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TEKNIK MESIN
FAKULTAS TEKNOLOGI INDUSTRI
UNIVERSITAS MERCU BUANA
MODUL KE SATU
HEAT TRANSFER
NANANG RUHYAT
BASICS OF HEAT TRANSFER
Basics of Heat Transfer
In the simplest of terms, the discipline of heat transfer is concerned with only two things:
temperature, and the flow of heat. Temperature represents the amount of thermal energy
available, whereas heat flow represents the movement of thermal energy from place to
place.
On a microscopic scale, thermal energy is related to the kinetic energy of molecules. The
greater a material's temperature, the greater the thermal agitation of its constituent
molecules (manifested both in linear motion and vibrational modes). It is natural for regions
containing greater molecular kinetic energy to pass this energy to regions with less kinetic
energy.
Several material properties serve to modulate the heat tranfered between two regions at
differing temperatures. Examples include thermal conductivities, specific heats, material
densities, fluid velocities, fluid viscosities, surface emissivities, and more. Taken together,
these properties serve to make the solution of many heat transfer problems an involved
process.
Heat Transfer Mechanisms
Heat transfer mechanisms can be grouped into 3 broad categories:
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Heat Equation (Temperature Determination)
The temperature profile within a body depends upon the rate of its internally-generated heat,
its capacity to store some of this heat, and its rate of thermal conduction to its boundaries
(where the heat is transfered to the surrounding environment). Mathematically this is stated
by the Heat Equation,
along with its boundary conditions, equations that prescribe either the temperature T on, or
the heat flux q through, all of the body boundaries.
In the Heat Equation, the power generated per unit volume is expressed by qgen. The thermal
diffusivity
is related to the thermal conductivity k, the specific heat c, and the density
For Steady State problems, the Heat Equation simplifies to,
Derivation of the Heat Equation
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by,
The transfer of heat is normally from a high temperature object to a lower temperature
object. Heat transfer changes the internal energy of both systems involved according to the
First Law of Thermodynamics.
Heat transfer from a cold to a hotter region
Heat Convection
Convection is heat transfer by mass motion of a fluid such as air or water when the heated
fluid is caused to move away from the source of heat, carrying energy with it. Convection
above a hot surface occurs because hot air expands, becomes less dense, and rises (see
Ideal Gas Law). Hot water is likewise less dense than cold water and rises, causing
convection currents which transport energy.
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