3 - Steady Heat Conduction Flashcards
How is heat transfer through a plane wall modelled?
Steady
One-dimensional
Equation for the rate of energy change of the wall
rate of heat transfer into the wall - rate of heat transfer out of the wall
Proportionality relationships of the rate of heat conduction through a plane wall
Proportional to the average thermal conductivity, wall area and temperature difference.
Inversely proportional to wall thickness.
Define conduction resistance of a wall
Thermal resistance of the wall against heat conduction.
What does the thermal resistance of a medium depend on?
Geometry and thermal properties of the medium.
How are the rate of heat transfer, thermal resistance and temperature difference related to electrical concepts?
Rate of heat transfer -> electric current
Thermal resistance -> electrical resistance
Temperature difference -> voltage difference
Define convection resistance of a surface
Thermal resistance of the surface against heat convection.
What happens to the convection resistance when the convection heat transfer coefficient is very large?
Convection resistance becomes zero
Therefore the surface offers no resistance to convection, and thus it does not slow down the heat transfer process.
Define radiation resistance of a surface
Thermal resistance of the surface against radiation.
What are the assumptions for solving complex multidimensional heat transfer problems by treating them as one-dimensional using the thermal resistance network?
Any plane wall normal to the x-axis is isothermal (i.e. assume the temperature varies in the x-direction only).
Any plane parallel to the x-axis is adiabatic (i.e. assume heat transfer occurs in the x-direction only).
How does additional insulation in a cylindrical pipe or spherical shell affect the conduction and convection resistances?
Increases conduction resistance.
Decreases convection resistance due to increase in outer surface area for convection.
What does Newton’s Law of Cooling reflect?
The rate of heat transfer from a surface to the surrounding medium.
When Ts and Tinfinity are fixed, what are the two ways to increase the rate of heat transfer?
Increase the convection heat transfer coefficient (h) - may require the installation of a pump or fan.
Increase the surface area (A) - add fins to the surface made of highly conductive materials such as aluminium.
How is heat transfer of the exposed surfaces of a fin related to heat conduction of the fin at the base under steady conditions?
They are equal.
What should the properties of a fin be?
Thermal conductivity (k) should be as high as possible.
Ratio of the perimeter to the cross-sectional area should be as high as possible.
Low convection heat transfer coefficient (h).
