流体力学与 及传热课件Heat Transfer and Its Applications.pptVIP

Chapter 4 Heat Transfer and Its Applications ;4.1.1 Nature of heat flow ;Steady-State Heat Transfer ;Basic Mechanisms of Heat Transfer;4.1.2 Conduction ;In solids that are poor conductor of electricity and in most liquids, thermal conduction results from momentum transfer between adjacent vibrating molecules or atoms.; In gases, conduction occurs by the random motion of molecules. ;4.1.3 Convection ; The convection flux is usually proportional to the difference between the surface temperature and temperature of the fluid, as stated in Newton’s law of cooling; Note that the linear dependence on the temperature driving force tw-tf is the same as that for pure conduction in a solid of constant thermal conductivity.; The heat-transfer coefficient is not an intrinsic property of the fluid, but depends on the flow patterns determined by fluid mechanics as well as on the thermal properties of the fluid.;The forces used to create convection currents in fluids are of two types:;Natural convection;forced convection;4.1.4 Radiation;If , however, matter appears in its path, the radiation will be transmitted, reflected, or absorbed. It is only the absorbed energy that appears as heat.; The energy emitted by a black body is proportional to the fourth power of the absolute temperature.;4.2 Heat Transfer by Conduction ;The basic relation for heat flow by conduction is the proportionality between heat flux and the temperature gradient.;The partial derivative calls attention to the fact that the temperature may vary with both location and time. ; In using equation it must be clearly understood that the area A is that of a surface perpendicular to the flow of heat and distance n is the length of path measured perpendicularly to area A.;For steady one-dimensional flow. Equation may be written;Thermal Conductivity ;k vary over a wide range. They are highest for metals and lowest for finely powdered materials from which air has been evacuated.;Fourie’s law states that