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Một vấn đề thường gặp trong thiết kế truyền nhiệt là sự kết hợp của nhiều lớp vững chắc để cung cấp cho cách nhiệt, hoặc các lớp rắn và chất lỏng như trong thiết kế bộ trao đổi nhiệt. | 1.1.2 Effect of Shape on Calculation of Conduction Heat Transfer Evaluation of the integral between two specific points in the direction of heat transfer allows for the calculation of the macroscopic amount of heat. For an object with constant cross-sectional area in the direction of heat flow integration of Eq. 2 gives q k T1 - T2 A S2 - S1 5 Instead for a cylindrical object heat flow in the direction of the radius finds a constantly changing cross-sectional area. Thus integration of Eq. 2 with A 2nrL gives _ T1 - T2 q ln r2 r1 2nkl 6 which involves a logarithmic distance radius difference instead of the thickness of the medium involved. 1.1.3 Combined Resistances A common problem in heat transfer design is the combination of several layers of solid to provide heat insulation or layers of solid and fluid as in heat exchanger design. For a rectangular geometry two combined resistances to heat transfer can be expressed as q T1 - t2 A L1 k1 L2 k2 7 where L and k are the thickness and the thermal conductivity of components 1 and 2 respectively and T1 and T2 are the temperatures at the external surfaces of the combined wall. Each additional layer of material increases by one the resistances to heat transfer added in the denominator of Eq. 7 . The effect of geometry on the combined resistances to heat transfer can be obtained by integrating Eq. 2 for a double-layered cylinder T2 - T1__ q ln r2 r1 2nk1L tn r3 r2 2nk2L 8 1.1.4 Relative Magnitude of Values of Thermal Conductivity The values of the thermal conductivity depend on the phase of the material considered Copyright 2002 by Marcel Dekker Inc. All Rights Reserved. kgas kliquid ksolid Thus solid materials are good heat conductors while for heat insulation trapped gases are the best option. Good electric conductor metals are the best selection for heat conductors. The design of heat insulation follows the criteria for air entrapment in fabrics or ceramics that could be resistant to