The effect of flow maldistribution on the evaluation of axial dispersion and thermal performance during the single-blow testing of plate heat exchangers

A single-blow transient test technique based on axial dispersion model is proposed for the determination of both heat transfer coefficient and axial dispersion coefficient in plate heat exchangers, characterized by NTU and dispersive Peclet number respectively. The present experimental analysis deals with the effect of flow maldistribution on the transient temperature response for U-type plate heat exchangers. The experiments are carried out with uniform and non-uniform flow distributions for various flow rates and two different numbers of plates. Special effort has been made to differentiate the deviation from plug flow due to flow maldistribution and fluid backmixing. The fluid axial dispersion is used to characterize the backmixing and other deviations from plug flow. Due to unequal distribution of the fluid, the velocity of the fluid varies from channel to channel and hence the heat transfer coefficient variations are also taken into consideration. The computed outlet fluid temperatures are compared with experimental outlet temperatures, and the values of dispersive Peclet numbers are estimated. The results indicate that in order to get parameters independent of the number of plate used in single-blow experiment, it is essential to isolate flow maldistribution from backmixing. This paper has brought out a practical way in which this isolation can be done in the process of data reduction through suitable computational model.     

Hyperbolic axial dispersion model for heat exchangers

Flow maldistribution in heat exchangers for steady-state and transient processes can be described by dispersion models. The traditional parabolic model and the proposed hyperbolic model which includes the parabolic model as a special case can be used for dispersive flux formulation. Instead of using the heuristic approach of parabolic or hyperbolic formulation, these models can be quantitatively derived from the axial temperature profiles of heat exchangers. In this paper both the models are derived for a shell-and-tube heat exchanger with pure maldistribution (without back mixing) in tube side flow and the plug flow on the shell side. The Mach number and the boundary condition which plays a key role in the hyperbolic dispersion have been derived and compared with previous investigation. It is observed that the hyperbolic model is the best suited one as it compares well with the actual calculations. This establishes the hyperbolic model and its boundary conditions.     

Experimental Investigation on Port-to-Channel Flow Maldistribution in Plate Heat Exchangers

Experiments have been conducted to analyze the flow and pressure distribution in a plate heat exchanger by measuring local port pressure distribution in a commercial plate heat exchanger. Flow rate in channel and channel pressure drops are evaluated by measuring the pressure inside the inlet and exit ports at different locations for different port dimensions. In these experiments, the measurement of pressure is done without disturbing the fluid flow inside the port. This technique also offers the option of manipulating port size without changing the plate characteristics. Direct experimental measurement provides the scope for eliminating other effects, such as gasket, end losses, and improper wetting of channels from the flow maldistribution effect. The measurements indicate the existence of non-uniform flow distribution that increases with flow rate and decreases with port diameter. Results clearly show that it is important to consider the flow maldistribution for better design of plate heat exchangers.     

Comparative Study of Thermal Performance of Parallel Plate and Rectangular Microchannel Counter Flow Heat Exchangers

This paper is aimed at comprehensive investigations of the thermal performance of parallel plate and rectangular microchannel counter flow heat exchangers based on axial conduction, number of transfer units, and non-dimensional power density. The geometrical parameters of the two configurations are optimized for a given heat transfer rate, effectiveness, and pressure drop. A reduced order model of rectangular micro channel counter flow heat exchanger is developed in which it is transformed into a hydrodynamically and thermally equivalent parallel plate micro heat exchanger. To improve the accuracy of the model, correction factors obtained from detailed computational fluid dynamics model are introduced. Various factors affecting the dimensionless power density of both the counter flow micro heat exchangers are studied. It is found that the axial conduction plays an important role on the performance of rectangular channel counter flow micro heat exchanger. In the limiting case where the channel aspect ratio tends to zero, the dimensionless power density of rectangular channel is found to approach that of a parallel plate counter flow micro heat exchanger.     

Effect of Flow Distribution to the Channels on the Thermal Performance of the Multipass Plate Heat Exchangers

Over last two decades, plate heat exchangers (PHEs) have presented themselves as a viable alternative to the conventional shell and tube heat exchangers in the process and power industries. The thermal theory available for plate heat exchangers in the literature largely works on the assumption of equal flow in each channel. However, it is well known that the distribution of fluid from port to channel in PHE is far from being uniform. The present study brings about this port to channel flow distribution effect on the thermal behavior of multipass plate heat exchangers. The variation of the heat transfer coefficient due to flow variation from channel to channel has also been taken into consideration. Heat exchangers with both equal and unequal passes of the fluids have been studied. The results indicate that the flow maldistribution severely affects the performance of plate heat exchangers, and multipassing can act as an important tool to reduce the deterioration in performance due to maldistribution. The results show that with a low number of passes, the increase of velocity of fluid may be counterproductive in terms of heat transfer enhancement. Also, adding plates in order to increase the heat transfer surface may not be effective due to an increase in flow maldistribution. The correlations for 1-1, 1-2, 2-2, and 2-3 pass plate heat exchangers with the maldistribution index as a parameter are also presented.     

Dynamics of plate heat exchangers subject to flow variations

A predictive model has been presented to suggest the transient response of plate heat exchangers, subjected to a step flow variation. The work also brings out the effect of the port to channel maldistribution on the performance of plate heat exchangers under the condition of flow variation. The results indicate that flow maldistribution affects the performance of the plate heat exchangers in the transient regime. A wide range of the parametric study has been presented which brings out the effects of NTU and heat capacity rate ratio on the response of the plate heat exchanger, subjected flow perturbation.To verify the presented theoretical model, appropriate experiments have been carried out. Experiments include the responses of the outlet temperatures subjected to inlet temperature transient in the circuit followed by a sudden change in flow rate in one of the fluids. Simulated performance has been compared to the performance measured in the experiments. Comparisons indicate that theoretical model developed for flow transient is capable of predicting the transient performance of the plate heat exchangers satisfactorily, under the given conditions of changed flow rates.     

Second law analysis of crossflow heat exchanger in the presence of axial dispersion in one fluid

In the present paper second law analysis of crossflow heat exchangers has been carried out in the presence of non-uniformity of flow. This non-uniformity is modeled with the help of axial dispersion model and takes into account the back mixing and flow maldistribution. An analytical model for exergy destruction has been evaluated for the cross-flow configuration. A wide range of study of the operating parameters and non-uniform flow on exergetic behavior of crossflow heat exchangers has been carried out. The results clearly bring out not only the reason behind the maximum entropy paradox in heat exchangers but also the proper perspective of exergy destruction and the consequent optimization of crossflow heat exchangers from the second law viewpoint.     

Effects of Distributor Configuration on Flow Maldistribution in Plate-Fin Heat Exchangers

In this paper, an original concept of a design that adds a complementary fluid cavity in the distributor is presented. The experimental investigation of the effects of distributor configuration parameter on the fluid flow maldistribution in the plate-fin heat exchanger is completed. The correlation of the dimensionless flow maldistribution parameter and the Reynolds number is obtained under different distributor configuration parameters. The experimental studies prove that the performance of flow distribution in heat exchangers can be effectively improved by the optimum design of the distributor's configuration parameter. The ratio of the maximum velocity and the minimum velocity in the channels of the plate-fin heat exchanger can drop from 2.57–3.66 to 2.08–2.81 for various Reynolds numbers. The conclusions are of great significance on the optimum structure design of the plate-fin heat exchangers and can effectively improve the performance of the heat exchangers.     

Experimental Investigation on Fluid Flow Maldistribution in Plate-Fin Heat Exchangers

The plate-fin heat exchanger is normally designed with the assumption that the fluid is uniformly divided among all the parallel passages. In practice, however, the design of the exchanger, the heat transfer process, the operation of the external system, etc., may create high flow maldistribution. The performance deterioration of plate-fin heat exchangers due to flow maldistribution may be serious. In this review, the flow distribution performance in a plate-fin heat exchanger has been experimentally studied and the distribution performance of different distributors' inlet angles has been measured. The combined effects of the inlet angle and mass flow rate on flow maldistribution have been studied. The study is useful in the optimum design of plate-fin heat exchangers.     

Thermal analysis of plate condensers in presence of flow maldistribution

Flow maldistribution in plate heat exchangers causes deterioration of both thermal and hydraulic performance. The situation becomes more complicated for two-phase flows during condensation where uneven distribution of the liquid to the channels reduces heat transfer due to high liquid flooding. The present study evaluates the thermal performance of falling film plate condensers with flow maldistribution from port to channel considering the heat transfer coefficient inside the channels as a function of channel flow rate. A generalized mathematical model has been developed to investigate the effect of maldistribution on the thermal performance as well as the exit quality of vapor. A wide range of parametric study is presented, which shows the effects of the mass flow rate ratio of cold fluid and two-phase fluid, flow configuration, number of channels and correlation for the heat transfer coefficient. The analysis presented here also suggests an improved method for heat transfer data analysis for plate condensers.     

连续螺旋折流板管壳式换热器动态特性研究及预测

建立了可进行壳管式换热器动态特性试验研究系统,通过试验研究的方法对水-油为换热工质的连续螺旋折流板管壳式换热器动态特性进行了试验研究,进口流量扰动为等百分比流量特性,研究了4种流量扰动方式下水和油出口温度的动态响应。同时研究了在一定Re数下,不同的流体扰动量对换热器进出口温升的影响,得到了换热器进出口温升与流体扰动量之间的关联式。实验表明,液液换热系统温度的动态响应时间比较长,研究发现在正负的流量扰动下,换热器进出口温度变化呈现线性变化,进出口温升在正负流量扰动下其变化曲线具有对称特征。分别建立了有限差分数值预测模型及人工神经网络模型对换热器油侧的出口温度进行了动态预测,预测结果与试验值符合良好,人工神经网络的预测结果要好于数值模拟预测,其偏差绝对值在1.3%以内,表明人工神经网络在进行复杂的系统辨识时具有一定的参考及应用价值。     

Fluid Flow Distribution and Heat Transfer in Plate-Fin Heat Exchangers

Single-phase and two-phase flow distribution in plate-fin heat exchangers and the influence of nonuniform fluid flow distribution on the thermal performance of such heat exchangers were experimentally investigated. The experimental results show that flow maldistribution can be a serious problem in plate-fin heat exchangers because of nonoptimized header configurations. The uneven distribution of two-phase flow in plate-fin heat exchangers is more pronounced than that of single-phase flow. It is shown that the uneven distributions result in a significant deterioration of the heat transfer performance. The relationship between the flow maldistribution characteristics and the resulting loss in heat exchanger effectiveness has been studied in this work. Certain improved header configurations with perforated plates were proposed in order to solve the maldistribution problem. It was found that the new header configurations could effectively improve the thermal performance of plate-fin heat exchangers. By changing the header configuration, the degree of flow and temperature nonuniformity in the plate-fin heat exchanger was reduced to 16.8% and 74.8%, respectively, under the main test condition.     

Pressure Drop in Multi-Parallel Channels of Corrugated Plate Steam Condensers

An experimental investigation has been carried out to find the pressure difference of the process of steam condensation across the port to channel in plate heat exchangers. In the present study, low corrugation angle (30°) plates have been used for different number of channels, namely, 10 and 80. The process steam entered at 1 bar with a small degree of superheat. Water has been used as the cold fluid. The pressure probes are inserted through the plate gasket into both the inlet and exit ports of the channel. The pressure drop of the process steam has been measured and recorded at the first, middle, and last channels at different flow and exit conditions for each plate package of the heat exchanger. Also, the overall pressure drop has been measured at different conditions at the outlet of the process steam, i.e., full and partial condensation. The pressure drop measurements have indicated that there is a considerable variation in pressure drop from the first channel to the last channel due to flow maldistribution. The experimental data has been analyzed to show how the flow maldistribution affects the pressure drop of a plate condenser.     

An analytical solution for a cyclic regenerator in the warm-up period in presence of an axially dispersive wave

Regenerative heat exchangers are important components of energy intensive sectors such as chemical process, power, metallurgical and cryogenic industry. The challenge of simulating regenerators accurately is considerable in view of the transient cyclic process taking place in it. The simulation of warm-up period is even more challenging due to the change in temperature profiles after each cycle. In the present work a modern technique based on the “axial dispersion model” has been utilized to simulate the regenerative heat exchanger both in the warm-up and pseudo-steady state operation. The advantage of this model is that it takes all the flow maldistribution and backmixing effects into consideration instead of idealizing the flow to be so called “plug flow”. In contrast to previous studies with dispersion, in the present study the dispersion is considered to propagate with a finite propagation velocity following a hyperbolic law which is physically more consistent. The effect of different parameters on the cyclic response has been brought out and the results have been verified by comparing results of a rotary regenerator available in literature. The technique utilized in the present study can act as a tool for modelling regenerators where non-uniformity in flow distribution is significant.     

新型蜂窝板式换热器实验与模拟分析

板式换热器在工业生产中广泛应用,采用数值模拟和实验相结合的方法对新型蜂窝板式换热器进行研究。结果表明:数值模拟和实验结果在合理的误差范围内,验证了数值模拟的可靠性。新型板式换热器内部蜂窝结构附近的速度场出现规律的周期性变化,流体湍流强度增加,Nu和阻力系数f随焊孔直径和蜂窝直径的增加而增大,通过对蜂窝板在3 000≤Re≤25 000的数据结果进行关联,得到了蜂窝板流动换热的准则关系式,为蜂窝板式换热器的优化提供了理论依据,并为工业生产提供参考。     

开缝布置方式对开缝翅片管换热器 传热与阻力特性的影响

为了获得开缝布置方式对开缝翅片管换热器传热与阻力特性的影响规律,对5种不同翅片管换热器进行了数值模拟研究,并进行了模化试验验证。结果表明：增加开缝会提高翅片管换热器的传热性能,但阻力也随之增加;与开缝位置相比,开缝数量对开缝翅片管换热器传热与阻力特性的影响更大;在Re=4800~7500日时,开缝翅片管换热器综合流动传热性能 随着Re数的增大而增大;在5种翅片中,开缝翅片的综合流动传热性能高于普通平直翅片;数值模拟与试验结果偏差较小,采用数值模拟方法能够比较准确地分析开缝翅片管换热器的传热与阻力特性。     

Transient behavior of co-current parallel flow three-fluid heat exchanger

Transient behavior of co-current parallel flow three-fluid compact heat exchangers with the effect of two-dimensional longitudinal heat conduction through the separating sheet and axial dispersion in fluids has been investigated numerically by using the Gauss–Seidel iterative technique for step excitation provided to hot fluid inlet temperature. The results reveal that the performance of the heat exchanger is affected when two-dimensional longitudinal conduction in separating sheets and axial dispersion in fluids are considered.     

Effect of inlet flow maldistribution on the thermal performance of a three-fluid crossflow heat exchanger

This study investigates the effect of flow maldistribution on the thermal performance of a three-fluid crossflow heat exchanger by the numerical method. In the inlets of three fluid streams, this study considers four modes of flow nonuniformity arrangement by using three flow maldistribution models. According to the results of temperature fields, effectiveness and deterioration factor, this study discusses the deterioration or promotion due to the flow maldistribution in the heat exchanger. The results indicate that there is a best one in choice between the four maldistribution modes and the best flow maldistribution mode promotes the thermal performance of a three-fluid crossflow heat exchanger when NTU and heat capacity rate ratios are large.     

Experimental investigation of header configuration improvement in plate–fin heat exchanger

Flow characteristics in the header of a plate–fin heat exchanger have been investigated by means of Particle Image Velocimetry (PIV). A series of velocity vector and streamline graphs of different cross-sections are obtained in the experiment. The experimental results indicate that performance of fluid maldistribution in a conventional header is very serious, while the improved header configuration with a punched baffle can effectively enhance the uniformity of flow distribution. The flow maldistribution parameter and the ratio of the maximum velocity to the minimum in a plate–fin heat exchanger decreases by installing the punched baffle. Further heat exchange experiments indicate that the temperature is distributed more uniformly in the improved heat exchanger core and the heat exchanger effectiveness can be effectively enhanced. The conclusion of this paper is of great significance in the optimum design of plate–fin heat exchanger.     

Thermal Design of Multistream Plate Fin Heat Exchangers—A State-of-the-Art Review

Multistream plate fin heat exchangers have replaced two-stream heat exchangers in diverse applications due to their compactness, capacity of handling multiple fluid streams in a single unit, and possibilities of having intermediate entry and exit of the streams. Unique features of such heat exchangers like direct/indirect crossover in temperatures due to several thermal communications among the fluid streams and the dependence of the thermal performance on “stacking pattern” have no equivalent in two-stream modules. As a consequence, an extension of the commonly used design/simulation techniques like ?-NTU or the LMTD method, applicable for two-stream exchangers, fails miserably in the case of multistream units. Though several techniques have been suggested over the years, in reality, no universally accepted methodology exists for the “thermal design” of multistream plate fin heat exchangers to date. In this communication, a state-of-the-art review of the thermal design of multistream plate fin heat exchanger is provided. Reported techniques based on heuristics, extension of the analysis applicable for two-stream heat exchangers, differential analysis, network analysis, and rigorous numerical solutions are briefly reviewed. Advantages and limitations of such techniques are also critically judged. The method of “area splitting” and “successive partitioning” proposed by the present research group is also elaborated. Apart from the basic design methodology, the techniques adopted for accounting for variable fluid properties, axial heat conduction in the solid matrix, and thermal communication with the environment have been discussed. Further, the suggested methodologies for optimizing the thermal design are reviewed. Finally, comments have been made indicating the future need of research in this topic.