High Wall Superheat Research Articles

Secondary size distributions for single drop impacts at high wall superheat

The impingement of liquid sprays on hot walls is used extensively in both spray-cooling systems and in combustor fuel injection applications. At low and moderate wall temperatures, the secondary size distributions have been reported in the literature. For high wall superheat conditions, particularly for real multicomponent fuels, this secondary size distribution has received less attention. Understanding the resultant size distribution for a spray-wall impact is key to capturing vaporization and local mixture for fuel-spray impingement. In this study, single drop impacts for a range of single-component (n-decane) and multicomponent jet fuel (F-24) are characterized through dual-view imaging. Secondary droplets are captured for impact Weber numbers of 100–600 and wall temperatures spanning the nucleate and film boiling (Leidenfrost) regimes. Imaging through a transparent sapphire substrate is used to capture the impact phenomena and impact-induced breakup of impacting drops. We report empirical correlations for the secondary droplet size for single-component (n-decane) and multicomponent (F-24) liquid fuels with varying wall temperature to provide validation datasets for spray-wall simulations.

Unveiling the underlying physics of two-phase boiling heat transfer enhancement through shearing of coalescing bubbles

The upcoming energy scarcity problem has driven research toward developing energy-efficient two-phase heat exchangers essential for various cooling applications. This research is rooted in the principles of pool boiling, essential for effective heat transfer in various heat exchangers. A well-known reported problem in heat exchangers is the dry-out phenomena of heated surfaces due to bubble coalescence. To tackle this undesirable problem, an innovative technique has been introduced in this study, which involves the shearing of bubbles through liquid jet impingement over the heated surface. The study has been carried out in a two-dimensional domain numerically, in the wall superheat range of 9–16 K. To study the underlying physics involved in this pool boiling phenomenon, the bubble dynamics parameters such as departure frequency, bubble diameter, cold spot (bubble base) temperature, and vapor volume fraction have been analyzed. The results show that with the jet shearing effect, a maximum enhancement of 25% in heat transfer rate is observed at higher wall superheat. The investigation also highlights that the liquid jet enhances vapor volume fraction, indicating enhanced steam generation, particularly an enhancement of 27% observed at elevated wall superheat. An early onset necking effect is also observed with the shearing effect, which leads to the formation of smaller bubbles with higher departure frequencies. This study is a benchmark to the fundamental physics of enhancing two-phase heat transfer through bubble shearing, offering promising insights for energy conservation in two-phase heat exchanger design, particularly within the context of pool boiling.

Numerical study on 3D nucleate boiling over an oscillating base plate with high wall superheat and multiple nucleation sites

This paper is on 3D numerical study on nucleate boiling at high wall superheat of 13.5 K, over an oscillating base plate, using a sharp interface dual-grid level set method (SI-DGLSM)-based in-house code. These studies are performed for two nucleation sites, with two configurations of the computational domain. A micro-layer evaporation model has been used to enable 3D simulations on a coarser grid and higher wall superheats. The size of the computational domain is chosen based on the nucleation site density, reported in experiments, and the present numerical results compare well with the published experimental results; for a stationary horizontal plate under atmospheric pressure. For nucleate boiling on an oscillating plate, bubbles departing from both the nucleation sites synchronize and lock-on with the plate oscillation. Enhancement in heat transfer coefficient is observed in presence of base plate oscillations, which is attributed to increase in rate of phase change as compared to increase in rate of convective heat transfer. Non-uniform distribution of nucleation sites on the base plate leads to formation of local convergence zones on the plate, which in turn affects the motion of bubbles as they rise within the computational domain. The present work is significant as it presents 3D simulations for nucleate boiling under the effect of base plate oscillations—as a promising technique for heat transfer enhancement.

Heat transfer enhancement of different ribbing pitched semi-closed microstructure tube bundles fabricated by deformational cutting method

The study investigates the limitations posed by low-value heat transfer coefficients and higher wall superheat in conventional tube bundles, which adversely impact the performance of heat exchangers. To address this challenge, semi-closed microstructure tubes, created via deformational cutting at ribbing pitches of 300 µm and 400 µm, are proposed as a potential solution. The research centers on analyzing the two-phase boiling heat transfer performance of these microstructure tubes arranged in 2 × 3 staggered tube bundles. A comparative analysis is conducted, evaluating the thermal effectiveness of the microstructure tube bundles against a 2 × 3 smooth tube bundle and a copper-coated tube bundle under varying mass flux, heat flux, and pitch-to-diameter ratios. The experimental findings indicate the superior heat transfer performance of both microstructure tube bundles, with reduced wall superheat compared to other tube bundles. The findings further indicate that the 2 × 3 semi-closed microstructure tube bundle with a higher ribbing pitch outperforms the microstructure with a lower ribbing pitch. The 2 × 3 high-pitched microstructure tube bundle exhibits a maximum enhancement of 420 % over the 5 × 3 smooth tube bundle and 340 % over the 2 × 3 smooth tube bundle. Moreover, it surpasses the performance of copper-coated tube bundles by 230 % and outperforms its lower-pitch counterpart by 70 %. Particularly, the use of microstructure tube bundles allows for a reduction in the number of tubes required in the heat exchanger configuration without compromising its performance, promoting compactness and cost-effectiveness. Overall, the study highlights the feasibility of implementing these tubes in the two-phase heat exchanger owing to their superior performance compared to conventional tubes.

Electric field inspired alternating surface wettability for enhancing pool boiling heat transfer performance: A lattice Boltzmann method study

Electric field and surfactants have been proven to potentially alter surface wettability on demand. In this work, alternating wettability inspired by an electric field has been used to enhance boiling heat transfer performance in terms of critical heat flux (CHF) over a rough surface by using two-dimensional lattice Boltzmann simulations. In alternating wettability, the surface wettability changes with time between hydrophobic and hydrophilic. The effects of the parameters, including alternating wettability time-period and wettability contrast on the boiling process have been revealed. The simulation results show that for the alternating wettability: when surface is hydrophobic bubble expansion is faster, and then wettability transition to hydrophilic quickly retracts three-phase contact line leading to faster bubble shrinking as compared to constantly hydrophobic and constantly hydrophilic surfaces. Furthermore, the rapid shrinking of the bubble tends to disintegrate vapor film on the surface at high wall superheats, resulting in a 53% enhancement in CHF compared to a constantly hydrophilic surface. The bubble departure takes much longer time for too small/large time-periods of alternating wettability. The CHF increases with time-period, becomes maximum at a certain value and then reduces. The CHF also increases with wettability contrast, becomes largest for hydrophilic and hydrophobic contact angles 55o and 107o, respectively, and decreases with further increase in wettability contrast due to high potential for vapor film formation.

Characterizing effect of particle size on flow boiling in sintered porous-microchannels

Flow boiling in porous microchannels draws extensive attention recently owing to its great potential in high heat flux applications. Subcooled flow boiling of deionized and degassed water was carried out to characterize transport performance. Porous microchannels were sintered from six-sized dendritic-like copper particles: 10, 30, 50, 90, 120 and 150 μm with the same layer thickness of 400 μm. The dimensions of 23 parallel porous microchannels are 600 μm × 1200 μm × 2800 μm (width × depth × length). The inlet subcooling degree and mass flux was maintained at 40 K and 142 kg/m2·s, respectively. Experimental results show that the particle size has great effects on the heat transfer coefficient (HTC) and Critical heat flux (CHF) of flow boiling in porous microchannels. Medium particle size (90 μm or 120 μm) remains higher performance in HTC and CHF. In heat fluxes between 60∼160 W/cm2, average HTCs of both PM-90 and PM-120 could reach almost 120 kW/(m2·K) or so. The visualization study reveals that the underlying heat transfer mechanism in porous microchannels is dominated by the nucleate boiling in low heat flux and then, by the thin film evaporation from moderate to high heat flux. Moreover, two boiling crisis phenomena have been observed: (a) in microchannels sintered from optimum size particles, the rewetting-dryout cycle has been well established and sustained without being interrupted by occasionally explosive boiling events, indicating capillary limits; (b) in non-optimum samples, high wall superheat in the rear part usually leads to explosive boiling persistently, which interrupts periodic oscillation modes with heat flux exceeding to moderate level and results in CHF conditions. An optimal ratio of bottom wall thickness and particle diameter, δ/d, is found in the range of 3∼5 for the flow boiling in sintered porous microchannels.

Performance enhancement of intermittent spray cooling with a self-rewetting fluid for relatively high-temperature applications

Comparing to continuous spray, intermittent spray is able to offer effective cooling by consuming less liquid. Targeting at relatively high-temperature applications, this study explores the influences of pulse duration and duty cycle (DC) in the cooling performance of the intermittent spray of a self-rewetting fluid, the 1.5% wt.% aqueous solution of 1-pentanol. Unlike water, using the self-rewetting fluid in intermittent spray is able to induce the inverse Marangoni convection which helps to replenish the hot spot with cool liquid even though the surface temperature exceeds 145°C. This leads to a further reduction in fluid consumption because comparable cooling rate can be delivered by spraying the self-rewetting fluid at a much lower frequency. Although dryout can still occur in the low DC regime, the inverse Marangoni convection is proven to sustain effective cooling during spray-off period in the high DC regime. As a result, cooling rate is nearly independent of DC in the high DC regime. For the self-rewetting fluid, cooling performance is not benefited from increasing the pulse duration as excess fluid simply drains out without contributing much to heat removal. Hence, a pulse duration of 1.5 s with a DC of 15% is recommended for the intermittent spray cooling of 1-pentanol/water mixtures at high wall superheats. Since the spray cooling efficiency of the self-rewetting fluid is always higher than that of water in this study, it opens up new possibility in spray quenching with more rapid cooling, better thermal uniformity, and further reduction in liquid consumption.

Nucleate and Film Boiling Performance of Ethanol-Based Nanofluids on Horizontal Flat Plate: An Experimental Investigation

In this paper, the heat transfer characteristics of nanofluid nucleate and film boiling are studied experimentally. For this purpose, Al2O3 and SiO2 ethanol-based nanofluids prepared with three volumetric concentrations of 0.1 %, 0.3 %, and 0.5 %. The boiling experiments were conducted on a circular and polished copper surface with a diameter of 25 mm. The results showed that the addition of nanoparticles to the base fluid reduced the heat transfer coefficient of nucleate boiling. The critical heat flux of ethanol-based nanofluids was significantly higher than that of pure ethanol. The Al2O3 ethanol-based nanofluid with a volumetric concentration of 0.5 % had the best performance, with a critical heat flux of 42.36 % higher than that of pure ethanol. The presence of nanoparticles in the ethanol-based nanofluid improved the heat transfer coefficient of film boiling. The results showed that the stable film boiling for nanofluids starts at higher wall superheat temperature difference than pure ethanol. Among the investigated concentrations, volumetric concentration of 0.5 % had the best performance for both nanofluids, so that the minimum heat flux of Al2O3 and SiO2 ethanol-based nanofluids were increased by 45.96 % and 45.67 % compared to pure ethanol, respectively.

Lattice Boltzmann study of nucleation site interaction and nucleate boiling heat transfer on a hybrid surface with multiple cavity-pillar structures

In this work, nucleation site interaction and nucleate boiling heat transfer are studied on a hybrid rough surface, a surface with multiple cavities and a pillar on sides of each cavity, using a two-dimensional pseudopotential phase-change lattice Boltzmann method (LBM). To elucidate the effects of incorporation of pillars, the results of the hybrid rough surface are compared with a simple rough surface (a surface with multiple cavities). First, bubble nucleation, growth and departure processes are investigated above the simple and hybrid rough surfaces with three cavities and bubble interactions and their effects on cavity activation/deactivation at low superheat are discussed. The results showed a complete suppression of the central cavity at small pitch distance and left/right cavities at large pitch distance, which are caused by combined suppression effect of adjacent cavities and edge effects of the heater, respectively, for the simple rough surface. But, with the incorporation of pillars the heat transfer area is increased and the effects of induced flow are reduced, resulting in the activation of suppressed cavities, and thus, the higher heat flux is observed for the hybrid rough surface than the simple rough surface. The cavity suppression is also observed for unequal cavity depths or widths. The complete suppression of the central cavity occurs, when the central cavity is narrow or left/right cavities are deep. On the other hand, the highest heat flux is observed for the hybrid rough surface with central deep cavity and left/right shallow cavities due to the activation of all nucleation sites and additional heat transfer from the central deep cavity. Increasing pillar height or decreasing cavity-pillar spacing results in the stability and growth of residual bubbles to the next cycle without waiting periods, which increase the heat flux. Next, heat transfer in the nucleate boiling region is investigated by comparing saturated boiling curves obtained through simulations. The heat flux in the nucleate boiling including the critical heat flux (CHF) is higher for the hybrid rough surface than the simple rough surface. The heat flux also found to be increased in developing region of nucleate boiling, when pillar height is increased. The CHF increases with pillar height up to a certain height. But, with tall pillars at high wall superheats, the excessive nucleation of bubbles can also occur in the inter-pillar spacings, in addition to cavities, increasing the susceptibility of merger of closely spaced bubbles, which reduce the CHF.

Enhanced Pool Boiling Performance of Microchannel Patterned Surface with Extremely Low Wall Superheat through Capillary Feeding of Liquid

ABSTRACT The pool boiling performance plays a key role in the development of high heat flux dissipating applications. The high critical heat flux and low wall superheat are two of the critical factors that affect the long-term life of devices. In this paper, enhanced pool boiling performance can be achieved by well-designed microchannels in copper surfaces using a precision diamond dicing method. The microchannel patterned surface with the channel length of 0.4 mm obtains a critical heat flux of 169.8 W/cm2, which has a 193% enhancement compared to the plain surface. Besides, the extremely low wall superheat of 3 K has been achieved, and thus the heat transfer coefficient reaches 51.8 W/cm2·K, about 738% larger than that of the plain surface. Herein, the microcavity has increased the nucleation site, the surface can promote the bubbles escape, and then the channel can continuously supply the liquid. Hence, the extremely low wall superheat at high heat flux occurs due to the rapid bubble departure and enhanced capillary feeding of liquid replenishment to active nucleation sites on the surface. The above results provide an effective way for the realization of high-performance two-phase microchannel patterned heat sinks via optimizing the microstructure geometry.

Heat transfer during drop impingement onto a hot wall: The influence of wall superheat, impact velocity, and drop diameter

The present work addresses the influence of the wall superheat, drop impact velocity, and impact diameter on hydrodynamics, heat transport, and evaporation during drop impingement onto a heated solid wall in a pure vapor atmosphere. A generic experimental setup has been designed and built with a temperature-controlled cell that allows investigation of drop impingement in a pure vapor atmosphere. Therein a single drop is generated and falls onto a heated surface due to gravity. The experiments are conducted with refrigerant FC−72. The heated surface is formed by a thin metallic layer coated onto an infrared transparent glass, so that the temperature field of the solid-fluid interface can be observed from below with an infrared camera at high spatial and temporal resolution. The heat flux field is derived from the temperature field using a dedicated post-processing procedure. The dynamic evolution of contact line radius is derived using image analysis. The drop shape is observed with a high speed camera, which is synchronized with the infrared camera. Experimental and numerical results for contact line radius and heat flow evolution are compared with each other. This gives an insight to the governing heat transport mechanism during different phases of drop impingement. Experimental and numerical parameter studies reveal that higher wall superheats, higher impact velocities, or larger drop diameters each result in increasing heat flow after the impact. The maximum spreading radius after impingement is increasing with rising impact velocity or impact diameter, and decreasing with rising wall superheat.

Experimental Study on Flow Boiling Heat Transfer Characteristics of Ammonia in Microchannels

To solve the heat dissipation problem of high heat flux components on spacecraft, experiments were carried out to investigate the flow boiling heat transfer characteristics of ammonia in a microchannel heat sink. Verified by gravity-independent criteria, the experimental results can be used to predict the performance of flow boiling heat transfer in microgravity environment. The heat sink consisted of 37 V-shaped microchannels with hydraulic diameters of 280 μm and channel lengths of 45 mm. Saturated flow boiling experiments were conducted with heat fluxes of 60.2~134.3 W/cm2, mass fluxes of 165~883 kg/m2s and saturation temperatures of 25 and 35 °C, as well as subcooled flow boiling experiments with inlet subcooling of 5 °C as a comparison. According to the experimental results, the following conclusions can be drawn. (1) As the mass flow rate increases, higher wall superheat is needed to trigger nucleate boiling. (2) In a lower mass flux range, the heat transfer coefficient changes drastically with mass flux, showing convective boiling characteristics, while the value varies slightly in a higher mass flux range, indicating that the nucleate boiling mechanism is dominant. (3) Under the experimental conditions in this work, the average heat transfer coefficient of ammonia reaches its peak when the outlet vapor quality is between 0.21 and 0.31. (4) A subcooled inlet condition has a suppression effect on the average heat transfer capacity of the working fluid at high mass flux, and increasing the saturation temperature reduces the average heat transfer coefficient. In addition, a comparison between the experimental results and existing correlations demonstrated that the correlation proposed by Fang predicted the experimental results well, and then a modified Fang correlation was proposed based on the experimental data obtained in this study. The results could provide references for the prediction of flow boiling heat transfer performance in aerospace environments and the design of heat dissipation systems.

On the transition between contact line evaporation and microlayer evaporation during the dewetting of a superheated wall

We present experimental results on the transition between contact line evaporation and microlayer evaporation during the dewetting of a superheated wall. The length and thickness of the liquid microlayer is measured, both showing a dependency on the wall superheat, dewetting velocity, and heating power.This work falls within the continuing efforts of understanding the formation of the so called microlayer: a thin liquid film, that is observed for example during boiling underneath a growing vapor bubble. The short lived and small scale nature of the phenomenon make accurate measurements difficult, therefore a novel experimental set-up is proposed that allows high resolution measurements in a steady-state manner. The time resolved formation of the microlayer is described in detail and the influence of the dewetting velocity, wall superheat, and heating power is discussed. Microlayer formation is observed only if a critical velocity is exceeded for a given combination of wall superheat and heating power. Below this threshold velocity, contact line evaporation dominates the heat transfer. The dependency of the microlayer length and thickness on the dewetting velocity is described using power laws. Instability effects near the contact line are observed at high dewetting velocities and high wall superheat. It is concluded that microlayer evaporation and contact line evaporation exist in separate regimes, obviously separated by a parameter combination of dewetting velocity, wall superheat, and heating power.

3D simulations of pool boiling above smooth horizontal heated surfaces by a phase-change lattice Boltzmann method

Pool boiling heat transfer from smooth horizontal hydrophilic and hydrophobic heaters having a finite thickness under constant bottom wall temperature conditions is simulated numerically with a three-dimensional liquid-vapor phase-change lattice Boltzmann method. For a small heater, single bubble dynamics and temporal variations of heater’s top wall temperature in contact with the bubble are investigated. It is found that top wall temperature drops during initial bubble growth period at low wall superheats on a hydrophilic heater due to the microlayer evaporation process. Because of a residual bubble is left on the top of the heater after bubble departure, no wall temperature drop is found on a hydrophilic surface at high wall superheats nor on a hydrophobic surface at any wall superheats. 3D pool boiling curves in dimensionless forms for smooth horizontal heaters of infinite extent from nucleation to critical heat flux through transition boiling to stable film boiling are presented. It is found that 2D and 3D simulated heat fluxes are identical in nucleate boiling and stable film boiling regimes, although transition boiling heat fluxes differ greatly. It is also found that the 3D simulated CHF is in better agreement with existing theories and correlation equations than 2D simulated CHF. Detailed 3D multiple bubbles dynamics above hydrophobic and hydrophilic surfaces and temporal variations of dry spots distribution on these surfaces at CHF are presented. Effects of wettability and thickness of the heater on boiling curves for smooth heaters are investigated. It is founded that only wettability play predominate roles in nucleate boiling regime from smooth superheated surfaces while all factors influence the transition boiling regime (including maximum and minimum heat fluxes) greatly.

Enhanced pool boiling heat transfer during quenching of water on superhydrophilic porous surfaces: Effects of the surface wickability

Porous surfaces were prepared by chemical etching with hydrofluoric acid on stainless steel spheres. The etched surfaces were shown to be superhydrophilic with near zero static contact angles as well as wickable by taking advantage of the capillary microstructures. Using the capillary tube method, the surface wickability of the etched samples was characterized by the transient absorbed liquid volume and the wicked volume flux. The wickability was able to be varied by controlling the etching time. Quenching experiments were then performed on the unmodified and etched samples in saturated water at atmospheric pressure. It was found that the quenching process is significantly accelerated in the presence of the etched wickable surfaces, because of the enhancement of boiling heat transfer, especially within the transition boiling regime, and improvement in both the critical heat flux (CHF) and Leidenfrost point as well. Even at a relatively high wall superheat, the vapor film is highly destabilized due to the localized evaporation of the wicked liquid in the capillary structures on the etched surfaces, in addition to the effects of increased surface roughness and wettability. It was also shown that the extent of quenching acceleration is closely related to the surface wickability. In the present work, the most wickable surface leads to the fastest quenching with the mostly improved boiling heat transfer. Based on the classical Kandlikar’s model, a linear correlation was proposed between the increase ratio of the CHF and the square of the wicking number that quantifies the surface wickability.

Heterogeneous bubble nucleation model on heated surface based on free energy analysis

Heterogeneous bubble nucleation on heated surfaces, as the first stage of boiling, is a fundamental and prevalent process in industrial systems. A nucleation model based on chemical potential energy is revisited and a Gibbs free energy based model is developed. The nucleation models cover two types of bubble nucleation, i.e. nucleation with or without prefer nucleation site on heated surfaces which are denoted as type I bubble nucleation and type II bubble nucleation, respectively. The effect of surface wettability is also considered. Analytical solutions such as active nucleation size boundary, critical nucleation radius and heat flux at onset of bubble nucleation are obtained without the assumption that the temperature at the tip or center of nucleation bubbles is higher than saturated temperature which was commonly applied in previous studies. The high wall superheat usually found for type II bubble nucleation is illustrated. Bubble nucleation under flow condition is studied by analyzing the effects of heat transfer coefficient, fluid subcooling and pressure on bubble nucleation. For surfaces that cannot provide relative large initial bubbles, bubble nucleation transits from type I to type II with the increase of heat transfer coefficient, fluid subcooling or pressure due to the decrease in the critical bubble radius. Comparisons with previous nucleation models and experiments indicate that the Gibbs free energy analysis is a very promising method in the prediction of bubble nucleation.

Confined jet impingement boiling in a chamber with staggered pillars

The jet impingement boiling heat transfer is investigated experimentally and numerically in a cylindrical chamber with the dense circular pillars, which is a simplified model of the shell-side chamber of the phase-change heat exchanger in a thermoacoustic Stirling engine. The saturated or subcooled liquid water is impinged onto the horizontal pillars from the bottom inlet of the boiling chamber, whereas the two-phase flow is exhausted through the outlet on the top of the chamber. Aside from the measurements of the wall heat flux and wall temperature, the boiling induced bubbly flow patterns are optically observed with a high-speed camera through the transparent window. In the simulation, the boiling heat transfer is predicted with Rensselaer Polytechnic Institute (RPI) boiling model, in which the wall heat flux is composed of three parts. The predicted boiling curve is compared with the measured one, and the calculated distributions of the vapor volume fraction adjacent to the boiling wall are compared with the captured images. The reasonable agreements have been reached. Three different regimes are identified based on the captured images. The jet velocity plays less significant role in the low heat flux cases for the saturated jets, but more in the high heat flux scenarios for the subcooled jet impingement boiling. The latent heat of phase change weakens the effect of jet velocity when crossing the boiling point. Moreover, a higher velocity of the saturated jet results in a later transition to the fully established boiling regime with a higher wall superheat. After the transition point, the predicted wall heat flux components are almost independent of wall heat flux or wall superheat, indicating that they are the good indicators to determine the boiling regime in the simulation.

Experimental and analytical study on nucleate pool boiling heat transfer of R134a/R245fa zeotropic mixtures

In the present study, the nucleate pool boiling heat transfer characteristics of zeotropic binary mixtures (R134a/R245fa) with different blending ratios, as well as their pure components, were experimentally investigated in a visualized pressure vessel. For each test refrigerant, the experiments were performed on a horizontal plain copper surface (10 mm × 10 mm) at the same evaporating temperature of 26 °C with the heat flux ranging from 1.2 kW/m2 to 360 kW/m2. The boiling curves of test refrigerants were subdivided into up to four zones from natural convection region to bubble column region according to the visualization results. The slope of the boiling curve in bubble coalescence region was greater than that in isolated bubble region. The onset of nucleate boiling (ONB) point of zeotropic mixtures obviously lagged behind that of pure components, namely, the higher wall superheat was required, which presented the consistent variation with the temperature glide. The boiling heat transfer coefficient of zeotropic mixtures, although increasing with the increase of heat flux, was degraded compared to the corresponding ideal heat transfer coefficient, except for some specific cases with high heat flux. Based on the analysis of heat transfer degradation factors and additional mass transfer resistance, the nucleate pool boiling process of zeotropic mixtures was identified as three stages due to the presence of two inflection points, pseudo fully developed nucleate boiling (P-FDNB) point and pseudo double boiling (P-DB) point. The wall superheat of test mixtures at P-DB point was stable at a certain value and did not vary with the blending ratio. The thermodynamic fluctuation theory was introduced to describe the effect of mass transfer resistance for zeotropic mixtures at P-DB point and the calculation results showed good agreement with the experimental data.

Phase Change Characteristics of Ultra-Thin Liquid Argon Film over different Flat Substrates at High Wall Superheat for Hydrophilic/Hydrophobic Wetting Condition: A Non-Equilibrium Molecular Dynamics Study

Non-equilibrium molecular dynamics simulations have been conducted to understand the effect of solid-liquid interfacial wettability and surface material on the phase change phenomena of the thin liquid argon film placed over flat substrate at high wall superheat. The molecular system consists of a three phase simulation domain involving solid wall, liquid argon and argon vapor. After the system is thermally equilibrated at 90K and kept in equilibrium for a while, a high wall superheat (250K that is far above the critical temperature of argon) is induced at the liquid boundary so that the liquid undergoes ultrafast heating. Both hydrophilic and hydrophobic surfaces were considered in the present study in order to observe the effect of surface wettability on phase change characteristics for three different solid substrate materials namely, Platinum (Pt), Silver (Ag) and Aluminium (Al). Results obtained in the present study are discussed in terms of transient atomic distribution inside system domain, heat flux characteristics across the solid-liquid interface together with evaporative mass flux from liquid argon. Simulation results show that, depending on the surface wetting condition, the phase change process appears to be very different (explosive/ diffusive) for all three substrate materials under consideration. Among three materials considered herein, Al is found to offer the least favourable condition for phase change process while Pt and Ag show similar heat and mass transfer characteristics for both hydrophilic and hydrophobic wetting conditions. Surface wettability effect is found to be more prominent than the effect of substrate material in thin film liquid phase change phenomena.Journal of Chemical Engineering, Vol. 29, No. 1, 2017: 49-55

Flow Boiling Heat Transfer of Subcooled Water on Sintered Microporous Surfaces

Electronic devices such as battery packs in electric vehicles and LED lights require advanced control in temperature uniformity for their optimum performance and prolonged lifetime. Flow boiling heat transfer of subcooled water (∆Tsub = 20K) in a 300 mm long minichannel with the cross section of 20×10 mm2 was investigated to improve the temperature uniformity over the entire minichannel. The minichannel was uniformity heated from the bottom copper surface. A 10 mm thick Pyrex glass was used for the top plate of the channel to visualize two-phase flow during the experiment. Microporous coating was fabricated by sintering copper particles on the top surface of the copper block. The average particle size was 50 μm, the average coating thickness was 300 μm, and the porosity was 41%, respectively. At the heat flux of 100 kW/m2, more bubbles are shown on the microporous surface compared with plain surface, resulting in better boiling heat transfer performance. These bubbles were large and stationary as liquid is evaporated and condensed to transport the heat as if heat pumps. As heat flux increases, bubble nucleation becomes more intensive, however, the larger stationary bubbles observed at 100 kW/m2 started to decrease. Most of the generated bubbles flowed through the downstream and they shrank quickly upon departure from the wall due to the 20K subcooling. High speed video showed some streaks of these small bubbles, and more streaks were observed as the heat flux increased. As shown in the left graph above, at 50 kW/m2 in subcooled flow boiling, both plain and microporous surfaces show similar local wall temperature because both are placed in the single-phase regime. In contrast, the difference of wall superheat between plain and porous surface is relatively large at higher heat flux of 500 kW/m2. Sintered microporous surface showed smaller increase in wall superheat compared with plain surface at higher wall superheat. [This study was supported by National Research Council of Science and Technology (NST) grant, Korea (Grant No. KIMM-NK203B)].