# Interview Short Question Answers : Conduction-3

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**Interview Short Question Answers : Conduction-3**

**Discuss thermal diffussivity.**

**Thermal diffussivity** 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 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 required to establish the thermal equilibrium in the specimen.

The rate of change of temperature depends on its numerical value.

METHODS TO FIND THERMAL DIFFUSION

(i) Flash Method

(ii) Infrared detectors

(iii) Intrinsic thermo-couples

**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 conduction.

**List at least four examples of multi-dimensional heat conduction.**

- Cooling of I.C. Engines
- Heat transfer in air conditioning ducts.
- Heat transfer in an industrial chimney.
- During various heat treatment processes.

**List 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

**List the methods used in the analysis of 3 dimensional steady state conductive heat transfers.**

** There are three methods.**

- Analytical method
- Analogical Method
- Numerical Method

**what are the assumptions used in Lumped capacity analysis?**

- Solid materials have infinite (very large) value of ‘k’.
- The conductance resistance or internal resistance is negligible as compared to convective resistance or external resistance.
- Temperature is assumed to be constant at a given time in such solids.
- Thus rate of change of internal energy is linked with convective heat exchange at the surface of the solid.

**What is a conduction shape factor?**

In Convection q^{.} =h A dT

In conduction, q^{.} = -k A (dT/dx)

Compare and write conduction equation as q^{.} = k S dT

Here S is the shape factor and S = – k/dx in conduction

S =h in convection

**What is LUMPED CAPACITY?**

It the product of Biot number and Fourier number as given below.

hA_{s}t /ρVC = (hV/ (kA_{s})) (A_{s}^{2}kt/ (ρV^{2}C)) = (hL_{c}/k) (αt/L_{c}^{2}))

**List some of the areas which are covered under the discipline of heat transfer.**

(i) Fins in compressors, in motors, on condensers and evaporators, on transformers

(ii) Heat exchanger like condenser, evaporator and a boiler

(iii)Transfer of Heat from the Sun, from fire, from furnaces, from radiators.

**State the assumptions used in Fourier law of heat conduction.**

- Conduction under steady state conditions.
- The heat flow is 1 dimensional.
- The temperature gradient is constant and the temperature profile is linear.
- There is no internal heat generation.
- The bounding surfaces are at respective constant temperature.
- The material is homogeneous and isotropic (The value of ‘k’ is constant in all directions).

**List some 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.

**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.