Boiling Heat Transfer Process Research Articles

Investigation of boiling heat transfer process in the barrel of a gravity heat pipe type extruder

Gravity heat pipe barrels produce gas films during the processing of thermoplastic materials, which affect the heat transfer, so there is an urgent need to study the nucleate boiling process and mechanism at different locations in the barrel. The results showed that the local heat flux at the vapor-liquid-solid three-phase contact was the largest, and different process conditions had different effects on the departure diameter and departure time of boiling bubbles. The tail vortex disturbance generated by bubble departure drives the merging and growth of bubbles to departure, which in turn improves the heat transfer. The rounded bottom of the barrel reduced the air film and increased the heat transfer efficiency compared to the horizontal bottom of the barrel. This study can deepen the understanding of the mechanism of bubble boiling heat exchange in gravity heat pipe-type barrels and provide theoretical support for more relevant application scenarios.

Molecular insights into the role of tetrafluoromethane in mitigating the boiling crisis suffered by liquid nitrogen in superconducting apparatus

Nitrogen/tetrafluoromethane (N2/CF4) has emerged as an effective refrigerant mixture, which plays a significant role in various high-Tc superconducting (HTS) power devices and energy systems. Several studies have documented the superior performance of such zeotropic binary mixtures in heat transfer and energy conservation compared to pure liquids. However, boiling heat transfer characteristics of liquid mixtures on the molecular scale are not fully understood, especially the mechanisms associated with the role of the additive in regulating thermal properties. Here, we performed the molecular dynamics simulations for the pure N2 and the mixtures containing 20 and 40 mol% of CF4 to probe into the boiling heat transfer process on an ideal copper substrate. Analyses suggest that the boiling process of the N2/CF4 mixture shares similar features with the pure N2, but the binary refrigerant manifests advantages at higher substrate temperatures. Specifically, the additive CF4 delays the onset of film boiling, and the operational temperature range could be enlarged by almost 40% compared to pure N2 in terms of the 40 mol% mixture. Moreover, the mixture with CF4 additive maintains a small interfacial resistance even if the substrate temperature exceeds the critical value of film boiling for pure N2, highlighting the potential to use such mixtures for devices that may suffer from high heat flux levels. Finally, analyses regarding the mixture composition and the solid-liquid interactions confirm the essential role of the additive CF4 in mitigating the mismatch of the vibrational density of states at a high substrate temperature, which may reveal the mechanisms concerning the CF4 improving the heat transfer performance of the original N2. These findings provide a better understanding of the advantage of N2/CF4 at high substrate temperatures, and they lay a foundation for designing the cooling systems of HTS apparatuses.

Enhanced liquid replenishment for pool boiling heat transfer and its CHF mechanism on patterned freeze-casted surfaces

In the current investigation, the pool boiling heat transfer process of patterned porous surfaces, as well as a smooth surface, in ethanol was examined from an academic perspective. The freeze casting (also known as ice-templating) processes was used to fabricate the structured surfaces. It was discovered that freeze-casted coatings significantly improved the performance of boiling heat transfer in comparison to the smooth surface (SMS). The most remarkable increase in Critical Heat Flux (CHF) was achieved by checkerboard porous surface(CHECKERBOARD), with an improvement of 81.6% relative to that of SMS. The maximum Heat Transfer Coefficient (HTC) enhancement was obtained by full porous surface (FULLY-COVERED), with an HTC value 215.8% higher than that of the SMS. For the semicircular porous surface (SEMICIRCULAR), striped porous surface (STRIPED), annular porous surface (ANNULAR) and checkerboard porous surface(CHECKERBOARD) with 50% coating coverage, CHECKERBOARD demonstrated the best heat transfer performance, with CHF and HTC increased by 32.3% and 48.9%, respectively, compared to the SEMICIRCULAR surface. The mechanism of liquid supply at the CHF of the patterned porous surfaces was investigated through phenomenological observations of the boiling phenomena. A modified model, accounting for the coalesced bubble departure frequency and capillary wicking effects, was proposed for CHF prediction. It is noteworthy that the fluid replenishment considers both vertical and lateral replenishment of porous coatings. The CHF data from this study, along with existing literature, were utilized to validate the model, and the predicted results exhibited good agreement with the experimental data, with errors within ±8%.

Study of pool boiling on hydrophilic surfaces developed using electric discharge coating technique

The emergence of high performing micro-sized devices in the recent decades have led to significant increment in heat generation rate, which is challenging to be dissipated by single-phase cooling schemes such as air or liquid cooling. As such, two-phase mechanism like boiling heat transfer have gained prominence to resolve this challenge. The boiling heat transfer process is usually assessed on the basis of parameters such as critical heat flux (CHF) and nucleate boiling heat transfer coefficient (NBHTC). The process of boiling is most efficient in the nucleate boiling phase which involves bubble generation and detachment. The occurrence of bubble generation can be increased by development of micro/nano-structured surfaces, finally leading to significant enhancement in boiling performance. A variety of surface modification techniques have been adopted in the past to improve boiling heat transfer. But certain issues could be found in few of the techniques. In order to address those issues, the present work has employed the method of Electric Discharge Coating (EDC) for developing microstructured boiling surfaces. The coating deposited was a mixture of zinc-copper (Zn-Cu) composite microparticles, at different current settings, on a copper surface. The EDC process performs the deposition in a layer by layer manner, in subsequent pulse-on and pulse-off cycles, thus ensuring a stable layer. Surface preparation was followed by surface characterization that involved wettability analysis, porosity and pore size estimation, mean roughness measurement and elemental analysis. The surfaces obtained were hydrophilic in nature where the contact angle varied from 62° to 40°. Consequently, the CHF and NBHTC showed enhancements of 48.5% and 195% respectively for the best performing surface in comparison with the bare/uncoated surface.

Shell-side heat transfer characteristics of floating liquefied natural gas spiral wound heat exchanger under sloshing conditions

To study the influence of offshore sloshing motion on the performance of a spiral wound heat exchanger (SWHE) installed on a floating liquefied natural gas (FLNG) platform and to explore potential methods to improve heat transfer efficiency, a sloshing experimental platform was established. This platform facilitated the experimental study of the boiling heat transfer process of a subcooled falling film on the outer surface of the heat exchanger tube. A quantitative analysis of the change in the heat transfer coefficient with respect to the amplitude and period of the rolling and swaying motion was performed. The experimental results indicated that both rolling and swaying changed the liquid film morphology, leading to a decrease in the heat transfer coefficient of the heat exchanger tube. The fluctuation in the heat transfer coefficient became more drastic with an increase in amplitude, but the effect of the swaying motion was less than that of the rolling motion. When the rolling angle was 12° and the Re number was 569, the average heat transfer coefficient of the round tube decreased by 55% compared with that at the static condition. Furthermore, the research encompassed the design, processing, and rigorous testing of elliptical and egg-shaped tubes to improve their heat transfer performance, and the results were compared with those of round tubes. Under the same rolling or swaying conditions, the heat transfer coefficients of the egg-shaped and elliptical tubes were 5.1–21.5% higher than that of the round tubes. The results of this study can be used as a reference for the operational and structural design of SWHEs under sloshing conditions.

Characterization of bubble population morphology in direct contact heat transfer process under different flow regimes and its influence on heat transfer performance

In this study, the effect of bubble population morphological characteristics and heat transfer performance in the direct contact boiling heat transfer process under two different flow modes are comparatively investigated. Based on the visualization method and homogeneity theory, the relationships between the bubble population morphological characteristics, distribution homogeneity, specific interface area, and volumetric heat transfer coefficient were investigated. The obtained results show that the bubble morphology tends to flatten with the increase of the dispersed phase flow rate in both flow modes. The more uniform the distribution of the bubble population, the better the specific interface area, and the trend is more obvious in the two-phase reverse flow mode. In addition, it was found that the specific interface area was positively correlated with the volumetric heat transfer coefficient. The synergistic relationship between the uniformity of bubble population distribution and the improvement of heat transfer is explained from the perspective of a specific interface area. It is concluded that different flow patterns affect the morphological characteristics of the bubble population and its distribution, which in turn affects the specific interface area of the two-phase contact heat transfer and ultimately the heat transfer performance. The morphology of the bubble population and its uneven distribution has a strong correlation with the flow pattern. This thesis provides a new idea to study the influence of bubble cluster morphology characteristics and heat transfer performance in direct contact boiling heat transfer process under different flow patterns and provides a new way for heat exchanger design.

Extension of GeN-Foam to modeling of boiling water and validation against the OECD/NRC PSBT benchmark

The OpenFOAM-based GeN-Foam solver has recently been extended to allow simulating water boiling based on a porous-medium approach. This paper describes such extension and preliminarily validates it based on the OECD/NRC PSBT benchmark. Three benchmark exercises are included: steady-state single sub-channel exercise; steady-state rod bundle exercise; and transient rod bundle exercise. Model sensitivity analysis is conducted, with a simple empirical turbulent model that is developed and calibrated to account for the mixing effect caused by the interaction of the two phases. The calculated results are compared with the experimental data, and most of the predictions fall within the ± 2σexp error bar. The results indicate that the solver can simulate water boiling heat transfer process and predict void fraction distribution accurately, at least for the cases of the PSBT benchmark.

Silver nanorod-induced porous networks: A pathway to efficient thermal energy transfer via pool boiling heat transfer

With the continuous trend of miniaturization in the electronics sector, high heat flux management is becoming increasingly important for ensuring efficient and safe operation. For thermal management of high heat flux devices, nucleate boiling is a prominent passive heat transfer approach. The two fundamental variables that can be varied to improve the effectiveness and capacity of the nucleate boiling process are the working fluid and surface properties such as surface roughness, wettability, area, and capillary. Boiling-induced interconnected porous network of silver nanowires coating is identified in this research as an efficient coating for enhanced nucleate boiling performance. The maximal ability of heat transmission, known as critical heat flux, and the efficiency of the boiling heat transfer process, represented by the heat transfer coefficient, are explored for the newly developed surfaces. Using a nucleate boiling testing setup, three distinct concentrations of silver nanowires-based nanofluids, 0.01%, 0.001%, and 0.0001%, were examined as working fluid. The working fluid resulted in the deposition of the nanowires over the heating surface and resulted in enhanced heat transfer performance. The maximum enhancement of 200% and 93.7% has been reported for critical heat flux and heat transfer coefficient, compared to deionized liquid on the unmodified Cu substrate. A detailed characterization has been performed to investigate the topology, wettability and morphology of the surfaces and understand the enhancement mechanisms.

Flow boiling heat transfer mechanisms and flow characteristics of pump-driven two-phase flow systems used in data center cooling

Pump-driven two-phase flow systems are commonly used in data center cooling owing to their outstanding heat-transfer performance. The evaporator is a key component of the cooling system, and its design significantly affects the overall cooling efficiency of the system. The boiling heat transfer process in the evaporator is primarily affected by the boiling heat transfer mechanism. Such a mechanism is mainly influenced by the type of coolant, inner wall structure of the tube (threaded tube or smooth tube), heat flux, and mass flow rate. In this study, we experimentally investigated the heat transfer mechanisms, capacity, and flow characteristics of the boiling process in a pump-driven two-phase flow system commonly used in data center cooling. The heat transfer capacity involved in this experiment was 140–1150 W, and the mass flow rate was 40–270 kg/h. Different coolants (R134a and R410A) and tube structures (threaded and smooth) were tested. The main novelty of this study is as follows. First, under the working conditions in this experiment, the inside-tube flow boiling mechanism was nucleate boiling for both the smooth and threaded tubes, and the boiling heat transfer coefficient remained constant as the mass flow rate decreased and vapor quality increased. Second, although it is commonly believed that a threaded pipe can significantly enhance the heat transfer ability, our study found that under data center cooling applications, the threaded tube had little strengthening effect on the heat transfer performance. Third, commonly used coolants, including R134a and R410A, were tested and the above conclusions were verified.

Experimental investigation of multiwall carbon nanotubes/water nanofluid pool boiling on smooth and groove surfaces

Boiling is an essential process for many industrial applications, such as refrigeration, distillation, and chemical processes. The effectiveness of the heat transfer processes determines the efficiencies of these applications. This study presents the experimental data analysis for pool boiling performance of 0.10, 0.15, and 0.20 wt.% of multiwall carbon nanotubes/water nanofluid on smooth and straight, square, and circular grooved surfaces. According to the experimental results, the configuration S4 with a 30 mm deep circular groove inclined at a 45° angle and 33% higher than the base fluid on the smooth surface had the greatest enhancement in boiling heat transfer coefficient. The result indicated that the inclination of the circular groove had the potential to enhance significantly the pool boiling heat transfer process. Furthermore, this study demonstrated that analyzing the effectiveness of nanofluids in a variety of concentrations and geometrical configurations of heat transfer surfaces is still essential and desirable.

Transient heat transfer modeling of an ice on coil thermal storage system with boiling heat transfer process of a refrigerant for diary preservation application

A numerical investigation has been made on transient heat transfer modeling of an ice on coil type thermal storage system based on a double u-tube borehole thermal network concept. Four distinct concentric tube has been considered inside a milk storage tank, where a refrigerant streams inside the inner tubes of the concentric tubes, while an ice will be formed at outer tube to store a cold energy. The complex boiling heat transfer of the cooling refrigerant has been performed by utilizing a set of empirical equations with iterative process. The cooling refrigerant cools down the milk inside the storage tank as well as it freezes the water inside the outer concentric tubes at the same time. In this analysis, temperature profile of the working fluids along the vertical position has been determined for specified times. Also temperature history of the working fluids has been determined at specific position of the thermal storage system. Finally the temperature variation of the refrigerant at the exit of the tubes is determined, and it is found that for the first 400 s the refrigerant leaves the exit of the tubes as superheated state, while the refrigerant exits the tubes as saturated state afterwards.

Lattice Boltzmann investigation of flow boiling in a microchannel

A hybrid multiphase lattice Boltzmann model is adopted to investigate the flow boiling heat transfer process in a horizontal microchannel with consideration of bubble dynamics. The flow pattern transition in a microchannel, involving single-phase flow, bubbly flow, slug flow, and contact-slug flow, is reproduced by varying the heat flux. The influencing parameters, including the liquid mass flux, the heating wall wettability, and the confinement height of channels, on dynamic bubble behaviors and heat transfer performance are discussed. The results indicate that a sequence of single-phase flow, bubbly flow, slug flow, and contact-slug flow occurs in a microchannel with increasing heat flux. Correspondingly, the dominant heat transfer mechanism experiences the liquid convection (single-phase flow), nucleate boiling (bubbly flow), the microlayer evaporation (slug flow), and the hybrid gas convection and microlayer evaporation (contact-slug flow). The increase of mass flux or confinement height extends the range of heat flux for high-efficiency bubbly and slug flow. The hydrophilic heating wall is preferred for better heat transfer performance in a microchannel due to the strengthened microlayer evaporation and less vapor occupation of the effective nucleation area.

Machine learning prediction of critical heat flux on downward facing surfaces

Application of external reactor vessel cooling (ERVC) in-vessel retention (IVR) was an important measure to ensure the integrity of the lower head of a reactor pressure vessel (RPV). As a typical boiling heat transfer process, the critical heat flux (CHF) plays a crucial role in the safety margin of a RPV's IVR-ERVC strategy. Although there have been a lot of correlations and experiments about the CHF of pool boiling on downward facing surfaces, their application range was relatively limited. To further expand the usability, machine learning was introduced in this research after collecting most accessible CHF data of pool boiling on downward facing surfaces. Considering the small amount of these experimental data, some pseudo data obtained by fitting the existed experimental data were added. Three machine learning methods, the ε-support vector machine (ε-SVM), back propagation neural network (BPNN) and random forest were used to predict CHF. Among the three methods, ε-SVM provided the best accuracy in the prediction. The effects of orientation, surface dimensions, heating surface material and pressure on the downward facing surface boiling crisis were predicted by ε-SVM, the results showed that in general, the CHF increased with the increase of orientation angle, and increased with the pressure from 1 bar to 10 bar. Under atmospheric pressure, the CHF decreased with increasing the width of the heating surface, but there was a width value that could eliminate the influence of width. In addition, the CHF seemed to increase with the increase of thermal effusivity. However, there are still some inexplicable phenomena that need to be revealed by further research. Overall, this method is expected to be widely used in predicting the CHF of pool boiling on downward facing surfaces.

Experimental investigation of flow boiling characteristics in counter-flow microchannels with different mass flux distributions

The cooling of high-power electronics urgently requires efficient and controllable heat dissipation techniques. In this work, a novel bidirectional counter-flow microchannels (CFMC) with mass flux distribution being regulated is proposed to achieve this goal. Flow boiling experiments are conducted in the CFMC and compared with traditional unidirectional parallel-flow microchannels (PFMC) by using deionized water as the working fluid. The effects of mass flux distributions on the boiling heat transfer process are experimentally investigated by changing the mass flux on one side of CFMC from 118 kg/m2·s to 370 kg/m2·s while keeping the mass flux on the other side unvaried. It is found that CFMC excels in heat transfer enhancement, as compared to traditional PFMC, the critical heat flux (CHF) can be increased by 42.9%∼53.8%. Furthermore, the average and local heat transfer coefficient (HTC) of CFMC in the convective boiling stage can be enhanced by 33.5%∼62.0% and 39.3%∼89.4%, respectively. In the nucleate boiling stage, the average enhancement of HTC for CFMC can be up to ∼170%. It should be noted that due to the shortening of the drying-out length and ensuring the wetting on one side, compared with the results under even mass flux distribution, the CHF can be further improved for CFMC with uneven mass flux distribution, but HTC has deteriorated. Surprisingly, all heat transfer enhancement results are achieved with a 53.4%∼66.7% reduction in two-phase pressure drop. Equally important, precise regulation of wall temperature distributions, heat transfer characteristics, and two-phase pressure drops can be realized by adjusting the mass fluxes on both sides. The statistical analysis of the visualization results reveals that the mass flux deployment also allows for regulating the wetting and drying times within the CFMC. This work presents essential value for optimizing the flow boiling process in microchannels and has great potential in practical applications.

Experimental investigation on flow boiling heat transfer characteristics of endothermic hydrocarbon fuel and its visualization at high-temperature conditions

The endothermic hydrocarbon fuel is used as a coolant to realize active cooling for the high-heat-flux components of air-breathing hypersonic vehicles. Because of its complex composition and severe thermal boundary conditions, the unstable heat transfer and coking are usually the major problems. Flow boiling heat transfer of a kind of endothermic hydrocarbon fuel named EHF-204 and its visualization were experimentally investigated in mini-channels with 2.0 mm internal diameter at heat flux of 250 kW/m2 and 500 kW/m2. It is found that the heat transfer enhancement and heat transfer deterioration would occur obviously during the flow boiling heat transfer process at subcritical and near-critical pressures of 1.0 MPa and 2.5 MPa. After the fuel was completely vaporized, the heat transfer coefficient would slowly increase again with the increasing fuel temperature. The heat transfer enhancement and heat transfer deterioration process at subcritical temperatures were not existed at the supercritical pressure of 4.0 MPa. The flow patterns of bubbly flow, slug flow, and churn flow were clearly observed at the subcritical pressures. Flow pattern visualization proved that the constant wall temperature heat transfer processes at subcritical pressures were subcooled boiling and saturation boiling flow. The bigger bubbles were formed at lower pressures, which caused higher heat transfer coefficient. The bubbly flow, slug flow caused the heat transfer enhancement, and the appearance of churn flow was a feather that made the boiling heat transfer transition from heat transfer enhancement to heat transfer deterioration. Some interesting visualization phenomena were observed in this study including the twice vaporization during flow boiling.

Experimental investigation of bubble group and temperature distribution uniformity in the direct contact boiling heat transfer process

In the present study, a new method is proposed to measure the uniformity of temperature distribution and the phase distribution in direct contact boiling heat transfer. The proposed method is applied to explore the relationship between uniformity of bubble distribution and temperature distribution. Based on the uniformity theory, the relationship between temperature distribution uniformity, temperature field uniformity, two-phase mixing uniformity and volumetric heat transfer coefficient is explored. Then the uniformity of temperature distribution is obtained through thermal imaging under specific conditions. The obtained results show that the actual temperature distribution uniformity and the correlation coefficient of the uniformity through the gray value are higher than 99%. Moreover, the obtained relationship between bubble group distribution and temperature distribution reveals the essence of the uniformity of bubble group distribution in improving heat transfer. It is concluded that the temperature non-uniformity mainly originates from improper mixing of different streams with different temperatures when entering the heat exchanger, which reduces the thermal performance of a heat transfer process. This non-uniformity has a strong correlation with the flow pattern. The main objective of the present study is to provide a new method to evaluate bubble group distribution and temperature distribution in boiling heat transfer.

A case study in the field of sustainability energy: Transient heat transfer analysis of an ice thermal storage system with boiling heat transfer process for air-conditioning application

A numerical study on transient heat transfer of shell and tube type ice thermal storage system is carried out based on the concept of a single u-tube borehole thermal network. Two tubes are arranged in the thermal storage tank in which glycol solution flows in one of the tubes, while a refrigerant flows in the other tube. In this study a boiling heat transfer of the refrigerant is considered to model the heat exchange among the ice storage, the refrigerant and the glycol solution which can be used for air-conditioning application. Temperature variation of all the three substances of the ice thermal storage system (i.e. the ice/water, glycol solution and the refrigerant) with depth has been determined for specified times. Also temperature history of all the three substances has been determined at specific depth of the thermal storage system. Finally the effect of mass flow rate of the refrigerant on the solidification process has been investigated with the numerical method. The transient boiling heat transfer of the refrigerant in the saturated region, the transient single-phase heat transfer process of the glycol solution and the refrigerant at the superheated region, and the solidification process of the ice thermal storage system were all investigated in this study.

Dynamics of bubble growth during boiling at microgravity

The purpose of this investigation is to study the mechanisms of boiling heat transfer in microgravity conditions. The RUBI (Reference mUltiscale Boiling Investigation) is an experiment where the basic phenomena of boiling heat transfer processes on a heated surface are investigated on the ISS (International Space Station). The special focus is paid to the coupling of macroscopic bubble dynamics from nucleation, growth and detachment combined with the microscopic phenomena in the thin films and micro layers on the heater, underneath the boiling bubbles. The image treatment program has been developed in order to extract the bubble volume as well as the contact angle from the experimental images. The first data of the bubble growth dynamics have been obtained and analysed.

Lattice Boltzmann mesoscopic modeling of flow boiling heat transfer processes in a microchannel

Microchannel flow boiling has received increasing attention for its high heat dissipation intensity with a small temperature difference. In the present study, a multi-relaxation-time (MRT) phase change lattice Boltzmann model is employed to investigate the flow boiling heat transfer processes in a vertical microchannel under constant heat flux boundary conditions. The bubble dynamic behavior including nucleation, growth, slide, coalescence and departure is obtained and the periodic variations of wall superheat and heat transfer coefficient are evaluated using the Jacob (Ja) number and Nusselt (Nu) number. It is demonstrated that the cavity microstructure on the heating surface can enhance the flow boiling heat transfer, further the cavity height and cavity distance are optimized with single-cavity model and multi-cavity model, respectively. Phase change heat transfer and flow boiling curves from single-phase forced convection, nucleate boiling regime, transition boiling regime to stable film boiling regime are numerically predicted for both the single-cavity structure and multi-cavity structures. The simulation results show that increasing Reynolds (Re) number accelerates the bubble departure frequency and intensifies the external forced convection, thereby achieving enhanced flow boiling heat transfer performance. These findings advance our understanding of the bubble dynamics and heat transfer mechanisms in the microchannel flow boiling processes.

Investigating of the Heat Transfer Characteristics of Impinging Flow on a Downward-Facing Surface

Critical heat flux (CHF) is an important heat transfer deterioration phenomenon during boiling heat transfer. It has been extensively studied, especially in the field of nuclear energy. Previous research has found that downward-facing heat transfer is worse than upward-facing and should be paid more attention. In this paper, the boiling heat transfer process under different flow rates and inlet distances is investigated. Seven experimental cases were made including a pool boiling case. The experiment studied the effect of inlet distances under small flow rates, which is not covered by previous research. Analysis of the CHF mechanism included surface temperature curves, boiling curves, bubble behaviors, and heat transfer coefficient. The fluctuation of the surface temperature of forced convention cases was observed due to the bubbles sliding along the heating surface. The phenomenon of vapor film fragmentation could also be found. The results show that even at small flow rates, CHF occurring time is postponed and the CHF value increases. Reducing inlet distance or increasing flow rate can both promote boiling heat transfer, thereby enhancing CHF.