开孔折流板对列管式换热器传热性能的影响研究

针对由传统弓形折流板结构带来的壳程流动死区,从而引起的流动阻力大、传热效率低等问题,本文对折流板进行开孔,通过数值模拟的方法研究开孔折流板结构对列管式翅片换热器壳侧流体流动、传热及阻力性能的影响。研究发现,折流板开孔后,壳程流动死区明显减小,壳程传热系数及压降同比开孔前降低了;综合换热性能同比开孔前提升了。壳程压降随开孔率及板间距的增大而减小,壳程努塞尔数Nu随板间距的增大逐渐增大。从综合换热性能及场协同的角度分析发现,开孔率x=0.229、折流板间距H=85 mm的列管式换热器综合传热性能最佳。     

The effects of design and operating factors on the frost growth and thermal performance of a flat plate fin-tube heat exchanger under the frosting condition

An experimental study of the effects of various factors (fin pitch, fin arrangement, air temperature, air humidity, and air velocity) on the frost growth and thermal performance of a fin-tube heat exchanger has been conducted under the frosting condition. It is found that the thermal performance of a heat exchanger is closely related to the blockage ratio of the air flow passages due to the frost growth. The maximum allowable blockage ratio is used to determine the criteria for the optimal operating conditions of a fin-tube heat exchanger. It is also shown that heat transfer rate of heat exchanger with staggered fin arrangement increases about 17% and the time required for heat transfer rate to reach a maximum value becomes longer, compared with those of an inline fin-tube heat exchanger under the frosting condition. The energy transfer resistance between the air and coolant decreases with the increase of inlet air temperature and velocity and with decreasing inlet air humidity.     

Heat transfer characteristics and entropy generation for wing-shaped-tubes with longitudinal external fins in cross-flow

A numerical study is conducted to clarify heat transfer characteristics, effectiveness and entropy generation for a bundle of wingshaped-tubes attached to Longitudinal fins (LF) at downstream side. The air-side Rea ranged from 1.8 x 10³ to 9.7 x 10³. The fin height (h_f) and fin thickness (δ) have been changed as: (2 mm ≤ h_f ≤ 12 mm) and (1.5 mm ≤ δ ≤ 3.5 mm). The analysis of entropy generation is based on the principle of minimizing the rate of total entropy generation that includes the generation of entropy due to heat transfer and friction losses. The temperature field around the wing-shaped-tubes with (LF) is predicted using commercial CFD FLUENT 6.3.26 software package. Correlations of Nu_a, St_a, and Bejan number (Be), as well as the irreversibility distribution ratio (Φ) in terms of Re_a and design parameters for the studied bundle are presented. Results indicated that, installing fins with heights from 2 to 12 mm results in an increase in Nua from 11 to 36% comparing with that of wing-shaped tubes without fins (NOF). The highest and lowest values of effectiveness (ε) at every value of the considered Re_a range are occurred at h_f = 6 mm and (NOF), respectively. The wing-shaped-tubes heat exchanger with h_f = 6 mm has the highest values of (ε), efficiency index (η) and area goodness factor (G _a) and also the lowest values of Φ and hence the best performance comparing with other arrangements. The minimum values of Φ are occurred at h_f = 6 mm. (Be) decreases with increasing Re_a for all studied h_f. The heat transfer irreversibility predominates for (1800 ≤ Re_a ≤ 4200) while the opposite is true for (6950 < Re_a ≤ 9700). δ has negligible effect on Nu_a and heat transfer irreversibility. Comparisons between the experimental and numerical results of the present study and those, previously, obtained for similar available studies showed good agreements.

     

Characteristics of fluid flow and heat transfer in a fluidized heat exchanger with circulating solid particles

The commercial viability of heat exchanger is mainly dependent on its long-term fouling characteristic because the fouling increases the pressure loss and degrades the thermal performance of a heat exchanger. An experimental study was performed to investigate the characteristics of fluid flow and heat transfer in a fluidized bed heat exchanger with circulating various solid particles. The present work showed that the higher densities of particles had higher drag force coefficients, and the increases in heat transfer were in the order of sand, copper, steel, aluminum, and glass below Reynolds number of 5,000.     

Numerical analysis of entropy generation in concentric curved annular ducts

The entropy generation has been numerically investigated in concentric curved annular square ducts under constant wall temperature boundary condition. The problem has been assumed to be steady, hydrodynamically and thermally fully developed and incompressible laminar flow with constant physical properties. The solutions of discretized equations for continuity, momentum and energy have been obtained by using an elliptic Fortran Program based on the SIMPLE algorithm. Solutions have been achieved for i) Dean numbers ranging from 3.6 to 207.1, ii) Annulus dimension ratios of 5.5, 3.8, 2.9 and 2.36, and iii) Prandtl number of 0.7. In this regard, local entropy generation as well as overall entropy generation in the whole flow field has been analyzed in detail. Moreover, the effects of Dean number and annulus dimension ratio on entropy generation arising from the friction and heat transfer have been investigated. Accordingly, it is concluded that the effect of volumetric entropy generation that is a result of fluid frictional irreversibility can be neglected as compared with volumetric entropy generation due to heat transfer irreversibility. As Dean number increases, the distribution of volumetric entropy generation coming out from the heat transfer irreversibility is formed by the temperature field, which is depending on the curvature.     

A study on development of a plugging margin evaluation method taking into account the fouling of shell-and-tube heat exchangers

As the operating time of heat exchangers progresses, fouling caused by water-borne deposits and the number of plugged tubes increase and thermal performance decreases. Both fouling and tube plugging are known to interfere with normal flow characteristics and to reduce thermal efficiencies of heat exchangers. The heat exchangers of Korean nuclear power plants have been analyzed in terms of heat transfer rate and overall heat transfer coefficient as a means of heat exchanger management. Except for fouling resulting from the operation of heat exchangers, all the tubes of heat exchangers have been replaced when the number of plugged tubes exceeded the plugging criteria based on design performance sheet. This paper describes a plugging margin evaluation method taking into account the fouling of shell-and-tube heat exchangers. The method can evaluate thermal performance, estimate future fouling variation, and consider current fouling level in the calculation of plugging margin. To identify the effectiveness of the developed method, fouling and plugging margin evaluations were performed at a component cooling heat exchanger in a Korean nuclear power plant.     

Experimental study on heat transfer characteristics of water-spray-bed heat exchanger

An experimental study was conducted on a water-spray-bed heat exchanger to investigate the heat transfer characteristics. A laboratory-scale test rig was built and its heat transfer characteristics were investigated with respect to various design and operation parameters such as the water spray flow rate, exhaust gas flow rate and number of tube rows. It was found that the implementation of the water spray increased the heat transfer rate to about 1.3 - 2.2 times that of a heat exchanger without water spray, although with a slight increase in the pressure loss. It was thus confirmed that the water spray was effective for enhancing condensing heat recovery from an exhaust gas.

     

Air to air fixed plate enthalpy heat exchanger, performance variation and energy analysis

The thermal performance of a Z shape enthalpy heat exchanger utilising 70 gsm Kraft paper as the heat and moisture transfer surface has been investigated. Effects of different inlet air humidity ratio conditions on the heat exchanger effectiveness and on the energy recovered by the heat exchanger have been the main focus of this investigation. A typical air conditioning cooling coil which incorporates an enthalpy heat exchanger has been modelled for tropical climate. Under test conditions, results have shown that latent effectiveness and the moisture resistance coefficient have strong dependency on the inlet air humidity ratio. Moreover, the latent effectiveness has been found to be strongly dependent on the moisture resistance coefficient rather than the convective mass transfer coefficient. Finally, annual energy analysis for Singapore weather conditions have also shown that energy recovered under variable inlet air conditions is 15% less than that recovered under constant inlet air conditions for the same heat exchanger.     

飞机环境控制系统热交换器优化研究

一种以热力学第二定律为基础的熵产分析方法,应用于飞机环境控制系统的部件级优化设计.在给出的设计条件下,通过分析空气在环控系统中的流动过程,得出系统熵产与给定初始条件的函数关系,结合热交换器设计计算过程,得到热交换器热边流动长度、冷边流动长度和高度之间比例关系对整个系统熵产的影响,并应用该分析方法对某型飞机环控系统进行了优化计算验证.分析结果对飞机环境控制系统的优化设计具有一定指导作用.     

Effects of equivalence ratio and carbon dioxide concentration on premixed charge compression ignition of gasoline and diesel-like fuel blends

The effects of fuel/air equivalence ratio and CO₂ concentration in fuel/air charge on the ignition process of gasoline and diesel-like fuel (n-heptane) blends on a rapid compression machine are investigated in this study. Results showed that the effects of equivalence ratio on ignition delays of two ignition stages are varied. As equivalence ratio increases from 0.3 to 0.5, the first stage ignition delay slightly increases because the increased equivalence ratio improves the mixture heat capacity, reducing the in-cylinder temperature and weakening the low-temperature heat release process of the fuel. The second stage ignition delay is shortened with the increased equivalence ratio because increased fuel concentration facilitates mixture reactivity. CO₂ addition to the cylinder charge can effectively reduce the peak cylinder pressure and the two stage pressure rise rates, as well as extend the durations of ignition delays of two ignition stages.     

微型制冷系统的熵产模型及分析

介绍了微型制冷系统的研制及熵产模型,分析了随着制冷系统尺寸的减小系统中各个部件熵产率的变化情况.制冷系统尺寸减小以后,压缩机内的熵产率及系统内不可逆热传导引起的熵产率增加;而换热器内由温差引起的熵产率减小,由摩擦引起的熵产率增加.对于微型制冷系统,提高微型压缩机的加工精度及减少系统内部热漏是系统成功运转的关键.     

新型同轴套管式换热器强化传热研究

同轴套管式地热能开采技术是用于中深层地热能开采的有效方式。为了增强同轴套管式换热器的换热效率,提出了一种带螺旋翅片的新型同轴套管式换热器。基于有限体积法,对比分析新型与普通型同轴套管式换热器的换热性能,得到螺旋翅片结构对换热性能的影响,揭示其强化传热机理。结果表明：通过增加螺旋翅片的翅高或减小螺旋翅片的螺距,可有效增强同轴套管式换热器内流体流动的湍流动能,达到提高换热性能的目的;与普通光滑管换热器相比,翅高为19 mm,螺距为300 mm的新型换热器在雷诺数为27000时,努赛尔数提高了35.5%,摩擦系数提高了91%,热性能系数达到最高1.093;增加翅片和减小螺距都可以增加采出温度和采热功率,翅高为19 mm,螺距为300 mm换热器较光滑管换热器采热温度提高了5.4 K,采热功率提高了32.4%。为高效同轴套管式换热器设计提供了理论依据。     

Impacts of fouling and cleaning on the performance of plate fin and spine fin heat exchangers

An experimental study was conducted to investigate the effects of air-side fouling and cleaning on the performances of various condenser coils used in unitary air-conditioning systems. A total of six condenser coils with different fin geometry and row number were tested. Performance tests were performed at three different conditions: clean-as-received, after fouling, and after cleaning. In all cases, it was observed that the fouling was mostly confined to the frontal face of the heat exchanger as reported in the previous investigations. The amount of deposited dust was more dependent on fin geometry for the single-row heat exchangers than for the double-row heat exchangers. The predominant effect of fouling was to cause a more significant increase in air-side pressure drop than a degradation in heat transfer performance. For the single-row heat exchangers, the pressure drop increased by 28 to 31%, while the heat transfer performance decreased by 7 to 12% at the standard air face velocity of 1.53 m/s depending on fin shape. For the double-row heat exchangers, the pressure drop increased by 22 to 37%, and heat transfer performance decreased by only 4-5% at the same air face velocity. Once the contaminated coils were cleaned according to the given cleaning procedure the original performance of the heat exchangers could almost be recovered completely. The pressure drop could be restored within 1 to 7% and the heat transfer performance could be recovered to within 1 to 5% of the originally clean heat exchangers. Therefore, it is concluded that a periodic application of the specified cleaning technique will be effective in maintaining the thermal performance of the condenser coils.     

LRM 增强传热结构研究

采用LRM架构的电子设备具有良好的升级换代和扩展能力,通过更换新一代板卡即可提高设备工作能力。但处理板卡的发展日新月异,发热量飞速增加,现有LRM安装架的散热能力有限,使得新板卡的使用选择受到较大限制。文中介绍了一种LRM增强传热结构,该结构在不影响维修性的前提下,使模块与安装架的接触热阻有效降低40％以上,为提高LRM散热能力提供了一条途径。     

几何参数对绕管式换热器传热特性影响的数值研究

通过数值模拟的方法对绕管式换热器壳侧传热进行研究,重点研究了管径、径向比、缠绕角度、轴向比、盘绕圈数对传热性能的影响。结果表明:小的管径、小的径向比、大的轴向比对传热是有利的;缠绕角度、每层不同的圈数对传热的影响很小。     

板式换热器传热板片的波纹结构设计分析

板式换热器的核心部件是传热板片,传热板片的波纹结构设计直接关系到板式换热器的成型质量、传热性能以及承压能力。根据多年的设计经验,对影响板片传热性能、承压能力的板片局部关键位置包括密封槽外周边波纹、密封槽、导流区以及主传热区的设计结构、相关设计参数配比进行了设计分析和讨论,对板式换热器传热板片的结构设计具有指导意义。     

Analysis of heat transfer inside wall-joint-fins systems

Heat transfer inside wall-joint-fins systems is analyzed. The coupled two-dimensional energy equations of the wall and the joint-fin are solved numerically using an iterative high order scheme finite volume method. Advanced fine analytical solution is proposed and various closed form equations for different heat transfer augmentation indicators are obtained. Excellent agreement is noticed between the numerical and the analytical results. Wall-joint-fins systems are more effective in transferring thermal energy when the joint-fin is made of a highly conductive material. Moreover, varying the joint-fin lengths ratio may increase the system effectiveness by a factor of 1.2. In addition, the maximum reported system effectiveness is 925% above that when both wall and joint-fin have same thermal conductivity. The maximum system effectiveness which occurs at specific geometrical aspect factors increases as convective heat transfer coefficients increase. Furthermore, the effectiveness and efficiency of the wall-joint-fins system increase as the relative joint-fins to wall volume ratio increases. The wall-joint-fins efficiency is least affected by the joint-fin lengths ratio. Eventually, the heat transfer coefficient between the joint-fin and the wall is identified. Finally, wall-joint-fin systems are recommended as heat transfer enhancing elements.     

R134a冷水机组用高效换热管的试验研究及应用

通过换热管测试台对A、B2种双侧强化蒸发管的换热性能进行了测试研究。研究结果显示：对于同一管型而言,在工况不变的情况下,管内流速在1．0-2．0m／s时,换热管的综合换热系数会随着管内水流速的增加而呈上升趋势：两者的综合换热系数与其外齿周向齿数和轴向齿数有很大关系,外内齿数多综合换热系数高：新开发的高效换热蒸发管-B比原仃的蒸发管-A换热性能高6．6％。通过采用新开发的高效蒸发管-B每年可为公司节省成本约256．6余万元。     

Evaporation heat transfer and pressure drop characteristics of r-134a in the oblong shell and plate heat exchanger

The evaporation heat transfer coefficienthr and frictional pressure drop δp_f of refrigerant R-134a flowing in the oblong shell and plate heat exchanger were investigated experimentally in this study. Four vertical counterflow channels were formed in the oblong shell and plate heat exchanger by four plates of geometry with a corrugated sinusoid shape of a 45° chevron angle. Upflow of refrigerant R-134a boils in two channels receiving heat from downflow of hot water in other channels. The effects of the refrigerant mass flux, average heat flux, refrigerant saturation temperature and vapor quality of R- 134a were explored in detail. Similar to the case of a plate heat exchanger, even at a very low Reynolds number, the flow in the oblong shell and plate heat exchanger remains turbulent. The results indicate that the evaporation heat transfer coefficienthr and pressure drop Δp_f increase with the vapor quality. A rise in the refrigerant mass flux causes an increase in theh _r and Δp_f. But the effect of the average heat flux does not show significant effect on the h_r and Δp_f. Finally, at a higher saturation temperature, both theh _r and Δp_f are found to be lower. The empirical correlations are also provided for the measured heat transfer coefficient and pressure drop in terms of the Nusselt number and friction factor.     

An analytical investigation into the Nusselt number and entropy generation rate of film condensation on a horizontal plate

This study investigates the heat transfer characteristics and entropy generation rate of a condensate film formed on a horizontal plate with suction at the wall. Applying the minimum mechanical energy principle, the dimensionless liquid film thickness along the plate is found to vary as a function of the Rayleigh number, the Jakob number, the Prandtl number and the suction parameter. The governing differential equation of the condensate thickness is solved numerically by using a finite-difference shooting method. Closed-form analytical expressions are derived for the Nusselt number and the dimensionless overall entropy generation number. When there is no suction at the wall, the results obtained from the analytical expression for the Nusselt number are found to be in good agreement with those presented in the literature. This paper was recommended for publication in revised form by Associate Editor Dae Hee Lee Tong-Bou Chang received the Ph.D. degree in Mechanical En-gineering from National Cheng Kung University, Tainan, Taiwan, in 1997. From 1997 to 2001, he was a researcher at Yuloon-Motor Group (Taiwan), whose job function includes design and characterization of the thermal and fluid flow systems for vehicle. Since 2002, he has been as a Professor at the Department of Mechanical Engineering, Southern Taiwan University. His current research interests include heat transfer with phase change, energy-system optimization, heat and mass transfer in porous medium, enhancement heat transfer and high performance heat exchangers.