- 1. A steel ball of diameter 60 mm is initially in thermal equilibrium at 1030 C in a furnace. it is suddenly removed from the furnace and cooled in ambient air at 30 C, with convective heat transfer coefficient h=20 W/m3k. the thermo physical properties of steel are : density ρ = 7800 kg/m2, conductivity k= 40 W/mk and specific heat c= 600j/kgk. the time required in seconds to cool the steel ball in air from 1030 C to 430 C is
- 2. Which one of the following configuration has the highest fin effectiveness?
- 3. A fin has 5 mm diameter and 100 mm length. the thermal conductivity of fin material is 400 Wm−1 K−1. One end of the fin is maintained at 130 C and its remaining surface is exposed to ambient air at 30 C. if the convective heat transfer coefficient is 40 Wm−2 k−1, the heat loss (in W) from the fin is
- 4. The average heat transfer coefficient on a thin hot vertical plate suspended in still air can be determined from observation of the change in plate temperature with time as it cools. Assume the plate temperature to be uniform at any instant of time and radiation heat exchange with the surroundings negligible. the ambient temperature is 25 C, the plate has a total surface area of 0.1 m2 and a mass of 4kg. the specific heat of the plate material is 2.5 kJ/kgK. the convective heat transfer coefficient in W/m2K, at the instant when the plate temperature is 225 C and the change in plate temperature with time dT/dt = -0.02K/s, is
- 5. A small copper ball of 5 mm diameter at 500K is dropped into an oil bath whose temperature is 300K. the thermal conductivity of copper is 400W/mK, its density 9000kg/m3 and its specific heat 385 J/kgK. if the heat transfer coefficient is 250 W/m2K and lumped analysis is assumed to be valid, the rate of fail of the temperature of the ball at the beginning of cooling will be, in K/s,
- 6. Two rods one of length L and other of length 2L are made of the same material and have the same diameter. The two ends of the longer rod are maintained at 100 C. One end of the shorter rod is maintained at 100 C while the other end is insulated. Both the rods are exposed to the same environment at 40 C. the temperature at the insulated end of the shorter rod is measured to be 55 C. The temperature at the mid point of the longer rod would (in degree Celsius)
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