环路热虹吸管间歇沸腾可视化实验研究

针对两相环路热虹吸管中出现的间歇沸腾不稳定现象,分别以R134a、水和无水乙醇作为工质,通过流场可视化实验观测,探究了间歇沸腾出现的条件及其对环路传热特性的影响.实验结果表明,在中等充液率和中等加热热流密度条件下更容易发生间歇沸腾现象;流型的周期性变化引起环路内部压力和温度波动,同时会增加环路的均温性;流型变化和波动特...     

Heat transfer performance of a toluene loaded two-phase thermosyphon

Large dimension thermosyphons are efficient heat transfer components in heat recovery systems. Their performance limits depend on the following parameters: geometrical (length, diameter, inclination angle), physical (fluid, fill charge), thermal (temperature, heat flux).An experimental investigation was carried out with a large dimension, closed, two-phase thermosyphon which correspond to a device used in industrial recuperators. A vertical or inclinded steel thermosyphon, 3 m long and 27 mm inner diameter, was tested at temperatures varying from 100°C to 300°C with toluene as the working fluid. The lower part of the pipe was electrically heated along a variable length and the upper zone was cooled with an air stream whose flow rate and temperature were controlled. The maximum heat flux was measured as a function of temperature for different liquid fill charges and inclination angles. From these experimental data, boiling and condensation heat transfer coefficients were deduced. It was observed that the critical heat flux depends little on the fill ratio unless the charge is less than 20% for which a local dry-out occurs. The optimal fill charge was found to be between 20% and 50%. Experimental data have been compared with existing theories. The inclination effects have been taken into account with an empirical formula.     

不同工质中汽泡周围电场特性研究

为探明外加电场作用下,不同工质对强化沸腾换热的影响,本文采用了FC-72、R11、R113和R123四种工质,定量研究了两相系统中汽泡周围电场分布特性,探讨了工质介电常数对汽泡的作用规律．结果表明:随着工质介电常数的增加,电场非均匀性增强,电场对汽泡的作用加剧．这为不同工质对EHD沸腾强化效果不同提供了理论依据．     

Heat pipe research and development in east european countries

The paper presents a review, made on the basis of available publications, of the research, development and application of heat pipes and closed two-phase thermosyphons in East European countries, i.e. Bulgaria, Czechoslovakia, German Democratic Republic, Hungary, Poland, Rumania and Yugoslavia from 1984 to 1986.     

Heat Transfer Analysis between R744 and HFOs inside Plate Heat Exchangers

Plate heat exchangers (PHE) are used for a wide range of applications, thus utilizing new and unique heat sources is of crucial importance. R744 has a low critical temperature, which makes its thermophysical properties variation smoother than other supercritical fluids. As a result, it can be used as a reliable hot stream for PHE, particularly at high temperatures. The local design approach was constructed via MATLAB integrated with the NIST database for real gases. Recently produced HFOs (R1234yf, R1234ze(E), R1234ze(Z), and R1233zd(E)) were utilized as cold fluids flowing through three phases: Liquid-phase, two-phase, and gas-phase. A two-step study was performed to examine the following parameters: Heat transfer coefficients, pressure drop, and effectiveness. In the first step, these parameters were analyzed with a variable number of plates to determine a suitable number for the next step. Then, the effects of hot stream pressure and cold stream superheating difference were investigated with variable cold channel mass fluxes. For the first step, the results showed insignificant differences in the investigated parameters for the number of plates higher than 40. Meanwhile, the second step showed that increasing the hot stream pressure from 10 to 12 MPa enhanced the two-phase convection coefficients by 17%, 23%, 75%, and 50% for R1234yf, R1234ze(E), R1234ze(Z), and R1233zd(E), respectively. In contrast, increasing the cold stream superheating temperature difference from 5 K to 20 reduced the two-phase convection coefficients by 14%, 16%, 53%, and 26% for R1234yf, R1234ze(E), R1234ze(Z), and R1233zd(E), respectively. Therefore, the R744 is suitable for PHE as a driving heat source, particularly at higher R744 inlet pressure and low cold stream superheating difference.     

Heat pipe research and development in the U.S.S.R.

The review covers a number of works on heat pipes designed in the U.S.S.R. between 1970 and 1987, and consists of two parts. The first part deals with fundamental studies of energy and substance transfer in closed evaporator-condenser systems. The second one contains the results of integration of heat pipes, thermosyphons and units on their base in different branches of engineering, agriculture and building.     

CO_2两相回路启动过程阶段蒸发和干烧现象分析

采用并联蒸发器作为传热元件,并联冷凝器作为散热部件,建立了包括热量收集、传递与排散的机械泵驱动两相回路集成试验系统,并对该系统的热启动进行实验研究.实验结果表明:(1)在冷工况、低热流密度的情况下,采用长程毛细管结构的蒸发器对分布式小热源进行集热时,会出现阶段性形核沸腾蒸发,并且上游过热液体形核沸腾后会导致下游出现干烧现象.(2)对于该系统,并联蒸发器相同热量同时启动是相对较好的启动方式.     

聚光型光伏电池冷却热管蒸发端的数值模拟

针对碟式聚光型太阳能光伏电池效率受温度制约的问题,采用两相闭式热虹吸管(重力热管)散热的方式,工质采用水.热管的蒸发端与聚光太阳能电池接触,其温度场对电池性能和热管效率影响显著.通过FLuENT建立了蒸发端底部的数学模型,计算过程中考虑热流密度、蒸汽饱和温度以及充液量对蒸发端性能的影响.计算结果表明,在聚光倍数为140...     

INFLUENCE OF FLUID FLOW ON NUCLEATE BOILING FROM A TUBE

Abstract

The mechanisms of nucleate boiling on the outside of a horizontal tube differ fundamentally from those on a flat plate. In this experimental work the variation of heat transfer coefficient around the periphery of a tube is measured with the aim of clarifying these mechanisms. A specially designed tube is used in which local variations are not masked by conduction through the metal surface. The tube diameter is 27 mm, and the working fluid is R113 under saturated conditions at 1 atm.

When there is no imposed velocity the peripheral variations are typically 10–20% with the maximum heat transfer coefficient at the base of the tube. At very low velocity of upward flow ( ? 0.1 m/s) there is a marked change in the variation, with the maximum coefficient occurring at a point about 70° from the base. At higher velocities there is a slight increase in angle to the maximum point with maximum peripheral variations in heat transfer coefficient of around 25%.

The variations are explained in terms of nucleate boiling at the base and top and flow boiling at the sides. The thin layer of two-phase bubbly flow at the sides leads to the predominance of heat transfer caused by sliding bubbles aver other mechanisms in this region. The complex mix of mechanisms involved in boiling on tubes implies an inherent limit to the accuracy of predictive correlations.     

Evaporative Heat Transfer Characteristics of R404a and R134a under Varied Heat Flux Conditions

The two-phase heat transfer coefficients of R404A and R134a in a smooth tube of 7.49-mm inner diameter were experimentally investigated at low heat and mass flux conditions. The test section is a 10-m-long counter-flow horizontal double-tube heat exchanger with refrigerant flow inside the tube and hot fluid in the annulus. The heat transfer coefficients along the length of the test section were measured experimentally under varied heat flux conditions between 4 and 18 kW m^?2 and mass flux ranging between 57 and 102 kg m^?2 s^?1 (2.5 to 4.5 g s^?1) for saturation temperatures of ?10°C, ?5°C, and 0°C. The saturation temperatures correspond to pressures of 4.4, 5.2, and 6.1 bar for R404A and 2.0, 2.4, and 3.0 bar for R134a, respectively. The results showed that under the tested conditions, the contribution of the nucleate boiling mechanism is predominant in the heat transfer coefficient throughout the flow boiling process. The Kattan–Thome–Favrat flow pattern maps confirm the occurrence of stratified and stratified-wavy flow patterns for all of the tested conditions. The average heat transfer coefficient of R404A is estimated to be 26 to 30% higher than that of R134a for the same saturation temperature.     

Two-phase flow boiling heat transfer of FC-72 in parallel micro-channels

Experimental research is performed on two-phase flow boiling heat transfer in micro-channels. FC-72 is used as the working fluid. In order to analyze the heat transfer mechanism during two-phase flow boiling, the dimensionless parameters, e.g., boiling number and convection number, are used, and the effect of these parameters on the heat transfer can be confirmed during flow boiling in the micro-channel. In addition, the transition criterion from bubbly/slug flow to annular flow is proposed from the modified Weber number. Based on the boiling heat transfer mechanism obtained from the experiments, a new correlation is proposed to predict the heat transfer coefficient. The new correlation predicts well the experimental results within a mean absolute error of 5.2%.     

Experimental Investigation of Saturated Flow Boiling Heat Transfer to TiO2/R141b Nanorefrigerant

Heat transfer in two-phase flow boiling of a dilute mixture of TiO₂ nanoparticles in R141b base fluid in a smooth tube is investigated experimentally. Examining the obtained results reveals that enhancement of the convective heat transfer coefficient for the particle volume fractions of 0.01% and 0.03% in comparison with pure R141b is more pronounced for a higher volume fraction. The measured data also show that at higher vapor qualities, the improvement in heat transfer coefficient is greater. Moreover, the heat transfer coefficient decreases substantially with mass flux while an increase in saturation temperature leads to an improvement in this coefficient.     

利用重力增压的新型有机工质热力发电循环

与水蒸气朗肯循环给水泵相比,有机朗肯循环工质泵存在技术难度大、效率低、易气蚀和单位功率成本高等问题。本文提出了一种利用重力增压的新型有机工质热力发电循环,冷凝器出口工质不经过泵而依靠重力增压,然后进入蒸发器气化。分别采用R113、R123和R245fa三种干工质分析了不同蒸发温度和冷凝温度下循环所需的重力增压高度。并基于泵的实验数据,比较了该热力循环与泵增压有机朗肯循环的性能。结果表明,相同工作温度下沸点和密度越高的工质所需的重力增压高度越小。在蒸发温度100℃和冷凝温度50℃时,若采用R113,新型循环所需的重力增压高度为22.2 m,热效率为8.1%,比泵增压循环效率高约0 8%。该重力增压循环显示了应用于热电联供领域的潜力。     

竖直微槽道内沸腾换热CHF实验研究

对于沸腾换热,一个主要的约束条件就是临界热流密度(Critical Heat Flux,简称CHF)。这个约束条件对沸腾换热量有一个最高值的限制。文中对矩形微槽道中的流动沸腾临界热流密度进行了实验研究。实验数据是在不同尺寸(0.15mm;0.4mm;1mm)微槽道中,在较大范围的面积质量流速和不同进口过冷度下,以去离子水为工质得到的。实验过程中发现,达到CHF时,靠近出口壁面温度会突然升高,此时传热效率迅速下降。实验数据分析结果表明:CHF随质量流量的增加而增加;进口过冷度对CHF没有明显影响;CHF随着出口干度的增加而降低。     

Upward and downward heat and mass transfer with miniature periodically operating loop thermosyphons

Upward and downward two-phase heat and mass transfer has been considered in the present paper. The heat and mass transfer with the condenser located below the evaporator has been obtained by inserting an accumulator tank in the liquid line of a loop thermosyphon and enforcing a pressure pulsation. In previous papers these heat transfer devices have been called pulsated two phase thermosyphons (PTPT). A mini PTPT has been experimentally investigated. It has shown a stable periodic heat transfer regime weakly influenced by the position of the condenser with respect to the evaporator. In contrast a classical loop mini thermosyphon (diameter of connecting pipes 4 mm) did not achieve a stable functioning for the investigated level differences between evaporator and condenser lower than 0.37 m. The present study shows that the functioning of a PTPT device does not directly depend on the level difference or the presence of noncondensable gas. In order to obtain a natural circulation in mini or micro loops, a periodically operating heat transfer regime should therefore be considered.     

Process industry heat exchanger R&D with possible implications for aerospace technology

Important European co-ordinated research activities in heat exchanges are within the European Community Non-Nuclear Energy R&D Programme entitled JOULE, while national governments and a wide variety of industrial laboratories and companies also carry out work in these areas.This paper will cover the past and present JOULE activities in a number of ways. R&D projects within this programme include gas-gas, gas-liquid, gas-solid and liquid-liquid heat exchangers, with high, medium and low temperature terrestrial process applications. Examples of past projects which are described include compact plate heat exchangers, novel high capacity thermosyphons, the use of jet impingement to enhance heat transfer, fouling and complementary activities such as expert systems. Projects currently being studied within JOULE concentrate mainly on two-phase heat transfer enhancement and fouling of both gas-side and liquid-side heat transfer equipment.In order to maintain the competitiveness of industry, the paper underlines the necessity to continue the R&D activities in heat exchanger technologies.     

INVESTIGATION OF BOILING OF REFRIGERATING MEDIUM UNDER CONDITIONS OF IMPULSE DISTURBANCES

This article describes investigations of bubbly boiling in flow of a refrigerating medium under conditions of impulse disturbances occurring in refrigeration engineering systems. The development and decay of boiling is induced by impulsively changing parameters, such as pressure and mass flux density, of the two-phase system in a tubular channel. It is confirmed that the two-phase systems exhibit wave properties and the induced disturbances propagate with a finite velocity. Correlations are elaborated to evaluate the propagation velocity of disturbance signals during the development and decay of boiling in a coil tube. Investigations are conducted by feeding under pressure a heat exchanger with an environment-friendly refrigerant R404A.     

Advection and Thermal Diode

We prove that under the condition of closed boundary to mass flux, pure advection is not a valid mechanism to make a practical thermal diode. Among the various designs of thermal diodes, many of them involve circulating fluid flow, such as in thermosyphons. However, those designs often employ natural convection, which is basically a nonlinear process. It thus remains unclear how the pure advection of temperature field induced by a decoupled velocity field influences the symmetry of heat transfer. Here we study three typical models with pure advection:one with open boundary, one with closed boundary at unsteady state, and one with closed boundary at steady state. It is shown that only the last model is practical, while it cannot become a thermal diode. Finally, a general proof is given for our claim by analyzing the diffusive reciprocity.     

INTERNAL FLOW PATTERNS OF AN INCLINED CLOSED TWO-PHASE THERMOSYPHON AT CRITICAL STATE: CASE STUDY II,EFFECT OF BOND NUMBER

The effects of Bond number (Bo) on internal flow patterns of an inclined, closed, two-phase thermosyphon at critical state are considered in this article. The thermosyphons used employed a glass tube for the evaporator and the adiabatic section, and a copper tube for the condenser section. The internal diameters of the thermosyphon were 10, 12, and 28.5 mm. The aspect ratios were 5 and 30, with inclination angles of 90 degrees, 60 degrees, and 5 degrees (against the horizontal axis). R123 was used as working fluid, with a filling ratio of 80% and 150% of the evaporator section. A solution of water and ethylene glycol was used to carry heat from the condenser, while water was employed to supply heat to the thermosyphon. A video camera was used to record all the flow patterns, while a still camera was used to record the internal flow patterns at specific times. Heat transfer rate was measured by means of a calorimeter in the condenser section. It was found from the experiments that, as under normal operating conditions, an Le/d of 10 is an important value at which changes of internal flow patterns can be observed. At an Le/d< 10 and with low Bo, the internal flow pattern at critical state changed from bubble flow at vertical to slug flow at an inclined position. Although the flow patterns were the same as with high Bo, the bubbles generated were smaller and appeared only on the upper side. In cases of Le/d>10, the pattern changed from annular flow at vertical to stratified flow at inclined positions for all Bo. Generated bubbles were comparatively small at low Bo. A flow pattern map at each angle was also created by using modified Kutateladze numbers and Reynolds numbers of the vapor.