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.     

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.     

Fluid Flow and Heat Transfer in Plate-Fin and Tube Heat Exchangers in a Transitional Flow Regime

The unsteady behaviors of fluid flow and heat transfer in plain plate-fin and tube heat exchangers with a wide range of fin spacings from 2.06 mm to 16.48 mm and tube diameter 8.28 mm are studied by a large eddy simulation technique (LES). Velocity fluctuations and vortex sheddings induced by the tubes in the channel are modeled. The results found that the flow in passages of large spacings is quite different from that of small spacings. The flow is co-determined by two effects: the duct effect and the tube bank effect. The tube bank effect is more dominant with increasing fin spacings.     

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.     

Heat Transfer and Fluid Flow Analysis for Plate-Fin and Oval Tube Heat Exchangers With Vortex Generators

This article investigates the effectiveness of embedded vortex generators in enhancing the heat transfer performance of a plate-fin heat exchanger with a four-row staggered oval tube bundle. Two different types of vortex generator are considered, namely annular and inclined block. Numerical simulations are performed to analyze the effects of the three-dimensional turbulence induced by the vortex generators on the heat transfer and fluid flow characteristics of the heat exchanger. The results indicate that compared to a plate-fin heat exchanger with circular tubes, the use of oval tube fins and vortex generators increases the heat transfer rate by 3 to 16% and reduces the pressure drop by 17 to 35% for inlet velocities in the range of 1 to 8 m/s. Furthermore, the vortex generators make possible an average area reduction ratio of 14 to 18%. Overall, the results show that the inclined block shape vortex generators yield the greatest improvement in the heat transfer performance at medium to high inlet velocities.     

Calculation of Two-Phase Convective Heat Transfer Coefficients of Cryogenic Nitrogen Flow in Plate-Fin Type Heat Exchangers

The two-phase convective heat transfer coefficients for nitrogen inside the flow path of plate-fin type heat exchangers operating at cryogenic temperatures are calculated using CFX Release 13.0. Using a homogeneous two-phase model, the governing equations are solved to find pressure, velocity, and enthalpy distributions for three types of fin geometries: plain, wavy, and serrated. The results are further processed to evaluate the wall shear stress and heat flux, which in turn yield the friction coefficients and convective heat transfer coefficients. The coefficients are presented as functions of system pressure, flow rate, and local quality. The results can be used for the design of plate-fin type exchangers with the same fin configurations and operating conditions as the calculation.     

Mathematical Modeling of Cross-Flow Tube Heat Exchangers With a Complex Flow Arrangement

The general principles of mathematical modeling of heat transfer in cross-flow tube heat exchangers with complex flow arrangements that allow the simulation of multipass heat exchangers with many tube rows are presented. The finite-volume method is used to solve the system of differential equations for temperature of the both fluids and the tube wall with appropriate boundary conditions. A numerical model of a multipass steam superheater with 12 passes is presented. The convection and radiation heat transfer on the flue gas side are accounted for. In addition, the deposit layer is assumed to cover the outer surface of the tubes. Comparing the computed and measured steam temperature increase over the entire superheater allows for determining the thermal resistance of the deposits layer on the outer surface of the superheater. The developed modeling technique can especially be used for modeling tube heat exchangers when detailed information on the tube wall temperature distribution is needed.     

Single- and Multi-Objective Design Optimization of Plate-Fin Heat Exchangers Using Jaya Algorithm

This paper explores the use of Jaya algorithm for the single- and multi-objective design optimization of plate-fin heat exchangers (PFHEs). Design of PFHEs involves a number of geometric and physical parameters with high complexity. The general design approaches are based on trial and error and become tedious and time consuming and do not guarantee the achievement of an optimal design. Therefore, advanced optimization algorithms are preferred. The Jaya algorithm is a newly developed simple algorithm and it does not have any algorithmic-specific parameters to be tuned and this aspect reduces the designer's effort in tuning the parameters to arrive at the optimum value of the objective function. The Jaya algorithm is proposed for the design optimization of PFHEs by minimizing the total surface area of heat transfer, total annual cost, and total pressure drop of the system and maximizing the effectiveness. Seven design parameters are considered which are imposed by constraints on the design. Single- as well as multi-objective design optimization is carried out using the proposed algorithm. The results obtained by Jaya algorithm are compared with the results of latest reported algorithms. These comparisons revealed that the Jaya algorithm can be successfully applied for the design optimization of PFHEs.     

Parallel Flow Asymmetries in Heat Exchangers

The effect of flow asymmetry was observed experimentally in vertical parallel channels of a continuous direct ohmic heater. While downward-oriented flow is evenly distributed at isothermal conditions, the flow rate in parallel channels differs in the case of heating due to buoyancy effects. Three different configurations of parallel flow (with and without internal heating in channels) are analyzed, giving critical values of Gr/Re or Ri = Gr/Re² and ensuring stable and even distribution of flow into parallel channels of vertically oriented heat exchangers or heaters. Experimental verification was based upon flow visualization (injection of a colored tracer and monitoring the tracer by a camera), measurement of temperature profiles, and stimulus response experiments using KCl as a tracer for conductivity methods (2 Pt conductivity probes) and Tc99 as a radioisotope tracer (collimated scintillation detectors).     

A Large Eddy Simulation of Plate-Fin and Tube Heat Exchangers with Small Diameter Tubes

Plate-fin and tube heat exchangers are extensively studied both experimentally and numerically. However, data on the fluid flow and heat transfer in the exchanger passage with small diameter tubes have not been accumulated enough. With a large eddy simulation technique (LES), this study performs a detailed investigation of the fluid flow and heat transfer in a plate-and-tube channel with tubes of diameters as small as 5.2 mm. The conservation equations for mass, heat, and momentum were solved by the proposed LES model. It was found that the LES model is appropriate to predict the fluid flow and heat transfer. Compared to heat exchangers of larges tubes, the heat exchangers exhibit much higher heat transfer coefficients with small tubes. The fin efficiencies are improved with small tubes.     

Risk-Based Design Margin Selection for Heat Exchangers

For more than a quarter century, business and industry have used risk-based matrices to quantify probability and consequences in decision making. However, this tool has not yet been applied to the heat exchanger design process. Adding a design margin to the calculated size of an exchanger is common practice. This margin represents the added heat exchanger area necessary to provide confidence that the exchanger will operate as required throughout its run cycle. An assumption is made that the additional area will not have a deleterious impact on performance. This report introduces the concept of a risk-based design margin selection process as a quantitative aid in separating the individual components that comprise the uncertainty in heat exchanger design. In addition, it provides a technique to help the designer determine a reasonable, cost-effective margin to apply to the heat exchanger. Two example cases show the application of the procedure.     

Tube Vibration and Flow Distribution in Shell-and-Tube Heat Exchangers

Avoiding flow-induced vibration damage is a major concern to the designers and operators of shell-and-tube heat exchangers. This paper discusses the characteristics of tube vibration as determined from a research program featuring tests of an industrial-size exchanger. The state of the art in understanding and predicting tube vibration and shell-side flow distribution is briefly reviewed. Finally, research needs are identified and discussed.     

Research Needs in Low Reynolds Number Flow Heat Exchangers

Low Reynolds number flows prevail in some process, power, automotive, aircraft, and other industrial heat exchangers such as shell-and-tube, plate, and compact heat exchangers. Laminar or low Reynolds number turbulent flows are the results of either high viscosity fluids, compact flow passages (i.e., small hydraulic diameter), or low fluid velocities. Significant advancements have been made in the past 100 years in understanding and predicting flow and heat transfer in such internal flows. This paper summarizes the research needs to further advance the science of low Reynolds number flow heat exchangers. Emphasis is primarily given to the thermal design aspects; research needs related to mechanical design, manufacturing, material selection, and other nonthermal design aspects are not covered. Also the coverage is restricted primarily to single-phase applications. The outlined research needs are based on the input from invited lecturers and some participants, experts in the field, at the Fourth NATO Advanced Study Institure in Ankara, Turkey, July 1981.     

Performance Analysis of Aluminum and Graphite Foam Heat Exchangers Under Countercurrent Arrangement

Due to the increasing power requirement and the limited available space in the vehicles, a countercurrent heat exchanger (HEX) is proposed for the position on the roof of the vehicle compartment. Furthermore, a new material, graphite foam with high thermal conductivity and low density, is a potential material for HEXs in vehicles. In order to evaluate the performance of the graphite foam HEX, the CFD computational fluid dynamics (CFD) approach is applied in a comparative study between the graphite foam and the aluminum HEXs under countercurrent flow condition. The analysis is conducted for the thermal performance (heat transfer coefficient) and the pressure loss. The simulation results show that the graphite foam HEX proves higher thermal performance than the aluminum HEX. However, due to the high pressure loss in the graphite foam HEX, the coefficient of performance in the graphite foam HEX is much lower than that of the aluminum HEX. A specific case study is carried out to evaluate the performance of graphite foam HEX as well. Useful recommendations are highlighted and provided to promote the development of the countercurrent flow HEXs in vehicles.     

Efficiency of a Combined Desalination and Power System Utilizing a Two-Phase Flow Multistream Heat Exchanger

ABSTRACT

The aim of this study is to simplify the process of discharge thermal energy combined desalination with power system by integrating the two existing heat exchangers (condensers) into a new multistream one. This system is a heat recovery unit, which is used to cogenerate water and power. Two shell-and-tube condensers operate in a closed power cycle and a desalination system for cooling an ammonia mixture as a working fluid and condensing a pure vapor, respectively. Here, a two-phase flow multistream condenser is utilized instead of the two low-exergy-efficiency shell-and-tube condensers. The results proved that the proposed technique leads to improving its exergy efficiency by 15%. The performance of the proposed condenser was analyzed by applying parametric optimization.     

Two-Phase Flow Distribution to Tubes of Parallel Flow Air-Cooled Heat Exchangers

Abstract

This paper addresses two-phase flow distribution phenomena in multiple header–tube junctions used in heat exchangers. Because of phase separation, it is very difficult to obtain uniform two-phase flow distribution to the branch tubes. The flow distribution is strongly influenced by the header orientation (horizontal or vertical) and the number of branch tubes. Other factors that influence the flow distribution are the flow direction in the header (upflow or downflow), the header shape and tube end projection into the header, and the location and orientation of the inlet and exit connections. The source of maldistribution is the flow in the dividing headers. Work performed by the authors and others (including patents) are discussed. The possibilities for eliminating two-phase flow maldistribution are identified and discussed. This investigation shows that solutions, which provide uniform flow distribution, are very design-specific. Change of the geometry or operating parameters will require modification of the design.     

Review of various Types of Flow Maldistribution in Heat Exchangers

The types of maldistributions and their causes are discussed. Where possible, means of-avoiding or curing the problem are given. While the performance loss is often small, there are associated mechanical problems that can be severe.     

Flow Patterns for Isothermal Condensation in One-Pass Air-Cooled Heat Exchangers

An analytic method is developed to describe the flow in each row of tubes of an air-cooled one-pass isothermal condenser. This leads to a better understanding of some common steam condenser problems, including freezing of condensate, rapid tube-side corrosion, and worse performance than predicted by any reasonable condensing coefficient. The NTU approach is used to derive expressions for pressure drop in each row in terms of exchanger effectiveness. The resulting simultaneous equations are solved numerically, and the solutions, presented graphically, show the flow pattern to be solely a function of effectiveness. For steam condensers contaminated by traces of air, it is found that air pockets will develop inside the tubes of the lower rows, blanketing the surface and preventing condensation. Several designs are described that attack the problem of noncondensable accumulation, including the most recent developments in the air-cooled condenser field.     

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.