内表面烧结型多孔管的流动沸腾换热

采用流动沸腾传热试验平台,研究了2 m长铁基烧结型内表面多孔管竖直管内流动沸腾传热特性,利用流动沸腾传热学基本原理及公式计算了传热过程中的热通量、沸腾传热系数及相关参数,并考察了过热度和流速对多孔管流动沸腾传热性能的影响.结果表明:烧结型表面多孔管的流动沸腾传热能力优于同条件下的光滑管,内表面沸腾传热系数是同尺寸光滑管...     

竖直矩形窄通道内流动沸腾局部换热特性实验研究

为了明确竖直矩形窄通道内各阶段流动沸腾的换热特性,优化换热器性能,以去离子水为工质,对尺寸为720 mm×250 mm×3.5 mm的单面电加热竖直矩形窄通道内的流动沸腾换热进行实验研究,分析了质流密度、进口温度、热流密度对流动沸腾局部换热特性的影响。并在已有流动沸腾传热关联式的基础上,对实验数据进行非线性回归分析,得到适用于实验工况下的新流动沸腾传热关联式。结果表明：质流密度增大对流动沸腾段换热特性有强化作用,对核态沸腾段换热特性有削弱作用;热流密度对核态沸腾影响剧烈,但对流动沸腾的影响不明显;入口温度越高,流体会越早进入过冷沸腾阶段,但对局部传热系数的影响不明显;新流动沸腾传热关联式与实验值的平均相对误差为23.87%,其中74.19%的预测值在±25%内,83.87%的预测值在±50%以内,能很好地预测本实验工况下矩形窄通道内流动沸腾的局部传热系数。     

垂直矩形窄通道流动沸腾换热特性实验研究

流动沸腾换热是典型的两相流问题。窄通道与常规通道相比较,其流动沸腾换热系数有较大提高,换热机理也更加复杂。针对截面为250 mm×5 mm的竖直矩形窄缝通道,在低压、入口温度过冷、不同质量流速及加热功率密度的条件下,对水流动沸腾换热特性进行实验研究。通过实验分析可知:入口温度27～60℃、质量流速2.22～3.49 kg/(m2.s)及加热功率密度0～12 kW/m2对饱和沸腾起始点和过冷段长度有重要影响;高的空泡份额和通道结构的限制使汽液两相流动不稳定而影响换热系数,换热系数随着功率的增大而减小,流体进入完全对流沸腾阶段;由于实验段通道顶部结构的限制,干度的增加不会出现干涸点,换热不会得到恶化,换热系数随着功率的增大基本不变。     

斜扩张管催化器流场三维数值模拟和结构优化设计

采用计算流体力学(CFD)软件,对具有良好流动特性的斜扩张管催化器进行了三维稳态流动数值模拟,并研究了扩张管倾斜角和入口管布置对流动特性的影响.研究结果表明扩张管倾斜角在60°附近可使催化器获得较均匀的流速分布和较小的压力损失;入口管应布置成与扩张管在同一中心线上.     

复间冷系统凝汽器管内换热的数值模拟与优化

对电站空冷凝汽器管壳式换热管内氨进行汽液两相流蒸发沸腾的数值模拟,将换热器简化为研究单根水平换热管,分析了不同管壁温度、液氨进口流速、入口温度对沸腾传热性能的影响。确定最优管壁温度、进口流速、入口温度的组合形式。结果表明:壁面温度为302.96K、进口流速为0.1m/s、入口温度为278.15K时换热管的换热效果最佳。     

自然循环热水锅炉水动力研究

本文提出的自然循环热水锅炉水动力计算方法包括了简单回路、并联回路、结构不均的典型回路和下降管入口水温的计算,并且考虑了热力不均及水力不均的影响。通过试验分析提出了防止气泡贴壁和产生过冷沸腾的条件。     

非能动余热排出系统C型管换热器数值模拟

为保证在事故工况下非能动余热排除系统有效导出余热,对其主要设备PRHR热交换器进行换热特性研究,建立了非能动余热排出系统C型管换热器的内外耦合传热分析模型,采用一维均相流模型计算管内冷凝换热与CFD程序分析水池空间的自然对流。研究进口质量流量、进口流体含气率、管倾角和水箱温度对C型管换热器换热特性的影响。结果表明:C型管换热器入口倾斜段管内始终为饱和的两相流体,在竖直段与出口倾斜段,管内流体温度逐渐下降;管内压力、流体焓值和换热系数沿管长逐渐降低;大约在冷凝70 s后,管内流体参数趋于稳定;管壁温度在入口倾斜段迅速下降,在竖直段和出口段趋于平缓。增大进口质量流量与进口流体含气率,流体温度、流体焓以及管内外换热系数增加,并且沿流动方向受两者的影响逐渐减小;若管倾斜角度增大20°,出口倾斜段管内流体温度下降约3℃;当水箱温度升高10℃,汽泡生成与脱离速度加快,水箱内部换热增强,入口倾斜段外壁温升高2℃左右,出口倾斜段外壁温大约升高0.2℃。CFD模拟结果展示出水箱内汽泡大部分聚集在C型管上部并逐渐向上流动,致使热流体向上运动,冷流体向下流动,形成自然循环。     

缸盖冷却水的单相流沸腾模型

针对缸盖水腔内的冷却水流动沸腾传热计算,本文介绍了两种单相流沸腾模型。模型认为流动沸腾总传热量等于泡核沸腾和单相对流传热之和,其中泡核沸腾传热计算采用修正后的容积沸腾传热计算公式。BDL模型在Chen模型的基础上作了改进,考虑了冷却水局部流动参数及饱和状态的影响,适用于局部流动传热计算。     

垂直下降管内液膜沸腾蒸发及相界面波动研究

采用数值模拟的方法对垂直下降管内液膜沸腾蒸发流动和传热特性进行研究。分析入口雷诺数Re和热流密度的耦合作用对液膜流动和传热的影响,结果表明:壁面生成的汽泡呈现液滴状;大汽泡表面分割、脱离出小汽泡;汽泡生成、脱离强化了沸腾传热效率;热流密度越大,液膜表面的稳定性越差;Re的提高能够增强相界面稳定性;降膜沸腾传热方式的不同对传热系数影响很大;在计算工况范围内,绘制出传热模态分布图,为工程应用提供基础。     

缸盖冷却水的单相流沸腾模型

针对缸盖水腔内的冷却水流动沸腾传热计算,本文介绍了两种单相流沸腾模型.模型认为流动沸腾总传热量等于泡核沸腾和单相对流传热之和,其中泡核沸腾传热计算采用修正后的容积沸腾传热计算公式.BDL模型在Chen模型的基础上作了改进,考虑了冷却水局部流动参数及饱和状态的影响,适用于局部流动传热计算.     

Experimental Investigation on Observed Scattering in Heat Transfer Characteristics for Flow Boiling in a Small Diameter Tube

The effects of flow fluctuation at the test section inlet on flow boiling heat transfer were investigated for FC72 by the use of a horizontal circular tube with a diameter of 0.51 mm. Flow fluctuation was minimized by employing a high-powered syringe pump in one experiment and intensified in another experiment by the connection of an auxiliary tank exposed to atmosphere allowing the reverse flow. In the experiments of strict inlet flow rate regulation, heat transfer characteristics were similar to those observed in normal size tubes. However, if the flow rate fluctuated by up to ± 20% of the total under the weak inlet flow rate regulation, the trend of the heat transfer coefficient increasing with increasing vapor quality in moderate vapor quality region disappeared, and heat transfer deterioration due to partial dryout started at lower vapor quality. Boiling heat transfer characteristics in minichannels could be changed considerably by the existence of flow fluctuation caused by the rapid axial growth of elongated bubbles at low vapor quality. The regulation of inlet flow rate seems to be a key parameter in reducing the scattering in heat transfer data encountered in flow boiling of minichannels. Experimental data obtained by using a pump of weak power or data using a liquid reservoir can never be inherently consistent with those obtained for the constant inlet flow rate conditions. Although such a weak restriction of inlet conditions is actually encountered in application systems, the difference in heat transfer characteristics between the normal and mini-tubes should be clarified, as the first objective of the research, under the same inlet conditions without flow fluctuation.     

Experimental Study on the Effect of Stabilization on Flow Boiling Heat Transfer in Microchannels

The effect of flow instabilities on flow boiling heat transfer in microchannels is investigated using water as the working fluid. The experimental test section has six parallel rectangular microchannels, each having a cross-sectional area of 1054 × 197 microns. Flow restrictors are introduced at the inlet of each microchannel to stabilize the flow boiling process and avoid the backflow phenomena. The mass flow rate, inlet temperature of water, and the electric current supplied to the resistive cartridge heater are controlled to provide quantitative heat transfer information. The results are compared with the unrestricted flow configuration.     

Heat transfer and pressure drop during flow boiling of pure refrigerants and refrigerant/oil mixtures in tube with porous coating

The experimental stand and procedure for flow boiling investigations are described. Experimental data for pure R22, R134a, R407C and their mixtures with polyester oil FUCHS Reniso/Triton SEZ 32 in a tube with porous coating and smooth, stainless steel reference tube are presented. Mass fraction of oil was equal to 1% or 5%. During the tests inlet vapour quality was set at 0 and outlet quality at 0.7. Mass velocity varied from about 250 to 500 kg/m²s. The experiments have been conducted for average saturation temperature 0 °C. In the case of flow boiling of pure refrigerants, the application of a porous coating on inner surface of a tube results in higher average heat transfer coefficient and simultaneously in lower pressure drop in comparison with the flow boiling in a smooth tube for the same mass velocity. Correlation equation for heat transfer coefficient calculation during the flow boiling of pure refrigerants inside a tube with porous coating has been proposed.     

Transient behavior simulation of fin-and-tube heat exchangers for the variation of the inlet temperatures of both fluids

In this article, a transient behavior simulation of fin-and-tube heat exchangers has been studied. Energy equation for fluid flow and tube wall is derived and solved numerically. The variation of the temperatures of both fluids with time and position are obtained for a step-change in the inlet temperatures of the water and air fluids. The results show that in step-change of inlet water-side temperature, outlet water-side temperature will get steady faster than air-side, while in step-change of inlet air-side temperature, outlet air-side temperature will become steady state faster than water-side. Also, the time constant (the time interval that the flow will reach steady state) of the system is not influenced by the step-change amplitude of inlet air and water temperatures. The inlet water temperature expands along the tube and after a time interval, it reaches the outlet section of the tube. But, the inlet air temperature reaches the outlet section without time delay.     

Correlation of high critical heat flux during flow boiling for water in a small tube at various subcooled conditions

High critical heat fluxes (CHFs) for subcooled boiling of water in a small tube were investigated experimentally. A platinum tube with an inner diameter of 1.0 mm and a length of 40.9 mm was used in the experiment. The upward flow velocity, the subcooling of water, and the outlet pressure of the experimental tube were varied to enable a parametric study of the CHFs. The flow velocity ranged from 9 to 13 m/s and the inlet subcooling ranged from 69 to 148 K. The boiling number decreased with increasing Weber number. The boiling number is also dependent on a non-dimensional parameter and the density ratio of liquid to vapor. A correlation for the high CHF of the small tube was obtained based on the experimental data. Finally, the high CHF correlation was evaluated using the CHF data obtained by other researchers.     

Flow Boiling in Rectangular Microchannels: 1-D Modeling of the Influence of Inlet Resistance on Flow Reversal

Pressure changes caused by the growth of confined bubbles during flow boiling in mini-/microchannels lead to transient flow reversal in the presence of inlet (upstream) compressibility. A one-dimensional (1-D) model is presented to study the effect of inlet resistance on maximum flow reversal distance, local pressure fluctuations for different initial upstream compressible volumes, channel dimension, locations of nucleation site, heat flux, and initial channel velocity for water and FC-72 at atmospheric pressure and R134a at 800 kPa. The two upstream compressibility models considered are condensable vapor in a subcooled boiling region and trapped noncondensable gas.     

Boiling of water and FC-72 in microchannels enhanced with novel features

A microchannel test section comprised of parallel square microchannels with a 25 × 25 μm and 50 × 50 μm cross section was manufactured. Boiling of perfluorinated dielectric fluid FC-72 and water in microchannels was studied. Troublesome occurrences associated with flow boiling in microchannels were reduced or eliminated with inlet/outlet restrictors, inlet/outlet manifolds and potential nucleation cavities incorporated in the array of microchannels. The gradual reduction of channel cross section in the manifolds ensured a uniform distribution of the working fluid among the microchannels. The flow restrictors provided a higher upstream pressure drop in comparison with the downstream pressure drop which favors vapor flow in the downstream direction and consequentially suppresses the vapor backflow present in flow boiling. The superheat of the microchannel wall necessary for the onset of boiling was decreased significantly with the incorporation of properly sized artificial cavities. Experimental results confirmed the benefits of the etched features, as there was (i) an even working fluid distribution (ii) without dominating backflows of vapor (iii) at a low temperature of the onset of boiling. Bubble growths as well as other events in the microchannels were visualized with a high-speed imaging system which captured images at over 87,000 frames per second. Results exhibit boiling hysteresis dependence of the working fluid and its mass flux through the microchannels. The temperature of the onset of boiling is highly dependent on the working fluid, microchannel size and its roughness.     

方形分离器结构优化试验研究

在冷态试验台上对特征尺寸D=300 mm的入口带加速段的方形分离器进行结构优化研究。结果表明:分离效率随着芯筒直径(d)、芯筒插入深度(s)的增大均呈先增大后减小的趋势;入口高宽比(a/b)与直段高度(h)对分离效率的影响存在交互作用;随着a/b的增大,分离效率先增大后减小;不同入口高宽比时,分离效率随直段高度的变化趋势不同,当a/b>5.92时,随着直段高度的增大,分离效率先减小后增大,在h/D=2.3时最低;当a/b<5.92时,分离效率随着直段高度的增大而减小;分离器阻力随着入口高宽比的增大而增大,随着直段高度的增大而减小。4个参数的最优值分别为:d=0.4D、s=0.6D、a/b=8和h=1.8D,此时对应的分离器阻力为1.22 kPa。     

Study of flow boiling characteristics of R134a in annulus of enhanced surface tubing

The paper presents an experimental study of the flow-boiling heat-transfer characteristics of R12 and R134a in the annulus of a horizontal enhanced-surface-tubing evaporator. The test section has an inner-tube bore diameter of 17.5 mm, an envelope diameter of 28.6 mm and an outer smooth tube of 32.3 mm inside diameter. The ranges of heat flux and mass velocity covered in the tests were 5–25 kW/m² and 180–290 kg/m²/s, respectively, at a pressure of 365 kPa. In order to establish the flow regime conditions at the inlet to the test section, the test rig allows for the visualization of refrigerant flow through the preheater. The experiments show two regions of heat transfer: a nucleate boiling region where the heat transfer depends mainly on heat flux, and a forced convective region where the heat transfer depends only on the refrigerant flow rate.     

R1234yf Flow Boiling Heat Transfer Inside a 2.4-mm Microfin Tube

ABSTRACT

This 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.