*$_In free convection heat transfer transition from laminar to turbulent flow is governed by the critical value of the?

Reynold's number, Grashoff's number

Prandtl number, Grashoff's number

*$_Thermal conductivity of non-metallic amorphous solids with decrease in temperature?

May increase or decrease depending on temperature

*$_Metals are good conductors of heat because?

Their atoms are relatively far apart

*$_The insulation ability of an insulator with the presence of moisture would?

May increase/decrease depending on temperature and thickness of insulation

*$_Unit of thermal conductivity in M.K.S. units is?

K cal/hr m¬¨‚â§ ¬¨‚àûC

K cal/kg m¬¨‚â§ ¬¨‚àûC

K cal m/hr m¬¨‚â§ ¬¨‚àûC

K cal/hr m¬¨‚â§ ¬¨‚àûC

*$_Thermal conductivity of solid metals with rise in temperature normally?

May increase or decrease depending on temperature

*/*_In free convection heat transfer, Nusselt number is function of?

Prandtl number and Reynold number

Grashoff number and Reynold number

Grashoff number and Prandtl number

Prandtl number and Reynold number

Grashoff number, Prandtl number and Reynold number

*/*_A perfect black body is one which?

Absorbs heat radiations of all wave lengths falling on it

*/*_Thermal diffusivity is?

A physical property of the material

*/*_According to Prevost theory of heat exchange?

Heat transfer in most of the cases takes place by combination of conduction, convection and radiation

It is impossible to transfer heat from low temperature source to t high temperature source

Heat transfer by radiation requires no medium

All bodies above absolute zero emit radiation

Heat transfer in most of the cases takes place by combination of conduction, convection and radiation

*/*_According to Newton's law of cooling, the heat transfer from a hot body to a cold body is?

Directly proportional to the surface area

Directly proportional to the difference of temperatures between the two bodies

*/*_The energy distribution of an ideal reflector at higher temperatures is largely in the range of?

Wavelength has nothing to do with it

*/*_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

*/*_Heat transfer takes place as per?

Second law of the thermodynamics

_ A grey body is one whose absorptivity$?

Does not vary with temperature and. wavelength of the incident ray

Varies with wavelength of the incident ray

Does not vary with temperature and. wavelength of the incident ray

_ Moisture would find its way into insulation by vapour pressure unless it is prevented by$?

Less thermal conductivity insulator

_ The critical temperature is the temperature$?

Below which a gas does not obey gas laws

Below which a gas is always liquefied

Above which a gas will never liquefied

_ Kirchhoff's law states that$?

The total radiation from a black body per second per unit area is directly proportional to the fourth power of the absolute temperature

The wave length corresponding to the maximum energy is proportional to the absolute temperature

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

_ Thermal diffusivity of a substance is$?

Directly proportional to the thermal conductivity

Inversely proportional to density of substance

Inversely proportional to specific heat

_ The heat transfer by conduction through a thick cylinder (Q) is given by (where T‚Äö√á√Ö = Higher temperature, T‚Äö√á√á = Lower temperature, r‚Äö√á√Ö = Inside radius, r‚Äö√á√á = Outside radius, l = Length of cylinder, and k = Thermal conductivity)$?

Q = 2.3 log (r‚Äö√á√á/r‚Äö√á√Ö)/[2≈ì√Ñlk (T‚Äö√á√Ö - T‚Äö√á√á)]

Q = [2≈ì√Ñlk (T‚Äö√á√Ö - T‚Äö√á√á)]/2.3 log (r‚Äö√á√á/r‚Äö√á√Ö)

Q = 2.3 log (r‚Äö√á√á/r‚Äö√á√Ö)/[2≈ì√Ñlk (T‚Äö√á√Ö - T‚Äö√á√á)]

Q = [2≈ì√Ñ (T‚Äö√á√Ö - T‚Äö√á√á)]/2.3 lk log (r‚Äö√á√á/r‚Äö√á√Ö)

Q = = 2≈ì√Ñlk/2.3 (T‚Äö√á√Ö - T‚Äö√á√á) log (r‚Äö√á√á/r‚Äö√á√Ö)

_ Which of the following would lead to a reduction in 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.

_ According to Stefan Boltzmann law, ideal radiators emit radiant energy at a rate proportional to$?

Fourth power of absolute temperature

_ Thermal conductivity of water _________ with rise in temperature.$?

May increase or decrease depending upon temperature

_ 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¬¨‚àûC

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¬¨‚àûC

Heat conducted in unit time across unit area through unit thickness when a temperature difference of unity is maintained between opposite faces

_ The use of heat exchangers is made in$?

Condensers and boilers in steam plants

Condensers and evaporators in refrigeration and air conditioning units

362 + Mcqs in Heat And Mass Transfer in Mechanical Engineering Page 5 McqOptions

201.	*$_Log mean temperature difference in case of counter flow compared to parallel flow will be?
A.	Same
B.	More
C.	Less
D.	Depends on other factors
Answer» C. Less

202.	*$_In free convection heat transfer transition from laminar to turbulent flow is governed by the critical value of the?
A.	Reynold's number
B.	Grashoff's number
C.	Reynold's number, Grashoff's number
D.	Prandtl number, Grashoff's number
Answer» E.

203.	*$_The ratio of surface convection resistance to the internal conduction resistance is known as?
A.	Grashoff number
B.	Biot number
C.	Stanton number
D.	Prandtl number
Answer» C. Stanton number

204.	*$_Thermal conductivity of non-metallic amorphous solids with decrease in temperature?
A.	Increases
B.	Decreases
C.	Remain constant
D.	May increase or decrease depending on temperature
Answer» C. Remain constant

205.	*$_Two long parallel surfaces each of emissivity 0.7 are maintained at different temperatures and accordingly have radiation heat exchange between them. It is desired to reduce 75% of the radiant heat transfer by inserting thin parallel shields of emissivity 1 on both sides. The number of shields should be?
A.	One
B.	Two
C.	Three
D.	Four
Answer» D. Four

206.	*$_The emissivity for a black body is?
A.	0
B.	0.5
C.	0.75
D.	1
Answer» E.

207.	*$_Metals are good conductors of heat because?
A.	Their atoms collide frequently
B.	Their atoms are relatively far apart
C.	They contain free electrons
D.	They have high density
Answer» B. Their atoms are relatively far apart

208.	*$_An electric cable of aluminium conductor (k = 240 W/mK) is to be insulated with rubber (k = 0.15 W/mK). The cable is to be located in air (h = 6 W/m¬¨‚â§). The critical thickness of insulation will be?
A.	25 mm
B.	40 mm
C.	160 mm
D.	800 mm
Answer» B. 40 mm

209.	*$_The insulation ability of an insulator with the presence of moisture would?
A.	Increase
B.	Decrease
C.	Remain unaffected
D.	May increase/decrease depending on temperature and thickness of insulation
Answer» C. Remain unaffected

210.	*$_Thermal diffusivity of a substance is given by (where h = Thermal diffusivity, ≈ì√Ö = Density of substance, S = Specific heat, and k = Thermal conductivity)?
A.	h = k/ ≈ì√ÖS
B.	h = ≈ì√ÖS/k
C.	h = S/≈ì√Ök
D.	h = k≈ì√Ö/S
Answer» B. h = ≈ì√ÖS/k

211.	*$_Unit of thermal conductivity in M.K.S. units is?
A.	K cal/kg m¬¨‚â§ ¬¨‚àûC
B.	K cal m/hr m¬¨‚â§ ¬¨‚àûC
C.	K cal/hr m¬¨‚â§ ¬¨‚àûC
D.	K calm/hr ¬¨‚àûC
Answer» C. K cal/hr m¬¨‚â§ ¬¨‚àûC

213.	/_Heat transfer in liquid and gases takes place by?
A.	Conduction
B.	Convection
C.	Radiation
D.	Conduction and convection
Answer» C. Radiation

220.	/_The expression Q = ≈ì√Ö AT4 is called?
A.	Fourier equation
B.	Stefan-Boltzmann equation
C.	Newton Reichmann equation
D.	Joseph-Stefan equation
Answer» C. Newton Reichmann equation

223.	/_When heat is transferred from one particle of hot body to another by actual motion of the heated particles, it is referred to as heat transfer by?
A.	Conduction
B.	Convection
C.	Radiation
D.	Conduction and convection
Answer» B. Convection

224.	/_The most commonly used method for the design of duct size is the?
A.	Velocity reduction method
B.	Equal friction method
C.	Static regains method
D.	Dual or double method
Answer» D. Dual or double method

212.	*$_Thermal conductivity of solid metals with rise in temperature normally?
A.	Increases
B.	Decreases
C.	Remain constant
D.	May increase or decrease depending on temperature
Answer» C. Remain constant

214.	/_In free convection heat transfer, Nusselt number is function of?
A.	Grashoff number and Reynold number
B.	Grashoff number and Prandtl number
C.	Prandtl number and Reynold number
D.	Grashoff number, Prandtl number and Reynold number
Answer» C. Prandtl number and Reynold number

215.	/_The critical radius is the insulation radius at which the resistance to heat flow is?
A.	Maximum
B.	Minimum
C.	Zero
D.	None of these
Answer» C. Zero

216.	/_Which of the following property of air does not increase with rise in temperature??
A.	Thermal conductivity
B.	Thermal diffusivity
C.	Density
D.	Dynamic viscosity
Answer» D. Dynamic viscosity

217.	/_A perfect black body is one which?
A.	Is black in colour
B.	Reflects all heat
C.	Transmits all heat radiations
D.	Absorbs heat radiations of all wave lengths falling on it
Answer» E.

218.	/_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?
A.	First law of thermodynamics
B.	Newton's law of cooling
C.	Newton's law of heating
D.	Stefan's law
Answer» C. Newton's law of heating

219.	/_Thermal diffusivity is?
A.	A dimensionless parameter
B.	Function of temperature
C.	Used as mathematical model
D.	A physical property of the material
Answer» E.

221.	/_According to Prevost theory of heat exchange?
A.	It is impossible to transfer heat from low temperature source to t high temperature source
B.	Heat transfer by radiation requires no medium
C.	All bodies above absolute zero emit radiation
D.	Heat transfer in most of the cases takes place by combination of conduction, convection and radiation
Answer» D. Heat transfer in most of the cases takes place by combination of conduction, convection and radiation

222.	/_A non-dimensional number generally associated with natural convection heat transfer is?
A.	Grashoff number
B.	Nusselt number
C.	Weber number
D.	Prandtl number
Answer» B. Nusselt number

225.	/_The product of Reynolds number and Prandtl number is known as?
A.	Stanton number
B.	Biot number
C.	Peclet number
D.	Grashoff number
Answer» D. Grashoff number

Explore topic-wise MCQs in Mechanical Engineering.

*$_Log mean temperature difference in case of counter flow compared to parallel flow will be?

*$_In free convection heat transfer transition from laminar to turbulent flow is governed by the critical value of the?

*$_The ratio of surface convection resistance to the internal conduction resistance is known as?

*$_Thermal conductivity of non-metallic amorphous solids with decrease in temperature?

*$_The emissivity for a black body is?

*$_Metals are good conductors of heat because?

*$_An electric cable of aluminium conductor (k = 240 W/mK) is to be insulated with rubber (k = 0.15 W/mK). The cable is to be located in air (h = 6 W/m¬¨‚â§). The critical thickness of insulation will be?

*$_The insulation ability of an insulator with the presence of moisture would?

*$_Thermal diffusivity of a substance is given by (where h = Thermal diffusivity, ≈ì√Ö = Density of substance, S = Specific heat, and k = Thermal conductivity)?

*$_Unit of thermal conductivity in M.K.S. units is?

*$_Thermal conductivity of solid metals with rise in temperature normally?

*/*_Heat transfer in liquid and gases takes place by?

*/*_In free convection heat transfer, Nusselt number is function of?

*/*_The critical radius is the insulation radius at which the resistance to heat flow is?

*/*_Which of the following property of air does not increase with rise in temperature??

*/*_A perfect black body is one which?

*/*_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?

*/*_Thermal diffusivity is?

*/*_The expression Q = ≈ì√Ö AT4 is called?

*/*_According to Prevost theory of heat exchange?

*/*_A non-dimensional number generally associated with natural convection heat transfer is?

*/*_When heat is transferred from one particle of hot body to another by actual motion of the heated particles, it is referred to as heat transfer by?

*/*_The most commonly used method for the design of duct size is the?

*/*_The product of Reynolds number and Prandtl number is known as?

*/*_According to Newton's law of cooling, the heat transfer from a hot body to a cold body is?

*/*_The energy distribution of an ideal reflector at higher temperatures is largely in the range of?

*/*_The ratio of the energy absorbed by the body to total energy falling on it is called?

*/*_The amount of heat flow through a body by conduction is?

*/*_Heat transfer takes place as per?

_ The transfer of heat by molecular collision is known as$?

_ Depending on the radiating properties, a body will be opaque when$?

_ A grey body is one whose absorptivity$?

_ Moisture would find its way into insulation by vapour pressure unless it is prevented by$?

_ The critical temperature is the temperature$?

_ The thermal diffusivities for gases are generally$?

_ The thickness of thermal and hydrodynamic boundary layer is equal if Prandtl number is$?

_ Kirchhoff's law states that$?

_ The concept of overall coefficient of heat transfer is used in heat transfer problems of$?

_ Thermal diffusivity of a substance is$?

_ The heat transfer by conduction through a thick cylinder (Q) is given by (where T‚Äö√á√Ö = Higher temperature, T‚Äö√á√á = Lower temperature, r‚Äö√á√Ö = Inside radius, r‚Äö√á√á = Outside radius, l = Length of cylinder, and k = Thermal conductivity)$?

_ The critical thickness of insulation for a sphere is$?

_ Pick up the wrong case. Heat flowing from one side to other depends directly on$?

_ Which of the following would lead to a reduction in thermal resistance?$?

_ 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$?

_ According to Stefan Boltzmann law, ideal radiators emit radiant energy at a rate proportional to$?

_ Thermal conductivity of water _________ with rise in temperature.$?

_ Thermal conductivity of a material may be defined as the$?

_ The use of heat exchangers is made in$?

/_Heat transfer in liquid and gases takes place by?

/_In free convection heat transfer, Nusselt number is function of?

/_The critical radius is the insulation radius at which the resistance to heat flow is?

/_Which of the following property of air does not increase with rise in temperature??

/_A perfect black body is one which?

/_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?

/_Thermal diffusivity is?

/_The expression Q = ≈ì√Ö AT4 is called?

/_According to Prevost theory of heat exchange?

/_A non-dimensional number generally associated with natural convection heat transfer is?

/_When heat is transferred from one particle of hot body to another by actual motion of the heated particles, it is referred to as heat transfer by?

/_The most commonly used method for the design of duct size is the?

/_The product of Reynolds number and Prandtl number is known as?

/_According to Newton's law of cooling, the heat transfer from a hot body to a cold body is?

/_The energy distribution of an ideal reflector at higher temperatures is largely in the range of?

/_The ratio of the energy absorbed by the body to total energy falling on it is called?

/_The amount of heat flow through a body by conduction is?

/_Heat transfer takes place as per?

226.	/_According to Newton's law of cooling, the heat transfer from a hot body to a cold body is?
A.	Directly proportional to the surface area
B.	Directly proportional to the difference of temperatures between the two bodies
C.	Either (A) or (B)
D.	Both (A) and (B)
Answer» E.

227.	/_The energy distribution of an ideal reflector at higher temperatures is largely in the range of?
A.	Shorter wavelength
B.	Longer wavelength
C.	Remain same at all wavelengths
D.	Wavelength has nothing to do with it
Answer» B. Longer wavelength

228.	/_A designer chooses the values of fluid flow rates and specific heats in such a manner that the heat capacities of the two fluids are equal. A hot fluid enters the counter flow heat exchanger at 100¬¨‚àûC and leaves at 60¬¨‚àûC. A cold fluid enters the heat exchanger at 40¬¨‚àûC. The mean temperature difference between the two fluids is?
A.	20¬¨‚àûC
B.	40¬¨‚àûC
C.	60¬¨‚àûC
D.	66.7¬¨‚àûC
Answer» B. 40¬¨‚àûC

229.	/_The ratio of the energy absorbed by the body to total energy falling on it is called?
A.	Absorptive power
B.	Emissive power
C.	Absorptivity
D.	Emissivity
Answer» B. Emissive power

230.	/_The amount of heat flow through a body by conduction is?
A.	Directly proportional to the surface area of the body
B.	Directly proportional to the temperature difference on the two faces of the body
C.	Dependent upon the material of the body
D.	All of the above
Answer» E.

231.	/_Heat transfer takes place as per?
A.	Zeroth law of thermodynamics
B.	First law of thermodynamic
C.	Second law of the thermodynamics
D.	Kirchoff's law
Answer» D. Kirchoff's law

232.	_ The transfer of heat by molecular collision is known as$?
A.	Conduction
B.	Convection
C.	Radiation
D.	None of these
Answer» C. Radiation

233.	_ Depending on the radiating properties, a body will be opaque when$?
A.	P = 0, x = 0 and a = 1
B.	P=1, x = 0 and a = 0
C.	P = 0, x = 1 and a = 0
D.	X = 0, a + p = 1
Answer» E.

234.	_ A grey body is one whose absorptivity$?
A.	Varies with temperature
B.	Varies with wavelength of the incident ray
C.	Is equal to its emissivity
D.	Does not vary with temperature and. wavelength of the incident ray
Answer» D. Does not vary with temperature and. wavelength of the incident ray

235.	_ Moisture would find its way into insulation by vapour pressure unless it is prevented by$?
A.	High thickness of insulation
B.	High vapour pressure
C.	Less thermal conductivity insulator
D.	A vapour seal
Answer» E.