Convection overview(Contd) Same principal applies to any arbitrary shape, not Just a flat plate Average convection heat transfer coefficient hdA. or, for unit width hdx S So. we need to know how h varies with x. the distance from the leading edge What do you think key parameters that might influence h? Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 6 Convection overview (Cont’d) • Same principal applies to any arbitrary shape, not just a flat plate • Average convection heat transfer coefficient: So, we need to know how h varies with x, the distance from the leading edge…….. What do you think key parameters that might influence h? q dAs As Ts , or, for unit width: = As s s hdA A h 1 = L hdx L h 0 1
Key parameters Transfer potential: forced flow or free flow Phase change: boiling and condensation Flow conditions laminar or turbulent flow Geometries: shape, size, position and roughness Properties: density, viscosity, thermal conductivity, specific heat, and so on Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 7 Key parameters • Transfer potential: forced flow or free flow • Phase change: boiling and condensation • Flow conditions: laminar or turbulent flow • Geometries: shape, size, position and roughness. • Properties: density, viscosity, thermal conductivity, specific heat, and so on
E Xample Given Experimental results for measured local heat transfer coefficient h for flow over a flat plate with a rough sur face where: a=coefficient h,(x)=ax 0.3 x= distance from leading edge Find expression for average heat transfer coefficient. and the relation of average heat transfer coefficient to the local coefficient sh Jar 03 dx=jx. dx 0 0.7 0.7x Distance from leading edge Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 8 Example Given • Experimental results for measured local heat transfer coefficient h for flow over a flat plate with a rough surface • where: a = coefficient x = distance from leading edge – Find expression for average heat transfer coefficient, and the relation of average heat transfer coefficient to the local coefficient ( ) −0.3 h x = ax x 0.7 0 0.3 0 0.3 0.7 1 1 x x a h x dx x a a x dx x h x x x x = = = − −