INTERVIEW SHORT QUESTION ANSWERS-CONDUCTION-1

INTERVIEW SHORT QUESTION ANSWERS-CONDUCTION-1 

 Q. List the basic laws which govern heat transfer.

  1.   First law of thermodynamics i.e. Law of conservation of energy
  2.  Second law of thermodynamics: Heat flows along negative temperature gradient.
  3. Law of conservation of mass
  4.  Newton’s Law of motion
  5. The rate equations

Q. What is the significance of heat transfer?

Like transfer of a person from one city to another city, heat transfer is heat energy in motion.

   Q. Definition of thermal conductivity

From Fourier equation

k=q. with dT=1 K, dx=1 m and A=1 m2

 Thus thermal conductivity is the rate of heat transfer in a solid of unit area, unit thickness with unit temperature difference. It is property of a solid for conducting heat.  Its symbol is k. Its unis are W/mK.

Q.  How is the thermal resistance of a rectangular slab change under the following conditions if

(i)    The thermal conductivity is increased ?—Resistance will decrease.

(ii)  The cross sectional area is increased ? –Resistance will decrease.

(iii) The thickness of the slab is increased ?– Resistance will increase.

Q. State the condition which considers heat transfer in a fin as one-dimensional.

When Biot number is < 0.01,

Where Bi =hl/ksolid

 Q. Define and state the physical interpretation of the Biot number.

It is defined as

Bi = hL/ksolid

Where

h = convective heat transfer coefficient

k = thermal conductivity of the solid

L = characteristic length.

Q. What is a lumped system ?

In a lumped system dT/dx=0, dT/dy=0, dT/dz=0.

Temperature is a function of time only T=f(t) i.e. unsteady state heat transfer.

 When is the temperature in a body considered uniform ?

When the Biot number is small (Bi << 0.1).

 Q.   What is basic equation of conduction?

Fourier equation is the fundamental equation of conduction.

It is used in following three forms.

a. Difference form of equation

Q=-k A (Th -Tl)/(x2—x1)

It is used when temperatures at the two ends are fixed and distance between the two ends is also fixed.

b. Differential form of equation

Q=-k A dT/dx

It is used when temperature is a function of x.

c. Second order differential equation

d2T/dx 2 + d2T/dy 2+ d2T/dz 2 + qg/k =(1/α) dT/dt

It is three dimensional non steady state conduction equation.

T is the temperature

t is the time.

K is thermal conductivity

α  is thermal diffusivity

qg is heat generated per unit volume within the system

Q.  What is thermal resistance in conduction and convection respectively?

Conductive rate of heat transfer q.= –dT/x/kA

thermal resistance in conduction Rcond= x /(kA), measured in K W−1  

Convective heat transfer: q = ΔT/(1/hA)

Thermal resistance to convective heat transfer= R th conv = 1/hA

Q. What is difference between heat and temperature?

Heat is the energy in motion or transition. It flows spontaneously from high temperature to low temperature. It varies with temperature. Rises with the increase of temperature and decreases with fall of temperature. The common symbol for heat is Q. Units of heat are kJ.

Temperature is the measurement of heat energy in a body. It is the degree of hotness or coldness of a body. It decides the direction of heat transfer which is always from high temperature to low temperature. The common symbol for temperature is t or T. Its units are oC or K.

Q. Where does internal heat generation found?

When electric current flow in wires/cables.

During chemical reactions

Nuclear reactions

Heat treatment processes.

Q. What are a temperature gradient, velocity gradient and pressure gradient?

Temperature gradient

dt/dx, dt/dy and dt/dz are temperature gradients i.e. temperature variation w.r.t. x, y, z.

Velocity gradient          dv/dx, dv/dy and dv/dz are velocity gradients i.e. velocity variation w.r.t. ‘x’, y, z.

Pressure gradient    dp/dx, dp/dy and dp/dz are pressure gradients i.e. pressure variation w.r.t. ‘x’, y, and z.

Q. Write the temperature distributions for a plane wall, cylinder and a sphere.

Firstly, for a plane wall (TX—Th)/ (Tl—Th) = x/δ                Linear variation

Secondly, for a cylinder    (TR—Th)/ (Tl—Th) = ln(r/ri) / ln (r0/ri)                Logarithmic variation

Thirdly, for a sphere       (TR—Th)/ (Tl—Th) = (ro/r) (r–ri)/ (ro—ri)             Hyperbolic variation