An array of power transistors, dissipating 6W of power each, are to be cooled by mounting them on a 25cm x 25xm square aluminum plate and blowing air at 35o over the plate with a fan at a velocity of 4m/s. The average temperature of the plate is not to exceed 65oC. Assuming the heat transfer from the back side of the plate to be negligible and disregarding radiation, determine the number of transistors that can be placed on the plate. Hint 1: This is a flat plate question. DO NOT treat the transistors as a tube bank. How much heat is dissipated across the flat side of the plate (opposite the side the transistors are mounted on). Hint 2: Use the average Reynold’s and Nusselt numbers

An array of power transistors, dissipating 6W of power each, are to be cooled by mounting them on a 25cm x 25xm square aluminum plate and blowing air at 35o over the plate with a fan at a velocity of 4m/s. The average temperature of the plate is not to exceed 65oC. Assuming the heat transfer from the back side of the plate to be negligible and disregarding radiation, determine the number of transistors that can be placed on the plate. Hint 1: This is a flat plate question. DO NOT treat the transistors as a tube bank. How much heat is dissipated across the flat side of the plate (opposite the side the transistors are mounted on). Hint 2: Use the average Reynold’s and Nusselt numbers

Principles of Heat Transfer (Activate Learning with these NEW titles from Engineering!)

8th Edition

ISBN:9781305387102

Author:Kreith, Frank; Manglik, Raj M.

Publisher:Kreith, Frank; Manglik, Raj M.

Chapter5: Analysis Of Convection Heat Transfer

Section: Chapter Questions

Problem 5.65P

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An array of power transistors, dissipating 6 W ofpower each, are to be cooled by mounting them on a 25-cm *25-cm-square aluminum plate and blowing air at 35°C overthe plate with a fan at a velocity of 4 m/s. The average temperatureof the plate is not to exceed 65°C. Assuming theheat transfer from the back side of the plate to be negligibleand disregarding radiation, determine the number of transistorsthat can be placed on this plate.
A 4-m x 4-m flat plate maintained at a constant temperature of 80°C is subjected to parallel flow of air at 1 atm, 20°C, and 10 m/s. The total drag force acting on the upper surface of the plate is measured to be 2.4 N. Using momentum heat transfer analogy, determine the average convection heat transfer coefficient, and the rate of heat transfer between the upper surface of the plate and the air
In the effort to increase the removal of heat from a hot surface at 120°C, a cylindrical pin fin (kf = 237 W/m.K) with diameter of 5 mm is attached to the hot surface. Air at 20°C (1 atm) is flowing across the pin fin with a velocity of 10 m/s. Determine the maximum possible rate of heat transfer from the pin fin. Evaluate the air properties at 70°C.
Hot carbon dioxide exhaust gas at 1 atm is being cooled by flat plates. The gas at 220oC flows in parallel over the upper and lower surfaces of a 1.5-m-long flat plate at a velocity of 3m/s. if the flat plate surface temperature is maintained at 80oC, determine (a) the local convection heat transfer coefficient at 1 m from the leading edge, (b) the average convection heat transfer coefficient over the entire plate, and (c) the total heat flux transfer to the plate.
In a geothermal power plant, the used geothermal water at 80°C enters a 15-cm-diameter and 400-m-long uninsulated pipe at a rate of 8.5 kg/s and leaves at 70°C before being reinjected back to the ground. Windy air at 15°C flows normal to the pipe. Disregarding radiation, determine the average wind velocity in km/h.
Hot carbon dioxide exhaust gas at 1 atm is being cooled by flat plates. The gas at 220°C flows in parallel over the upper and lower surfaces of a 1.5-m-long flat plate at a velocity of 3 m/s. If the flat plate surface temperature is maintained at 80°C, determine (a) the local convection heat transfer coefficient at 1 m from the leading edge, (b) the average convection heat transfer coefficient over the entire plate, and (c) the total heat flux transfer to the plate.
Metal plates (k = 150 W/m·K, ρ= 2800 kg/m3, and cp = 900 J/kg·K) with a thickness of 2 cm exiting an oven are conveyed through a 10-m long cooling chamber at a speed of 4 cm/s, as shown in the figure. The plates enter the cooling chamber at an initial temperature of 500°C. The air temperature in the cooling chamber is 15°C, and the plates are cooled with blowing air and the convection heat transfer coefficient is given as a function of the air velocity h(V) = 33V0.8, where h is in W/m2·K and V is in m/s. To prevent any hazard to workers handling the plates, it is necessary to design the cooling process such that the plates exit the cooling chamber at a relatively safe temperature of 50°C or less. Determine the air velocity V and the heat transfer coefficient h such that the temperature of the plates exiting the cooling chamber is at 50C.
Hot air leaving the stove in the kitchen of a house at 70 0C, with a square section of 22 cm x 22 cm. sheet metal enters the 12 m long section of the channel with a speed of 6 m/s. Emissivity 0.3 The outer surface of the duct, whose thermal resistance is neglected, is located in the basement of the house at 14 0C and 12 W/m2 . It is in contact with cold air with 0C convection heat transfer coefficient. Basement Calculate the exit temperature of the hot air from the basement by taking the wall temperatures of the floor to 14 0C .
Air enters a 7-m-long section of a rectangular duct of cross section 15 cm X 20 cm at 50°C at an average velocity of 7 m/s. If the walls of the duct are maintained at 10°C, determine: (a) the outlet temperature of the air, (b) the rate of heat transfer from the air, and (c) the fan power needed to overcome the pressure losses in this section of the duct.
Consider an air solar collector that is 1 m wide and 5 m longand has a constant spacing of 3 cm between the glass cover andthe collector plate. Air enters the collector at 30°C at a rate of0.15 m3/s through the 1-m-wide edge and flows along the 5-mlong passage way. If the average temperatures of the glasscover and the collector plate are 20°C and 60°C, respectively,determine (a) the net rate of heat transfer to the air in thecollector and (b) the temperature rise of air as it flows throughthe collector.
Four power transistors, each dissipating 12 W, are mounted on a thin vertical aluminum plate (k = 237 W/m?K) 22 cm * 22 cm in size. The heat generated by the transistors is to be dissipated by both surfaces of the plate to the surrounding air at 20°C, which is blown over the plate by a fan at a velocity of 250 m/min. The entire plate can be assumed to be nearly isothermal, and the exposed surface area of the transistor can be taken to be equal to its base area. Determine the temperature of the aluminum plate. Evaluate the air properties at a film temperature of 40°C and 1 atm.
An average person generates heat at a rate of 84 W while resting. Assuming one-quarter of this heat is lost from the head and disregarding radiation, determine the average surface temperature of the head when it is not covered and is subjected to winds at 10°C and 25 km/h. The head can be approximated as a 30-cm-diameter sphere. Assume a surface temperature of 15°C for evaluation of ms. Is this a good assumption?