Heat And Mass Transfer — Part 1 MCQs with Explanations

an object that absorbs all radiation falling on it, at all wavelengths, is called a black body. When a black body is at a uniform temperature, its emission has a characteristic frequency distribution that depends on the temperature. Its emission is called black-body radiation.

The heat is transferred by conduction, convection and radiation in Boiler Furnaces

Heat is transferred by all three modes of transfer, viz. conduction, convection and radiation in Boiler

When a black body is at a uniform temperature, its emission has a characteristic frequency distribution that depends on the temperature. Its emission is called black-body radiation

The amount of heat flow through a body by conduction is - Directly proportional to the surface area of the body - Directly proportional to the temperature difference on the two faces of the body - Dependent upon the material of the body

According to wein's displacement law, Emissive power from black body is a function of temperature. So, we can say that emissive power depends on temperature and wavelength.

Radiation is a method of heat transfer that does not rely upon any contact between the heat source and the heated object It depends on nature of the body, its temperature and kind & extent of its surface.

Film coefficient is defined as Inside diameter of tube Thermal conductivity Equivalent thickness of film Specific heat × Viscosity

Thermal diffusivity is a measure of the transient thermal response of a material to a change in temperature

Static regain is the third sizing method for ductwork included in Design Master HVAC. The calculation works by keeping the static pressure in the ductwork constant throughout the system.

The value of Prandtl number for air is about 0.7

We should select a material which have low heat transfer capacity. So we should select a material which thermal conductivity is low. So that heat will pass through material is minimum. We should use low thermal conductivity material both side because after all we need to decrease the rate of heat transfer.

Total Heat is the thermal equivalent of the energy required to convert unit mass of a liquid at one temperature (as the melting point of the substance) into saturated vapor at any other given temperature.

Sensible heat is the energy required to change the temperature of a substance with no phase change. The temperature change can come from the absorption of sunlight by the soil or the air itself.

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.

Thermal radiation generates from the emission of electromagnetic waves. These waves carry the energy away from the emitting object. Radiation occurs through a vacuum or any transparent medium (either solid or fluid). Thermal radiation is the direct result of random movements of atoms and molecules in matter.

It is used in calculating the heat transfer, typically by convection or phase transition between a fluid and a solid. The heat transfer coefficient has SI units in watts per squared meter kelvin: $$\frac{{\text{W}}}{{{{\text{m}}^2}{\text{K}}}}$$ . The heat transfer coefficient is the reciprocal of thermal insulance.

Convection is the process of heat transfer by the bulk movement of molecules within fluids such as gases and liquids. The initial heat transfer between the object and the fluid takes place through conduction, but the bulk heat transfer happens due to the motion of the fluid

Stefan-Boltzmann law is applicable for radiation it states that the total radiant heat power emitted from a surface is proportional to the fourth power of its absolute temperature. The law applies only to blackbodies, theoretical surfaces that absorb all incident heat radiation.

Kilocalorie per hour per square meter per degree Celsius (kcal/m 2 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 × 10 10 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)

A measure of the ability of a material to transfer heat. Given two surfaceson either side of the material with a temperature difference between them,the thermal conductivity is the heat energy transferred per unit time andper unit surface area, divided by the temperature difference. It is measured in watts per degree Kelvin.

The rate of heat flow through a body is $${\text{Q}} = \frac{{{\text{kA}}\left( {{{\text{T}}_1} - {{\text{T}}_2}} \right)}}{{\text{x}}}.$$ The term $$\frac{{\text{x}}}{{{\text{kA}}}}$$ is known as Thermal resistance.

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

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

Stefan-Boltzmann law, statement that the total radiant heat power emitted from a surface is proportional to the fourth power of its absolute temperature. The expression Q = ρ AT 4 is called Stefan-Boltzmann equation

In heat transfer, conductance equals conductivity (kcal/hr/sq.m/°C/cm) divided by K.cal (heat)

The Time Constant of a thermocouple is defined as the time required for the sensor to respond to 63.2% of its total output signal when subjected to a step change in temperature.

Total emissive power refers to the total amount of thermal radiation emitted by a surface (typically a black body) across all wavelengths per unit time and per unit area. This concept is governed by Stefan-Boltzmann law , which states: E = σT⁴ Where: E = Total emissive power σ = Stefan-Boltzmann constant T = Absolute temperature of the black body As per the definition, the total emissive power is expressed in units of watts per square meter (W/m²) , indicating that it is radiation emitted per unit area. Therefore, the correct answer is: Area (Option C).

Heat transfer takes place as per Second law of the thermodynamics it states that Heat transfer occurs spontaneously from higher- to lower-temperature bodies but never spontaneously in the reverse direction. The law states that it is impossible for any process to have as its sole result heat transfer from a cooler to a hotter object.

Thermal Diffusivity is defined as the ratio of heat conducted to the heat energy stored per unit volume of a material. Its SI unit is m 2 /s. It describes how fast heat can be transferred across the medium. Greater the value of thermal diffusivity, faster is the propagation of heat through the medium.

Thermal diffusivity. In heat transfer analysis, thermal diffusivity is the thermal conductivity divided by density and specific heat capacity at constant pressure. It measures the rate of transfer of heat of a material from the hot end to the cold end. It has the SI derived unit of m 2 /hr.