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تاريخ التسجيل : 06/01/2011
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 | موضوع: طرق انتقال الحرارة Modes of Heat Transfer  الأحد 19 يونيو 2011 - 14:40 | |
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Modes of Heat Transfer
OBJECTIVES:
-- To study different modes of heat transfer.
-- To determine rate of heat transfer in food and non-food materials
Why study heat transfer؟
-- to examine how foods are heated and cooled
-- to calculate the rate of heating and cooling
-- to design new heat transfer equipment
-- to assess the performance of existing heat exchange equipment
CONDUCTION
-- Energy transfer at a molecular level
-- No physical movement of the material
-- Heating/Cooling of the solid material
The rate of heat flux (rate of heat transfer per unit area) in a solid object is proportional to the temperature gradient, this can be stated mathematically as,
We may remove the proportionality by using a constant 'k', to obtain, Fourier’s Law
where
qx = rate of heat transfer in the x direction by conduction, W
k = thermal conductivity, W/mC
A = area (normal to x-direction) through which heat flows, m2
T = temperature, C
x = length, variable, m
SIGN CONVENTION
-dT/dx
Direction of heat flow
Temperature
Distance
Thermal Conductivity, k unit: W/mC
Metals: k = 50-400 W/mC
Water: k = 0.597 W/mC
Air : k = 0.0251 W/mC
Insulating materials: k = 0.035 - 0.173 W/mC
For foods
k = 0.25 mc + 0.155 mp + 0.16 mf + 0.135 ma + 0.58 mm
Where m is mass fraction and subscripts c: carbohydrate, p: protein, f: fat, a: ash, m: moisture.
CONVECTION
Fluid flow over a solid body -- heat transfer between a solid and a fluid.
Newton’s Law of Cooling:
q = h A (Tp-Ta
where: h is convective heat transfer coefficient (W/m2C), A is area (m2), Tp is plate surface temperature (°C), Ta is surrounding fluid temperature (°C).
Forced Convection - artificially induced fluid flow
Free (Natural) Convection -- caused due to density differences
Fluid condition h (W/m2C)
Air, free convection 5-25
Air, forced convection 10-200
Water, free convection 20-100
Water, forced convection 50-10,000
Boiling water 3,000-100,000
Condensing water vapor 5,000-100,000
RADIATION
Heat transfer between two surfaces by emission and later absorption of electromagnetic radiation
n requires no physical medium
n Stefen-Boltzmann Equation:
( q = A s e (T24 – T14
where s = Stefen-Boltzmann's constant, 5.669x10-8 W/m2K4
e = emissivity, (varies from 0 to 1) dimensionless
A = area, m2
T1 = temperature of surface 1, Absolute
T2 = temperature of surface 2, Absolute | |
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