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Heat Transfer

source: general lecture book about Thermofluida & thermodynamic

what is heat transfer ?

heat transfer is energy in transit due to a temperature difference

how is energy transferred by heat ?

by difference type of heat transfer processes, there are conduction, convection, and radiation.

·         conduction is the heat transfer that will accur across between two different temperatures medium.

- Thermal conductifity (k) is proportionality constant k is a transport property.

- Heat flux (q”_X ) is the rate of heat transfer per unit area.

   

·         convection is heat transfer that will occur between a surface and moving or stationary fluid when they are at different temperatures. 

·         radiation is heat transfer that will occur by thermal radiation between two surface different temperatures.

Example 1

The wall of  an industrial furnance is constructed from o,15 m thick fireclay brick having a thermal conductivity of 1,7 W/m.K. measurements made during steady state operation reveal temperatures of 1400 and 1150 K at inner and outer surface, respectively what is the rate of heat transfer through a wall is 0,5 m by 1,2 m on a side ?

·         known: steady-state conditions with prescribed wall thickness, area, thermal conductivity, and surface temperatures

·         find : heat transfer rate through the wall

·         answer

The heat flux represents the rate of heat transfer through a section of unit area, and it is uniform across the surface of the wall. The heat rate through the wall of area A= H  W is then,

Example 2

An uninsulated steam pipe passes through a large room in which the air and walla are at 25⁰C. the outside diameter of the pipe is 70 mm, and its surface temperature and emissivity are 200⁰C and 0,8 , respectively. What are surface emissive power and irradiation ? if the coefficient associated with free convection heat transfer from the surface to the air is 15 W/m².K and the surface is gray, what is the rate of heat transfer from the surface per unit length of pipe ?

·         known : uninsulated pipe of prescribed diameter, emissivity, and surface temperature in a large room with fixed wall and air temperatures.

·         Find : surface emissive power, E, and irradiation, G, pipe heat transfer per unit length, q’.

Heat transfer from the pipe is by convectionto the room air and by radiation exchange with the walls. Hence, q = q_conv + q_rad and from equation 15.10, with A = DL

 the heat transfer per unit length of pipe is then