Heat and Mass Transfer Exam

Question 1

The variation of thermal conductivity of a metal with temperature is often correlated using an expression of the form
`k = k_o + aT`.
Where `k` is the thermal conductivity, and `T` is the temperature in `K`. The units of '`a`' in the SI system will be

(a) `W/m·K`
(b) `W/m`
(c) `W/m·K²`
(d) None; '`a`' is just a number

Answer: C

Question 2

Walls of a cubical oven are of thickness `L`, and they are made of material of thermal conductivity `k`. The temperature inside the oven is `100 °C` and the inside heat transfer coefficient is `3k/L`. If the wall temperature on the outside is held at `25 °C` what is the inside wall temperature in degrees `C`?

(a) `35.5`
(b) `43.75`
(c) `81.25`
(d) `48.25`

Answer: C

Question 3

The heat flux (from outside to inside) across an insulating wall with thermal conductivity `k = 0.04 W/m·K` and thickness `0.16 m` is `10 W/m²`. The temperature of the inside wall is `-5 °C`. The outside wall temperature is

(a) `25 °C`
(b) `30 °C`
(c) `35 °C`
(d) `40 °C`

Answer: C

Question 4

A composite flat wall of a furnace is made of two materials `A` and `B`. The thermal conductivity of `A` is twice of that of material `B`, while the thickness of layer `A` is half of that of `B`. If the temperature at the two sides of the wall are `400` and `1200 K`, then the temperature drop (in `K`) across the layer of material `A` is

(a) `125`
(b) `133`
(c) `150`
(d) `160`

Answer: D

Question 5

The critical radius `r` of insulation on a pipe is given by

(a) `r = 2k/h`
(b) `r = k/h`
(c) `r = k/(2h)`
(d) `r = h/k`

Answer: B

Question 6

The thermal conductivity is minimum for:

(a) Silver
(b) Chrome-nickel steel
(c) Aluminum
(d) Carbon steel

Answer: B

Question 7

A layer of insulation is applied on a spherical metallic tank containing a cryogenic liquid. The rate of heat leakage into the tank

(a) always decreases with increase in insulation thickness
(b) increases with insulation thickness and remains constant beyond a critical thickness
(c) may increase upto a thickness and then decreases, depending upon the thermal conductivity of the insulating material
(d) may increase upto a thickness and then decrease depending upon the thermal conductivity of the metal

Answer: C

Question 8

A metal wire of `0.01 m` dia and thermal conductivity `200 W/m·K` is exposed to a fluid stream with a convective heat transfer coefficient `1000 W/m²·K`. The Biot number is

(a) `5.6`
(b) `0.0125`
(c) `3.5`
(d) `0.0035`

Answer: B

Question 9

In pipe flow, heat is transferred from hot wall to the liquid by

(a) conduction only
(b) forced convection only
(c) forced convection and conduction
(d) free and forced convection

Answer: C

Question 10

Heat transfer occurs by natural convection because change in temperature causes differences in

(a) viscosity
(b) density
(c) thermal conductivity
(d) heat capacity

Answer: B

Question 11

The Grashof number is defined as the ratio of

(a) buoyancy to inertial forces
(b) bouyancy to viscous forces
(c) inertial to viscous forces
(d) bouyancy to surface tension forces

Answer: B

Question 12

For turbulent flow in a tube, the heat transfer coefficient is obtained from Dittus-Boelter correlation. If the tube diameter is halved and the flow rate is doubled, then the heat transfer coefficient will change by a factor of

(a) `1`
(b) `1.74`
(c) `6.1`
(d) `37`

Answer: C

Question 13

In natural convection heat transfer the correlating parameter is

(a) Graetz number
(b) Eckert number
(c) Grashof number
(d) Bond number

Answer: C

Question 14

The hydrodynamic and thermal boundary layers will merge when

(a) Prandtl number is one
(b) Schmidt number tends to infinity
(c) Nusselt number tends to infinity
(d) Archimedes number is greater than 10,000

Answer: A

Question 15

When the ratio of the Grashof number to the square of the Reynolds number is one, the dominant mechanism of heat transfer is

(a) free convection
(b) entry length problem in laminar forced convection (developing thermal boundary layer)
(c) mixed convection (both free and forced)
(d) forced convection

Answer: C

Question 16

Prandtl number is the ratio of

(a) mass diffusivity to thermal diffusivity
(b) momentum diffusivity to thermal diffusivity
(c) thermal diffusivity to mass diffusivity
(d) thermal diffusivity to momentum diffusivity

Answer: B

Question 17

The widely used Dittus-Boelter equation is valid provided

(a) `2100 < Re < 10,000` and the properties of the fluid are evaluated at the average film temperature
(b) `Re < 2100` and the properties of the fluid are evaluated at the bulk temperature
(c) `10,000 < Re < 120,000` and the fluid properties are evaluated at the bulk temperature
(d) none of the above

Answer: C

Question 18

Grashof number is associated with:

(a) bouyancy effects
(b) free convection
(c) forced convection
(d) high temperature difference

Answer: B

Question 19

The rate of diffusion of momentum relative to the rate of diffusion of heat is:

(a) proportional to Prandtl number
(b) inversely proportional to Prandtl number
(c) Proportional to the Colburn's `j_H` factor
(d) proportional to Stanton number

Answer: A

Question 20

Heat transfer coefficient in a helical coil compared to that in a striaght pipe is:

(a) Lower
(b) Higher
(c) Same

Answer: B

Question 21

To determine the heat transfer coefficient Dittus-Boelter equation is valid for:

(a) Laminar flow
(b) For liquid metals
(c) Turbulent flow
(d) Natural convection

Answer: C

Question 22

Heat transfer by natural convection is enhanced in systems with

(a) high viscosity
(b) high coefficient of thermal expansion
(c) low temperature gradients
(d) low density change with temperature

Answer: B

Question 23

The Sieder-Tate correlation for heat transfer in turbulent flow in a pipe gives `Nu ∝ Re^0.8`, where `Nu` is the Nusselt number and `Re` is the Reynolds number for the flow. Assuming that this relation is valid, the heat transfer coefficient varies with the pipe diameter (`D`) as

(a) `D^-1.8`
(b) `D^-0.2`
(c) `D^0.2`
(d) `D^1.8`

Answer: B

Question 24

As the difference between the wall temperature and bulk temperature increases, the boiling heat transfer coefficient

(a) continues to inrease
(b) continues to decrease
(c) goes through a minimum
(d) goes through a maximum

Answer: D

Question 25

Gibbs phase rule finds application when heat transfer occurs by

(a) conduction
(b) convection
(c) radiation
(d) condensation

Answer: D

Question 26

Saturated steam is condensed as a film on a vertical plate of height `50 cm`. If the local heat transfer coefficient at a distance of `50 cm` from the top is `7200 W/m²·°C` the average heat transfer coefficient over the entire plate in `W/m²·°C` is

(a) `5400`
(b) `7200`
(c) `9600`
(d) `14400`

Answer: C

Question 27

For condensation of pure vapours, if the heat transfer coefficients in filmwise and dropwise condensation are respectively `h_f` and `h_d`, then

(a) `h_f = h_d`
(b) `h_f > h_d`
(c) `h_f < h_d`
(d) `h_f could be greater or smaller than h_d`

Answer: C

Question 28

In a heat exchanger, floating head is provided to

(a) facilitate cleaning of the exchanger
(b) increase the heat transfer area
(c) relieve stresses caused by thermal expansion
(d) increase the log mean temperature gradient

Answer: C

Question 29

The advantage of using a 1-2 shell and tube heat exchanger over a 1-1 shell and tube heat exchanger is

(a) Lower tube side pressure drop
(b) Lower shell side pressure drop
(c) Higher tube side heat transfer coefficient
(d) Higher shell side heat trabsfer coefficient

Answer: C

Question 30

In a finned tube heat exchanger:

(a) only heat transfer area is augmented
(b) only film coefficient is augmented
(c) both heat transfer area and film coefficient are augmented
(d) none of the above

Answer: A

Heat and Mass Transfer

The heat flux (from outside to inside) across an insulating wall with thermal conductivity k = 0.04 W/m·K and thickness 0.16 m is 10 W/m2. The temperature of the inside wall is -5 °C. The outside wall temperature is (a) 25 °C (b) 30 °C (c) 35 °C (d) 40 °C

The critical radius r of insulation on a pipe is given by (a) r = 2k/h (b) r = k/h (c) r = k/(2h) (d) r = h/k

The thermal conductivity is minimum for: (a) Silver (b) Chrome-nickel steel (c) Aluminum (d) Carbon steel

A metal wire of 0.01 m dia and thermal conductivity 200 W/m·K is exposed to a fluid stream with a convective heat transfer coefficient 1000 W/m2·K. The Biot number is (a) 5.6 (b) 0.0125 (c) 3.5 (d) 0.0035

In pipe flow, heat is transferred from hot wall to the liquid by (a) conduction only (b) forced convection only (c) forced convection and conduction (d) free and forced convection

Heat transfer occurs by natural convection because change in temperature causes differences in (a) viscosity (b) density (c) thermal conductivity (d) heat capacity

The Grashof number is defined as the ratio of (a) buoyancy to inertial forces (b) bouyancy to viscous forces (c) inertial to viscous forces (d) bouyancy to surface tension forces

For turbulent flow in a tube, the heat transfer coefficient is obtained from Dittus-Boelter correlation. If the tube diameter is halved and the flow rate is doubled, then the heat transfer coefficient will change by a factor of (a) 1 (b) 1.74 (c) 6.1 (d) 37

In natural convection heat transfer the correlating parameter is (a) Graetz number (b) Eckert number (c) Grashof number (d) Bond number

The hydrodynamic and thermal boundary layers will merge when (a) Prandtl number is one (b) Schmidt number tends to infinity (c) Nusselt number tends to infinity (d) Archimedes number is greater than 10,000

Prandtl number is the ratio of (a) mass diffusivity to thermal diffusivity (b) momentum diffusivity to thermal diffusivity (c) thermal diffusivity to mass diffusivity (d) thermal diffusivity to momentum diffusivity

Grashof number is associated with: (a) bouyancy effects (b) free convection (c) forced convection (d) high temperature difference

The rate of diffusion of momentum relative to the rate of diffusion of heat is: (a) proportional to Prandtl number (b) inversely proportional to Prandtl number (c) Proportional to the Colburn's jH factor (d) proportional to Stanton number

Heat transfer coefficient in a helical coil compared to that in a striaght pipe is: (a) Lower (b) Higher (c) Same

To determine the heat transfer coefficient Dittus-Boelter equation is valid for: (a) Laminar flow (b) For liquid metals (c) Turbulent flow (d) Natural convection

Heat transfer by natural convection is enhanced in systems with (a) high viscosity (b) high coefficient of thermal expansion (c) low temperature gradients (d) low density change with temperature

As the difference between the wall temperature and bulk temperature increases, the boiling heat transfer coefficient (a) continues to inrease (b) continues to decrease (c) goes through a minimum (d) goes through a maximum

Gibbs phase rule finds application when heat transfer occurs by (a) conduction (b) convection (c) radiation (d) condensation

Saturated steam is condensed as a film on a vertical plate of height 50 cm. If the local heat transfer coefficient at a distance of 50 cm from the top is 7200 W/m2·°C the average heat transfer coefficient over the entire plate in W/m2·°C is (a) 5400 (b) 7200 (c) 9600 (d) 14400

For condensation of pure vapours, if the heat transfer coefficients in filmwise and dropwise condensation are respectively hf and hd, then (a) hf = hd (b) hf > hd (c) hf < hd (d) hf could be greater or smaller than hd

In a heat exchanger, floating head is provided to (a) facilitate cleaning of the exchanger (b) increase the heat transfer area (c) relieve stresses caused by thermal expansion (d) increase the log mean temperature gradient

The advantage of using a 1-2 shell and tube heat exchanger over a 1-1 shell and tube heat exchanger is (a) Lower tube side pressure drop (b) Lower shell side pressure drop (c) Higher tube side heat transfer coefficient (d) Higher shell side heat trabsfer coefficient

In a finned tube heat exchanger: (a) only heat transfer area is augmented (b) only film coefficient is augmented (c) both heat transfer area and film coefficient are augmented (d) none of the above

The heat flux (from outside to inside) across an insulating wall with thermal conductivity `k = 0.04 W/m·K` and thickness `0.16 m` is `10 W/m²`. The temperature of the inside wall is `-5 °C`. The outside wall temperature is

(a) `25 °C`
(b) `30 °C`
(c) `35 °C`
(d) `40 °C`

The critical radius `r` of insulation on a pipe is given by

(a) `r = 2k/h`
(b) `r = k/h`
(c) `r = k/(2h)`
(d) `r = h/k`

The thermal conductivity is minimum for:

(a) Silver
(b) Chrome-nickel steel
(c) Aluminum
(d) Carbon steel

A metal wire of `0.01 m` dia and thermal conductivity `200 W/m·K` is exposed to a fluid stream with a convective heat transfer coefficient `1000 W/m²·K`. The Biot number is

(a) `5.6`
(b) `0.0125`
(c) `3.5`
(d) `0.0035`

In pipe flow, heat is transferred from hot wall to the liquid by

(a) conduction only
(b) forced convection only
(c) forced convection and conduction
(d) free and forced convection

Heat transfer occurs by natural convection because change in temperature causes differences in

(a) viscosity
(b) density
(c) thermal conductivity
(d) heat capacity

The Grashof number is defined as the ratio of

(a) buoyancy to inertial forces
(b) bouyancy to viscous forces
(c) inertial to viscous forces
(d) bouyancy to surface tension forces

For turbulent flow in a tube, the heat transfer coefficient is obtained from Dittus-Boelter correlation. If the tube diameter is halved and the flow rate is doubled, then the heat transfer coefficient will change by a factor of

(a) `1`
(b) `1.74`
(c) `6.1`
(d) `37`

In natural convection heat transfer the correlating parameter is

(a) Graetz number
(b) Eckert number
(c) Grashof number
(d) Bond number

The hydrodynamic and thermal boundary layers will merge when

(a) Prandtl number is one
(b) Schmidt number tends to infinity
(c) Nusselt number tends to infinity
(d) Archimedes number is greater than 10,000

Prandtl number is the ratio of

(a) mass diffusivity to thermal diffusivity
(b) momentum diffusivity to thermal diffusivity
(c) thermal diffusivity to mass diffusivity
(d) thermal diffusivity to momentum diffusivity

Grashof number is associated with:

(a) bouyancy effects
(b) free convection
(c) forced convection
(d) high temperature difference

The rate of diffusion of momentum relative to the rate of diffusion of heat is:

(a) proportional to Prandtl number
(b) inversely proportional to Prandtl number
(c) Proportional to the Colburn's `j_H` factor
(d) proportional to Stanton number

Heat transfer coefficient in a helical coil compared to that in a striaght pipe is:

(a) Lower
(b) Higher
(c) Same

To determine the heat transfer coefficient Dittus-Boelter equation is valid for:

(a) Laminar flow
(b) For liquid metals
(c) Turbulent flow
(d) Natural convection

Heat transfer by natural convection is enhanced in systems with

(a) high viscosity
(b) high coefficient of thermal expansion
(c) low temperature gradients
(d) low density change with temperature

As the difference between the wall temperature and bulk temperature increases, the boiling heat transfer coefficient

(a) continues to inrease
(b) continues to decrease
(c) goes through a minimum
(d) goes through a maximum

Gibbs phase rule finds application when heat transfer occurs by

(a) conduction
(b) convection
(c) radiation
(d) condensation

For condensation of pure vapours, if the heat transfer coefficients in filmwise and dropwise condensation are respectively `h_f` and `h_d`, then

(a) `h_f = h_d`
(b) `h_f > h_d`
(c) `h_f < h_d`
(d) `h_f could be greater or smaller than h_d`

In a heat exchanger, floating head is provided to

(a) facilitate cleaning of the exchanger
(b) increase the heat transfer area
(c) relieve stresses caused by thermal expansion
(d) increase the log mean temperature gradient

The advantage of using a 1-2 shell and tube heat exchanger over a 1-1 shell and tube heat exchanger is

(a) Lower tube side pressure drop
(b) Lower shell side pressure drop
(c) Higher tube side heat transfer coefficient
(d) Higher shell side heat trabsfer coefficient

In a finned tube heat exchanger:

(a) only heat transfer area is augmented
(b) only film coefficient is augmented
(c) both heat transfer area and film coefficient are augmented
(d) none of the above