Transferência de Calor por Convecção Profa. Flávia Zinani PPGENGMEC fzinani@unisinos.br – sala 6A-234 Introduction Introdução Introduction Introdução Introdução Introduction Introduction Introdução Local and Average Coefficients Distinção entre Coeficiente de Transferência de Calor Local e Médio • Coeficiente e Fluxo de Calor Local: qs h Ts T Fluxo de Calor Médio e Coeficiente para Temperatura Uniforme na Superfície • q hAs Ts T q As qdAs Ts T A hdAs s 1 h As hdAs As Boundary Layer Features Boundary Layers: Physical Features Boundary Layer Features Boundary Layers: Physical Features • Velocity Boundary Layer – A consequence of viscous effects associated with relative motion between a fluid and a surface. – A region of the flow characterized by shear stresses and velocity gradients. – A region between the surface and the free stream whose thickness increases in the flow direction. – Why does increase in the flow direction? – Manifested by a surface shear stress s that provides a drag force, FD . – How does s vary in the flow direction? Why? u y u u s y 0.99 y 0 FD s dAs As Boundary Layer Features (cont.) • Thermal Boundary Layer – A consequence of heat transfer between the surface and fluid. – A region of the flow characterized by temperature gradients and heat fluxes. – A region between the surface and the free stream whose thickness t increases in the flow direction. – Why does t increase in the flow direction? – Manifested by a surface heat flux q s and a convection heat transfer coefficient h . – If Ts T is constant, how do q s and h vary in the flow direction? t Ts T y Ts T T qs k f y h 0.99 y 0 k f T / y Ts T y 0 Transition Boundary Layer Transition • How would you characterize conditions in the laminar region of boundary layer development? In the turbulent region? • What conditions are associated with transition from laminar to turbulent flow? • Why is the Reynolds number an appropriate parameter for quantifying transition from laminar to turbulent flow? • Transition criterion for a flat plate in parallel flow: u x Re x ,c c critical Reynolds number xc location at which transition to turbulence begins 105 Re x ,c 3 x 106 ~ ~ Transition (cont.) What may be said about transition if ReL < Rex,c? If ReL > Rex,c? Transition (cont.) • Effect of transition on boundary layer thickness and local convection coefficient: Why does transition provide a significant increase in the boundary layer thickness? Why does the convection coefficient decay in the laminar region? Why does it increase significantly with transition to turbulence, despite the increase in the boundary layer thickness? Why does the convection coefficient decay in the turbulent region? Do que depende o Coeficiente de Transferência de Calor por Convecção? h h(Ts ,T , , , k , c p , u, x,geometria, dimensões) Utilizando o teorema dos Pi de Buckingham e tornando o problema adimensional • Nu Nu ( x*,Re,Pr ) hL Nu k Nu Nu (Re,Pr ) VL Re Pr Significado dos números adimensionais • Número de Reynolds Significado dos números adimensionais • Número de Prandtl: razão entre a difusividade de quantidade de movimento e a difusividade térmica. Está relacionado ao crescimento relativo entre as camadas-limite fluidodinâmica e térmica: • Pr<<1: metais líquidos, difusão térmica mais eficiente que difusão de momentum, t>>. • Pr1: gases, t. • Pr>>1: óleos, difusão de momentum mais eficiente que difusão térmica, t<<. Significado dos números adimensionais • Número de Nusselt: representa o gradiente de temperatura adimensional na superfície, mede a transferência de calor por convecção que ocorre nesta superfície. (provar) Significado dos números adimensionais Boundary Layer Equations The Boundary Layer Equations • Consider concurrent velocity and thermal boundary layer development for incompressible flow with constant fluid properties , c p , k . • Conservação da Massa: Equação da Continuidade div u 0 u v w 0 x y z ui ui ,i iui 0 xi • Equação de Navier-Stokes Du 2 p u g Dt u 1 [u]u p 2u g t What is the physical significance of each term in the foregoing equation? Referência para a dedução das equações de Navier-Stokes e da continuidade: Bejan, Adrian - Convective Heat Transfer Fox, McDonald e Pritchard – Introdução à Mecânica dos Fluidos