B.E. /B.TECH DEGREE EXAMINATION, APRIL/MAY 2008.
Sixth Semester
(Regulation 2004)
Mechanical Engineering
ME1351 - HEAT AND MASS TRANSFER
Time: Three hours Maximum: 100 marks
PART A - (10*2 = 20 MARKS)
1. A temperature difference of 500 C is applied across a fire-clay brick, 10cm thick having a thermal conductivity of 1W/mK. Find the heat transfer rate per unit area.
2. Write the general 3-D heat conduction equation in cylindrical coordinates.
3. Biot number is the ratio between _________and _________
4. Define bulk temperature.
5. A vertical flat plate is maintained at a temperature lower than the surrounding fluid. Draw the velocity and temperature profiles assuming natural convection.
6. What is burnout point? Why is it called so?
7. What is a compact heat exchanger? Give examples.
8. What is thermal radiation and what is its wavelength band?
9. What are radiation shields?
10. Explain the physical meaning of Schmidt number?
PART B - (5*16 = 80 marks)
11. (a) A composite wall is formed of a 2.5 cm copper plate (k=355W/mK), a 3.2 mm layer of asbestos (k=0.110 W/mK) and a 5cm layer of fiber plate (k=0.049 W/mK). The wall is subjected to an overall temperature difference of 560 C (560 C on the Cu plate side and 0 C on the fiber plate side).Estimate the heat flux through this composite wall and interface temperature between asbestos and fiber plate.
OR
(b) When a thermocouple is moved from one medium to another medium at a different temperature, sufficient time must be given for the thermocouple to come to the thermal equilibrium with the new conditions before a reading is taken. Consider a 0.1 cm diameter copper thermocouple wire originally at 150 C. Find the thermometer response (i.e. an approximate plat of temperature vs time for intervals of 0, 40 and 120 seconds) when this wire is suddenly immersed in
(i) water at 40 C (h=80 W/m2K)
(ii) air at 40 C (h=40 W/m2K)
Assume unit length of wire.
12. (a) Air at 400 K and 1atm pressure flows at a speed of 1.5 m/s over a flat plate of 2 m long. The plate is maintained at a uniform temperature of 300 K. If the plate has a width of 0.5 m, estimate the heat transfer coefficient and the rate of heat transfer from the air stream to the plate. Also estimate the drag force acting on the plate.
OR
(b) 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
(i) Horizontal position
(ii) Vertical position
13. (a) 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 108 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.
OR
(b) Define effectiveness of a heat exchanger. Derive an expression for the effectiveness of a double pipe parallel flow heat exchanger. State the assumptions made.
14. (a) (i) Discuss briefly the variation of black body emissive power with wavelength of different temperatures. (8)
(ii) The spectral emissive function of an opaque surface at 800 K is approximated as
e1 = 0.30 0 = ? < 3µm
e? = e2 = 0.80 3µm = ? < 7µm
e3 = 0.10 7µm = ? < 8
Calculate the average emissivity of the surface and its emissive power. (8)
OR
(b) Explain the following: (5+5+6)
(i) Specular and diffuse reflection
(ii) Reflectivity and transmissivity
(iii) Reciprocity rule and summation rule.
15. (a) Discuss briefly the following: (4+6+6)
(i) Fick’s law of diffusion
(ii) Equimolar counter diffusion
(iii) Evaporation process in the atmosphere.
OR
(b) (i) What are the assumptions made in the 1-D transient mass diffusion problems? (4)
(ii) An open pan, 20 cm diameter and 8 cm deep contains water at 25°C and is exposed to dry atmospheric air. Estimate the diffusion coefficient of water in air, if the rate of diffusion of water is 8.54 × 10-4 kg/h. (12)
------THE END------
ANNA UNIVERSITY, CHENNAI 25Sixth Semester
(Regulation 2004)
Mechanical Engineering
ME1351 - HEAT AND MASS TRANSFER
Time: Three hours Maximum: 100 marks
PART A - (10*2 = 20 MARKS)
1. A temperature difference of 500 C is applied across a fire-clay brick, 10cm thick having a thermal conductivity of 1W/mK. Find the heat transfer rate per unit area.
2. Write the general 3-D heat conduction equation in cylindrical coordinates.
3. Biot number is the ratio between _________and _________
4. Define bulk temperature.
5. A vertical flat plate is maintained at a temperature lower than the surrounding fluid. Draw the velocity and temperature profiles assuming natural convection.
6. What is burnout point? Why is it called so?
7. What is a compact heat exchanger? Give examples.
8. What is thermal radiation and what is its wavelength band?
9. What are radiation shields?
10. Explain the physical meaning of Schmidt number?
PART B - (5*16 = 80 marks)
11. (a) A composite wall is formed of a 2.5 cm copper plate (k=355W/mK), a 3.2 mm layer of asbestos (k=0.110 W/mK) and a 5cm layer of fiber plate (k=0.049 W/mK). The wall is subjected to an overall temperature difference of 560 C (560 C on the Cu plate side and 0 C on the fiber plate side).Estimate the heat flux through this composite wall and interface temperature between asbestos and fiber plate.
OR
(b) When a thermocouple is moved from one medium to another medium at a different temperature, sufficient time must be given for the thermocouple to come to the thermal equilibrium with the new conditions before a reading is taken. Consider a 0.1 cm diameter copper thermocouple wire originally at 150 C. Find the thermometer response (i.e. an approximate plat of temperature vs time for intervals of 0, 40 and 120 seconds) when this wire is suddenly immersed in
(i) water at 40 C (h=80 W/m2K)
(ii) air at 40 C (h=40 W/m2K)
Assume unit length of wire.
12. (a) Air at 400 K and 1atm pressure flows at a speed of 1.5 m/s over a flat plate of 2 m long. The plate is maintained at a uniform temperature of 300 K. If the plate has a width of 0.5 m, estimate the heat transfer coefficient and the rate of heat transfer from the air stream to the plate. Also estimate the drag force acting on the plate.
OR
(b) 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
(i) Horizontal position
(ii) Vertical position
13. (a) 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 108 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.
OR
(b) Define effectiveness of a heat exchanger. Derive an expression for the effectiveness of a double pipe parallel flow heat exchanger. State the assumptions made.
14. (a) (i) Discuss briefly the variation of black body emissive power with wavelength of different temperatures. (8)
(ii) The spectral emissive function of an opaque surface at 800 K is approximated as
e1 = 0.30 0 = ? < 3µm
e? = e2 = 0.80 3µm = ? < 7µm
e3 = 0.10 7µm = ? < 8
Calculate the average emissivity of the surface and its emissive power. (8)
OR
(b) Explain the following: (5+5+6)
(i) Specular and diffuse reflection
(ii) Reflectivity and transmissivity
(iii) Reciprocity rule and summation rule.
15. (a) Discuss briefly the following: (4+6+6)
(i) Fick’s law of diffusion
(ii) Equimolar counter diffusion
(iii) Evaporation process in the atmosphere.
OR
(b) (i) What are the assumptions made in the 1-D transient mass diffusion problems? (4)
(ii) An open pan, 20 cm diameter and 8 cm deep contains water at 25°C and is exposed to dry atmospheric air. Estimate the diffusion coefficient of water in air, if the rate of diffusion of water is 8.54 × 10-4 kg/h. (12)
------THE END------
MODEL QUESTION PAPER
B.E. MECHANICAL ENGINEERING, VI SEMESTER
ME340 – HEAT AND MASS TRANSFER
TIME – 3 Hours MAXIMUM : 100 Marks
(Use of steam table, Mollier Chart and HMT data book is permitted)
PART – A (10 x 2 = 20 Marks)
ANSWER ALL QUESTIONS
1. Write down the equation for heat conduction through a hollow
cylinder for one dimensional steady state without heat generation.
2. What is meant by critical radius of Insulation?
3. State the Wien’s displacement law.
4. Define radiation shape factor.
5. Give two examples of the application of free convection.
6. What is the significance of Nusselt Number?
7. What is the difference between film-wise and drop-wise condensation?
8. List out the different types of Heat exchangers.
9. Define Fick’s Law of Diffusion.
10. Give two practical examples of mass transfer.
PART – B (5 x 16 = 80 Marks)
11. Water enters a cross flow Heat exchanger (both fluids unmixed) at 5oC and flows at the rate of 4600 kg/h to cool 4000 kg/h of air that is initially at 40oC. Assume the over all heat transfer coefficient value to be 150 W/m2K. For an exchanger surface area of 25m2, Calculate the exit temperature of air and water. (16)
12. a) A steel tube k=43.26 W/mK of 5.08 cm ID and 7.62 cm OD 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 316oC with heat transfer coefficient ha=284 W/m2K while the outer surface of Insulation is exposed to atmosphere air at 38oC with heat transfer coefficient of 17 W/m2K. Calculate heat loss to atmosphere for 3 m length of the tube and temperature drop across each layer. (16)
( OR )
12. b) (i) A plane wall 20 cm thickness generates heat at the rate of 5 x 104 W/m3 when an electric current is passed through it. The convective heat transfer coefficient between each face of the wall and the ambient air is 60 W/m2K. Determine.
· The surface temperature (4)
· The maximum temperature in the wall. Assume ambient air
temperature to be 25oC and the thermal conductivity of the
wall material to be 16 W/mK. (4)
(ii) A steel ball 100 mm diameter was initially at 500oC and is placed
in air which is at 35oC. Calculate time required to attain 400oC
and 300oC. (8)
k steel = 35 W/mK
c = 0.46 kJ/ kgK
p = 7800 kg/m3
h = 10 W/m2K
13. a) (i) Two parallel, infinite grey surface are maintained at temperature of 127oC and 227oC respectively. If the temperature of the hot surface is increased to 327oC, by what factor is the net radiation exchange per unit area increased?. Assume the emissivities of cold and hot surface to be 0.9 and 0.7 respectively. (8)
(ii) Two equal and parallel discs of diameter 25 cm are separated by a distance of 50 cm. If the discs are maintained at 600oC and 250oC. Calculate the radiation heat exchange between them. (8)
( OR )
b) Two large parallel planes with emissivities 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. (16)
14. a) State the Buckingham’s p- Theorem. Explain the various parameters used in forced convection and using dimensional analysis obtain an expression for Nusselt number interms of Reynolds number and Prandtl number. (16)
( OR )
b) A circular disc heater 0.2m in diameter is exposed to ambient air at 25oC. One surface of the disc is insulated at 130oC. Calculate the amount of heat transferred from the disc when it is.
(i) Horizontal with hot surface facing up (5)
(ii) Horizontal with hot surface facing down (5)
(iii) Vertical (6)
15. a) CO2 and air experience equimolar counter diffusion in a circular tube whose length and diameter are 1m and 50mm respectively. The system is at a toal pressure of 1 atm and a temperature of 25oC. The ends of the tube are connected to large chambers in which the species concentration are maintained at fixed values. The partial pressure of CO2 at one end is 190mm of Hg while at other end is 95mm of Hg. Estimate the mass transfer rate of CO2 and air through the tube. The diffusion coefficient for CO2 – air combination is 0.16 x 10 –4 m2/s. (16)
( OR )
b) (i) Define the non dimensional numbers in mass transfer (6)
(ii) Dry air at 27oC and 1 atm flows over a wet flat plate 50cm long at a velocity of 50m/sec. Calculate the mass transfer co-efficient of water vapour in air at the end of the plate. Take D=0.26x10–4 m2/sec.
b = 1.1614 kg/m3
m = 184.6 10-7 Ns/m2
Pr = 0.707 (10)
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