1. Chapter 7 Natural convection systems

2. 7-1 Introduction  natural or free convection: the motion of the fluid due to density changes arising from heating process  Buoyancy forces that given rise to the free-convection currents are body forces  Examples: radiator, refrigerator and electronic components  In infinite spaces ( 大空间自然对流 ) No body have influence on the natural convection  in enclosed spaces ( 有限空间自然对流 ) The natural convection is affected by somebody

3. 7-2 Free-convection heat transfer on a vertical flat plate  Velocity profile is different from forced convection  Temperature profile is similar to forced convection  Out of boundary layer, the T=const. u=0  The flow may be laminar flow, or turbulent flow Solutions Differential equation Integral equation

4. Where 7-3 Empirical relations for free-convection  volume coefficient of expansion For ideal (perfect) gas pv=RT Reference temperature

5.  Characteristic dimensions  The values of the constants C and m are listed in table  Rayleigh number  the length L for vertical surface  Diameter d for horizontal cylinder  A vertical cylinder may be treated as a vertical flat plate when

7. 7-4 Free convection from vertical planes and cylinders 7-5 Free convection from horizontal cylinders 7-6 Free convection from horizontal plates 7-7 Free convection from inclined surfaces 7-8 Nonnewtonian fluids 7-9 Simplified equations for air 7-10 Free convection from spheres canceled

8. 7-11 Free convection in enclosed spaces Grashof number based on the distance  Correlations (7-54), (7-55) and (7-60)

9. Momentum equation Criterion for free or forced convection Free convection effects should be considered Free convection is of primary importance 7-12 Combined free and forced convection