Heat And Mass Transfer MCQ Questions & Answers | Mechanical Engineering

Degree of approach in a heat exchanger is a measure of how close the outlet temperatures of the two fluids get to each other.

Specifically, it is defined as the temperature difference between the hot fluid outlet and the cold fluid outlet.

Option A: Cold water inlet and outlet → This represents the temperature rise of the cold water, not the degree of approach.

Option B: Hot medium inlet and outlet → This represents the temperature drop of the hot medium, not the degree of approach.

Option C: Hot medium outlet and cold water inlet → This does not directly indicate how close the fluids are to thermal equilibrium at the outlet.

Option D: Hot medium outlet and cold water outlet → This is correct. It shows how efficiently the heat exchanger transfers heat. A smaller degree of approach means the fluids are approaching the same temperature at the exit, indicating better heat transfer performance.

The fouling factor represents the theoretical resistance to heat flow due to a build-up of a layer of dirt or other fouling substance on the tube surfaces of the heat exchanger

Natural convection air-cooled condensers find applications in various cooling systems. The correct option is All of these, which means that they are used in domestic refrigerators, water coolers, and room air conditioners.

Natural convection refers to the heat transfer process where the movement of fluid (air, in this case) is driven by density differences due to temperature variations. Air-cooled condensers utilize natural convection to dissipate heat from the refrigerant to the surrounding air.

Therefore, in the context of the question, natural convection air-cooled condensers are utilized in domestic refrigerators, water coolers, and room air conditioners.

The heat transfer coefficient is the reciprocal of thermal insulance.
The concept of overall coefficient of heat transfer is used in heat transfer problems of Conduction and convection

Critical radius of insulation for cylinder = k/h.
K = thermal conductivity of cylinder
$$\frac{{0.15}}{6} = 0.025\,{\text{m}} = 25\,{\text{mm}}{\text{.}}$$

The logarithmic mean temperature difference ($${{\text{t}}_{\text{m}}}$$) is given by $${{\text{t}}_{\text{m}}} = \frac{{\Delta {{\text{t}}_1} - \Delta {{\text{t}}_2}}}{{{{\log }_{\text{e}}}\left( {\frac{{\Delta {{\text{t}}_1}}}{{\Delta {{\text{t}}_2}}}} \right)}}$$
Where $$\Delta {{\text{t}}_1}$$ and $$\Delta {{\text{t}}_2}$$ are temperature differences between the hot and cold fluids at entrance and exit.

White body reflects all incident radiation;
i.e., its absorptivity is zero.
Accordingly, its emissivity is also zero.
So any black or even "grey" body will have a higher emissivity,

The sun heats the earth through radiation. Since there is no medium in space, radiation is the primary way that heat travels in space.

The thermal diffusivity of a substance or material is the thermal conductivity divided by the density and specific heat capacity at constant pressure and is a measure the rate of transfer of heat of a substance or material from the hot side to the cold side.

The insulation ability of an insulator with the presence of moisture would Decrease