Transferência de Calor por Convecção

1. Transferência de Calor por Convecção Profa. Flávia Zinani PPGENGMEC fzinani@unisinos.br– sala 6A-234

2. Introdução Introduction

3. Introdução Introduction

4. Introdução Introduction

5. Introdução Introduction

6. Distinção entre Coeficiente de Transferência de Calor Local e Médio Local and Average Coefficients • Coeficiente e Fluxo de Calor Local: • Fluxo de Calor Médio e Coeficiente para Temperatura Uniforme na Superfície

7. Boundary Layers: Physical Features Boundary Layer Features

8. 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 that provides a drag • force, . • How does vary in the flow • direction? Why? Boundary Layers: Physical Features Boundary Layer 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.

9. 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 heat • flux and a convection heat • transfer coefficient h . • If is constant, how do and • h vary in the flow direction? 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.

10. Boundary Layer Transition 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:

11. Transition (cont.) What may be said about transition if ReL < Rex,c? If ReL > Rex,c?

12. 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?

13. Do que depende o Coeficiente de Transferência de Calor por Convecção? • Utilizando o teorema dos Pi de Buckingham e tornando o problema adimensional

14. Significado dos números adimensionais • Número de Reynolds

15. 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>>. • Pr1: gases, t. • Pr>>1: óleos, difusão de momentum mais eficiente que difusão térmica, t<<.

16. 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)

17. Significado dos números adimensionais

18. The Boundary Layer Equations Boundary Layer Equations • Consider concurrent velocity and thermal boundary layer development for incompressible flow with constant fluid properties . • Conservação da Massa: Equação da Continuidade

19. Equação de Navier-Stokes 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