Performance of Counterflow Microchannel Heat Exchangers Subjected to External Heat Transfer

This article analyzes the effect of external heat transfer on the thermal performance of counterflow microchannel heat exchangers. Equations for predicting the axial temperature and the effectiveness of both fluids as well as the heat transferred between the fluids, while operating under external heating or cooling conditions, are provided in this article. External heating may decrease and increase the effectiveness of the hot and cold fluids, respectively. External cooling may improve and degrade the effectiveness of the hot and cold fluids, respectively. For unbalanced flows, the thermal performance of the microchannel heat exchanger subjected to external heat transfer depends on the fluid with the lowest heat capacity. At a particular number of transfer units (NTU), the effectiveness of both the fluids increased with decrease in heat capacity ratio when the hot fluid had the lowest heat capacity. When the cold fluid had the lowest heat capacity, the effectiveness of both fluids increased with decrease in heat capacity ratio at low values of NTU but at high values of NTU the effectiveness increased with increase in heat capacity ratio. A term called the “performance factor” has been introduced in this article to assess the relative change in effectiveness due to external heat transfer.     

Effect of Partition Wall on Heat Transfer Characteristics of a Gas-to-Gas Counterflow Microchannel Heat Exchanger

The effect of a partition wall on heat transfer characteristics of a two-stream gas-to-gas counterflow microchannel heat exchanger has been numerically investigated. The flow passages of the microchannel heat exchanger are plane channels of 100 μm in height and 20 mm in length. The material of the partition wall is assumed to be stainless steel. The computations were performed for a wide range of flow rate to investigate heat transfer characteristics of the microchannel heat exchanger. Moreover, computations for various partition wall thicknesses were conducted to investigate the effect of the wall thickness. The thickness ranged from 200 μm to 6 μm while the channel height was fixed at 100 μm. Numerical results show that heat transfer characteristics of a gas-to-gas counterflow microchannel heat exchanger are affected by partition wall thickness. Computations for various partition wall thicknesses and thermal conductivities of the partition wall were performed. The results were compared with those of a single microchannel with constant wall temperature. Applicability of the assumption of constant wall temperature was revealed.     

Numerical Simulation of Re-Entry Flow: Heat Flux Evaluation

Abstract

The problem of heat transfer in a four channel plate heat exchanger involving the effect of unsymmetrical heat transfer in the outer two channels is studied analytically and experimentally. An energy balance over a control volume yields the governing system of differential equations that has been solved exactly for the cases of parallel flow and counterflow to give the temperature distribution in the channels. The results show that zero or even reversed heat flow may be obtained at the middle plate of the heat exchanger. Expressions for the heat exchanger efficiency and the log-mean temperature difference correction factor in terms of the heat capacity rate ratio and the number of transfer units are presented. Experiments carried out with a counterflow plate heat exchanger show reasonable agreement between the experimentally observed and the theoretically calculated efficiencies and mean driving temperature differences.     

Numerical study of conjugated heat transfer in metal foam filled double-pipe

The two equation numerical model has been applied for parallel flow double-pipe heat exchanger filled with open cell metal foams. The model fully considered solid–fluid conjugated heat transfer process coupling heat conduction and convection in open cell metal foam solid matrix, interface wall and fluid in both inner and annular space in heat exchanger. The non-Darcy effect and the wall thickness are also taken into account. The interface wall heat flux distribution along the axial direction is predicted. The numerical model is firstly verified and then the influences of solid heat conductivity, metal foam porosity, pore density, relative heat conductivity and inner tube radius of the heat exchanger on dimensionless temperature distribution and heat transfer performance of heat exchanger are numerically studied. It is revealed that the proposed numerical model can effectively display the real physical heat transfer process in the double pipe heat exchanger. It is expected to provide useful information for the design of metal foam filled heat exchanger.     

Heat Transfer Characteristics of One-and Two-Tube-Pass Split-Flow Heat Exchangers

The heat transfer process in a split-flow heat exchanger designated as G type by TEMA standards is analyzed for two cases: with one tube pass and with two tube passes. Equations are presented for the shell fluid temperature distribution as a function of exchanger length or surface area. The thermal performance of the two-tube-pass exchanger is compared with those of a 1:1 counterflow exchanger and a one-pass split-flow exchanger. Results are presented in terms of temperature efficiency, reduced thermal flux (NTU), and thermal flow rate ratio.     

Heat Transfer and Exergy Analysis of a Spiral Heat Exchanger

The heat transfer effectiveness of a countercurrent spiral heat exchanger is expressed as a function of number of transfer units, ratio of flow capacity rates, number of spiral turns, and dimensionless start-point angle of spiral (dimensionless angular angle of the start point of a spiral curve constituting the solid wall of the heat exchanger). The heat transfer effectiveness is weakly dependent on the dimensionless start-point angle of spiral, but moderately increases with the number of spiral turns. As the number of spiral turns is larger than 20, the heat transfer effectiveness of the spiral heat exchanger approaches that of a counterflow heat exchanger. The heat transfer effectiveness of the spiral heat exchanger has a maximum. The optimum number of transfer units at the maximum heat transfer effectiveness increases with the number of spiral turns, whereas it increases with a decrease of the ratio of flow capacity rates. In the second-law analysis, an optimum hot flow-to-cold flow capacity-rate ratio is found. For obtaining a large net recovered exergy rate, the spiral heat exchanger needs to possess a large number of transfer units (greater than 2.0) and operate at a near balanced-flow condition. In addition, a small consumed mechanical power is demanded.     

Performance of Counterflow,Parallel Plate Heat Exchangers under Laminar Flow Conditions

Multilayered heat exchangers were analyzed theoretically and their heat transfer characteristics were clarified. The problem was treated as a two-dimensional, conjugated one with three phases-two fully developed laminar flows and the exchanger wall. From numerical results, the exchanger effectiveness was found to be definitely influenced by the following parameters: Graetz number, heat capacity flow rate ratio, dimensionless wall thickness, and conductance ratios between fluid and wall and between both fluids. Examination of mixed-mean temperature distributions in the exchanger showed that longitudinal wall conduction significantly reduces exchanger effectiveness in the low-Graetz-number region. Experimental results were in fairly good agreement with theoretical predictions.     

Parallel-Flow Heat Exchangers with Ambient Thermal Interaction

A mathematical model is developed to study the performance of a parallel-flow heat exchanger in which both fluid streams are interacting thermally with the surroundings. The fluid temperatures are found to be dependent on the magnitude of the ambient temperature relative to fluid inlet temperatures, the ratios of conductances between the fluids and the ambient and the interfluid conductance, the ratio of minimum to maximum fluid capacities, and the number of transfer units, NTU, for the heat exchanger. Two heat exchanger effectiveness criteria, one each for the hot and cold fluids, are used to study performance. The effectiveness is found to be adversely affected by increasing conductance ratios, increasing NTU, and increasing temperature difference between the ambient and the fluid of interest. For very high values of the conductance ratios, the heat exchanger will not perform as expected and both fluid temperatures will approach that of the ambient. The parallel-flow arrangement is compared to counterflow and is found to be less effective under the external heat transfer condition.     

一种监测换热器污垢的新方法

在考虑污垢对换热器传热性能影响的基础上,提出换热器当量总污垢热阻和污垢函数的概念,并给出换热器当量总污垢热阻的监测方法,讨论了换热流型、传热有效度ε和冷热流体热容量率比R对换热器当量总污垢热阻的影响。     

考虑污垢时换热器热力学性能的评价

在分析污垢对换热器传热性能影响的基础上,在考虑污垢时采用Ｙｏｎｇ损率这一指标对换热器的热力学性能进行了评价,讨论了传热数和冷热流体热容量率比对其性能的影响,并把结果与不考虑污垢时的情况进行了比较,得到了一些有益的结论。     

Heat transfer enhancement for the power-law fluids through a parallel-plate double-pass heat exchangers with external recycle

The conjugated Graetz problem of the double-pass heat transfer through a parallel-plate device with uniform wall temperature and external recycle in the outlet channel was solved analytically with the use of the orthogonal expansion technique for the power-law fluids. The mathematical formulation of the heat transfer problem was developed for fully developed laminar velocity profiles through the parallel-plate channels by ignoring axial conduction and assuming fluid properties of temperature independence. The constant outer wall temperature and continuous temperature and heat flux between the two subchannels with inserting impermeable sheet were considered for thermal boundary conditions. The analytical results show that the recycle ratio and impermeable sheet position play significant influences on the efficiencies of the heat transfer as compared to that in a single pass (without an impermeable sheet inserted and without recycle). The outlet temperature of the heat exchanger seems to be independent of the power-law index of the fluid, while the average Nusselt number could not be enhanced significantly with the lowering power index. The power consumption increment owing to the cross-sectional area reduction from single-pass to double-pass was also taken into account for comparisons.     

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.     

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.     

Effectiveness–NTU Relations for Counterflow Heat Exchangers: The Effect of Kinetic Energy Variation and Heat Leak From Outside

The effectiveness–number of transfer units (NTU) relations are useful data for designing and performance evaluation of heat exchangers with fluids having considerable variation in velocities in the presence of heat leak. In this article, the closed-form (benchmark) solutions for counterflow heat exchangers, when the heat leak is either on the hot or cold side of the heat exchanger in the presence of kinetic energy variation, are presented. It was found that the effectiveness depends on NTU and fluid capacity ratio along with six other dimensionless variables that reflect the magnitude and axial distribution of the kinetic energy and heat leak on the hot and cold sides of the heat exchanger. The results are also presented in a graphical form exhibiting the variation of effectiveness of the heat exchanger with the already-mentioned parameters. It was demonstrated that when the dimensionless heat leak and kinetic energy terms approach zero, the solution reduces to the classical effectiveness–NTU relations for counterflow heat exchangers.     

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.     

A New Method for Analyzing Heat Exchangers- Matching of Temperature Field

In heat exchangers, the magnitude of Nu of each duct is influenced by the temperature field, since the ratio of heat capacity rate will influence the matching status of the temperature field between contacting ducts, the total heat transfer coefficient is related with the ratio of heat capacity rate. Considering this relationship, a new method for analyzing heat exchanger is proposed - matching of temperature field. First, for a single duct with the temperature field varying exponentially along the flow direction, its Nu is calculated. Then under the hypothesis that the thermal resistance of the wall is negligible, the matching condition was set like this: both the temperature and heat flux are equal for the hot and cold fluids at the wall, so the matching relationship of parameter that describes the temperature field of the hot and cold fluids, was obtained. Finally the relationship between the total Nu and the ratio of heat capacity rate along with the ratio of inherent thermal resistance is obtained. Compared with traditional analyzing methods, the temperature matching method can be used to get the total heat transfer coefficient directly, and also be used for optimization of heat exchanger design. For a parallel flow, the optimal ratio of heat capacity rate is reciprocal to the ratio of inherent thermal resistance, and for a counter flow, the optimal ratio of heat capacity rate is zero or infinity.     

An analytical study of power-law fluids in double-pass heat exchangers with external recycle

The influence of the recycle effect on heat transfer to the power-law fluids from a double-pass parallel-plate heat exchanger with uniform wall temperature has been studied analytically. The governing equations are solved by an orthogonal expansion technique in terms of power series. The heat transfer problem is solved for fully developed laminar flow through parallel-plate channels by ignoring axial conduction, with fluid properties of temperature independence. The dependence of the average Nusselt number on the Graetz number has been studied in detail. Analytical results show that the recycle effect can enhance the heat transfer efficiency as compared with that in a single pass (without an impermeable plate inserted and without recycle). Considerable heat-transfer enhancement is obtained by introducing the recycle-effect concept in designing such double-pass operations. The effects of the impermeable-sheet position and the power-law index on the enhancement of device performance as well as the power consumption increment have also been delineated.     

Thermally dependent viscosity and non-Newtonian flow in a double-pipe helical heat exchanger

A double-pipe helical heat exchanger was numerically studied to determine the effects of thermally dependent viscosity and non-Newtonian flows on heat transfer and pressure drop for laminar flow. Thermally dependent viscosities were found to have very little effect on the Nusselt number correlations for Newtonian fluids; however significant effects on the pressure drop in the heat exchanger were predicted. Changing the flow rate in the annulus can significantly affect the pressure drop in the inner tube, since the average viscosity of the fluid in the inner tube would change due to the change in the average temperature.The effects of non-Newtonian power law fluids on the heat transfer and the pressure drop were determined for laminar flow in the inner tube and in the annulus. The Nusselt number was correlated with the Péclet number for heat transfer in the inner tube. For the annulus, the Nusselt number was found to correlate best with the Péclet number and the curvature ratio. Pressure drop data were compared by using ratios of the pressure drop of the non-Newtonian fluid to a Newtonian fluid at identical mass flow rates and consistency indices.     

换热器系统的热力学性能评价

引入可用能损率这一指标对串联组合的换热器系统的热力学性能进行了 分析和评价,得到了换热器系统可用能损率的一般计算式,讨论了换热器系统的总流动趋势、冷热流体热容量流率比、传热单元数及单台换热器的流型、传热有效度和数目等对换热器系统可用能损率的影响。     

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.