وزارة التعليم العالي و البحث العلمي

Ministry of Higher Education&

Scientific Research

University of Technology

Materials Engineering Department لوجيةوالجامعة التكن

قسم هندسة المواد

Date: / / 2012

Final Examination

(2011-2012) Class:2

nd

Subject: heat Transfer &Fluid

Allowed time: 3Hr

Examiner: Dr. Ayad K. Hassan

Note: answer five questions

Use Tables Allowed

Q1:- A house has a composite wall of wood, fiberglass insulation, and plaster board as

indicated in the sketch. On a cold winter day the convection heat transfer coefficients are

ho=60 W/m2 .K and hi=30 W/m

2 .K. the total wall surface area is 350 m

2.

a- Determine a symbolic expression for the total thermal resistance of the wall,

including inside and outside convection effects for the prescribed conditions.

b- Determine the total heat loss through the wall.

c- If the wind were blowing violently, raising ho to 300 W/m2 .K, determine the

percentage increase in the heat loss. ....( 20 mark)

Thermal conductivity of plaster board Kp=0.17 W/m.K ,

fiberglass insulation Kb=0.038 W/m.K and plywood siding Ks=0.12 W/m.K

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Q2:- The temperature of a gas stream is to be measured by a thermocouple whose

junction can be approximated as a 1-mm-diameter sphere, as shown below. The properties

of the junction are K= 35 W/m·°C, ρ= 8500 kg/m3, and Cp =320 J/kg · °C, and the

convection heat transfer coefficient between the junction and the gas is h= 210 W/m2·°C.

Determine how long it will take for the thermocouple to read 99 percent of the initial

temperature difference. Assume Ti=0oC, T∞= 100

oC and T(t)=99

oC ….(20 mark)

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Q3:- An aluminum pin fin has a diameter of 4mm and length of 20 mm. calculate the heat

dissipated when the base temperature is 600 K, the fluid temperature is 400 K, and heat

transfer coefficient is 100 W/m2.K . take KAL=180 W/m.K Assume that the fin is insulated

end ….(20 mark)

Q4:- A: Derive the nodal temperature equation for the case of an exterior corner node

with one adjacent side insulated and one adjacent side to a convective heat transfer, as

shown in Fig bellow. (10 marks)

B: Two large parallel plate are at T1=800K, ϵ1=0.3, T2=400K, ϵ2=0.7. Calculate the heat

transfer rate between the two plans. where σ =5.67X10-8

W/m2.K

4 (10 marks)

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Q5:- Air at 27 oC and 1 atm flows over a flat plate at speed of 2 m/s. Calculate the

hydrodynamic boundary layer thickness and thermal boundary layer thickness at

distance 20 cm from the leading edge of the plate. Assume that the plate is heated over its

entire length to a temperature of 60 oC. Calculate the heat transferred in a first 20 cm

Assume unit depth in the z-direction. …. (20 mark) ----------------------------------------------------------------------------------------------------------------------------

Q6 -: A thin (16 cm) diameter circular horizontal plate is maintained at 130oC in a large

room at (24oC) the plate convicts heat from both its top and bottom surfaces. Determine

the rate of heat input in to the plate necessary to maintain the temperature of (130oC)

….(20 mark)

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List of laws

Forced convection

For laminar flow over plate :

For turbulent flow over plate

Natural convection

Horizontal plate

Nuavrg =cRam

Upper surface heat: c=0.15, m=1/3

Lower surface heat: c=0.27, m=1/4

GOOD LUCK

m,n

m,n-1

m-1,n

h,T∞

dx

dy