Ain Shams University

Faculty of Engineering

MANF475: Heat Transfer

New Engineering Programs

Manufacturing Department

Dr. GamilW. Younan

Mid-Term Examination. L9 December 2O12

Answer the Foltowins Questions: Time attowed:1Trhrs.

Question One [8 Marks]:An infinite slab wall, LBmm thick is at uniform temperature of 3OoC is suddenly one face

raised to a temperature of 21OoC by usihg a good contact electric heater. If that face is

maintained at this temperature, while the other face is perfectly insulated.

Calculate:i-The time required to raise the temperature of the insulated face to 1,20oC.

ii-The electric heater capacity in kW/m2.iii- Draw the temperature distribution through the slab thickness at that time in (iJ, three

positions at least. Take for slab material c =4.85x10't m2/s and k=0.50 W/m.K

Question Two [10 Marksl;The external annular channel oftube in heat-exchanger carries water enters at temperature of 20'Cand heated to30'C through the tube length of 4.65m. If the outer surface temperature of the innertube is at 65"C, while the outer and inner diameters of the annular channel are 60mm and 40mm,

respectively. Calculate the mean water velocity and mass flow rate in Kg/min. Also calculate the

heat-transfer rate to water in kW. Assume thattheflow through the annular channel is laminorflow and check that later.

Question Three [7 Marksl:A steam duct passes vertically through a room. The exterior surface of the duct is at B6'C

and the still air temperature in the room is 20"C. The dross section dimensions of the duct

are 500mm length with 300mm width, while the height of the duct is 2.75m. Determine theheat transfer rate from the duct to still air. By assuming that the exterior surface of the

room is completely insulated, calculate how Iong it would take for the steam to raise the

temperature of the room air by 5oC, take the room value as 270m3.

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