement due to formation of the thin curved liquid film and surface waves is presented. The characteristics of heat transfer during subcooled flow boiling in microchannel heat sink designed for high heat flux removal are discussed and method for heat transfer prediction is proposed.