Indian Institute of Technology Kharagpur (IIT-K) 2007 B.Tech Chemical Technology Transport process-2 - Question Paper

   Department of Chemical Engineering

'    IIT Kharagpur

End Spring Semester Examination 2007 Transport Processes - II (CH24002)

Open Book Examination, class notes not allowed

An aluminum plate, heated to a uniform temperature is allowed to cool while vertically suspended in a room where the ambient air and surroundings are at 27 C. The plate is 0.3 m by 0.3 m with a thickness of 15 mm and an emissivity of 0.25. This plate is used in an experiment to determine the free convection heat transfer coefficient. At an instant of time when the plate temperature was 127 C, the time rate of change of this temperature was observed to be - 0.0465 K/s. What is the corresponding free convection heat transfer coefficient? Compare this result with an estimate based on any standard empirical correlation. Properties: A1 alloy : p= 2770 kg/m³, c_p= 925 J/kg.K, Air: kinematic viscosity = 20.92x10

0.02 W/m.K, a = 29.9x10" ni /s, Pr = 0.7

An experiment is conducted to determine the average mass transfer coefficient of a small droplet using a heater controlled to operate at a constant temperature. The power history required to completely evaporate the droplet at a temperature of 37 C is shown in the figure. It was observed that, as the droplet dried, its wetted diameter on the heater surface remained nearly constant at a value of 4 mm.

(a)    Calculate the average mass transfer convection coefficient based on the wetted area during the evaporating process when the droplet, heater, and the dry ambient air are at 37 C.

(b)    How much energy will be required to evaporate the droplet if the dry ambient air temperature is 27 C, while the droplet-heater temperature remains at 37 C?

Properties: Saturated water (37 oC): hfg - 2414 kJ/kg, p_{A saI} 1/vg = 1/22.93 = 0.04361 kg/m-*, Air-water vapor (Ts = 37 C, 1 atm): DB - 0.276x10 6 /s Air (T_aVg = (27+37)/2=305K, 1 atm): p ~ 1.1448 kg/m³, c_p - 1008 J/kg.K, y = 16.39x10 - ⁶ m²/s, Pr = 0.706    9

0

4, (a) A stationary cart of empty mass Mo contains water with initial height ho. The water tank cross section area is A_t. Water issues out with a velocity V through a hole of cross section area A. The tank is pressurized such that velocity of water jet V is assumed to be constant. Find out the range of coefficient of static friction between the wheel of the cart and the road surface such that the call does not move. Density of water is p .    (7)

(b) A cart vane assembly of mass M moves to the right with an initial velocity uo on a rough surface with coefficient of kinetic friction . At t=0, a water jet of area A and velocity V falls on the vane at an angle 9_] with the vertical. Water goes out of the vane with an angle 0₂ with vertical maintaining the same area of cross section. Find the distance traveled by the assembly before it comes to a stop.        (8)

5 & A stream is heated from 25 to 75C as it moves at 0.2 m/s through a straight thin walled stainless steel tube of 12.7 mm diameter. A uniform heat flux is maintained by an electrical resistance heater around the outer surface of the tube. If tube is 10 m long, what is the required heat flux? If fluid enters the tube with a fully developed velocity profile and uniform temperature profile, what is the surface temperature at the tube exit and at a distance 0.5 m from the entrance? Constant fluid properties are: density 1000 kg/m^J; c_p=4000 J/kgK; viscosity 2x10'' Pa.s. thermal conductivity 0.48 W/mK and Pr=10. (5)

& A horizontal electrical cable of 25 mm diameter has a heat dissipation rate of 30 W/m. If the ambient air (stationary) is at 27C. estimate the surface temperature of the cable.

(5)

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Earning:   Approval pending.