STUPIDSID
1 (a)
What is thermal diffusivity? Explain importance in heat conduction problem.
4 M
1 (b)
Describe different types of boundary conditions applied to heat Conduction problems.
4 M
1 (c)
Consider a one dimensional steady state heat conduction in a plate with constant thermal conductivity in a region 0≤ ×≤ L. A plate is exposed to uniform heat flux q W/m2 at x= 0 and dissipates heat by convection at x-L with heat transfer coefficient h in the surrounding air at T∞. Write the mathematical formulation of this problem for the determination of one dimensional, steady temperature distribution within the wall.
4 M
1 (d)
An industrial freezer is designed to operate with an internal air temperature of-20°C when the external air temperature is 25°C and the internal and external heat transfer coefficients are 12 W/m2°C and 8 W/m2°C, respectively. The wall of the freezer are composite construction, comprising of an inner layer of plastic 3 mm thick with thermal conductivity of 1 W/m°C. An outer layer of stainless steel of thickness 1 mm and thermal conductivity of 16W/m°C. Sandwiched between these layers is a layer of insulation material with thermal conductivity of 0.07 W/m°C. Find the width of the insulation required to reduce the convective heat loss to 15 W/m2.
8 M
2 (a)
What is critical thickness of insulation on a small diameter wire or pipe? Explain its physical significance and derive an expression for the same.
10 M
2 (b)
A set of aluminium fins (K=180 W/mK) that are to be fitted to a small air compressor. The device dissipates 1 KW by convecting to the surrounding air which is at 20°C. Each fin is 100 mm long, 30 mm high and 5 mm thick. The tip of each fin may be assumed to be adiabatic and a heat transfer coefficient of 15 W/m2K acts over the remaining surfaces. Estimate the number of fins required to ensure the base temperature does not exceed 120°C.
10 M
3 (a)
What are Biot and Fourier numbers? Explain their physical significance.
6 M
3 (b)
What are Heisle charts? Explain their significance in solving transient convection problems.
6 M
3 (c)
The temperature of a gas stream is measured with a thermocouple. The junction may be approximated as a sphere of diameter I mm, K=25 W/m°C, ρ = 8400 kg/m3 and C= 0.4 kJ/kg°C. The heat transfer coefficient between the junction and the gas stream is h= 560 W/m2°C. How long will it take for the thermocouple to record 99% of the applied temperature difference?
8 M
4 (a)
Establish a relation between Nusselt, Prandtl and Grashof numbers using dimensional analysis.
8 M
4 (b)
Explain velocity and thermal boundary layer.
6 M
4 (c)
A 30 cm long glass plate is hung vertically in the ak at 27°C while its temperature is maintained at 77°C. Calculate the boundary layer thickness at the trailing edge of the plate. Take properties of air at mean temperature K-28.15× 10-3W/mK, γ =18.41 × 10-6m2/s, Pr=0.7,β = 3.07 × 10-3 K-1
6 M
5 (a)
Explain the significance of i) Reynolds number ii) Prandtl number, iii) Nusselt number, iv) Stanton number.
8 M
5 (b)
Atmospheric air at 275 K and free stream velocity 20 m/s flows over a flat plate of length 1.5 m long maintained at 325 K. Calculate:
i) The average heat transfer coefficient over the region where the boundary layer is laminar.
ii) Find the average heat transfer over the entire length 1.5 m of the plate.
iii) Calculate the total heat transfer rate from the plate to the air over the length of 1.5 m and width 1 m. assume transition occurs at a Reynolds number 2×105. Take air Properties at mean temperature of 300K
K-0.026W/m°C, Pr=0.708,γ=16.8×10-6 m2/s, μ=1.98×10-5kg/ms
i) The average heat transfer coefficient over the region where the boundary layer is laminar.
ii) Find the average heat transfer over the entire length 1.5 m of the plate.
iii) Calculate the total heat transfer rate from the plate to the air over the length of 1.5 m and width 1 m. assume transition occurs at a Reynolds number 2×105. Take air Properties at mean temperature of 300K
K-0.026W/m°C, Pr=0.708,γ=16.8×10-6 m2/s, μ=1.98×10-5kg/ms
12 M
6 (a)
Derive an expression for the,effectiveness of a parallel flow heat exchange.
10 M
6 (b)
Engine oil is to be cooled from 80°C to 5°C by using a single pass counter flow , concentric-tube heat exchanger with cooling water available at 20°C. Water flows inside a tube with an internal dia of 2.5cm with a flow rate of 0.08 kg/s and oil flows through the annulus at a rate of 0.16kg/s. The heat transfer coefficient for the water side and oil side are respectively 1000 W/m2°C and 80W/m2C. The fouling factors are 0.00018m2°C/W and 0.00018 m2°C/W, the tube wall resistance is negligible. Calculate the tube length required. Take specific heat of water as 4180 J/kg°C and for oil, 2090J/kg°C.
10 M
7 (a)
Explain film wise and drop wise condensation.
4 M
7 (b)
Draw the boiling curve and discuss the different regimes of boiling.
8 M
7 (c)
Derive an expression for the total mass of water vapour diffused from a water column to the air passing over the water container.
8 M
8 (a)
Explain briefly the concept of a black body.
4 M
8 (b)
State: (i) Kirchoff's law, ii) Plank's law, iii) Wien's displacement law.
6 M
8 (c)
Calculate the net radiant heat exchange per mm2 area for two large parallel plates at temperature of 427°C and27°C respectively ε for hot plates is 0.9 and for cold plate it is 0.6. If polished aluminium shield is placed between them, find percentage reduction in the heat transfer. Assume ε for shield :0.4.
10 M
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