纳米尺度固体悬浮颗粒强化池沸腾换热的实验研究

文中对纳米尺度的固体悬浮颗粒对泡状池沸腾换热的影响进行了实验研究，固体颗粒为粒径0.0016mm的黄沙和28nm的三氧化二铝粉末。实验分析了悬浮颗粒的沸腾特性，并讨论了颗粒大小、颗粒浓度对换热的影响。实验结果表明，在池沸腾条件下加入纳米尺度的固体颗粒能够有效的强化换热，强化效果与固体颗粒的大小及颗粒密度有关。     

R134a在水平微小圆管内流动沸腾过程中压降对换热的影响

微小通道内流动沸腾换热研究进程制约着紧凑式微小通道蒸发器的进一步开发和应用.针对HFC134a在1.0 mm水平圆管内流动沸腾换热的研究,设计并建立了开放直流式实验装置;在对测试数据分析的基础上,提出了局部饱和温度沿管长呈线性降低的假定;表明了压降对换热系数的较大影响,最后对实验不确定度进行了分析.     

声空化场下纳米颗粒对沸腾传热影响的实验研究

作者以添加有纳米颗粒的丙桐溶液为工质,首次通过实验系统地研究了声空化场方向及强度,液体过冷度,纳米材料浓度和光照射等对水平圆管沸腾传热的影响,对其进行了机理分析。     

微细管内R290流动沸腾换热与干涸特性研究

对内径为1、2、3 mm的水平不锈钢圆管内R290两相流动沸腾换热特性进行了理论与实验研究。分析了热流密度为15~35 kW/m~2、质量流率为76~200 kg/(m~2·s)、饱和温度为16~36℃、干度为0~1时的管内传热特性。研究结果表明:热流密度的增加促进管内核态沸腾,换热得到强化,从而导致换热系数随之增加;质量流率的增加促进管内由核态沸腾换热向对流换热转化,换热系数也随之增加;饱和温度的增加促进管内气泡核心的形成速率加快,强化管内沸腾换热;管径的减小导致微尺度效应增加,从而导致换热系数随之增加;在整个换热过程中干涸前平均换热系数、干涸过程中的平均换热系数分别占总换热系数的40%、37%。     

纳米颗粒及其接触角对沸腾传热系数的影响

选取具有不同接触角的SiO_2纳米流体进行沸腾实验,探究纳米颗粒接触角对沸腾传热系数的影响。通过观察沸腾过程中气泡大小及沸腾实验后加热面上的沉积层形态,分析纳米颗粒接触角对沸腾气泡和沉积层形态的影响,进而揭示了纳米颗粒表面的润湿性对沸腾传热系数的影响机理。结果表明:纳米颗粒的接触角会影响纳米流体沸腾时气泡的稳定性和加热面上纳米沉积层的形态,不同表面形态具有不同的换热效果;中等亲水的纳米颗粒能吸附在气泡的气液界面上,使沸腾传热系数增大;加热面上沉积的中等亲水纳米颗粒会增大加热面粗糙度,产生更多的汽化核心,使沸腾传热系数增大。     

紧凑满液型叉排管束蒸发换热器内水的沸腾强化换热特性

采用紧凑满液型蒸发换热器,利用水平传热管叉排管束狭窄空间内早期沸腾强化换热机理将中小热负 荷条件下的自然对流换热转化为旺盛核沸腾换热,换热性能大大优于传统的降膜式蒸发换热器。对水平传热管 管束在受限空间内沸腾强化换热进行实验研究,确认了紧凑满液式水平管蒸发换热器具有良好的换热性能,传 热管在管束中的位置对换热特性已经没有明显影响,随着压力增加,受限空间内沸腾强化换热强化效果显著增 加。     

水平微细管道内R717沸腾换热特性研究

搭建了氨(R717)沸腾换热测试台,对内径3 mm水平光管内R717的沸腾换热特性进行了测试,分析热流密度、干度、饱和温度及质量流率对沸腾换热及换热方式的影响。实验热流密度15～40 kW/m~2,质量流率40～160 kg/(m~2·s),饱和温度-5、0和5℃,干度0.1～0.9。结果表明:在氨制冷剂管内沸腾换热的过程中,质量流率过低和热流密度过高会导致干涸传热恶化,换热形式由核态沸腾换热向气态氨制冷剂强制对流换热转变,同时也影响干涸的起始干度;在干涸发生前,沸腾换热系数随着干度的增加而增大,逐渐达到峰值;在干涸发生后,传热恶化导致换热系数急剧降低;饱和温度升高会加快核态沸腾气泡生成速率,强化沸腾换热,但干涸的起始干度随着饱和温度升高而降低。     

“球囊夹紧法”取出锁骨下动脉支架推送杆断裂残端一例

针对水平光滑管和微肋管,基于FLUENT平台对制冷剂管内沸腾传热特性进行了数值模拟,研究质量流量、热流密度及干度等因素对制冷剂R245fa沸腾换热系数的影响。模拟结果表明:沸腾换热系数随着制冷剂质量流速与热流密度的增加而提高;随着干度的增加,换热系数先增加再降低,并在x=0.7时达到极大值;相比光滑管,微肋管内制冷剂的沸腾传热系数能提高10%~25%。     

水平管内制冷剂沸腾传热特性的数值模拟

针对水平光滑管和微肋管,基于FLUENT平台对制冷剂管内沸腾传热特性进行了数值模拟,研究质量流量、热流密度及干度等因素对制冷剂R245fa沸腾换热系数的影响。模拟结果表明:沸腾换热系数随着制冷剂质量流速与热流密度的增加而提高;随着干度的增加,换热系数先增加再降低,并在x=0.7时达到极大值;相比光滑管,微肋管内制冷剂的沸腾传热系数能提高10%~25%。     

国内能源环保文献索引

02 0 40 1　赵言冰 ,施明恒 .纳米尺度固体悬浮颗粒强化池沸腾换热的实验研究 [Ｊ] .能源研究与利用 ,2 0 0 2 ,(3 ) :18-2 0 .实验分析了悬浮颗粒的沸腾特性 ,并讨论了颗粒大小、颗粒浓度对换热的影响。实验结果表明 ,在池沸腾条件下加入纳米尺度的固体颗粒能够有效的强化换热 ,强化效果与固体颗粒的大小及颗粒密度有关。0 2 0 40 2　施慧琴 .冷凝水回收系统改造 [Ｊ] .能源研究与利用 ,2 0 0 2 ,(3 ) :2 6-2 8.分析了扬子石化公司工艺外管网蒸汽伴热系统存在的问题 ,介绍了冷凝水回收系统改造情况以及改造后取得的经济效益 ,并对尚存问题…     

Nucleate pool-boiling enhancement outside a horizontal bank of twisted tubes with machined porous surface

Nucleate pool boiling of refrigerants is of important application in the flooded evaporator of refrigeration and air-conditioning system. Many surface geometries involve machined porous surface have been adopted to enhance the nucleate pool boiling heat transfer of refrigerants. Nucleate pool-boiling performance of R134a and R142b outside a horizontal bank of twisted tubes with machined porous surface (T-MPS tubes) was investigated in this paper. The experimental results showed that the T-MPS tube bank could enhance boiling heat transfer evidently. The enhancement ratios of R134a from the T-MPS tube bank were 1.4–1.7 and the maximum enhancement ratio of R142b could reach up to 4.4. Analyzing the tube bank effects of boiling heat transfer for R134a and R142b, the overall trend showed that the boiling heat transfer performance of the T-MPS tube bank was inferior to that of single T-MPS tube slightly.     

Similarity solutions for pressure-driven boiling flows in capillary porous media

A similarity transformation has been proposed to study the two-phase boundary layer equations derived by Wang and Beckermann for capillary porous media. By virtue of the present transformation, the set of governing equations and boundary conditions for the pressure-driven boiling flow over a body of arbitrary shape reduces to the one for the flow over a flat plate. The dryout heat fluxes for pressure-driven boiling flows over a plate, a wedge, a cone, a sphere and a horizontal circular cylinder are correlated with the fluid thermophysical properties and porous medium parameters. The study confirms that the capillary force plays an important role in boiling flows in porous media.     

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.     

Nucleate pool boiling characteristics from coated tube bundles in saturated R-134a

Nucleate pool boiling heat transfer from plasma coated copper tube bundles with porous copper (Cu) immersed in saturated R-134a was experimentally studied. The bundle is composed of 15 tubes (of which the number of heated/instrumented tubes was varied) arranged in four different configurations with a pitch-to-diameter ratio of 1.5. The influences of various parameters, for instance, bundle arrangements and heat flux were clarified. Tests were conducted with both increasing and decreasing the heat flux. The data presented indicated that at low heat fluxes, the vertical-in-line tube bundles have the highest bundle factor. A configuration factor was proposed which can be used to characterize the geometric arrangements of the bundles.     

表面强化管外池沸腾传热特性研究

为实现节能降耗,开发了多种强化沸腾传热的高效换热管。以水为工质,在0.1MPa下对垂直光管、烧结多孔管和T槽管进行了池沸腾传热实验研究,并分析了沿管子轴向的温度分布。实验结果表明,烧结多孔管与T槽管能显著降低起始沸腾过热度、强化沸腾传热:烧结多孔管和T槽管的起始沸腾过热度比光管的低1.5K左右;烧结多孔管和T槽管的核态沸腾传热系数分别为光管的2.4～3.2倍和1.6～2.0倍。此外,烧结多孔管和T槽管能降低相同热流密度下的壁面温度,且有利于降低管子轴向的温差。     

LDV assisted bubble dynamic parameter measurements from two enhanced tubes boiling in saturated R-134a

LDV measurements and heat transfer experiments for nucleate pool boiling from two horizontal enhanced tubes with porous copper on copper surfaces immersed in saturated R-134a were conducted. The influence of tube position (alignment) and tube pitch on bubble dynamics and boiling characteristics were studied. Photographs indicate that the average number of bubbles increases with heat fluxes, which is the same as those in previous studies of single tube. However, the bubble departure diameters of the upper tube show an opposite trend with an increase as compared to previous single-tube studies. LDV measurements show that the present tube arrangement has significant influence on local velocity in both magnitude and trend. The heat transfer mechanism and modeling for the upper tube were studied and developed.     

Numerical study and sensitivity analysis on convective heat transfer enhancement in a heat pipe partially filled with porous material using LTE and LTNE methods

An improved design for convective heat transfer in a heat pipe partially filled with porous medium is presented. In the present study, porous media is used to increase heat transfer in laminar flow inside the tube with a constant heat flux boundary condition. The porous segments are set in different positions in the tube, while the ratio of porous volume to total volume of tube is considered to be constant. A performance evaluation criteria (PEC), which takes account of both heat transfer and pumping power, is defined to find the enhanced mode of porous media arrangement. According to the current results, PEC increases with the number of porous segments. Moreover, the sequence of unequal segments arrangement within the tube is from the largest to the smallest part for a higher PEC. Effects of parameters including porosity, Darcy number, and ratio of effective coefficient of thermal conductivity to coefficient of thermal conductivity of fluid (TKR) are investigated for sensitivity analysis. Simulations are conducted using the local thermal equilibrium method. In addition, the local thermal nonequilibrium is also used for comparison. For TKR numbers less than 10, these models show the same results with negligible differences except for TKR more than 10.     

Pool Boiling of Water–Al2O3 and Water–Cu Nanofluids Outside Porous Coated Tubes

This paper deals with pool boiling of water–Al₂O₃ and water–Cu nanofluids on porous coated, horizontal tubes. Commercially available stainless-steel tubes having 10 mm outside diameter and 0.6 mm wall thickness were used to fabricate a test heater. Aluminum porous coatings 0.15 mm thick with porosity of about 40% were produced by plasma spraying. A smooth tube served as a reference tube. The experiments were conducted under different absolute operating pressures of 200 kPa, 100 kPa, and 10 kPa. Nanoparticles were tested at concentrations of 0.01%, 0.1%, and 1% by weight. In all cases tested, enhancement heat transfer was always observed during boiling of water–Al₂O₃ and water–Cu nanofluids on smooth tubes compared to boiling of distilled water. Contrary to smooth tubes, addition of even a small amount of nanoparticles resulted in deterioration of heat transfer during pool boiling of water–Al₂O₃ and water–Cu nanofluids on porous coated tubes in comparison with boiling of distilled water.     

Numerical modeling of high-temperature shell-and-tube heat exchanger and chemical decomposer for hydrogen production

Numerical simulations of shell-and-tube heat exchanger and chemical decomposer with straight tube configuration and porous media were performed using FLUENT6.2.16 to examine the percentage decomposition of sulfur trioxide. The decomposition process can be a part of sulfur–iodine (S–I) thermochemical water splitting cycle, which is one of the most studied cycles for hydrogen production. A steady-state, laminar, two-dimensional axisymmetric shell-and-tube model with counter flow and parallel flow arrangements and simple uniform cubical packing was developed using porous medium approach to investigate the fluid flow, heat transfer and chemical reactions in the decomposer. As per the investigation, the decomposition percentage of sulfur trioxide for counter flow arrangement was found to be 93% and that of parallel flow was 92%. Also, a high pressure drop was observed in counter flow arrangement compared to parallel flow. The effects of inlet velocity, temperature and the porous medium properties on the pressure drop across the porous medium were studied. The influence of geometric parameters mainly the diameter of the tube, diameter of the shell and the length of the porous zone on the percentage decomposition of sulfur trioxide in the tube was investigated as well. A preliminary parametric study of the mentioned configuration is conducted to explore effects of varying parameters on the decomposition of sulfur trioxide. From the performed calculations, it was found that the Reynolds number played a significant role in affecting the sulfur trioxide decomposition. The percentage decomposition decreases with an increase in Reynolds number. Surface-to-volume area ratio and activation energy were also the important parameters that influenced the decomposition percentage.     

Laminar film condensation on an elliptical tube embedded in porous media

Laminar film condensation of saturated vapor flowing over an isothermal elliptical tube embedded in a porous medium is analyzed for conditions of free and forced convection. The flow field in the porous medium is described by the Darcy–Brinkman–Forchheimer model. The effect of vapor shear on condensation is determined by simultaneous solution of the two phase vapor boundary layer and condensate film momentum equations. The numerical results, which are presented in the form of local film thickness and local Nusselt number, show a dependence of these physical parameters on practical dimensionless parameters such as Reynolds number, Darcy number, Bond number and eccentricity.