流体力学与传热（伍钦）08chapter4-3.pptVIP

For flow inside helical coils and Re above 104, the predicted film coefficient for straight pipes should be increased by the factor (1 + 3.5d/Dcoil). Heat-Transfer Coefficient for Transition Flow Inside a Pipe In the transition region for a Re between 2100 and 6000, the empirical equations are not well defined, just as in the case of fluid friction factors. No simple equation exists for accomplishing a smooth transition from heat transfer in laminar flow to that in turbulent flow. A graphical method therefore is used. From R. H. Perry and C. H. Chilton, Chemical Engineers Handbook. 5th ed. New York: McGraw-Hill Book Company, 1973. With permission. Substituted for the Graetz number, using Eqs. (4.4-13), is the quantity (πD/4L)RePr. The result is (4.4-35) Heat-Transfer Coefficient for Noncircular Conduits A heat-transfer system often used is one in which fluids flow at different temperatures in concentric pipes. The heat-transfer coefficient of the fluid in the annular space can be predicted by using the same equations as for circular pipes. However, the equivalent diameter must be used. For other geometries, an equivalent diameter can also be used. For an annular space, de is the ID of the outer pipe D1 minus the OD of the inner pipe D2. Entrance-Region Effect on Heat-Transfer Coefficient Near the entrance of a pipe where the fluid is being heated, the temperature profile is not fully developed and the local coefficient h is greater than the fully developed heat-transfer coefficient h for turbulent flow. At the entrance itself, where no temperature gradient has been established. The value of h drops rapidly and is approximately the same as h at L/D≈60, where L is the entrance length. problem The Prandtl number of gases is almost （ ） of temperature because the viscosity and thermal conductivity both increase with temperature at about the same rate. Assuming that fluid flows through a pipe in turbulent flow,