# HEAT TRANSFER

INTRODUCTION
Heat transfer is defined as the form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference.

convection.Mechanisms of Heat Transfer Heat can be transferred in three different ways: conduction. . All modes of heat transfer are from the hightemperature medium to a lower temperature one. All modes of heat transfer require the existence of a temperature difference. and radiation.

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The better the conductor. . the more rapidly heat will transfer.CONDUCTION Conduction is the transfer of heat between substances that are in direct contact with each other.

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FOURIER’S LAW OF HEAT CONDUCTION .

72 W/mK. The right face temperature of the brick is 900C. .Example 1 : A flat wall is composed of 20 cm of brick having a thermal conductivity kt = 0. and the left face temperature of the brick is 20C. Determine the rate of heat conduction through the wall per unit area of wall.

72 A x m K 0.Tright = 900C Tleft = 20C 20 cm T  Qcond  kt A x  Q T W (900 20)K cond  kt  0.2m W  3168 2 m .

Exercise 1 : .

52 W/mK. . Calculate the rate of heat transfer per unit area through the wall if the wall thickness is 300mm with the thermal conductivity 12.Exercise 2 : The temperature at the surface in and out of a stone wall are respectively 50 ° C and 30 ° C.

If it is determined that heat is transferred through the plate at a rate of 500 W/m2. respectively. .Exercise 3 : Two surfaces of a 2-cm-thick plate are maintained at 0°C and 100°C. determine its thermal conductivity.

Convection heat transfer is the mode of energy transfer between a solid surface and the adjacent liquid or gas that is in motion. .CONVECTION Convection is the up and down movement of gasses and liquids caused by heat transfer. and it involves the combined effects of conduction and fluid motion.

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Example: A cup of hot coffee left to cool by itself.FREE CONVECTION Fluid movement caused by buoyancy forces that occur from differences in density due to fluid temperature difference. Example: A cup of hot coffee quickly blown to cool . FORCED CONVECTION Fluid motion caused by an external source.

NEWTON’S LAW OF COOLING .

Example 2 : Consider a person standing in a breezy room at 20°C. respectively. and the convection heat transfer coefficient is 6 W/ m2 · °C .6 m2 and 29°C. Determine the total rate of heat transfer from this person if the exposed surface area and the average outer surface temperature of the person are 1.

If the convection heat transfer coefficient is 55 W/ m2 · °C.Exercise 4 : Hot air at 80°C is blown over a 2-m X 4-m flat surface at 30°C. . determine the rate of heat transfer from the air to the plate.

. Determine the rate of heat loss from the pipe by convection. 10-m-long hot-water pipe at 80°C is losing heat to the surrounding air at 5°C by natural convection with a heat transfer coefficient of 25 W/m2 · °C.Exercise 5: A 5-cm-external-diameter.

. The radiative energy transferred is proportional to the difference in the fourth power of the absolute temperatures of the bodies exchanging energy.RADIATION Radiative heat transfer is energy in transition from the surface of one body to the surface of another due to electromagnetic radiation.

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HEAT FLOW THROUGH THE DIFFERENT MEDIA value of heat transfer rate per unit area is same for each media .

Then calculate the temperature at the outer surface of the tank. Calculate the rate of heat loss of the tank when the atmospheric temperature is 30 ° C. The thermal conductivity of steel is 50 W / mK and the coefficient of heat transfer inside and outside of each tank is 2800 W/ m2 K and 25 W/ m2 K. .Example 3 : A steel tank with a wall thickness of 5mm containing water temperature of 90 ° C.
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