tailieunhanh - Heat Transfer Handbook part 3

Heat Transfer Handbook part 3. The Heat Transfer Handbook provides succinct hard data, formulas, and specifications for the critical aspects of heat transfer, offering a reliable, hands-on resource for solving day-to-day issues across a variety of applications. | 10 BASIC CONCEPTS 4A de ---- P In the inlet zones of such parallel-plate channels and along isolated plates the heat transfer coefficient varies with the distance from the leading edge. The low-velocity or laminar flow average convective heat transfer coefficient along a surface of length L for Re 3 x 105 is found to be h 2 Pr1 3 W m2 K where k is the fluid thermal conductivity L the characteristic dimension of the surface and Re the Reynolds number based on L namely VL v. A similar relation applies to a flow in tubes pipes annuli or channels with the equivalent diameter de serving as the characteristic dimension in both the Nusselt and Reynolds numbers. For laminar flow Re 2100 hde de 1 3 0 14 i Re Pr pLj which is attributed to Sieder and Tate 1936 and where ro is the viscosity of the convective medium at the channel wall temperature. Observe that this relationship shows that the heat transfer coefficient attains its maximum value at the inlet to the channel and decreases as de L decreases. In higher-velocity turbulent flow along plates the dependence of the convective heat transfer coefficient on the Reynolds number increases and in the range Re 3 x 105 h 8 Pr1 3 W m2 K In pipes tubes annuli and channels turbulent flow occurs at an equivalent diameter-based Reynolds number of 10 000 with the regime bracketed by 2100 Re 10 000 usually referred to as the transition region. For the transition region Hausen 1943 has provided the correlating equation e -k. Re - 125 Pr1 3 1 and Sieder and Tate 1936 give for turbulent flow 7 7 II -k. 8 Pr1 3 Í Forced convection in internal and external flows is treated in greater detail in Chapters 5 and 6. HEAT TRANSFER FUNDAMENTALS 11 Phase-Change Heat Transfer Boiling heat transfer displays a complex dependence on the temperature difference between the heated surface and the saturation temperature boiling point of the liquid. Following Rohsenow 1952 the heat