微细圆管内CO2两相沸腾换热实验研究

本文对CO_2在水平微细管内流动沸腾特性进行实验研究。实验结果表明:热流密度增加对强化核态沸腾换热和高干度区域流型转变具有显著影响,随着热流密度的增加换热系数增加,对摩擦压降影响很小;质量流率对于换热系数的影响较小,但随着质量流率的增加摩擦压降大幅增加,质量流率的大小直接决定了换热过程所经历流态;饱和温度升高换热系数相应升高,摩擦压降减小,且对流态转变特性有重要影响。在同样工况下摩擦压降最大值先于换热系数最大值出现,理论分析采用的流态形式与实际CO_2管内流动流动沸腾换热流态基本一致。     

水平微细管内CO_2流动沸腾换热与干涸特性研究

为分析饱和温度、热流密度、质量流率和管径对CO_2流动沸腾换热特性和干涸特性的影响,对水平微细管内CO_2流动沸腾换热进行了实验研究。实验工况:饱和温度-40～25℃,热流密度5～40 kW/m~2,质量流率180～1 400 kg/(m~2·s),管径0. 50、1. 0和1. 5 mm。实验结果表明:热流密度的增加显著影响核态沸腾换热,加快干涸发展进程的同时,降低干涸起始干度;换热系数受质量流量的影响较小,但质量流率的增加会降低干涸起始干度,干涸后的换热系数有所增加;不同饱和温度下换热特性差异的主要原因是CO_2的热物性受饱和温度的影响较大,饱和温度升高后干涸起始干度具有降低的趋势,且干涸后换热系数下降更为剧烈;在符合微尺度效应的前提下,管径的减小会极大地提高换热系数,同时降低干涸起始干度,干涸后换热系数下降剧烈。     

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%。     

微细通道内R290流动沸腾换热特性实验研究

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%。     

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。     

R290微细通道流动沸腾摩擦压降特性研究

研究R290在管径为3、2和1 mm的水平不锈钢微细通道内,质量流率为73～505 kg/(m~2·s)、热流密度为12. 74～66. 05 k W/m~2、饱和温度为-10～25℃、干度为0～1的范围内的摩擦压降特性,分析R290流动沸腾过程中的摩擦压降变化。结果表明:换热管径尺度微型化使相同条件下的管内压降剧烈上升,质量流率对压降的影响最显著;热流密度的增加对压降的影响很小,几乎为零; R290不同饱和温度物性的改变是造成其不同温度时压降特性差异的主要原因,随着饱和温度的升高,摩擦压降变小;压降随着干度的变化在某个干度存在极值。     

R290在微细圆管内流动沸腾摩擦压降特性研究

对制冷剂R290在微细圆管内流动沸腾摩擦压降梯度进行了定性的理论分析和定量的实验研究,分析不同影响因素下其变化规律。实验工况：质量流率50~1 020 kg/(m2•s)、热流密度1~70 kW/m2、管径1~3 mm、饱和温度-10~25 ℃、干度0~1。实验结果表明：质量流率的增大和换热管径的减小,都会造成摩擦压降梯度和增长幅度大幅增加;热流密度值的变化不影响摩擦压降梯度,但会影响摩擦压降达到最大值的时间;摩擦压降梯度随着饱和温度和管径的减小而增大;摩擦压降梯度在中低干度时快速增加,在高干度时增速减小趋于平稳,直至达到最大值后缓慢减小。     

泡沫金属圆管内沸腾传热的数值模拟

为了解泡沫金属沸腾传热原理,建立了泡沫金属圆管三维物理模型,采用BrinkmanForchheimer扩展达西动量方程和C语言编写气液两相质量传递和能量传递的自定义函数,对泡沫金属圆管中沸腾传热现象进行数值模拟,分析了质量流率、干度对流型、压降和沸腾传热系数的影响。模拟结果表明,在一定质量流率下,单位长度压降随着干度的增大成非线性增长趋势;低质量流率时,随着干度的增大,管内流型由分层流过渡到波状流进而过渡到稳定的波状流,传热系数变小;高质量流率时,随着干度的增大,管内流型由弹状流过渡到环状流,传热系数变大。     

3mm微细通道内R134A沸腾换热压降特性研究

环保型制冷剂R134A作为R22的替代品已广泛应用于各种制冷技术,在不断探索更完美的制冷剂替代品过程中,微细通道换热技术也逐渐成为近些年的研究热点。为了研究R134A在3 mm紫铜管内沸腾换热过程中的压降特性,在饱和温度为0～20℃、热流密度为5～10 kW/m~2、干度变化在0～1、质量流率在300～500 kg/(m~2·s)的实验工况下进行实验,通过对压力、温度和干度等重要物理参数的控制和试验数据的分析,得出以下结论:压降在相同干度区随质量流率的增大而增大,但在高干度区和低干度区的增幅不同;干度对压降的影响很大程度上与沸腾过程中的流动型态发生变化相关;饱和温度与压降的关系主要呈现负相关;而热流密度在压降影响中的占比则是很小的一部分。     

微细通道CO_2流动沸腾换热临界热流密度研究

CO2在微细通道内流动沸腾换热过程所具有的临界热流密度(CHF)对于其换热系数有着重要影响。根据国内外现有发表的公开文献的实验数据分析了质量流量、饱和温度、管径等对临界热流密度的影响,并对理论模型与试验数据进行误差分析。发现Bowring预测关联式对小于3 mm管径内临界热流密度预测精度较高,在30%误差范围内可以达到70%预测精度,Wojtan预测关联式具有较小的平均绝对误差。提出了今后CO2在微细通道内沸腾换热CHF的研究方向。     

平行流蒸发器内气液两相流分配均匀性实验研究

平行流蒸发器内气液两相(特别是液相)在各扁管间的分配对其传热性能影响较大,如果各扁管间的气液分配不均匀其传热性能将显著地下降.在不同气-液流量下实验研究了6种不同形式的平行流蒸发器的分支管液体流量分配情况,实验中观察到流型以环状流为主.研究发现,对于竖直向下流动和竖直向上流动,用通过增加管径的方法不能改善液体流量在各分支管的分配,而主管中气液入口的位置对于流量分配均匀性影响较大.     

发动机气道稳流试验流动过程的相似分析

通过对发动机气道内气体流动过程的相似分析，在雷诺Re准则数足够大的前提下，非定性准则欧拉Eu数和相似不变量无因次涡流NR保持不变。这就证实通过稳流试验来测量气道的流通性能是可行的。通过数理推导，评价参数流量系数μσ真实就是欧拉Eu准则数的一种表现形式。     

利用T型三通测量气液两相流体的流量和干度

利用T型三通的相分离特性 ,从被测气液两相流体中分流分离出一部分单相气体 ,通过测量这部分单相气体的流量确定被测气液两相流体的流量或干度。这种方法把两相流体的流量测量转化成了单相流的测量 ,测量仪表的稳定性和可靠性能得到显著改善 ,测量精度得以大幅度提高。试验结果证实 ,分流系数与被测两相流的干度成正比。在本文实验范围内 ,流量和干度的平均测量误差小于± 5 %。     

锥阀空化现象研究及结构优化

为研究锥阀空化现象,运用ANSYS Fluent流体力学有限元计算软件,分析锥阀在内流和外流两种情形下的流场,得到锥阀在内流和外流两种流动方式下空化区域0~3s内气体的体积分数。计算对比发现,在液体流速和压力的突变区域空化现象明显,外流较内流空化区域大,空化程度比内流严重。为降低空化现象对锥阀型腔造成的损伤,优化锥阀型腔阀芯结构,优化模型1可避免内流时空化的发生,优化模型2在内流和外流情况下均能有效避免空化的发生,优化模型2的结构形式更为合理。研究成果可为锥阀设计提供参考。     

多级精馏装置的节能研究与分析

探讨了几十种基本的多级精馏装置的流程，并从节能的角度研究和分析它们的特点及适用条件。     

Shape of isolated bubble in intermittent flows in a horizontal pipe

An experimental study on the shape of a single bubble similar to those observed in a horizontal plug/slug flow was performed using visual observation and conductance probes. The results indicated that the shapes of the bubble nose and the bubble body depend on the Froude number defined by gas/liquid mixture velocity, whereas the shape of the back region of the bubble depends on both the Froude number and bubble length. The photographic images showed that the structural feature of the bubble head is related to the motion characteristics of the bubble. The transition from plug flow to slug flow occurs when the tail of the bubble changes from a staircase to hydraulic jump pattern with the increasing of the Froude number and bubble length. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(5): 276– 285, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20161     

Computer-aided numerical solution for the flow of compressible fluid through a series of identical annular orifices

The paper represents a general semi-empirical model for the calculation of the pressure distribution and the mass flow rate of a compressible fluid flowing through a set of concentric annular orifices in series. The flow through a single orifice is discussed taking into consideration the variation of the discharge coefficient and the effect of kinetic energy being carried from one throttling to the next one. The study is extended to a set of orifices resulting in the general model. A computer program based on this model is described which is applicable for different geometries and overall pressure ratios. Comparison with available experimental results shows that the model gives fairly accurate results.     

Experimental Studies on The Mechanism and Control of Secondary Flow Losses in Turbine Cascades

This paper summarizes the results of the authors' 4 year experimental studies on the secondary flow losses in turbine cascades. Cascade wind tunnel experiments were carried out concerning the influence of aspect ratios, incidence, turning angles and outer endwall divergent angles in order to unveil the evolution mechanism of secondary flow losses in turbine cascades without end clearance. Some methods for controlling the secondary flows are investigated including the blade leaning, blade cambering, endwall convergence and leading edge extension at two ends of the blade.     

Measurement and analysis of tip clearance unsteady flow spectrum in axial-flow fan rotor

The dynamic pressure measurement device and test technology are described in this study. The tip clearance unsteady flow development from the inlet to the outlet of an axial-flow rotor was revealed by analyzing pressure frequency spectrum acquired from measuring the unsteady pressure field of the tip endwall. The experiment provides test basis for thoroughly understanding the tip clearance unsteady flow and building interaction models of tip clearance flow and main flow. __________ Translated from Compressor, Blower & Fan Technology, 2007, (5): 12–15 [译自: 风机技术]     

Response compensation of fine‐wire thermocouples and its application to multidimensional measurement of a fluctuating temperature field

A novel sensor technique is proposed to visualize readily the spatiotemporal behavior of a fluctuating temperature field of air flow. Two temperature probes were fabricated: a rod‐type probe consisting of 24 two‐thermocouple sensors set in line, and a plane‐type probe with 64 two‐thermocouple sensors arrayed on a two‐dimensional grid (8 × 8 points). Each two‐thermocouple sensor used in these probes was composed of two fine‐wire thermocouples (type K) of unequal diameters of 25 and 51 μm, respectively. An adaptive response‐compensation scheme was applied to accurately reconstruct rapidly‐changing airflow temperatures. The plane‐type probe enables visualization of fluctuating temperature fields of an artificially disturbed hot‐air jet and of the adequate capture of spatiotemporal behavior of a rapid circular motion of a hot‐air jet blown out from a hair dryer. A time‐constant estimation scheme was proposed to estimate instantaneous time‐constants which serve as a basis for a real‐time response compensation technique for multidimensional temperature measurement. In addition, by scanning a temperature field with the rod‐type probe, the temperature distribution can be reconstructed in one‐dimensional space and time. This quasi two‐dimensional visualization can become a prototype of a “scanner” for fluid temperature fields. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20355