Experimental Work

Abstract

The test rig used consists of air duct and control and display units. An assembly of vertical finned-tube were fixed into the air duct by a shaft and aligned vertically in the direction of gravity. The air duct has a transparent part (12cm×12cm) which makes the finned-tube visible.In order to calculate the heat transfer, thin thermocouples wires of 1.5mm outer diameter including fiber glass insulation of type K with welded tips were embedded into the vertical tube wall and top surface of the circular fins and adjusted to keep its tip at the surface of the tube and fin.The heater inserted in the vertical circular tube to act as heat source. The thermocouples were distributed on the fins and tube as can be seen in figure (1). Figure (1) Thermocouples locations The air flows from the inlet and drugged up by a fan to the outlet in case of forced convection and flows by the difference of air density in free convection. Heater wires were connected with the (Variac)volt meterto control the input voltage to the heater. Thermocouples were connected to digital temperature recorder to read the temperature of each thermocouple. The experiment was performed at different temperatures of heater (40, 50, and 60) °C. The voltage across the heater was measured by voltmeter and the resistance of the heater was calculated.The input power was estimated from the relation: Q total =) The steady state was established after about half hour, then the readings of thermocouples and input power were recorded to study the steady state free and forced convection heat transfer subjected to constant heat flux boundary conditions. Natural convection phenomenon is characterized by dimensionless combinations named Rayleigh number which was studied with variation in Nusselt number to characterize natural convection. Reynolds number is a dimensionless quantitywhich is used to study forced convection heat transfer. 2.1 Heat transfer analysis: The power input to the heater is equal to the total heat transfer from vertical finned-tube to the surrounding at steady state condition. The net heat is transferred from a heated surface to its surrounding by both convection and radiation, Conduction and Radiation heat transfer were neglect here for their small quantities. 2.2 Convection heat transfer: Free convection heat transfer can be calculated from the equationbelow: