INTERVIEW SHORT QUESTION ANSWERSCONDUCTION
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INTERVIEW SHORT QUESTION ANSWERS
CONDUCTION
It is one of mode of heat transfer. It takes place in solids by physical contact. It depends on the thermal conductivity. Thus there is more heat conduction in metals. There is least conduction in non metals.
Q1. Define critical thickness of insulation.
The thickness of insulation for which rate of heat transfer is maximum, is called critical thickness of insulation.
Adding insulation to a wall type surface always reduces the rate of heat transfer because thermal resistance (R_{th}=x/KA) increases.
While in case of cylindrical and spherical bodies the situation is different. Adding insulation increases thermal resistance in conductance (x/kA) while decreases thermal resistance in convection (R_{th}=1/hA). As a result the overall resistance decreases and the rate of heat transfer increases. The thickness of insulation for which rate of heat transfer is maximum, is called critical thickness of insulation. The critical radius (outer radius with insulation) is given by for a
Cylindrical surface r_{c} = k/h_{o}
Spherical surface r_{c} =2k/h_{o}
Q2. Explain the significance of Fourier number.
Fourier number = Fo = αt/L_{c}^{2}.
It signifies the degree of penetration of cooling or heating through a solid.
It is a Non dimensional number.
Q3. Differentiate thermal conductance and thermal resistance.
_{ }When rate of heat transfer is compared with electrical flow of current, the concept of thermal resistance comes into existence. Thermal resistance is R_{th}
Rate of heat transfer q^{.} ΔT R_{th}=ΔT/q^{.}
Charge q^{.}=current= I ΔV R = ΔV/I
Thermal conductance is reciprocal of thermal resistance. It is represented by C. C=1/R_{th}. Its units are W/K. The unit of thermal resistance will be K/W.
Q4. What is log mean area as applied to a hollow cylinder?
Heat transfer through a cylinder is in the radial direction. Radius is changing from inner radius to outer radius. Therefore area is changing. Thus mean area must be used. Now mean area can be arithmetic mean or logarithmic mean. By experiments it has been found that log mean area gives better results.
For a cylinder, log mean area
ln ( A_{m}) = (A_{o} — A_{i})/ln (A_{o}/A_{i}) = (r_{o}–r_{i)}/ ln (r_{o}/r_{i})
For a sphere, log mean area
ln (A_{m}) = (A_{o} –A_{i})^{0.5}= 4 r_{i} r_{o}
Q5.Discuss thermal diffusivity.
Thermal diffusivity is expressed as the thermal conductivity divided by the by the product of density and specific heat. In a substance with high thermal diffusivity, heat moves rapidly through the solid. It is because the substance conducts heat quickly relative to its volumetric heat capacity. If it is higher, then less time is required for certain heat transfer to take place through the solid. Further, thermal diffusivity is typically measured in mm²/s. Its symbol is α. The thermal diffusivity of a material indicates the rate of heating and rate of cooling of a material under transient conditions. The physical signiﬁcance of this quantity lies in the fact that the inverse of thermal diffusivity is a measure of time. This is he time required to establish the thermal equilibrium in the specimen. The rate of change of temperature depends on its numerical value.
Q6. Difference between thermal conductivity and thermal diffusivity?
Thermal conductivity (k) represents its ability to conduct heat. Whereas thermal diffusivity (α) indicates how fast the heat is conducted?
Q7. Quote at least four examples of multidimensional heat conduction.

Cooling of I.C. Engines

Heat transfer in air conditioning ducts.

Heat transfer in an industrial chimney.

During various heat treatment processes.
Q8. Name the methods used in the analysis of 2 Dimensional steady state conductive heat transfers.
There are four methods.

Analytical method

Graphical Method

Analogical Method

Numerical Method
Q9. Methods used in the analysis of 3 dimensional steady state conductive heat transfers.
There are three methods.

Analytical method

Analogical Method

Numerical Method
Q10. Discuss conduction shape factor?
Convection q^{.} =h A dT
conduction, q^{.} = k A (dT/dx)
Compare and write conduction equation as q^{.} = k S dT
Here S is the shape factor
S = – k/dx in conduction
S =h in convection
Q11. State the assumptions used in Fourier law of heat conduction.

Conduction under steady state conditions.

Heat flow is 1 dimensional.

Temperature gradient is constant and the temperature profile is linear.

There is no internal heat generation.

The bounding surfaces are at respective constant temperature.

Material is homogeneous and isotropic (The value of ‘k’ is constant in all directions).
Q13. Give a list of the essential features of Fourier Law.

This law is applicable to solids, liquids and gases.

Fourier law is based on experimental data and hence cannot be derived from first principle.

Heat transfer is vector expression and indicates that the rate of flow of heat in the temperature decreasing direction.

It gives the definition of thermal conductivity, a transport property.Q15. What is thermal contact resistance?.
When two solid bodies come in contact, heat flows from the hotter body to the colder body. There is a temperature drop at the common surfaces in contact. It is due to the thermal contact resistance existing because of imperfect contact between the two contacting surfaces. Further it is due to irregular surfaces. Thermal contact resistance is the ratio of temperature drop and the average heat flow across the contacting surfaces.
Q16. MENTION METHODS TO FIND THERMAL DIFFUSION
(i) Flash Method
(ii) Infrared detectors
(iii) Intrinsic thermocouples