pls do a, b and c in a detailed and easily understandable manner..thanks Q.C8. (
ID: 2073380 • Letter: P
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pls do a, b and c in a detailed and easily understandable manner..thanks
Q.C8. (a) List all the mechanisms of heat transfer that can occur through the following: . . . A translucent solid A vacuum A flowing liquid A stationary gas . (4 marks) (b) During a cold winter day, wind at 55 km/h is blowing parallel to a 4 m high and 10 m long wall of a house. If the air outside is 5 °C and the surface temperature of the wall is 12 °C, determine the heat loss from the wall by convection (c) What would the answer be if the wind velocity was doubled? Properties of air at the mean temperature: (10 marks) (6 marks) 1 25 kg/m k = 0.024 W/m· = 1.8 x 10-5 kg/m·s Pr 0.73 Equations to note: Critical Re:5x10 For Laminar Flow: h L For Turbulent Flow: hl. 0.037 Re Pr 0.037(Re 871)Pr Nu For Laminar and Turbulent 7:9Explanation / Answer
a) There are three types of heat transfer mechanisms:
They are ,
1)
Conduction
If someone pours you a hot cup of coffee, you're likely going to pick it up from the handle instead of around the main part of the mug. This is because the coffee mug will conduct heat from the hot liquid inside to your hand. This direct transfer of heat from one object to another is called conduction.
2)
Convection
This is due to the way heat flows in fluids, and both gases (like air) and liquids (like water) are considered fluids because of the way they move. When heat is transferred by currents in a fluid this is called convection.
3) Radiation
Radiation is a method of heat transfer that does not rely upon any contact between the heat source and the heated object as is the case with conduction and convection. Heat can be transmitted through empty space by thermal radiation often called infrared radiation. This is a type electromagnetic radiation .
b) The heat loss from the wall by convection =?
we know that, heat loss, Q = 2h1 A(T1 -T2 )
h1 =?
Nu = hx L / K
Nu = 0.664 Re0.5 Pr1/3
= 0.664 X 0.550.5 X 0.731/3
= 0.443
h = Nu X K / L
= 0.443 X 0.024 / 10
= 1.06 X 10-3
h1 = 2h
= 2 X 1.06 X 10-3
=2.12 X 10-3
Q = 2 X 2.12 X 10-3 X 10 X 4 ( 12-5)
= 1.18
c)
The heat loss would be approximately remains same if the speed of air doubles..it is not proportional to the air.
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