# Anna University Chennai 2007 B.A Arabic ME1351 - HEAT AND MASS TRANSFER, /'2008 - Question Paper

Saturday, 23 February 2013 02:45Web

B.

**E.**/B.TECH DEGREE EXAMINATION, APRIL/MAY 2008.

Sixth Semester

(Regulation 2004)

Mechanical Engineering

ME1351 - HEAT AND MASS TRANSFER

**Time:**3 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. obtain the heat transfer rate per unit area.

**2.**Write the general 3-D heat conduction formula in cylindrical coordinates.

**3.**Biot number is the ratio ranging from _________and _________

**4.**describe bulk temperature.

**5.**A vertical flat plate is maintained at a temperature lower than the surrounding flui

**d.**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? provide examples.

**8.**What is thermal radiation and what is its wavelength band?

**9.**elaborate radiation shields?

**10.**discuss the physical meaning of Schmidt number?

**PART B**- (5*16 = 80 marks)

**1**

**1.****(**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 ranging from asbestos and fiber plate.

**a)**OR

**(**When a thermocouple is moved from 1 medium to a different medium at a various temperature, sufficient time must be provided for the thermocouple to come to the thermal equilibrium with the new conditions before a studying is taken. Consider a 0.1 cm diameter copper thermocouple wire originally at 150

**b)****C.**obtain 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

**(**water at 40 C (h=80 W/m2K)

**i)****(**air at 40 C (h=40 W/m2K)

**i****i)**presume unit length of wire.

**1**

**2.****(**Air at 400 K and 1atm pressure flows at a speed of 1.5 m/s over a flat plate of two 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 plat

**a)****e.**Also estimate the drag force acting on the plate.

OR

**(**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 one C and a pressure of one bar. Determine the cooling rates when the cans are kept in

**b)****(**Horizontal position

**i)****(**Vertical position

**i****i)****1**

**3.****(**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

**a)****C.**The diameter of the bottom of the pan is 30 cm. Assuming Csf = 0.0130, compute

**(**the rate of heat transfer to the water, and

**i)****(**the rate of evaporation of water.

**i****i)**OR

**(**describe effectiveness of a heat exchanger. Derive an expression for the effectiveness of a double pipe parallel flow heat exchanger. State the assumptions made.

**b)****1**

**4.****(**

**a)****(**explain briefly the variation of black body emissive power with wavelength of various temperatures. (8)

**i)****(**The spectral emissive function of an opaque surface at 800 K is approximated as

**i****i)**e1 = 0.30 0 = ? < 3µm

e? = e2 = 0.80 3µm = ? < 7µm

e3 = 0.10 7µm = ? < 8

compute the avg. emissivity of the surface and its emissive power. (8)

OR

**(**discuss the following: (5+5+6)

**b)****(**Specular and diffuse reflection

**i)****(**Reflectivity and transmissivity

**i****i)****(**Reciprocity rule and summation rule.

**i****i****i)****1**

**5.****(**explain briefly the following: (4+6+6)

**a)****(**Fick’s legal regulations of diffusion

**i)****(**Equimolar counter diffusion

**i****i)****(**Evaporation process in the atmosphere.

**i****i****i)**OR

**(**

**b)****(**elaborate the assumptions made in the 1-D transient mass diffusion problems? (4)

**i)****(**An open pan, 20 cm diameter and eight 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.

**i****i)****(**

**1****2)**finished

Earning: Approval pending. |