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Air at p 1 atm enters a thin-walled (D 5-mm diameter) long
Chapter , Problem 8.3(choose chapter or problem)
Air at p 1 atm enters a thin-walled (D 5-mm diameter) long tube (L 2 m) at an inlet temperature of Tm,i 100 C. A constant heat flux is applied to the air from the tube surface. The air mass flow rate is m . 135 106 kg/s. (a) If the tube surface temperature at the exit is Ts,o 160 C, determine the heat rate entering the tube. Evaluate properties at T 400 K. (b) If the tube length of part (a) were reduced to L 0.2 m, how would flow conditions at the tube exit be affected? Would the value of the heat transfer coefficient at the tube exit be greater than, equal to, or smaller than the heat transfer coefficient for part (a)? (c) If the flow rate of part (a) were increased by a factor of 10, would there be a difference in flow conditions at the tube exit? Would the value of the heat transfer coefficient at the tube exit be greater than, equal to, or smaller than the heat transfer coefficient for part (a)? 8.31 T
Questions & Answers
QUESTION:
Air at p 1 atm enters a thin-walled (D 5-mm diameter) long tube (L 2 m) at an inlet temperature of Tm,i 100 C. A constant heat flux is applied to the air from the tube surface. The air mass flow rate is m . 135 106 kg/s. (a) If the tube surface temperature at the exit is Ts,o 160 C, determine the heat rate entering the tube. Evaluate properties at T 400 K. (b) If the tube length of part (a) were reduced to L 0.2 m, how would flow conditions at the tube exit be affected? Would the value of the heat transfer coefficient at the tube exit be greater than, equal to, or smaller than the heat transfer coefficient for part (a)? (c) If the flow rate of part (a) were increased by a factor of 10, would there be a difference in flow conditions at the tube exit? Would the value of the heat transfer coefficient at the tube exit be greater than, equal to, or smaller than the heat transfer coefficient for part (a)? 8.31 T
ANSWER:(a) The heat rate entering the tube is given by
q = m * Cp (Tm,i - Ts,o)
where Cp is the specific heat capacity of the air.
Using the given conditions, we can calculate q as follows:
q = 135 x 10-6 x 1000 x (100 - 160) = -1.08 x 105 W
(b) If the length of the tube is reduced to 0.2 m, then the flow co