heat transfer exam 3

define process of boiling

the liquid to vapor phase change process that occurs at solid liquid interface when the surface is heated above saturation temperature of liquid

rank boiling regimes by which order they occur in vertical tube during flow boiling

1. forced convection of liquid

2. bubbly flow

3. slug flow

4. annular flow

5. transition flow

6. mist flow

7. forced convection of vapor

match definitions

natural convection boiling : fluid motion is governed by natural convection current

nucleate : bubbles form at various preferential sites and rise to top

film: heated surface is completely covered by continuous stable vapor film

transition : part of surface covered by vapor film

the burnout during boiling is caused by the heater surface being blanketed by a continuous layer of vapor film at increased heat fluxes (t/f?)

true

with reference to film condensation and drop wise condensation, identify wrong statement

WRONG: film condensation is much more effective mechanism of heat transfer

RIGHT : in film condensation, thickness of liquid film increases in flow direction as more vapor condenses on the film

in film condensation, the condensate sets the surface and slides down due to gravity

in dropwise condensation, the condensed vapor forms droplets on surface, and surface is covered by countless droplets of varying diameters

identify characteristics of counter flow heat exchangers

the hot and cold fluids move in opposite directions

the hot and cold fluids enter heat exchanger on opposite ends

what are benefits of shell and tube heat exchanger?

they force the shell side fluid to flow across the shell to enhance heat transfer

they maintain uniform spacing between the tubes

the eliminate dead spots

mixed flow vs unmixed flow

mixed : when the fluid is free to move in transverse direction

unmixed: when the plate fins force the fluids to flow through particular interfin spacing and prevent it from moving in transverse direction

the type of fouling where heat exchangers fouled by growth of algae is

biological fouling

the effectiveness of a heat exchanger is defined as the ratio of max possible heat transfer rate to actual heat transfer rate (t/f?)

false, effectiveness of heat exchanger is defined as the ratio of actual heat transfer to the max possible heat transfer rate