Assignment -05

Problem 1

A 10-m- long section of a 6-cm-diameter horizontal hot-water pipe passes through a large room whose temperature is 220C. If the temperature and the emissivity of the outer surface of the pipe are 650C and 0.8, respectively, determine the rate of heat loss from the pipe by

(a) Natural convection and

(b) radiation.

Problem 2

Consider a 15-cm ´ 20-cm printed circuit board (PCB) that has electronic components on one side. The board is placed in a room at 200C. The heat loss from the back surface board is negligible. If the circuit board is dissipating 8 W of power in steady operation, determine the average temperature of the hot surface of the board, assuming the board is

(a) vertical

(b) horizontal with hot surface facing up and

(c) horizontal with hot surface facing down.

Take the emissivity of the surface of the board to be 0.8 and assume the surrounding surfaces to be at the same temperature as the air in the room.

Problem 3

Flat- plate solar collectors are often tilted up toward the sun in order to intercept a greater amount of direct solar radiation. The tilt angle from the horizontal also affects the rate of heat loss from the collector. Consider a 2-m-high and 3-m-wide solar collector that is tilted an angle q from the horizontal. The back side of the absorber is heavily insulated. The absorber plate and the glass cover, which are spaced 2.5 cm from each other, are maintained at temperatures of 800C and 400C, respectively. Determine the rate of heat loss from the absorber plate by natural convection for q = 00, 200, and 900.

Problem 4

Consider an array of vertical rectangular fins which is to be used to cool an electronic device mounted in quiescent, atmospheric air at T¥ = 27 0C. Each fin has L = 20 mm and H = 150 mm and operates at an approximately uniform temperature of Ts = 770C.

Viewing each fin surface as a vertical plate in an infinite, quiescent medium, estimate the rate of heat transfer from a fin by free convection. Comment on the effect of boundary layer formation on specifying the spacing between fins.