机械搅拌对非牛顿流体速度场与温度场影响的研究

Study on the effect of mechanical agitation on the velocity and temperature fields of non-Newtonian fluids

热沙浴所用沙是一种非牛顿流体,高表观黏度使其传热特性较低,对其均匀加热带来不便。该文使用计算流体动力学分析了不同转速下三根螺旋搅拌轴对沙体产生的流动特征,研究了搅拌轴转速、离底高度、轴间距等结构参数对搅拌过程中速度场、温度场、有效搅拌体积分数、加热时间等的影响。研究结果表明:沙体有效搅拌体积分数随转速的增大而增大;未搅拌时沙体受热后的最大温差为10℃,而当有效搅拌体积分数最高时沙体受热最大温差为3℃;螺旋搅拌轴在初始安装位置转速为40r/min时,将Z=23cm平面加热到42~45℃所需时间为10 h,与静态加热相比,加热速率提高了120%,调整安装位置后加热时间缩短至8 h,加热速率提高了175%,缩短了加热时间,消除了热量分布不均问题。

[Objective]The sand used in the hot sand bath is a non-Newtonian fluid,and its high apparent viscosity leads to low heat transfer characteristics,hindering uniform heating.Traditional sand therapy beds require preheating for a long time before use.The heat is easily dissipated,which wastes electricity and fails to accomplish the desired heating outcome.This paper aims to improve the heat exchange between the sand body and the heating medium by adding a mechanical stirring structure to increase the heating rate of the sand body.[Methods]To solve the problem of nonuniform temperature distribution of the traditional sand therapy bed during static heating,this paper examined the flow characteristics of the sand body generated by the three helical stirring shafts under diverse rotational speeds by using computational fluid dynamics,focusing on the influences of the structural parameters such as the stirring shaft rotational speed,off-bottom height,and shaft spacing on the velocity field,the temperature field,the fraction of the effective mixing volume,and the heating time of the sand body during stirring.Taking the overall heating time,the percentage of effective mixing zone with velocity greater than 0.001 m/s,and the temperature difference as the indices,the sand body during mechanical mixing was analyzed using FLUENT fluid simulation software,and the results were combined with those of the experiments.[Results]Results reveal that the effective stirring volume fraction of the sand body increased with the stirring shaft rotational speed.When the sand body was in a static state,i.e.,without adding a mechanical stirring structure,the maximum temperature difference of the heated sand body could reach 10℃.When the effective stirring volume fraction of the sand body was at its maximum,the maximum temperature difference could be reduced to 3℃.In addition,when the spiral stirring shaft was located in the initial installation position,and the rotational speed was 40 rpm,the time required for heating to 42–45℃on a plane with a height of Z=23 cm above the bottom was 10 h,which increased the heating rate by 120%compared with static heating.After adjusting the mounting position,the heating time was shortened to 8 h,and the heating rate was increased by 175%,which further shortened the heating time and eliminated the problem of uneven heat distribution.[Conclusions]This paper is of great relevance for sand heating in industrial production.By optimizing the structural parameters and operating conditions of the stirring shaft,the heating efficiency of the sand can be remarkably improved,thus lowering the production cost and increasing the production efficiency.Moreover,in-depth research on parameters such as stirring shaft rotation speed,height from the bottom,and shaft spacing can provide important theoretical guidance for optimizing the design and operation of hot sand bath equipment.Therefore,the results of this paper will bring immense improvements to sand heating in industrial production and provide crucial support and guidance for engineering practice and technological innovation in related fields.