Single-blow Method Research Articles

Experimental Study of the Convective Heat Transfer and Local Thermal Equilibrium in Ceramic Foam

Foam materials have been widely used in various industrial applications, where higher and higher heat and mass transfer performances are pursued. However, the mechanism of many factors on the heat transfer performances is still unclear. The main purpose of this article is to investigate how the porous properties, porosity, cell size and the sample thickness affect the volumetric convective heat transfer. In this study, the single-blow method is used to determine the volumetric heat transfer coefficient of ceramics foam in the temperature range from 283 K to 323 K. In particular, sensitivity analysis of the foam porosity, cell size, velocity and the sample thickness on the volumetric heat transfer coefficient within the ceramics foam were all conducted. The results indicate that the sample thickness has a significant effect on the volumetric heat transfer coefficient which decreases with the sample thickness. In addition, the local thermal equilibrium phenomenon is verified and its influence on the volumetric heat transfer coefficient discussed. Based on the experimental data, a new correlation is proposed that includes sample thickness, porosity, superficial velocity and fluid properties. This study is crucial to the theory of the convective heat transfer inside the porous media, and can be used to guide the design and optimization of volumetric solar air receivers, compact heat exchangers, heat sinks, heat regenerators, packed bed reactors and so on.

Transient investigation of mini-channel regenerative heat exchangers: Combined experimental and CFD approach

This paper presents a combined approach based on experiment and CFD as an alternative to single-blow method to investigate heat transfer and flow friction of three mini-channel regenerative heat exchangers (MCRHX), having channel hydraulic diameters of 1.5, 1 and 0.5mm. Experiments were conducted to measure pressure drop and the transient thermal responses at the inlet and outlet of each MCRHX. The thermal response predicted from CFD, based on transient conjugated heat transfer simulations, was iteratively solved until it matched the experimental data within an acceptable deviation. The results showed that 0.5mm MCRHX had the highest interstitial heat transfer coefficient due to the increased specific surface area. However, the penalty is the increased pressure drop. Moreover, Nusselt number and friction factor correlations were obtained for each configuration to correlate mini-channel heat transfer characteristics.

多孔質体内熱流動のモデリングの進展

Recent advances in mathematical modeling of heat and fluid flow in porous media were discussed starting from an introduction of volume averaging theory and leading to various engineering applications. Mathematical interpretations of tortuosity, thermal dispersion and interstitial heat transfer are provided with their models based on solvable volume averaged quantities. A useful concept “effective porosity” was introduced to account for the tortuosity. Moreover, an inverse relationship between the interstitial heat transfer coefficient and thermal dispersion coefficient was explained, which enables us to estimate the thermal dispersion simply by measuring the interstitial heat transfer using a single blow method. Engineering applications based on the local space sharing concept will be discussed ranging from bioheat transfer to ion transport through a membrane.

An Accurate Experimental Determination of Interstitial Heat Transfer Coefficients of Ceramic Foams Using the Single Blow Method

An exhaustive experimental investigation has been carried out for accurate determination of interstitial heat transfer coefficients of ceramic foams in forced convective flows. The single-blow method was used, in which, the fluid temperature varies with time and convective heat transfer becomes time-dependent. The method results in a transient and conjugate heat transfer phenomena between the foam and fluid. These transient temperature data are then compared with the theoretical results to obtain the corresponding interstitial heat transfer coefficients between the fluid and foam solid surface. The interstitial coefficients thus determined are compared against available sets of experimental data, so as to ex- amine the consistency among the reported experimental data.

Conjugate Heat Transfer Computation for Evaluation of Single-Blow Method for Compact Fin-Tube Heat Exchangers

Computations have been carried out to evaluate heat transfer coefficients given by the single-blow method that is characterized by a transient and conjugate heat transfer problem between the fluid and the solid. Both heat conduction and convection equations are solved numerically to obtain the transient fluid and fin temperature distributions. Finite volume solutions indicate that the fin temperature distribution of the single-blow method varies with time and position and that the fluid temperature distribution of the single-blow method is close to that observed in the steady-state computation specified with the constant wall temperature condition. It is found that the local heat transfer coefficient resulting from the single-blow method is almost identical to that from the steady-state constant wall temperature computation and is nearly time-independent.

Performance of Wire Springs as Extended Heat Transfer Surface for Compact Heat Exchangers

Use of thin metal wire structures as a new type of extended heat transfer surface is proposed. As one of the most basic shapes of such wire structures, heat transfer performance of spring shaped fins is experimentally investigated under relatively low Reynolds number conditions. The averaged heat transfer coefficient is evaluated by a single-blow method while the pressure drop is measured at a steady state flow condition. The effects of the geometric parameters such as the wire diameter, the spring pitch and the pitch ratio were systematically examined and the obtained data were compared with that of a conventional offset fin, which is commercially available. It was found that the geometric parameters of the spring fins and the arrangement of spring fins in the test section affect their heat transfer performance. Some types of spring fins showed better heat transfer performance than a conventional offset fin, when they are evaluated in terms of the total heat transfer at a constant pumping power.

A Novel Semi-empirical Model for Evaluating Thermal Performance of Porous Metallic Foam Heat Sinks

A novel semi-empirical model with an improved single blow method for exploring the heat transfer performance of porous aluminum-foam heat sinks in a channel has been successfully developed. The influencing parameters such as the steady-state air preheating temperature ratio, Reynolds number and medium porosity on local and average heat transfer behavior of porous aluminum-foam heat sinks in a channel are explored. The heat transfer enhancement of using a porous heat sink in a channel to a hollow channel is, (Nu¯b)ss∕(Nu¯b)ε=1, much greater than unity and generally decrease with increasing Re. Furthermore, two new correlations of (Nu¯b)ss and (Nu¯i)ss in terms of ϴ,Re,Da,γ and ε are proposed. As compared with the results evaluated by the transient liquid crystal method, the channel wall temperatures predicted by the present semi-empirical model have a more satisfactory agreement with the experimental data, especially for the cases with smaller porosities. The limitations with relevant error maps of using the transient liquid crystal method in porous aluminum foam channels are finally postulated.

Heat transfer and flow characteristics of fin-tube bundles with and without winglet-type vortex generators

The objective of this research is to investigate the effect of longitudinal vortices that can be applied to the heat transfer enhancement for fin-tube heat exchangers such as air-cooled condensers. A multichannel test core was designed and fabricated for the determination of overall heat transfer and pressure loss with circular tubes and winglet vortex generators. Heat transfer results were obtained using a transient method referred to as the modified single-blow method. For a three-row tube bundle in an in-line arrangement without winglets, the heat transfer and the pressure loss were 72% and 210% higher, respectively, than for a multichannel test core without any built-in tube or winglet. These increases were caused by vortices around the tube banks. The corresponding increases for a staggered tube bundle are 95% and 310%, respectively. The triangular winglets recommended by the previous studies in a fin-tube bundle in an in-line arrangement increase the overall heat transfer 10–25% and the pressure loss 20–35% for the Reynolds numbers ranging from 300 to 2700.

Evaluation of Heat Transfer and Pressure Loss Performances of Louvered Fins by Modified Single-blow Method

Evaluation of Heat Transfer and Pressure Loss Performances of Louvered Fins by Modified Single-blow Method

Flow channeling effect on a regenerator's thermal performance

The aim of this study is to obtain a general formula from the measured heat transfer results in three regenerator matrices for evaluating the thermal performance of a regenerator matrix with various degrees of flow channeling. In this formula, a velocity ratio, φ and an area ratio, ψ are determined from a nonuniform velocity profile at the regenerator exit. The nonuniform velocity distribution results from the flow channeling occurring at the contact surface between the regenerator matrix and the holding tube. A transient single-blow method was employed to measure the heat transfer rate in three inhomogeneous stacked-screen regenerator matrices. For a regenerator matrix with flow channeling, the thermal performance can be evaluated from the general formula and the measured radial velocity profile at the regenerator exit. The results show that the flow channeling effect may reduce the thermal performance of a regenerator matrix.

Measurements of Thermal Performance of the Regenerator of a Cryocooler with a high Number of Transfer Units Value

Hundreds of stacked wire screens are used in the regenerator matrix of a common cryocooler. The number of transfer units of such a matrix (denoted as NTUm) may well exceed 60. However, most of the earlier studies reported are limited to studies of regenerators with NTUm values less than 60, as the single-blow method was employed to measure the NTUm value of the regenerator matrix. Furthermore, in these earlier studies, the effect of heat transfer from the working fluid to the external tube and the Joule-Thomson effect were neglected. In the present study, three regenerators having high NTUm values have been constructed and a transient single-blow technique has been employed to measure the friction factor and the heat transfer performance of these regenerators. In addition, an improved model has been adopted to correct the shortcomings of the earlier studies. Empirical correlations have been provided for the relation between the friction factor and Reynolds number and between the Nusselt number and Reynolds number. The correlation with smaller NTUm values agreed well with those reported in earlier studies.

Experimental prediction of heat transfer coefficients by use of a double-blow method

A new method is proposed for evaluating heat transfer coefficients in a heat exchanger matrix. In comparison with the well known single-blow method, a new double-blow method offers prediction of heat transfer coefficients on both sides of the heat exchanger wall by using one run only, because temperatures of both fluids flowing through the heat exchanger matrix are changed and measured.

Development of a modified single-blow method. An Application to parallel plate heat transfer surfaces.

An automatic heat transfer measuring system is designed and fabricated to demonstrate the usefulness of the transient heat transfer testing method. New techniques are utilized in both air-heating and air-temperature measuring systems. A computer system is incorporated to calculate the temporally and spatially averaged heat transfer coefficient using the measured disturbance and response of the fluid temperature. The present system was applied to evaluating the heat transfer performance of parallel plate stacks. The test results are compared with the existing theoretical and experimental data. It was confirmed that the present system not only has high accuracy (within 5% ) but also advantages of time and labor saving over conventional steady-state methods.

Thermal Performance of Cross-Inclined Tube Bundles Measured by a Transient Method

By considering both theoretical and experimental aspects of an exponential inlet temperature response, significant improvement in the transient single blow method for measurement of heat transfer coefficients in matrices becomes possible. Specific requirements for success with the method were met in a test programme to evaluate the shell side thermal performance of a variety of tube bundle configurations. Results for conventional geometries of known worth confirm the precision of the technique, and new data on cross-inclined tube bundles are presented.