Heat And Mass Transfer MCQ Questions & Answers | Mechanical Engineering

Conduction transfers heat via direct molecular collision. An area of greater kinetic energy will transfer thermal energy to an area with lower kinetic energy. Higher-speed particles will collide with slower speed particles. The slower-speed particles will increase in kinetic energy as a result. Conduction is the most common form of heat transfer and occurs via physical contact.

Wien's law formula
The equation describing Wien's law is very simple:
$${\lambda _{\max }} = \frac{{\text{b}}}{{\text{T}}}$$
where,
• $${\lambda _{\max }}$$  is the aforementioned peak wavelength of light
• T is an absolute temperature of a black body
• b = 2.8977729 mm$$ \cdot $$K is the Wien's displacement constant
Although the relation between wavelength and frequency of electromagnetic waves is fairly simple ($$\lambda $$ × f = c), we can't work out the peak frequency f_max by this analogy. The reason is that spectral radiance is a sort of energy density function, so its shape and maximum depend on the argument (wavelength or frequency in our case).
Knowing that the formula for the peak frequency is:
f_max = k × T,
where k = 5.8789232 × 10¹⁰ Hz/K is a numerical constant.
Wien's law formula can't be derived from classical physics. Numerous observations that confirm this law are among experiments (e.g., photoelectric effect), which contribute to the creation of quantum mechanics.

The amount of radiation mainly depends on Nature of body ,Temperature of body , Type of surface of body

The heat transfer by conduction through a thick cylinder (Q) is given by
$${\text{Q}} = \frac{{2\pi l{\text{k}}\left( {{{\text{T}}_1} - {{\text{T}}_2}} \right)}}{{2.3\log \left( {\frac{{{{\text{r}}_2}}}{{{{\text{r}}_1}}}} \right)}}$$
Where T₁ = Higher temperature, T₂ = Lower temperature, r₁ = Inside radius, r₂ = Outside radius, $$l$$ = Length of cylinder and k = Thermal conductivity.

Thermal Diffusivity is defined as the ratio of heat conducted to the heat energy stored per unit volume of a material.
The thermal conductivity of a substance divided by the product of its density and its specific heat capacity.
Thermal diffusivity of a substance is given by $${\text{h}} = \frac{{\text{k}}}{{\rho {\text{S}}}}$$
(Where h = Thermal diffusivity, $$\rho $$ = Density of substance, S = Specific heat and k = Thermal conductivity)

The thermal conductivity of ice is about 2.22W/mk, about one order higher than the thermal conductivity of traditional polymer.

Thermal conductivity is a measure of a material's ability to conduct heat.

Among the given options — Aluminium, Steel, Brass, and Copper — Copper has the highest thermal conductivity.

Copper is widely used in heat exchangers, electrical wiring, and cookware precisely because of its excellent thermal conductivity.

Typical thermal conductivity values at room temperature are:

Copper: ~400 W/m·K

Aluminium: ~205 W/m·K

Brass: ~110 W/m·K

Steel: ~50 W/m·K

Conclusion: Since Copper has the maximum thermal conductivity among the given materials, Option D is correct.

Option A: Density
In free convection, the density of the fluid plays a crucial role in determining the buoyancy forces that drive the flow.
When the fluid heats up, its density decreases, leading to an upward flow due to the buoyancy effect, which is a key characteristic of free convection.

Option B: Coefficient of viscosity
The coefficient of viscosity also influences the flow in free convection.
Viscosity affects the resistance to flow and the development of velocity profiles in the fluid.
Higher viscosity tends to slow down the flow, whereas lower viscosity allows for a more efficient convection process.

Option C: Gravitational force
The gravitational force is essential in free convection, as it is responsible for the buoyancy effect.
The difference in temperature within the fluid creates density variations, and the gravitational force acts on these density differences to drive the convection currents.

Option D: All of these
The correct answer is Option D: All of these because free convection is influenced by all the factors mentioned—density, viscosity, and gravitational force.
All of these factors work together to determine the behavior and efficiency of convection flow in a fluid.

Critical thickness of insulation means maximum heat loss with min thermal resistance. This concept is useful in the situation when surface area of the base object increases with the insulation.
The critical thickness of insulation for a sphere is 2k/h₀.

Thermal Conductivity is the quantity of heat that passes in unit time through unit area of a substance whose thickness is unity, when its opposite faces differ in temperature by one degree.
Unit of thermal conductivity in M.K.S. units is K cal m/hr m² °C.