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
# 9 Heat Transfer Su Yongkang School of Mechanical Engineering 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
Example Given Experimental results for measured local heat transfer coefficient h for flow over a flat plate with a rough surface 0.3 where:a=coefficienth(x)=ax x=distance from leading edge Find expression for average heat transfer coefficient,and the relation of average heat transfer coefficient to the local coefficient 万x= I jax03ds=ajx3d X0 hx 1ax07 0.7x 2.5 1.5 050 2 Distance from leading edge Heat Transfer Su Yongkang School of Mechanical Engineering
# 10 Heat Transfer Su Yongkang School of Mechanical Engineering 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 = = ∫ = ∫ − −
The Convection Boundary Layers Velocity Boundary Layer Free stream 一6(x) Velocity boundary layer For fluid flow over a flat plate,which disturbs the fluid flow: -ASy→0： u=uo where u is velocity in x-direction Asy-→0： u=0 (no-slip condition) The boundary layer thickness is defined as the value at which: u(y)=0.99u。 - The boundary layer thickness 8 varies with x Shear Stress Bu ay Dynamic viscosity y=0 Local friction coefficient u 2 Heat Transfer Su Yongkang School of Mechanical Engineering
# 11 Heat Transfer Su Yongkang School of Mechanical Engineering The Convection Boundary Layers Velocity Boundary Layer • For fluid flow over a flat plate, which disturbs the fluid flow: – As y→∞: where u is velocity in x-direction – As y→0: (no-slip condition) – The boundary layer thickness is defined as the value at which: – The boundary layer thickness δ varies with x • Shear Stress • Local friction coefficient =0 ∂ ∂ = y s y u τ µ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ = ∞ 2 2 u C s f ρ τ = ∞ u u u = 0 = ∞ u( y) 0.99u Dynamic viscosity