A. Existence of thermal boundary layer
B. Temperature gradient produced due to density difference
C. Buoyancy of the bubbles produced at active nucleation site
D. None of these
Related Mcqs:
- 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 - Fouling factor for a heat exchanger is given by (where, U1 = heat transfer co-efficient of dirty surface U2 = heat transfer co-efficient of clean surface) ?
A. U1 – U2
B. 1/U1 – 1/U2
C. 1/U2 – 1/U1
D. U2 – U1 - A 10 cm dia steam pipe, carrying steam at 180°C, is covered with an insulation (conductivity = 0.6 W/m.°C). It losses heat to the surroundings at 30°C. Assume a heat transfer co-efficient of 0.8 W/m2.°C for heat transfer from surface to the surroundings. Neglect wall resistance of the pipe and film resistance of steam. If the insulation thickness is 2 cms, the rate of heat loss from this insulated pipe will be__________________?
A. Greater than that for un-insulated steam pipe
B. Less than that of the un-insulated steam pipe
C. Equal to that of the un-insulated steam pipe
D. Less than the steam pipe with 5 cms insulation - Nusselt number is a function of Prandtl number and ______________ number of fluid in natural convection heat transfer?
A. Grashoff
B. Biot
C. Stanton
D. Reynolds - For a laminar flow of fluid in a circular tube, ‘h1’ is the convective heat transfer co-efficient at velocity ‘V1’. If the velocity is reduced by half and assuming the fluid properties are constant, the new convective heat transfer co-efficient is________________?
A. 1.26 h1
B. 0.794 h1
C. 0.574 h1
D. 1.741 h1 - The rate of heat transfer from a vertical plate by natural convection depends upon the temperature differences (ΔT) between wall and outside bulk. The proportionality is given as______________?
A. (ΔT)1/4
B. (ΔT)1/2
C. (ΔT)5/4
D. (ΔT)3/4 - In natural convection heat transfer, the correlating parameter is the_______________________?
A. Graetz number
B. Eckert number
C. Grashoff number
D. Bond number - Heat transfer occurs by natural convection because change in temperature causes difference in__________________?
A. Viscosity
B. Density
C. Thermal conductivity
D. Heat capacity - Heat transfer by natural convection is enhanced in system with__________________?
A. High viscosity
B. High co-efficient of thermal expansion
C. Low temperature gradients
D. Low density change with temperature - The inner wall of a furnace is at a temperature of 700°C. The composite wall is made of two substances, 10 and 20 cm thick with thermal conductivities of 0.05 and 0.1 W.m-1.°C- 1 respectively. The ambient air is at 30°C and the heat transfer co-efficient between the outer surface of wall and air is 20 W.m-2.°C-1. The rate of heat loss from the outer surface in W.m-2is__________________?
A. 165.4
B. 167.5
C. 172.5
D. 175
