Amity University 2010-4th Sem B.E Mechanical Engineering HEAT

BTM-402


B.TECH(M.A.E.)

FOURTH SEMESTER END TERM exam
APRIL,2010
HEAT & MASS TRANSFER

TIME:3 Hrs [Maximum marks 70]




part -A [ Marks 30]

Attempt any five questionss.
every ques. carry six marks.



1. Derive the Fourier 3 dimensional conduction formula in cylindrical co-ordinates.



2. (a) A 160mm diameter pipe carrying saturated steam is covered by a layer of a few material of thickness 40mm & K=0.8 W/m 'C. Later an extra layer of identical material having thickness 10mm & K=1.2 W/m 'C.If the surrounding tempratureremains constant and heat transfer co-efficient is 10W/m.m 'C,determine the percentage change in the rate of heat loss due to the extra layer.

(b)What do you mean by thermal diffusivity?What is its physical significance.



3. (a)An electrical conductor of 10mm diameter and having 2mm thick insulator (K=0.18 W/m-K) covering is located in air
at 25'C having convection heat co-efficient of eight W/m.m -K.Ifthe base conductor has specific resistance of 72 u.omega-cm and its surface temperature is 80'c determine,
(1) Current capacity of conductor.
(2)Critical thickness of insulation.
(3)Maximum current carrying capacity.
(b)What do you mean by the term log mean area of a cylinder?




4.(a) Air flows over a plate 0.5m long and 0.6m wide with a velocity of 4m/s.The velocity profileis



{u/U} = Sin{(pie/2)(y/delta)}

{delta=thickness of boundary layer}
If density=1.24 Kg/m-cube and kinematic viccousity = 0.15 * 10{power -4} m-square /s.
calculate:
(1)Boundary layer thickness at the end of the plate.
(2)Shear stress at 250mm from th leding edge.
(3)Drag force on 1 side of the plate.
(b)Define Prandtl no. and nusselts no.


5. (a) describe the subsequent terms:
(1) Total emiisive power
(2) Monochromtic emiisive power
(3) Emissivity
(4) Intensity of radiation

(b) How does a gray body differ from a black body?





6.(a)What do you mean by effectiveness and efficiency of an heat exchanger?

(b)What do you mean by fouling in a heat exchanger?





SECTION-B

Attempt any 2 ques..
every ques. carry 10 marks.





7. (a)One end of a long rod 35mm in diameter is inserted in a furnace with the other end projectin in the outside air.After the steady state is reached the temperature of the rod is measured at 2 points 180mm apart and obtained to be 140'C and 180'C.The atomspheric air temperature is 25'C.If the heat temperature co-efficient is 65W/M-square'C,calvulate the thermal conductivity of the rod.

(b)DErive the 2 special case of heat conduction through a rectangular fin when

(1)The tip of the fin is insulated.
(2)The fin is infinitely long.



8 (a)Explain the formation of hydrodynamic boundary layer and thermal boundary layer due to natural convetion.


(b)State the advantages and limits of dimensional analysis.


9. The flow rates of hot water and cold water streams running through a parallel flow heat exchanger are .2kg/s and .5kg/s.
The inlet temprature on the cold and heat sides are 75'C and 20'C respectively. The inlet temperature of hot and cold sides are 75'C and 20'C respectively.The exit ttemperatureof hot water is 45'C.If the individualheat transfer co-efficients om both sides is 650W/m-square 'C,calculate the area of heat exchanger.




SECTION-c

(Compulsory)



10.(a)The radiation shape factor of the circular surface of a thin holow cylinder of 10cm diameter and 10cm diameter and
10cm length is 0.1716.What is the shape factor of the curved surface of cylinder with respsct to itself? [marks 6]


(b)What is the radiation shield? [marks 2]


(c)Derive the energy formula for thermal boundary layer in cartesian co-ordinates. [marks 8]

(d)Atmospheric air at 20'C is flowing parallel to a flat plate at a velocity of 2.8m/s.Assuming cubic velocity profile and using exact BLAUSSIUS solution,estimate the boundary layer thickness and the local co-efficient of drag at x=1.2m from the leading edge of plate.Also obtain the deviation of the approximate solutionfrom the exact solution.Take the knematic viscousity at 20'C=1.54*10{power-6} m-square/s. [marks 4]

Earning:   Approval pending.