HEAT TRANSFER BY CONVECTION
For a fluid at a mean temperature to over a surface at a temperature Ts Newton proposed the following heat convection equation.
q = Q/A = h(Ts – T)
from the fig. it is seen than velocity of a fluid layer at the wall is zero (no slip condition) so the heat must be transferred only by conduction over there. It has been found that the heat transfer coefficient h varies significantly with the type of fluid and temperature.
Typical values of convection heat transfer coefficient
Types of flow h, W/m2o or W/m2.K
Free convection air 5 – 28
Forced convection air 10 – 500
Water 100 – 17,000
Boiling Water 2,500 – 57,000
Condensation of steam 5000 -110,000
The coefficient of heat transfer ‘h’ (also known as film heat transfer coefficient ) may be defined as “the amount of heat transmitted for a unit temperature difference between the fluid and unit area of surface in unit time”.
The value of ‘h’ depends on the following factors :
- Thermodynamic and transport properties (e.g. viscosity, density, specific heat etc.,)
- Nature of heat flow.
- Geometry of surface
- Prevailing thermal conditions
Since ‘h’ depends upon several factors, it difficult to frame single equation to satisfy all the variation; however, by dimensional analysis as equation for the purpose can be obtained.