《传热学》NATURAL CONVECTION SYSTEMS.ppt

CHAPTER 7 NATURAL CONVECTION SYSTEMS INTRODUCTION VERTICAL FLAT PLATES EMPIRICAL RELATIONS VERTICAL PLATES AND CYLINDERS HORIZONTAL CYLINDERS ENCLOSED SPACES Introduction What is natural convection? No external forces Temperature difference induced Density changes Gravity field The body forces/buoyancy forces that give rise to the free-convection currents The influencing factors Temperature difference Fluid properties Size of solid surfaces Geometry The thermal boundary conditions Constant surface temperature Constant wall heat flux Configuration The influences of configurations The influences of configurations The Boundary Layer The Development of the Boundary Layer g the acceleration of gravity, m/s2 ? the volume coefficient of expansion, 1/K The Grashof Number The Velocity Profiles The velocity profile of natural convection boundary layers is quite different from that of forced convection boundary layers Heat Transfer The Nu is a function of Gr and Pr Nu=f(Gr, Pr) Nu=C (GrPr)m The Fluid Properties Temperature Tf =?(Tw+T?) The Characteristic Dimensions Vertical plates cylinders: height of surface L Horizontal cylinders: diameter d The other geometry: given by the researchers Free Convection from Vertical Planes And Cylinders Isothermal Surfaces The classical correlation Free Convection from Horizontal Cylinders The Classical Correlation: See T7-1pp346 The Churchill-Chu Correlation Free Convection in Enclosed Spaces Limited or enclosed spaces The development of the boundary layer is confined in a given space. Effective thermal conductivity Effective thermal conductivity From which one can get Heat Transfer Relations Heat Transfer Correlations Experimental results are usually expressed in the form of Example A 50-mm-diameter hot water tube having a length of 3m is maintained at a constant temperature of 70?C. It is exposed to air at 1 atm and 20?C, Calculate the free convection heat loss if it is placed horizontally vertically Explain your re