非牛顿流体在椭圆管束外螺旋流动的二次流耦合强化传热机理及传热与流阻特性

Aiming at the high viscosity and special rheology property of non-newtonian fluid, the effective heat transfer enhancement technology can not be found for the shell and tube heat exchanger by now.In this project,The continuous hecial baffle combined with elliptical tubes as one novel technology was produced.The coupling secondary flow heat transfer enhancement can be obtained by combining the surface curvature effect of elliptical tube with Dean effect by centrifugal force.The flow field is investigated using particle image velocimetry for non-newtonian fluid flowing in the shell side. The flow and heat transfer mathematical models are built in the cyclinder coordinate system, CFD soft "Fluent" is used to solve numerically the equations. Velocity, temperature and pressure fields distribution are obtained.The effects of structure paremeters and operation condition of heat exchanger on flow flieds are analysised, too. Based on flow flied observation,numerical simulation,heat transfer and pressure drop calculation, the heat transfer mechansim of coupling secondary flow and the key factors affecting flow and heat transfer are ascertained; Nusselt and friction correlations relating Dean effect, surface curvature effect and rheology property of non-newtonian fluid are conducted.It can give a guidance to a novel heat exchanger with continuous hecial baffle combined with elliptical tubes for the optimization design.

针对非牛顿流体粘度大及具有特殊流变特性，目前在壳管式换热器中尚无高效强化传热手段的"瓶颈"问题，本项目提出采用连续螺旋隔板与椭圆管搭配, 利用椭圆管表面曲率效应形成二次流与离心力作用下迪恩效应形成二次进行耦合强化传热。采用PIV技术对非牛顿流体在壳程的流态进行直观的测量分析；建立柱状坐标体系下壳程流动与传热的数学模型，并采用计算流体力学软件Fluent对模型进行数值求解，获得壳程速度场、温度场和压力场的分布情况，以及换热器结构参数和实验操作条件的变化对速度场、温度场和压力场的影响规律；基于流态观察、数值模拟和传热与压降的测量计算，探明非牛顿流体的二次流耦合强化传热机理及影响传热与流阻性能的关键因素；建立反映迪恩效应、曲率效应及非牛顿流体流变特性对换热器传热与流阻性能影响的努塞尔特准数和摩擦系数关系式，指导新型螺旋隔板椭圆管换热器的优化设计。

在化工、食品、塑料加工等行业常采用壳管式换热器对非牛顿流体进行加热或冷却，由于非牛顿流体的粘度较高，低的雷诺常数和高的普兰特常数导致传热系数低，必须进行传热强化。本项目提出采用连续螺旋隔板与椭圆管搭配, 利用椭圆管表面曲率效应形成二次流与离心力作用下迪恩效应形成二次进行耦合强化传热。采用PIV技术对非牛顿流体在壳程的流态进行直观的测量分析；建立柱状坐标体系下壳程流动与传热的数学模型，并采用计算流体力学软件Fluent对模型进行数值求解，获得壳程速度场、温度场和压力场的分布情况，以及换热器结构参数和实验操作条件的变化对速度场、温度场和压力场的影响规律；基于流态观察、数值模拟和传热与压降的测量计算，探明非牛顿流体的二次流耦合强化传热机理及影响传热与流阻性能的关键因素；建立了换热器传热与流阻的努塞尔特准数和摩擦系数关系式，并对换热器的结构参数进行了优化。在此基础上，还在非牛顿流体中添加多壁碳纳米管进行传热强化。研究结果表明，对于非牛顿流体的传热，螺旋隔板椭圆管换热器的壳程传热系数比普通光滑管换热器提高11.4-16.6%，压降降低29.2-36.9%，综合热性能系数提高了30-35%，证明椭圆管比圆管能显著强化非牛顿流体的传热性能。添加质量分率为1%的多壁碳纳米管，壳程传热系数提高了35%，综合热性能系数提高了24-26%。螺旋隔板椭圆管换热器在酱油杀菌工艺中进行了成功应用，显著提高了生产效率，降低了能耗。