IINTERVIEW SHORT QUESTION ANSWERS-BOILING & CONDENSATION
https://www.mesubjects.net/wp-admin/post.php?post=3781&action=edit Q.A. Pool boiling
https://www.mesubjects.net/wp-admin/post.php?post=2742&action=edit Q.Ans Convection
INTERVIEW SHORT QUESTION ANSWERS-BOILING & CONDENSATION
Q. Write note on the critical diameter of a bubble.
Critical diameter of a bubble is the maximum diameter of a bubble. It is given by an equation given below:
dc = Cβ( σlv / σls) ( σlv /(g(ρL —ρv))0.5
Where C = constant = 0.0148 for a water bubble
β=angle formed by a bubble with the horizontal surface = 300 = for a water bubble
σlv = surface tension between liquid and vapor N/m
σls = Surface tension between liquid and solid surface N/m
dc = maximum or critical diameter of a bubble
ρ = density, kg/m3
What is sub-cooled boiling?
When bubbles are formed from the hot solid surface and try to rise to the free surface of the liquid but are condensed within the fluid. Bubbles do not reach the free surface.It is called sub-cooled boiling.
Q. Why rate of heat transfer is higher in drop wise condensation than that in film condensation .
Because in drop wise condensation rate of heat transfer is due to phase change and is thus due latent heat which is very large as compared to sensible heat transfer. In film condensation heat transfer is by sensible means. It has been found that rate of heat transfer in drop wise condensation is 10 times the rate of heat transfer in film condensation.
Q. What is pool boiling?
The change of stationary liquid phase into vapor phase at the saturation temperature is called pool boiling. In this motion is caused by the density difference or by the buoyant force.
Q. Explain the mechanism of bubble formation and collapse in pool boiling.
When excess temperature (Tsur –Tsat) is > 5 0C and <100C, vapor formation starts from the heated surface due to absorbing of latent heat. But the liquid above the bubble is not so hot and hence bubble collapse as it tries to rise. It is in pool boiling since liquid is at rest.
Q. Assumptions used in film condensation on a vertical plate.
1.The vapor is pure, dry and saturated.
2.The flow of condensate is laminar and under action of gravity.
3.The liquid film is in good thermal contact with the cooling surface and therefore the temperature of the film is equal to the temperature of the cooling surface.
4.The temperature at the liquid-vapor interface is the saturation temperature.
5.There is a velocity gradient in the film but the velocity gradient (hence shear stress) at the liquid vapor interface is zero.
6.The heat transfer across the film is by pure conduction.
7.The temperature distribution across the film is linear.
8.Heat transfer is under steady state condition.
Q. Explain the phenomenon of drop condensation.
When a vapor changes to liquid at saturation temperature by a cooling surface at temperature below the saturation temperature. The liquid formed should not wet the surface. It is possible on a highly polished surface or surface is coated with oil or grease. It is due release of latent heat. Rate of heat transfer in drop wise condensation is 10 times the rate of heat transfer in film wise condensation. But drop wise condensation is difficult to maintain because oil or grease will be washed away after some time. Therefore only film wise condensation is used in the design of condensers.