Heat And Mass Transfer — Part 2 MCQs with Explanations

Thermocouple can easily sense temperature of 540°C or even upto 1200°C. For measuring any temperature above 1200°C a radiation pyrometer type thermometer is generally used because this type of temperature sensors is not required to be brought in touch with the object whose temperature to be measured.

The total radiation from a black body per second per unit area is Directly proportional to fourth power of the absolute temperature. This statement is known as Stefan Boltzmann law.

The use of heat exchangers is made in Radiators in automobile, Condensers and boilers in steam plants, Condensers and evaporators in refrigeration and air conditioning units

The heat transfer from a hot body to a cold body is directly proportional to the surface area and difference of temperatures between the two bodies. This statement is called Newton's law of cooling

Diathermanous body is a body which transmits all the incident radiation without absorbing or reflecting. (A body with transmitivity 1, reflectivity 0 and absorptivity 0) Air, up to certain extent can be considered as the diathermanous body because, it transmits all the radiation without hearing itself.

Planck's law is a formula for the spectral radiance of an object at a given temperature as a function of frequency (Lf) or wavelength (Lλ). Planck’s law holds good for black bodies. It has dimensions of power per solid angle per area per frequency or power per solid angle per area per wavelength.

Thermal conductivity of a material may be defined as the: - Quantity of heat flowing in one second through one cm cube of material when opposite faces are maintained at a temperature difference of 1°C. - Quantity of heat flowing in one second through a slab of the material of area one cm square, thickness 1 cm when its faces differ in temperature by 1°C. - Heat conducted in unit time across unit area through unit thickness when a temperature difference of unity is maintained between opposite faces.

A perfect blackbody is one that absorbs all incoming light and does not reflect any. At room temperature, such an object would appear to be perfectly black . However, if heated to a high temperature, a blackbody will begin to glow with thermal radiation

Film coefficient is defined as the ratio of Thermal conductivity to the equivalent thickness of the film of fluid The heat transferred by convection per unit area per degree temperature difference between the surface and the fluid.

Thermal diffusivity is the thermal conductivity of the material divided by the product of density and specific heat capacity, at constant pressure. The highest thermal diffusivity is of Lead

Nusselt number is the ratio of convection heat transfer to conduction heat transfer. Nu >> 1 , More effective Convection Nu = 1, For fluid layer represent heat transfer by pure conduction across layer

Thermal conductivity is a measure of a material’s ability to conduct heat. In the case of liquids like water, thermal conductivity depends on the molecular motion and interactions between the molecules. As temperature rises, the molecules in water gain kinetic energy and move more rapidly. This enhanced molecular activity leads to better energy transfer between molecules, resulting in an increase in thermal conductivity over most of the liquid temperature range. For water, thermal conductivity generally increases with temperature up to the boiling point. Therefore, the correct answer is: Increases (Option C).

Density of air does not increase with rise in temperature

Thermal conductivity of glass wool varies from sample to sample because of variation in Composition,Density and Porosity

Two plates spaced 150 mm apart are maintained at 1000°C and 70°C. The heat transfer will take place mainly by Radiation

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.

It is the radius of insulation before heat transfer increases and after heat transfer decreases. The critical radius is the insulation radius at which the resistance to heat flow is Minimum

The process of heat transfer from one particle of the fluid to another by the actual movement of the fluid particles caused by some mechanical means, is known as Forced convection Forced convection is a special type of heat transfer in which fluids are forced to move, in order to increase the heat transfer. This forcing can be done with a ceiling fan, a pump, suction device,

Planck's constant, symbolized h, relates the energy in one quantum (photon) of electromagnetic radiation to the frequency of that radiation. In the International System of units (SI), the constant is equal to approximately 6.626176 x 10-34 joule-seconds.

Kirchhoff's law states that the emissivity and the absorptivity of a surface at a given temperature and wavelength are equal. The ratio of the emissive power and absorptive power of all bodies is the same and is equal to the emissive power of a perfectly black body. This statement is known as Kirchhoff's law

In free convection heat transfer, Nusselt number is function of Grashoff number and Prandtl number the Nusselt number (Nu) is the ratio of convective to conductive heat transfer at a boundary in a fluid.

A regenerative heat exchanger, or more commonly a regenerator, is a type of heat exchanger where heat from the hot fluid is intermittently stored in a thermal storage medium before it is transferred to the cold fluid.

Heat Flow occurs when two object at different temperatures are in contact with each other. Heat flows from the warmer to the cooler object until they are both at the same temperature.

The ratio of the thickness of thermal boundary layer to the thickness of hydrodynamic boundary layer is equal to $$ - \frac{1}{3}.$$

Following Steps are use to reduce the thermal resistance: In conduction, reduction in the thickness of the material and an increase in thermal conductivity. In convection, stirring of the fluid and cleaning the heating surface. In radiation, increasing the temperature and reducing the emissivity.

When a fluid, such as air or a liquid, is heated and then travels away from the source, it carries the thermal energy along. This type of heat transfer is called convection.

Kirchhoff's law states that the emissivity and the absorptivity of a surface at a given temperature and wavelength are equal. The ratio of the emissive power and absorptive power of all bodies is the same and is equal to the emissive power of a perfectly black body. This statement is known as Kirchhoff's law

Thermal conductivity is a material property. It will not differ with the dimensions of a material, but it is dependent on the temperature, the density and the moisture content of the material. The thermal conductivity of a material depends on its temperature, density and moisture content.

The intensity of radiation is defined as the rate of emission of radiation in a given direction from a surface per unit solid angle and per unit projected area of a radiating surface on a plane perpendicular to the direction of radiation.

Radiation refers to the emission of energy in rays or waves. Heat moves through space as energy waves. It is the type of heat one feels when sitting in front of a fireplace or around a campfire.

Absorptive power is a measure of heat absorbed by an object. When heat is incident on the surface an object, a part of it is absorbed and the remaining part is reflected. Usually the absorbed radiation is converted to thermal energy, increasing the object's temperature.

Sensible Heat Ratio (SHR) - is used to describe the ratio of sensible heat load to total heat load and can be expressed as: $$\eqalign{ & {\text{SHR}} = \frac{{{{\text{q}}_{\text{s}}}}}{{{{\text{q}}_{\text{t}}}}} \cr & = \frac{{{{\text{q}}_{\text{s}}}}}{{{{\text{q}}_{\text{s}}} + {{\text{q}}_l}}}......\left( {\text{1}} \right) \cr} $$ where, SHR = sensible heat ratio (ratio of sensible heat load to total heat load) $${{{\text{q}}_{\text{s}}}}$$ = sensible heat load (kW, Btu/hr) $${{{\text{q}}_{\text{t}}}}$$ = total heat load - sensible heat and latent heat (kW, Btu/hr) $${{{\text{q}}_l}}$$ = latent heat load (kW, Btu/hr) For an air flow (1) can be modified to: $${\text{SHR}} = \frac{{{{\text{c}}_{\text{p}}}\left( {{{\text{t}}_{\text{o}}} - {{\text{t}}_{\text{i}}}} \right)}}{{{{\text{h}}_{\text{o}}} - {{\text{h}}_{\text{i}}}}}......\left( {\text{2}} \right)$$ where, $${{{\text{c}}_{\text{p}}}}$$ = specific heat air (1.005 kJ/kg°C, 0.240 Btu/lb°F) $${{{\text{t}}_{\text{o}}}}$$ = outlet air temperature (°C, °F) $${{{\text{t}}_{\text{i}}}}$$ = inlet air temperature (°C, °F) $${{{\text{h}}_{\text{o}}}}$$ = outlet moist air enthalpy (kJ/kg, Btu/lb) $${{{\text{h}}_{\text{i}}}}$$ = inlet moist air enthalpy (kJ/kg, Btu/lb)

Counter flow heat exchangers are inherently more efficient than parallel flow heat exchangers because they create a more uniform temperature difference between the fluids, over the entire length of the fluid path.

The automobile radiator is a heat exchanger of Cross flow type A radiator is a type of heat exchanger. It is designed to transfer heat from the hot coolant that flows through it to the air blown through it by the fan.

A grey body is an idealized physical body used in thermal radiation studies. It is defined as a body whose absorptivity and emissivity remain constant across all wavelengths and temperatures , although the value is less than 1 (unlike a black body which has a value of 1). Option A: Varies with temperature This is incorrect because a grey body’s absorptivity does not vary with temperature. It is assumed to remain constant for analysis purposes. Option B: Varies with wavelength of the incident ray This is incorrect. If the absorptivity varies with wavelength, the body is not grey. Grey body assumes constant absorptivity across all wavelengths. Option C: Is equal to its emissivity While Kirchhoff’s law states that emissivity equals absorptivity at thermal equilibrium, this is not the defining property of a grey body. This condition can apply to other bodies as well. Option D: Does not vary with temperature and wavelength of the incident ray This is correct. A grey body is defined as one whose absorptivity does not change with wavelength or temperature. Correct Answer: Option D: Does not vary with temperature and wavelength of the incident ray

In free convection heat transfer transition from laminar to turbulent flow is governed by the critical value of the Prandtl number, Grashoff's number

The process of heat transfer from one particle of the body to another is called conduction, when the particles of the body Do not move actually

Thermal diffusivity is defined as the ratio of thermal conductivity to the product of density and specific heat capacity at constant pressure. It indicates how quickly a material can conduct thermal energy relative to its ability to store it . Solids generally have much higher thermal conductivity compared to liquids and gases , and although their density and specific heat may also be higher, the conductivity dominates, resulting in higher thermal diffusivity. Liquids and gases typically have lower thermal conductivity and, in many cases, similar or lower specific heat, which makes their thermal diffusivity values lower than that of solids. Option A: Less than those for gases Incorrect. Gases usually have very low thermal diffusivity. Option B: Less than those for liquids Incorrect. Liquids have lower thermal diffusivity than most solids. Option C: More than those for liquids and gases Correct. Solids, particularly metals, typically have higher thermal diffusivity than both liquids and gases. Option D: More or less same as for liquids and gases Incorrect. Thermal diffusivity varies significantly among solids, liquids, and gases. Correct Answer: Option C: More than those for liquids and gases

When a fluid, such as air or a liquid, is heated and then travels away from the source, it carries the thermal energy along. This type of heat transfer is called convection.

Kirchhoff's law states that the emissivity and the absorptivity of a surface at a given temperature and wavelength are equal. The ratio of the emissive power and absorptive power of all bodies is the same and is equal to the emissive power of a perfectly black body. This statement is known as Kirchhoff's law.