Convection
Convection
Convection ▣ Definition:between fluid in motion and a bounding surface when the two are at different temperatures ■ Natural Convection (water boiling):gravity- induced ■ Forced convection(disturbing,blowing, Breathing):pressure gradient Boiling and Condensation:Latent heat Exchange
Convection Definition: between fluid in motion and a bounding surface when the two are at different temperatures Natural Convection（water boiling）: gravityinduced Forced convection( disturbing, blowing, Breathing): pressure gradient Boiling and Condensation: Latent heat Exchange
Convective Transfer Coefficient ▣ Newton's Law of cooling g =h(T:-T) h=fn(flow condition,properties,surface geometry)
Convective Transfer Coefficient Newton’s Law of cooling ( ) '' = T − T∞ q h s h=fn( flow condition, properties, surface geometry)
Free stream T 6,x) Thermal boundary layer T+ Thermal Boundary Layer development on an isothermal flat plate Thickness T.-T T.-T. =0.99
Thermal Boundary Layer development on an isothermal flat plate δ t = 0.99 − − T T∞ T T s s Thickness
Convective Heat transfer Coefficient h= T、-T y=0 h= Empirical or Nusselt c N- Semi-empirical Number
0 0 ( ) ( ) = − − ∞ ∂ = ∂ − = − − ⋅ = ∞ y LyT T T T cf s y y T f cs s dd Lk T T k h Nu Nusselt Number f u c k N h L = Empirical or Semi-empirical Convective Heat transfer Coefficient
Local and Average heat transfer coefficients 9。-j∬9.aS=Jj小q.dkd gs=h(x,y)(I,-T) q,=(T,-T.)h(x,y)dxdy =h(T:-T)A 万=(x,d
Local and Average heat transfer coefficients q q dS q dxdy s s s s s = = '' '' ( , )( ) '' = T − T∞ q h x y s s h T T A q T T h x y dxdy s s s s ( ) ( ) ( , ) ∞ ∞ = − = − h x y dxdy A h s = ( , ) 1
Convection Over the Body Surface Forced Convection: L=D 万≈12.1ua 4a<2.6m/s Colin Houdas,1967; Winslow et.al.,1939 Natural Convection 万≈2.38|T,-Tp25T。一Air Temporatur Nielsen and Pedersen,1953
Convection Over the Body Surface Forced Convection: Natural Convection L c = D 0.5 h ≈12.1 uair 0.25 2.38 | | ≈ T − T∞ h s T∞ Air Temperature u m s air < 2.6 / Colin & Houdas, 1967; Winslow et. al., 1939 Nielsen and Pedersen, 1953
Boiling regimes Free convection Nucleate Transition Film Isolated Jets and bubbles columns 107 Critical heat flux,gmax -qmax 106 105 B D q品in Leidenfrost point.qmin 104 ONB ATe.A ATe.B △Te,c △Te.D 103 5 10 30 120 1000 ATe=T;-Tsat (C) FIGURE 10.4 Typical boiling curve for water at 1 atm:surface heat flux q"as a func- tion of excess temperature,△T。=T,-Tat
T.<Tut Vapor 0 Drop 0 0 0 0 Film Fog (a) (b) (c） Liquid Vapor Vapor Liquid spray 0000 0000 o0 Droplets 0000 Liquid (d) m=7 9 hLA(Tsa-T:) h
( ) ' ' L sat s fg fg q h AT T m h h • − = =
Mass Transfer thru Sweat “Humid and Hot'”or“Dry and Hot”？ Latent heat:Energy required for the phase change (i.e. qeap=hg·meap 侣 liquid-->gas)J/Kg Mevap Evaporation rate Kg/s ip=inA.[par(T,）-pPar(T】 五n Mass Transfer Coefficient in analogy to hf(uir.) Ts skin surface temp.;T air temp
Mass Transfer thru Sweat evap fg evap q hm = ⋅ fg h mevap “Humid and Hot” or “Dry and Hot” ? [ ( ) ( )] m hA T T evap m s sat s sat = − ρ ϕρ∞ ∞ Latent heat : Energy required for the phase change (i.e. liquid-->gas) J/Kg Evaporation rate Kg/s m h Mass Transfer Coefficient in analogy to ~( ) n air h fu Ts skin surface temp.; T∞ air temp