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This paper presents an improved flow pattern map for flow boiling in horizontal micro-fin tubes considering the effect of helix angle. The proposed map is modified from the one developed for horizontal smooth tubes by Wojtan et al. (2005) and takes the early transition to annular flow due to micro-fin geometry into account. Visualization of R410A flow boiling is carried out through the transparent smooth and micro-fin tubes with 0°, 10° and 18° helix angle and plotted on the proposed map for comparison. The result shows that the flow pattern map for micro-fin tube with 0° helix angle is generally similar to that for the smooth tube. For the tube with 10° and 18° helix angle, the annular flow occurs at lower vapor quality and lower mass flux. The proposed map shows good agreements when compared with the visualization and reflects the downward shift (to lower mass flux and vapor quality conditions) of the SW-WA transition with the increase of the helix angle. In order to further validate the proposed model, the flow pattern transitions calculated from the equations are also compared with the experimental data of the micro-fin tube in literature. The results show that the modified map increase the prediction accuracy of the flow patterns in the micro-fin tube.

Heat transfer measurements of flow boiling Carbon Dioxide (CO$_2$) in vertical upward direction have been carried out with a dedicated test facility at the European Organization for Nuclear Research (CERN). The investigation covers saturation temperatures within a range of $-25 \degree \textrm{C} \leq T_{\mathrm{sat}} +5 \degree \textrm{C}$, mass velocities from 100kg m$^{-2}$ s$^{-1}$ $\leq G \leq $ 450 kg m$^{-2}$ s$^{-1}$ and heat fluxes of 5.3 kW m$^{-2}$ and 11.4 kW m$^{-2}$. The experiments have been conducted in a vertical upward evaporator of 8 mm inner diameter and 8 m length and a database of more than 1900 measurements has been compiled within the present study. The heat transfer mechanism is dominated by the nucleate boiling contribution and dryout is observed as a function of saturation temperature, mass velocity and heat flux. A correlation is proposed to predict the dryout inception within the experimental range where the onset of dryout has been observed. The results suggest that most commonly used heat transfer prediction models underpredict the heat transfer mechanisms of CO$_2$. Moreover, the heat transfer coefficients of CO$_2$ increase in vertical upward direction, compared to the data of horizontal studies. For that reason, a vertical multiplier is suggested to capture the trends of vertical upflow with two existing prediction models.

Nucleate boiling on downward facing heating walls such as those found in IVR-ERVC and core catcher systems of advanced LWRs is characterized by bubbles of markedly different scales (small bubbles and vapor slugs). The heat transfer governing mechanism of deformable vapor slugs is different from that of small spherical bubbles. Therefore, the two-fluid wall-boiling model developed based on the characteristic of small-scale nucleate bubbles might not be sufficient for prediction of wall boiling heat transfer on a downward facing heating surface. In this article, a hybrid wall-boiling model is formulated and implemented in the solution framework of the Eulerian-Eulerian model coupled with a large-scale interface model and adaptive interface sharpening scheme. This approach gives a realistic representation of the physical phenomena present in flow boiling on a downward facing heating wall, and the validation against experimental data showed similar performance to the two-fluid model in terms of the prediction of wall boiling heat transfer.

Due to the different parameters affecting critical heat flux (CHF), four models were proposed in this study to estimate this parameter. The presented models were validated for mass fluxes of 100 400 [kW/m2] was 4.5%. Using the unique correlation for microchannels instead of using the Webber number coefficient, the model accuracy significantly increased.

In this study, symmetrical and dichotomous fractal flow channel designs with various configurations in terms of the number of channels being split into smaller channels, were investigated experimentally for their flow boiling heat transfer performance with FC-72 as the working fluid. A multi-channel parallel channel design depicted as Parallel was used as a benchmark. The channels were fabricated by Selective Laser Melting (SLM). Three mass fluxes were studied, viz., 600, 900 and 1200 kg/m2⋅s. The peak heat transfer coefficient and pressure drop were investigated. High-speed visualization studies were also employed to examine the two-phase behaviors in the channels. The fractal flow channels were found to perform comparably with the Parallel channel at low mass fluxes but achieved higher heat transfer coefficients at higher mass fluxes, with the design n = 2, i.e., channel splitting into smaller channels twice, being the highest. The pressure drops of the fractal flow channel designs are higher than the Parallel channel due to their longer channel lengths, and the flow impacting multiple bifurcations at an angle, with n = 4, i.e., channel splitting into smaller channels four times, experiencing the highest pressure drop for all mass fluxes. Visualization studies show that fractal flow channels cause an early annular flow, enhancing heat transfer. Flow reversals in the Parallel channel were observed especially at higher mass fluxes, which suggest no enhancement in the heat transfer coefficient. The fractal flow channels, however, do not experience any flow reversal which could be attributed to the acceleration of the two-phase flow after each bifurcation, which in turn increases the inertia force to overcome the force from vapor expansion. The visualization studies also suggest that increasing the complexity does not result in better flow boiling heat transfer performance. This is due to the multiple flow separations which resulted in no two-phase heat transfer interaction and thus, under-utilization of the entire channel surface area for heat transfer. A comparison with a general correlation for flow boiling gives mean absolute errors of 31.6% and 35.3% for the fractal flow channels and Parallel, respectively. National Research Foundation (NRF) Accepted version The SLM 250 HL equipment used in this research is supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Medium-Sized Centre funding scheme.

The flow boiling characteristics of ammonia/miscible oil mixture are experimentally studied. Experiments are conducted in an 8 mm tube at oil concentration from 0 to 5.78%, mass flux from 51 to 99.5 kg m −2 s −1 , heat fluxes of 9 and 21 kW m −2 , and temperature from −5.5 to −5 °C. The heat transfer is basically deteriorated with the influence of miscible oil. Specifically, the effect of miscible oil is small in low vapor quality region, and with a rise in vapor quality, the oil causes significant heat transfer deterioration at smaller mass flux. Meanwhile, Gungor and Winterton (1986) correlation can predict 98.5% of the experimental data within ±30% error band. In addition, the experimental results reveal that the pressure drop are insensitive to the oil concentration.

International audience; An experimental study of heat transfer during R245fa flow boiling is carried out in a 1.6 mm inner diameter circular channel (198 original data points). The test section is composed of a sapphire tube coated with ITO, which enables a total transparency of the evaporator in the visible spectrum. The acquisition of the outer wall temperature field is ensured by means of an infrared camera and the heat transfer coefficient is calculated by means of a local thermal model. The effect of the inclination on the heat transfer is presented and discussed for various vapour qualities and mass velocities and a saturation temperature of 81°C, corresponding to a Bond number of 4.1. For each experimental condition, the mean heat transfer coefficient is calculated for various inclinations from the vertical downward flow (-90°) to the vertical upward flow (+90°). The variation of the heat transfer coefficient are first presented and analyzed in the horizontal case. The heat transfer coefficient variations with the flow parameters are interpreted in terms of nucleate and convective boiling and these observations are confirmed by the visualisation of the nucleation phenomenon in the evaporator. The inclination has almost no effect on the heat transfer and this lack of effect is finally interpreted in terms of shift between the annular to intermittent flow pattern transition coupled with the nucleate to convective boiling transition.

Surfactant additives can prevent bubbles merging and change the flow mode in flow boiling. This work presented the effect of bubbles merging on flow boiling heat transfer performance of water. Flow boiling experiment and lattice Boltzmann method (LBM) were carried out in a rectangular channel. With a high speed camera, the flow boiling process has been recorded. D2Q9 model and the Peng-Robinson (P-R) equation of two phase state (EOS) were used to simulate flow boiling process. The present experimental results showed that surfactant addition could enhance the flow boiling heat transfer and increased the critical heat flux (CHF). And there were more nucleation sites, larger bubbles density and smaller bubble size in surfactant solution. The present simulation results showed that the flow boiling without bubbles merging has higher and more stable average Nu number. The bubbles merging made the middle nucleation sites lose effectiveness. It implies that preventing bubbles merging is another factor of surfactant addition enhancing boiling heat transfer.

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%.

Critical Heat Flux (CHF) is the maximal limit of heat flux in two-phase nucleate boiling heat transfer; therefore, an understanding of CHF under a wide range of conditions is important for safe system operation. In this work, CHF experiments are conducted over a range of subatmospheric system pressures in a vertical square channel that is heated on one side. The experimental conditions cover a pressure range of 20 kPa to 108 kPa, a mass flux range of 45–190 kg/m2-s, and an inlet subcooling range of 0–14 K. Heat flux is gradually increased until an excursion of the wall temperature occurs, indicating CHF. For the experimental conditions considered, CHF increases with rising system pressure, mass flux, and inlet subcooling, although the effects of mass flux and inlet subcooling are weak. A new correlation for CHF is developed and found to predict the data with an average error of ±15.6%.

Ammonia-sodium thiocyanate (NH3-NaSCN) solution is an efficient absorption refrigeration working pair. This paper presents the experimental results of the flow boiling heat transfer coefficients (HTCs) of ammonia-sodium thiocyanate solution in horizontal tubes with three different inner diameters (4, 6 and 8 mm). The experimental measurements were conducted with different heat fluxes, mass fluxes, boiling temperatures and vapor qualities to determine their impact on the flow boiling heat transfer coefficients of the tested working pair. Comparison of NH3-NaSCN and NH3-LiNO3 solution has been performed and higher flow boiling HTCs of NH3-NaSCN solution than that of NH3-LiNO3 solution are obtained under similar working conditions. The results also indicate that the heat flux has the most significant influence on the flow boiling HTCs. The updated flow boiling HTCs prediction correlation for ammonia-salt working pair proposed by the authors’ previous work has been validated by the present experimental results. The present study help to extend the basic researching results of NH3-NaSCN working pair, which is of great significance to the design and optimization of generators in the absorption refrigeration units, and will ultimately be beneficial to energy saving for refrigeration area.

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.

The relation between flow instability and friction pressure drop of flow boiling in multichannel was investigated by visualization experiments and theoretical analysis. Based on visualization observation, both slug flow and liquid-plug flow were the dominating flow patterns for all operation conditions, and combing with flow instability simultaneous. With theoretical analysis, the vapor slug extension growth velocity would affect the shear force which was the main reason for friction pressure drop. In addition, the channels number and the channel length had significant effects on flow instability. With the simplified vapor growth model and the analysis on the collected database statistically, the modified friction factor, finter, and the structure factor, fstr, were proposed for the revised homogenous friction pressure drop correlation. By comparing with the database, the modified pressure drop correlation predicted the collected experimental data in a good agreement.

Oblique-finned microchannels have been reported to enhance heat transfer performance and suppress the flow boiling instability, but they can also cause an increase in pressure drop. Since the stepped microchannels are beneficial to reduce the pressure drop with enhanced heat transfer, herein we designed stepped oblique-finned microchannels (SOFMC) that uptake the comprehensive merit of oblique fins and steps, and compared the heat transfer and fluid regime with rectangular microchannels (RMC). The visualization experiment was performed using deionized water as the coolant with the temperature of 25 °C at different mass fluxes (400 kg/m2s, 600 kg/m2s and 800 kg/m2s) for different heat fluxes varying from 13.6–418.8 W/cm2. The stratified slug flow and stratified annular flow were observed in SOFMC, which were essential to enhance the heat transfer. Compared to RMC, the SOFMC showed a better heat transfer performance and a smaller pressure drop at high heat flux. The increase of nucleation sites, intensive disturbance by oblique fins and additional flow area along the flow direction for bubbles provided by steps contributed to suppress the flow boiling instability and enhance the flow boiling heat transfer for SOFMC. The large pressure drop and severe flow boiling instabilities are the main problems for the further improvement of heat transfer performance; hence the integration of steps and oblique fins in the microchannels provides promising potential for high heat flux dissipation due to the enhancement of heat transfer, reduction of pressure drop and suppression of flow boiling instabilities.

Bubbly flow patterns for subcooled flow boiling of carbon dioxide at high reduced pressures were experimentally studied. Development of boiling patterns starting from scant nucleations of bubbles to fully developed boiling was observed. The pressure effect was examined, and it was found that as the critical pressure approached, the bubble diameter and growth rate decreased. Moreover, the bubbles became elongated at reduced pressures above 0.95 as a result of the diminishing surface tension. In addition, bubble diameter, velocity, and shape were measured, and results were compared to the asymptotic solutions of the Rayleigh-Plesset equation. Isolated bubbles and bubble-bubble interactions were considered. It was found that the asymptotic thermally driven solution of the Rayleigh-Plesset equation predicted the bubble dynamics well for isolated bubbles, but deviations were noted when bubbles started interacting. The Mikic model was modified to account for bubble-to-bubble interactions and an additional term was added to the model that is linearly proportional to time (i.e., a·t). A modified model based on experimentally fitted coefficients was developed and used effectively to predict additional set of experiments. The modified model reduced the average error of the diameter from 13.5 μm to 6.6 μm — a reduction of 51%.

Pressure drop in microchannels can be considerably high, which can cause significant changes in thermophysical properties. Thus, the existing pressure drop modelling approach for flow boiling in microchannels, which neglects the effects of property gradients and flashing, suffers from large errors in case of high pressure drop. To mitigate this, a new modelling approach has been developed for comprehensive evaluation of pressure drop along a microchannel with flow boiling. It is based on the separated flow model, incorporating the evaluation of thermophysical properties at local pressure, the effect of flashing on thermodynamic quality, and the effect of heat flux on the two-phase multiplier. Relations based on the Clausius–Clapeyron Equation were employed for evaluation of local thermophysical properties. This approach was applied along with different empirical correlations from the literature to predict flow boiling pressure drop. Experiments were conducted on flow boiling of DI water in a single microchannel of hydraulic diameter 111 µm, with mass fluxes of 205–410 kg/ m2s and heat fluxes of 7.5–19.5 W/cm2, and the pressure drop recorded was 10–36 kPa. It was found that the new modelling approach improves the accuracy of pressure drop predictions significantly in case of high pressure drop.

Flow boiling pressure drop and heat transfer performance of R452A in horizontal single tubes have not been investigated or reported yet in scientific literature. In this paper, local heat transfer coefficient and pressure gradient for R452A are experimentally investigated. Tests have been carried-out in a 6.0 mm internal diameter single horizontal stainless-steel tube from the onset of boiling up to the dry-out conditions at 25°C as average saturation temperature. Mass flux values of 150 and 400 kg/m2s and heat fluxes equal to 10 and 20kW/m2 are considered for the experimental analysis.

In this article, we report on a theoretical analysis and experimental investigations on critical heat flux (CHF) in subcooled flow boiling. Commonly, CHF is considered as a local phenomenon. A validated CHF- concept recently developed in our group indicated that CHF may be initiated in two different ways, that is, locally and globally. We designed and conducted an experiment to verify this hypothesis. The experimental results agree well with the expectations from our CHF- modelling and confirm the two mechanisms. Following that, we continued to clarify the role of different parameters, such as channel orientation, channel length and hydraulic diameter. The new concept of CHF is useful to explain and predict CHF at conditions of low pressure and low fluid velocity.

To improve the closure relations employed for component-scale Computational Fluid Dynamics simulation of boiling flows, a first-principles method for predicting bubble departure diameters in flow boiling has been developed. The proposed method uses minimisation of the free energy of a system in thermodynamic equilibrium to predict the contact angle and the resistance to sliding of a vapour bubble attached to a surface in the presence of a forced liquid flow. Predictions of the new method are compared with measurements from existing experimental databases, and agreement with data is shown to be comparable or superior to that obtained with previous bubble departure models that have generally used a force-balance approach. The main advantages of the energy-based method over the previous force-based methods are that its formulation is simpler, and that the new model does not require the use of ad hoc tunable parameters to define force terms, or geometrical characteristics of the attached bubble such as its base area, which cannot be confirmed experimentally. This increases confidence in the validity of the new approach when applied outside the rather limited range of current test data on bubble departure in flow boiling.

The study of the enhancement of critical heat flux (CHF) has a long history, with extensive experiments devoted in the last several decades to searching for approaches to expand thermal margin. It continues to be a promising topic in heat-transfer research fields, such as nuclear energy engineering, where higher heat-removal efficiency can improve the ultimate heat utilization as well as delay or even avoid the occurrence of accidents or system failures. In this paper, we present a comprehensive overview of CHF enhancement experiments, focusing on four broad categories of approaches. The first approach considered is amelioration of fluid properties by adding nanoparticles into the base fluid, by which both flow boiling and pool boiling achieve significant improvements in CHF. The second prevailing method recently advanced to increase CHF is surface modification, where various nano/microstructures on the surface were fabricated by advanced techniques. Third, we review the effect of various modified channel structures on the boiling process. Finally, some creative and notable hybrid approaches are presented. Based on this review of the state-of-the-art in CHF enhancement, future research directions are also proposed. In addition, according to the conclusions and results of the comparisons of many experiments, we provide optimal design options for heat-transfer systems. Moreover, following the analysis of experimental data, a better understanding of the highly complex mechanism of enhanced CHF can be realized.

Dynamics of pressure drop oscillations during flow boiling inside minichannel with diameter of 1 mm was experimentally investigated. High speed camera was used to identify two-phase flow patterns. The chaotic pressure drop oscillations occurring with a low and high frequencies were observed. The analysis of the dynamics of chaotic pressure oscillations and recorded videos show that a certain dominant sequences of flow patterns can be identified. These sequences depend on the mean value of pressure drop. The map of dynamical properties of two-phase flow patterns was constructed using the windowed recurrence quantification analysis. The clusters on this map identified the four kinds of dominant sequences of flow patterns. These were: liquid flow - small bubbles flow - confined bubbles flow, small bubbles flow - slug flow - reverse slug flow, churn flow - wavy annular flow - reverse slug flow, churn flow - wavy annular flow. There has been shown that each dominant sequence of flow boiling can exist during a certain time period in the non-stationary flow boiling, therefore such sequence can be treated as temporary boiling pattern.

In this study, flow boiling of refrigerant R245fa is investigated in a microgap of height 200 μm populated with a staggered pin fin array of diameter 150 μm and spacing 200 μm. For heat fluxes up to 498 W/cm2, mass flux values up to 7896 kg/m2 s, and inlet temperatures of 13 °C and 18 °C, average two-phase heat transfer coefficient up to 60 kW/m2 K are measured. High speed flow visualizations at frame rate of 2229 fps elucidate the flow boiling patterns inside the microgaps, including bubbly and foggy that are generated in the pin finned area. Surface temperatures are measured for heat fluxes up to 498 W/cm2 which enable determination of heat transfer characteristics.

The self-rewetting fluid is the liquid which have an anomalous surface tension increasing with temperature when the temperature exceeds a certain value. This particular property can cause the subcooled liquid to be drawn towards the heated surface if a dry patch appears, thus making it possible for the self-rewetting fluids to become promising working fluids. This paper presents an overview of the recent developments of the research on boiling heat transfer using self-rewetting fluids. Thermophysical properties, pool and flow boiling experiments and heat pipe applications of self-rewetting fluids have been reported in this paper. It can be found that the use of self-rewetting fluids in a wide range of applications appears promising. However, further detailed and valuable theoretical and numerical investigations are necessary for us to better understand the boiling phenomenon of self-rewetting fluids.

This work studies gas–liquid flow of small bubbles ( −1 ) that is encountered in human bloodstream during Decompression Sickness and is also relevant to two-phase applications such as flow boiling in macro-channels. Two fundamental parameters are experimentally investigated: Bubble Size Distribution ( BSD ) and void fraction. Experiments are conducted in co-current upward bubbly flow. Water and blood simulant are used as test liquids, while bubble size is controlled using prescribed surfactant ( SDS ) concentrations. BSD s are determined employing digital image analysis of bubbly flow images captured at three radial positions across the flow cross-section. Volumetric and cross-sectional area averaged void fraction is measured at three axial locations along the flow by Differential Pressure ( ΔP ) and Electrical Resistance Tomography ( ERT ), respectively. BSD s are well-fitted by the log-normal distribution. ERT and ΔP measurements are in fair agreement, with void fraction being practically equal along the flow. The influence of gas/liquid phase velocities and surfactant concentration on the measured void fraction and BSD s’ average value and width is discussed in detail. Interestingly, high SDS concentration in blood simulant results in the formation of bubble clusters, whose role on the examined parameters is investigated.

In previous research on nuclear reactors, the effect of irradiation on heat transfer has rarely been studied. We investigated the electron beam irradiation effect on downward-facing flow boiling heat transfer and critical heat flux. All the critical heat flux values in flow boiling decreased after irradiation, particularly at low doses, whereas heat transfer coefficient almost remained the same. Low dose electron beam irradiation substantially increased the nucleation sites on the heated copper surface, and the critical heat flux was inversely related to the nucleation site density in downward-facing flow boiling.

Critical heat flux (CHF) experiments are conducted to investigate the flow boiling CHF enhancement by additives and a heater material for a wide range of exit qualities and void fractions and for pressure conditions of 1 and 2 bar using the rectangular curved flow channel. For the wettability improvement to enhance the CHF, the additives of boric acid and tri-sodium phosphate (TSP, Na3PO4 12H2O) and the heater material of low alloy steel (SA508) are used. The trend of the CHF enhancement in terms of the flow conditions is discussed for the slug flow regime. The CHF is observed in the bubbly-slug and slug flow through flow visualization works. For each condition of pressure, the CHF is enhanced in most of the experimental cases under relatively high exit quality condition due to the improvement effect of surface wettability. The CHF is not enhanced under the relatively low exit quality condition, because the sufficient liquid supply outplays the difference of wettability. The relationship between CHF enhancement ratio and exit quality, which is observed in each pressure condition, is not effective in comparison with those for the different pressure condition. The void fraction is additionally considered to discuss the CHF enhancement according to void condition. The CHF enhancement ratio is clearly varied by the void fraction for all pressure conditions. The CHF enhancement ratio is proportional to the void fraction for the void fraction of ∼0.5 and above, and it is close to 1 in the void fraction of ∼0.5.

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.

The prediction of flow boiling is essential owing to its broad applications and complexity. However, the characteristics of flow boiling have not been fully elucidated. In particular, the selection of a model for the bubble lift-off diameter on which the evaporative heat flux is described by a cubic dependence remains ambiguous. In this study, we measure the bubble lift-off diameter on a flat heated surface in a vertical square channel under subcooled flow boiling conditions. Experiments are performed at the atmospheric pressure for mass fluxes of 75–200 kg/m2s, heat fluxes of 50–300 kW/m2, and an inlet subcooling degree of 4.5 K using demineralized water. Bubbles are categorized into lifting-off and ejecting bubbles based on their detaching behaviors from the wall. More than 1600 bubbles are analyzed to measure the lift-off diameter and lift-off velocity. The experimental results show a significant difference in the bubble lift-off diameter between the two types of bubbles mentioned above. We propose a modified model of the bubble lift-off diameter that can predict the average size based on the population ratio of each group.

Flow boiling in microchannels have widely been studied in order to design more efficient cooling systems with numerical simulations forming a crucial part to deal with the areas that cannot be investigated experimentally. Previously published research largely focussed on the behaviour of a single bubble. Here, we focus on the behaviour of multiple bubbles. In this study, the influence of the distance between the bubbles (liquid slug length) is investigated in both an axisymmetric and a planar domain for two and three bubbles present. In this regard, an interface-tracking adaptive mesh refinement model was implemented to improve simulation time. Results show that the heat transfer was improved with sequential bubbles, and a 50% increase in heat transfer coefficient was observed for the cases investigated with three bubbles present. The heat transfer also improved the closer the bubbles were together.

Flow boiling has been applied in various conventional and microscale systems owing to its high efficiency. A major flow boiling system drawback is two-phase flow instabilities that can cause heat transfer performance deterioration and mechanical vibration. The origin of two-phase flow instabilities has not been identified. In this study, we classified sliding and ejecting bubbles based on bubble behavior. We observed that the periodic behavior of bubbles can be induced by the existence of ejecting bubbles. Furthermore, we found that flow instability can occur when the periodic behavior of bubbles exists. This finding can advance the understanding of two-phase flow instabilities, which should be controlled for improved flow boiling system application.

Surface wettability has a significant effect on the flow boiling heat transfer coefficient in metal foam filled tubes. In the present study, the flow boiling characteristics of metal foam filled tubes with different wettability were experimentally investigated, covering the surface wettability of uncoated, hydrophobic and hydrophilic surfaces. The results showed that, for the hydrophilic metal foam filled tubes, the heat transfer coefficient and pressure drop are decreased by 2%–18% and 4%–15%, respectively than those for the uncoated ones; for the hydrophobic metal foam filled tubes, both the heat transfer coefficient and pressure drop are larger than those for uncoated ones, and the increments in the heat transfer and pressure drop are 6%–30% and 8%–35%, respectively. Based on the experimental data, the correlations for heat transfer and pressure drop of flow boiling in metal foam filled tubes with different wettability were developed with the deviations of ±6.4% and ±9.7%, respectively.

Modifying the surface wettability at the solid-liquid interface can significantly improve flow boiling heat transfer. The graphene nanoplatelets (GNPs) and epoxy additives form nanocomposites coatings with tunable wettability (superhydrophilic, superhydrophobic and hydrophobic) via a thermal curing process. They are applied for enhancing the subcooled flow boiling heat transfer in a minichannel. By comparing with the uncoated surface, the superhydrophilic GNPs coating shows the highest boiling heat transfer coefficient with a maximum enhancement of 64.9 %. The nanocapillary-networks of the superhydrophilic GNPs coatings facilitate the fast water permeation effect and induce an effective filmwise boiling heat transfer. Moreover, the superhydrophilic GNPs coatings facilitate bubble detachment process and suppress the formation of dry spot during the boiling process. On the contrary, the hydrophobic and superhydrophobic GNPs coatings act as impervious blankets for the intercalation of water and deteriorate the flow boiling rates. The effects of the subcooled degree and the mass flux of the bulk flow on the performance enhancement are also investigated. This study aims to investigate the flow boiling enhancement attributed to the graphene functionalized coatings and provides interesting insights of the ultrafast water permeation mechanism of graphene into the subcooled flow boiling heat transfer.

Two-phase flow in mini/micro-channels can meet the high heat dissipation requirements of many state-of-the-art cooling solutions. However, there is lack of accurate universal methods for predicting parameters like pressure drop in these configurations. Conventional ways of predicting pressure drop employ either Homogeneous Equilibrium Model (HEM) or semi-empirical correlations. This current study leverages the availability of data collected over the past few decades to build several machine learning models to demonstrate the efficacy and ease of building and deploying such models. A consolidated database of 2787 data points for flow boiling pressure drop in mini/micro-channels is amassed from 21 sources that includes 10 working fluid, reduced pressures of 0.0006 –0.7766, hydraulic diameters of 0.15–5.35 mm, mass velocities of 33.1

In the present paper, a closed-loop thermosyphon cooling system for power electronics has been developed and tested. The evaporator is a multi-microchannel heat sink designed for high heat fluxes. The power electronics module is emulated by four independent electrical heaters, which can generate a non-uniform heat flux on the evaporator. The condenser is air-cooled and is tested in both natural convection and forced convection modes. R1234ze has been selected as the working fluid. Although in natural convection mode the cooling system experiences a maximum thermal resistance of approximately 0.6 K/W, the complete passiveness makes such a system very attractive. With values of heat fluxes of 107 W/cm(2) and 25 W/cm(2) equally distributed on the four heaters, the maximum heater temperature measured is 53 degrees C at a saturation temperature of 45 degrees C. In forced convection mode, two different values of air flow have been tested. A reduction of 50% on the overall thermal resistance has been measured with an imposed air flow rate of 87.8 m(3)/h, consuming only 5.29 W to drive the fan for dissipating 70.2 W. At this condition, the maximum heater temperature is reduced to 36 degrees C. In summary, it has been demonstrated that a thermosyphon coupled with air cooling can dissipate high heat fluxes of power electronics with no or little energy consumption. (C) 2017 Elsevier Ltd. All rights reserved.

Enhancing critical heat flux (CHF) of flow boiling without escalating pressure drop is highly desirable in thermal management of high power-density electronic devices. Usually, an improved CHF can be achieved by restricting flow or at a higher mass velocity, leading to a higher pressure drop. In this study, compared to the two-nozzle microchannel configuration, the improved microchannel configuration as detailed in the Part (I) of this study can enhance CHF without sacrificing pressure drop. In this part, CHF is experimentally evaluated together with the pressure drop with mass flux ranging from 120 kg/m 2 s to 600 kg/m 2 s. Compared to the two-nozzle configuration, our study shows that CHF can be enhanced up to 32% with a ∼53% reduction of pressure drop at a mass flux of 325 kg/m 2 s. The bubble collapse-removal process is significantly improved because more micronozzles are integrated. The enhanced pumping effect, which is created by rapid bubble collapse processes in the entire main channels, enables a more sustainable liquid supply and hence delays the CHF conditions. Moreover, two-phase flow in terms of pressure drop fluctuations is more stable owing to the effective management of bubble confinement in the entire channel.

Hydrocarbons (HCs) present favorable thermodynamic and transport properties that make them attractive for use in refrigeration, air conditioning and heat pump systems. The major issue related to HCs is their flammability and hence the realization of low charge heat transfer devices is a key factor for their market placement. Although flammable refrigerants call for charge minimization, propylene heat transfer data inside minichannels is new in the open literature. In the present paper, condensation heat transfer, flow boiling heat transfer and two-phase pressure drop measurements taken with propylene (R1270) inside a 0.96 mm diameter minichannel are presented and compared against correlations. A comparative analysis of the condensation performance of R1270, R290 and two HFCs is made with the objective of minimizing the exergy losses, due to the driving temperature difference between saturation and wall and to the frictional pressure drop. An estimation of the condenser refrigerant charge is also performed. The flow boiling heat transfer performance of R1270 is discussed in comparison to the one of R290.

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.

An experimental study of natural circulation flow instability has been carried out to investigate instability modes at low pressure. Water in the primary loop is heated in a single macro-channel and condensed in a tube-shell shape condenser by circulating coolant in the secondary loop. Two kinds of flow instability modes are identified: flow instability induced by flow boiling in the heated channel and flow instability induced by loop circulation. Flow boiling instabilities are found at three exit vapor qualities: xe

In the past few years, flow boiling has achieved a lot of attention in many industrial applications and thermal systems. To enhance the efficiency of the thermal systems, various working fluids like water, nanofluids, nanorefrigerants, etc. were used and the investigations were carried out using different operating conditions. The main purpose of this literature review is to study the heat transfer rate in different working fluids in the process of flow boiling. In this study, the systematic review of the numerical and experimental investigation of the flow boiling process has been carried out. A Systematic Literature Review (SLR) approach given by Denyer and Tranfield was utilized in this study. More than a hundred research articles were selected from the publications of Elsevier, SpringerLink, Taylor & Francis, and the American Society of Mechanical Engineers (ASME) and analyzed. It was concluded that the previous studies focused on the flow boiling heat transfer in different working fluids and the development of new correlations between the input and output parameters in the flow boiling investigation. Finally, the concluding remarks on the review of the numerical and experimental investigation of the flow boiling process are given.

Subcooled flow boiling is an important thermal hydraulic phenomenon in PWR core analysis, especially for accident analysis. Generally, RELAP5/MOD3.4 is adequate for modeling high-pressure subcooled boiling. However, there is a lack of assessing the code’s capability of modeling low-pressure sub-cooled boiling. In this paper, the capability of RELAP5/MOD3.4 of modeling low-pressure subcooled boiling is analyzed and assessed, and the results show that it is not applicable. In order to improve the performance, code modifications are made and evaluated. The assessment results show that the performance of modified code of modeling subcooled boiling is improved obviously.

Accepted: 2017-03-30, 資料番号: SA1160356000

Developments in many modern applications are encountering rapid escalation in heat dissipation, coupled with a need to decrease the size of thermal management hardware. These developments have spurred unprecedented interest in replacing single-phase hardware with boiling and condensation counterparts. While computational methods have shown tremendous success in modeling single-phase systems, their effectiveness with phase change systems is limited mostly to simple configurations. But, given the complexity of phase change phenomena important to many modern applications, there is an urgent need to greatly enhance the capability of computational tools to tackle such phenomena. This article will review the large pool of published papers on computational simulation of boiling and condensation. In the first part of the article, popular two-phase computational schemes will be discussed and contrasted, which will be followed by discussion of the different methods adopted for implementation of interfacial mass, momentum and energy transfer across the liquid-vapor interface. This article will then review papers addressing computational modeling of bubble nucleation, growth and departure, film boiling, flow boiling, and flow condensation, as well as discuss validation of predictions against experimental data. This review will be concluded with identification of future research needs to improve predictive computational capabilities, as well as crucial phase change phenomena found in modern thermal devices and systems that demand extensive computational modeling.

Bubble behavior in horizontal ultra-shallow microchannels displays distinct features in flow boiling heat transfer process, which is embodied as the fast elongation and deformation. It expands the range of the microlayer region and causes effective disturbances to the main flow, and therefore contributes to the enhancement in heat transfer. However, too fast deformation of the confined bubbles also results in rapid reducing of the liquid film thickness, causing the early appearance and short span of the annular flow. The enhancement in the heat transfer capability is then limited or even weakened. These distinct features lead to unsuitable predictions of classic correlations for the experimental data. The average error of the classic correlations exceeds 50% and the largest deviation reaches 125%. On the ground of the visualization experimental results, a theoretical model based on Chen’s model is proposed to calculate the flow boiling heat transfer coefficient in horizontal ultra-shallow microchannels. The presented model can be applied for both the elongation bubble flow and the annular flow as the heat transfer mechanism for these two flow patterns are specially considered respectively. Verified by the measured data, the average error of the model successfully drops to ±15%, from ±52.4% of the Chen’s model whose prediction fitted experimental data the best among the classic correlations.

The objective of the present work is to study the influence of curvature on local boiling heat transfer coefficient and two-phase pressure drop in helically coiled tubes with water as the working medium. In helically coiled tubes, the geometric parameters like pipe diameter and coil diameter vary the curvature and hence secondary flow which affects the heat transfer distribution and pressure drop. The present work investigates the characteristics of flow boiling in helically coiled tubes. Very less information is available on the influence of curvature on flow boiling system. The local wall temperature is measured not only in the axial direction (along the length of coil) but also in the circumferential direction using thermal imaging technique. Experiments are performed with six helically coiled test sections made of SS 304 tubes having inner diameters varying from 6 to 10 mm. The coil diameter to the tube diameter ratio ranges from 14 to 58 and coil pitch is 50 mm. The effect of geometric and operating parameters like tube diameter, coil diameter, heat flux and mass flux on local boiling heat transfer coefficient and two-phase pressure drop is analysed. In the subcooled region and in a low quality region, boiling heat transfer coefficient in helical coils is much higher (12%-100%) than in straight tubes. However, in a high quality region, the boiling heat transfer coefficient in helical coils is same as in straight tubes. A correlation is suggested for two-phase pressure drop in helical coils. (C) 2016 Elsevier Masson SAS. All rights reserved.

Gravity-driven cooling systems employing microchannel flow boiling can become a scalable and viable long term solution for the cooling of datacenter servers. In order to design this emerging type of cooling system, a new dynamic simulation tool using interconnected PDEs is described in the first part of the present paper followed by the validation of the modeling for both steady and dynamic regimes using a first-of-a-kind 15 cm-height thermosyphon test bench. A corrected error of 0.4 K was found for 7 different steady-state mean chip temperatures and heat load disturbances were predicted with the same 2-stage process presented here as the experimental results. The code was then used to predict the behavior of a mini-thermosyphon that would fit within the height of a 2U server. Multiple steady-state solutions were found with both stable and unstable operating states. A sensitivity study demonstrated that a 30% increase in the riser internal diameter led to 10-60% increase in the mass flow rate depending on the heat flux. Then, simulations with unbalanced heat loads showed that the flow gets self redistributed into the CPU with the highest heat load, e.g. 1.7 times larger flow rate for a heat flux 3 times larger. Finally, heat load and water coolant flow rate disturbances were also simulated and discussed. (C) 2016 Elsevier Ltd. All rights reserved.

Under subcooled and low quality flow boiling conditions, a general critical heat flux (CHF) mechanistic model is essential to the thermal-hydraulic design and safety analysis, especially at the conceptual design stage of a new pressurized water reactor (PWR). In this study, an overview of the physical mechanisms behind the most common CHF models was carried out and the additional effects to be considered in a new CHF model were indicated. Then, based on the mechanism of superheated liquid layer depletion, the net mass transfer from the bulk liquid to the superheated liquid layer caused by turbulent velocity fluctuations of flowing bubbles was determined and a new calculation method of superheated liquid layer thickness was developed. After matching with other constitutive equations, a novel CHF mechanistic model for subcooled and low quality flow boiling conditions has been developed. Furthermore, the present CHF mechanistic model was validated with 5 × 5 rod bundle CHF experimental data of Nuclear Power Institute of China (NPIC) and compared with other CHF models and CF-DRW correlation. Finally, the distribution of M/P was evaluated and the advantage of the present CHF mechanistic model was discussed. The results show that the present CHF model performs as well as the CF-DRW correlation and is superior to other well established CHF models without any correction factors or empirical constants from experimental data.

Flow boiling in microchannels is one of the most desirable cooling solutions for high power electronics. However, it is challenging to promote the flow boiling performance, particularly critical heat flux (CHF), due to their unfavorable liquid rewetting. Inlet restrictors (IRs) have been successfully used to manage two-phase instabilities in flow boiling in microchannels. However, IRs would result in extremely high pressure drop. In this study, without using IRs, parallel microgrooves (W=50 µm, H=250 µm, L=10 mm) are fabricated on the bottom surface of five parallel microchannels (W=200 µm, H=250 µm, L=10 mm) to effectively manage two-phase flow instabilities without sacrificing pressure drop. Our visualization study shows that these microgrooves can enable rewetting at highhigh frequencies up to 151 Hz and enhance thin film evaporation with the highest thin film ratio of 0.65. A highly desirable periodic rewetting mechanism can substantially delay CHF conditions and enhance heat transfer rates. Flow boiling in the present microchannel configuration has been systematically characterized with mass flux ranging from 100 kg/m2s to 600 kg/m2s. Compared to plain-wall microchannels, flow boiling heat transfer coefficient (HTC) is enhanced up to ~72% at a mass flux of 210 kg/m2s due to the sustainable thin film evaporation. Moreover, CHF is substantially enhanced by ~155% at a mass flux of 389 kg/m2s due to the rapid and periodic rewetting enabled by these decorated microgrooves. We also observed that the enhancement of HTC fades when mass flux exceeds 346 kg/m2s due to flooding.

Boiling heat transfer enhancement using nanoscale techniques has received considerable attention in recent years. This paper provides a comprehensive review of published articles addressing such techniques for both pool boiling and flow boiling enhancement. Discussed are heat transfer merits or drawbacks (in regards to both heat transfer coefficient and critical heat flux) for each technique, with particular focus on reliability concerns stemming from any time-dependent variations in boiling performance. Despite the many studies reporting measurable improvement in heat transfer performance, significant inconsistencies exist regarding both dominant enhancement mechanism and heat transfer trends. And lacking most in the vast majority of studies are long-term and repeatability experiments. To tackle this critical shortcoming, the authors are proposing a step-by-step strategy for future nanoscale enhancement studies. Without this type of strategy, it is believed two-phase nanoscale research will continue to entail simplistic curiosity experiments, lacking conformance to testing norms learned from decades of two-phase research, or convincing evidence of viability of enhancement schemes to practical cooling applications.

Experiments to determine critical heat flux (CHF) for cryogen flow in uniformly heated, round tubes have been performed throughout the globe during the past sixty years. However, experimental CHF data for cryogens are rarely published, remaining in the archives of authors, or in obscure technical reports of an organization or other inaccessible sources. In the present study, the Purdue University-Boiling and Two-Phase Flow Laboratory (PU-BTPFL) Cryogen Flow Boiling CHF Database is consolidated from world literature dating back to 1959. With 2312 data points for LH2, LHe, LN2 and LCH4, it represents the largest cryogen CHF database assembled to date. The database encompasses diameters from 0.5 to 14.1 mm, critical length-to-diameter ratios from 2.5 to 230.8 (term ‘critical’ refers to axial location where CHF is detected), system pressures from 0.01 to 4.07 MPa, reduced pressures from 0.1 to 0.93, mass velocities from 2.2 to 8203.9 kg m−2 s−1, inlet subcoolings from 0 to 78.9 K, inlet qualities from -2.06 to 0.95, critical subcoolings from 0 to 48.5 K, critical qualities from -1.23 to 1.00, critical void fractions from 0 to 1, and CHF values from 0.05 to 8203.9 kW m−2. The consolidated database represents an invaluable tool for development of CHF correlations – a primary goal for the present study – as well as future analytic and computational models. Using this database, new universal CHF correlations are constructed for two distinct CHF mechanisms, Departure from Nucleate Boiling (DNB) and Dryout, after careful physics-based segregation of the data. With mean absolute errors below 30%, the new CHF correlations are shown to provide good predictive agreement with the database.