传热学教学课件电子教案全套课件一.pptx

HEAT TRANSFERCHAPTER 6Introduction to convection # CH6 – INTRODUCTIONWhere we’ve been ……Basic Conduction Heat Transfer FinishedFourier’s law:Where we’re going:Begin study of convective heat transfer.Newton’s law of cooling: # Convective transfer problem # CH6 INTRODUCTIONKEY POINTS THIS CHAPTERWhat are the key variables when analyzing convection heat transfer?Review boundary layer concept and significanceGeneral idea of relationship between velocity and thermal profiles in a boundary layer.Effect of laminar versus turbulent flow on heat transfer potentialBoundary layer similarityThis 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 overviewConsider 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 coefficientTotal heat transfer rate:average heat transfer coefficientDetermination of ‘h’ will rely on analytical as well as empirical data # Convection overview (Cont’d)Same principal applies to any arbitrary shape, not just a flat plateAverage 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 flowPhase change: boiling and condensationFlow conditions: laminar or turbulent flowGeometries: shape, size, position and roughness.Properties: density, viscosity, thermal conductivity, specific heat, and so on. # ExampleGivenExperimental results for measured local heat transfer coefficient h for flow over a flat plate with a rough surfacewhere: a = coefficient x = distance from leading edgeFind expression for average heat transfer coefficient, and the relation of average heat transfer coefficient to the local coefficient # The Convection Boundary LayersVelocity Boundary LayerFor f