INTERVIEW SHORT QUESTION ANSWERS-CONVECTION

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INTERVIEW SHORT QUESTION ANSWERS-CONVECTION

  1. Define convective heat transfer coefficient.

From Newton’s Law of Cooling

  1. Q.= h A dT        

Where Q.= Rate of heat transfer, W

 h = convective heat transfer coefficient (W/m2K or W/m2 oC)

A = Surface area, m2

 dT = temperature difference between the surface and the bulk fluid (oC)

  1. Q.= h with A =1m2and dT=1oC

Therefore convective heat transfer coefficient is the rate of convective heat transfer through a unit area and unit temperature difference.

Q . Mention the numerical data on convective heat transfer coefficient.

Type of convection Fluid used Experimental data on ‘h’, W/m2K
Free Convection Air 5 – 25
Free Convection Water 20 – 100
Forced Convection Air 10 – 200
Forced Convection Water 50 – 10000
Boiling Water 3000 – 100,000
Condensation Water vapor 5,000 – 100,000

It is highest during condensation.

  1. Define mean film temperature.               

It is arithmetic mean of two temperatures involved in heat transfer. For example the two temperatures are 100 and 200C, then mean film temperature is (100+20)/2=600C. At this temperature the properties of the fluid are considered.

  1. Difference between natural and forced convection
Sr.No. Natural convection Forced convection
 1. The motion of the fluid is only due to density difference. It is due to buoyancy force. No external pump or blower is used in this. Motion of the liquid is caused by a pump. Motion of the vapor/gas is caused by a blower.
 2. It is a slow process of heat transfer It is fast process
3. It involves Grashoff’s number to decide laminar/ turbulent flow It involves Reynolds number to decide laminar/ turbulent flow
4., Nu = f( Gr, Pr) Nu = f( Re, Pr)

Q. Differentiate Nusselt number and Biot number.

It is a non-dimensional number.

Nu = q.conv in fluid/q.cond in static fluid = hAΔT/–kf AΔT/dx

Nu= hLc/kf

Where kf is the thermal conductivity of the fluid

Biot number = Bi= hLc/ ks

Where ks is the thermal conductivity of the solid

It is also a Non dimensional number

 Q. Write the formula for Grashoff’s number and discuss its importance.

Gr = Buoyant force x Inertia force/ (viscous force)2

Gr  = L3gβΔT/ν2

Where β=1/ (Tmean+273)

and ν=µ/ρ

Grashoff’s number used in the analysis of free convection (laminar flow).