Practice Pakistan's most useful MCQs for competitive exams

A metal ball of radius 0.1 m at a uniform temperature of 90°C is left in air at 30°C. The density and the specific heat of the metal are 3000 kg/m3 and 0.4 kJ/kg.K respectively. The heat transfer co-efficient is 50 W/m2.K Neglecting the temperature gradients inside the ball, the time taken (in hours) for the ball to cool to 60°C is_________________?

• (A)  555
• (B)  55.5
• (C)  0.55
• (D)  0.15

Answer

Submitted By :Ali Uppal

The ratio of momentum diffusivity to thermal diffusivity is the ______________ number?

• (A)  Prandtl
• (B)  Nusselt
• (C)  Stanton
• (D)  Grashoff

Answer

Submitted By :Ali Uppal

Heat produced when a steady state current, I passes through an electrical conductor having resistance, ‘R’ is___________________?

• (A)  IR
• (B)  I2R
• (C)  IR2
• (D)  I2R2

Answer

Submitted By :Ali Uppal

The heat transfer by radiation from a mild steel surface is to be reduced by reducing the emissivity of the surface. This can be best achieved by__________________?

• (A)  Painting the surface black
• (B)  Painting the surface white (with aluminium paint)
• (C)  Giving the surface a mirror finish
• (D)  Roughening the surface

Answer

Submitted By :Ali Uppal

Fourier’s law applies to the heat transfer by__________________?

• (A)  Convection
• (B)  Radiation
• (C)  Conduction
• (D)  All

Answer

Submitted By :Ali Uppal

When vaporisation takes place directly at the heating surface, it is called________________?

• (A)  Film boiling
• (B)  Nucleate boiling
• (C)  Vapour binding
• (D)  None of these

Answer

Submitted By :Ali Uppal

Extended heat transfer surface like fins are used to increase the heat transfer rate. Fin efficiency is defined as the ratio of heat transferred across the fin surface to the theoretical heat transfer across an equal area held at the________________?

• (A)  Surrounding temperature
• (B)  Average temperature of the fin
• (C)  Temperature of the fin end
• (D)  Constant temperature equal to that of the base

Answer

Submitted By :Ali Uppal

The actual temperature drop across the heating surface in an evaporator depends on the_________________?

• (A)  Feed
• (B)  Depth of liquid over heating surface
• (C)  Pressure difference between steam chest and vapour space
• (D)  All

Answer

Submitted By :Ali Uppal

Heat flux is the time rate of heat transfer per unit_______________?

• (A)  Length
• (B)  Area
• (C)  Volume
• (D)  None of these

Answer

Submitted By :Ali Uppal

The steam ejector is used to__________________?

• (A)  Remove condensate from the steam pipelines
• (B)  Create vacuum
• (C)  Superheat the steam
• (D)  None of these

Answer

Submitted By :Ali Uppal