流体力学与 及传热课件Heat Transfer to Fluids without Phase Change.pptVIP

4.4 Heat Transfer to Fluids without Phase Change ; Introduction; In most industrial processes where heat transfer is occurring, heat is being transferred from one fluid to a second fluid. ; The convective coefficient for heat transfer through a fluid is given by ;The type of fluid flow of the individual fluid has a great effect on the heat-transfer coefficient h. The more turbulence, the greater the heat-transfer coefficient. ;To correlate these data for heat-transfer coefficients, dimensionless numbers such as the Reynolds and Prandtl numbers are used. The Prandtl number is defined as:;Values of the Pr for gases range from about 0.5 to 1.0. Values for liquids range from about 2 to well over 104. ;Nusselt number, Nu, is used to relate data for the heat-transfer coefficient h to the thermal conductivity k of the fluid and a characteristic dimension d: ;The Graetz number is commonly used in treating heat transfer to fluids and is defined by the equation ;The Graetz number can also be calculated from the Reynolds and Prandtl numbers and the D/L ratio ;Empirical equation;4.4.2 Heat Transfer by Forced Convection in Laminar Flow ;Different types of correlations for the convective coefficient are needed for laminar flow, for fully turbulent flow, and for the transition region.;4.4.2.1 Laminar flow heat transfer to flat plate ;The average coefficient is ;4.4.2.2 Laminar flow heat transfer in tubes;This equation holds for (Re·Pr·d/L) 100. If used down to (Re·Pr·d/L) 10, it still holds to ±20%. For (Re ·Pr·d/L) 100, another expression is available.;The decreasing length is a result of the change in shape of the temperature profile. For short lengths, the thermal boundary layer is very thin, and the steep temperature gradient gives a high local coefficient. ;With increasing distance from the entrance, the boundary layer becomes thicker and eventually reaches the center of the pipe, giving a nearly parabolic temperature profile. ;The local coefficient is approximatel