•Flow boiling of R134a/R245fa zeotropic mixtures in a micro-channel was studied.•The delay of flow pattern transition and two types of bubble coexistence were observed.•The heat transfer performances were analyzed based on the visualization results.•A new prediction method considering Marangoni and capillary effects was proposed.In the present study, the flow boiling characteristics of R134a/R245fa zeotropic mixtures with three mass fractions (10/90, 30/70 and 70/30 by wt%) in a single rectangular micro-channel and their pure components were experimentally investigated. The flow boiling in micro-channel was visualized from the top view and heated on three sides with the cross-sectional area of 1 mm × 1 mm and length of 106 mm. For each test refrigerant, the visualization results and heat transfer coefficients were obtained under the parametric conditions comprising the heat flux range of 30–120 kW/m2 and mass flux range of 60–1100 kg/m2 s at the same evaporating temperature of 18.5 °C. The corresponded outlet vapor quality was in the range of 0–1. Results showed that almost all of the test refrigerants experienced five typical flow patterns. Different bubble coexistence phenomena were observed for the pure and mixed refrigerants, respectively. The hysteresis of flow pattern transition for R134a/R245fa zeotropic mixtures was strongly affected by both of the temperature glide and blending ratio. Based on the comparison with pure components, flow boiling heat transfer characteristics of test zeotropic mixtures were analyzed and discussed in detail by considering the mixture effects. Taking account of capillary and Marangoni effect in confined space, as well as other impact factors, the developed prediction method exhibited satisfactory accuracy in predicting the heat transfer coefficient of zeotropic mixtures.

•Flow boiling characteristics of R134a and R407C were studied in a micro-channel.•The phenomena of advance into churn-annular flow and coexistent bubbles were observed for R407C.•The nucleate boiling heat transfer of R407C is suppressed during bubbly flow.•A correlation with Marangoni number was developed for zeotropic mixtures.In the present study, the flow boiling characteristics of pure refrigerant R134a and zeotropic mixture R407C are experimentally investigated in a single visualized rectangular micro-channel heated on three sides with the cross-sectional area of 1 mm × 1 mm and length of 106 mm. Boiling heat transfer coefficients are obtained at the saturation temperature of 21 °C under the heat flux and mass flux ranging from 30–150 kW/m2 and 35–1400 kg/m2 s, respectively. The boiling curves of the two refrigerants are also discussed. Based on the visualization results, seven flow types are identified and the flow pattern maps are plotted. Through the comparative study, the phenomena of advance into churn-annular flow and coexistent bubbles with the flow patterns from confined bubble to annular are observed for R407C. The boiling heat transfer coefficient of R407C is slightly higher than that for R134a at lower vapor quality while the opposite situation appears with the increasing vapor quality after that. During churn-annular to annular flow stage, the boiling heat transfer coefficient of R407C presents a declining trend which is obviously different from the relatively stable value of R134a. The nucleate boiling heat transfer of R407C is suppressed during bubbly flow but promoted in confined bubble to slug flow stage compared with R134a. The CHF of R407C is higher than that of R134a. A correlation for the flow boiling heat transfer coefficient of mixtures is proposed in consideration of Ma number and predicts satisfactorily the database of R407C and R404A.

•A new structure of rack with inner duct and pulsating heat pipe was suggested.•The simplified thermal conductivity model of pulsating heat pipe is introduced.•The start-up of pulsating heat pipe (PHP) leads to the temperature of CPUs decrease.•The temperature field is improved and some “hot spots” are removed in the rack.A rack cooling system with the pulsating heat pipe and inner duct in data center is suggested and the heat transfer performance is numerically investigated in this paper. The inner structure of the rack is designed and analyzed. The simplified thermal conductivity model of the pulsating heat pipe is introduced in numerical simulation. The temperature of CPUs (Central Processing Units) is evaluated based on the rack electricity power, the working state of the pulsating heat pipe, the temperature of the cooling air and the wind pressure of the rack fans. The results show that the CPUs temperature increases with the rack heating power. The start-up of the pulsating heat pipe (PHP) leads to the temperature of CPUs decrease and the distribution of temperature of CPUs uniformly. The temperature of CPUs decreases with the decrease of the temperature of the cooling air or the increase of wind pressure. The temperature field of the inner air is improved and some “hot spots” are removed by installing an inner duct in the rack. The temperature of CPUs is no more than 60 °C in the rack cooling system with load 1380 W.Download high-res image (114KB)Download full-size image

•Visualization of refrigerant R410a condensation in a microchannel is investigated.•One dimension annular flow model is established for the condensation in microchannel.•The film thickness, dimensionless velocity and local heat transfer coefficient are obtained.A visualization experimental investigation on the condensation heat transfer of refrigerant R410a in a rectangular micro-channel with hydraulic diameter of 0.67 mm is conducted in this paper. Experiments are conducted with saturation temperatures of 36–41 °C, and the refrigerant mass fluxes is in the range of 109–1042 kg/(m2 s) over the entire range of vapor qualities. One-dimension annular flow model is established to study the condensation heat transfer characteristics inside micro-channel. The condensate film thickness, dimensionless condensation liquid flow velocity, local heat transfer coefficient and average heat transfer coefficient are obtained to analyze the condensation heat transfer mechanism in micro-channel. The predicted value of heat transfer coefficient has great agreement with experimental data in this study. Visualization experimental results agree with condensation flow patterns map in micro-channel proposed by other researcher. Annular flow, wavy-annular flow, slug flow and bubble flow are observed along the flow direction in micro-channel under the low mass flux. Saturation pressure, mass flux, sub-cooled and vapor quality of R410a are considered. The results show that the heat transfer coefficient increases with mass fluxes and vapor quality remarkably. Experiment results are 17% higher than other research results, especially in high mass flux and vapor quality. In addition, the significant influence of sub-cooled is proposed and discussed in micro-channel condensation study. The lower sub-cooled will bring higher heat transfer coefficient.

•Condensing heat transfer and pressure drop of refrigerant R410A for flow upward are investigated.•The enhancement of heat transfer and pressure drop penalty of micro-fin tube are discussed.•Correlations for condensation heat transfer and the pressure drop in micro-fin tube are proposed.Heat transfer and pressure drop performance of refrigerant R410A condensation during vapor flowing upward in a smooth tube with inner diameter of 8.32 mm and a micro-fin tube with fin root diameter of 8.76 mm are experimentally investigated. The test section is a 0.5 m long double tube with refrigerant flowing in the inner tube and cooling water flowing in the annulus in the same direction. Experiments are conducted at saturation temperatures 40 °C, 45 °C and 48 °C with mass fluxes in the range of 80–345 kg/m2 s. Four different cooling water flow conditions in the test section are set up to compare the heat transfer enhancement factors and pressure drop penalty factors of the smooth tube and micro-fin tube. The total pressure drops across the test section are directly measured by a differential pressure transducer. The experimental data are compared with some well-known heat transfer correlations of micro-fin tube. The correlation suggested by Chamra and Mago shows the best agreement with the present data and exhibits an average deviation of 31.08 and a mean deviation of 35.74. Based on the experimental data, two revised correlations for condensation heat transfer and the frictional pressure drop in micro-fin tube are proposed, within ±15% and ±25% deviation band respectively.

•A new type of BTM system based on flow boiling in mini-channel are presented.•Uniform temperature and volume distribution of battery module are obtained.•The temperatures of battery cell are maintained around 40 °C.•There exists an appropriate Re number range for boiling heat transfer in mini-channel.In order to guarantee the safety and prolong the lifetime of lithium-ion power battery within electric vehicles, thermal management system is essential. A new type of thermal management system based on flow boiling in mini-channel utilizing dielectric hydrofluoroether liquid which boiling point is 34 °C is proposed. The cooling experiments for battery module are carried out at different discharge rates and flow Re number. The cooling effect and the influence of battery cooling on the electrochemical characteristics are concerned. The experimental results show that the thermal management can efficiently reduce maximum temperature of battery module and surface maximum temperature difference. A relatively uniform temperature and voltage distributions are provided within the battery module at higher discharge rate benefit from the advantage of boiling heat transfer with uniform temperature distribution on cold plate. It is shown that the voltage decreases with the increase of Re number of fluid due to the reducing of temperature. There exist slight fluctuations of voltage distribution because of the non-uniformity of temperature distribution within the battery module at higher discharge rates. For different discharge rate, there also exists an appropriate Re number range during which the mode of heat transfer is mainly in boiling heat transfer mode and the cooling result can be greatly improved.

Flow boiling experiments were conducted in a vertical annular channel to study bubble departure characteristics. Deionized water was used as the working fluid, and the tests were performed at atmospheric pressure. Bubble departure diameters were obtained from the images which were captured by a high-speed digital camera. The relationship between bubble contact diameter and departure diameter was discussed. A new model base on force balance analysis, taking bubble contact diameter into account for predicting bubble departure diameter is proposed in this study. A good agreement between predicted and measured results is achieved.

•The voltage response is proportional to ambient temperature and current density.•There exists a time constant for the response to be stable.•The initial membrane water content is beneficial to the cell performance.Cold start and operation of a proton exchange membrane fuel cell (PEMFC) at the cold temperatures are crucial to the commercialization of it in the field of transportation. A 32 cm2 two cell stack is prepared to conduct the experiments at subzero temperatures, including cold start processes and cell performance testing, aiming of the characteristics of the cell. The startup study under subfreezing temperatures is conducted by galvanostatic method at various operation conditions, i.e. ambient temperature (−3 and −5 °C), current density and anode stoichiometry. The results show that the voltage evolutions are proportional to the operating current densities under the former two conditions, but the relationship becomes the opposite at the last condition. It is also found that the time constant for the cell to reach steady status is no more than 100 s and highly depends on the startup mode. In addition, the performance of the cell is tested at the temperature of 0 °C and −3 °C. The comparison of pre-humidification and normal operations indicate that the initial water content of membrane affects the cell performance.

A new mass transfer model is developped to predict the volatile organic compounds (VOCs) from fresh wet building materials. The dry section of wet materials during the process of VOC emission from wet building materials is considered in this new model, differing from the mass transfer-based models in other literatures. The mechanism of effect of saturated vapor pressure on the surface of wet building materials in the process of VOC emission is discussed. The concentration of total volatile organic compounds (TVOC) in the building materials gradually decreases as the emission of VOCs begins, and the vapor pressure of VOCs on the surface of wet building materials decreases in the case of newly wet building materials. To ensure the partial pressure of VOCs on the surface of wet building materials to be saturated vapor pressure, the interface of gas-wet layer is lowered, and a dry layer of no-volatile gases in the material is formed. Compared with the results obtained by VB model, CFD model and the experiment data, the results obtained by the present model agree well with the results obtained by CFD model and the experiment data. The present model is more accurate in predicting emission of VOC from wet building materials than VB model.

The bubble dynamics during subcooled flow boiling were investigated in a single rectangular cross-section (0.5 mm × 1 mm) microchannel. Degassed deionized water was used as the working fluid with the confined bubble behavior observed using a high-speed CCD camera. The microchannel wall confines the bubble growth in the bubble height direction when the bubble departure diameter was larger than the microchannel cross section, resulting in strong deformation of the bubble top interface even before the liquid–vapor interface touched the microchannel wall. The microchannel confinement effect works by exerting a wall confining force on the top interface of the growing bubble. The negative feedback between the velocity of the top interface and the wall confining force is the main reason for the fluctuations in the increasing maximum local void fraction and the bubble aspect ratio during the confined bubble growth period.