传热学上海交通大学Heat Exchangrs 3.pptVIP

HEAT TRANSFER CH6 – INTRODUCTION Where we’ve been …… Basic Conduction Heat Transfer Finished Fourier’s law: Where we’re going: Begin study of convective heat transfer. Newton’s law of cooling: Convective transfer problem CH6 INTRODUCTION KEY POINTS THIS CHAPTER What are the key variables when analyzing convection heat transfer? Review boundary layer concept and significance General idea of relationship between velocity and thermal profiles in a boundary layer. Effect of laminar versus turbulent flow on heat transfer potential Boundary layer similarity This chapter will be taught in two lectures: the first includes text book sections §6.1 to 6.4 the other includes text book sections §6.5 to 6.10 Convection overview Consider a flat plate of length L, in air flow with velocity u? and temperature T? Local heat flux is: where h is the  local heat transfer coefficient Total heat transfer rate: 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? 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 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 The Convection Boundary Layers Velocity Boundary Layer For fluid flow over a flat plate, which disturbs the fluid flow: As y??: