Heat And Mass Transfer MCQ Questions & Answers | Mechanical Engineering

Kilocalorie per hour per square meter per degree Celsius (kcal/m² hr °С) is a metric unit of the heat transfer coefficient. The heat transfer coefficient has SI units in watts per squared meter kelvin: $$\frac{{\text{W}}}{{{{\text{m}}^2}{\text{K}}}}$$ . Heat transfer coefficient is the inverse of thermal insulance.

Thermal conductivity is measured in watts per meter-kelvin W/m °K and J/m °K sec.

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.

air has the least value of conductivity

According to Prevost theory of heat exchange:
Every material body, at any temperature above absolute zero, radiates heat to the surroundings and at the same time absorbs heat from the surroundings. The rate of emission of heat by a body depends upon its absolute temperature.

Conduction is the process by which heat energy is transmitted through collisions between neighboring atoms or molecules. Conduction occurs more readily in solids and liquids, where the particles are closer to together, than in gases, where particles are further apart.

According to Dalton's law of partial pressures, the total pressure by a mixture of gases is equal to the sum of the partial pressures of each of the constituent gases. The partial pressure is defined as the pressure each gas would exert if it alone occupied the volume of the mixture at the same temperature -According to Dalton's law of partial pressures, Pb = pa + pv.
(Where pb = Barometric pressure, pa = Partial pressure of dry air and pv = Partial pressure of water vapour)

Conductance = $$ = {\text{k}} \times \frac{{\text{A}}}{l}$$
Where A = slab area, $$l$$ = slab thickness and k = conductivity.
The equation for series conductance is :
$${\text{cond}}\left( {{\text{total}}} \right) = \frac{{{\text{con}}{{\text{d}}_1} \times {\text{con}}{{\text{d}}_2}}}{{{\text{con}}{{\text{d}}_1} + {\text{con}}{{\text{d}}_2}}}$$
Because the geometry of the slabs is the same:
$${\text{k}}\left( {{\text{total}}} \right) = \frac{{2\left( {{{\text{k}}_1} \times {{\text{k}}_2}} \right)}}{{{{\text{k}}_1} + {{\text{k}}_2}}}$$

A polished silver surface has an emissivity of about 0.02 near room temperature. Black soot absorbs thermal radiation very well; it has an emissivity as large as 0.97

Depending on the radiating properties, a body will be black when, p = 0, x = 0 and a = 1
(Where a = absorptivity, p = reflectivity, x = transmissivity)