A sphere, a cube and a thin circular plate, all made of the same material and having the same mass are initially heated to the temperature of 250 degree Celsius. When left in air at room temperature, what will be their response to cooling?

They will cool at the same rate

Cube will cool faster than sphere but slower than the circular plate

They will cool at the same rate

Circular plate will cool at the slower rate

On a summer day, a scooter rider feels more comfortable while on the move than while at a stop light because

An object in motion captures less radiation

Air has a low specific heat and hence it is cooler

More heat is loss by convection and radiation while in motion

Air is transparent to radiation and hence it is cooler than the body

A fin will be more effective when Biot number is

\(between\;\frac{1}{4}\;and\;\frac{3}{4}\)

Prandtl number is expressed by

\(\frac{Inside\ diameter\ of\ tube}{equivalent\ thickness\ of\ film}\)

\(\frac{specific\ heat\ \times\ viscosity}{thermal\ conductivity}\)

\(\frac{thermal\ conductivity}{equivalent\ thickness\ of\ film}\)

\(\frac{molecular\ diffusivity\ of\ momentum }{thermal\ diffusivity}\)

Biot number signifies the ratio of

Inertia force to viscous force in the fluid

Convective resistance in the fluid to conductive resistance in the solid

Conductive resistance in the solid to convective resistance in the fluid

Inertia force to viscous force in the fluid

Buoyancy force to viscous force in the fluid

In a pipe, laminar flow in a fully developed region with constant heat flux from the pipe wall, bulk mean temperature of fluid

and pipe wall temperature decrease in the flow direction

and pipe wall temperature increase in the flow direction

and pipe wall temperature decrease in the flow direction

remains constant, but pipe wall temperature: increases. inflow direction

increases but pipe wall temperature remains constant

Nusselt number may be characterised as:

the ratio of the convective to the conductive resistance

the temperature (dimensional) gradient at the surface

the dimensionless temperature gradient at the surface

the ratio of the convective to the conductive resistance

the ratio of the rate of heat transfer due to convection to the rate of the store of the energy

Biot number signifies

the ratio of heat conducted to heat convected

the ratio of heat conducted to heat conducted

the ratio of heat convective resistance to internal conductive resistance

the ratio of internal conductive resistance to external convective resistance

For calculation of heat transfer by natural convection from a horizontal cylinder, what is the characteristic length in Grashof number?

Circumference of the base of the cylinder

Half the circumference of the base of the cylinder

Combined natural and forced convection analysis is done only when (Gr = Grashof number and Re = Reynold number)

\(0.1 < \frac{{{G_r}}}{{{R_e}}} < 10\)

\(0.1 < \frac{{{G_r}}}{{R_e^4}} < 10\)

\(0.1 < \frac{{{G_r}}}{{R_e^3}} < 10\)

\(0.1 < \frac{{{G_r}}}{{R_e^2}} < 10\)

\(0.1 < \frac{{{G_r}}}{{{R_e}}} < 10\)

For flow of viscous fluid over a flat plate, if the fluid temperature is the same as the plate temperature, the thermal boundary layer is

thinner than the velocity boundary layer

thicker than the velocity boundary layer

of the same thickness as the velocity boundary layer

Nukiyama’s Boiling curve is plotted between

Boiling temperature vs excess temperature

Boiling heat flux vs boiling temperature

Boiling temperature vs boiling pressure

Boiling heat flux vs excess temperature

Nusselt number is the ratio of

Wall heat transfer rate to mass heat flow rate

Convective and conductive resistances

Conductive and convective resistances

Wall heat transfer rate to mass heat flow rate

Kinetic viscosity to thermal diffusivity

For materials, where Prandtl number (Pr) 1

Heat diffuses at faster rate than momentum diffusion through the medium

Heat and momentum diffuses at almost same rate through the medium

Thermal boundary layer is much thicker than hydrodynamic boundary layer

Thermal boundary layer is much thinner than hydrodynamic boundary layer

86 + Mcqs in Types Convection in Heat Transfer Page 1 McqOptions

1.	A sphere, a cube and a thin circular plate, all made of the same material and having the same mass are initially heated to the temperature of 250 degree Celsius. When left in air at room temperature, what will be their response to cooling?
A.	Cube will cool faster than sphere but slower than the circular plate
B.	They will cool at the same rate
C.	Sphere will cool faster
D.	Circular plate will cool at the slower rate
Answer» B. They will cool at the same rate

Discussion

2.	On a summer day, a scooter rider feels more comfortable while on the move than while at a stop light because
A.	An object in motion captures less radiation
B.	Air has a low specific heat and hence it is cooler
C.	More heat is loss by convection and radiation while in motion
D.	Air is transparent to radiation and hence it is cooler than the body
Answer» E.

Discussion

3.	Large heat transfer coefficients for vapour condensation can be achieved by promoting
A.	Film condensation
B.	Dropwise condensation
C.	Cloud condensation
D.	Dew condensation
Answer» C. Cloud condensation

Discussion

4.	Air at 20°C blows over a hot plate of 50 × 60 cm2 made of carbon steel maintained at 220°C. The convective heat transfer coefficient is 25 W/m2.K. What will be the heat loss from the plate?
A.	1500 W
B.	2500 W
C.	3000 W
D.	4000 W
Answer» B. 2500 W

Discussion

5.	If heat and mass transfer take place simultaneously, the ratio of heat transfer coefficient to the mass transfer coefficient is a function of the ratio of
A.	Schmidt and Reynolds number
B.	Schmidt and Prandtl numbers
C.	Nusselt and Lewis numbers
D.	Reynolds and Lewis numbers
Answer» C. Nusselt and Lewis numbers

Discussion

6.	\(Nu = CR_e^mP_r^n\) represents heat transfer under
A.	Free convection
B.	Forced convection
C.	Combined convection
D.	None of the above
Answer» C. Combined convection

Discussion

7.	A fin will be more effective when Biot number is
A.	Greater than 1
B.	Equal to 1
C.	\(between\;\frac{1}{4}\;and\;\frac{3}{4}\)
D.	Less than 1
Answer» E.

Discussion

8.	Prandtl number is expressed by
A.	\(\frac{Inside\ diameter\ of\ tube}{equivalent\ thickness\ of\ film}\)
B.	\(\frac{specific\ heat\ \times\ viscosity}{thermal\ conductivity}\)
C.	\(\frac{thermal\ conductivity}{equivalent\ thickness\ of\ film}\)
D.	\(\frac{molecular\ diffusivity\ of\ momentum }{thermal\ diffusivity}\)
Answer» E.

Discussion

9.	For flow over a flat plate the hydrodynamic boundary layer thickness is 0.5 mm. The dynamic viscosity is 25 × 10-6 Pa-s, specific heat is 2.0 kJ/kgK and thermal conductivity is 0.05 W/mK. The thermal boundary layer thickness would be
A.	0.1 mm
B.	0.5 mm
C.	1 mm
D.	2 mm
Answer» C. 1 mm

Discussion

10.	Biot number signifies the ratio of
A.	Convective resistance in the fluid to conductive resistance in the solid
B.	Conductive resistance in the solid to convective resistance in the fluid
C.	Inertia force to viscous force in the fluid
D.	Buoyancy force to viscous force in the fluid
Answer» C. Inertia force to viscous force in the fluid

Discussion

11.	In a pipe, laminar flow in a fully developed region with constant heat flux from the pipe wall, bulk mean temperature of fluid
A.	and pipe wall temperature increase in the flow direction
B.	and pipe wall temperature decrease in the flow direction
C.	remains constant, but pipe wall temperature: increases. inflow direction
D.	increases but pipe wall temperature remains constant
Answer» B. and pipe wall temperature decrease in the flow direction

Discussion

12.	A fluid having a density of 1 g/cc is in a state with Grashof number 2 × 106 and Prandtl number 0.7. Assuming acceleration due to gravity as 10 m/s2, calculate the Rayleigh number?
A.	1.4 × 106
B.	2.86 × 106
C.	3.7 × 105
D.	8.4 × 106
Answer» B. 2.86 × 106

Discussion

13.	Hydrodynamic and thermal boundary layer thickness is equal when Prandtl number
A.	0
B.	< 1
C.	1
D.	> 1
Answer» D. > 1

Discussion

14.	Nusselt number may be characterised as:
A.	the temperature (dimensional) gradient at the surface
B.	the dimensionless temperature gradient at the surface
C.	the ratio of the convective to the conductive resistance
D.	the ratio of the rate of heat transfer due to convection to the rate of the store of the energy
Answer» C. the ratio of the convective to the conductive resistance

Discussion

15.	\(\frac{{Nusselt~number}}{{Reynold's~number \times Prandlt~number}}\)
A.	Weber number
B.	Stanton number
C.	Biot number
D.	Mach number
Answer» C. Biot number

Discussion

16.	Grashoff number has a significant role in heat transfer by
A.	Conduction
B.	Natural convection
C.	Forced convection
D.	Radiation
Answer» C. Forced convection

Discussion

17.	In a laminar developing flow through a pipe with constant wall temperature, the magnitude of the pipe wall inner surface convective heat transfer coefficient shall be maximum at the:
A.	Middle length of flow
B.	Beginning of flow
C.	End of flow
D.	None of the above
Answer» C. End of flow

Discussion

18.	Biot number signifies
A.	the ratio of heat conducted to heat convected
B.	the ratio of heat conducted to heat conducted
C.	the ratio of heat convective resistance to internal conductive resistance
D.	the ratio of internal conductive resistance to external convective resistance
Answer» E.

Discussion

19.	For calculation of heat transfer by natural convection from a horizontal cylinder, what is the characteristic length in Grashof number?
A.	Diameter of the cylinder
B.	Length of the cylinder
C.	Circumference of the base of the cylinder
D.	Half the circumference of the base of the cylinder
Answer» B. Length of the cylinder

Discussion

20.	Combined natural and forced convection analysis is done only when (Gr = Grashof number and Re = Reynold number)
A.	\(0.1 < \frac{{{G_r}}}{{R_e^4}} < 10\)
B.	\(0.1 < \frac{{{G_r}}}{{R_e^3}} < 10\)
C.	\(0.1 < \frac{{{G_r}}}{{R_e^2}} < 10\)
D.	\(0.1 < \frac{{{G_r}}}{{{R_e}}} < 10\)
Answer» D. \(0.1 < \frac{{{G_r}}}{{{R_e}}} < 10\)

Discussion

21.	Boiling point of water is ______.A. 210° FB. 212° FC. 214° FD. 208° F
A.	A
B.	B
C.	D
D.	C
Answer» C. D

Discussion

22.	In transition boiling, heat flux decreases due to which of the following?1. Low value of film heat transfer coefficient at the surface during 100°C to 120°C surface temperature.2. Major portion of heater surface is covered by vapour film which has smaller thermal conductivity as compared to liquid.3. Nucleate boiling occurs very fast.Select the correct answer using the code given below.
A.	1 only
B.	2 only
C.	3 only
D.	1, 2 and 3
Answer» C. 3 only

Discussion

23.	An average Nusselt number in laminar natural convection from a vertical wall at 180°C with still air at 20°C is found to be 48. If the wall temperature becomes 30°C, all other parameters remaining same, the average Nusselt number will be
A.	16
B.	8
C.	24
D.	32
Answer» D. 32

Discussion

24.	In the laminar flow of air (Pr = 0.7) over a heated plate if δ and δT denote, respectively, the hydrodynamic and thermal boundary layer thicknesses, then
A.	δ = δT
B.	δ > δT
C.	δ < δT
D.	δ = 0 but δT ≠ 0
Answer» D. δ = 0 but δT ≠ 0

Discussion

25.	Heat is lost from a 100 mm diameter steam pipe placed horizontally in ambient at 30°C. If the Nusselt number is 25 and thermal conductivity of air is 0.03 W/mK, then the heat transfer coefficient will be
A.	7.5 W/m2K
B.	16.2 W/m2K
C.	25.2 W/m2K
D.	30 W/m2K
Answer» B. 16.2 W/m2K

Discussion

26.	In the case of turbulent flow through a horizontal isothermal cylinder of diameter ‘D’, the free convection heat transfer coefficient for the cylinder will
A.	be independent of diameter
B.	vary as D3/4
C.	vary as D1/4
D.	vary as D1/2
Answer» B. vary as D3/4

Discussion

27.	For flow of viscous fluid over a flat plate, if the fluid temperature is the same as the plate temperature, the thermal boundary layer is
A.	thinner than the velocity boundary layer
B.	thicker than the velocity boundary layer
C.	of the same thickness as the velocity boundary layer
D.	not formed at all
Answer» E.

Discussion

28.	Natural convection heat transfer coefficients over surface of a vertical pipe and vertical flat plate for same height and fluid are equal. What is/are the possible reasons for this?1. Some height2. Both vertical3. Same fluid4. Same fluid flow patternSelect the correct answer using the code given below
A.	1 only
B.	1 and 2
C.	3 and 4
D.	4 only
Answer» B. 1 and 2

Discussion

29.	Consider a two-dimensional laminar flow over a long cylinder as shown in the figure below.The free stream velocity is U∞ and the free stream temperature T∞ is lower than the cylinder surface temperature Ts. The local heat transfer coefficient is minimum at point
A.	1
B.	2
C.	3
D.	4
Answer» C. 3

Discussion

30.	In liquid metals, thermal boundary layer develops much faster than velocity boundary layer due to
A.	lower value of Nusselt number
B.	higher value of Prandtl number
C.	lower value of Prandtl number
D.	higher value of Nusselt number
Answer» D. higher value of Nusselt number

Discussion

31.	Analogy between momentum and heat transfer is known as
A.	Stanton-Prandtl analogy
B.	Grassoff-Meyer analogy
C.	Chilton-Colburn analogy
D.	None of the above
Answer» D. None of the above

Discussion

32.	A coolant fluid at 30°C flows over a heated flat plate maintained at a constant temperature of 100°C. The boundary layer temperature distribution at a given location on the plate may be approximated as T = 30 + 70exp (−y) where y (in m) is the distance normal to the plate and T is in ° C. If thermal conductivity of the fluid is 1.0W/mK, the local convective heat transfer coefficient (in W/m2K) at that location will be
A.	0.2
B.	1
C.	5
D.	10
Answer» C. 5

Discussion

33.	Consider the following statements:For the laminar condensation on a vertical plate, the Nusselt theory says that1. inertia force in the film is negligible compared to viscosity and weight2. heat flow is mainly by conduction through the liquid film, convection in liquid film as well as in vapour is neglected.3. velocity of vapour is very highWhich of the above statements are CORRECT?
A.	1, 2 and 3
B.	1 and 2 only
C.	1 and 3 only
D.	2 and 3 only
Answer» C. 1 and 3 only

Discussion

34.	For a hydrodynamically and thermally fully developed laminar flow through a circular pipe of constant cross-section, the Nusselt number at constant wall heat flux (Nuq) and that at constant wall temperature (NuT) are related as
A.	Nuq > NuT
B.	Nuq = NuT
C.	Nuq < NuT
D.	Nuq = (NuT)2
Answer» B. Nuq = NuT

Discussion

35.	If \(q_w'' = 2500x,\) where x is in m and in the direction of flow (x = 0 at the inlet), the bulk mean temperature of the water leaving the pipe in °C is
A.	42
B.	62
C.	74
D.	104
Answer» C. 74

Discussion

36.	Boiling point of water at sea level is ______.
A.	210° F
B.	212° F
C.	208° F
D.	214° F
Answer» C. 208° F

Discussion

37.	Nukiyama’s Boiling curve is plotted between
A.	Boiling temperature vs excess temperature
B.	Boiling heat flux vs boiling temperature
C.	Boiling temperature vs boiling pressure
D.	Boiling heat flux vs excess temperature
Answer» E.

Discussion

38.	In liquid metal heat transfer for a fluid flowing through a tube, Nusselt number is a function of
A.	Reynold Number
B.	Prandtl Number
C.	Peclet Number
D.	Reynolds and Peclet Number
Answer» D. Reynolds and Peclet Number

Discussion

39.	In turbulent flow, one can estimate the Nusselt number using the analogy between heat and momentum transfer. (Colburn analogy). This analogy relates the Nusselt number to the co-coefficient of friction, Cf as.
A.	(Nu)=0.5Cf(Re)(Pr)1/3
B.	(Nu)=0.5Cf(Re)(Pr)2/3
C.	(Nu)=Cf.Re.(Pr)1/3
D.	(Nu)=Cf.Re1/2(Pr)1/3
Answer» B. (Nu)=0.5Cf(Re)(Pr)2/3

Discussion

40.	Nusselt number is the ratio of
A.	Convective and conductive resistances
B.	Conductive and convective resistances
C.	Wall heat transfer rate to mass heat flow rate
D.	Kinetic viscosity to thermal diffusivity
Answer» C. Wall heat transfer rate to mass heat flow rate

Discussion

41.	A dimensionless quantity that connects the link between velocity flow field and the temperature field is
A.	Nusselt number
B.	Prandtl number
C.	Reynolds number
D.	Grashof number
Answer» C. Reynolds number

Discussion

42.	For materials, where Prandtl number (Pr) 1
A.	Heat diffuses at faster rate than momentum diffusion through the medium
B.	Heat and momentum diffuses at almost same rate through the medium
C.	Thermal boundary layer is much thicker than hydrodynamic boundary layer
D.	Thermal boundary layer is much thinner than hydrodynamic boundary layer
Answer» E.

Discussion

43.	Nucleate boiling regime is formed approximately between[ΔTexcess = excess temperature]
A.	5°C ≤ ΔTexcess ≤ 10°C
B.	5°C ≤ ΔTexcess ≤ 50°C
C.	50°C ≤ ΔTexcess ≤ 80°C
D.	80°C ≤ ΔTexcess ≤ 100°C
Answer» C. 50°C ≤ ΔTexcess ≤ 80°C

Discussion

44.	Match List-I with List-II and select the correct answer:List-IList-IIAFourier number1Surface TensionBWeber number2Forced convectionCGrashoff number3Natural convectionDSchmid number4Radiation5Transient heat conduction6Mass Diffusion
A.	A-2, B-1, C-3, D-5
B.	A-5, B-1, C-3, D-6
C.	A-5, B-2, C-3, D-1
D.	A-5, B-1, C-3, D-4
Answer» C. A-5, B-2, C-3, D-1

Discussion

45.	Mist is formed as a result of:
A.	Condensation
B.	Circulation
C.	Evaporation
D.	Sublimation
Answer» B. Circulation

Discussion

46.	Laminar sub-layer acts as
A.	An insulating medium
B.	Good conductor of heat
C.	Refractory substance
D.	Heat absorber
Answer» B. Good conductor of heat

Discussion

47.	Match the followingP: Compressible flowU: Reynolds numberQ: Free surface flowV: Nusselt numberR: Boundary layer flowW: Weber numberS: Pipe flowX: Froude numberT: Heat convectionY: Mach number Z: Skin friction coefficient
A.	P-U; Q-X; R-V; S-Z; T-W
B.	P-W; Q-X; R-Z; S-U; T-V
C.	P-Y; Q-W; R-Z; S-U; T-X
D.	P-Y; Q-X; R-Z; S-U; T-V
Answer» E.

Discussion

48.	Given that Nu = Nusselt number, Re = Reynolds number, Pr = Prandtl number, Sh = Sherwood number, Sc = Schmidt number and Gr = Grashoff number. The number relationship for free convective heat transfer is given as,
A.	Nu = f(Gr, Pr)
B.	Sh = f(Sc, Gr)
C.	Nu = f(Re, Pr)
D.	Sh = f(Re, Sc)
Answer» B. Sh = f(Sc, Gr)

Discussion

49.	______ occurs when vapour pressure is less than pressure above a liquid surface at a given temperature.
A.	Freezing
B.	Condensation
C.	Boiling
D.	Gauging
Answer» D. Gauging

Discussion

50.	For laminar forced convection over a flat plate, if the free stream velocity increases by a factor of 2, the average heat transfer coefficient
A.	Remains same
B.	Decreases by a factor of √2
C.	Rises by a factor of √2
D.	Rises by a factor of 4
Answer» D. Rises by a factor of 4

Discussion

Explore topic-wise MCQs in Heat Transfer.

A sphere, a cube and a thin circular plate, all made of the same material and having the same mass are initially heated to the temperature of 250 degree Celsius. When left in air at room temperature, what will be their response to cooling?

On a summer day, a scooter rider feels more comfortable while on the move than while at a stop light because

Large heat transfer coefficients for vapour condensation can be achieved by promoting

Air at 20°C blows over a hot plate of 50 × 60 cm2 made of carbon steel maintained at 220°C. The convective heat transfer coefficient is 25 W/m2.K. What will be the heat loss from the plate?

If heat and mass transfer take place simultaneously, the ratio of heat transfer coefficient to the mass transfer coefficient is a function of the ratio of

\(Nu = CR_e^mP_r^n\) represents heat transfer under

A fin will be more effective when Biot number is

Prandtl number is expressed by

For flow over a flat plate the hydrodynamic boundary layer thickness is 0.5 mm. The dynamic viscosity is 25 × 10-6 Pa-s, specific heat is 2.0 kJ/kgK and thermal conductivity is 0.05 W/mK. The thermal boundary layer thickness would be

Biot number signifies the ratio of

In a pipe, laminar flow in a fully developed region with constant heat flux from the pipe wall, bulk mean temperature of fluid

A fluid having a density of 1 g/cc is in a state with Grashof number 2 × 106 and Prandtl number 0.7. Assuming acceleration due to gravity as 10 m/s2, calculate the Rayleigh number?

Hydrodynamic and thermal boundary layer thickness is equal when Prandtl number

Nusselt number may be characterised as:

\(\frac{{Nusselt~number}}{{Reynold's~number \times Prandlt~number}}\)

Grashoff number has a significant role in heat transfer by

In a laminar developing flow through a pipe with constant wall temperature, the magnitude of the pipe wall inner surface convective heat transfer coefficient shall be maximum at the:

Biot number signifies

For calculation of heat transfer by natural convection from a horizontal cylinder, what is the characteristic length in Grashof number?

Combined natural and forced convection analysis is done only when (Gr = Grashof number and Re = Reynold number)

Boiling point of water is ______.A. 210° FB. 212° FC. 214° FD. 208° F

An average Nusselt number in laminar natural convection from a vertical wall at 180°C with still air at 20°C is found to be 48. If the wall temperature becomes 30°C, all other parameters remaining same, the average Nusselt number will be

In the laminar flow of air (Pr = 0.7) over a heated plate if δ and δT denote, respectively, the hydrodynamic and thermal boundary layer thicknesses, then

Heat is lost from a 100 mm diameter steam pipe placed horizontally in ambient at 30°C. If the Nusselt number is 25 and thermal conductivity of air is 0.03 W/mK, then the heat transfer coefficient will be

In the case of turbulent flow through a horizontal isothermal cylinder of diameter ‘D’, the free convection heat transfer coefficient for the cylinder will

For flow of viscous fluid over a flat plate, if the fluid temperature is the same as the plate temperature, the thermal boundary layer is

Consider a two-dimensional laminar flow over a long cylinder as shown in the figure below.The free stream velocity is U∞ and the free stream temperature T∞ is lower than the cylinder surface temperature Ts. The local heat transfer coefficient is minimum at point

In liquid metals, thermal boundary layer develops much faster than velocity boundary layer due to

Analogy between momentum and heat transfer is known as

For a hydrodynamically and thermally fully developed laminar flow through a circular pipe of constant cross-section, the Nusselt number at constant wall heat flux (Nuq) and that at constant wall temperature (NuT) are related as

If \(q_w'' = 2500x,\) where x is in m and in the direction of flow (x = 0 at the inlet), the bulk mean temperature of the water leaving the pipe in °C is

Boiling point of water at sea level is ______.

Nukiyama’s Boiling curve is plotted between

In liquid metal heat transfer for a fluid flowing through a tube, Nusselt number is a function of

In turbulent flow, one can estimate the Nusselt number using the analogy between heat and momentum transfer. (Colburn analogy). This analogy relates the Nusselt number to the co-coefficient of friction, Cf as.

Nusselt number is the ratio of

A dimensionless quantity that connects the link between velocity flow field and the temperature field is

For materials, where Prandtl number (Pr) 1

Nucleate boiling regime is formed approximately between[ΔTexcess = excess temperature]

Match List-I with List-II and select the correct answer:List-IList-IIAFourier number1Surface TensionBWeber number2Forced convectionCGrashoff number3Natural convectionDSchmid number4Radiation5Transient heat conduction6Mass Diffusion

Mist is formed as a result of:

Laminar sub-layer acts as

Match the followingP: Compressible flowU: Reynolds numberQ: Free surface flowV: Nusselt numberR: Boundary layer flowW: Weber numberS: Pipe flowX: Froude numberT: Heat convectionY: Mach number Z: Skin friction coefficient

Given that Nu = Nusselt number, Re = Reynolds number, Pr = Prandtl number, Sh = Sherwood number, Sc = Schmidt number and Gr = Grashoff number. The number relationship for free convective heat transfer is given as,

______ occurs when vapour pressure is less than pressure above a liquid surface at a given temperature.

For laminar forced convection over a flat plate, if the free stream velocity increases by a factor of 2, the average heat transfer coefficient