(Aktu Btech) Heat and Mass Transfer Important Unit-4 Thermal Radiation

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Important Questions For Heat and Mass Transfer:
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Q1. What do you understand by radiation heat transfer ? Also write the surface emission properties. 

Ans. A. Radiation Heat Transfer: 

• 1. It is described as the transfer of energy from a system via an electromagnetic mechanism that is brought on by a difference in temperature. 
• 2. Factors on which rate of emission of radiation by a body depends are as follows: 
  • i. Temperature of the surface,
  • ii. The nature of the surface, and 
  • iii. Wavelength of radiation. 

B. Surface Emission Properties: 

a. Total Emissive Power (E): 

• 1. It is defined as the total amount of radiation emitted by a body per unit area and time. 
• 2. According to Stefan Boltzmann “the total emissive power of a black body is proportional to the fourth power of its absolute temperature.” 

b. Monochromatic Emissive Power (E_𝝀): The rate of energy radiated per unit area of the surface and per unit wavelength is how it is defined.

Total emissive power is:

c. Emission from real surface-emissivity: The emissive power from a real surface is given by

Where,                    E = Emissivity of the material. 

d. Intensity of radiation. 

e. Radiation density and pressure. 

f. Radiosity (J): It refers to all of the radiant energy leaving a surface.

Q2. Define the properties: 

i. Emissivity. 

ii. Absorptivity. 

iii. Reflectivity. 

iv. Transmissivity.

Ans. i. Emissivity (𝜺): It is described as a body’s capacity to emit heat. It can alternatively be described as the difference between a body’s emissive power and the emissive power of a black body with an identical temperature. 

ii. Absorptivity (𝜶): It is the ratio of amount of radiation absorbed to the total incident radiation.

                                          𝜶  = G_𝜶 /G 

iii. Reflectivity (𝝆): It is the ratio of amount of radiation reflected to the total incident radiation. 

                                          𝝆 = G_𝝆 / G 

iv. Transmissivity (𝝉): It is the ratio of amount of radiation transmitted to the total incident radiation.

                                         𝝉 = G_𝝉 / G

Q3. What do you understand by radiation shields and derive the relation for radiation network for two parallel infinite planes separated by one shield. 

Ans. A. Radiation Shield: Radiation shields are those that decrease the heat transmission from radiation by essentially raising surface resistances while doing so without really removing any heat from the system. 

B. Derivation:  

1. Let us consider two parallel plates, 1 and 2, each of area A(A₁ = A₂ = A) at temperature t₁ and t₂ respectively with a radiation shield placed between them as shown in Fig.

2. With no radiation shield, the net heat exchange between the parallel plates is given by,  

5. Since the radiation shield does not deliver or remove heat from the system, 

Q4. Write a short note on solar radiation. 

Ans.

• 1. Solar radiation is made up of energy that the sun’s many layers emit.
• 2. It is a result of the surface’s geometric relationship to the sun, which is constantly changing on a daily and annual basis.
• 3. Some of the solar radiation that reaches the earth’s surface is reflected, some of it is absorbed, and the remaining energy is transferred thermally to the land and oceans, chemically to vegetation through the process of photosynthesis, and mechanically in the form of wind. 
• 4. The approximate distribution of the flow of sun’s energy to the earth’s surface is:
  • i. 9%is scattered. 
  • ii. 15 % is absorbed in the atmosphere and out of it 4 % reaches the earth’s surface by convection. 
  • iii. 43 % is transmitted to the earth directly and by diffuse radiation. 
  • iv. 33%is reflected back to space.  

Q5. What is the green house effect ? Why is it a matter of great concern among environmental scientists ?

Ans. A. Green House Effect: 

• 1. The green house effect is the mechanism through which green house gases, such as carbon dioxide, methane, water vapour, etc., absorb radiant energy leaving a planetary surface.
• 2. They convey this energy to the atmosphere’s other elements, where it is radiated once more in all directions.
• 3. This energy transfer to the surface raises environmental temperatures and causes global warming.

B. Reason to Consider it as a Matter of Great Concern: It is matter of great concern among environmental sCientists because of the following reasons:

• 1. The global temperature will rise if greenhouse gas emissions persist.
• 2.  It results in floods. 
• 3. The pattern of rainfall will change as a result of global warming.

Q6. A flat plate 5 m² receives normally radiant energy with an intensity of 660 W/m². The absorptivity of the plate is 2 times its transmissivity and 3 times its reflectivity. Find the energy absorbed, transmitted and reflected in watts.

Ans. 

To Find: Energy absorbed, transmitted and reflected. 

2. Energy absorbed per unit area, 

Total energy absorbed = 360 x 5 = 1800 W 

3. Energy reflected per unit area,

Total energy reflected = 120 x 5 = 600 W

4. Energy transmitted per unit area, 

Total energy transmitted = 180 x 5 = 900 W

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