Thursday 9 November 2017

ME6502 Heat and Mass Transfer Question bank

ME6502-HEAT AND MASS TRANSFER
Unit-1 Conduction
Part-A
1.      State Fourier’s law of heat conduction.
2.      Define thermal conductivity and thermal resistance.
3.      Write three dimensional steady state heat conduction equation for Cartesian and cylindrical coordinates.
4.      Write Laplace equation and Poisson equation.
5.      Define thermal diffusivity.
6.      Define overall heat transfer coefficient.
7.      What is meant by critical radius of insulation? Give it’s expression.
8.      Define fin efficiency and fin effectiveness.
9.      What do you understand the term extended surfaces or fins? Give it’s applications.
10.  Differentiate steady state and unsteady state heat conduction with examples.
11.  Define Biot number. Mention it’s significance.
12.  What are Heisler charts?
13.  What is meant by lumped heat analysis? When it is used?
14.  Differentiate semi-infinite solids and Infinite solids.

Unit-1 Conduction
Part-B(First part)
1.      `Derive heat conduction equation in Cartesian Coordinates.

2.      Derive an expression for the heat conduction equation in cylindrical coordinates.

3.      A composite wall consists of three layers of thicknesses 300 mm, 200mm and 100mm with thermal conductivities 1.5, 3.5 and is W/m K respectively. The inside surface is exposed to gases at 1200°C with convection heat transfer coefficient as 30W/m2K. The temperature of air on the other side of the wall is 30°C with convective heat transfer coefficient 10 W/m2K. If the temperature at the outside surface of the wall is 180°C, calculate the temperature at other surface of the wall, the rate of heat transfer and the overall heat transfer coefficient.

4.      A furnace wall consists of three layers. The inner layer of 10 cm thickness is made of firebrick
(k =1.04 W/mK). The intermediate layer of 25 cm thickness is made of masonry brick (k = 0.69 W/mK) followed by a 5 cm thick concrete wall (k = 1.37 W/mK). When the furnace is in continuous operation the inner surface of the furnace is at 800°C while the outer concrete surface is at 50°C. Calculate the rate of heat loss per unit area of the wall, the temperature at the interface of the firebrick and masonry brick and the temperature at the interface of the masonry brick and concrete.

5.      A steel pipe. Inside diameter 100 mm, outside diameter 120 mm (k 50 W/m K) IS Insulated with a40mm thick high temperature insulation(k = 0.09 W/m K) and another Insulation 60 mm thick (k = 0.07 W/m K). The ambient temperature IS 25°C. The heat transfer coefficient for the inside and outside surfaces is 550 and 15 W/m2K respectively. The pipe carries steam at 300oC. Calculate (a) the rate of heat loss by steam per unit length of the pipe (b) the temperature of the outside surface.

6.      A steel tube k=43.26 W/mK of 5.08 cm 10 and 7.62 cm 00 is covered with 2.54 cm of asbestos insulation k=0.208 W/Mk. The inside surface of the tube receives heat by convection from a hot gas at a -temperature of 316°C with heat transfer coefficient h=284 W/m2K while the outer surface of Insulation is exposed to atmosphere air at 38°C with heat transfer coefficient of 17 W/m2 K Calculate heat loss to atmosphere for 3 m length of the tube and temperature drop across each layer.

7.      (i) An electric current is passed through a plane wall of thickness 150 mm which generates heat at the rate of 50,000 W/m3. The convective heat transfer coefficient is 65 W/m2K. The ambient air is at 28°C and the thermal conductivity of the wall material is 22 W/mK. Calculate surface temperature and maximum temperature in the wall.
(ii) A copper wire of 40 mm diameter carries 250 A and has a resistance of 0.25x10-4 ohm cm/length surface temperature of wire is 250°C and the ambient temperature is 10°C. if the thermal conductivity of wire is 175 W/mK. Calculate heat transfer coefficient and maximum temperature of the wire.

Unit-1 Conduction
Part-B (Second part)
8.      (i) A Circumferential rectangular profile fin on a pipe of 50 mm outer diameter is 3 mm thick and 20 mm long. Thermal conductivity is 45 W/mK. Convection coefficient is 100 W/m2 K. base and surrounding temperatures are 120°C and 35°C respectively. Determine heat flow rate per fin, Fin efficiency and Fin effectiveness.

(ii) An aluminium rod (k =204 W/mK) 2 cm in diameter and 20 cm long protrudes from a wall which is maintained at 300°C. The end of the rod is insulated and the surface of the rod is exposed to air at 30°C. The heat transfer coefficient between the rod's surface and air is 10 W/m2 K. Calculate the heat lost by the rod and the temperature of the rod at a distance of 10 cm from the wall.

9.      (i) Aluminums fins 1.5 cm wide and 10 mm thick are placed on a 2.5 cm diameter tube to dissipate the heat. The ambient temperatures are 20°C and the tube surface temperature is 170°C calculate the heat loss per fin. Take h = 130 W/m2°C and K = 200 W/m2°C for aluminums.
(ii) Write short notes on fins.

10.  A heating unit made in the form of a cylinder is 5 cm diameter and 1 m long. It is provided with   10 longitudinal fins of material having k=120 W/mK of 0.76 mm thick which protrudes 1.27 cm from the surface of the cylinder. The surface temperature of the cylinder is 150°C and the ambient temperature is 45°C. The heat transfer coefficient is 17 W/m2K. Calculate the rate of heat transfer and the temperature at the end of the fins.

11.  (i) A large steel plate 5 cm thick is initially at a uniform temperature of 400°C. It is suddenly exposed on both sides to a surrounding at 60°C with convective heat transfer coefficient of 285W/m2K. Calculate the centre line temperature and the temperature inside the plate 1.25 cm from the mid plane after 3 minutes.
Take k for steel is 42.5 W/mK, α for steel is 0.043 m2/hr

(ii) A long steel cylinder 12 cm diameter and initially at 20°C is placed into furnace at 820°C with h=140 W/m2K. Calculate the time required for the axis temperature to reach 800°C. Also calculate the corresponding temperature at a radius of 5.4 cm at that time.
Take k for steel is 21 W/mK, α for steel is 6.11x10-6 m2/s.

12.  A large slab initially at a temperature of 600°C and its surface temperature is suddenly lowered to 50°C. Calculate a) temperature at a depth of 3 cm after 6 minutes b) How much time required, the temperature at a depth of 3 cm will reach to 350 °C c)The quantity of cumulative heat passed through the plate at a depth of 3 cm within first one hour.
Take k=1.2 W/mK, α= 0.004 m2/hr.

13.  Alloy steel ball of 12 mm diameter heated to 800°C is quenched in a bath at 100 °C. The material properties of the ball are k=2015 kJ/m hr K, density is 7860 kg/m3, Cp=0.45 kJ/kg K, h=150 KJ/hr m2 K. Determine (a) Temperature of a ball after 10 sec (b) Time for ball to cool to 400°C.

Unit-2
Convection
Part-A
1.      Define convection. Give its types.
2.      What is meant by free convection? Give examples.
3.      What is meant by forced convection? Give examples.
4.      State Newton’s law of convection.
5.      What are the dimensionless parameters used in forced and free convection?
6.      Indicate the significance of boundary layer.
7.      Define boundary layer thickness.
8.      Define hydrodynamic and thermal boundary layers.
9.      Define Nusselt number and Reynolds number. Mention its significance.
10.  Define Prandtl number and Grashof number. Give its significance.
11.  What is meant by laminar flow and turbulent flow?
12.     In which mode of heat transfer is the convection heat transfer coefficient usually higher, natural or forced convection? Why?

Unit-2 Convection

Part-B (First part)

1.         Air at 40° C is flows over a flat plate of 0.9 m at a velocity of 3 m/s. Calculate the following:   (a)Overall drag coefficient (b) Average shear stress, (c) Compare the average shear stress with local shear stress (shear stress at the trailing edge)

2.         Air at 20° C at atmospheric pressure flows over a flat plate at a velocity of 3 m/s. if the plate is 1 m wide and 80° C calculate the following at x = 300. a. Hydrodynamic boundary layer thickness, b. Thermal boundary layer thickness, c. Local friction coefficient, d. Average friction coefficient, e. Local heat transfer coefficient  f. Average heat transfer coefficient, g. Heat transfer.

3.         Air at 20 °C with a free stream velocity of 6 m/s.°C flows over a flat plate at 60°C Determine the value of the average convective heat transfer coefficient up to a length of 1 m in the flow direction.

4.         Engine oil flows through a 50 mm diameter tube at an average temperature of 147°C .The flow velocity is 80 cm/s. Calculate the average heat transfer coefficient if the tube wall is maintained at a temperature of 200°C and it is 2 m long.

5.         For a particular engine, the underside of the crank case can be idealized as a flat plat 20 cm. The engine runs at 80 km/hr and the crank case is cooled by air flowing past it at the same speed. Calculate the loss of heat from the crank case surface of 75°C and the ambient air temperature 25°C. Assume the boundary layer becomes turbulent from the loading edge itself.

6.         Air at 25°C flows over 1 m x 3 m (3 m long) horizontal plate maintained at 200°C at 10m/s. Calculate the average heat transfer coefficients for both laminar and turbulent regions.
Take Re (critical) = 3.5 x 105

Unit-2 Convection
Part-B (Second part)

7.      A steam pipe 10 cm OD runs horizontally in a room at 23° C. Take outside temperature of pipe as 165 ° C. Determine the heat loss per unit length of the pipe.Pipe surface temperature reduces to 80° C with 1.5 cm insulation. What is the reduction in heat loss?

8.      Cylindrical cans of 150 mm length and 65 mm diameter are to be cooled from an initial
temperature of 20°C by placing them in a cooler containing air at a temperature of 1°C and
a pressure of 1 bar. Determine the cooling rates when the cans are kept in horizontal and
      vertical positions.

9.      The water is heated in a tank by dipping a plate of 20 cm X 40 cm in size. The temperature of
the plate surface is maintained at 100°C. Assuming the temperature of the surrounding water is
      at 30° C, Find the heat loss from the plate 20 cm side is in vertical plane.
10.  A steam pipe 10 cm outside diameter runs horizontally in a room at 23°C. Take the outside
surface temperature of pipe as 165°C. Determine the heat loss per unit length of the pipe.

11.  A large vertical plate 5 m high is maintained at 100°C and exposed to air at 30°C. Calculate the
convection heat transfer coefficient.

12.Sketch the boundary layer development of a flow over a flat plate and explain the significance
of the boundary layer.

Unit-3Phase Change Heat Transfer and Heat Exchangers
Part-A
1.Define boiling and condensation.
2.Give the applications of boiling and condensation.
3.What is meant by nucleate and film boiling?
4.Define film wise and drop wise condensation.
5.Draw different regions of boiling.
6.List out the types of heat exchangers.
7.What is meant by compact heat exchanger?
8.Define NTU approach and state it's use.
9.What is meant by LMTD with it's expression.
10.What is meant by effectiveness?
11.What is meant by fouling factor?
12.Sketch the temperature variations in parallel and counter flow heat exchangers.
13. What is meant by pool boiling?
14. What is meant by flow boiling?

Unit-3 Phase Change Heat Transfer and Heat Exchangers
Part-B (First Part)


1.         Water is to be boiled at atmospheric pressure in a mechanically polished stainless steel pan
placed on top of a heating unit. The inner surface of the bottom of the pan is maintained at
  l08°C. The diameter of the bottom of the pan is 30 cm. Assuming Csf = 0.0130. Calculate
(i)the rate of heat transfer to the water and ii) the rate of evaporation of water.

2.      (i) It is desired to boil water at atmospheric pressure on a copper surface which electrically heated.Estimate the heat flux from the surface to the water, if the surface is maintained at 10°C andalso the peak heat flux.
(ii) With a neat sketch explain about regimes of boiling.

3.      A tube of 2 m length and 25 mm outer diameter is to be used to condense saturated steam at   100°C while the tube surface is maintained at 92°C. Estimate the average heat transfercoefficient and the rate of condensation of steam if the tube is kept horizontal. The steamcondenses on the outside of the tube.

4.      Dry saturated steam at a pressure of 2.45 bar condenses on the surface of a vertical tube ofheight 1 m. The tube surface temperature is kept at 117°C. Estimate the thickness of thecondensate film and the local heat transfer coefficient at a distance of 0.2m from the upper endof the tube.

5.      A heating element cladded with metal is 8 mm diameter and emissivity of 0.92. The element is horizontally immersed in a water bath. The surface temperature of the metal is 260°C under steady state boiling conditions. Calculate the power dissipation per unit length of the heater.

6.      A steam condenser consisting of a square array of 900 horizontal tubes each 6 mm in diameter. The tubes are exposed to saturated steam at a pressure of 0.18 bar and the tube surface temperature is maintained at 23°C. Calculate heat transfer coefficient and the rate of steam is condensed.

7.      Steam condenses at atmospheric pressure on the external surface of the tubes of a steam condenser. The tubes are 12 in number and each is 30 mm in diameter and 10 m long. The inlet and outlet temperatures of cooling water flowing inside the tubes are 25°C and 60°C respectively. If the flow rate is 1.1 kg/s, calculate (i) The rate of condensation of steam (ii) The number of transfer units (iii) The effectiveness of the condenser.


Unit-3 Phase Change Heat Transfer and Heat Exchangers
Part-B (Second Part)

8.      (i)Give the classification of heat exchangers.

(ii)It is desired to use a double pipe counter flow heat exchanger to cool 3 kg/s of oil (Cp = 2.1 kJ/kgK) from 120°C. Cooling water at 20°C enters the heat exchanger at a rate of 10 kg/so The overall heat transfer coefficient of the heat exchanger is 600 W/m2Kand the heat transfer area is 6 m2 .Calculate the exit temperatures of oil and water.

9.      (i) Compare LMTD and NTU method of heat exchanger analysis.

(ii) Hot exhaust gases which enters a finned tube cross flow heat exchanger at 300°C and leave at 100°C, are used to heat pressurized water at a flow rate of 1 kg/s from 35 to 125°C. The exhaust gas specific heat is approximately 1000 J/kg.K, and the overall heat transfer co-efficient based on the gas side surface area is Uh = 100W/m2K. Determine the required gas side surface area  using the NTU method. Take Cp,c at Tc = 80°C is 4197 J/kg.K and Cp,h = 1000 J/kg.K.

10.  Water enters a cross flow Heat exchanger (both fluids unmixed) at 5°C and flows at the rate of 4600 kg/h to cool 4000 kg/h of air that is initially at 40°C. Assume the overall heat transfer coefficient value to be 150 W/m2K For an exchanger surface area of 25m2 Calculate the exit temperature of air and water.

11.  (i) Describe the principle of parallel flow and counter flow heat exchangers showing the axial temperature distribution.

(ii) A parallel flow heat exchanger has hot and cold water stream running through it, the flow rates are 10 and 25 kg/min respectively. Inlet temperatures are 75° C and 25° C on hot and cold sides. The exit temperature on the hot side should not exceed 50° C. Assume hi = ho = 600W/m2K. Calculate the area of heat exchanger using E -NTU approach.

12.  Water enters a cross flow Heat exchanger (both fluids unmixed) at 5°C and flows at the rate of 4600 kg/h to cool 4000 kg/h of air that is initially at 40°C. Assume the overall heat transfer coefficient value to be 150 W/m2K For an exchanger surface area of 25m2 Calculate the exit temperature of air and water.

13.  Derive an expression of LMTD for counter flow heat exchanger and parallel flow heat exchanger.

14.  In a cross flow heat exchanger, both fluids unmixed, hot fluid with a specific heat of 2300 J/KgK enters at 380°C and leaves at 300°C. Cold fluids enter at 25°C and leaves at 210°C. Calculate the area required. Take overall heat transfer coefficient is 750 W/m2K. Mass flow rate of hot fluid is 1 kg/s.

Unit-4Radiation
Part-A
1.Define emissive power.
2.What is meant by absorptivity,reflectivity,transmissivity?
3.what is blackbody and gray body?
4.Stateplanck's distribution law.
5.State Wien's displacement law.
6.State Stefan Boltzmann law.
7.Define emissivity and intensity of radiation.
8.What is meant by shape factor and mention its significance.
9.StateKirchoff's law of radiation.
10.Differentiateradiosity and irradiation.
11. What is the use of radiation shield and state its assumptions to calculate radiation exchange between the surfaces?
12.Discuss the radiation characteristics of carbon dioxide and water vapour.

Unit-4 Radiation
Part-B (First Part)
1.       (i)Define emissivity, absorptivity and reflectivity .
(ii)Describe the phenomenon of radiation from real surfaces.

2.       A black body at 3000 K emits radiation. Calculate the following a)Monochromatic emissive power at 1 micro meter wave length b) Wavelength at which emission is maximum c) Maximum emissive power d) Total emissive power e)Intensity f) Total emissive power if it is assumed as a real surface having emissivity 0.85.

3.       Derive the radiation heat exchange between two parallel planes with radiation shield.

4.       800 W/m2 of radiant energy is incident upon a surface, out of which 300 W/m2 is absorbed, 100 W/m2 is reflected and the remainder is transmitted through the surface. Calculate absorptivity, Reflectivity and transmissivity.

5.       Write short notes Shape factor and radiation shield.

Unit-4 Radiation
Part-B (Second Part)
1.      Two parallel, infinite grey surface are maintained at temperature of 127°C and 227°C respectively. If the temperature of the hot surface is increased to 327°C, by what factor is the net radiation exchange per unit area increased? Assume the emissivity’s of cold and hot surface to be 0.9 and 0.7 respectively.

2.      Two large parallel planes with emissivity’s 0.35 and 0.85 exchange heat by radiation. The planes are respectively 1073K and 773K. A radiation shield having the emissivity of 0.04 is placed between them. Find the percentage reduction in radiation heat exchange and temperature of the shield.


3.      Two equal and parallel discs of diameter 25 cm are separated by a distance of 50 cm. If the discs are maintained at 600°C and 250°C. Calculate the radiation heat exchange between them.

4.      Discuss how the radiation from gases differs from that of solids.


5.      Two very large parallel plates with emissivities 0.5 exchange heat. Determine the percentage reduction in the heat transfer rate if a polished aluminium radiation shield of emissivity= 0.04 is placed in between the plates.
6.      A thin aluminium sheet with an emissivity of 0.1 on both sides is placed between two very large parallel plates that are maintained at uniform temperatures Tl = 800 K and T2 = 500 K and have emissivities 0.2 and  0.7 respectively. Determine the net rate of radiation heat transfer between the two plates per unit surface area of the plates and compare the result to that without shield.


Unit-5Mass Transfer
Part-A
1.What is mass transfer and give any two examples.
2.What is molecular diffusion.
3.Define convective masss transfer.
4.State Fick's law of diffusion.
5.What is free convective mass transfer.
6.What is forced convective mass transfer
7.Write any two examples of convective mass transfer.
8.What is Schmidt number.
9.What are the dimensionless parameter used in convective mass transfer.
10.StateScherwood number.
11.Define the terms(i)Mass density (ii) Molar density
12.Define the terms(i)Mass fraction (ii) Molar fraction

Unit-5Mass Transfer
Part-B (First Part)
1. A steel sphere of radius 60 mm which is initially at a uniform temperature of 325°C is suddenly exposed to an environment at 25°C; with convection heat transfer coefficient 500 W/m2K. Calculate the temperature at a radius 36 mm and the heat transferred 100 seconds after the sphere is exposed to the environment.
2. The tire tube of a vehicle has a surface area 0.62 m2 and wall thickness 12 mm. The tube has air filled in it at a pressure 2.4 x 105 N/m2 . The air pressure drops to 2.3 x 105 N/m2 in 10 days. The volume of air in the tube is 0.034 m3 . Calculate the diffusion coefficient of air in rubber at the temperature of 315K. Gas constant value = 287. Solubility of air in rubber tube = 0.075m3 of air/m3 of rubber tube at one atmosphere
 3. Define mass concentration, molar concentration, mass fraction and mole fraction.(4) The diffusivity of CCl4 in air is determined by observing the steady state evaporation of CCl4 in a tube of 1 cm diameter exposed to air. The CCl4 liquid level is 10 cm below the top level of the tube. The system is held at 25°C and 1 bar pressure. The saturation pressure of CCl4 at 25°C is 14.76 kPa. If it is observed that the rate of evaporation of CCl4 is 0.1 kg/hour determine the diffusivity of CC14 into air.
4. An open pan 20 cm in diameter and 8 cm deep contains water at 25°C and is exposed to dry atm air. If the rate of diffusion of water vapour is 8.54x10-4 kg/hr. Estimate the diffusion coefficient of water in air.
5. Explain Fick's first and second laws of diffusion.
Unit-5Mass Transfer
Part-B (Second Part)
6. Discuss the analogy between heat and mass transfer.
7. Explain the phenomenon of equimolar counter diffusion. Derive an expression for equimolar counter diffusion between two gases or liquids.
 8. Dry air at 20°C (p = 1.2 kg/m3 , v = 15 x l0-6 m2 /s, D = 4.2 x l0-5 m2 /s) flows over a flat plate of length 50 cm which is covered with a thin layer of water at a velocity of 1 m/s. Estimate the local mass transfer coefficient at a distance of 10 cm from the leading edge and the average mass transfer coefficient.
9. A mixture of 02 and N2 with their partial pressures in the ratio 0.21 to 0.79 is in a container at 25°C. Calculate the molar concentration, the mass density, the mole fraction and the mass fraction of each species for a total pressure of 1 bar. What would be the average molecular weight of the mixture?
10. Air at 1 atm and 25°C containing small quantities of iodine flow with a velocity of 6.2 m/s inside a 35mm diameter tube. Calculate mass transfer coefficient. The thermo physical properties of air are ( v = 15 x l0-6 m2 /s, D = 4.2 x l0-5 m2 /s)