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Issue 4, Volume 2, October 2007
Print ISSN: 1790-5044
E-ISSN: 2224-3461







Title of the Paper: Heat Transfer and Pressure Drop Performance Comparison of Finned-Tube Bundles in Forced Convection


Authors: Rene Hofmann , Friedrich Frasz, Karl Ponweiser

Abstract: In this paper, heat transfer and pressure drop at different transverse serrated and solid finned-tubes were investigated in cross-flow with aim of optimizing heat exchanger performance. Three different finned-tube shapes were investigated. The I-shaped and U-shaped fin geometries under consideration have varying geometrical constants, i.e. fin height, fin pitch, fin thickness, and fin width. The heat exchanger consists of eight consecutive finned-tube rows and eleven tubes on top of each other. The finned tubes are arranged in a staggered formation at equal transverse and longitudinal pitch. The experimental setup, measurement technique and measurement uncertainties are presented. The design of an optimum heat exchanger must take into account the advantages and disadvantages of geometrical factors which influence heat transfer and pressure drop. After measurement validation, the derived correlations for the Nusselt number and the pressure drop coefficient were compared with experimental results and equations from literature. The difference between solid and serrated finned tubes is shown with the help of equations for a special configuration from literature. Additionally, a performance evaluation criterion for single-phase flows, developed by Webb [14], was carried out for the three different serrated and solid fin geometries. To evaluate the uncertainty of pressure drop measurement, the analogy to the “generalized Lévêque equation” cited in Martin and Gnielinski [13] was used.

Keywords: Finned tube, Heat transfer, Pressure drop, Serrated fin, Solid fin, Performance evaluation criterion, Experimental setup, Lévêque Analogy, Turbulent flow, Helical finned tubes.

Title of the Paper: Numerical Analysis of Marangoni Convection with Free-slip Bottom under Magnetic Field


Authors: Norihan MD. Arifin and Haliza Rosali

Abstract: In this paper, we use a numerical technique to analyze the onset of Marangoni convection in a horizontal layer of electrically-conducting fluid heated from below and cooled from above in the presence of a uniform vertical magnetic field. The top surface of a fluid is deformable free and the bottom boundary is rigid and free-slip. The critical values of the Marangoni numbers for the onset of Marangoni convection are calculated and later it is found to be critically dependent on the Hartmann, Crispation and Bond numbers. We found that the presence of Magnetic field always has a stabilizing effect of increasing the critical Marangoni number when the free surface is non-deformable. If the free surface is deformable, then there is a range where the critical Marangoni number will have unstable modes no matter how large magnetic field becomes.

Keywords: Marangoni Convection, Magnetic Field, Free-slip.

Title of the Paper: Developments in the Data Evaluation of the EDPS Technique to Determine Thermal Properties of Solids


Authors: Bashir M. Suleiman, Svetozár Malinarič

Abstract: Developments to the data evaluation procedures of the Extended Dynamic Plane Source (EDPS) Technique have been introduced. This technique has been used for simultaneous measurements of the thermal conductivity, diffusivity and the specific heat. The theoretical principle and the experimental arrangement of the technique are highlighted. The technique has the potential to determine these three parameters from a single transient recording of the temperature increase. Within the total time of this transient recording, and by using the difference analysis model, it is possible to select a correct “optimal” time sub-interval for the evaluation procedures. The difference analysis model is based on a mathematical procedures that provides the selection of the optimal time interval within the total measuring time and thus to obtain more accurate and reliable results. The selected time interval is defined as ( tB , tB + tS ). The beginning and the size of the interval are represented by tB and tS , respectively. The procedures consider tB as the varying variable within the selected tS values. The results are plotted versus tB and the optimal time interval is the interval within which the fitting is not sensitive to the interval size that cause a plateau in the plot. Measurements on (Polymethlmethacrylate) PMMA has been performed and analysed based on the difference analysis model. The estimated uncertainties in measurement were 3.6% for thermal conductivity and 2.7% for thermal diffusivity. The results were compared with those obtained from the sensitivity coefficients (parameter estimation) model.

Keywords: Difference Analysis; Dynamic Plane Source Technique; Thermal Conductivity; Thermal Diffusivity; (Polymethlmethacrylate) PMMA.

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