Experimental Research on Flow Maldistribution in Plate-Fin Heat Exchangers

The flow maldistribution and the effect of different inlet configuration on the flow distribution in platefin heat exchangers were studied experimentally. It is found that the flow maldistribution is serious because of the defects of inlet configurations, while the inlet configuration and Reynolds number are the main factors affecting the flow distribution. The improved inlet configurations, which are the header with a two-stage distributing configuration and the guide vane with a fluid complementary cavity were proposed and tested in this paper. The experimental results show that the improved inlet configurations can effectively improve the performance of flow distribution in heat exchangers.

Numerical Simulation of the Effect of Air Distribution on Turbulent Flow and Combustion in a Tubular Heating Furnace

A three-dimension full-size numerical simulation of the effect of air distribution on turbulent flow and combustion in a tubular heating furnace was carried out. A standard k –ε turbulent model, a simplified PDF combustion model and a discrete ordinate transfer radiation model were used. The hybrid grid combining a structured and a non-structured grid was generated without any simplification of the complicated geometric configuration around the burner. It was found that the multistage combustion could reduce and control the peak value of temperature. At the same time, it was concluded that the amount of primary air had little effect on the global distribution of velocity and temperature in the furnace, but a great effect on that around the burner. It is recommended that 45% - 65% of the total amount of air be taken in in primary air inlets in the furnace. All the results are important to optimize the combustion progress.

NUMERICAL STUDY ON FLOW DISTRIBUTION IN PLATE-FIN HEAT EXCHANGERS

Objective To investigate the flow distribution in plate fin heat exchangers and optimize the design of header configuration for plate fin heat exchangers. Methods A mathematical model of header was proposed. The effects of the header configuration on the flow distribution in plate fin heat exchangers were investigated by CFD. The second header configuration with a two stage distributing structure was brought forward to improve the performance of flow distribution. Results It is found that the flow maldistribution is very serious in the direction of header length for the conventional header used in industry. The numerical predictions indicate that the improved header configurations can effectively improve the performance of flow distribution in plate fin heat exchangers. Conclusion The numerical simulation confirms that CFD should be a suitable tool for predicting the flow distribution. The method has a wide variety of applications in the design of plate fin heat exchangers.

Distribution performance of gas–liquid mixture in the shell side of spiral-wound heat exchangers

The non-uniformity of gas–liquid mixture is a critical issue which leads to the heat transfer deterioration of spiralwound heat exchangers(SWHEs).Two-phase mass flow rate and the content of gas are important parameters as well as structural parameters which have prominent influences on flow distribution uniformity of SWHE shell side.In order to investigate the influences of these parameters,an experimental test system was built using water and air as mediums and a novel distributor named"tubes distributor"was designed.The effects of mass flow rate and the content of gas on two-phase distribution performance were analyzed,where the mass flow rate ranged from 28.4 to 171.9 kg·h-1 and the content of gas changed from 0.2 to 0.8,respectively.The results showed that the mixture mass flow rate considerably influenced the liquid distribution than that of gas phase and the larger mass flow rate exhibited the better distribution uniformity of two-phase flow.It was also found that the tubes distributor had the better two-phase uniformity when the content of gas was around 0.4.Tube diameter played an important role in the distribution of gas phase and slit width was more significant for the uniformity of liquid phase.

Heat transfer in boundary layer stagnation-point flow towards a shrinking sheet with non-uniform heat flux

In this paper, the effect of non-uniform heat flux on heat transfer in boundary layer stagnation-point flow over a shrinking sheet is studied. The variable boundary heat fluxes are considered of two types： direct power-law variation with the distance along the sheet and inverse power-law variation with the distance. The governing partial differential equations （PDEs） are transformed into non linear self-similar ordinary differential equations （ODEs） by similarity transformations, and then those are solved using very efficient shooting method. The direct variation and inverse variation of heat flux along the sheet have completely different effects on the temperature distribution. Moreover, the heat transfer characteristics in the presence of non-uniform heat flux for several values of physical parameters are also found to be interesting.

Boundary Layer Flow of an Unsteady Dusty Fluid and Heat Transfer Over a Stretching Sheet with Non-Uniform Heat Source/Sink

An analysis has been carried out to study the effect of hydrodynamic laminar boundary layer flow and heat transfer of a dusty fluid over an unsteady stretching surface in the presence of non-uniform heat source/sink. Heat transfer characteristics are examined for two different kinds of boundary conditions, namely 1) variable wall temperature and 2) variable heat flux. The governing partial differential equations are transformed to system of ordinary differential equations. These equations are solved numerically by applying RKF-45 method. The effects of various physical parameters such as magnetic parameter, dust interaction parameter, number density, Prandtl number, Eckert number, heat source/sink parameter and unsteadiness parameter on velocity and temperature profiles are studied.

Refrigerant Distribution Characteristics in Vertical Header of Flat-Tube Heat Exchanger without Internal Protrusion

A heat exchanger that arranges flat tubes horizontally has a vertical header that distributes the refrigerant to each tube. When the heat exchanger works as an evaporator, differences in flow conditions at each branch, such as the ratio and distribution of vapor and liquid, due to the differences in densities and momentums of vapor and liquid in the two-phase flow make equal distribution difficult. This paper describes the distribution characteristics of a four-branch header that has a rectangular cross-section without the internal protrusion of flat tubes in the case of the inflow of the refrigerant R32 from the bottom of the header by using an equipment that can estimate the distribution ratio of the liquid and vapor phase to each branch. This paper also discusses the distribution characteristics on the basis of the flow visualization in the header. The flow visualization shows that a liquid level that contains vapor phase exists in the header and affects the distribution greatly.

Effect of viscous dissipation and heat source on flow and heat transfer of dusty fluid over unsteady stretching sheet

This paper investigates the problem of hydrodynamic boundary layer flow and heat transfer of a dusty fluid over an unsteady stretching surface. The study considers the effects of frictional heating （viscous dissipation） and internal heat generation or ab- sorption. The basic equations governing the flow and heat transfer are reduced to a set of non-linear ordinary differential equations by applying suitable similarity transformations. The transformed equations are numerically solved by the Runge-Kutta-Fehlberg-45 order method. An analysis is carried out for two different cases of heating processes, namely, variable wall temperature （VWT） and variable heat flux （VHF）. The effects of various physical parameters such as the magnetic parameter, the fluid-particle interaction pa- rameter, the unsteady parameter, the Prandtl number, the Eckert number, the number density of dust particles, and the heat source/sink parameter on velocity and temperature profiles are shown in several plots. The effects of the wall temperature gradient function and the wall temperature function are tabulated and discussed.

Non-uniform operative temperature distribution characteristics and heat-source-controlled core-area range of local heating radiators

Heating the whole space,which is currently used in northern China,leads to high energy consumption and substantial pollution.A transition to local heating has the potential to help address this problem.In this paper,the effects of radiator-related parameters(position,power,and size)and room-related parameters(aspect ratio and height)on local heating were studied.Two evaluation indices,the effective coefficient of operative temperature(OTEC)and the effective coefficient of local heating(LHEC),were proposed.In addition,the heat source-control core-area(HSCCA)was proposed,and the effect range of heat sources in the space was evaluated by the attenuation of operative temperature.The findings demonstrated that the radiator position has a greater influence on local heating than size.When the position of the radiator was changed from"close to the inner wall"to"close to the outer wall",the LHEC(the interior one-quarter of room is a local heating zone)was found to decrease by 73%.The size of the radiator,which is close to the inner wall,doubled or quadrupled,and the LHEC increased by 9%and 18%.Moreover,rooms with a larger aspect ratio or small room height were found to be the most optimal for local heating applications.The area of the HSCCA decreased as the position of the radiator approached the outer wall.The findings of this study can be used as a design reference for the radiator when the heating mode changes from"full-space heating"to"local heating".

竖直铯热管传热特性的实验和数值模拟

近年来,有关中温铯热管的研究较少,主要因为铯热管造价昂贵且铯金属化学性质活泼,开展实验难度较大,因此通过数值模拟方法对铯热管启动特性与内部相变等过程进行分析预测具有重要意义。首先,本文开展铯热管冷启动实验,并基于目前的热管模拟模型,利用UDF建立蒸发与冷凝的自适应模型,对比实验热管稳态壁温与数值模拟计算壁温,最大温差小于20K,且较好地呈现了热管末端的温度突降,证明了模型的准确性。其次,基于竖直铯热管变功率冷态启动实验,模拟获得铯热管启动过程中热管壁面温度及内部工质相态分布的演变过程;并利用壁面温度的平均差数值评价有效长度上的均温性。最后,通过模拟研究改变热管蒸发段长度(200mm和120mm)、加热功率(1028.2W和844.4W)及充液率(8.8%、12%和15%),对比分析其相态分布、温度分布及压力分布;综合热管有效工作长度、壁面均温性、蒸发段液相堆积与干烧现象,分析蒸发段长度与充液率的最佳配比,为热管的优化设计提供基础。

基于热负荷均匀分配原则的直接空冷系统轴流风机转速灵活调节策略

在直接空冷电站中,轴流风机功耗占厂用电比例较高。为获得更为高效的风机运行调整策略,以某典型2´600 MW直接空冷机组为例,基于热负荷均匀分配原则,采用数值模拟方法,研究轴流风机转速灵活调节策略。获得不同环境温度和风速条件下,各轴流风机转速分布规律以及通过空冷凝汽器单元的冷却空气质量流量分布规律。最终,计算得到不同环境气象条件下不同轴流风机转速调节策略风机总功耗变化规律。结果表明,基于热负荷均匀分配原则的轴流风机转速灵活调整策略明显优于轴流风机转速整体调节策略,结果可为直接空冷机组优化运行和深层次节能提供参考。

低温微通道等差树形分流数值模拟及实验研究

针对微通道换热器进出口流量分配与压力损失问题,提出了等差树形分流法,设计了不同分流结构的进出口导流通道,构建了不同分流结构的微通道换热板模型;建立了压力损失理论计算模型,组建了微通道换热器总体三维模型,并进行了流量分配及压力损失数值模拟。制造了不同分流结构的微通道换热器实验模型,并对理论计算模型和数值模型进行了实验验证。研究表明:依据等差树形分流法设计的不同分流结构的导流通道能够解决流量分配不均的问题;提出的压降计算方法能够为不同结构微通道换热器压力损失计算提供依据;梯形分流结构的流量均匀性最佳且压力损失最小。

两相并联换热通道间流量分配特性实验研究

为提高两相流动换热通道的换热能力及系统性能,探究其并联换热通道中流量分配不均的特征,开展了以水为工质的两并联通道换热实验,对流量分配不均所带来的参数变化进行研究。实验参数变化主要包括进口温度、热流密度、进口流量,以及外部扰动、集管结构带来的流量分配差异。将实验结果与模型计算结果进行了对比,结果表明:并联换热通道间流量分配不均时,优先沸腾通道中流量较少,未沸腾通道出口温度明显降低;增加进口温度、增加热流密度、降低进口流量会导致流量分配不均的出现,其产生和消失对应的进口温度、热流密度、进口流量分别变化了24.8℃、175.6 W/m、19.8%,并存在滞后现象;扰动会加速流量分配不均的出现,对应进口温度相差3.2℃;集管结构会影响优先沸腾通道的位置,单相状态下流量较少的通道倾向于优先沸腾。研究结果可为提高并联换热通道流量分配的均匀性提供参考。

高速飞行器边界层质量引射降热减阻技术流量分区优化研究

边界层质量引射被认为是解决临近空间高超声速飞行器关键部位热防护问题和减小其飞行阻力的有效途径之一.然而,已有研究主要关注边界层质量引射对热流或阻力的单独影响,少有研究对边界层质量引射能够带来的降热减阻效果进行综合评估.针对高空高超声速层流条件下钝楔与钝锥外形的边界层质量引射降热减阻问题开展数值模拟研究,并采用多目标优化方法开展了边界层质量引射的分区流量优化设计,在满足减阻性能的同时,提高了整体的热防护效果,实现了给定冷却剂流量条件下的降热减阻综合性能优化.研究结果表明,层流条件下,边界层质量引射通过改变速度边界层和温度边界层能够显著降低壁面摩阻和热流.在单位面积引射质量流量为28.028 g/(m^(2)·s)时,对于钝楔外形,采用均匀引射方案能够减阻3.60%,同时峰值热流下降12.06%,而采用分区优化方案能够将减阻效果提高到4.30%,同时峰值热流下降91.01%;对于钝锥外形,采用均匀引射方案会造成阻力增大2.60%,同时峰值热流仅下降8.57%,而采用分区优化方案能够在减阻19.75%的情况下同时将峰值热流降低99.95%.

混合板换热器板片结构对流道内触点分布及流动换热性能的影响

混合板式换热器作为余热利用系统中高效节能设备,其换热性能对系统效率及稳定性有重要影响。该研究基于RNG k-?湍流模型,对M型和H型板式换热器流道内流动及传热过程展开数值模拟,研究板片结构对板间触点分布的影响,及雷诺数Re和板片组合对横向流道内流动及换热特性的影响规律。结果表明:M型板间触点呈“方格”,H型呈“菱形”,且H型流流体速度较大、温度场分布较均匀。随Re数增大,压降△P及平均努塞尔数■均增大;Re数较低时,H型的■明显提高,且△P增幅较小;而高Re数(Re=6000)时,与M型相比,H型■的增幅仅为△P的25%左右。随波纹间距s减小,触点数明显增多,△P和■均增大;s为12~16 mm较合适。研究结果为混合板式换热器的设计及优化提供理论依据。

并联通道中超临界RP-3航空煤油流量分配特性数值研究

针对热流偏差下空-油换热器的应用,进行了三种类型并联通道中超临界压力RP-3航空煤油流量分配数值研究,探究了进口压力、进口温度、进口总流量、进出口布置对双通道流量分配的影响机制。通过摩擦压降和加速压降的演变机制揭示了流量非对称分配的原因,讨论了通道质量流速和温度的分布特征及其对换热的影响,进行了(火积)耗散效应分析。根据热流密度比值和运行参数提出了流量偏差的预测准则,预测偏差处于±15%范围内。结果表明:随着双通道热流差别增大,两者流量偏差增大,源于加速压降和摩擦压降的平衡机制。热流比值小于1时,摩擦压降主导流量分配;热流比值大于1时,加速压降主导流量分配。热流比值等于1时,双通道流量等量分配。提高进口压力、进口温度、进口总流量双通道流量偏差均受到抑制。高热流偏差下通道出现流量不连续和偏差管温度反向分层问题。相对流量偏差和(火积)耗散偏差的变化规律一致,I型布置相比U型和Z型布置更好。

旋流式微通道热沉传热特性研究

随着微电子设备高度集成化和小型化发展,电子器件发热量增加,其热管理问题是目前的研究热点。本文针对旋流式微通道热沉传热和流动特性开展了数值模拟的研究,讨论了冷却剂流量和均热板材料导热系数的影响。研究结果表明,与常规微通道不同,随冷却剂流量增加,旋流式微通道热沉内冷却剂旋律程度增强,流程增加,导致高温向热沉中心出口位置迁移,高低温差降低。热沉底部均热材料导热系数增加时,加热面上方径向和周向导热增强,导致加热面平均温度和高低温差降低。

气侧均流装置对冷却三角单元流动传热特性影响的实验研究

对于间接空冷系统冷端基本传热元件−冷却三角单元,通过在其加装及相邻未加装气侧均流装置冷却三角单元内的两侧冷却柱沿散热管束横向布置风速、风温、壁温测点,进一步在左右两侧出水柱布置壁温测点,对比分析两侧冷却柱沿散热管束横向迎风风速、风温、壁温及出水壁温的变化规律,研究气侧均流装置对冷却三角单元流动传热特性及出水壁温分布的影响机制。研究结果表明:加装气侧均流装置后冷却三角单元两侧冷却柱沿散热管束横向迎风风温、壁温分布更加均匀,两侧出水柱壁温均值更低,且降低了两侧出水柱壁温差,气侧均流装置优化了两侧冷却柱的换热均匀性。

大型环盘折流板反应器壳程流动模拟及结构设计

环盘结构是大型换热式反应器中最常用的壳程导流结构之一,针对环盘折流板结构参数的选择影响反应器壳程流场特性的情况,分析了环盘型折流板的结构特性,发现大型反应器折流板相对板距d*的取值通常为0.15~0.20;在此基础上建立了不同相对缺口面积S^(*)条件下的模型进行流场模拟;详细分析折流板结构参数对壳程流体流动形态、停留时间分布和压降特性的影响,综合考虑后提出S^(*)=0.15~0.30是大型反应器相对缺口面积的合理设计范围;最后,根据现有研究中单弓型折流板换热器的数据,在模拟的结构条件范围内对比了环盘和单弓结构的性能,为工程设计提供指导。

集成方位跟踪太阳能塔式接收器热流密度分布特性研究

为研究接收器与竖直方向夹角和射线集中点的位置对热流密度值与分布的影响,我们以集成方位跟踪太阳能集热实验装置中集热单元为研究对象,基于SolTrace软件和射线追踪技术搭建了定日镜和接收器光学模型。以光学模型为基础,通过改变接收器的角度和位置来改变热流密度值与分布,得到接收器在不同位置时的热流密度值与分布状态。结果表明:接收器上热流密度值在接收器与竖直方向夹角为0°~90°时,随夹角的增大先增后减,夹角为40°时其值达到最大;射线焦点在接收器开口焦点2位置附近时热流密度分布较为均匀,射线可全部进入接收器内,热流密度峰值只有70 kW/m^(2)左右。建模仿真结果对集热单元中接收器安装和位置的调节提供了有益参考。