环形窄缝通道内水流动换热特性数值研究/Numerical Study on the Characteristic of Flow and Heat Transf

2018-06-16 01:54:37

heat inner 换热 wall outer



狭缝传热技术是近几年发展起来的一门新兴的传热技术,具有传热效率高、结构紧凑等优点。由于其在现代工业和高新技术领域具有广泛的应用背景而受到人们的重视,是当前流动与传热研究的重要课题。迄今为止,对于狭缝通道的流动与传热特性已进行了许多研究,并发现了一些不同于常规流道的流动与传热现象,但是所得结论存在较大的差异,甚至相互矛盾。至今对狭缝传热的机理及影响因素仍未获得清晰统一的认识。 本文对环形窄缝通道内单相水的流动与换热特性进行了数值分析。针对通道内层流和湍流两种不同的流动状态分别建立了理论模型,并对模型进行了数值求解。通过对层流模型的求解,得到了环形窄缝通道内层流换热系数与内外管热流比的关系曲线;通过对湍流模型的求解,得到了通道内速度分布,温度分布,脉动动能分布,粘性耗散分布和轴向换热系数的理论计算值。把换热系数的理论值和试验值进行了比较,得到了比较一致的结果。 研究结果表明,对于层流对流换热,不同的加热热流密度比对环形通道内、外壁与流体的对流换热特性有着重大影响。当内、外壁加热热流密度之比较小时,内壁的对流换热特性较外壁的对流换热特性好。当内、外壁加热热流密度比值大到一定程度时,外壁的对流换热系数值将超过内壁的对流换热系数值。 对于湍流对流换热,流体截面平均温度沿轴向线性增加,而内外壁温度变化曲线在起始阶段斜率很大,以后逐渐减小,最后基本不变,其大小和流体温度增长率几乎完全一样.由于脉动动能耗散的作用,通道内速度分布梯度越来越小。通道内脉动动能和脉动动能的耗散有十分相似的分布规律,都是在壁面附近区域比较大,而在远离壁面区域减小。随Re数的增加,局部Nu数均呈增加趋势,而且在Re数相同的情况下,内管的Nu数大于外管,说明内管的换热强于外管。在内外管热流相同的情况下,Re增大,内外管Nu数的差距减小。随狭缝减小,局部Nu数增大。当外管的热流密度恒定时,随内管热流密度的增大,外管的Nu数有所减小。而当内管的热流密度恒定时,随外管的热流密度的增大,内管的Nu数稍有增大。而且这种情况随窄缝尺寸的减小而变得更加明显。

During recent years, narrow channel heat transfer technique has been successfully used in many practical situations. It has notable advantages of high heat transfer efficiency and compact configuration. It became very important in modern hydrodynamic and heat transfer field because of its broad application in so many different areas. So many researchers investigate on this problem but the conclusions are very different. Its influence factor and mechanism of heat transfer have not been understood yet. The characteristics of single-phase convective heat transfer in narrow annular channel are numerically calculated in this paper. The theoretical model is based on two different flow characteristics of laminar and turbulent flows. Through numerically solving the equations of laminar model, relation curve of the heat transfer coefficient and the ratio of the inner wall heat flux to the outer wall heat flux is got. Through numerically solving the equations of turbulent model, radial velocity and temperature distribution, radial kinetic energy and dissipation rate distribution, axial heat transfer coefficient of inner and outer tubes are obtained. The predicted results are compared with the experimental data and good agreements between them are found. Calculation results show that the ratio of the inner wall heat flux to the outer wall heat flux has great influences on the heat transfer characteristics to laminar flow in bilaterally heated narrow annuli. With the increase of the inner wall heated flux, the heat transfer coefficient at the outer wall of the inner tube will decrease, while the heat transfer coefficient at the inner wall of outer tube will increase. Sectional mean temperature of fluid increased linearly along axis to turbulent flow in bilaterally heated narrow annuli. Curve of the inner wall and the outer wall temperature has great slope in the beginning. The slope decreases gradually later and become stably at last. Then the curve of the inner wall and the outer wall temperature and the curve of sectional mean temperature of fluid have almost the same slope. The radial velocity gradient is smaller and smaller because of dissipation. Radial kinetic energy is bigger near the wall than far from wall. Dissipation rate distribution is similar to the radial kinetic energy distribution. Local Nusselt number increases with the increasing of Re. Nusselt number of the inner wall is bigger than Nusselt number of outer wall when they have the same Re. It shows that the heat transfer of the inner wall is greater than that of the outer wall. Local Nusselt number increases with the decreasing of gap size. Nusselt number of the outer wall decreases with the increasing of the inner wall heat flux when the outer wall heat flux is invariable. Nusselt number of the inner wall increases with the increasing of the outer wall heat flux when the inner wall heat flux is invariable. It becomes obvious with the decreasing of gap size.