tailieunhanh - Advanced Engineering Mathematics 2011 Part 12

Tham khảo tài liệu 'advanced engineering mathematics 2011 part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | The Heat Equation 429 Figure The temperature within the portion of a slab 0 x L 1 at various times a2t L2 if the faces of the slab radiate to free space at temperature zero and the slab initially has the temperature 1. The parameter hL 1. Therefore the inversion of U x s is Ws. t 1 - Í1 - 2h V cos An c exp q2A21 1 I An l ỉÃ sin AnZ AnZcos A J or u x t 2h n l cos Xnx exp q2A2t An l i sin AnZ XnL cos AnẢ We can further simplify by using h xn tan AnL and hL AnZtan AnẢ . Substituting these relationships into and simplifying u x t 2 n l sin AnẢ cos Anx exp q2A2t XnL sin AnẢ cos AnL Example Heat dissipation in disc brakes Disc brakes consist of two blocks of frictional material known as pads which press against each side of a rotating annulus usually made 430 Advanced Engineering Mathematics of a ferrous material. In this problem we determine the transient temperatures reached in a disc brake during a single brake If we ignore the errors introduced by replacing the cylindrical portion of the drum by a rectangular plate we can model our disc brakes as a one-dimensional solid which friction heats at both ends. Assuming symmetry about X 0 the boundary condition there is ur 0 t 0. To model the heat flux from the pads we assume a uniform disc deceleration that generates heat from the frictional surfaces at the rate N i Mt where M and N are experimentally determined constants. If u x t K and a2 denote the temperature thermal conductivity and diffusivity of the rotating annulus respectively then the heat equation is du 7d2u T a idr Q x L G t dt dx2 with the boundary conditions and a ỵ W l-MÍ . 0 dx dx The boundary condition at X L gives the frictional heating of the disc pads. Introducing the Laplace transform of u x t defined as U x s u x t e sidt Jo the equation to be solved becomes s 0 ax2 a subject to the boundary conditions that dU L s Nil M -2 - 0 and - - . dx dx K s .

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