Radiative heat transfer is intended between the inner surfaces of two very large isothermal parallel metal plates. While the upper plate (designated as plate 1) is a black surface and is the warmer one being maintained at 727ºC, the lower plate (plate 2) is a diffuse and gray surface with an emissivity of 0.7 and is kept at 227ºC. Assume that the surfaces are sufficiently large to form a two-surface enclosure and steady state conditions to exist. Stefan Boltzmann constant is given as 5.67×10-8W/m2K4.

The irradiation (in kW/m2) for the upper plate (plate 1) is

1.   2.5 

2.   3.6 

3.   17.0 

4.   19.5

For the three dimensional object shown in the figure below, five faces are insulated. The sixth face (PQRS), which is not insulated, interacts thermally with the ambient, with a convective heat transfer coefficient of 10 W/m2.k The ambient temperature is 300C. Heat is uniformly generated inside the object at the rate of 100 W/m3. Assuming the face PQRS to be at uniform temperature, its steady state temperature is

1.   100C 

2.   200C 

3.   300C 

4.   400C

A hollow enclosure is formed between two infinitely long concentric cylinders of radii 1 m and 2 m, respectively. Radiative heat exchange takes place between the inner surface of the larger cylinder (surface–2) and the outer surface of the smaller cylinder (surface–1). The radiating surfaces are diffuse

and the medium in the enclosure is non–participating. The fraction of the thermal radiation leaving the larger surface and striking itself is 

1.   0.25      

2.   0.5 

3.   0.75 

4.   1

A small copper ball of 5 mm diameter at 500 K is dropped into an oil bath whose temperature is 300 K. The thermal conductivity of copper is 400 W/m.K, its density 9000 kg/m3 and its specific heat 385 J/kg.K. If the heat transfer coefficient is 250 W/m2.K and lumped analysis is assumed to be valid, the rate of fall of the temperature of the ball at the beginning of cooling will be

1.   8.7 K/s 

2.   13.9 K/s 

3.   17.3 K/s 

4.   27.7 K/s

An un-insulated air conditioning duct of rectangular cross section 1 m × 0.5 m, carrying air at 20°C with a velocity of 10 m/s, is exposed to an ambient of 30°C. Neglect the effect of duct construction material. For

air in the range of 20-30°C, data are as follows: thermal conductivity =0.025 W/m.K; viscosity = 18  Pa.s; Prandtl number =0.73; density = 1.2 kg/m3. The laminar flow Nusselt number is 3.4 for constant wall temperature conditions and, for turbulent flow, Nu=0.023 Re0.8 Pr0.33.

The Reynolds number for the flow is

1.   444 

2.   890 

3.   4.44 × 105 

4.   5.33 × 105

Consider a laminar boundary layer over a heated flat plate. The free stream velocity is  . At some

distance x from the leading edge, the velocity boundary layer thickness is   and the thermal boundary

layer thickness is  . If the Prandtl number is greater than 1, which of the following is true?

1.     >   

2.    >  

3.    ~ ( x)–1/2 

4.    ~ x –1/2

Consider steady-state heat conduction across the thickness in a plane composite wall (as shown in the figure) exposed to convection conditions on both sides.

Given: hi = 20W / m2 k; h0 = 55 W/ m2K;  = 20C; k1 = 20W / mk; k2 = 500 W/ mk; L1 = 0.30am and L2 = 0.15 m.

Assuming negligible contact resistance between the wall surfaces, the interface temperature, T (in °C), of the two walls will be

1.   - 0.50 

2.   2.75 

3.   3.75 

4.   4.50

Steady two–dimensional heat conduction takes place in the body shown in the figure below. The normal temperature gradients over surfaces P and Q can be considered to be uniform. The temperature

gradient   at surface Q is equal to 10 K/m. Surfaces P and Q are maintained at constant temperatures as shown in the figure, while the remaining part of the boundary is insulated. The body has constant

thermal conductivity of 0.1 W/m.K. The values of   at surface P are

1.     

2.     

3.     

4.   

With an increase in thickness of insulation around a circular pipe, heat loss to surroundings due to

1.   convection increases, while that due to conduction decreases 

2.   convection decreases, while that due to conduction increases 

3.   convection and conduction decreases 

4.   convection and conduction increases

A solid cylinder (surface 2) is located at the center of a hollow sphere (surface 1). The diameter of the sphere is 1m, while the cylinder has a diameter and length of 0.5m each. The radiation configuration factor F11 is

1.   0.375 

2.   0.625 

3.   0.75      

4.   1

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

1.   parallel flow 

2.   double pipe 

3.   counter flow 

4.   cross flow

Heat is being transferred by convection from water at 48°C to a glass plate whose surface that is exposed to the water is at 40°C. The thermal conductivity of water is 0.6 W/mK and the thermal conductivity of glass is 1.2 W/mK. The spatial gradient of temperature in the water at the water-glass interface is dT/dy = 1×104 K/m.

The value of the temperature gradient in the glass at the water-glass interface is

1.   -2 × 104 K/m 

2.   0.0 K/m 

3.   0.5 × 104 K/m 

4.   2 × 104 K/m