学校代码： 10128学 号： 031203060 本科毕业论文外文翻译（ 题 目：空冷热交换器和空冷塔 学生姓名： 学 院：电力学院 系 别：能源与动力工程 专 业：热能与动力工程 班 级：动本2003 指导教师： 二 七 年 六 月内蒙古工业大学本科毕业论文Air-cooled Heat Exchangers and Cooling TowersD.G.KROGER Sc.D. (MIT)(This text is a part of MR KROGERs book. include 8.4, 9.3, 9.4)8.4 RECIRCULATIONHeated plume air may recirculate in an air-cooled heat exchanger, thereby reducing the cooling effectiveness of the system. Figure 8.4.1 depicts, schematically, a cross-section of an air-cooled heat exchanger. In the absence of wind, the buoyant jet or plume rises vertically above the heat exchanger. A part of the warm plume air may however be drawn back into the inlet of the tower. This phenomenon is known as recirculation. Plume recirculation is usually a variable phenomenon influenced by many factors, including heat exchanger configuration and orientation, surrounding structures and prevailing weather conditions. Because of higher discharge velocities, recirculation is usually less in induced draft than in forced draft designs. Figure 8.4.1: Air-flow pattern about forced draft air-cooled heat exchanger.Lichtenstein 51LI1 defines a recirculation factor as (8.4.1)where mr is the recirculating air mass flow rate, while ma is the ambient air flow rate into the heat exchanger.Although the results of numerous studies on recirculation do appear in the literature, most are experimental investigations performed on heat exchangers having specific geometries and operating under prescribed conditions e.g. 74KE1, 81SL1. Gunter and Shipes 72GUll define certain recirculation flow limits and present the results of field tests performed on air-cooled heat exchangers. Problems associated with solving recirculating flow patterns numerically have been reported 81EP1. Kroger et al. investigated the problem analytically, experimentally and numerically and recommend a specific equation with which the performance effectiveness of essentially two-dimensional mechanical draft heat exchangers experiencing recirculation, can be predicted 88KR1, 89KR1, 91DU1, 93DU1, 95DU1.8.4.1 RECIRCULATION ANALYSISConsider one half of a two-dimensional mechanical draft air-cooled heat exchanger in which recirculation occurs. For purposes of analysis, the heat exchanger is represented by a straight line at an elevation Hi above ground level as shown in figure 8.4.2(a). Figure 8.4.2: Flow pattern about heat exchanger.It is assumed that the velocity of the air entering the heat exchanger along its periphery is in the horizontal direction and has a mean value, vi (the actual inlet velocity is highest at the edge of the fan platform and decreases towards ground level). The outlet velocity, vo, is assumed to be uniform and in the vertical direction.Consider the particular streamline at the outlet of the heat exchanger that diverges from the plume at 1 and forms the outer boundary of the recirculating air stream. This streamline will enter the platform at 2, some distance Hr below the heat exchanger. For purposes of analysis it will be assumed that the elevation of 1 is approximately Hr above the heat exchanger. If viscous effects, mixing and heat transfer to the ambient air are neglected, Bernoullis equation can be applied between 1 and 2 to give (8.4.2)It is reasonable to assume that the total pressure at I is approximately equal to the stagnation pressure of the ambient air at that elevation i.e. (8.4.3)At2 the static pressure can be expressed as (8.4.4)Furthermore, for the ambient air far from the heat exchanger (8.4.5)Substitute equations (8.4.3), (8.4.4) and (8.4.5) into equation (8.4.2) and find (8.4.6)Due to viscous effects the velocity at the inlet at elevation Hi is in practice equal to zero. The Velocity gradient in this immediate region is however very steep and the velocity peaks at a value that is higher than the mean inlet velocity. Examples of numerically determined inlet velocity distributions for different outlet velocities and heat exchanger geometries are shown in figure 8.4.3 95DU1. Since most of the recirculation occurs in this region the velocity v2 is of importance but difficult to quantify analytically. For it will be assumed that v2 can be replaced approximately by the mean inlet velocity, vi, in equation (8.4.6). Thus