Dryout Phenomenon Research Articles

Influence of Subcooling and pressure on flow boiling in a vertical mini-channel: Heat transfer analysis using inverse method

The background of this study is the problem of thermal runaway in the battery cells of electric vehicles. Increasingly powerful and compact battery packs require highly efficient thermal management. One potential solution is the direct liquid cooling of lithium-ion battery cells using a dielectric liquid which boils over a safety limit temperature. This approach intends to prevent in a first step the cell thermal runaway and if it fails in a second step its propagation to its neighbors: the liquid boiling helps to mitigate the cell temperature rise and to transport and spread the heat. The arrangement of the cells in the pack and their close spacing induce a scientific problem of convective boiling in a mini-channel. To enhance designs using this solution, a comprehensive understanding of flow boiling and a precise local heat transfer quantification is essential. The HFE-7100 has been selected as a working fluid for its attractive thermophysical properties and a suitable saturation temperature.The present paper introduces new experimental results of flow boiling dielectric fluid, the HFE-7100, in a vertical rectangular mini-channel. Measurements are carried out from start of boiling to dry-out. Boiling curves are obtained for a mass flux of 391 kg/(m².s), at different pressures (0.7, 1, 1.5 bar) and at different subcoolings (ΔTsub=0;15;30;45∘C). The test section is an aluminum block instrumented with three lines of five K-type thermocouples located all along the flow. The visualization of the flow is achieved through a glass pane with a high-speed camera to identify different flow patterns. The local heat transfer coefficient is calculated by a resolution of a 2D inverse heat conduction method.The experiment mainly aims at determining the heat transfer coefficient, the critical heat flux (CHF) and the pressure drop and at characterizing both the flow regimes and the dry-out phenomenon. The influence of subcooling and pressure on flow boiling is analyzed. An increase in subcooling enhances the CHF and the heat transfer coefficient, and delays the dry-out. Subcooling has an impact on the flow pattern and modifies the type of CHF. In the operating conditions tested, a decrease in pressure enhances CHF and delays dry-out. On the other hand, the heat transfer coefficient increases with rising pressure. Similarly, pressure drops are lower with higher pressure.

Study on two-phase flow instability of parallel helical tubes in steam generator of small modular reactors

The flow instability phenomenon within parallel helical tubes is experimentally studied. The oscillation characteristics of wall temperature are comprehensively discussed. In the experiment, the pressure of the test section is in the range of 2.7–6.0 MPa, the mass flux is in the range of 100–300 kg/m2s, the degree of inlet subcooling is in the range of 10–75 °C, and the inlet throttling is in the range of 82–328. This study reveals different effects of inlet subcooling. Specifically, increasing the inlet subcooling under high mass flux enhances the stability of parallel helical tubes. However, increasing the inlet subcooling under low mass flux and low subcooling weakens the stability. The oscillation period to mixture transit time ratio is small at high inlet subcooling and increases with decreasing subcooling. Furthermore, this study shows that the onset of wall temperature oscillation is closely related to the dryout phenomenon, and that the oscillation amplitude increases with increasing local quality. Finally, this study demonstrates that the flow instability can be accurately predicted using time-domain method. The simulation results show satisfactory accuracy, with the prediction errors of ± 15% for the instability threshold and oscillation period, and ± 20% for the wall temperature oscillation amplitude.

Dry-out characteristics of debris bed under different water injection conditions

A porous debris bed may form through the interaction between melting corium and coolant during a pressurized water reactor severe accident, which continues to emit large amounts of decay heat. The cooling of debris bed is a critical phenomenon and serves as a potential mitigation measure to halt the accident. In this paper, an experimental facility DBC was constructed to study the cooling properties and dry-out heat flux under different water injection conditions and system pressures. The debris bed consisted of stone particles ranging in diameter from 1～10 mm, with a porosity of 0.37. In the debris bed, a total of thirty-three electrical heating elements, twelve optical fibers equipped with 204 temperature sensors, and thirty-three thermocouples were embedded. Additionally, a computer program MIDEC was developed to investigate the vapor-liquid flow characteristics and dry-out mechanism. The experimental results indicated that the cooling of debris bed can be significantly improved by increasing system pressure. Furthermore, it was observed that the dry-out heat flux under bottom water injection conditions was considerably higher compared to that of top water reflooding. Under top reflooding conditions, the counter-current flow of vapor-liquid impeded the downward flow of coolant, resulting in insufficient coolant supply and leading to a dry-out phenomenon at the bottom region. However, under bottom injection conditions, both vapor and liquid flowed in an upward direction. As a result, the void fraction reached its maximum value at the top region of the debris bed. The distinct vapor-liquid flow characteristics under top and bottom injection conditions resulted in notable discrepancies in dry-out heat flux and dry-out positions. Based on the comparison between the calculated dry-out heat flux (DHF) and experimental results, it was observed that the Reed model accurately predicted the cooling of debris bed under top-flooding conditions when compared with measured data obtained from DBC experiments. However, for very small particle sizes, the Lipinski model would be more suitable and provided better predictions.

Mesoscopic numerical study of the startup characteristics of grooved heat pipe under high heat flux

The dry-out phenomenon that occurs in the evaporation section under high thermal load can lead to heat pipe startup failure, which considerably affects the safe and efficient operation of microelectromechanical systems. In this study, the startup characteristics of a grooved heat pipe are investigated with the mesoscopic lattice Boltzmann method. The focus is on the effects of wettability, inclination angle, and liquid filling volume on the evolution of the liquid supply velocity and dry-spot area. Results show that nucleate boiling can be formed by starting the heat pipe at high heat flux, and the vapor jetting generated by bubble bursting can reduce the liquid supply velocity by more than 70%. Capillary pressure can be increased, and the perturbation of bubbles to the meniscus region can be suppressed by enhancing the wettability of the capillary wick, thus promoting the return of the condensate to the evaporation section. The pressure on the liquid side in the evaporation section decreases as the inclination angle increases, which shortens the waiting time for vapor core formation and improves the stability of boiling behavior. A non-monotonic relationship exists between the liquid filling volume and heat transfer limit time of the heat pipe, which reaches the maximum only when the initial liquid layer fills the capillary wick exactly.

Study of flow boiling in a vertical mini-channel with surface structuring: Heat transfer analysis using inverse method

Using a boiling fluid to cool components is a very efficient process, which is often used in micro/mini channel flows. A proposed application is the direct contact liquid cooling of lithium-ion battery cells, where a dielectric fluid is required and where the effects a cell thermal runaway could be mitigated by a convective boiling flow between the cells. For such applications, better designs require both to understand confined convective boiling with coupled flow phenomena and to quantify the local heat transfer accurately. The present paper introduces new experimental results of flow boiling heat transfer of a dielectric fluid, the HFE-7100, in a vertical rectangular 1 mm deep, 30 mm wide and 120 mm long mini-channel. The test section is an aluminum block instrumented with three lines of five K-type thermocouples located all along the flow. The experiment mainly aims at determining the heat transfer coefficient and CHF and at characterizing both the flow regimes and the dry-out phenomenon. Measurements are carried out from start of boiling to dry-out, at different mass fluxes (140, 390 and 648 kg/(m2.s)) and at a pressure of 1.1 bar. These tests were performed on a reference unmodified surface and compared to two biphilic surfaces including coating techniques. The visualization of the flow is achieved through a glass pane with a high-speed camera to identify different flow patterns. The local heat transfer coefficient is calculated by a resolution of a 2D inverse heat conduction method. This approach uses a numerical Finite Difference Method (FDM) for the spatial discretization and a Tikhonov's method for regularization. The inverse 2D method permits to follow the evolution of the heat transfer coefficient along the channel with the visualized flow pattern. The experimental results show a small sensitivity of the mass flux on the heat transfer coefficient and a predominance of nucleate boiling. On the other hand, there is a significant sensitivity of the mass flux on the dry-out occurrence. Finally, the sensitivity of two techniques of biphilic surface coating on boiling and on the dry-out occurrence is quantified and discussed.

Theoretical and experimental research on flow boiling heat transfer in microchannels for IGBT modules

Two-phase microchannel heat sink has received extensive attention due to its small size, high heat transfer efficiency, good temperature uniformity, and low working fluid charge. Heat flux and mass flow rate are important parameters that affect the heat transfer performance of two-phase microchannel heat sink. For different heat fluxes, if the mass flow rate is not reasonable, dry-out may occur, which will significantly reduce the heat transfer performance. In order to further explore the influence of mass flow rate and heat flux on heat transfer characteristics, the flow boiling heat transfer performance and flow regimes under different mass flow rates and heat fluxes were studied for a two-phase microchannel heat sink using R245fa as the working fluid. The results showed that when the mass flow rate was 100 kg/(m2·s), the boiling heat transfer coefficient of R245fa showed a trend of increasing firstly and then decreasing with the increase of the heat flux. At a higher heat flux, the local dry-out in the microchannels began to appear, and the heat transfer performance decreased significantly. And the higher the heat flux was, the more serious the dry-out was. There was no obvious dry-out phenomenon when the mass flow rate was 150–300 kg/(m2·s) and the heat flux was below 500 kW/m2. The research provided a valuable reference for exploring the influence of the mass flow rate and the heat flux on the flow boiling heat transfer characteristics of the microchannel heat sink.

Pre-test simulation and a scoping test for dryout and remelting phenomena of an in-vessel debris bed

The present study is intended to investigate the dryout and remelting phenomena of a debris bed during the late phase of an in-vessel severe accident progression. The SIMECO-2 facility at KTH is adapted to conduct the experimental investigation. For selection of an appropriate debris bed in the facility, pre-test simulations are performed by using the COCOMO code to determine: (i) simulant materials of debris particles; (ii) debris bed particle diameters; (iii) configuration and geometry of the debris bed (e.g., shape, layers, dimensions). Candidate particulate beds packed with different mixtures of particles are identified and simulated to obtain their thermal hydraulics in the hemispherical slice test section with radius of 500 mm and width of 120 mm. Based on the simulation results, a particulate bed is chosen and loaded in the SIMECO-2 facility for a scoping investigation. FBG probes with multiple measurement points of each probe are employed to acquire the temperature field of the particulate bed inductively heated. A video recording is applied to detect the dryout and remelting phenomena. In the scoping test, the dryout phenomenon occur first at the elevation of 5 cm from the bed surface under the induction heating power of 14.8 kW, which are comparable with the data predicted by the COCOMO code (6 cm from the bed surface under the heating power of 13.8 kW) in the pre-test simulations.

The Effect of Self-Rewetting Fluids and Surface Wettability Modification on the Thermal Performance of a Two-Phase Flat-Shaped Thermosyphon

Abstract Two-phase flat-shaped thermosyphons have been studied to optimize their thermal performance and reduce the hot spot temperature. A two-dimensional model was employed to observe the evaporation and condensation process in the thermosyphon. Simulating the phase change process was achieved through an in-house user-defined function (UDF), which obtains mass and energy source terms, added to ansysfluent code. Utilizing 1-Buthanol aqueous solution instead of water as the working fluid induces a Marangoni flow to a hotter region and prevents the dry-out phenomenon in the evaporator section. Also, employing a super hydrophilic evaporator makes the liquid spread on the evaporator more evenly, and hinders drying out. Using a super hydrophobic condenser accelerates the detachment of the condensed liquid from the condenser and reduces the accumulation of liquid on the condenser. Furthermore, it accelerates the return of the liquid to the evaporator section to recharge the liquid and prevent the dry-out phenomenon. It has been observed that by using the 1-Buthaol solution, the condensed liquid is more likely to fall back on the center part of the evaporator, which has the highest temperature. Altering the working fluid to 1-Buthanol aqueous solution yields a temperature drop of 10 K for the maximum temperature for a 100 W heat input. The temperature drop increased to 39 K by utilizing a super hydrophobic condenser and super hydrophilic evaporator instead of a bare copper. It has been observed that increasing the filling ratio in the thermosyphon increases the thermal inertia and reduces the hot spot temperature.

Effects of surfactant CTAB on performance of flat-plate CLPHP based on PEMFC cooling

In this study, passive flat-plate closed-loop pulsating heat pipe (CLPHP) cooling is proposed to address the structural characteristics and cooling requirements of proton exchange membrane fuel cells (PEMFCs). Through visual images, non-visualized temperature and pressure measurements, and theoretical analysis, the effect of surfactant cetyltrimethylammonium bromide (CTAB) on the performance of a binary non-azeotropic mixed working medium flat-plate CLPHP with a 40% filling ratio (FR) and 5:1 mass ratio of methanol and deionized water was studied. It was found that the addition of surfactant CTAB reduced the surface tension and increased the shear friction force of the working fluid. The optimal CTAB concentration was 20 ppm, at which the start-up critical heat flux of the flat-plate CLPHP was reduced. At a heating power of 400 W, the heat transfer resistances reduced by 29.9%, which improved the heat transfer power of the flat-plate CLPHP. The backflow of the liquid-phase working medium at the bottom of the elbow in the evaporation section was promoted, dry-out phenomenon was avoided, temperature fluctuation of the flat-plate CLPHP was reduced, and temperature stability of the PEMFC was improved.

Experimental investigation of dryout phenomena under oscillatory flow conditions

In water-cooled nuclear reactors, the maximum power which can be extracted from the core is limited by critical heat flux (CHF). CHF in the high-quality region is known as dryout. In advanced nuclear reactors, the coolant flow occurs solely by virtue of natural circulation; however, instabilities may occur during off-normal operations. This may lead to premature dryout due to lower coolant flow rates seen by the heater during such oscillations. This paper describes the experimental investigation on the effect of flow oscillations on the CHF with the time period of 120 s, which is observed typically in the large-scale natural circulation system. Based on observations made with respect to temperature transient, the continuous dryout is preceded by the transient dryout for higher flow oscillations. But as flow fluctuation decreases, the transient dryout phenomenon is found to disappear. The applicability of the look-up table to predict CHF under oscillatory flow conditions using suitable correction factors (CFs) for premature dryout has been evaluated. CFs for the CHF under oscillations suggested by previous authors have been compared. The maximum possible degradation in CHF value suggested by previous authors has been found to agree with the present experimental data. Percentage fluctuation in heat transfer coefficient (HTC) at fully developed annular flow conditions has been evaluated, and it is found that fluctuation in HTC is in phase with the fluctuation in flow.

Development of an analytical model for the dryout characteristic in helically coiled tubes

In helically coiled tubes, the geometry-induced centrifugal force exerts upon the fluid to make the dryout phenomenon complicated. Hence, it is essential to understand the dryout mechanism and develop a model to accurately predict the dryout quality and dryout pattern in helically coiled tubes that can cover a wide range of parameters. In this paper, 17 helically coiled tubes with different coil diameters (160 ∼1350 mm), inner diameters (9 ∼14 mm) and inclined angles (2.5 ∼ 10°) are used to conduct the experimental study of the dryout characteristic under different conditions, including pressure between 2 and 7 MPa, mass flow rate between 100 and 1000 kg/(m2·s), heat flux between 50 and 700 kW/m2. Three types of dryout patterns in helically coiled tubes were observed by analyzing the circumferential distribution of the wall temperature, including the gravity-dominated pattern, the secondary-flow-dominated pattern and the centrifugal-force-dominated pattern. Then the effects of the operational and geometric parameters of helically coiled tubes on the dryout characteristic have been studied, such as system pressure, mass flow rate, heat flux, tube diameter and coil diameter. Finally, a “dryout map” was developed based on the experimental data (297 data points), and an analytical model of dryout quality was proposed in terms of dimensionless groups for each zone. The correlations were verified by comparing with the experimental results in the open literature. The proposed correlations in this paper can give a better prediction for different experimental results.

Boiling Heat Transfer and Visualization for R717 in a Horizontal Smooth Mini-tube

The paper presents an experimental and theoretical investigation on the two-phase flow boiling heat transfer with ammonia (R717) as the refrigerant in a horizontal smooth mini-tube with inner diameter of 3mm. The experimental data were obtained in the following condition, the heat flux density is 10∼30 kW · m − 2 , the mass flux is 40∼200 kg · m − 2 · s − 1 , the saturation temperature is -10∼10 ℃, and the range of vapor quality is 0.1 ∼1. The experimental results show that the heat transfer coefficient before dryout phenomenon increases and gradually reaches a peak with the increase of vapor quality, but occurs heat transfer deterioration after dryout phenomenon. Combined with the transform of flow patterns, the effect of mass flux, heat flux density, and saturation temperature on the heat transfer coefficient and friction pressure drop are analyzed. The theoretical gas phase flow velocity of ammonia is much higher than other refrigerants, and the flow boiling heat transfer for ammonia is more dependent on convective boiling. The existing correlations of two-phase flow are compared with the measured data of ammonia. According to the experimental and comparative results, Kew and Conwell correlation and Müller-Steinhagen and Heck correlation were updated respectively, and new boiling heat transfer correlation and frictional pressure drop correlation were obtained.

Numerical Investigation of Excess Capillary Pressure Created by Dry-Out and Replenishment Phenomenon in Geothermal Reservoirs

Numerical Investigation of Excess Capillary Pressure Created by Dry-Out and Replenishment Phenomenon in Geothermal Reservoirs

Characterization of a Closed Loop Pulsating Heat Pipe Using Ethanol with Different Angles

In low-temperature difference, a closed-loop pulsating heat pipe (CLPHP) can be used as a cooling device due to its capability to transfer heat. The thermal performance of the CLPHP is affected by the working fluids. In this work, the effects of some operating parameters such as using ethanol as working fluid with 0.5 filling ratio, orientation, and power inputs are offered based on experimental study. Where the CLPHP was constructed and tested to achieve a better vision into the effect of orientation of 0°, 15°, 30°, 45°, and 90°, and power input of 50 W, 115 W, 215 W, and 450 W on the heat transfer characteristics and the thermal performance. The results indicated that the minimum thermal resistance can be reached at 0.1585 (℃/W) with an orientation of 90° and a power input of 450 W. The results revealed that the inclination angles and power inputs had considerable influence on the enhancement of the thermal performance of the CLPHP. For the low boiling temperature of the working fluid, the power input is more favorable because of fast startup compared with a high power input that leads to some difficulties like the dry-out phenomenon.

Flow boiling heat transfer and dryout characteristics of ammonia in a horizontal smooth mini-tube

A boiling two-phase flow heat transfer experimental system for ammonia in a horizontal smooth tube with an inner diameter of 3 mm was established. The purpose is to explore the characteristics of flow boiling and dryout phenomenon. The experimental conditions: heat flux density is 10–30 kW·m−2, the mass flux is 40–200 kg·m−2·s−1, the saturation temperature is −10–10 °C, and the range of vapor quality is 0.1–1. The results show that the dominant heat transfer in different flow patterns is different, intermittent flow with nucleate boiling, annular flow and mist flow with convective boiling; the increase of mass flux can reinforce the convective boiling, the heat flux density can enhance the nucleate boiling, the increase of saturation temperature has no significant effect on the nucleate boiling, but it can weaken the convective boiling. The increase of mass flux and heat flux density will increase the friction pressure drop, and the increase of saturation temperature can decrease the friction pressure drop. The dryout visualization results and attenuation of heat transfer show that the increase of saturation temperature, heat flux density and mass flux can decrease the initial vapor quality of dryout, which is consistent with the results of Mori et al. model. Compared with the experimental data, the existing two-phase flow correlations are not suitable for the post-dryout prediction for ammonia, and the Wojtan et al. model and Mori et al. model can provide a reference for the dryout prediction.

Understanding dry-out mechanism in rod bundles of boiling water reactor

Present work reports numerical understanding of interfacial dynamics during co-flow of vapor and liquid phases of water inside a typical Boiling Water Reactor (BWR), consisting of a nuclear fuel rod bundle assembly of 7 pins in a circular array. Two representative spacings between rods in a circular array are used to carry out the simulation. In literature, flow boiling in a nuclear reactor is dealt with mechanistic models or averaged equations. Hence, in the present study using the Volume of Fluid (VOF) based multiphase model, a detailed numerical understanding of breaking and making in interfaces during flow boiling in BWR is targeted. Our work will portray near realistic vapor bubble and liquid flow dynamics in rod bundle scenario. Constant wall heat flux for fuel rod and uniform velocity of the liquid at the inlet patch is applied as a boundary condition. The saturation properties of water are taken at 30 bar pressure. Flow boiling stages involving bubble nucleation, growth, merging, local dry-out, rewetting with liquid patches, and complete dry-out are illustrated. The dry-out phenomenon with no liquid presence is numerically observed with phase fraction contours at various axial cut-sections. The quantification of the liquid phase fraction at different axial planes is plotted over time, emphasizing the progressive dry-out mechanism. A comparison of liquid-vapor distribution for inner and outer rods reveals that the inner rod's dry-out occurs sooner than that of the outer rod. The heat transfer coefficient to identify the heat dissipation capacity of each case is also reported.

An experimental comparison of thermal performance: Smooth and inner-grooved closed loop pulsating heat pipes in different angles

Pulsating Heat Pipes are ideal for compact cooling applications such as electronic cooling. In this experimental research, the thermal performance of Inner-Grooved Pulsating Heat Pipes (IGPHP) and Smooth Pulsating Heat Pipes (SPHP) compared using distilled water in different angles. The inclination angles of 0o, 5 o, 15 o, 30 o, 50 o, 70 o, and 90 o, and the input heat range of 50-300 W were studied. The results show that the optimum filling ratio is 60% for IGPHP and SPHP. The study shows that in constant Input heat, the thermal resistance of IGPHP is lower than SPHP and the effective thermal conductivity of IGPHP is higher compared to SPHP. The average reduction of the thermal resistance of IGPHP compared to SPHP was found 21% across all angles and input heat powers; however, the average reduction was 49% for the angle of 5 o across all input heat powers. The dryout phenomenon for IGPHP took place at 0o and the input heat of 100 W, while for SPHP, this occurred at 5o and 250 W. The overall comparison shows that IGPHP is an even better alternative to SPHP in higher capacity applications.

Numerical Investigation on Bubble Growth and Merger in Microchannel Flow Boiling With Self-Rewetting Fluid

Abstract There exist some problems such as flow instability and critical heat flux (CHF) caused by the local dryout phenomenon, which is an obstacle to the application of microchannel flow boiling heat sink. Utilizing self-rewetting fluid is one of the promising ways to minimize the dryout area, thus increasing the heat transfer coefficient and CHF. To investigate the heat transfer performance of self-rewetting fluid in microchannel flow boiling, a numerical investigation is carried out in this study using the volume of fluid (VOF) method, phase-change model, and continuum surface force model with surface tension versus temperature. A three-dimensional numerical investigation of bubble growth and merger is carried out with water and 0.2 wt % heptanol solution. The single bubble growing cases, two x-direction/y-direction bubbles' merging cases, and three bubbles' merging cases are conducted. Since the bubbles never detach the heated walls, the dryout area and regions nearby the contact line with thin liquid film dominated the heat transfer process during the bubbles' growth and merger. The self-rewetting fluid can minimize the local dryout area and achieve larger thin liquid film area around the contact line due to the Marangoni effect and thermocapillary force, thus resulting in higher wall heat flux. The two x-direction bubbles' merging case performed best for heat transfer in the microchannel, in which self-rewetting fluid achieves heat transfer enhancement for over 50%.

Design aspects of forced circulation evaporators of high salty water from the view point of scale formation

In this paper, dryout phenomenon was studied for vertical tubes working at the operating condition of forced circulation evaporator in zero liquid discharge systems to find the maximum allowable value of temperature difference between evaporator effects in which scale formation and fouling do not occur. Eulerian two-fluid approach coupled with Rensselaer Polytechnic Institute wall boiling model was used to simulate upward flow subcooled boiling. Critical near-wall void fraction was used as the criterion of dryout. The value of the critical temperature difference was investigated for 36 cases. Values varied from 4.8 to 10.7 °C for different amounts of tube length, tube diameter and mass flux. In addition, the effect of tube length, tube diameter and mass flux on critical temperature difference was analyzed. The results showed that critical temperature difference decreased approximately linearly with an increase in tube length. Furthermore, the value of critical temperature difference decreased with the increase in tube diameter and decrease in mass flux and the effect of tube diameter is more significant at higher mass fluxes.

Flow boiling heat transfer performances of R1234ze(E)/R152a in a horizontal micro-fin tube

ABSTRACT The boiling heat-transfer performances of R1234ze(E)/R152a (0.4/0.6 by mass) flowing inside a horizontal micro-fin tube of 9.52 mm outer diameter were researched by experiment. Under the experimental conditions of mean vapor quality between 0.02 and 0.98 with the mass flux of 50 ~ 100 kg/(m2·s), saturation temperature from 12 to18°C and heat flux from 5 to 20 kW/m2, and the impacts of mass flux, heat flux, saturation temperature, and mean vapor quality on flow boiling heat-transfer coefficient (HTC) and frictional pressure drop were mainly investigated; furthermore, the impact of mass transfer resistance on flow boiling process was discussed. The results indicate that R1234ze(E)/R152a obviously reveals the dry-out phenomenon in the flow boiling process, the boiling HTCs increase as the saturation temperature and heat flux, decrease and then increase as the mass flux; the frictional pressure drop rises as the mass flux and declines as the saturation temperature, while the frictional pressure drop is scarcely affected by the heat flux; it is also found that the boiling HTCs take on a decreased trend whole within the entire range of mean vapor quality, which implies that the mass transfer resistance brings about the heat-transfer degradation during the flow boiling process.