1. Differentiate irradiation and radiosity.

Irradiation:

It is defined as the total radiation incident upon a surface per unit time per unit area.

It is expressed in W/m²

Radiosity:

It is used to indicate the total radiation leaving a surface per unit time per unit area.

It is expressed in W/m²

2. What are the assumptions made to calculate radiation exchange between the surfaces?

(i) All surfaces are considered to be either black or gray.

(ii) Radiation and reflection process are assumed to be diffuse.

(iii) The absorptivity of a surface is taken equal to its emissivity and independent of temperature of the source of the incident radiation.

3. Discuss the radiation characteristics of carbon dioxide and water vapor.

The CO₂ and H₂O both absorb and emit radiation over certain wavelength regions called absorption bands.

The radiation in these gases is in a volume phenomenon.

 The emissivity of CO₂ and the emissivity of H₂O at a particular temperature increases with partial pressure and mean beam length.

4. What is meant by shape factor and mention its significance.

 It is defined as the fraction of the radiate energy that is diffused from one surface element and strikes the other surface directly with no intervention reflections.

 It is used to analyse the radiative heat exchange between two surfaces.

5. What is meant by mass transfer? Give examples.

The process of transfer of mass as a result of the species concentration difference in a mixture is known as mass transfer.

Ex:

(i) Humidification o air in a cooling tower

  (ii) Evaporation of petrol in the carburetor of an IC engine.

  (iii) The transfer of water vapor into dry air.

6. What are the modes of mass transfer?

(i) Diffusion mass transfer

(ii) Convective mass transfer

7. Differentiate molecular diffusion and eddy diffusion.

Molecular diffusion:

The transport of water on a microscopic level as a result of diffusion from a region of higher concentration to the region of lower concentration in a mixture of liquids or gases is known as molecular diffusion.

Eddy diffusion:

When one of the diffusion fluids is in turbulent motion, eddy diffusion will takes place.

8. What is convective mass transfer?

It is the process of mass transfer that will occur between a surface and a fluid medium when they are at different concentrations.

9. State Flick’s law of diffusion.

 It states that molar flux of an element per unit area is directly proportional to concentration gradient.

            M_a/A = -D_ab (dc_c/dx)

Where

            M_a/A = Molar flux – Kg-mole/sm²

            D_{ab =} Diffusion coefficient of species a and b in m²/s

            dc_c/dx = Concentration gradient in kg/m³

10. What is free convective mass transfer?

 If the fluid motion is produced due to change in density resulting from concentration gradients, the mode of mass transfer is said to be free or natural convective mass transfer.

Ex: Evaporation of alcohol.