Numerical investigation on the thermal performance of cross-cocurrent and countercurrent three fluid heat exchanger with flow maldistribution at the inlet

Flow maldistribution at the inlet of a heat exchanger (HX) is a significant parameter that needs to be considered to judge the performance of the same. In this paper, four arrangements of three-fluid cross-flow heat exchanger (3FCFHX) with different flow conditions are considered as shown in Figure 1 in which is the performance of cross-countercurrent and cross-cocurrent arrangements for uniform and flow maldistribution at the inlet. In addition, the effect of different inlet fluid flow models on the HX performance are investigated numerically. Among the three fluids in these arrangements, the central fluid is considered to be the hot fluid. From the principles of conservation of energy, the governing equations for three fluids are generated and solved using the finite element method. Four different inlet fluid-flow models are considered for the analysis. Performance is judged using hot fluid effectiveness and the number of transfer units for a different range of governing parameters. The effects of inlet flow maldistribution (IFM) are measured using degradation factors. The results show that the performance of cross-cocurrent arrangement is found to be superior to the cross-countercurrent arrangement. In addition, the IFM enhances thermal performance. Further, it is determined that the flow maldistribution at the inlet will enhance the hot fluid effectiveness by 4%–4.5% and 1.8%–2% in cross-cocurrent and countercurrent arrangements, respectively. The results give a thorough insight into the significant concerns involved in the design of such HXs. Application of the finite element method proves the ease of determining the exit temperatures of the fluids in the HX. This approach is indeed time-saving and gives insights when compared to the CFD approach.     

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.     

Flow maldistribution and thermal performance deterioration in a cross-flow air to air heat exchanger with plate-fin cores

The inlet and outlet duct geometry in an air to air compact heat exchanger is always irregular. A skewed Z-type arrangement is popular between the impinging flow and the core. Such duct placements usually lead to a non-uniform flow distribution on core surface. In this research, the flow maldistribution and thermal performance deterioration in cross-flow air to air heat exchangers are investigated. The inlet duct, the core and the outlet duct are combined together to calculate the flow distribution on core inlet face. First, a CFD code is used to calculate the flow distribution, by treating the plate-fin core as a porous media. Then a heat transfer model between the two air flows in the plate-fin channels is set up. Using the flow distribution data predicted, the heat exchange effectiveness and the thermal performance deterioration factor are calculated with finite difference scheme. Experiments are performed to validate the flow distribution and heat transfer model. The results indicate that when the channel pitch is below 2.0 mm, the flow distribution is quite homogeneous and the thermal deterioration due to flow maldistribution can be neglected. However, when the channel pitch is larger than 2 mm, the maldistribution is quite large and a 10–20% thermal deterioration factor could be found. The study proves that the inlet duct, the outlet duct, and the core should be coupled together to clarify flow maldistribution problems.     

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 Compensation Effect of Passage Arrangement Design for Inlet Flow Maldistribution in Multiple-Stream Plate-Fin Heat Exchanger

ABSTRACT

Passage arrangement design in fin channels is an efficient methodology for the reduction of the thermal deterioration influence of inlet flow maldistribution in multiple-stream plate-fin heat exchangers. In this work, the thermal compensation effects of passage arrangement design under different statistical parameters of inlet flow maldistribution are investigated. The inlet flow maldistribution in inlet header is analyzed and represented with distribution types, mean, and standard deviation of inlet mass flow rate entering the fin channels. A thermal calculation model based on integer–mean temperature difference method is established, and then, the passage arrangement under inlet flow maldistribution is optimized using a hybrid particle swarm algorithm. The thermal compensation effects for different inlet flow maldistributions and passage arrangements are calculated and compared. The results indicate that the compensation effect of optimization design of passage arrangement increases from 1.1% to 3.9% as the standard deviation of inlet mass flow rate increases from 0.06 kg/s to 0.12 kg/s. The results presented in this study can be used by other researchers to guide the passage arrangement design of actual heat exchanger with inlet flow maldistribution.     

The combined effects of longitudinal heat conduction,flow nonuniformity and temperature nonuniformity in crossflow plate-fin heat exchangers

An analysis of a crossflow plate-fin compact heat exchanger, accounting for the combined effects of two-dimensional longitudinal heat conduction through the exchanger wall and nonuniform inlet fluid flow and temperature distribution is carried out using a finite element method. A mathematical equation is developed to generate different types of fluid flow/temperature maldistribution models considering the possible deviations in fluid flow. Using these models, the exchanger effectiveness and its deterioration due to the combined effects of longitudinal heat conduction, flow nonuniformity and temperature nonuniformity are calculated for various design and operating conditions of the exchanger. It was found that the performance variations are quite significant in some typical applications.     

The comparison of longitudinal wall conduction effect on the crossflow heat exchangers including three fluid streams with different arrangements

This study investigates the effect of longitudinal wall conduction on a crossflow heat exchanger including three fluid streams with three different arrangements. By using numerical method, this study calculates the exit mean temperature of each fluid stream and then computes the deterioration factor of each fluid stream in each arrangement. The results indicate that the effect of longitudinal wall conduction for fluid stream 3 in first arrangement is more severe than that in second arrangement when the heat capacity rate ratio of fluid stream 1 is same to that of fluid stream 3. The deterioration factor of fluid stream 2 is affected slightly by the change of inlet temperature of fluid stream 3 in first and second arrangements. Besides, the longitudinal wall conduction strongly deteriorates the thermal performance of heat exchangers including three fluid streams at lower heat capacity rate ratios.     

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.     

The Optimal Aspect Ratio on the Thermal Performance in a Crossflow Heat Exchanger with Longitudinal Wall Conduction

This study investigates the effect of aspect ratio on the thermal performance of a crossflow heat exchanger with longitudinal wall conduction. This study uses a numerical method to calculate the exit mean temperatures and effectiveness for various aspect ratios. The results indicate that there exists a maximum effectiveness along with an increase of aspect ratio. The maximum occurs when the aspect ratio is equal to one when the flows are balanced, and it moves to where the aspect ratio is larger than one when the flows are unbalanced. For industrial applications, this study suggests that the square shape is the best choice for the design of a crossflow heat exchanger.     

Performance Deteriorations from Flow Maldistribution in Air-to-Air Heat Exchangers: A Parallel-Plates Membrane Core Case

Parallel-plates membrane cores have been widely used in air-to-air heat exchangers for fresh air heat and moisture recovery, which are also called total heat exchangers. Flow maldistribution and the consequent performance deteriorations are of great interest. The flow maldistribution is predicted with a CFD code, by treating the parallel-plates core as a two-dimensional porous media. Then, a coupled heat and moisture transfer model between the two air flows in the parallel-plates channels is set up. Using the CFD predicted flow distribution data on core face, the sensible heat and moisture exchange effectiveness and the performance deterioration factors are calculated with finite-difference scheme. Experiments are performed to validate the heat mass transfer model. The results indicate that for the current structure, when the channel pitch is below 2.0 mm, the flow distribution is quite homogeneous and the sensible and latent performance deteriorations due to flow maldistribution are below 9% and can be neglected. However, when the channel pitch is larger than 2 mm, the maldistribution is quite large and the consequent thermal and latent performance can be deteriorated by 28%. The flow maldistribution the consequent thermal and latent effectiveness deteriorations for parallel-plates cores are more serious than those for plate-fin cores previously reported.     

Thermal performance criteria of elliptic tube bundle in crossflow

In this work, the thermofluid characteristics of the elliptic tube bundle in crossflow have been investigated. Experimental and numerical investigations of the turbulent flow through bundle of elliptic tubes heat exchanger are carried out with a particular reference to the circular tube bundle. The investigation covers the effects of key design parameters of Reynolds numbers (5600–40,000), minor-to-major axis ratios (0.25, 0.33. 0.5 and 1) and flow angles of attack (0–150°). Five bundles of elliptic tube heat exchangers with different axis ratios were designed and manufactured in staggered manner. Numerical CFD modeling using finite volume discretization method was conducted to predict the system performance extensively. Four methods were presented to resort a metric that expresses the thermal performance criteria of the elliptic tube bundle. The results indicated that, increasing the angle of attack clockwise until 90° enhances the convective heat transfer coefficient considerably. The maximum thermal performance under constraint of a fixed pumping power or a mass flow rate was obtained at a zero angle of attack and the minimum thermal performance occurred at an angle of attack equals 90°. The best thermal performance of the elliptic tube heat exchanger was qualified with the lower values of Reynolds number, axis ratio and angle of attack.     

Transient response of plate heat exchangers considering effect of flow maldistribution

Plate heat exchangers have been playing important role in the power and process industries in the recent past. Hence, it is important to develop simulation strategies for plate heat exchangers accurately. This analysis represents the dynamic behaviour of the single pass plate heat exchangers, considering flow maldistribution from port to channel. In addition to maldistribution the fluid axial dispersion is used to characterise the back mixing 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 variation is also taken into consideration. Solutions to the governing equations have been obtained using the method of Laplace transform followed by numerical inversion from frequency domain. The results are presented on the effects of flow maldistribution and conventional heat exchanger parameters on the temperature transients of both U-type and Z-type configurations. It is found that the effect of flow maldistribution is significant and it deteriorates the thermal performance as well as the characteristic features of the dynamic response of the heat exchanger. In contrast to the previous studies, here the axial dispersion describes the inchannel back mixing alone, not maldistribution, which is physically more appropriate. Present method is an efficient and consistent way of describing maldistribution and back mixing effects on the transient response of plate heat exchangers using an analytical method without performing intensive computation by complete numerical simulation.     

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.     

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.     

Airflow in Air-Cooled Heat Exchangers

The problems encountered in the air-side performance of air-cooled heat exchangers are discussed with particular reference to airflow measurement. The variability of the airflow in and around the plenum and tube bundle of forced draft exchangers is described, as well as the difficulty in using static pressure determinations as an indication of fan and system efficiency. Measuring techniques and instrumentation are dealt with, and a recommendation is made for the best method and type of instrument currently available for determining airflow. The effects of air-side flow reduction and maldistribution on the thermal performance are also examined by reference to a typical forced draft exchanger in both condensing and cooling duty. In each case the condenser is more sensitive to flow variations than the cooler is. Generally, the effect of flow reduction is more serious than maldistribution unless crosswinds become dominant.     

Effect of ambient heat-in-leak on the performance of a three fluid heat exchanger,for cryogenic applications,using finite element method

In most cryogenic applications, heat in leak from the ambient is a significant factor for the degradation in the performance of heat exchangers. The effect of heat in leak to the cold fluid in a three-fluid heat exchanger, for a cryogenic application, involving thermal interaction between all the three fluids, has been investigated using both the analytical and finite element methods. Cooling of the hot fluid has been identified as the objective of the three fluid heat exchanger. Seven non-dimensional parameters, including one to account for ambient heat in leak to the cold fluid, have been identified and their effects on hot fluid behaviour – temperature profile, effectiveness and degradation factor – have been studied. The results presented give valuable inputs towards better understanding of the behaviour of the hot fluid in this class of heat exchangers.     

Transient temperature field in a parallel-flow three-fluid heat exchanger with the thermal capacitance of the walls and the longitudinal walls conduction

Transient temperature response of a parallel-flow three-fluid heat exchanger with the thermal capacitance of the walls and the longitudinal heat conduction through the walls is investigated numerically, by the implicit MacCormack method, for a step change in flow rate of one fluid. The impact of thermal properties of the walls on temperature field is examined. The results of calculations show that as the thermal diffusivity of the walls decreases, the effect of the walls increases.     

Effect of temperature and flow nonuniformity on transient behaviour of crossflow heat exchanger

The transient temperature response of a crossflow heat exchanger is carried out using finite difference method accounting for the effect of temperature and flow nonuniformity at different input conditions. Beta flow maldistribution model has been introduced for the flow nonuniformity. The responses are found dependent on the relative position of the individual temperature streams and the position of the fluid moving device for the temperature and flow nonuniformity, respectively. Combined effect of temperature and flow nonuniformity has also been analysed and compared with the other cases.     

Experimental Investigation of Heat Transfer and Resistance Characteristics of a Finned Oval-Tube Heat Exchanger With Different Air Inlet Angles

The air inlet flow direction is not orthogonal to the heat exchanger surface in many cases. To study the performance of the heat transfer and pressure drop of a heat exchanger with different air inlet angles, this paper shows the experimental system about a finned oval-tube heat exchanger inclined toward the air incoming flow direction. The heat transfer and pressure drop characteristics of four air inlet angles (90°, 60°, 45°, and 30°) are studied separately for the Reynolds number ranging from 1300 to 13000 in this study. The experimental correlations of Nusselt number and resistance coefficient of the air side are acquired. The results show that the overall heat transfer coefficients become smaller and smaller with the decrease of the air inlet angles, while the pressure drops have significant changes. The heat transfer performances of the heat exchanger under the three inclined air inlet angles are worse than that at 90°. Among the three inclined angles, the performance at 45° is the best under identical mass flow rate criterion and at low Reynolds number under identical pressure drop criterion; that at 60° is the best at large Reynolds under identical pressure drop criterion. Finally, some conclusions are attained about the effects of the air inlet angles on the heat transfer and pressure drop performance of the finned oval-tube heat exchanger.     

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.