摘要
传热强化技术对提高换热设备传热性能与减少换热设备重量具有十分重要的意义,扭带作为强化传热的主要措施之一,其相关的基础与应用研究一直备受重视。
本文建立了不同余隙率及扭率的扭带换热管以及螺旋和直板组合式扭带换热管的数学模型及物理模型,采用大型流体分析软件Fluent中的RNG k-ε模型分别对它们进行数值模拟研究,通过观察换热管中流体的速度场、温度场、压力场及流线图,分析了各种插入件换热管的换热特性及阻力特性,并在此基础上通过综合性能评价标准分别对其进行综合性能分析,得出的主要结论如下:①同一Re数下,普通扭带换热管的Nu数和f数均随着扭带余隙率和扭率的增大而减小;当扭带余隙率CR=0.1,扭率Y=5时换热管具有最佳的综合传热性能,强化传热效果最明显。
②对于内置螺旋和直板组合式扭带换热管,平均表面换热系数和压力损失均随着螺旋段长度比例的增加而增大,螺旋段长度约占组合式扭带长度1/4时,换热管的综合强化传热性能最佳。组合式扭带直板段的宽度对Nu数影响不大,但f随着直板宽度的增加而增大。对于直板和螺旋间隔排列的组合式扭带换热管,直板长度为螺旋长度1.5～2倍时换热管能够发挥最佳的强化传热性能。
③在整个模拟Re数范围内,螺旋和直板组合式扭带的强化传热性能均优于普通扭带,但在高Re数下,特别是当Re数大于25000时,换热管中内置插入件由于其强旋流,会产生较大的压力损失,从而使综合强化传热性能削弱,如果对负荷有限制的话则不适宜采用插入件式换热管。
Heat transfer enhancement technology has very important significance to improve the heat transfer performance and decrease the weight of heat exchange equipment, the related basic and applied research of twist tape has caught much attention as one of heat transfer enhancement measures.
Mathematical model and physical model of many insert heat exchanger tubes were established, the insert include: twisted tape of different clearance ratio and twist ratio, modular twisted tape which is made up spiral and straight. The Renormalized Group(RNG) k-εturbulence model was used to simulate the heat exchange tube. The velocity field ,temperature field ,pressure field and streamlines of heat exchanger tubes are obtained and the heat transfer and pressures drop characteristics are investigated through the software fluent. On this basis, the comprehensive performance is respectively derived through an integrated performance evaluation criteria, some important results and conclusions are as follows:
①The Nu number and f number of the heat transfer tube decreased with the increasing of CR and Y. The heat transfer tube which fit with twist tape of clearance rate CR=0.1and twist ratio Y =5 has the most obvious heat transfer enhancement effect and hold the best heat transfer performance.
②The heat transfer efficiency and pressure loss are increased with the increasing of spiral length for heat exchanger tubes with modular twisted tape, and heat exchanger tubes have the best heat transfer performance while the length of spiral accounts for 1/4 of the length of modular twisted tape. The width of straight have no meaning for Nu number, however f is increased with the increasing of width. For the tubes with the modular twisted tape which the straight and twisted placed intevail, the tubes have the best heat transfer performance while the straight is 1.5 or 2 times as length as spiral.
③The modular twisted tapes have better heat transfer performance than common twisted tapes in the whole rang of Re number simulated. Due to the strong swirl caused by the insert in the heat exchanger, the pressure loss is huge while at high Re number, especially when Re>25000, this would impair comprehensive heat transfer performance. If the load is limited, the insert-tape heat exchanger is not suitable.
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