17-5C Consider steady heat transfer through the wall of a of t room in winter. T

Question

17-5C Consider steady heat transfer through the wall of a of t room in winter. The convection heat transfer coefficient at the the outer surface of the wall is three times that of the inner sur- face as a result of the winds. On which surface of the wall do radi you think the temperature will be closer to the surrounding 0.5 air temperature? Explain 17-6C How is the combined heat transfer coefficient 17 defined? What convenience does it offer in heat transfer cal- 0.40 culations? pera 17-7C Why are the convection and the radiation resistances at a surface in parallel instead of being in series? Two ers tim tain 17-8C Consider steady one-dimensional heat transfer mea through a plane wall exposed to convection from both sides oute to environments at known temperatures T and T, with for known heat transfer coefficients h, and h. Once the rate of of h heat transfer has been evaluated, explain how you would of t determine the temperature of each surface. 17-9C So. Someone comments that a microwave oven can viewed as a conventional oven with zero convectiorn sistance at the surface of the food. Is this an accurate statement? 17-10C Consider two cold canned drinks, one wrapped in a bl unket and the other placed on a table in the same room Which drink will warm up faster? 17-11C Con tion and radi urface of area A at which the convec- fer coefficients are hoom and h you would determine (a) the single

Explanation / Answer

Q. 17.5 C

Given that heat transfer coefficient of outer side is three times that of inner side.

therefore ho =3×hi

Resistance to heat transfer is given by

for outer side

Ro = 1/ (ho.A) = 1/(3.hi.A)

Here A= Area of heat transfer

for inner side resistance

Ri = 1/(hi.A)

therefore

Ro = Ri/3

Resitance to heat transfer in outer side is one third of inner side. Therefore temperature gradient is very less between the air and the wall.

Therefore in outer side the Temperature of air is close to the temperature of wall.

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