Lower Critical Heat Flux Research Articles

Improved flow boiling performance and temperature uniformity in counter-flow interconnected microchannel heat sink

Microchannel flow boiling is one of the most effective solutions to the problem of heat dissipation caused by high power electronic devices. However, inherent drawbacks such as low critical heat flux due to dry-out of the liquid film near the channel exit, relatively high two-phase flow pressure drop, severe temperature nonuniformity and boiling instability at high heat flux still prevent widespread commercial application. In this study, based on the idea of uniform vapor quality along the flow direction, a copper Counter-Flow Interconnected Microchannel (CFIM) was proposed to address the main issues currently faced by two-phase microchannel heat sink. A comprehensive comparative study of CFIM, Co-Current Microchannel (CCM) and Counter-Flow Microchannel (CFM) was carried out from the aspects of boiling stability, boiling heat transfer characteristics and temperature uniformity. Three slots of 0.2 mm, 0.5 mm and 0.8 mm in width, denoted as IM0.2, IM0.5 and IM0.8, respectively, are considered. Experiments are conducted at mass fluxes of 291–618 kg/m2·s, saturation temperature of 40 °C and effective heat fluxes of 3–293 W/cm2, using the dielectric fluid R1233zd(E) as the working fluid. The results show that the CHF and average two-phase heat transfer coefficient (HTC) of CFIM are increased by 40.6 ∼ 100.5 % and 70.4–83.6 %, respectively, while the two-phase pressure drop is decreased by 17–32.6 % compared to CCM. The design of the counter-flow interconnected structure can manipulate the void fraction and two-phase flow pattern, resulting in a near uniform void fraction and flow pattern. The structure of these connecting slots promotes nucleate boiling and fluid mixing of neighboring channels. More importantly, the flow boiling instability and temperature non-uniformity in CFIM are well suppressed. In addition, IM0.2 has higher heat dissipation than IM0.5 and IM0.8 due to enhanced bubble nucleation, reducing superheat requirements and periodic rewetting to the center of the channel. The results of this research reveal that utilizing a microchannel heat sink with a counter-flow interconnected configuration can effectively improve heat transfer performance during flow boiling. Furthermore, it helps to mitigate issues such as flow boiling instability and temperature non-uniformity.

Fire behavior of hemp blocks: A biomass-based construction material

Hemp blocks, also known as hempcrete, are eco-friendly and sustainable construction materials composed of hemp, lime, and water. In this study, the fire behaviour and structural performance of hemp-based materials were experimentally investigated. Fire exposure scenarios using raw hemp shives, hemp blocks, and non-load-bearing hemp block walls were examined. Tests conducted include cone calorimeter, bomb calorimeter, standard furnace, heat-transfer rating inducing system (H-TRIS), and small-scale elevated temperature material tests. Hemp shives exhibit ignition with sustained flaming, a relatively high heat release rate (HRR), and a relatively low critical heat flux (CHF). However, the hemp blocks exhibited no flaming ignition, only smouldering combustion, and an HRR an order of magnitude lower. Hemp blocks and hemp shives produced minimal smoke. Hemp blocks charred, and associated discoloration zones have been documented. Tests indicate that limited structural capacity is lost up until 200 °C, whereas at 300 °C, the residual material strength is almost negligible. The hemp block walls maintained their structural stability and integrity for 2 h of standard fire testing. The ambient-temperature compressive strength of the hemp blocks was determined to be 1.0 MPa. This work is the first comprehensive study on the fire behaviour of hemp blocks and highlights their good performance, whereby they are likely to have a limited impact on fire risk in buildings. Plastered walls will have a fire performance exceeding those reported here.

Experimental Analysis of Bubble Behavior and Critical Heat Flux During Pool Boiling on Vertical Circular Tubes

Abstract The heat transfer during pool boiling incorporates a higher rate of heat dissipation capability at low-temperature differences. This technique is widely used in the nuclear industry for thermal management. In this study, the effect of tube diameter and length on critical heat flux (CHat atmospheric conditions in saturated water during pool boiling) is analyzed. The tubes of SS 304 are kept in the vertical orientation. The diameter of the tubes ranges from 1.2 mm to 9 mm. The experiments are performed with tubes having lengths varying from 50 mm to 1000 mm. It has been noted that tubes of smaller diameter show a greater magnitude of critical heat flux (CHF) for the given length. Compared to other tubes, the magnitude of CHF for a 1.2 mm diameter is higher. For a given diameter, a longer tube is found to have lower CHF than the ones having lesser length. The variation in the CHF magnitude is negligible for tubes with a diameter of more than 2.5 mm beyond a length of 200 mm. The location of occurrence of CHF is near the bottom end of the vertical tube. The study illustrates the behavior of bubble nucleation for various tube dimensions and heat fluxes. The inception and detachment of bubbles for different tubes are analyzed. The pool boiling regime is categorized and studied basing the behavior of the incepted and departed bubble while maintaining uniform heat flux. A mathematical relation that empirically accounts for the effect of tube dimensions , i.e., length and diameter on pool boiling CHF is proposed. The experimental CHF data obtained during pool boiling are tabulated toward contributing to the CHF databank.

Experimental study of falling film evaporation of refrigerants, R32, R1234yf, R410A, R452B and R454B on horizontal tubes

This study investigates the heat transfer performance of the falling film evaporation and pool boiling of low-GWP refrigerants, R452B and R454B, for the replacement of R410A. Fluids including pure refrigerants R32, R1234yf, and near-azeotropic mixtures R410A, R452B, and R454B were tested on smooth tubes and 60FPI fin tubes at various conditions of saturation temperature (5, 15 °C), volume flow rate (150∼300 ml/min), and heat flux (10∼50 kW/m2). For the falling film evaporation on smooth tubes, the R32 yielded the highest heat transfer coefficient among all test fluids. R410A has higher heat transfer among the three mixtures with the help of the component of R32, with lower Marangoni number and consequent lower critical heat flux. In addition, the heat transfer coefficients of film evaporation of R452B and R454B is about 18.2% and 14.5% lower than that of pool boiling, respectively, at Ref=833–865. The heat transfer coefficient of fin tubes is 2.84–3.87 folds that of the smooth tubes for R452B. Based on 339 sets of falling film test data of near-azeotropic fluids, an empirical correlation was established. It predicts experimental data with mean absolute error (MAD%) of 8.28%.

Comparison of pool boiling heat transfer performance between aqueous cationic and anionic surfactant solutions with similar ionic group

Pool boiling in aqueous surfactant solution is found to have a better heat transfer performance than that in distilled water. The present study intends to investigate the effect of functional ionic group in cationic or anionic surfactant on pool boiling performance. Two pairs of surfactants, namely, cationic surfactants of Dodecyltrimethylammonium bromide (DTAB) / Dodecyltrimethylammonium chloride (DTAC) and anionic surfactants of Sodium dodecyl sulfonate (SD-Sulfonate) / Sodium dodecyl sulfate (SD-Sulfate), are added into distilled water with a mass concentration of 100–3200 ppm. Boiling performance and thermophysical properties of the corresponded surfactant solutions are measured and compared with water. The bubble behaviour and surface wettability are also analyzed. Results indicate that the surfactant solutions can generally increase the heat transfer coefficient (HTC) compared with water and a higher concentration solution usually owns a greater HTC. The maximum relative HTC enhancement can reach up to 2.33 times. The reduction in surface tension and an additional repulsion force due to the addition of surfactant result in a denser nucleate site, a smaller bubble departure diameter and a faster bubble departure frequency, which results in a higher HTC. The surface tension reduction caused by surfactant addition and the boiling process make the copper surface more wettable, resulting in a higher critical heat flux (CHF). However, the surfactant also makes the fresh water supply difficult at a high heat flux and subsequently results in a lower CHF. The CHF is comprehensively affected by the surface wettability of boiling surface and surfactant concentration and their relative strength determines the final magnitude of CHF. The surfactants with a same or similar functional ionic group have similar boiling heat transfer performance in aqueous solutions. The present study can provide meaningful guidance for further pool boiling studies and relevant highly efficient heat exchanger design.

Enhanced pool boiling on micro-nano composited surfaces with nanostructures on micro-pin-fins

In the present study, the effect of the distribution of nanostructures on the pool boiling heat transfer performance of micro-nano composited surfaces in FC-72 was investigated. For this purpose, a micro-pin-fined surface (MPF), a micro-nano composited surface with nanoforests covering the top and bottom sides of micro-pin-fins (MPF-NF), a micro-pin-fined surface with nanostructures etched at the top corners of micro-pin-fins (MPF-TC), and a micro-pin-fined surface with nanostructures etched at the top edges of micro-pin-fins (MPF-TE) were fabricated. The pool boiling heat transfer performance was compared under the liquid subcooling (ΔTsub) of 0, 15, 25, and 35 K. It is found that MPF-NF has the lowest wall superheat at the onset of the nucleate boiling point due to its highest nucleation site density, while MPF-TE has the highest maximum heat transfer coefficient because its critical heat flux (CHF) is significantly higher than that of MPF-NF. The bubbles of MPF-TC and MPF-TE are mainly generated on the tops of micro-pin-fins, thereby alleviating the blockage of the capillary wicking channels by the bubbles in the high heat flux region. Furthermore, the condensation effect of the subcooled liquid on the bubbles generated on the tops micro-pin-fins, with a thinner thermal boundary layer, is stronger compared with that generated in the microchannels. On the contrary, the bubbles of MPF are mainly generated in the microchannels, leading to a blockage of the capillary wicking channels at high heat fluxes. Therefore, the CHF of MPF-TE at ΔTsub = 35 K is increased by 25% compared to MPF. However, the wake effect dominates the liquid supply and numerous lateral bubble coalescence on MPF-TE weakens the wake effect on the liquid supply, resulting in a lower CHF than that of MPF at ΔTsub = 0 K.

Pool boiling heat transfer enhancement of aqueous solution with quaternary ammonium cationic surfactants on copper surface

Pool boiling is a widely encountered heat transfer phenomenon and the enhancement of boiling heat transfer performance is of vital importance for real applications. Adding surfactant into distilled water is regarded to be a promising and effective approach for heat transfer improvement. The present study selected four kinds of quaternary ammonium cationic surfactants with an increased molecular weight, namely, decyltrimethylammonium bromide (DeTAB), dodecyltrimethylammonium bromide (DoTAB), myristylmethylammonium bromide (MTAB) and cetyltrimethylammonium bromide (CTAB), to investigate the pool boiling performance of their aqueous solutions on a copper heater with and without electric field under different concentrations. Their dynamic surface tensions and viscosities were also measured. The bubble dynamics during pool boiling and apparent contact angle on copper surface before and after boiling test were evaluated. Experimental results reveal that the aqueous surfactant solution can significantly enhance the heat transfer coefficient (HTC) compared with distilled water. The HTC exhibits an increment with concentration until critical micelle concentration (CMC) and then levels off. The maximum relative HTC enhancement can reach up to 209.3%, which is ascribed to the increased nucleate site and bubble departure frequency. In addition to the reduction in surface tension, a repulsion force caused by electrostatic interaction is also responsible for the changes in nucleate site and bubble departure frequency. However, the critical heat flux (CHF) shows a continuous decrement with surfactant concentration. In a low concentration surfactant solution, the CHF is even slightly higher than that of water, which is caused by an improvement in surface wettability after boiling test. The CHF in moderate and high surfactant concentration will be lower. The surfactant with a smaller CMC tends to have a higher HTC and a lower CHF under a low concentration. However, for a relatively high concentration comparative to CMC, the four kinds of quaternary ammonium cationic surfactant solutions tend to have pretty similar boiling curves. The electric field is found to have a negligible influence on the pool boiling performance in aqueous surfactant solutions. Results in present study can provide valuable guidance for surfactant selection during the design of highly effective heat exchangers.

Achieving ultra-high coefficient of performance of two-phase microchannel heat sink with uniform void fraction

Thermal management of high-power electronics/devices has been a very challenging issue for data centers, flourishing electric vehicle revolution as well as cooling of the first wall of a Tokamak fusion reactor. As a prominent cooling strategy, two-phase flow in microchannels has received extensive research over decades. However, inherent defects such as relatively low critical heat flux due to the dry out of liquid film near the exit of the channel and relatively high two-phase flow pressure drop at high heat flux still hinder extensive commercial applications. The present work proposes a robust microchannel heat sink design toward uniform void fraction distribution along the flow direction. The two-phase flow pressure drop will not increase significantly with increase in heat flux. This is made possible by an innovative and unique combination of diverging microchannel and counter-flow manifold, which enables extensive channel-to-channel heat transfer, especially near the end or inlet of the channels. Moreover, flow visualization reveals that nearly uniform void fraction distribution along the channel with relatively high bubble slug ratio is possible. For a relatively large area of 12 cm2 and under the limitation of wall temperature of 140 °C, a heat flux as high as 3525 kW/m2 is achieved without sign of reaching the critical heat flux with nearly negligible pressure drop increment comparing to single-phase convection for a relatively low mass flux of 600 kg/m2s. An ultra-high coefficient of performance of 75,675 is attained under a low inlet temperature and low mass flux.

Study on boiling heat transfer of surfactant solution on grooved surface

In the present study, the coupling effect of wire-cutting grooved surface and surfactant (Sodium dodecyl sulfate, SDS) in pool boiling heat transfer is studied experimentally. Based on visualization, the bubble growth at the onset of bubble formation is measured, and the results are found to yield the reported empiric laws in the functional form of heat diffusion growth. In addition, the surfactant is found to improve the bubble nucleation on the heated surface with a shorter growth time and smaller departure diameter. Moreover, the grooved surface is also easier to nucleate than the smooth one, that is, the onset of bubble formation on the grooved surface is advanced, and thus the coupling effect further enhances the bubble nucleation. For the heat transfer, the SDS solution presents good heat transfer performance at low and medium heat flux in comparison to the DI water, and the coupling effect with the grooved surface would lead to further enhancement, by which the heat transfer coefficient (HTC) is maximally increased by about 238%. In addition, there are three main forms of bubbles found during the boiling of SDS solution, including small bubbles, discrete bubbles and large bubbles. The discrete bubbles are found to be formed by the entrainment of the upper foam due to the liquid convection. At relatively high heat flux, there is bubble crowding in the boiling of surfactant solution, accompanied by the expansion and contraction of bubble clusters and significant fluctuation of near-wall temperature. Furthermore, the bubble crowding seems to cause the premature dryout and the low critical heat flux (CHF), and could be attributed to the good nucleation ability and the repulsion between bubbles, which also result in the heat transfer enhancement of SDS solution in low and medium heat flux. Therefore, the enhancement of HTC and CHF cannot be achieved simultaneously in the present method.

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.

Pool Boiling Performance of Water and Self-Rewetting Fluids on Hybrid Functionalized Aluminum Surfaces

The boiling performance of functionalized hybrid aluminum surfaces was experimentally investigated for water and self-rewetting mixtures of water and 1-butanol. Firstly, microstructured surfaces were produced via chemical etching in hydrochloric acid and the effect of the etching time on the surface morphology was evaluated. An etching time of 5 min was found to result in pitting corrosion and produced weakly hydrophilic microstructured surfaces with many microcavities. Observed cavity-mouth diameters between 3.6 and 32 μm are optimal for efficient nucleation and provided a superior boiling performance. Longer etching times of 10 and 15 min resulted in uniform corrosion and produced superhydrophilic surfaces with a micropeak structure, which lacked microcavities for efficient nucleation. In the second stage, hybrid surfaces combining lower surface energy and a modified surface microstructure were created by hydrophobization of etched aluminum surfaces using a silane agent. Hydrophobized surfaces were found to improve boiling heat transfer and their boiling curves exhibited a significantly lower superheat. Significant heat transfer enhancement was observed for hybrid microcavity surfaces with a low surface energy. These surfaces provided an early transition into nucleate boiling and promoted bubble nucleation. For a hydrophobized microcavity surface, heat transfer coefficients of up to 305 kW m−2 K−1 were recorded and an enhancement of 488% relative to the untreated reference surface was observed. The boiling of self-rewetting fluids on functionalized surfaces was also investigated, but a synergistic effect of developed surfaces and a self-rewetting working fluid was not observed. An improved critical heat flux was only obtained for the untreated surface, while a lower critical heat flux and lower heat transfer coefficients were measured on functionalized surfaces, whose properties were already tailored to promote nucleate boiling.

Flow-Controlled Spray Cooling Approaches for Dynamic Thermal Management

Abstract Two-phase spray cooling is considered as one of the most promising thermal management techniques characterized by high heat transfer coefficient (HTC) and critical heat flux (CHF), as well as near-uniform temperatures on target surface. Normally, spray cooling systems feature steady flow rate and continuous spray, and are designed to satisfy the maximum expected heat load. However, if the heat load varies due to operating conditions, such as cold starts and pulsing power cycles, a spray cooling system uses excessive coolant most of the time, and causes low cooling efficiencies because of lower utilization of phase-change heat transfer and higher pumping power. This study aimed to investigate variable and intermittent flow spray cooling characteristics for dynamic thermal management. Variable flow spray cooling scheme requires control of pump input voltage (or speed), and intermittent flow spray cooling scheme requires control of solenoid valve duty cycle and frequency. Tests were conducted on a closed-cycle system with HFE-7100 dielectric coolant at varying liquid flow rate and subcooling conditions, and using smooth and enhanced heat transfer surfaces. Results, compared to a baseline performance from steady flow case, indicated that the variable flow spray cooling conditions achieve similar cooling performance at low and moderate heat fluxes, while the intermittent flow spray conditions result in high temperature penalty and much lower CHF.

Combustion of a solid fuel material at motion

The present paper proposes a mathematical model for the ignition phenomenon of a non static combustible solid fuel bed under the exposure of a constant incoming heat flux impinging on the solid surface. The thermal behaviors in terms of temperature responses of the solids of thermally thin, thermally thick and thermally moderate (finite-depth) target materials are analytically evaluated. Having established asymptotic relations, analytical estimates for the ignition time and critical heat flux are derived together with further approximations for the full temperature distribution inside the solid material without necessitating any numerical treatment. The explicit findings clearly point that a solid fuel bed layer at motion possesses less ignition time with a lower critical heat flux as compared to a solid material at rest, which may constitute a practical significance in applications involving fire, forest fuel and waste burning ignition, since combustion is initiated rapidly for moving flammable targets.

Water immersion cooling of high power density electronics

Thermal management of power electronic systems is a key bottleneck to power densification. Single phase cooling is limited to low heat transfer coefficients (<2 kW/(m2 K)) while two-phase cooling such as flow boiling suffers from hydrodynamic instabilities. Immersion cooling has emerged as a potential solution to overcome these barriers by enabling the boiling of a cooling fluid directly from electronic components, thereby removing thermal interface materials and packaging constraints encountered in the aforementioned approaches. State-of-the-art (SOA) immersion cooling systems utilize dielectric heat transfer liquids due to electrical considerations, which presents fundamental disadvantages related to the relatively low boiling point, low critical heat flux (<20 W/cm2), and relatively poor thermophysical properties such as thermal conductivity, latent heat, and surface tension, when compared to higher performance fluids such as water. In this study, we propose an approach that uses immersion cooling directly in water. We use the electrically insulating nature of Parylene C coatings to insulate the printed circuit board (PCB) and electronic devices from the water. We demonstrate experimentally the effectiveness of conformal layers of Parylene C as thin as 1 μm in preventing current from leaking between the electronic components and the surrounding water when the system is subjected to voltages up to 200 Volts. Furthermore, we provide the heat flux and convection heat transfer coefficient obtained in 3 M Novec 72DE and 7300 dielectric fluids, water, and a 50/50 in volume mixture of water and ethylene glycol (WEG) as a function of hot-spot-to-fluid temperature difference in both natural convection and nucleate pool boiling regimes. Gallium Nitride (GaN) transistors with different board-mounting techniques and thermal pad locations are used as heat sources. Heat fluxes up to 562 W/cm2 are measured in water. As a proof of concept, water immersion cooling is tested successfully on a 2 kW power converter operating at 97.2% efficiency in deionized water. This study not only demonstrates immersion cooling in water of high-power density electronics, but also develops design guidelines for cooling of electronic components through the use of novel electrically insulating coatings coupled with attractive electrically conducting cooling media.

On the mechanism of boiling heat transfer enhancement by surfactant addition

In our previous study, bubble jet and bubble explosion around a heated wire were discovered in aqueous cetyltrimethyl ammonium chloride (CTAC) solutions (Wang et al., 2016), in which the bubble explosion process was explained. The bubble explosion phenomenon was proven to be a failed coalescence of unstable bubbles which caused local disturbance and enhanced boiling heat transfer. In this paper, comparative experiments of boiling around a heated wire for different types of surfactant solutions, ethanol and silicone oil were carried out in order to explore the mechanism of surfactant addition enhancing boiling heat transfer. All these fluids had lower surface tensions than that of water. Cationic surfactant of CTAC, anionic surfactant of sodium dodecylbenzenesulfonate (SDBS) and nonionic surfactant of alkyl polyglycoside (APG) were used. Bubble jet and bubble explosion phenomenon were found in all three kinds of surfactant solutions, exhibiting excellent heat transfer effect. For the pool boiling in ethanol, only bubble jet around the heated wire was found, showing better heat transfer efficiency than water. However, weaker boiling heat transfer occurred in silicone oil without bubble jet and bubble explosion. Due to the heat sensitivity characteristics, APG solution had a lower critical heat flux (CHF) than water, while CTAC and SDBS solution had higher CHF. With all phenomena and heat transfer performances, the mechanism of boiling heat transfer enhancement by surfactant addition was confirmed, i.e., the bubble jet and bubble explosion process enhanced the boiling heat transfer in surfactant solution rather than low surface tension.

Saturated pool boiling heat transfer from vertically oriented silicon surfaces modified with foam-like hexagonal boron nitride nanomaterials

In this work, the ability of a foam-like hierarchical hexagonal boron nitride (h-BN) nanomaterial to act as a scalable surface modifier for improved pool boiling performance is investigated. The h-BN foam samples were grown using an atmospheric pressure chemical vapor deposition process onto the surfaces of open-cell nickel foam followed by selective etching. Saturated pool boiling experiments were conducted in degassed deionized water for unaltered silicon surfaces, silicon surfaces modified with planar or foam-like h-BN materials plus adhesion layer, and a silicon sample with only the adhesion layer present. Results from the unaltered silicon surface obtained in this study were found to be in excellent agreement with previous studies. For the planar h-BN control sample the observed critical heat flux (CHF) occurred at 24.2±2.4W/cm2 at wall superheat of 28.0±2.1K, a factor of three lower CHF than for the unaltered Si surface. CHF for h-BN foam samples occurred at 69.1±6.9W/cm2 at wall superheat of 25.0±2.1K and 87.9±8.8W/cm2 at wall superheat of 24.9±2.1K. These CHF results are comparable to the bare silicon values but significantly enhanced compared to the adhesion layer only and planar h-BN samples, thereby indicating foam-like h-BN nanomaterials have the potential to facilitate significant CHF enhancement but that means of integration is critical to determining overall performance. Comparison of boiling curves reveals the h-BN foam samples demonstrated higher heat transfer coefficient at moderate and high heat flux relative to bare silicon, with CHF being reached at ∼30% lower wall superheat.

Flow boiling and critical heat flux in horizontal channel with one-sided and double-sided heating

This study explores flow boiling of FC-72 along a 5-mm high by 2.5-mm wide rectangular channel that is fitted with top and bottom heating walls. By activating one wall at a time, the opposing influences of gravity are examined for inlet velocities from 0.11 to 2.02m/s. Results for top wall and bottom wall heating are then compared to those for double-sided heating. For top wall heating, high speed video imaging proves gravity effects are dominant at low velocities, accumulating vapor along the heated wall and resulting in low critical heat flux (CHF) values. For bottom wall heating, buoyancy aids in vapor removal and liquid replenishment of the heated wall, resulting in higher CHF values. Higher velocities result in fairly similar interfacial behavior for top wall and bottom wall heating, and double-sided heating exhibiting greater symmetry between interfacial behaviors along the opposite walls. Overall, CHF values for all three configurations converge to one another above 1.5m/s. This convergence is clearly the result of high inertia negating the influence of gravity. It is shown that interfacial instability theory provides an effective means for assessing the influence of velocity on CHF for top wall versus bottom wall heating. For top wall heating, a stable interface at low velocities causes vapor accumulation against the top wall resulting in very low CHF. Instability theory shows that top wall heating becomes unstable above 1.03m/s, allowing liquid contact with the wall and improved wall cooling. For bottom wall heating, the interface is always unstable and favorable for liquid contact. Instability theory also shows that inertia dwarfs gravity around 1.5m/s, where critical wavelengths for top wall and bottom wall heating converge. Convergence of the CHF values for top wall and bottom wall heating also occurred at a similar value.

Flow impinging effect of critical heat flux and nucleation boiling heat transfer on a downward facing heating surface

Abstract Boiling heat transfer has a high heat removal capability in convective cooling. However, the heat removal capability of downward-facing boiling is significantly worse than that of upward-facing cases because of the confined buoyancy effect. This study was inspired by the conception of external reactor vessel cooling (ERVC) condition relevant to the in-vessel retention (IVR) design of Westinghouse AP1000 plant. In the present study, a small-scale test facility had been established to investigate the local phenomena of boiling heat transfer under a downward-facing horizontal heated surface with impinging coolant flow. In this study, the surface temperature, heat flux information and several specific scenes of bubbles are taken down throughout the boiling processes for detailed investigation. It is observed that bubbles are confined under the downward-facing heated surface, which causes a worse heat transfer rate and a lower critical heat flux (CHF) limit than upward-facing boiling. Nevertheless, the impinging coolant flow is found to disturb the thermal boundary layer formed by the heated surface, so the CHF increases with an increase of coolant flow rate. In addition, during nucleate boiling, it is discovered that the growth, combination and dissipation of bubbles induce turbulent wakes and therefore enhance the heat transfer capability.

Experimental study of pool boiling characteristic of an aluminized copper surface

Aluminizing is an appealing method of surface modification. Although this method is widely used in industry for surface modification, its effect on boiling heat transfer has not been reported yet. In the present study, effect of aluminizing of a copper surface on its pool boiling characteristic in the presence of De-Ionized water as working fluid is investigated experimentally. To perform the process of aluminizing the copper specimen was sunk in a powder that contained Al2O3, Al, and NH4Cl and was heated thereafter. The experimental results revealed that critical heat flux increases about 37% on aluminized surface compared with untreated copper surface while changes in boiling heat transfer coefficient is inconsiderable. Therefore some undesirable characteristic of copper like oxidation, and relatively low critical heat flux can be diminished by applying this coating method. In addition, AFM images were used to study some surface characteristics such as roughness, surface height and area ratio.

Effect of taper on pressure recovery during flow boiling in open microchannels with manifold using homogeneous flow model

Flow boiling in microchannels is considered an attractive cooling option due to its small hydraulic diameter, latent heat effect and relative uniformity of coolant temperature. Although various techniques have been successfully employed to provide stable boiling operation in microchannels, it suffers from poor heat transfer performance with low critical heat flux (CHF) and low heat transfer coefficient (HTC). The tapered manifold with open microchannel (OMM) geometry provides stable operation with high heat transfer performance coupled with a very low pressure drop (less than 10kPa). In this study, the pressure drop components from friction, acceleration and area changes during flow boiling of water at atmospheric pressure in the OMM geometry are evaluated using homogeneous flow model. Pressure recovery resulting from area change due to the taper is identified as a major factor in the extremely low pressure drops observed in this geometry. Seven viscosity averaging schemes are used to predict the frictional pressure drop. The best performing viscosity models are able to predict the pressure drop for the tapered configurations with a microchannel chip within an average MAE of less than ±30%. High speed visualization of the boiling phenomenon supports the applicability of the homogeneous flow model in the open microchannel geometry.