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ABSTRACTThis paper presents an experimental study on R1234yf flow boiling inside a mini microfin tube with an inner diameter at the fin tip of 2.4 mm. R1234yf is a new refrigerant with an extremely low global warming potential (GWP <1), proposed as a possible substitute for the common R134a, whose GWP is about 1300. The mass flux was varied between 375 and 940 kg m?2 s?1, heat flux from 10 to 50 kW m?2, and vapor quality from 0.1 to 1. The saturation temperature at the inlet of the test section was kept constant and equal to 30°C. The wide range of operative test conditions permitted highlighting the effects of mass flux, heat flux, and vapor quality on the thermal and hydraulic behavior during the flow boiling mechanism inside such a mini microfin tube. The results show that at low heat flux the phase-change process is mainly controlled by two-phase forced convection, and at high heat flux by nucleate boiling. The two-phase frictional pressure drop increases with increasing both mass velocity and vapor quality. Dry-out was observed only at the highest heat flux, at vapor qualities of around 0.94–0.95. 相似文献
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在1根光管、2根微肋管内对R1234yf两相流动冷凝换热进行实验,实验工况设定为冷凝温度(40±0.5)、(43±0.5)和(45±0.5)℃,质量流速为400~900 kg/(m~2·s),实验段进口制冷剂干度为0.80~0.85、出口制冷剂干度为0.15~0.20,进而从关联式拟合机理上分析各关联式对管内换热系数、压降的预测效果。结果表明:管内换热系数、压降均随流速的增加、冷凝温度的降低而增大,且微肋管内换热系数、压降均大于光管内换热系数及压降,其中,1号微肋管内换热系数最高,2号微肋管内压降最大;对于光管换热系数、压降,Thome关联式和Fridel关联式预测效果最佳,其预测平均误差均在3%以内,而Wang et al关联式和Chisholm et al关联式预测误差最大,其预测平均误差在25%以上;对于微肋管内换热系数、压降,Cavallini et al关联式和Haraguchi et al关联式分别表现出较好的预测效果,其平均预测误差分别-15.43%和-15.68%。 相似文献
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In this paper, we study the boiling heat transfer of upward flow of R21 in a vertical mini-channel with a size of 1.6 × 6.3 mm. The heat transfer coefficient was measured as a function of heat flux for a wide range of vapor quality and for two levels of mass flow rate, G = 215 kg/m2s and G = 50 kg/m2s. The standard deviation of wall superheat over channel perimeter and in time was determined from the measurement of the wall temperature along the channel perimeter. Different heat transfer mechanisms were revealed depending on flow patterns. The main heat transfer mode for large mass flux is convective boiling. We also figure out the mode when the evaporation of thin liquid films makes the essential contribution to heat transfer. The modified models of Liu & Winterton and Balasubramanian & Kandlikar describe the experimental data well for regime when the convective boiling makes the main contribution to the heat transfer. 相似文献
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Jacqueline B. Copetti Bruno de Sá Bekerle Mario Henrique Macagnan Júlio Cesar Passos Jeferson Diehl Oliveira 《传热工程》2017,38(3):323-331
ABSTRACTEvaporators of small and medium refrigeration systems, as in commercial and automobile air conditioning applications, are being studied to develop more compact and lighter equipment, that reaches good thermal performance and reliability, with low pressure drop. In this way, evaporators are being designed with small channels and materials like aluminum. Moreover, different refrigerants are being tested to substitute for hydrofluorocarbon (HFC) refrigerants, with different operational temperatures and pressures. Some of them, like hydrocarbons, although they present advantages with respect to their thermodynamic and transport properties, should be used with small charge in the system due to their flammability. This work presents the results of an experimental study to characterize the flow boiling of the refrigerant R600a (isobutane) in a multiport aluminum extruded tube with 7 parallel minichannels of 1.47 mm hydraulic diameter. The effects of mass velocity, heat flux, and vapor quality on heat transfer were investigated for constant saturation temperature and pressure. Heat fluxes in the range from 5 to 30 kW m?2, mass velocities set to discrete values in the range of 50 to 200 kg m?2 s?1, and saturation temperature of 20°C were considered. It was verified a significant effect of heat flux. Moreover, some images of flow patterns, in different conditions, are presented, and the main patterns identified were slug, intermittent, and annular. 相似文献
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This study focuses mainly on the prediction of saturated flow boiling heat transfer in microchannels. A wide range of experiments has been carried out with de-ionized water to obtain a comprehensive data set. Experiments of mass fluxes of 51–728.7 kg/m2s, wall heat fluxes of 36–221.7 kW/m2, vapor qualities of 0.01–0.69, liquid Reynolds number of 7.72–190, aspect ratios of 0.37–5.00 (with a constant hydraulic diameter of 100 µm) and hydraulic diameters of 100–250 µm (for constant aspect ratio = 1). A new correlation including the aspect ratio effect is proposed to predict the heat transfer coefficient for saturated flow boiling in microchannels. The proposed correlation shows very good predictions with an overall mean absolute error of 16.9% and 86.4%, 96.2% and 99.5% of the predicted data falling within ±30, ±40 and ±50% error bands, respectively. 相似文献
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Wall boiling and bubble population balance equations combined with a two-fluid model are employed to predict boiling two-phase flow in an inclined channel with a downward-facing heated upper wall. In order to observe the boiling behavior on the inclined, downward-facing heated wall, a visualization experiment was carried out with a 100 mm × 100 mm of the cross section, 1.2-m-long rectangular channel, inclined by 10° from the horizontal plane. The size of the heated wall was 50 mm by 750 mm and the heat flux was provided by Joule heating using DC electrical current. The temperatures of the heater surface were measured and used in calculating heat transfer coefficients. The wall superheat for 100 kW/m2 heat flux and 200 kg/m2s mass flux ranged between 9.3°C and 15.1°C. High-speed video images showed that bubbles were sliding, continuing to grow, and combining with small bubbles growing at their nucleation sites in the downstream. Then large bubbles coalesced together when the bubbles grew too large to have a space between them. Finally, an elongated slug bubble formed and it continued to slide along the heated wall. For these circumstances of wall boiling and two-phase flow in the inclined channel, the existing wall boiling model encompassing bubble growth and sliding was improved by considering the influence of large bubbles near the heated wall and liquid film evaporation under the large slug bubbles. With this improved model, the predicted wall superheat agreed well with the experimental data, while the RPI model largely overpredicted the wall superheat. 相似文献
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为快速且准确地计算气泡行为参数,本研究针对窄距形通道中欠热流动沸腾的气泡图像,提出了一种气泡分割方法.首先,对实验图像进行滤波,背景差值等操作消除背景噪声,提高两相之间对比度;然后,利用双阈值法获得气泡图像二值图;最后,依据气泡特征选择合适的结构元素,并利用形态学操作完成气泡图像分割.实验结果表明,该方法所得气泡图像分割结果有效消除复杂背景,气泡形状、大小及位置均与实验图像符合较好.因此,该方法能够对窄矩形通道中欠热流动沸腾的气泡图像进行快速有效的分割识别. 相似文献
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Experiments were conducted for a typical R134a compact European automotive air conditioning system equipped with an internally controlled variable displacement compressor, minichannel condenser, TXV, and minichannel evaporator. A “drop-in” R1234yf system was tested together with two modified R1234yf systems with the primary goal to document some laboratory results and their analyses which could prove useful in aiding manufacturers and researchers by indicating “minor” system modifications which could be implemented in existing air conditioning systems, with the aim to achieve with R1234yf similar capacity and efficiency as modern R134a systems. Since the experimental results indicate that, for a given cooling capacity, R1234yf systems present lower performance than the baseline R134a, numerical simulations were used to investigate the effects of “major” system modifications, such as, the use of an enhanced condenser and/or an enhanced evaporator. 相似文献
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Hemant Naik 《传热工程》2013,34(20):1774-1789
ABSTRACTPresent three-dimensional numerical study aims to investigate the effect of mounting rectangular winglet pair (RWP) on heat transfer enhancement in flow over a flat plate. Computations for incompressible flow of air have been carried out using commercial software ANSYS Fluent. Flow of air has been considered over the surface of an isothermally heated horizontal plate in presence of RWP in the range of Reynolds number from 400 to 2000. Common flow down configuration of RWP has been considered to study the effect of various geometric parameters, such as length of RWP, spacing between leading edges of the winglets and angle of attack of RWP, on flow characteristics and enhancement in heat transfer. Flow and temperature field characteristics have been presented using streamlines and temperature contours near the plate surface and streamlines in cross-stream planes. Enhancement in heat transfer in presence of RWP has been quantified using cross-stream variation of local Nusselt number, streamwise variation of span-averaged Nusselt number and surface-averaged overall Nusselt number. 相似文献
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对R290制冷剂在微细通道内的流动沸腾换热特性进行了实验研究。研究管径分别为1和2 mm,热流密度为20~65 k W/m~2,质量流率为100~200 kg/m~2·s,饱和温度为15和25℃,干度范围为0.1~0.9。通过实验数据分析管径、热流密度、质量流率、饱和温度对流动沸腾换热的影响。结果表明:随着管径的下降,换热系数呈现出大幅上升的趋势,其平均增幅为31%;随着热流密度的上升,换热系数呈现出大幅上升的趋势,其平均增幅达到了131%;随着质量流率的上升,换热系数呈现出小幅上升的趋势,其平均增幅为14%;随着饱和温度的上升,大部分换热系数呈现出小幅上升的趋势,其平均增幅为12.6%。 相似文献
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为研究柔性结构运动对通道换热性能的影响,采用任意拉格朗日-欧拉(Arbitrary Lagrangian-Eulerian, ALE)法,对通道中串列布置的倒转柔性体进行数值模拟。研究不同长宽比的柔性体在抗弯刚度系数相同时的拍动状态、流场特性以及对通道换热性能的影响。结果表明:随抗弯刚度系数变化柔性体存在稳定模式、大幅度拍动模式和偏转拍动模式3种运动模式;随抗弯刚度系数减小,小长宽比柔性体的拍动状态变化具有滞后性;大长宽比柔性体在净换热量提升上更有优势,在合理的机械能损失范围内拥有更高的热效率系数,相较小长宽比通道,平均净换热量提高了3.87倍,最高热效率系数可达1.3;在实际工程应用中,应采用大长宽比并处于大幅度拍动模式下才能有效提升通道内传热性能。 相似文献
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Mohammed Amine Moussaoui Mohammed Jami Hassan Naji 《Numerical Heat Transfer, Part A: Applications》2013,63(5):422-443
This article presents the application of the multiple-relaxation-time (MRT) lattice Boltzmann equation (LBE) method with nine-velocity model to the numerical prediction of a laminar and convective-heated transfer through a two-dimensional obstructed channel flow. The obstruction is carried out by three obstacles including two located on the upper wall and the other on the lower wall of the channel. The calculations are validated against results available in literature. Various physical arrangements are regarded as the size of the obstacles and the distance between the two upper obstacles to investigate their effects on thermal and flow characteristics. Results, presented for a Prandtl number equal to 0.71 and a Reynolds number ranging from 100 to 1200, showed that the heat transfer and the air flow depend both on the Reynolds number and geometric data of the configuration. 相似文献
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The boiling heat transfer and two-phase pressure drop of water in a microscale channel were experimentally investigated. The tested horizontal rectangular microchannel had a hydraulic diameter of 100 μ m and length of 40 mm. A series of microheaters provided heat energy to the working fluid, which made it possible to control and measure the local thermal conditions in the direction of the flow. Both the microchannel and microheaters were fabricated using a micro-electro-mechanical systems (MEMS) technique. Flow patterns were obtained from real-time flow visualizations made during the flow boiling experiments. Tests were performed for mass fluxes of 90, 169, and 267 kg/m2s and heat fluxes from 200 to 500 kW/m2. The effects of the mass flux and vapor quality on the local flow boiling heat transfer coefficient and two-phase frictional pressure gradient were studied. The evaluated experimental data were compared with existing correlations. The experimental heat transfer coefficients were nearly independent of the mass flux and vapor quality. Most of the existing correlations did not provide reliable heat transfer coefficient predictions for different vapor quality values, nor could they predict the two-phase frictional pressure gradient except under some limited conditions. 相似文献
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The flow of a rarefied gas in a square enclosure with one wall at high temperature and the other three walls at the same low temperature is investigated. The flow, characterized by the reference Knudsen number and ratio of the cold over the hot temperatures, is simulated both deterministically, using the nonlinear Shakhov kinetic model, and stochastically, using the DSMC method. Excellent agreement between the two approaches is obtained. It is found that along the side walls the gas velocity, depending on the flow parameters, may be either from cold to hot or from hot to cold regions. Furthermore, it is confirmed that the average heat flux departing from the hot plate exhibits a nonmonotonic behavior with regard to the temperature ratio, deducing a maximum heat flux at a temperature ratio of about 0.3. The flow and heat transfer characteristics are explained by computing the ballistic and collision parts of the total bulk quantities and by investigating the contribution of each part to the overall solution. 相似文献
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A pump is needed in an ejector refrigeration system, which makes the system high cost and complicated and hinders its application. A pump-free double heat source ejector refrigeration system using R1234yf as working fluid is proposed in which an injector driven by another heat source replaces the liquid circulating pump, making the system more affordable, simpler and more stable. The effect of different operation conditions on entrainment ratios and the influence of different factors on system p... 相似文献