Module 3. Dimensional analysis and heat transfer
Lesson 20
ASSIGNMENTS
Q. 1. Water flows with a velocity w = 0.2 m/s in a tube of diameter d = 4 mm and length l = 200 mm. The temperature of the tube wall tw=70°C. What will the temperature at the tube outlet be, if water inlet temperature tf1=10°C?
Answer
tf2= 27°C.
Q. 2. Oil, of the MK grade, flows in a tube with a diameter d = 10 mm. Oil temperature at the tube inlet tf1 = 80°C; the rate of oil flow = 120 kg/h. What should the length of the tube be, if it is desired that at a tube wall temperature tw = 30°C the temperature of oil at the tube outlet tf2 be equal to 76°C?
Answer
l = 1.66 m.
Q. 3. Determine the pressure drop across a tube in which oil flows under condition assumed for problem 2. Compare the calculated pressure drop with that, taking place when oil is in isothermal flow at the same tube inlet temperature.
Answer
The pressure drop across the tube ∆p = 2.55 × 104. In the event of isothermal flow ∆pi = 1.05 × 104 Pa which is about 2.5 times smaller.
Q. 4. Determine the hourly loss of heat from a bare vertical steam pipe whose diameter d = 60 mm and the length h = 6 m, if the wall temperature = 170°C and temperature of the surroundings = 35°C
Q. 5. Air flows inside a tube 50 mm in diameter and 2.4 m long at a velocity = 5m/sec. Find the heat-transfer co-efficient, if the mean air temperature = 110°C.
Q.6. Water flows inside a tube 40mm in diameter and 5 m long at a velocity = 0.8 m/sec. Determine the heat-transfer coefficient if the mean water temperature = 50°C and wall temperature = 70°C
Q.7. Determine the mean heat-transfer co-efficient and the quality of heat transferred in water flowing in a horizontal tube 3.2 mm in diameter and 2.5 m long. If w = 0.36 m/sec, tf = 60°C and tw =20°C