Two-fluid Heat Exchanger Research Articles

Enhancing thermal performance and optimization strategies of PCM-integrated slab-finned two-fluid heat exchangers for sustainable thermal management

This research investigates the influence of phase change material (PCM) distribution, fin density, and PCM type on the thermal performance of a slab-finned cross-flow PCM heat exchanger (HEX). The aim is to develop a highly efficient PCM-HEX system for enhanced thermal intermittency management. Computational fluid dynamics (CFD) simulations are employed to analyze the dynamic behavior of the HEX. Three PCM configurations, 15 variations in fin densities, and three paraffin-based PCMs with distinct melting temperatures are considered. The results reveal that the Type II configuration enhances heating time at high set point temperatures (SPTs), while the Type I configuration extends heating time at low SPTs. Increasing the air mass flow rate (AMFR) reduces the overall discharged thermal energy by 18–19 % for Type II and III configurations and by 10 % for the Type I configuration, indicating the lower sensitivity of Type I to AMFR variations. It is also shown that decreasing the PCM domain's fin density extends the heating time at low SPTs. Additionally, using a PCM with a higher melting temperature enhances the heating time at high SPTs and increases latent energy discharge power. Tetracosane demonstrates the highest efficiency at 54.9 % and maximum total discharged energy reaching 112.4 kJ. The findings of this study have significant implications for the development of optimized PCM heat exchangers in various applications, enabling more efficient and impactful thermal management.

Meshless Optimization of Triply Periodic Minimal Surface Based Two-Fluid Heat Exchanger

Triply Periodic Minimal Surface (TPMS) is suitable for the heat transfer of fluids, but it is difficult to preserve the integrality of independent fluids channels during optimization. We present an effective meshless optimization framework of Triply Periodic Minimal Surface based two-fluid heat exchangers, which can be represented, analyzed and optimized using function representation. Our method is directly executed on continuous functions instead of time-consuming meshing (tetrahedral/hexahedral), thereby substantially promotes the efficiency and controllability. Specifically, a valid TPMS based two-fluid heat exchanger is first constructed by function expression. The heat exchanger consists of two independent fully-connected spaces, which are used to infill different fluids, respectively. The two spaces are divided by a smooth and continuous separating wall, which can be expressed by functions and inherits several good properties of TPMS, such as large heat exchange surface area, high-order smoothness, independence and full connectivity of each fluid channel, good mechanical properties and controllability. Then, we formulate the optimization model by controlling the thickness and topology of the separating wall directly using the function representation. Also, we adapt an efficient automatic optimization of the modeling problem by maximizing thermal energy exchange under maximum pressure drop constraint. Finally, we obtain an optimized two-fluid heat exchanger with continuous geometry changes and proper topology changes. To our knowledge, this is the first work to provide both design and optimization for the TPMS based two-fluid heat exchangers. Various numerical results demonstrate the effectiveness and efficiency of our method.

Air-to-Liquid Heat Exchanger Fabricated Using Deposition-Based Additive Manufacturing Processes

Abstract This work describes the use of deposition-based additive manufacturing (AM) techniques to fabricate air-cooled, two-fluid heat exchangers. The project focused on a Heating, Ventilation and Air Conditioning application and used an industry-standard copper/aluminum heat exchanger manufactured with conventional technology as the basis for assessing performance. The manufacturing constraints associated with using deposition-based AM technology for this application include the need for a continuous tool path within each build layer that allows uninterrupted extrusion and therefore defect-free water channel walls that correspond to a reliably leak-tight heat exchanger. A geometry that respects these constraints was developed, simulated, optimized and finally manufactured and tested. The material used was a composite of polymer filled with conductive flakes in order to provide high conductivity in the direction that heat must flow in this heat exchanger, across the wall separating the fluids. The measured performance for several test coupons matched the predicted performance and the test coupons exhibited performance that approach and in some cases exceeds conventional technology

Maximum benefits from the use of enhanced heat transfer surfaces

The paper presents a critical review of the performance evaluation criteria (PEC) developed by Webb (Webb, 1981). These criteria evaluate the benefits that could be obtained in heat exchangers if their surfaces and inserts are designed to enhance the heat transfer, particularly in two-fluid heat exchangers in single-phase flow. This study revealed that the imposed constraint for fixed pumping power in the comparison of the augmented channel with the reference one is a major obstacle to achieving greater benefit. Accordingly, the elimination of the constraint of fixed pumping power and the objective of reduced pumping power decreases substantially the criteria of Webb. A new constraint related to the fixed augmentation entropy generation number has been involved replacing the constraint of fixed pumping power. The pumping power can be left to increase until the augmentation entropy generation number is less than unity. The maximum benefit can be obtained when the augmentation entropy generation number reaches the value of unity. The use of greater in tube heat transfer enhancement (with the use of compound heat transfer enhancement techniques instead of simple ones), and with an enhanced outer tube surface of the augmented exchanger is a useful instrument to increase the benefits, only if the augmentation entropy generation number does not cross the frontier of unity.

Heat transfer intensification in MEMS two-fluid parallel flow heat exchangers by embedding pin fins in microchannels

This work proposes implementation of a passive heat transfer enhancement technique in MEMS two-fluid parallel flow heat exchangers as well as model based investigation of the same; the technique involves embedding square in-line pin-fins in the microchannels of the heat exchanger. Model based study is conducted using MEMS two-fluid heat exchangers, with and without pin fins, for a wide range of Reynolds number and two heat capacity ratios. With increase in Reynolds number, the effectiveness of heat exchangers with pin fins increases in comparison with those without pin fins for all heat capacity ratios; moreover, at low Reynolds number the effectiveness of both types of heat exchangers are equal. The power consumption in heat exchangers with pin-fins is higher than in those without pin-fins; the difference in the power consumption increases with increase in Reynolds number irrespective of the heat capacity ratio. This enhancement technique allows MEMS two-fluid parallel flow heat exchangers operate at higher throughput without compromising the effectiveness. For purposes of simulation, microchannel dimensions are kept at 150 μm (width) by 150 μm (height) by 2 cm (length) and pin fin dimensions are held at 75 μm (width) by 500 μm (pitch) while the Reynolds number is varied between 50 and 1500 for heat capacity ratios of unity and 0.5. For the cases considered for simulation, the maximum increase in effectiveness, relative to the baseline case, achieved is 84% and 66% for balanced and unbalanced flow conditions, respectively. • Square pin-fins embedded in microchannels for heat transfer enhancement of MEMS heat exchangers. • Simulations conducted for Reynolds number from 50 to 1500 and heat capacity ratios of 1 and 0.5. • High effectiveness possible without compromising flow rate and size and weight of heat exchanger. • Penalty of approach is increased pumping power.

A Novel Equivalence Approximate Model for Second Law Based Optimization of Three-Fluid Cross-Flow Plate-Fin Heat Exchanger Using Genetic Algorithm

Abstract Number of entropy generation is a significant performance evaluation factor for heat exchanger as it results in trade-off between heat transfer and pressure drop. Optimization of the three-fluid heat exchanger (3−FHX) for minimum number of entropy generation unit requires ε−NTU relation which is unavailable for three-fluid cross-flow heat exchanger due to various complexities involved. A new approximate methodology for the optimization is presented by proving the equivalence of 3−FHX with a coupled two-fluid heat exchanger (2−FHX) followed by using ε−NTU relationship for 2−FHX. The experimental and numerical validation of the equivalence model is presented. Three-fluid cross-flow compact heat exchanger with offset strip fins is optimized for minimum number of entropy generation units using genetic algorithm (GA). Optimum design variables including heat distribution factor are determined for different cross-flow arrangements of 3−FHX with heat duty and pressure drop as constraint for both even and uneven fin dimensions case in all the passages. The present complex model including large number of continuous design variables can be handled well by GA. Validation of the results is carried out by using graphical techniques and very good agreement is observed. Additionally, sensitivity of the heat exchanger dimension has also been checked on the number of entropy generation along with the pressure drop in all the three fluids.

Experimental Performance of a Solid Heat Exchanger with Tree-Like Flow Passages

System performance of a solid single-fluid compact heat exchanger with tree-like flow passages has been experimentally examined. The results, presented in the form of commonly defined dimensionless parameters, demonstrate that system performance can be characterized in a mode similar to traditional compact heat exchanger designs. Pressure forces were found to dominate inertia forces at low Reynolds numbers. Correlations of the Euler number, Nusselt number, Colburn factor, and friction factor as a function of Reynolds number were utilized to compare system performance to traditional two-fluid compact heat exchangers.

Quantitative Assessment of the Overall Heat Transfer Coefficient U

This investigation was performed in order to quantify the validity of the assumed constancy of the overall heat transfer coefficient U in heat exchanger design. The prototypical two-fluid heat exchanger, the double-pipe configuration, was selected for study. Heat transfer rates based on the U = constant model were compared with those from highly accurate numerical simulations for 60 different operating conditions. These conditions included: (a) parallel and counter flow, (b) turbulent flow in both the pipe and the annulus, (c) turbulent flow in the pipe and laminar flow in the annulus and the vice versa situation, (d) laminar flow in both the pipe and the annulus, and (e) different heat exchanger lengths. For increased generality, these categories were further broken down into matched and unmatched Reynolds numbers in the individual flow passages. The numerical simulations eschewed the unrealistic uniform-inlet-velocity-profile model by focusing on pressure-driven flows. The largest errors attributable to the U = constant model were encountered for laminar flow in both the pipe and the annulus and for laminar flow in one of these passages and turbulent flow in the other passage. This finding is relevant to microchannel flows and other low-speed flow scenarios. Errors as large as 50% occurred. The least impacted were cases in which the flow is turbulent in both the pipe and the annulus. The general level of the errors due to the U = constant model were on the order of 10% and less for those cases. This outcome is of great practical importance because heat-exchanger flows are more commonly turbulent than laminar. Another significant outcome of this investigation is the quantification of the axial variations of the temperature and heat flux along the wall separating the pipe and annulus flows. It is noteworthy that these distributions do not fit either the uniform wall temperature or uniform heat flux models.

On observability gramian of a parallel flow heat exchanger equation with diffusive terms

This paper is concerned with observability and the related problem for a parallel flow two-fluid heat exchanger equation with diffusive terms. The observability results are stated under two kinds of boundary observations and the eigenvalues of the observability gramians are discussed. When one considers the inverse problem of estimating the initial states under the observations, it is expected that the reconstruction of the initial states under the observation at one outlet becomes more difficult than that under the observation at two outlets, from the physical point of view. The purpose of this paper is to explain the intuitive fact by using the eigenvalues of the observability gramians.

Operating characteristics and fabrication of a uniquely compact helical heat exchanger

A two-fluid heat exchanger of unusual compactness and high thermal performance is set forth in this paper. The underlying principle of the exchanger is a very tight helical intertwining of two small-diameter tubes (e.g., hypodermic tubes) such that tubes mutually envelope each other. A high-conductivity thermal bridge between the tubes is assured by the use of a braze material insinuated into interstices created by the helical windings. Heat transfer and pressure drop experiments carried out to validate the operational performance of the device are described. The uniqueness of the device has motivated a description of both the underlying design issues and fabrication processes. It is shown that the device can be manufactured by means of remarkably simple operations and machine tools.

New integral-mean temperature difference model for thermal design and simulation of parallel three-fluid heat exchanger

As we know, the mean temperature difference approach perfectly solves the problem of thermal design of a two-fluid heat exchanger. However, so far, the equivalent method has not been found for that of three-fluid heat exchanger. In this paper, a mean temperature difference method called IMTD (integral-mean temperature difference) is developed in detail for a parallel stream three-fluid heat exchanger with two communications. It includes the derivation of IMTD formulae, the illustration of design and simulation procedures of IMTD thermal model, the corresponding exemplary calculations and discussion, and the validation of IMTD thermal model. The research shows that the derived IMTD formulae and accelerated method proposed in the present IMTD model significantly improve the design process in the past analytical models. It realizes a fast convergence which is usually within 10 steps for all parallel stream flow arrangements. Moreover, the present model has the same accuracy as past exact analytical models and simultaneously can be comparable to past approximate analytical models in simplicity.

The influence of viscous heating on the entransy in two-fluid heat exchangers

In the present work, water and olive oil are taken as working fluids to study the influence of viscous heating on the entransy dissipation caused by heat transfer in two-fluid heat exchangers. The results show that the influence of viscous heating on the entransy loss associated with heat transfer can not be neglected for the liquids having large dynamic viscosity. The viscous heating effect maintains the heat transfer ability of the working fluids, relatively reduces the entransy loss in heat exchangers; the viscous heating effect relatively augments the entropy generation due to heat transfer and the available energy destruction in heat exchangers. For the working fluid having large dynamic viscosity, the increasing rates of the entransy and entropy generation contributed by the viscous heating are even larger than those contributed by heat transfer, when the mass flow rate of working fluid reaches a certain value under the fixed heat transfer area condition. Thus, the entransy loss rate decreases and the growth rate of entropy generation increases as the mass flow rate of the working fluid increases. Under the same other conditions, the heat transfer entransy loss rate and entropy generation rate per unit heat transfer rate obtained when the fluid having a smaller heat capacity rate is cold fluid are less than those obtained when the fluid having a smaller heat capacity rate is hot fluid.

The entransy dissipation minimization principle under given heat duty and heat transfer area conditions

Under given heat duty and heat transfer area conditions, the equipartition of the entransy dissipation (EoED) principle, the equipartition of the temperature difference (EoTD) principle, and the equipartition of the heat flux (EoHF) principle are applied to the optimization design of a heat exchanger with a variable heat transfer coefficient. The results show that the difference between the results obtained using the EoED and EoTD principles is very small, far smaller than that between the results obtained using the EoED and EoHF principles. The correct entransy dissipation minimization principle is chosen to optimize the parameters in the hot and cold fluids in a two-fluid heat exchanger, under given heat duty and heat transfer area conditions. The results indicate that the proper choice of the two alternative fluids has an important role in the successful application of the entransy dissipation minimization principle. The fluid that could improve the total heat transfer coefficient should be chosen, or the fluid that makes the temperature profiles of the hot and cold fluids parallel and decreases the temperature difference between the hot and cold fluids after optimization simultaneously, could be the proper one.

On observability of parallel-flow diffusive heat exchanger equations with boundary output

This paper is concerned with the observability problem of a parallel-flow two-fluid heat exchanger equation with diffusion term. First, the case where two fluid temperatures are measured at the outlet is considered. It is shown that the observed system with the measurements becomes observable on any interval of time through a concrete series expression of the solution. Next, the two cases where each one of two fluid temperatures is measured at the outlet are considered. It is also shown that the observed system with the only one measurement becomes observable on any interval of time except for the special cases of physical constants appearing in the equation. For the exceptional cases the unobservable subspace is finite dimensional and is characterized by using the eigenfunctions of heat equation with fluid transfer term.

Three-fluid heat exchangers with three thermal communications. Part A: general mathematical model

A general, simple and easy to implement analytical model that enables design and analysis of three-fluid heat exchangers with three thermal communications for all flow arrangements is developed. The model is shown to reduce to either a three-fluid, two thermal communications model or the traditional two-fluid heat exchanger model under proper conditions. Six non-dimensional design parameters are identified and their effect on the temperature distributions of the different fluid streams is presented. The model shows that the presence of three coupled thermal interactions makes temperature distributions of the different streams difficult to assess due to slight changes in the design parameters.

Flow maldistribution in heat exchangers

Based on the study of gross flow maldistribution in an experimental electrical heater, this paper presents the effect of flow nonuniformity on the performance of heat exchangers. First, it is shown that it is much more important to understand maldistributions for electrical heaters than for two-fluid heat exchangers. The study of the flow distribution in a particular heater shows that reverse flows may occur for poor inlet header design. Suggested here is a simple way to homogenize the flow distribution and a simple law to calculate, with good accuracy, the velocity ratio (ratio of the highest velocity in the tubes to the lowest velocity). The original fluid distribution is applied to heat exchangers (condensers, counterflow and crossflow heat exchangers), and it is shown that gross flow maldistribution leads to a loss of effectiveness of about 7% for condensers and counterflow heat exchangers, and up to 25% for crossflow exchangers, for velocity ratios up to 15.

A Review of Solution Methods for Determining Effectiveness-NTU Relationships for Heat Exchangers With Complex Flow Arrangements

A number of approaches have been employed for the design and analysis of two-fluid heat exchangers. These approaches include the effectiveness (ε or P)-number of transfer units (NTU) method, the mean temperature difference method, and the ones using the Mueller and Roetzel charts. A comprehensive review is presented of solution methods for obtaining effectiveness-NTU relationships for simple and complex heat exchangers and heat exchanger assemblies. These include analytical methods for obtaining exact solutions, approximate (including semi-numerical and analog) methods, numerical methods, matrix formalism, and additional methods based on the properties of the exchanger configurations. Additional details that pertain to the Laplace transforms and matrix formalism that includes the chain rule methodology are discussed. A step-by-step procedure that illustrates the chain rule methodology is presented. This review article includes 73 references.

An improved scheme for determining heat transfer correlations from heat exchanger regression models with three unknowns

A nonlinear regression scheme for determining heat transfer correlations from overall heat exchanger measurements is presented. The proposed scheme is valid for any two-fluid heat exchanger data where only one fluid's thermal resistance varies. The resulting regression model has three unknowns; two are associated with the variable resistance, and the third is the sum of all other resistances. The model can be analyzed only by using nonlinear regression because it cannot be transformed into linear form. Unlike existing approximate methods of analysis (known as modified Wilson plots), the proposed scheme is guaranteed to converge if a solution exists. The scheme was successfully tested by analyzing data collected on a tube-fin heat exchanger with continuous plain fins.

The Second Law Quality of Energy Transformation in a Heat Exchanger

This paper presents the entropy generation (irreversibility) concept as a convenient method for estimating the quality of the heat exchange process in heat exchanger analysis. The entropy generation caused by finite temperature differences, scaled by the maximum possible entropy generation that can exist in an open system with two fluids, is used as the quantitative measure of the quality of energy transformation (the heat exchange process). This measure is applied to a two-fluid heat exchanger of arbitrary flow arrangement. The influence of different parameters (inlet temperature ratio, fluid flow heat capacity rate ratio, flow arrangements) and the heat exchanger thermal size (number of heat transfer units) on the quality of energy transformation for different types of heat exchangers is discussed. In this analysis it is assumed that the contribution of fluid friction to entropy generation is negligible.

On the feasibility of heat-exchange processes in uninsulated equipment

Strictly speaking, the common two-fluid heat exchanger is a special version of a three-fluid exchanger because the environment also participates in the energy exchange. In a number of situations this effect is significant and leads to either reduced or increased area requirements, depending on the purpose of the exchange operation. With certain combinations of the system parameters it is even impossible to achieve an envisaged temperature change. For some typical flow arrangements these combinations are established in an analytic form so that one can rapidly assess the feasibility of a desired process without actually performing the rather tedious design calculations.