Cooling Electric Equipment

A novel trapezoid fin pattern applicable for air-cooled heat sink

Abstract: The present study proposed a novel step or trapezoid surface design applicable to air-cooled heat sink under cross flow condition. A total of five heat sinks were made and tested, and the corresponding fin patterns are (a) plate fin; (b) step fin (step 1/3, 3 steps); (c) 2-step fin (step 1/2, 2 steps); (d) trapezoid fin (trap 1/3, cutting 1/3 length from the rear end) and (e) trapezoid fin (trap 1/2, cutting 1/2 length from the rear end). The design is based on the heat transfer augmentation via (1) longer perimeter of entrance region and (2) larger effective temperature difference at the rear part of the heat sink. From the test results, it is found that either step or trapezoid design can provide a higher heat transfer conductance and a lower pressure drop at a specified frontal velocity. The effective conductance of trap 1/3 design exceeds that of plate surface by approximately 38 % at a frontal velocity of 5 m s−1 while retains a lower pressure drop of 20 % with its surface area being reduced by 20.6 %. For comparisons exploiting the overall thermal resistance versus pumping power, the resultant thermal resistance of the proposed trapezoid design 1/3, still reveals a 10 % lower thermal resistance than the plate fin surface at a specified pumping power.

Forced convection heat transfer correlations for flow in pipes, past flat plates, single cylinders, single spheres, and for flow in packed beds and tube bundles

Abstract: Previously obtained experimental heat transfer data have been collected and are illustrated along with minor variations of the standard correlations. Analysis of data for heat transfer in randomly packed beds and compact (void fraction less than 0.65) staggered tube bundles indicates that the Nusselt number for a wide range of packing materials and tube arrangements is given by provided NRe ≥ 50. The correlations presented in this paper are not necessarily the most accurate available; however, they have wide application, are easy to use, and are quite satisfactory for most design calculations.

Thermal contact conductance

Abstract: This paper considers the resistance to the flow of heat between two thick solid bodies in contact in a vacuum. Existing analyses of single idealized contacts are summarized and compared, and then applied, together with results of recent electrolytic analog tests, to predict the conductance of multiple contacts. “appropriately” or “inappropriately” distributed at the interface. Reconsideration of the theory of interaction between randomly rough surfaces shows how the parameters required to predict heat transfer can be determined in principle by simple manipulation of typical profiles of the mating surface, together with an approximation from deformation theory. It is also shown that this process depends more crucially than had been realized upon the distribution of the few high peaks of the surfaces, where the assumption of Gaussian distribution of heights is suspect. In place of that assumption, the use of describing functions is suggested. The few experimental data relevant to these theories are examined and compared with predictions of theory.

Wärmeabgabe von geheizten Drähten und Rohren im Luftstrom

Abstract: Es wird uber Versuche zur Bestimmung der Warmeabgabe senkrecht zum Luftstrom gestellter Drahte und Rohre berichtet. Die untersuchten Drahte und Rohre mit kreisrundem Querschnitt hatten 0,0198 bis 150 mm Dmr., die Geschwindigkeit des Luftstromes wurde von 2 bis 30 m/s geandert. Der dadurch erfaste Bereich liegt zwischen den Reynoldsschen Zahlen 2,1 und 231 000. Auserdem wurde die Warmeabgabe von Rohren mit quadratischem und sechseckigem Querschnitt im BereichRe = 5000 bis 100 000 bestimmt. Aus den Versuchen bei etwa 100°C Oberflachentemperatur last sich die Nuseltsche Kenngrose in Abhangigkeit von der Reynoldsschen darstellen durch die BeziehungNu =c·.(Re) m , wobei die Wertec undm in den Bereichen vonRe = 1 bis 4, 4 bis 40, 40 bis 4000, 4000 bis 40 000, 40 000 bis 250 000 jeweils konstant sind. Die an einem Draht bestimmten Warmeubergangszahlen sind bei 1000°C Oberflachentemperatur um etwa 6% hoher als bei 100°C. Bezeichnet man die Oberflachentemperatur mitT w ° abs. und die Lufttemperatur mitT 0, so lassen sich alle Versuchsergebnisse bei verschiedenen Geschwindigkeiten und verschiedenen Temperaturen vonRe = 1 bis 4000 erfassen durch die Beziehung: $$ Nu = c_1 \left( {Re \cdot \left( {\frac{{T_w }} {{T_0 }}} \right)^{{\raise0.5ex\hbox{$\scriptstyle 1$} \kern-0.1em/\kern-0.15em \lower0.25ex\hbox{$\scriptstyle 4$}}} } \right)^m $$ Die Warmeubergangszahlen von Profilrohren mit quadratischem und sechseckigem Querschnitt weichen in dem Versuchsbereich nur wenig von denen der kreisrunden Rohre von gleicher Oberflache ab. Auch fur diese Versuche wurden die Konstantenm undc der obigen Gleichung bestimmt.

Developing laminar free convection between vertical flat plates with asymmetric heating

Abstract: A numerical and experimental investigation of the developing laminar free convection heat transfer in vertical parallel plate channels with asymmetric heating is presented. Thermal boundary conditions of uniform wall heat tluxes (UHF) and uniform wall temperature (UWT) are considered. Solutions of the developing flow are obtained for air at different ratios of the wall heat fluxes and wall temperature differences (above the temperature of the fluid at the channel entrance). The numerical solutions are shown to approach asymptotically the closed form solution for fully developed flow. The present results indicate that for UHF. the difference between the maximum temperatures on the two walls diminishes as fully developed flow is achieved. For UWT. the Nusselt number characterizing the total heat transfer to the fluid is found to be related to the Rayleigh number very nearly by a universal curve for all ratios of wall temperature differences, providing the Nusselt and Rayleigh numbers are appropriately defined.