IAT2_hmt_2 MARKS

The ratio of energy transferred by convection to that by conduction is called

• Stanton number
• Nusselt number
• Biot number
• Preclet number

The average Nusselt number in laminar natural convection from a vertical wall at 180°C with still air at 20°C is found to be 48. If the wall temperature becomes 30°C, all other parameters remaining same, the average Nusselt number will be:

• 8
• 16
• 24
• 32

A 320 cm high vertical pipe at 150°C wall temperature is in a room with still air at 10°C. This pipe supplies heat at the rate of 8 kW into the room air by natural convection. Assuming laminar flow, the height of the pipe needed to supply 1 kW only is:

• 10 cm
• 20 cm
• 40 cm
• 80 cm

Drop wise condensation usually occurs on

• Glazed surface
• Smooth surface
• Oily surface
• Coated surface

In spite of large heat transfer coefficients in boiling liquids, fins are used advantageously when the entire surface is exposed to:

• Nucleate boiling
• Film boiling
• Transition boiling
• All modes of boiling

When a liquid flows through a tube with sub-cooled or saturated boiling, what is the process known?

• Pool boiling
• Bulk boiling
• Convection boiling
• Forced convection boiling

In a counter flow heat exchanger, for the hot fluid the heat capacity = 2 kJ/kg K, mass flow rate = 5 kg/s, inlet temperature = 150°C, outlet temperature = 100°C. For the cold fluid, heat capacity = 4 kJ/kg K, mass flow rate = 10 kg/s, inlet temperature = 20°C. Neglecting heat transfer to the surroundings, the outlet temperature of the cold fluid in °C is:

• 7.5
• 32.5
• 45.5
• 70

In a condenser, water enters at 30°C and flows at the rate 1500 kg/hr. The condensing steam is at a temperature of 120°C and cooling water leaves the condenser at 80°C. Specific heat of water is 4.187 kJ/kg K. If the overall heat transfer coefficient is 2000 W/m2K, then heat transfer area is:

• 0.707 m2
• 7.07 m2
• 70.7 m2
• 141.4 m2

Hot oil is cooled from 80 to 50°C in an oil cooler which uses air as the coolant. The air temperature rises from 30 to 40°C. The designer uses a LMTD value of 26°C. The type of heat exchanger is:

• Parallel flow
• Double pipe
• Counter flow
• Cross flow

Air enters a counter flow heat exchanger at 70°C and leaves at 40°C. Water enters at 30°C and leaves at 50°C. The LMTD in degree C is:

• 5.65
• 4.43
• 19.52
• 20.17