Chapter 5, Problem 5.23P

The next 8 questions are all related to the following problem statement and figure. Air at 27°C with a free stream velocity of u» = 10 m/s is used to cool electronic devices mounted on a printed circuit board (PCB). Each device dissipates 40 mW (rate of conversion of electrical energy into thermal energy), is thin, and can be modeled as isothermal with a square top that is 5 mm by 5 mm. Due to the low thermal conductivity of the PCB, all of the dissipated energy can be assumed to be transferred out of the devices from their tops. A turbulence promoter (turbulator) is located at the leading edge of the board, causing the boundary layer to be turbulent over the entire length of the PCB. Estimate the surface temperature of the fourth device located 15 mm from the leading edge of the board. Use properties of air at T= 52°C and atmospheric pressure. Air Turbulator Fourth device X = 15 mm Printed circuit board Itt

ricant An oil is acting as a lubricant for a pair of cylindrical surfaces such as those shown in 1. The angular velocity of the outer cylinder is 7908 rpmt. The outer cylinder has a radius of 5.06 cm, and the clearance between the cylinders is 0.027 cm. What is the maxi- mum temperature in the oil if both wall temperatures are known to be 158° F? The physical properties of the oil are viscosity density thermal conductivity 92.3 cp 1.22 g cm-3 0.0055 cal sec-1 cm-1 ° C-1

An object of irregular shape has a characteristic length of L = 1 m and is maintained at a uniform surface temperature of Ts = 375 K. When placed in atmospheric air at a temperature of T = 300 K and moving with a velocity of V = 100 m/s, the average heat flux from the surface to the air is 20,000 W/m². If a second object of the same shape, but with a characteristic length of L = 5 m, is maintained at a surface temperature of Ts 375 K and is placed in atmospheric air at T = 300 K, what will the value of the average convection coefficient be if the air velocity is V = 20 m/s? = h₂ = = || i W/m².K