CSVTU

Time: Three l{ours

le I l9rol

Code: ME 12712B. Tech. (Seventh Semester) Examination Nov.2012

Subject: Heat and Mass TransferMax marks: 80

Min Pass Marks: 28

Note: All qaestions carry equfll marks. Use of HMT data book and steam tables is permitted

Assume suilable data d any. Solve any t eo partsfrom all questions exceptfrom question No.4.

1 a. Define or explain the following terms

(i) Thermal conductivity (ii) thermal diffusivity

(iii) Write the 3-D general heat conduction equation and give the meaning of each term

-b. A saturated vapour refrigerant at -40oC flows through a copper pipe of 10 mm inside

diameter and wall thickness of 2 mm. A 40 mm thick of thermocole is provided on the outer

surface of the pipe to reduce the heat flow. Determine the heat leakage to the refrigerant per

meter length of pipe. Assume the internal and external heat transfer coefficients to be 500 and 5

Wm2K.

7L c. Deive the expression for the temperature distribution for the hollow spherical conduction

system for uniform thermal conductivity. State the assumptions taken. 08

04

0,4

)

08

I a. Consider a siab of thickness of L and constant thermal eonductivity k in which energy is

generdted at a constant rate of go Wm3The boundary at x : 0 is insulated andtheatx:Lat temperature T-dissipates heat by convection with heat transfbr coefficient h into a fluid

Develop expression for temperature distribution, and heat flux in the slab.

If L: 1cm, k = 20 w7m.oc, go: 8 x 10t w/m' , h:4000 wim2. oC and T*

the temperature at x: 0 and x = L cm

4V. A fin5 mm thick and 45 mm long has its base on a plane plate which is maintained at 125t.

The ambient temperature is 25 oC. The conductivity of the fin material is 55 WmoC and the heat

transfer coefficient is lal Wm2. oC. Determine (i) temperature at the end of the fin (ii)

temperature at the middle of the fin (iii) heat dissipated by the fin (per meter width) 08

: 100 oC then find

08

2 c" A slab of aluminum 10 cm thick is originally at 500 oc. It is suddenly immersed in a liquidat 100 oc resulting in heat transfer coefficient of 1200 wm2K. Determine the temperature at thecenter line and the surface I minute after the immersion. Also calculate the total thermal energyremoved per unit area of the slab during this period. The properties of aluminium are:p = 2700 kdm', c = 0.9 kJ/kg.K, k = 215 wmK, and *= g.4 x rO's m2ls 0g

3 a' Using von Karman integral momentum equation and cubic velocity profile derive theexpression for boundary layer thickness. state the assumptions taken 0g

3 b 0'5 Kg of water per minute is passed through a tube of 20 mm diameter it is found to beheated from 20 oc to 50 oc. The heating is accomplished by condensing steam on the surface ofthe tube and subsequently the surface temperature of the tube is maintained at g5 oc.

Determinethe length of the tube required for fully deveioped flow.Talce thermo-physical properties of the water ai 60oC as :

p = 983'2 Kg/mt, Cp =4.18 kJ/kg.K, k= 0.659 Wm.oC, u = 0.47gx 10-6 m2ls. 0S

3 c' A :fo mm long glass plate is hung vertically in ak at rL"" while its temperature ismaintained at f0 oc. calculate the boundary layer thickness

" ,{trailing edge of the plate. If

the similar plate is kept in the wind tunnel and air is blown over it at a velocity of 5 mls find theboundary layer thickness at the trailing edge. Also determine the heat transfer coefficient for thenatural convection.

0g

4 d' water is boiled at a tate o80 kg4r in a copper pan, 30 cm diameter at atmospheric pressure.Estimate the temperature of the bottom surface of the pan assuming nucleate pool boilingcondition. Determine the peak heat flux for the conditions given 0g/

t //

J6' A refrigerator is designed to cool1250 dgth othot liquid of (ip : 3350 J/kg K) at 1i0 0cusing a parallel flow arrangement. 1000(dhof cooling water is available for cooling purpose ata temperature 10 "C. tr the overall heat transfer coefficient is l iop wlmzK and ,*;;;;;the heat exchanger is 0'25 m2, calculate the outlet temperatur ,

"{{"'loot.a fiquid and water andalso the effectiveness of tf,e heat exchanger.

-- -- 'r'v vvvrvv . 0g

5a. What are radiation shape factors and why are they used? Also show that the shape factor F12

between two parallel circular discs fig. 1 is given as:

{tat^

X ={/+ B'+CIl,.J

{b,l.-1 .-+( l%Yl

ii'lftr( ,t. ;{n }_-tU_Fig 1.

5 b. Derive the expression for equivalent emissivity for

large concentric cylinders whose inner and outer surfaces

T2 and their emissivities 4r€ srand 12 respectively.

5 c. (i) An open pan20 cm in diameter and 8rcm deep contains water at

dry atmospheric air. If the rate of diffusion of water vapour is 8.54 x

diffirsion coefficient of water in air.

(ii) Define Fick's Law of diffusion.

08

the radiation exchange between two

are maintained at temperatures T1 and

.08

25 oC and is exposed to

l0-4 kg/h, estimate the

05

03

-.4*z cz