Chapter 7 Natural convection systems 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 ( ) No body have influence on the natural convection in enclosed spaces ( ) The natural convection is affected by somebody 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 Where 7-3 Empirical relations for free-convection volume coefficient of expansion For ideal (perfect) gas pv=RT Reference temperature 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 7-4 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 7-11 Free convection in enclosed spaces Grashof number based on the distance Correlations (7-54), (7-55) and (7-60) 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