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Efficient thermal system design is pivotal in cooling applications. The complex nature of boiling phenomena combined with the emergence of diverse flow patterns or regimes pose challenges. Despite advances in classification methods, machine learning (ML) models struggle to discern flow regimes from optical data obtained via high-speed cameras. This thesis presents a compelling case for learning from neuromorphic event representations of flow boiling, which offers insights into hidden properties of flow patterns. We delve into the application of diverse ML algorithms on event data for the task of flow regime classification. Through an analysis of convolutional neural networks, long short-term memory models, and event-based spiking neural networks, we highlight the superior accuracy and performance achieved by event data classification, as well as the unique insights provided by a novel Fourier-based classification approach. We evaluate the different ML algorithms on two event datasets — a video-to-event emulated dataset covering five flow boiling regimes, as well as a dataset combining the emulated data with a neuromorphic event camera dataset encompassing two regimes, for a total of seven flow boiling regimes. This research not only advances our understanding of flow boiling, but also showcases the potential of leveraging new data representations.

The study aims to numerically investigate the flow boiling of Al2O3/H2O and CuO/H2O nanofluids and water in pipes filled with copper metal foams. Four different values of porosity and three values of pore density have been used. To perform numerical simulation, the mixture model has been developed. For the first time, the effects of nanoparticle deposition on the wettability of heating surfaces were considered with the help of user-defined functions. Besides, the effect of metal foams with different porosities on the onset of nucleate boiling was evaluated. The thermal performance of metal foam pipes has been compared with each other by comparing the increase in heat transfer and pressure drop. As a result, by reducing the porosity from 0.95 to 0.80, the heat transfer coefficient was increased by 59 %, while the pressure drop increased by 28 %. Finally, by comparing the increase in heat transfer and pressure drop, results show that the metal foam pipe with 80 % porosity and 5 pores per inch has the best thermal performance. The results of this study are expected to be used for the optimization of advanced phase change cooling technologies.

The in-vessel retention system and ex-vessel retention system are very important to the safety of nu-clear power plants under severe accidents. While the success of such safety systems relies on well un-derstanding the corresponding physical mechanisms of boiling heat transfer and critical heat flux (CHF). Challenges till remain in accurately predicting the subcooled flow boiling curve especially in the low-pressure and low-flow conditions due to its complex boiling phenomenon. The present study introduces a theoretical model to predict the boiling curve and critical heat flux for subcooled flow boiling in in-clined downward heated rectangular channel. The proposed model well estimates the transition from forced convection, isolated bubble nucleate boiling to fully developed boiling regime by considering the growth and interaction of bubbles. Through probability analysis of bubbles' interaction, the proportion of heat flux in different boiling regimes is determined. In addition, the flow boiling CHF is predicted based on the probability analysis of dry spots. The new model is validated by the subcooled flow boil-ing experiments with vertical single-side heated channel under low-pressure and low-flow conditions. The predicted boiling curves are consistent with experimental results corresponding to different thermal-hydraulic parameters, such as pressure, mass flux, inlet subcooling and wall wettability (hydrophilic and hydrophobic), and the prediction error of CHF is within & PLUSMN;15%. Furthermore, the inclination effect on CHF is validated by the subcooled flow boiling experiments in inclined channel with the inclination angle varying from 0 & DEG; to 90 & DEG;, which shows the good applicability of the developed model., QC 20230706

Wettability patterning is a promising method to manipulate bubble dynamics in microscale boiling systems, allowing the transfer of large heat fluxes at low wall temperatures. Herein, we experimentally investigate the enhancement of flow boiling through exploitation of contact-line pinning using superbiphilic wettability patterns with a range of geometries and orientations. We compare the boiling performance on symmetrical (i.e., circular, square, and diamond-shaped) and asymmetrical (i.e., triangular) superhydrophobic patches and also create rings and chevrons through insertion of self-similar, recursive super hydrophilic cut-outs. Two main principles for boiling heat transfer enhancement are demonstrated: first, the ease of bubble departure from the superhydrophobic patches is shown to depend upon the interaction between the local contact angle and the bubble's tilt due to hydrodynamic drag; second, we find that ring-shaped superhydrophobic patches may trap droplets inside the forming bubbles, thus supplementing the heat transfer coefficient and critical heat flux through latent heat. Through application of these principles, the heat transfer coefficient and critical heat flux of heterogeneous surfaces was enhanced over the homogeneous analogues by 62% and 24%, respectively. Finally, we establish and validate a general model to estimate the ease of bubble departure through the use of geometric arguments.

Herein, we apply grayscale lithography to produce novel gradient microstructures, demonstrating the enhancement of flow boiling through the manipulation of drag forces upon the forming bubbles. The use of engineered 3D anisotropy allowed directional control of boiling enhancement, with respective critical heat flux and peak heat transfer coefficient enhancements of up to 20% and 93% observed over the unstructured substrate. It is our suspicion that the sheltering of bubbles inside of cavity microstructures deteriorates their departure characteristics, and that the combination of induced turbulence and available surfaces to capture passing liquid is beneficial for protruding microstructures. We demonstrate the promising novel implementation of gradient microstructures in the boiling discipline, which may serve as a useful foundation for future work. The enclosed approach to boiling enhancement shows a potential future direction for engineered boiling microstructures, wherein bubble dynamics are directly manipulated on bespoke, 3-dimensional substrates, allowing the transfer of large heat fluxes at low wall temperatures.

Flow boiling in microchannels offers a promising and attractive solution for thermal management of electronic devices and power systems. In this paper, microchannels composed of a hydrophilic surface with hydrophobic dots were studied to characterize the effects of non-wetting fraction and pitch distance of adjacent dots on flow boiling heat transfer and pressure drop. The pitch distances ranging from 122 µm to 172 µm were studied. Using deionized water as the working fluid, highly subcooled flow boiling experiments were conducted at different mass fluxes ranging from 41.1 to 246.6 kg/m2s over a heat flux up to 146.2 W/cm 2. Bubble dynamics and flow patterns were visualized using a high-speed camera. It was found that bubbles coalesced more easily, and flow patterns transited faster in the microchannel with smaller pitch distance. Heat transfer coefficient (HTC), critical heat flux (CHF) and pressure drop were found to significantly rely on the pitch distance of dots and the mass flux. Furthermore, based on a force-balance model, bubble detached diameters were predicted in hydrophilic, hydrophobic and wettability-patterned microchannels, respectively. This provides a useful insight to optimize the wettability pattern design and then improve flow boiling heat transfer in a microchannel.

Flow boiling in microchannels offers a promising and attractive solution for thermal management of electronic devices and power systems. In this paper, microchannels composed of a hydrophilic surface with hydrophobic dots were studied to characterize the effects of non-wetting fraction and pitch distance of adjacent dots on flow boiling heat transfer and pressure drop. The pitch distances ranging from 122 µm to 172 µm were studied. Using deionized water as the working fluid, highly subcooled flow boiling experiments were conducted at different mass fluxes ranging from 41.1 to 246.6 kg/m2s over a heat flux up to 146.2 W/cm 2. Bubble dynamics and flow patterns were visualized using a high-speed camera. It was found that bubbles coalesced more easily, and flow patterns transited faster in the microchannel with smaller pitch distance. Heat transfer coefficient (HTC), critical heat flux (CHF) and pressure drop were found to significantly rely on the pitch distance of dots and the mass flux. Furthermore, based on a force-balance model, bubble detached diameters were predicted in hydrophilic, hydrophobic and wettability-patterned microchannels, respectively. This provides a useful insight to optimize the wettability pattern design and then improve flow boiling heat transfer in a microchannel.

Flow boiling in microchannels is a promising technique to achieve large cooling rates and high heat transfer coefficients for electronic cooling. This work reports an effective approach to enhance flow boiling heat transfer in microchannels through expansion areas. Flow boiling experiments were conducted on plain microchannels and microchannels with single and three expansion areas for comparison. Experimental results show that the flow boiling heat transfer coefficient can be effectively enhanced by up to 43.3% through adding three expansion areas in microchannels, while the pressure drop variation is within 3 kPa. Expansion areas significantly reduce the inlet temperature fluctuation, indicating the suppression of flow boiling instability in microchannels. As disclosed by the visualization of flow patterns, the improved heat transfer coefficient can be attributed to the enhanced bubble nucleation and formation of a nonuniform liquid layer near the corners of expansion areas. © 2022 Elsevier Masson SAS

Flow boiling of refrigerants in narrow channels has a great potential to cope with future challenges in the thermal management of high heat flux electronics. This study investigated the flow boiling heat transfer of R1234yf, R1234ze(E), and R134a in multi-microchannels as the applied heat flux was varied from boiling incipience to the critical heat flux. A parametric analysis on the two-phase local heat transfer coefficients was conducted for channel mass fluxes from 415 kg/m2s to 1153 kg/m2s and saturation temperatures of 30.5 ∘C and 40.5 ∘C. The test section comprised 17 channels with inlet restrictions. The channels were 298 µm wide, 1176 µm deep and 10 mm long. Three boiling regimes (I, II, III) were identified according to the observed effect of the heat and mass flux. In boiling regime I, the two-phase local heat transfer coefficient was independent of the mass flux and increased with the heat flux. In boiling regime II, the flow departed from this behavior, and a significant effect of the mass flux appeared. Finally, in boiling regime III, the two-phase local heat transfer coefficient decreased steeply with the heat flux. Flow visualization showed that a predominantly bubbly flow was present in the channels during boiling regime I. In boiling regime II, churn flow was the predominant flow regime, but appeared with more frequent vapor slugs as the heat flux was increased, eventually leading to alternating churn and wispy-annular flow. In boiling regime III, the flow regime alternated between churn and wispy-annular, with a dryout incipience leading to the progressive decrease of the two-phase local heat transfer coefficient until the achievement of the critical heat flux. The experimental data for boiling regimes I and II, amounting to 2504 local values, were compared against the prediction of selected literature correlations.

Flow boiling is a promising method for the cooling of sensitive computational and industrial components, facilitating the transportation of large quantities of heat at near-constant temperature and in a small form factor. The prevention of vapor film formation is a fundamental challenge for the enhancement of boiling systems, and an impetus therefore exists for the discovery of new techniques to segregate nucleating bubbles during their formation. Herein, we utilize the strong capillary forces generated by nanostructures to pin the liquid/vapor interface in three dimensions and thereby control the coalescence and flow interactions of developing bubbles. We demonstrate this principle on both symmetrical and asymmetrical superbiphilic microstructures, showing enhancement of peak heat transfer coefficient by 81% and 113%, respectively, when compared to the best superhydrophilic and superhydrophobic analogues. Our approach shows a potential future direction for engineered boiling micro/nanostructures, wherein bubble dynamics are directly manipulated on bespoke, three-dimensional substrates.

Flow boiling in microchannels offers a promising and attractive solution for thermal management of electronic devices and power systems. In this paper, microchannels composed of a hydrophilic surface with hydrophobic dots were studied to characterize the effects of non-wetting fraction and pitch distance of adjacent dots on flow boiling heat transfer and pressure drop. The pitch distances ranging from 122 µm to 172 µm were studied. Using deionized water as the working fluid, highly subcooled flow boiling experiments were conducted at different mass fluxes ranging from 41.1 to 246.6 kg/m2s over a heat flux up to 146.2 W/cm 2. Bubble dynamics and flow patterns were visualized using a high-speed camera. It was found that bubbles coalesced more easily, and flow patterns transited faster in the microchannel with smaller pitch distance. Heat transfer coefficient (HTC), critical heat flux (CHF) and pressure drop were found to significantly rely on the pitch distance of dots and the mass flux. Furthermore, based on a force-balance model, bubble detached diameters were predicted in hydrophilic, hydrophobic and wettability-patterned microchannels, respectively. This provides a useful insight to optimize the wettability pattern design and then improve flow boiling heat transfer in a microchannel.

Flow boiling in microchannels is a promising technique to achieve large cooling rates and high heat transfer coefficients for electronic cooling. This work reports an effective approach to enhance flow boiling heat transfer in microchannels through expansion areas. Flow boiling experiments were conducted on plain microchannels and microchannels with single and three expansion areas for comparison. Experimental results show that the flow boiling heat transfer coefficient can be effectively enhanced by up to 43.3% through adding three expansion areas in microchannels, while the pressure drop variation is within 3 kPa. Expansion areas significantly reduce the inlet temperature fluctuation, indicating the suppression of flow boiling instability in microchannels. As disclosed by the visualization of flow patterns, the improved heat transfer coefficient can be attributed to the enhanced bubble nucleation and formation of a nonuniform liquid layer near the corners of expansion areas. © 2022 Elsevier Masson SAS

This experimental study investigated narrow and high aspect ratio multi-microchannels that were micro-machined in copper with thin separating walls during saturated flow boiling of refrigerants. The hypothesis was that these channels could increase the footprint critical heat flux and support the future development of thermal management systems for power electronics and other electronic packages. The measured footprint critical heat flux was as high as 678.5 W/cm2, which is twice as high as other investigations in the literature concerning saturated flow boiling of refrigerants. Two test samples were fabricated with a footprint area of (10 × 10) mm. The first had 25 rectangular channels (198 μm wide, 1167 μm high). The second had 17 rectangular channels (293 μm wide, 1176 μm high). The experimental investigation covered low-GWP replacement refrigerants (R1234yf, R1234ze(E)) as well as the well-examined R134a serving as a benchmark. A large data bank was obtained with 432 data points covering a wide range of typical inlet subcooling (1.3–14.7) K and mass fluxes (333–1260) kg/m2s as well as two nominal saturation temperatures (30 and 40) °C.The effect of inlet subcooling was found to be consistently significant at the higher mass fluxes. This was contradictory to other investigations that found moderate or insignificant effects. The result is suggested to be attributed to the two-phase stability induced by the upstream throttle valve, inlet orifices, isolated refrigerant in the inlet and outlet plenums as well as the short heated length causing higher importance of orifice to channel pressure drop importance. Finally, a new modified Katto and Ohno correlation including the effect of subcooling was proposed, achieving a 4.0% mean average error and predicting 93.3% of the data points within 10% error.

This paper presents the results of experimental investigations carried out with surfactant Tween 20 during flow boiling inside internal threaded copper tube. The boiling fluid was water with different concentrations of surfactant added to reduce surface tension. The effect of various concentration, heat flux, mass flux and inclination angle of tube on heat transfer coefficient was investigated. The experiments were conducted keeping constant fluid inlet temperature of 80°C. The test runs were carried out by varying mass fluxes from 100 to 300 Kg/m2s, heat fluxes from 10 to 90 kW/m2and inclination angles of 0° and 90°.The addition of small amount of surfactant showed heat transfer enhancement upto 100 ppm and above this concentration heat transfer enhancement was insignificant. The result showed that the maximum heat transfer coefficient increased by 16.34 % for horizontal position at mass flux 100 kg/m2s. The entire study is carried out at low mass flux values.

This paper presents the results of experimental investigations carried out with surfactant Tween 20 during flow boiling inside internal threaded copper tube. The boiling fluid was water with different concentrations of surfactant added to reduce surface tension. The effect of various concentration, heat flux, mass flux and inclination angle of tube on heat transfer coefficient was investigated. The experiments were conducted keeping constant fluid inlet temperature of 80°C. The test runs were carried out by varying mass fluxes from 100 to 300 Kg/m2s, heat fluxes from 10 to 90 kW/m2and inclination angles of 0° and 90°.The addition of small amount of surfactant showed heat transfer enhancement upto 100 ppm and above this concentration heat transfer enhancement was insignificant. The result showed that the maximum heat transfer coefficient increased by 16.34 % for horizontal position at mass flux 100 kg/m2s. The entire study is carried out at low mass flux values.

This paper presents the results of experimental investigations carried out with surfactant Tween 20 during flow boiling inside internal threaded copper tube. The boiling fluid was water with different concentrations of surfactant added to reduce surface tension. The effect of various concentration, heat flux, mass flux and inclination angle of tube on heat transfer coefficient was investigated. The experiments were conducted keeping constant fluid inlet temperature of 80°C. The test runs were carried out by varying mass fluxes from 100 to 300 Kg/m2s, heat fluxes from 10 to 90 kW/m2and inclination angles of 0° and 90°.The addition of small amount of surfactant showed heat transfer enhancement upto 100 ppm and above this concentration heat transfer enhancement was insignificant. The result showed that the maximum heat transfer coefficient increased by 16.34 % for horizontal position at mass flux 100 kg/m2s. The entire study is carried out at low mass flux values.

This paper presents the results of experimental investigations carried out with surfactant Tween 20 during flow boiling inside internal threaded copper tube. The boiling fluid was water with different concentrations of surfactant added to reduce surface tension. The effect of various concentration, heat flux, mass flux and inclination angle of tube on heat transfer coefficient was investigated. The experiments were conducted keeping constant fluid inlet temperature of 80°C. The test runs were carried out by varying mass fluxes from 100 to 300 Kg/m2s, heat fluxes from 10 to 90 kW/m2and inclination angles of 0° and 90°.The addition of small amount of surfactant showed heat transfer enhancement upto 100 ppm and above this concentration heat transfer enhancement was insignificant. The result showed that the maximum heat transfer coefficient increased by 16.34 % for horizontal position at mass flux 100 kg/m2s. The entire study is carried out at low mass flux values.

This paper presents the results of experimental investigations carried out with surfactant Tween 20 during flow boiling inside internal threaded copper tube. The boiling fluid was water with different concentrations of surfactant added to reduce surface tension. The effect of various concentration, heat flux, mass flux and inclination angle of tube on heat transfer coefficient was investigated. The experiments were conducted keeping constant fluid inlet temperature of 80°C. The test runs were carried out by varying mass fluxes from 100 to 300 Kg/m2s, heat fluxes from 10 to 90 kW/m2and inclination angles of 0° and 90°.The addition of small amount of surfactant showed heat transfer enhancement upto 100 ppm and above this concentration heat transfer enhancement was insignificant. The result showed that the maximum heat transfer coefficient increased by 16.34 % for horizontal position at mass flux 100 kg/m2s. The entire study is carried out at low mass flux values.

This paper presents the results of experimental investigations carried out with surfactant Tween 20 during flow boiling inside internal threaded copper tube. The boiling fluid was water with different concentrations of surfactant added to reduce surface tension. The effect of various concentration, heat flux, mass flux and inclination angle of tube on heat transfer coefficient was investigated. The experiments were conducted keeping constant fluid inlet temperature of 80°C. The test runs were carried out by varying mass fluxes from 100 to 300 Kg/m2s, heat fluxes from 10 to 90 kW/m2and inclination angles of 0° and 90°.The addition of small amount of surfactant showed heat transfer enhancement upto 100 ppm and above this concentration heat transfer enhancement was insignificant. The result showed that the maximum heat transfer coefficient increased by 16.34 % for horizontal position at mass flux 100 kg/m2s. The entire study is carried out at low mass flux values.

This paper presents the results of experimental investigations carried out with surfactant Tween 20 during flow boiling inside internal threaded copper tube. The boiling fluid was water with different concentrations of surfactant added to reduce surface tension. The effect of various concentration, heat flux, mass flux and inclination angle of tube on heat transfer coefficient was investigated. The experiments were conducted keeping constant fluid inlet temperature of 80°C. The test runs were carried out by varying mass fluxes from 100 to 300 Kg/m2s, heat fluxes from 10 to 90 kW/m2and inclination angles of 0° and 90°.The addition of small amount of surfactant showed heat transfer enhancement upto 100 ppm and above this concentration heat transfer enhancement was insignificant. The result showed that the maximum heat transfer coefficient increased by 16.34 % for horizontal position at mass flux 100 kg/m2s. The entire study is carried out at low mass flux values.

This study aims to characterize experimentally the heat transfer in micro-milled multi-microchannels copper heat sinks operating with flow boiling, in the attempt to contribute to the development of novel and high heat flux thermal management systems for power electronics. The working fluid was R-134a and the investigation was conducted for a nominal outlet saturation temperature of 30 degrees C. The microchannels were 1 cm long and covered a square footprint area of 1 cm(2). Boiling curves starting at low vapor quality and average heat transfer coefficients were obtained for nominal channel mass fluxes from 250 kg/m(2)s to 1100 kg/m(2)s. The measurements were conducted by gradually increasing the power dissipation over a serpentine heater soldered at the bottom of the multi-microchannels, until a maximum heater temperature of 150 degrees C was reached. Infrared thermography was used for the heater temperature measurements, while high-speed imaging through a transparent top cover provided visual access over the entire length of the channels. The average heat transfer coefficient increased with the dissipated heat flux until a decrease dependent on hydrodynamic effects occurred, possibly due to incomplete wall wetting. Depending on the channel geometry, a peak value of 200 kW/m(2)K for the footprint heat transfer coefficient and a maximum dissipation of 620 W/cm(2) at the footprint with a limit temperature of 150 degrees C could be obtained, showing the suitability of the investigated geometries in high heat flux cooling of power electronics. The experimental dataset was used to assess the prediction capability of selected literature correlations. The prediction method by Bertsch et al. gave the best agreement with a mean absolute percent error of 24.5%, resulting to be a good design tool for flow boiling in high aspect ratio multi-microchannels as considered in this study., QC 20210401

Organic Rankine cycle technology has gained worldwide acceptance as an efficient way to utilize low-grade heat sources. Plate heat exchangers are the most common type of heat exchanger employed as evaporators in small-scale organic Rankine cycle units, in which a high saturation temperature is the prevailing working condition. However, there is a lack of research on high temperature flow boiling in plate heat exchangers. This paper presents an experimental analysis on flow boiling heat transfer and pressure drop characteristics in a plate heat exchanger, and the development of prediction methods for the heat transfer coefficient and frictional pressure drop. Seven working fluids, R134a, R236fa, R245fa, R1234ze(E), R1233zd(E), propane and isobutane, were tested at the reduced pressures of 0.45, 0.55 and 0.65, corresponding to saturation temperatures ranging from 55 °C to 141 °C, and various mass fluxes. Two heat transfer mechanisms, nucleate boiling and thin-film evaporation, were identified in the heat transfer processes of the different working fluids, due to the diversity in their thermo-physical properties. Moreover, the results indicate that propane and isobutane have higher heat transfer coefficients than the other working fluids, while R236fa has the lowest heat transfer coefficient. The frictional pressure drops show the same characteristics for all the working fluids, increasing with the increase of the vapor quality and mass flux and the decrease of the saturation temperature. A superposition model presented in the paper achieves a good prediction for the heat transfer data, with a 12.8% mean absolute percentage deviation. A correlation developed in a previous work by the authors enables a prediction with an 11.1% mean absolute percentage deviation for the pressure drop data. The prediction methods presented in the paper will facilitate the modelling and design of plate heat exchanger evaporators in organic Rankine cycle units.

This study aims to characterize experimentally the heat transfer in micro-milled multi-microchannels copper heat sinks operating with flow boiling, in the attempt to contribute to the development of novel and high heat flux thermal management systems for power electronics. The working fluid was R-134a and the investigation was conducted for a nominal outlet saturation temperature of 30 ∘C. The microchannels were 1 cm long and covered a square footprint area of 1 cm2. Boiling curves starting at low vapor quality and average heat transfer coefficients were obtained for nominal channel mass fluxes from 250 kg/m2s to 1100 kg/m2s. The measurements were conducted by gradually increasing the power dissipation over a serpentine heater soldered at the bottom of the multi-microchannels, until a maximum heater temperature of 150 ∘C was reached. Infrared thermography was used for the heater temperature measurements, while high-speed imaging through a transparent top cover provided visual access over the entire length of the channels. The average heat transfer coefficient increased with the dissipated heat flux until a decrease dependent on hydrodynamic effects occurred, possibly due to incomplete wall wetting. Depending on the channel geometry, a peak value of 200 kW/m2K for the footprint heat transfer coefficient and a maximum dissipation of 620 W/cm2 at the footprint with a limit temperature of 150 ∘C could be obtained, showing the suitability of the investigated geometries in high heat flux cooling of power electronics. The experimental dataset was used to assess the prediction capability of selected literature correlations. The prediction method by Bertsch et al. gave the best agreement with a mean absolute percent error of 24.5%, resulting to be a good design tool for flow boiling in high aspect ratio multi-microchannels as considered in this study.

Flow boiling is a method of phase change heat transfer used widely in electronics cooling, refrigeration, air conditioning, and other areas where stable temperatures are needed. An area of interest is spaceflight systems, where efficient heat transfer is desired to minimize mass, power requirements, and cost. When compared to terrestrial gravity conditions, the heat transfer of flow boiling in microgravity typically depreciates. This depreciation has been documented across multiple experimental studies performed by teams using different fluids, tube geometries, and flow regimes over the past three decades. Though select experimental microgravity flow boiling heat transfer data are available in the literature, holistic results are sparse due to the cost and limited availability of microgravity research. The two-phase heat transfer mechanisms responsible for the depreciation are therefore not well known, and so heat transfer models for variable gravity flow boiling do not exist. The goal of the proposed study is to develop models for flow boiling heat transfer through a tube as a function of gravity by identifying the effect of gravity on different heat transfer mechanisms. The scope of this proposal involves modeling three microgravity flow regimes (bubbly, slug, and annular flow) to serve as baseline predictions for flow boiling heat transfer without the influence of gravity. Additional gravity effects can be identified using partial and hyper-gravity data. Experiments have been performed aboard parabolic flights and on the ground at various flow rates, heating rates, and inlet subcoolings in microgravity, hyper-gravity, Lunar gravity, Martian gravity, and terrestrial gravity. Results from the experiments showed that negligible slip velocity plays an important role in modeling flow boiling heat transfer. Simulations using modified single-phase models of an accelerating flow were performed which predicted microgravity flow boiling heat transfer well in the nucleate b

Flow boiling in a microchannel is an effective and attractive solution for thermal management. In this work, flow boiling was performed in microchannels with various wettability-patterned dotted surfaces. Pitch distances of dots were changed to study the wettability pattern effects on microchannel flow boiling. Glass-silicon-glass microchannels integrated with internal platinum heaters and various wettability patterns were fabricated and characterized using MEMS techniques. The transparent top of the channel allowed visualization of bubble dynamics and flow patterns. The hydraulic diameter of the microchannel is 338 μm, the side length of hydrophobic dots is 72 μm and the pitch distances of hydrophobic dots change from 122 μm to 172 μm. Bubble dynamics and flow patterns were visualized using a high-speed camera and a microscope. Heat transfer performance, including boiling curves and heat transfer coefficients (HTC), were explored experimentally under different mass flux. It is found that bubbles coalesce more frequently and flow pattern transitions are more intense with the decrease of pitch distance. In the low mass flux region, better heat transfer capability can be observed on the surface with larger pattern pitch distance, while with the increase of mass flux, a microchannel with smaller dot pitch distance presents better heat dissipation ability. Moreover, the critical heat flux (CHF) decreases with the decrease of dot pitch distance. The reasons behind these phenomena are related to the bubble coalescence, bubble density, and flow movement. © 2020, Avestia Publishing. All rights reserved.

An experimental study was performed to investigate both flow patterns and heat transfer during flow boiling of propylene (R-1270) at a saturation temperature of 25 °C in a small circular tube with an inner diameter of 1.0 mm. The heat flux was varied from 5 to 60 kW/m2 and the mass flux ranged from 240 up to 480 kg/m2 s, with vapor quality ranging from 0.01 to 0.99. Five flow patterns were identified: plug flow, slug flow, churn flow, wavy- and (smooth) annular flow. The influence of the heat flux, mass flux, vapor quality and flow pattern on the heat transfer coefficient are quantified and analyzed. The new experimental heat transfer coefficients were compared with predictions of correlations found in literature and recommendations are presented.

Steady state flow boiling experiments were carried out on several heated tubes with outer diameter 9.14 mm at outlet pressures 120, 200 and 300 kPa, inlet temperatures 64, 78 and 91 °C and approximately 400, 500, 600 and 800 kg/(m2·s) mass flux entering the vertically aligned test annulus until critical heat flux (CHF) was reached. The tubes were made of Inconel 625 with a length of 400 mm. The Inconel tubes were tested in three different modifications as smooth, abraded with 150 grit sandpaper and bead blasted. Multiple experiments were repeated on the same specimen to investigate the effect of surface characteristic changes (i.e., wettability, roughness and oxide layer morphology) on the occurrence of CHF. Despite the changes in initial wettability, the CHF dependency was not clearly observed, however, the changes in roughness led to an increase in CHF. The total number of 115 experimental runs were collected and the results were also compared with other literature experimental data.

Steady state flow boiling experiments were carried out on several heated tubes with outer diameter 9.14 mm at outlet pressures 120, 200 and 300 kPa, inlet temperatures 64, 78 and 91 °C and approximately 400, 500, 600 and 800 kg/(m2·s) mass flux entering the vertically aligned test annulus until critical heat flux (CHF) was reached. The tubes were made of Inconel 625 with a length of 400 mm. The Inconel tubes were tested in three different modifications as smooth, abraded with 150 grit sandpaper and bead blasted. Multiple experiments were repeated on the same specimen to investigate the effect of surface characteristic changes (i.e., wettability, roughness and oxide layer morphology) on the occurrence of CHF. Despite the changes in initial wettability, the CHF dependency was not clearly observed, however, the changes in roughness led to an increase in CHF. The total number of 115 experimental runs were collected and the results were also compared with other literature experimental data.

Steady state flow boiling experiments were carried out on several heated tubes with outer diameter 9.14 mm at outlet pressures 120, 200 and 300 kPa, inlet temperatures 64, 78 and 91 °C and approximately 400, 500, 600 and 800 kg/(m2·s) mass flux entering the vertically aligned test annulus until critical heat flux (CHF) was reached. The tubes were made of Inconel 625 with a length of 400 mm. The Inconel tubes were tested in three different modifications as smooth, abraded with 150 grit sandpaper and bead blasted. Multiple experiments were repeated on the same specimen to investigate the effect of surface characteristic changes (i.e., wettability, roughness and oxide layer morphology) on the occurrence of CHF. Despite the changes in initial wettability, the CHF dependency was not clearly observed, however, the changes in roughness led to an increase in CHF. The total number of 115 experimental runs were collected and the results were also compared with other literature experimental data.

Steady state flow boiling experiments were carried out on several heated tubes with outer diameter 9.14 mm at outlet pressures 120, 200 and 300 kPa, inlet temperatures 64, 78 and 91 °C and approximately 400, 500, 600 and 800 kg/(m2·s) mass flux entering the vertically aligned test annulus until critical heat flux (CHF) was reached. The tubes were made of Inconel 625 with a length of 400 mm. The Inconel tubes were tested in three different modifications as smooth, abraded with 150 grit sandpaper and bead blasted. Multiple experiments were repeated on the same specimen to investigate the effect of surface characteristic changes (i.e., wettability, roughness and oxide layer morphology) on the occurrence of CHF. Despite the changes in initial wettability, the CHF dependency was not clearly observed, however, the changes in roughness led to an increase in CHF. The total number of 115 experimental runs were collected and the results were also compared with other literature experimental data.

Steady state flow boiling experiments were carried out on several heated tubes with outer diameter 9.14 mm at outlet pressures 120, 200 and 300 kPa, inlet temperatures 64, 78 and 91 °C and approximately 400, 500, 600 and 800 kg/(m2·s) mass flux entering the vertically aligned test annulus until critical heat flux (CHF) was reached. The tubes were made of Inconel 625 with a length of 400 mm. The Inconel tubes were tested in three different modifications as smooth, abraded with 150 grit sandpaper and bead blasted. Multiple experiments were repeated on the same specimen to investigate the effect of surface characteristic changes (i.e., wettability, roughness and oxide layer morphology) on the occurrence of CHF. Despite the changes in initial wettability, the CHF dependency was not clearly observed, however, the changes in roughness led to an increase in CHF. The total number of 115 experimental runs were collected and the results were also compared with other literature experimental data.

Steady state flow boiling experiments were carried out on several heated tubes with outer diameter 9.14 mm at outlet pressures 120, 200 and 300 kPa, inlet temperatures 64, 78 and 91 °C and approximately 400, 500, 600 and 800 kg/(m2·s) mass flux entering the vertically aligned test annulus until critical heat flux (CHF) was reached. The tubes were made of Inconel 625 with a length of 400 mm. The Inconel tubes were tested in three different modifications as smooth, abraded with 150 grit sandpaper and bead blasted. Multiple experiments were repeated on the same specimen to investigate the effect of surface characteristic changes (i.e., wettability, roughness and oxide layer morphology) on the occurrence of CHF. Despite the changes in initial wettability, the CHF dependency was not clearly observed, however, the changes in roughness led to an increase in CHF. The total number of 115 experimental runs were collected and the results were also compared with other literature experimental data.

Steady state flow boiling experiments were carried out on several heated tubes with outer diameter 9.14 mm at outlet pressures 120, 200 and 300 kPa, inlet temperatures 64, 78 and 91 °C and approximately 400, 500, 600 and 800 kg/(m2·s) mass flux entering the vertically aligned test annulus until critical heat flux (CHF) was reached. The tubes were made of Inconel 625 with a length of 400 mm. The Inconel tubes were tested in three different modifications as smooth, abraded with 150 grit sandpaper and bead blasted. Multiple experiments were repeated on the same specimen to investigate the effect of surface characteristic changes (i.e., wettability, roughness and oxide layer morphology) on the occurrence of CHF. Despite the changes in initial wettability, the CHF dependency was not clearly observed, however, the changes in roughness led to an increase in CHF. The total number of 115 experimental runs were collected and the results were also compared with other literature experimental data.

Steady state flow boiling experiments were carried out on several heated tubes with outer diameter 9.14 mm at outlet pressures 120, 200 and 300 kPa, inlet temperatures 64, 78 and 91 °C and approximately 400, 500, 600 and 800 kg/(m2·s) mass flux entering the vertically aligned test annulus until critical heat flux (CHF) was reached. The tubes were made of Inconel 625 with a length of 400 mm. The Inconel tubes were tested in three different modifications as smooth, abraded with 150 grit sandpaper and bead blasted. Multiple experiments were repeated on the same specimen to investigate the effect of surface characteristic changes (i.e., wettability, roughness and oxide layer morphology) on the occurrence of CHF. Despite the changes in initial wettability, the CHF dependency was not clearly observed, however, the changes in roughness led to an increase in CHF. The total number of 115 experimental runs were collected and the results were also compared with other literature experimental data.

Steady state flow boiling experiments were carried out on several heated tubes with outer diameter 9.14 mm at outlet pressures 120, 200 and 300 kPa, inlet temperatures 64, 78 and 91 °C and approximately 400, 500, 600 and 800 kg/(m2·s) mass flux entering the vertically aligned test annulus until critical heat flux (CHF) was reached. The tubes were made of Inconel 625 with a length of 400 mm. The Inconel tubes were tested in three different modifications as smooth, abraded with 150 grit sandpaper and bead blasted. Multiple experiments were repeated on the same specimen to investigate the effect of surface characteristic changes (i.e., wettability, roughness and oxide layer morphology) on the occurrence of CHF. Despite the changes in initial wettability, the CHF dependency was not clearly observed, however, the changes in roughness led to an increase in CHF. The total number of 115 experimental runs were collected and the results were also compared with other literature experimental data.

An experimental study was performed to investigate both flow patterns and heat transfer during flow boiling of propylene (R-1270) at a saturation temperature of 25 °C in a small circular tube with an inner diameter of 1.0 mm. The heat flux was varied from 5 to 60 kW/m2 and the mass flux ranged from 240 up to 480 kg/m2 s, with vapor quality ranging from 0.01 to 0.99. Five flow patterns were identified: plug flow, slug flow, churn flow, wavy- and (smooth) annular flow. The influence of the heat flux, mass flux, vapor quality and flow pattern on the heat transfer coefficient are quantified and analyzed. The new experimental heat transfer coefficients were compared with predictions of correlations found in literature and recommendations are presented.

Flow boiling in microchannels is a promising technique for cooling high power-density electronic devices. In this study, a microchannel heat sink using ammonia as working fluid is developed, and its cooling efficiency is experimentally investigated under nonuniform high-density heat flux, which well simulates a practical heat dissipation scenario for microthermal systems. Diamond with high thermal conductivity of 1500 W/m·K is selected as the microchannel heat sink material. A total of 37 parallel triangular microchannels with aspect ratio of 5, channel length of 45 mm and hydraulic diameter of 280 μm are uniformly engineered on the diamond film by laser ablation processing. The significance of diamond substrate as a heat spreader to minimize the nonuniformity of heat flux imposed by a central hotspot is verified. The influences of heat flux and mass flux on the cooling efficiency are experimentally investigated. An optimal range of outlet vapor quality from 0.10 to 0.13 is found, within which the minimum heat source temperature can be achieved. Notably, the microchannel heat sink is capable of managing a central hotspot with heat flux of 267 W/cm2 while maintaining the heat source temperature at 53.3 °C for a mass flux of 320 kg/m2s.

A mechanistic model for bubble dynamics in flow boiling that is based on a force balance approach for a growing bubble is introduced. It considers evaporation of the microlayer underneath the bubble, thermal diffusion and condensation around the bubble cap as well as dynamic inclination and contact angles between the bubble and the heating wall. It requires no recalibration of parameters to predict the bubble growth. Validation against different experimental flow boiling data was carried out with no case-dependent recalibration and yielded good agreement. The simulations confirmed the dependency of bubble departure and lift-off diameters on different parameters, such as heat flux, liquid properties, subcooling temperature, system pressure, inclination angle of channel, channel geometry and mass flow rate.

A general heat transfer correlation to predict the thermal performance of zeotropic mixture flow boiling is essential for an optimal evaporator design in thermodynamic cycles using zeotropic mixtures as working fluids. This work aims at developing a new general correlation to predict the flow boiling heat transfer of zeotropic mixtures in horizontal plain tubes with a significantly better predictive performance than those of the existing correlations. In order to achieve this goal, a database containing 2091 experimental data points of macroscale flow boiling experiments with zeotropic mixtures in horizontal plain tubes from 22 independent research groups was collected. The predictive performances of the existing correlations were evaluated by comparing the predicted values calculated by the existing correlations with the experimental data points. Based on an analysis of the physical phenomena and on the results of the comparison of the existing correlations, two new heat transfer correlations were derived by following two different approaches: (i) modifying existing correlations and introducing a new dimensionless number (the ratio of the temperature glide of the mixture to the saturation temperature), and (ii) using dimensionless analysis coupled with multiple regression. The comparison results for the existing correlations indicate that most of the correlations overestimate the heat transfer coefficient, and only three correlations present a mean absolute percentage deviation below 50%, the lowest being 44.2%. The results suggest that the new correlation developed following the first approach, yields a mean absolute percentage deviation of 29.0%, while the correlation developed by following the second approach yields a mean absolute percentage deviation of 24.6%.

The Critical Heat Flux (CHF) causes a rapid reduction of heat transfer coefficient with a rapid increase of cladding temperature, which may induce physical failure of the heated material. Understanding CHF phenomena and reliable prediction of the boiling behavior are needed to design a heat transfer system including nuclear reactors. Due to the complex nature of CHF, it is still an active research topic of interest. With an increasing interest in Accident Tolerant Fuel (ATF), CHF of ATF is essential topic of study for the detailed design of the fuel – cladding element and for the reactor safety analysis. In this study, flow boiling experiments were conducted at atmospheric pressure with deionized water in an internal-tubular channel with a uniformly heated surface. With this heat flux controlled system, steady-state and transient CHF experiments were performed to gain a mechanistic understanding of the boiling nature. The experiments were conducted under the following conditions: (1) mass flux, G of 200 - 2000, (2) inlet subcooling of 54, 34, and 10, (3) atmospheric pressure of 84 kPa. Experiments at low mass flux were repeated on the same specimen to observe the effect of the surface characteristic change on CHF. Despite the notable changes in surface wettability and roughness, change in flow CHF was not observed, which implies the limited significance of surface characteristic on CHF under flow condition. To mechanistically understand observed CHF differences among the tested materials, material thermal parameters were considered. In addition, a statistical approach was made to analyze CHF differences among the tested materials covering a wide range of mass flow rate, which demonstrated a reducing relative CHF differences with increasing mass flow rate. It implies limited significance of material-sensitivity for the design of fuel and cladding and steady-state safety. Yet, observed CHF differences at low mass flux require a more detailed thermal-hydraul

This experimental study investigated the application of fluid flow pulsations for in-tube flow boiling heat transfer enhancement in an 8 mm smooth round tube made of copper. The fluid flow pulsations were introduced by a flow modulating expansion device and were compared with continuous flow generated by a stepper-motor expansion valve in terms of the time-averaged heat transfer coefficient. The cycle time ranged from 1 s to 7 s for the pulsations, the time-averaged refrigerant mass flux ranged from 50 kg m(-2) s(-1) to 194 kg m(-2) and the time-averaged heat flux ranged from 1.1 kW m(-2) to 30.6 kW m(-2). The time-averaged heat transfer coefficients were reduced from transient measurements immediately downstream of the expansion valves with 2 K and 20 K subcooling upstream, resulting in inlet vapor qualities at 0.05 and 0.18, respectively, and covered the saturated flow boiling range up to the dry-out inception. Averaged results of the considered range of vapor qualities, refrigerant mass flux and heat flux showed that the pulsations at low cycle time (1 s) improved the time-averaged heat transfer coefficients by 5.6% and 2.2% for the low and high subcooling, respectively. However, the pulsations at high cycle time (7 s) reduced the time-averaged heat transfer coefficients by 1.8% and 2.3% for the low and high subcooling, respectively, due to significant dry-out when the flow-modulating expansion valve was closed. Furthermore, the flow pulsations were visualized by high-speed camera to assist in understanding the time-periodic flow regimes and the effect they had on the heat transfer performance., QC 20180816

Flow boiling heat transfer in microspaces is of significant interest for thermal management applications, where the latent heat of phase change offers an efficient method to dissipate large heat fluxes in a compact device, such as a heat spreader or a heat pipe. However, a significant challenge for the implementation of microscale phase change heat spreaders is associated with micro/nano flow instabilities due to insufficient micro/nano bubble removal, leading to local liquid dry-out that severely limits the heat removal efficiency. Furthermore, for the various heat transfer mechanisms involved, it is difficult to predict the location of nucleation sites at which the onset of nucleate boiling (ONB) occurs. In this paper, we study the bubble dynamics and heat transfer in a flow boiling microchannel with a wettability patterned channel surface and compare it with a homogeneous hydrophilic channel surface. The bubble nucleation site and coalescence are studied experimentally. The effects of wettability patterned surfaces on bubble nucleation and contact line dynamics in a microchannel will be investigated utilizing high speed visualization techniques. Effects of mass flux on flow boiling in a wettability patterned microchannel are studied. Wettability patterned surfaces are manufactured on glass/silicon wafers. It is found that the heat transfer coefficient can be significantly enhanced by wettability patterns in comparison to a hydrophilic surface. The surface temperature with wettability patterned surfaces is lower than hydrophilic surfaces. The detailed experimental setup and results are presented in the paper.

Flow boiling heat transfer in microspaces is of significant interest for thermal management applications, where the latent heat of phase change offers an efficient method to dissipate large heat fluxes in a compact device, such as a heat spreader or a heat pipe. However, a significant challenge for the implementation of microscale phase change heat spreaders is associated with micro/nano flow instabilities due to insufficient micro/nano bubble removal, leading to local liquid dry-out that severely limits the heat removal efficiency. Furthermore, for the various heat transfer mechanisms involved, it is difficult to predict the location of nucleation sites at which the onset of nucleate boiling (ONB) occurs. In this paper, we study the bubble dynamics and heat transfer in a flow boiling microchannel with a wettability patterned channel surface and compare it with a homogeneous hydrophilic channel surface. The bubble nucleation site and coalescence are studied experimentally. The effects of wettability patterned surfaces on bubble nucleation and contact line dynamics in a microchannel will be investigated utilizing high speed visualization techniques. Effects of mass flux on flow boiling in a wettability patterned microchannel are studied. Wettability patterned surfaces are manufactured on glass/silicon wafers. It is found that the heat transfer coefficient can be significantly enhanced by wettability patterns in comparison to a hydrophilic surface. The surface temperature with wettability patterned surfaces is lower than hydrophilic surfaces. The detailed experimental setup and results are presented in the paper.

Flow patterns and local heat transfer coefficients were investigated experimentally during flow boiling of R-290 in a tube with inner diameter equal to 1.0 mm. The tests were conducted at heat fluxes ranging from 5 to 60 kW/m2, mass flux from 240 to 480 kg/(m2s) at 25 °C saturation temperature. Both mass flux and heat flux have a significant influence on the heat transfer coefficient. A new flow pattern identification method is proposed, using non-dimensional parameters based on phase velocities and heat flux. The main flow patterns identified were bubbly, plug, slug, churn, wavy- and smooth-annular flow. Convective boiling was found to be predominant for most flow patterns. Moreover, the resulting heat transfer coefficients were compared with predicting correlations available in the literature.

Flow boiling heat transfer in microspaces is of significant interest for thermal management applications, where the latent heat of phase change offers an efficient method to dissipate large heat fluxes in a compact device, such as a heat spreader or a heat pipe. However, a significant challenge for the implementation of microscale phase change heat spreaders is associated with micro/nano flow instabilities due to insufficient micro/nano bubble removal, leading to local liquid dry-out that severely limits the heat removal efficiency. Furthermore, for the various heat transfer mechanisms involved, it is difficult to predict the location of nucleation sites at which the onset of nucleate boiling (ONB) occurs. In this paper, we study the bubble dynamics and heat transfer in a flow boiling microchannel with a wettability patterned channel surface and compare it with a homogeneous hydrophilic channel surface. The bubble nucleation site and coalescence are studied experimentally. The effects of wettability patterned surfaces on bubble nucleation and contact line dynamics in a microchannel will be investigated utilizing high speed visualization techniques. Effects of mass flux on flow boiling in a wettability patterned microchannel are studied. Wettability patterned surfaces are manufactured on glass/silicon wafers. It is found that the heat transfer coefficient can be significantly enhanced by wettability patterns in comparison to a hydrophilic surface. The surface temperature with wettability patterned surfaces is lower than hydrophilic surfaces. The detailed experimental setup and results are presented in the paper.

Flow patterns and local heat transfer coefficients were investigated experimentally during flow boiling of R-290 in a tube with inner diameter equal to 1.0 mm. The tests were conducted at heat fluxes ranging from 5 to 60 kW/m2, mass flux from 240 to 480 kg/(m2s) at 25 °C saturation temperature. Both mass flux and heat flux have a significant influence on the heat transfer coefficient. A new flow pattern identification method is proposed, using non-dimensional parameters based on phase velocities and heat flux. The main flow patterns identified were bubbly, plug, slug, churn, wavy- and smooth-annular flow. Convective boiling was found to be predominant for most flow patterns. Moreover, the resulting heat transfer coefficients were compared with predicting correlations available in the literature.

This experimental study investigated the application of fluid flow pulsations for in-tube flow boiling heat transfer enhancement in an 8 mm smooth round tube made of copper. The fluid flow pulsations were introduced by a flow modulating expansion device and were compared with continuous flow generated by a stepper-motor expansion valve in terms of the time-averaged heat transfer coefficient. The cycle time ranged from 1 s to 7 s for the pulsations, the time-averaged refrigerant mass flux ranged from 50 kg m−2 s−1 to 194 kg m−2 s−1 and the time-averaged heat flux ranged from 1.1 kW m−2 to 30.6 kW m−2. The time-averaged heat transfer coefficients were reduced from transient measurements immediately downstream of the expansion valves with 2 K and 20 K subcooling upstream, resulting in inlet vapor qualities at 0.05 and 0.18, respectively, and covered the saturated flow boiling range up to the dry-out inception. Averaged results of the considered range of vapor qualities, refrigerant mass flux and heat flux showed that the pulsations at low cycle time (1 s) improved the time-averaged heat transfer coefficients by 5.6% and 2.2% for the low and high subcooling, respectively. However, the pulsations at high cycle time (7 s) reduced the time-averaged heat transfer coefficients by 1.8% and 2.3% for the low and high subcooling, respectively, due to significant dry-out when the flow-modulating expansion valve was closed. Furthermore, the flow pulsations were visualized by high-speed camera to assist in understanding the time-periodic flow regimes and the effect they had on the heat transfer performance.

Embedded cooling—an emerging thermal management paradigm for electronic devices—has motivated further research in compact, high heat flux, cooling solutions. Reliance on phase-change cooling and the associated two-phase flow of dielectric refrigerants allows small fluid flow rates to absorb large heat loads. Previous research has shown that dividing chip-scale microchannels into parallel arrays of channels with novel manifold designs can produce very high chip-scale heat transfer coefficients with low pressure drops. In such manifolded microchannel coolers, the coolant typically flows at relatively high velocities through U-shaped microgap channels, producing centripetal acceleration forces on the fluid that can be several orders of magnitude larger than gravity. Furthermore, the manifolded microchannels consist of high aspect ratio rectangular channels, short length to hydraulic diameter ratios (L/Dh < 100), and step-like inlet restrictions. The existing literature provides only limited information on each of these effects, and nearly no information on the combined effects, on fluid flow and heat transfer performance. This study provides fundamental insights into the impact of such channel features and coupled fluid forces on two-phase flow regimes and their associated transport rates. Moreover, because the flows in manifolded microchannel chip coolers are very small and optically inaccessible, a custom visualization test section was developed. The visualization test section was supported by a custom two-phase flow loop, which incorporated three power supplies, two chillers, and two dozen thermofluid sensors with live data monitoring. The fluid flow and wall heat transfer in the visualization test section was simultaneously imaged with a high-speed optical camera and a high-speed thermal camera. The results were reported with maps of the flow regime, wall temperature, wall temperature fluctuation, superheat, heat flux, and heat transfer coefficients under varying h

Multi bubbles interaction and merger in a micro-channel flow boiling has been numerically studied. Effects of mass flux (56, 112, 200, and 335 kg/m2 ∗ s), wall heat flux (5, 10, and 15 kW/m2) and saturated temperature (300.15 and 303.15 K) are investigated. The coupled level set and volume of fluid (CLSVOF) method and non-equilibrium phase model are implemented to capture the two-phase interface, and the lateral merger process. It is found that the whole transition process can be divided to three sub-stages: sliding, merger, and post-merger. The evaporation rate is much higher in the first two stages due to the boundary layer effects in. Both the mass flux and heat flux affect bubble growth. Specifically, the bubble growth rate increase with the increase of heat flux, or the decrease of mass flux., Correction in: International Communications in Heat and Mass Transfer, vol. 81 pages R1-R1, DOI: 10.1016/j.icheatmasstransfer.2017.01.001, WOS: 000397359200014, Scopus 2-s2.0-85013448409 Accidently republished as DOI: 10.1016/j.icheatmasstransfer.2016.12.011 Scopus 2-s2.0-85006515437QC 20170208

The optimal design of the evaporator is one of the key issues to improve the efficiency and economics of organic Rankine cycle units. The first step in studying the evaporator design is to understand the thermal-hydraulic performance of the working fluid in the evaporator of organic Rankine cycles. This paper is aimed at obtaining flow boiling heat transfer and pressure drop characteristics in a plate heat exchanger under the working conditions prevailing in the evaporator of organic Rankine cycle units. Two hydrofluoroolefins R1234yf and R1234ze, and one hydrofluorocarbon R134a, were selected as the working fluids. The heat transfer coefficients and pressure drops of the three working fluids were measured with varying saturation temperatures, mass fluxes, heat fluxes and outlet vapour qualities, which range from 60°C to 80°C, 86 kg/m2 s to 137 kg/m2 s, 9.8 kW/m2 to 36.8 kW/m2 and 0.5 to 1, respectively. The working conditions covered relatively high saturation temperatures (corresponding reduced pressures of 0.35-0.74), which are prevailing in organic Rankine cycles yet absent in the open literature. The experimental data were compared with existing correlations, and new correlations were developed that are more suitable for evaporation in organic Rankine cycles. The experimental results indicate that heat transfer coefficients are strongly dependent upon the heat flux and saturation temperature. Moreover, the results suggest better thermal-hydraulic performance for R1234yf than the other two working fluids at the same saturation temperatures. With the new heat transfer and pressure drop correlations, agreements within ±25% were obtained for experimental data in similar experiments with high saturation temperatures.