Direction: Water (specifie heat, c = 4.18 kJ/kgK) enters a pipe at a rate 0.01 kg/s and a temperature of 20°C. The pipe, of diameter 50 mm and length 3 m, is subjected to a wall heat flux q"w in W/m2.
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If q"w = 2500 x, where x is in m and in the direction of flow (x = 0 at the inlet), the bulk means temperature of the water leaving the pipe in °C is
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- 42
- 62
- 74
- 104
Correct Option: B
At the inlet of pipe
x = O
q "wi = 2500 × 0 = 0
At the exit of pipe,
x = l
| q "wi = 2500 × 1 × 3 = 7500 | m2 |
Average heat transfer,
| q "avg | = | = 3750 | ||||
| 2 | 2 | m2 |
Net heat transfer
Q = q "avg × surface area = 3750 × πdl
= 3750 × 3.14 × 0.05 × 3 = 1766.25 W
Also Q = mCp (T0 – Ti)
1766.25 = 0.01× 4180 (T0 –20)
By solving above equation, we get
T0 = 62° C
