反循环蒸发器内波动流体对螺旋的传热性能影响研究

发布时间：2018-05-01 03:03

  本文选题：蒸发器 + 反向循环　； 参考：《湘潭大学》2015年硕士论文

【摘要】：蒸发工艺广泛用于物料的浓缩、提纯等生产过程,是必不可少的高能耗生产工序;在蒸发浓缩过程中,蒸发器加热管非常容易附着大量的结晶颗粒以及由杂质产生的污垢,造成换热系数和生产效率降低,严重影响蒸发设备的长周期稳定进行。因此蒸发设备的防垢除垢技术研究具有重要的学术价值和工程应用意义。基于蒸发循环管内汽液两相流不稳定流动原理,建立了反向循环蒸发条件下的工作螺旋与柔性弹簧组合的往复螺旋清洗机构,通过理论研究、数值模拟以及实验研究相结合方法,研究了往复螺旋清洗机构对蒸发过程的影响,主要内容如下:(1)为了减小工作螺旋对管内流体的阻力,在单管冷模试验台上对工作螺旋的钢丝直径d和螺距P等结构参数对流动阻力进行了优化试验。试验结果表明,管内工作螺旋的钢丝直径d取1.8mm、螺距P大于35mm时的阻力较小。(2)为了更好地利用循环管内流体不稳定性能推动工作螺旋往复运动,建立了单管蒸发试验,并对往复螺旋清洗机构的柔性弹簧的弹性系数进行了优化。单管热模试验验证了循环管内流体的周期性波动推动工作螺旋往复运动的可行性;在???3000238 mm加热管中,柔性弹簧弹性系数为13~16 mN时,工作螺旋可以实现较好的往复运动效果。(3)探索了流体在循环管内形成不稳定性流动的机理,进一步模拟了流体在循环内的蒸发过程和气泡的聚合过程,模拟结果表明:流体在循环管内的蒸发过程呈现不均匀性,间歇性产生密集的小气泡,不同时间段内的气泡直径直方图表明气泡在循环管内上升的过程中逐渐聚合形成大气泡,造成管中局部流体压力的波动变化,由此形成不稳定的汽液两相流。(4)为了更加真实验证柔性弹簧在波动流体的作用下的往复性能,建立了14根换热管组成的反向循环蒸发器的中试试验台,中试结果表明:当加热蒸汽温度为122℃时,工作螺旋的往复行程达到了1.2倍螺距以上,满足反向循环蒸发器内的均匀清洗要求。并探索加热蒸汽的温度、反向循环管的直径、上管箱的静液面高度以及在循环管内插入碎泡器等四个因素对往复行程距离的影响,其中加热蒸汽的温度和循环管内插入碎泡器两个因素起主要作用。(5)以硫酸锰为试验介质,分别进行了工作螺旋外径以及空管与内插螺旋的对比影响试验,验证波动流体与往复螺旋相结合的在线清洗技术方案在反向循环蒸发器内的除垢及强化传热效果。试验结果表明:???3000238 mm的加热管中采用?29mm的工作螺旋除垢效果较好;内插往复螺旋具有较强的除垢和强化传热效果,其换热系数比空管高出76%。
[Abstract]:Evaporation process is widely used in material concentration, purification and other production processes, it is an essential high energy production process, in the evaporation and concentration process, evaporator heating tube is very easy to adhere to a large number of crystalline particles and dirt produced by impurities. The heat transfer coefficient and production efficiency are reduced, and the long period stability of evaporation equipment is seriously affected. Therefore, the research of scale prevention and scale removal technology of evaporation equipment has important academic value and engineering application significance. Based on the principle of unstable flow of vapor-liquid two-phase flow in evaporative circulation pipe, a reciprocating spiral cleaning mechanism combined with working helix and flexible spring under the condition of reverse circulation evaporation is established. The effects of reciprocating screw cleaning mechanism on the evaporation process are studied by numerical simulation and experimental study. The main contents are as follows: (1) in order to reduce the resistance of the working screw to the fluid in the pipe, The flow resistance of steel wire diameter d and pitch P of working screw was optimized on a single tube cold die test rig. The test results show that the diameter of steel wire in the pipe is 1.8mm, and the resistance is smaller when the pitch P is larger than 35mm.) in order to make better use of the fluid instability in the circulating pipe to promote the reciprocating movement of the working screw, a single pipe evaporation test was established. The elastic coefficient of flexible spring of reciprocating spiral cleaning mechanism is optimized. The experimental results of single tube thermo-model show that the periodic fluctuation of the fluid in the circulating tube can promote the working spiral reciprocating motion, and in the heating pipe of 3000238mm, the elastic coefficient of the flexible spring is 1316mN. The mechanism of the fluid forming instability flow in the circulating tube is explored, and the evaporation process and the bubble polymerization process of the fluid in the circulation are further simulated. The simulation results show that the evaporation process of the fluid in the circulation tube presents inhomogeneity, and the dense small bubbles are produced intermittently. The bubble diameter histogram in different time periods shows that the bubbles gradually converge to form large bubbles in the process of rising in the circulation tube. In order to verify the reciprocating performance of the flexible spring under the action of the fluctuating fluid, the fluctuating pressure of the local fluid in the tube is changed, and the unstable vapor-liquid two-phase flow is formed in order to verify the reciprocation performance of the flexible spring under the action of the fluctuating fluid. A pilot test bench of 14 reverse circulation evaporators composed of heat transfer pipes has been established. The results show that the reciprocating stroke of the working screw is more than 1.2 times when the heating steam temperature is 122 鈩，

本文编号：1827426