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H8 - Sample Calculation(Based on Sample1*q=V.I => 34.48...

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Sample Calculation (Based on Sample1): * q = V . I => ( 34.48 ) . ( 0.448 ) = 15.45 watts. * φ = q / A => ( 15.45 ) / ( 2.482x10 -3 ) = 6223.63 watts/m 2 . * T s - T a => ( 69.98 ) – ( 24.57 ) = 45.41 K. * h = φ / (T s - T a ) => ( 6223.63 ) / ( 45.41 ) = 137.05 W/m 2 .K. * U = 74.294 a d a P P T => 74.294 3 10 100 ) 92 . 6 )( 273 57 . 24 ( x + = 10.66 m/s. * Re = U.D/ ν => ( 10.66 ).( 1.58x10 -2 ) / ( 1.7 x 10 -5 ) = 9910.87 * Nu = h.D/ k => ( 35.51 ).( 1.58x10 -2 ) / ( 0.26 ) = 8.30
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Abstract: The objective of this experiment is to study the effect of Reynolds number on surface heat transfer coefficient in turbulent cross flow of air stream around a single copper tube. This experiment was performed with the aid of a computer linked to the equipment. Both the surface temperature T s and pressure P a were kept constant during the experiment at 70 o C and 752.5 mmHg. There were five runs in which iris damper position was changed from 8,7,6,5,4 in each run in order to get different air velocities (as position decreases, velocity increases). In order to keep T s constant at 70 o C, heater control was adjusted in each run (increased). Results show an increase in both heat flux ( φ ) and heat transfer coefficient (h) as the velocity (U) increases: e.g: U 1 = 10.66 m/s => φ 1 = 6223.63 W/m 2 , h 1 = 137.05 W/m 2 .K U 5 = 25.30 m/s => φ 5 = 8554.59 W/m 2 , h 5 = 189.89 W/m 2 .K Two plots were made: Figure1 : ln(Re) Vs ln(Nu). Figure2 : h Vs Re. Introduction: In order to transfer heat between two fluids, many forms of heat exchangers have been devised. In one of the most common arrangements, heat is transferred between a fluid flowing transversely over the out side of the tubes. Different tube layouts have been tried in order to improve the efficiency of heat exchange between the fluids thereby reducing the required physical size of the unit for a given heat transfer rate. The basic objective in all layouts is to increase turbulence in the fluid flowing across the tube bundle. The overall heat transfer coefficient in across flow heat exchanger is made up of three components: i) Tube side heat transfer coefficient. ii) Thermal conductivity and thickness of the tube material. iii) Shell side heat transfer coefficient. The first two components may be improved by increasing flow velocity in tubes and reducing the tube wall thickness or using a material of higher thermal conductivity.
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