Sensitivity analysis and numerical experiments on transient test of compact heat exchanger surfaces

A single-blow transient testing technique considering the effect of longitudinal heat conduction is suggested for determining the average convection heat transfer coefficient of compact heat exchanger surface. By matching the measured outlet fluid temperature variation with similar theoretical curves, the dimensionless longitudinal conduction parameter λ₁, the time constant of the inlet fluid temperature τ⁺, and the number of heat transfer units N _tu can be determined simultaneously using the Levenberg-Marquardt nonlinear parameter estimation method. Both sensitivity analysis and numerical experiments with simulated measurements containing random errors show that the method in the present investigation provides satisfactory accuracy of the estimated parameter N _tu, which characterizes the heat transfer performance of compact heat exchanger surfaces. __________ Translated from Journal of Power Engineering, 2007, 27(2): 227-232 [译自:动力工程]     

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.     

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.     

Theoretical prediction of longitudinal heat conduction effect in cross-corrugated heat exchanger

In the elementary heat exchanger design theory, the longitudinal heat conduction through the heat transfer plate separating hot and cold fluid streams is neglected, and only the transverse heat conduction is taken into account for the conjugate heat transfer problem. In the cross-corrugated heat exchanger, the corrugated primary surface naturally leads to the highly non-uniform convective heat transfer coefficient distribution on opposite sides of the plate. In such a case, the longitudinal heat conduction may play a significant role in the thermal coupling between high heat transfer regions located on opposite sides of the plate. In the present study CFD is used to perform a quantitative assessment of the thermal performance of a cross-corrugated heat exchanger including the longitudinal heat conduction effect for various design options such as different plate thickness and corrugation geometry for a typical operating condition. The longitudinal heat conduction effect is then predicted by the theoretical method using the ‘network-of-resistance’ in the wide range of the heat exchanger design space.     

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.     

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.     

用渐近分析法研究蜂窝蓄热体温度分布

1前言在冶金、机械和石化工业锻造炉、均热炉、连续加热炉、热处理炉、钢包烘烤炉、辐射管和熔铝炉上应用的高温空气燃烧(High Temperature Air Combustion,Hi-TAC)[1],具有热效率高、低NOx排放和燃烧放热均匀等特点。大多数的HiTAC应用了蜂窝蓄热系统[2]。温度变化和温度效率(     

Exact,analytical heat transfer from longitudinal radiating fins of rectangular profile in a tube/fin ensemble

In this technical note, the problem concerning the quantification of heat transfer rates from an array of longitudinal radiating fins of rectangular profile in a tube/fin ensemble to a nonzero temperature sink is investigated. Radiating fins constitute essential elements in the thermal control of spacecrafts and satellites. We consider quasi one-dimensional heat conduction in the longitudinal radiating fins and neglect radiative exchange between the fins and the tubes carrying a hot fluid. It is demonstrated that the governing nonlinear differential equation of second order with constant coefficients and nonhomogeneous can be solved in exact, analytical implicit form. The pertinent temperature distributions eventually provide the magnitudes of heat transfer rates and fin efficiencies influenced by the radiation–conduction parameter and the sink temperature.     

The effect of longitudinal heat conduction in cross flow indirect evaporative air coolers

The effect of the longitudinal heat conduction in the exchanger wall of a compact-plate cross flow indirect evaporative cooler is investigated. A NTU method is used to study the heat and mass transfer characteristics. A block iterative numerical method is used to solve the coupled conservation equations for the primary fluid, the secondary fluid and the liquid film. The model was validated using previously published data. The exchanger performance deterioration due to the conduction effect has been determined for various design and operating conditions. The results indicate that the thermal performance deterioration of the evaporative coolers may become significant for some typical operating conditions and could be as high as 10%, while it lies less than 5% for most conservative conditions.     

Numerical analysis of heat transfer and flow field around cross-flow heat exchanger tube with fouling

Fouling is one of the main problems of heat transfer which can be described as the accumulation on the heat exchanger tubes, i.e.; ash deposits on the heat exchanger unit of the boiler. A decrease in heat transfer rate by this deposition causes loss in system efficiency and leads to increasing in operating and maintenance costs. This problem concerns with the coupling among conduction heat transfer mode between solid of different types, conjugate heat transfer at the interface of solid and fluid, and the conduction/convection heat transfer mode in the fluid which can not be solved analytically. In this paper, fouling effect on heat transfer around a cylinder in cross flow has been studied numerically by using conjugate heat transfer approach. Unlike other numerical techniques in existing literatures, an unstructured control volume finite element method (CVFEM) has been developed in this present work. The study deals with laminar flow where the Reynolds number is limited in the range that the flow field over the cylinder is laminar and steady. We concern the fouling shape as an eccentric annulus with constant thermal properties. The local heat transfer coefficient, temperature distribution and mean heat transfer coefficient along the fouling surface are given for concentric and eccentric cases. From the results, we have found that the heat transfer rate of cross-flow heat exchanger depends on the eccentricity and thermal conductivity ratio between the fouling material and fluid. The effect of eccentric is dominant in the region near the front stagnation point due to high temperature and velocity gradients. The mean Nusselt number varies in asymptotic fashion with the thermal conductivity ratio. Fluid Prandtl number has a prominent effect on the distribution of local Nusselt number and the temperature along the fouling surface.     

sCO2太阳能热发电流化床固体颗粒/sCO2换热器建模与仿真研究

为了研究不同运行工况条件下的换热特性,利用分段集总参数法搭建流化床固体颗粒/sCO₂换热器的仿真数学模型,并对换热系统在不同输入变量扰动下的动态特性及对关键参数的敏感性进行分析和研究。结果表明：换热系统输入侧入口温度的扰动对换热器输出温度的影响幅度较大,而输入侧质量流量的扰动对输出温度的影响较小;小管径和低管数有利于获得较高的sCO₂侧换热系数;同时,在符合最小流化条件下,小粒径和较低的流化气体速度有利于颗粒侧传热系数的提高。     

Local effects of longitudinal heat conduction in plate heat exchangers

In a plate heat exchanger, heat transfer from the hot to the cold fluid is a multi-dimensional conjugate problem, in which longitudinal heat conduction (LHC) along the dividing walls often plays some role and can not be neglected. Large-scale, or end-to-end, LHC is always detrimental to the exchanger’s effectiveness. On the contrary, if significant non-uniformities exist in the distribution of either convective heat transfer coefficient, small-scale, or local, LHC may actually enhance the exchanger’s performance by improving the thermal coupling between high heat transfer spots located on the opposite sides of the dividing wall.     

Heat and mass transfer in plate-fin sinusoidal passages with vapor-permeable wall materials

Laminar forced flow and heat mass transfer in sinusoidal plate-fin small passages encountered in compact heat mass exchangers are investigated. The duct is similar to a traditional plate-fin heat exchanger, but vapor-permeable materials like polymer membranes, paper, and ceramics can be used as the duct materials so both sensible heat and moisture can be exchanged simultaneously. Heat conduction and mass diffusion in the fins and heat and moisture convection in the fluid are analyzed simultaneously as a conjugate problem. Their fully developed Nusselt and Sherwood numbers under various aspect ratios and fin conductance parameters are calculated. The results found that though fins extend the heat transfer area, they are less effective compared to a traditional compact heat exchanger with metal foils. Most unfortunately, fin efficiencies for moisture transfer are even much smaller than those for heat transfer due to the low fin mass conductance parameters. For such heat mass exchangers, the use of fins can be regarded mostly as supporting materials, rather than as mass intensification techniques.     

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.     

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 Analysis for a Concentric Tube Heat Exchanger Including the Wall Axial Conduction

The effect of wall axial conduction on the heat transfer in a concentric tube heat exchanger is examined for the inner flow laminar flow regime. The procedure used for the current analysis combines the analytical solution for the inner fluid with a numerical approximation for the wall conduction and has the capability of handling the temperature variation for the outer fluid. Both parallel and counterflow cases are evaluated for the analysis, and results are presented in terms of the axial variations of fluids and wall temperatures. Effects of the heat capacity rate ratio of the fluids on the temperature variations and on the mean heat flux are also pointed out. The effect of the exchanger length is included for the analysis. It is concluded that the total heat transfer between the fluids is greatly influenced by the wall axial conduction for the counterflow arrangement and is not ignorable when the heat capacity rate ratio of fluids are smaller than unity.     

Issues on the Testing of Small Cryogenic Recuperators and Experimental Studies on Perforated Plate Heat Exchangers

This paper deals with various issues on testing heat exchangers at cryogenic temperatures, the test setup and experimental studies on perforated plate heat exchangers. A number of fluid flow circuits are used in practice for testing a heat exchanger at cryogenic temperatures. These circuits are reviewed and a simplified fluid circuit is proposed with justifications. Effects of the fluid inlet/outlet tubes on the performance evaluation of the heat exchanger were investigated numerically. It is found that convection heat transfer between fluid and the tube as well as conduction heat transfer through the tube walls can cause significant errors in the evaluation. These errors are considered while designing the inlet/outlet tubes for the test setup. Details of the test setup including mounting of temperature sensors, instrumentation and data acquisition are discussed. Some selected data from our experimental studies on perforated plate heat exchangers are presented. Transient study on the heat exchanger by using the setup is also demonstrated. For pressure drop studies, a separate test setup is used.     

固体吸附式制冷系统中吸附床内传热过程的数值模拟

对一简化吸附床模型中传热过程进行理论分析,对吸附床内的热传导方程和换热管内流体的能量控制方程进行离散并利用控制容积法进行模拟数值计算,在计算模型中加入随时间变化的边界条件,得到吸附床内吸附剂和换热管内流体的两种互相耦合的温度分布,为吸附床的实际优化设计提供理论依据。     

The performance of a new gas to gas heat exchanger with strip fin

A compact gas to gas heat exchanger needs large heat transfer areas on both fluid sides. This can be realised by adding secondary surfaces. The secondary surfaces are plate fin, strip fin, and louvered fin, etc. The fins extend the heat transfer surfaces and promote turbulence.This paper presents a gas to gas heat exchanger with strip fins. The heat exchanger design and construction are based on a method to seal rectangular strip fins in slots in opposite walls of a rectangular pipe. Fins are fixed and sealed to the walls simultaneously by high temperature brazing of glass mixed with metals in a furnace. The additional advantage of glass is that it forms a coating on the heat transfer surface to protect the surface from corrosion.A number of measurements were carried out to test the performance of this heat exchanger. Not surprisingly, the measurement results indicate that heat transfer coefficient and pressure drop increase with the ratio of heat transfer area to volume (fin density). Colburn factor and friction factor versus Reynolds number are presented.     

Heat Transfer Analysis of Different Storing Media Using Oil as Working Fluid

Abstract

Thermal analysis of heat transfer through different storing media using oil as working fluid is presented. The storing medium is solid material in spherical shape. Steel, glass, and pebbles are selected as storing media and oil is selected as working fluid. The physical model is a heat exchanger in cylindrical shape, which is packed with each of the selected storing medium. The heat transfer through the heat exchanger is assumed to be one dimensional along its height. The flow of the working fluid is an axial direction from the top to downward. The problem is governed by two partial differential equations for the working fluid (oil) and the storing medium. Finite difference method and Thomas algorithm solver are used to solve the couple of the two partial differential equations along with their associated initial and boundary conditions. The modified computer program is used to obtain the solution of transient temperature distribution of the storing medium and the working fluid. The amount of absorbed heat inside each of the storing medium is obtained. The effect of special operating parameters on the amount of absorbed heat inside the storing medium, such as aspect ratio (the ratio between diameter and length of the heat exchanger), storing media, mass velocity, the number of charging cycles, and void fraction, is discussed. Therefore, the dimensionless heat transfer coefficient parameter (Nusselt number, Nu) provides a measure of the convection and conduction heat transfer at the surface of storing medium when the working fluid (oil) flows over a solid surface of the medium. The numerical results of transient temperature profiles and the amount of absorbed heat inside the storing medium for each system with respect to the operating parameters and the heat exchanger characteristics are illustrated. The results show that steel storing medium is charging by four cycles while the pebble storing medium is charging by two cycles only, this due to the thermal and physical properties of these materials. The absorbed heat inside storing medium, which has aspect ratio equals one (diameter of the heat exchanger equals its length) is higher than others. Increasing mass velocity increases absorbed heat inside the storing medium and decreasing the charging time. Increasing void fraction decreases absorbed heat inside the storing media due to the smaller volume of absorbing medium. The amount of absorbed heat (at certain time) inside the steel > glass > pebble is due to the thermal conductivity of these materials.