Question 4
Question 5: Heat is to be transferred to air at ‘atmospheric pressure’ and 27°C flowing at a velocity of 5m/s normal to a 35 mm diameter unfinned tube whose outer surface is maintained at a temperature of 60°C.
Find the rate of heat transfer per metre length of tube.
In order to increase the rate of heat transfer it is proposed to add fins to the outer surface and to change the air velocity, giving an estimated heat transfer coefficient of 75 W/m²K on all surfaces.
Two schemes are considered: a. 50 annular fins per metre of tube of 85 mm outside diameter and 2mm thickness, and, b. 7 longitudinal fins, 25 mm high and 2 mm thick. The thermal conductivity of the fin material is 50 W/m-K.
Find the revised rate of heat transfer in each of the two cases.
Question 6: The variation of heat flux with excess (surface) temperature for water is shown in the following diagram. Describe briefly the mechanisms involved in ANY THREE of the regions identified A to E
A domestic kettle transfers heat to its water contents at a rate of 2.5kW via a 100mm diameter horizontal stainless steel disk. Assuming the system to be at ‘atmospheric pressure’, find the equilibrium surface temperature of the disk if the kettle is left boiling.
What is the minimum disk diameter that could be used without incurring ‘burn-out’ and what would be the resulting surface temperature of the disk.
The surface tension of water may be taken as XXXXXXXXXXN/m and for a water/stainless steel combination, the Rosenhow coefficient, C
s , and index, n, are XXXXXXXXXXand 1.0 respectively.
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Question 5:
Heat is to be transferred to air at ‘atmospheric pressure’ and 27 °C flowing at a velocity of 5 m/s normal to a 35 mm diameter unfinned tube whose outer surface is maintained at a temperature of 60 °C.
Find the rate of heat transfer per metre length of tube.
In order to increase the rate of heat transfer it is proposed to add fins to the outer surface and to change the air velocity, giving an estimated heat transfer coefficient of 75 W/m²K on all surfaces.
Two schemes are considered: a. 50 annular fins per metre of tube of 85 mm outside diameter and 2 mm thickness, and, b. 7 longitudinal fins, 25 mm high and 2 mm thick. The thermal conductivity of the fin material is 50 W/m-K.
Find the revised rate of heat transfer in each of the two cases.
Question 6:
The variation of heat flux with excess (surface) temperature for water is shown in the following diagram. Describe briefly the mechanisms involved in ANY THREE of the regions identified A to E
A domestic kettle transfers heat to its water contents at a rate of 2.5 kW via a 100 mm diameter horizontal stainless steel disk. Assuming the system to be at ‘atmospheric pressure’, find the equilibrium surface temperature of the disk if the kettle is left boiling.
What is the minimum disk diameter that could be used without incurring ‘burn-out’ and what would be the resulting surface temperature of the disk.
The surface tension of water may be taken as XXXXXXXXXXN/m and for a water/stainless steel combination, the Rosenhow coefficient, Cs , and index, n, are XXXXXXXXXXand 1.0 respectively.