【作者】 王继红；

【导师】 王树刚； 张腾飞；

【作者基本信息】 大连理工大学 ， 供热、供燃气、通风及空调工程， 2013， 博士

【摘要】 在区域供冷系统中,以冰浆代替传统载冷介质是实现系统经济性投运的理想途径之一。冰浆的管道输送是冰浆在区域供冷系统中应用时重要的组成环节,伴随着浆体的管道输送,冰浆呈现出区别于纯流体的非均质性流动及潜热释放等特有的物理现象。这些物理现象的存在,极大地增进了对冰浆热流动规律的认知及刻画难度。因此,作为冰浆工程推广的基础性研究,获知冰浆输送中非均质流动的场特征、量化冰粒子非均匀分布对流变及流阻规律的改变、探索冰浆热流动的基本传热规律及其对流动特性产生的影响将具有重要的理论意义及实用价值。针对上述目标,本文采用两相流理论与计算流体力学相结合的方法,研究了不同形状管道内冰浆的流动特性及传热特性。对于冰浆流动特性,首先,将冰粒子间的相互作用类比于稠密气体分子间的相互作用,应用两相流欧拉-欧拉模型及颗粒动力学的相关理论研究了水平、垂直及90。弯管道内冰浆非均质性等温流动的场特征。其次,基于冰浆非均质性等温流动的场特征,分段考虑了因冰粒子非均匀分布对冰浆流变特性的改变,基于分段流变模型和混合模型求解出冰浆非均质性流动阻力分布规律。而后,结合两相流欧拉-欧拉三维微分模型,引入颗粒浓度扩散方程和非均质流动曲面方程,建立出适用于工程设计阶段的冰浆非均质性等温流动阻力“准二维”模型,并验证了数学模型的有效性。最后,根据对冰浆非均质性等温流动的场特征、流变特性及流动阻力规律所取得的理论认知,在专业实验室内搭建冰浆非均质性等温流动实验测试平台,基于实验研究方法进一步探讨了冰浆流动特性的基本规律。同时,对比了“准二维”模型、欧拉-欧拉模型、混合模型及拟单相流模型对冰浆流动阻力的预测性能。对于冰浆传热特性,采用热焓多孔介质模型与两相流欧拉-欧拉模型相结合的方式,研究了水平管道内冰浆热流动过程中冰粒子的相变传热特性及其对流动规律产生的影响。研究结果表明,对于冰浆流动特性,基于颗粒动力学理论的两相流欧拉-欧拉模型能够有效刻画出不同形状管道内冰浆非均质性等温流动的场特征。整体而言,随着流速的减小、冰粒子浓度的升高以及颗粒直径的增大,冰浆非均质性流动过程增强。在水平和竖直管道内,当局部区域内冰粒子浓度及流速高于某一限值时,冰粒子浓度分布出现反转。在90。弯管道内,冰浆流经弯管段时逐渐呈现出“二次流”现象,这一作用加剧了冰粒子与载流体间的混合。针对所选取的非均质性流动工况及冰浆构成,应充分考虑随运行工况变化时冰浆由牛顿向非牛顿流变特性的转变。当流速较高时,Thomas粘度方程能够较为准确地刻画出冰浆粘度。随着流速降低,可借助非牛顿宾汉流变模型加以描述。基于分段流变模型及混合模型,能够量化出管道顶部冰粒子聚集对剪切应力产生的影响,获得因冰粒子非均匀分布所产生的流阻规律。为了进一步提高冰浆非均质性流动阻力的求解效率,基于欧拉-欧拉三维微分模型,建立了冰浆非均质性流动阻力“准二维”模型。该模型充分考虑了非均质性流动的场特征,同时还能够分项量化出各流体相所占的阻力份额。对于冰浆传热特性,其对流换热过程将随着冰粒子浓度及流速的增大而增强。沿着管道流动方向冰粒子的相变使得冰浆的非均质性流动过程减弱,冰浆的流动阻力降低。更多还原

【Abstract】 In order to improve the efficiency and running cost of existing district cooling systems, it may be an ideal choice to use ice slurry as a substitute for traditional cold carrier fluid. When ice slurry is applied to district cooling systems, the pipe flow is an important component of ice slurry engineering application system. During ice slurry flow, the fluid presents special thermodynamic characteristics, such as heterogeneous flow patterns and latent heat release. And the special thermodynamic characteristics increase difficulty in characterizing ice slurry thermo-flow. Therefore, it is important to develop technologies that can describe ice slurry flow distribution, rheological behavior, flow resistance and heat transfer from the system design point of view.With the above objectives, this thesis uses solid-liquid flow theory and Computational Fluid Dynamics (CFD) as the main tool to study ice slurry thermodynamic characteristics in various pipes. Firstly, the present study treated ice particles interaction as the interaction of dense gas moleculars. And based on kinetic theory of granular flow, an Euler-Euler CFD model was applied to calculate ice slurry isothermal flow distribution. Then, the available flow characteristics were used to describe the rheological behavior piecewise. Meanwhile, the heterogeneous flow resistance was solved with Mixture model and piecewise rheological model. Further, by means of concentration diffusion equations and hypersurface equations, a ’quasi-two-dimensional’resistance model was derived from a three-dimensional Euler-Euler differential model. Finally, considering the heterogeneous ice slurry flow distribution characteristics and rheological properties, the ice slurry flow behaviour was experimentally investigated. And the experimental results were adopted to test the different flow resistance models. For ice slurry heat transfer behaviour, its phase change transfer characteristics and the effect on ice slurry flow distribution were discussed based on enthalpy-porosity model and Euler-Euler model.It is found that, based on kinetic theory of granular flow, the Euler-Euler CFD model was validated to be an effective tool for describing ice slurry flow in various pipes. In general, the ice slurry flow will become obviously heterogeneous with an increase of ice particle concentration and particle diameter but a decrease of flow velocity. In horizontal and vertical pipe flow, the phenomenon of ice particles repelling from the pipe wall appears in some conditions. In the horizontal90°elbow, it will gradually present a clear secondary flow image and the secondary flow action enhances the mixing between ice particles and carrier fluid. Considering the heterogeneous flow and the selected slurry composition, the ice slurry rheological behavior should be calculated piecewise for Newtonian and Non-Newtonian behaviour. When the flow velocity is high, Thomas equation can be appropriate for describing viscosity of ice slurry. With a decrease of flow velocity, the ice slurry can be treated as Bingham fluid. Based on the piecewise rheological model, the interaction between ice particles and pipe wall could be considered due to heterogeneous ice slurry flow. And the Mixture CFD model is able to calculate ice slurry flow resistance excellently. For further improving the solution efficiency of ice slurry flow resistance, a’quasi-two-dimensional’ resistance model was derived. The resistance model can not only realize the ice slurry flow resistance fast calculation, but also provide each flow phase resistance. For ice slurry heat transfer behaviour, the heat transfer process will be promoted with an increase of flow velocity and ice particle concentration. Meanwhile, melting of the ice particles makes the heterogeneous flow process be weakened, and the ice slurry flow resistance reduces along the pipe flow direction.更多还原