An experimental study of conduction heat transfer in rarefied polyatomic gases

Experimental data have been obtained for the conduction heat transfer through a rarefied polyatomic gas (benzene or n-hexane) in a concentric cylindrical geometry. The results are of direct relevance in evacuated solar collector technology. The data were fitted to within 7% over the whole transition region by a simple semi-empirical equation, previously used for monatomic gases. A Monte Carlo simulation of the system was undertaken to obtain heat conduction and temperature profiles over the transition region in benzene and n-hexane gases.     

Robust methodology for determination of heat transfer coefficient distribution in convection

To predict the microstructures, residual stresses and distortions in the heat treated metal components, it is important to accurately know the heat transfer coefficients (HTCs) between the hot work piece and cooling media. In this paper, a new method is presented to accurately determine the node-based HTC distribution by coupling computational fluid dynamics (CFD) with optimal weight functions and scale factors. With this new method, the predicted temperature profile of the work piece during quenching (rapid cooling) is in excellent agreement with experimental measurements. This new method can be also applied to accurately predict convection heat transfer in thermal equipment such as heat exchangers and refrigerators, building thermal design and other heat transfer related situations.     

Heat conduction in the hollow sphere with a power-law variation of the external heat transfer coefficient

The conduction phenomenon in an insulated sphere is re-worked through a dimensionless approach, where the heat transfer coefficient dependence on the external radius and on the surface temperature, as in the case of forced and free convection, is taken into account. Assuming a power law variation of the convection coefficient [1, 2], and using the results of Sparrow [3], equations and graphs for the most important dimensionless parameters are presented. The developed equations show, for example, that as the insulation thickness increases the heat transfer rate tends to a positive value, independent of the considered case: constant convection coefficient, forced or free convection.     

Experimental study and analysis on heat transfer coefficient of radial heat pipe

The experimental study was performed on five eccentric radial heat pipes with two outer-tube diameters.The test range can be given as follows,working fluid filling ratio Ω=44%～83%,heat flux q=10000W/m2～32000W/m2,and working temperature tv=50 ℃～120 ℃.The correlations between radial heat pipe heat transfer performance and filling ratio,heat flux,working temperature were studied in the experiment.Based on linear regression of experimental data,the relationship between heat pipe equivalent heat resistance R and working temperature tv,heat flux q and filling ratio Ω was obtained.     

An experimental heat transfer study for periodically varying-curvature curved-pipe

Experiments were conducted for water flowing through a varying-curvature curved-pipe inside a larger diameter straight pipe to form a double-pipe heat exchanger with water as the working medium. The heat transfer coefficients were obtained using the Wilson plot method. The effects of the Dean, Prandtl, Reynolds number and the curvature ratio on the average heat transfer coefficients and the friction factors are presented. A higher Dean number results in a higher heat transfer rate. It is found that the heat transfer rate may be increased by up to 100%, as compared with a straight pipe, while the friction coefficient increased by less than 40%. Therefore, it is promising to use S-shaped pipes instead of straight pipes for the performance enhancement for a heat exchanger such as a solar collector.     

Using genetic algorithms for the determination of an heat transfer coefficient in three-phase inverse Stefan problem

In the paper, an example is presented of the application of a genetic algorithm to a design inverse Stefan problem. The problem consists in the reconstruction of the function which describes the heat transfer coefficient, where the positions of phase change moving interfaces are well-known. In numerical calculations, the Tikhonov regularization, a genetic algorithm and a generalized alternating phase truncation method were used. The featured examples of calculations show a very good approximation of the exact solution.     

Analytical solutions of the 1-D heat conduction problem for a single fin with temperature dependent heat transfer coefficient - II. Recurrent direct solution

The recurrent direct solution of the 1-D heat conduction problem for a single straight fin and spine with power-law-type temperature dependent heat transfer coefficient has been derived using inversion of the closed-form solution obtained in the first part of the study. The expression with improving convergence to calculate accurately the dimensionless temperature excess T_e at the fin tip for a given values of the fin parameter N and exponent n in heat transfer equation has been obtained by a linearization method. Equation for the temperature excess distribution throughout the fin has also been derived. The obtained formula for T_e allows to calculate the fin base thermal conductance and augmentation factor. Obtained expressions are seen to be simple and convenient for the engineering design of the fins and finned surfaces.     

Analytical solutions of 1-D heat conduction problem for a single fin with temperature dependent heat transfer coefficient - I. Closed-form inverse solution

Closed-form solution of 1-D heat conduction problem for a single straight fin and spine of constant cross-section has been obtained. The local heat transfer coefficient is assumed to vary as a power function of temperature excess. The dependence of the fin parameter N on the dimensionless temperature difference T_e at the fin tip for a given exponent n was derived in a form N/N₀=T_e^−μn (where N₀ is a well-known N expression for n=0). Coefficient μ was found to be equal to 5/12 according to the exact solution at T_e→1 or to 0.4 according to the fitting procedure for the data of the numerical integration. Obtained formula serves as a basis for the derivation of the direct expressions for T_e vs N at given n, fin base thermal conductance and augmentation factor presented in the second part of the study.     

An experimental study of heat transfer enhancement with a pulsating flow

A pulsating flow in a pipe was experimentally investigated to determine the effect of pulsation on the rate of heat transfer. The influence of hydrodynamic parameters and characteristics of the pulsation on heat transfer was carefully studied. In order to adjust the pulsating parameters, a self‐oscillator was designed so the length of the resonator and the length of the outlet nozzle could be adjusted. The results show that the heat transfer rate is strongly affected by both the hydrodynamic parameters and the configuration of the resonator. With the increase of the flow rate of the liquid and the length of the chamber, heat transfer is enhanced. There is an optimal length at which the heat transfer enhancement attends to the best. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(5): 279–286, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20020     

Overall heat transfer coefficient in the presence of the axial dispersion coefficient for a forced draught cooling tower

An overall heat transfer coefficient was calculated for a forced draught counterflow cooling tower by using the pulse response technique. The presence of an axial dispersion coefficient for both gas and liquid was considered. Results indicate that, on neglecting the axial mixing and assuming a plug flow, the overall heat transfer coefficient is overestimated and can lead to errors in design applications.     

An experimental investigation of heat transfer enhancement for a shell-and-tube heat exchanger

For the purpose of heat transfer enhancement, the configuration of a shell-and-tube heat exchanger was improved through the installation of sealers in the shell-side. The gaps between the baffle plates and shell is blocked by the sealers, which effectively decreases the short-circuit flow in the shell-side. The results of heat transfer experiments show that the shell-side heat transfer coefficient of the improved heat exchanger increased by 18.2–25.5%, the overall coefficient of heat transfer increased by 15.6–19.7%, and the exergy efficiency increased by 12.9–14.1%. Pressure losses increased by 44.6–48.8% with the sealer installation, but the increment of required pump power can be neglected compared with the increment of heat flux. The heat transfer performance of the improved heat exchanger is intensified, which is an obvious benefit to the optimizing of heat exchanger design for energy conservation.     

螺旋管传热系数的研究

螺旋管传热器优越的结构和传热特性,使其在传热领域得到了广泛应用.采用实验方法对其对流传热系数进行了测定,并利用Fluent软件对螺旋管传热情况进行了模拟.结果表明,在设定的实验条件下,水-水螺旋管传热系数,自然对流情况下为350~550 W/(m2·K),强制对流(搅拌状态)下为400~650 W/(m2·K),所建立的Fluent软件模型能基本反映流场的实际情况,其计算结果与实验情况基本吻合.     

考虑污垢因素的凝汽器传热系数的计算研究

凝汽器冷却水系统运行过程中,由于难溶盐分的沉积和微生物污垢的生成,会在凝汽器的换热表面上形成污垢,使传热系数下降,真空降低。分析了凝汽器水侧污垢的特性,然后给出了考虑污垢因素的凝汽器传热系数的计算方法。     

Development of heat transfer coefficient correlation for concentric helical coil heat exchanger

The present study deals with developing a Correlation for heat transfer coefficient for flow between concentric helical coils. Existing Correlation is found to result in large discrepancies with the increase in gap between the concentric coils when compared with the experimental results. In the present study experimental data and CFD simulations using Fluent 6.3.26 are used to develop improved heat transfer coefficient correlation for the flue gas side of heat exchanger. Mathematical model is developed to analyze the data obtained from CFD and experimental results to account for the effects of different functional dependent variables such as gap between the concentric coil, tube diameter and coil diameter which affects the heat transfer. Optimization is done using Numerical Technique and it is found that the new correlation for heat transfer coefficient developed in this investigation provides an accurate fit to the experimental results within an error band of 3–4%.     

An experimental study in determining the local heat transfer coefficients for the plate finned-tube heat exchangers

A three-dimensional inverse problem in determining the local heat transfer coefficients for the plate finned-tube heat exchangers utilizing the steepest descent method (SDM) and a general purpose commercial code CFX4.4 is applied successfully in the present study based on the measured temperature distributions on fin surface by infrared thermography.Two different tube arrangements (i.e. in-line and staggered) with different fin pitch and air velocity are considered and the corresponding local heat transfer coefficients are to be determined. Results show that some interesting phenomena of the local heat transfer coefficients for the finned surface are found in the work and the averaged heat transfer coefficient of the staggered configuration is about 8–13% higher than that of the in-line configuration.     

Numerical study of local heat transfer coefficient and fin efficiency of wavy fin-and-tube heat exchangers

Three-dimensional numerical simulations were performed for laminar flow of wavy fin-and-tube heat exchangers by using body-fitted coordinates (BFC) method with fin efficiency effect accounted. The prediction results of average Nusselt number, friction factor and fin efficiency were compared with the related experimental correlations [R.C. Xin, H.Z. Li, H.J. Kang, W. Li, W.Q. Tao, An experimental investigation on heat transfer and pressure drop characteristics of triangular wavy fin-and-tube heat exchanger surfaces, J. Xi'an Jiaotong Univ. 28 (2) (1994) 77–83] and Schmidt approximation [T.E. Schmidt, Heat transfer calculations for extended surfaces, Refrigerating Engineering (April 1949) 351–357]. For Reynolds numbers based on the tube outside diameter ranging from 500 to 4000, the mean deviation is 3.3% for Nusselt number, 1.9% for friction factor and 3.6% for fin efficiency. The distributions of local Nusselt number and fin efficiency on fin surface were studied at wavy angle equal to 0° (plain plate fin), 10° and 20° respectively. The local Nusselt number decreases along the air flow direction, but fin efficiency increases in general. The wavy angle can greatly affect the distributions of local Nusselt number and fin efficiency, and make the distributions present fluctuation along the flow direction. The result also shows that the fin efficiency at the inlet region of wavy fin is larger than that of plain plate fin at the same region. With the increase of Reynolds number, the effects of wavy angle on the distributions of local Nusselt number and fin efficiency are more and more significant.