【作者】 张博；

【导师】 周宏基；

【作者基本信息】 武汉理工大学 ， 轮机工程， 2016， 硕士

【摘要】 板式降膜蒸发器作为一种新型高效换热设备,其利用降膜蒸发传质带动传热的原理来提高换热效果,在石油、化工、和海水淡化等领域都中有着很大的发展前景。由于板外降膜蒸发是一种复杂的传热和传质过程,目前还缺乏对降膜蒸发过程比较深入的了解,因此对传热传质特性的研究有很强的必要性和现实意义。论文以流体力学基本理论和数值模拟方法为基础,借助流体力学模拟仿真软件FLUENT中的VOF模型建立了平板式降膜蒸发二维仿真模型,初步研究不同工况条件下对其传热性能的影响,通过数值模拟所获取的主要研究内容:(1)合理的提高液相入口温度和壁面热流量,可以提高相应界面温度,增强蒸发效应,提高传热性能;(2)提高入口空气的流速和降低液相进口流率可以强化蒸发换热,提高蒸发器的换热效果,但是要选择合适的液相进口流量和空气入口流速。接下来此基础上再定义了气相对液膜的剪应力作用以及表面张力效应的源项来建立了波纹板二维降膜蒸发仿真模型,深入的模拟研究其流动和传热传质特性,得到以下结论:(1)波纹板壁面对流场的影响较大,在波纹凹陷处气相的速度相对较小,液相的速度在重力的作用下越向下速度越大;(2)在液膜向下流动过程中,液相的进口部分液膜的厚度较大,而随着流动的进行,液膜的厚度不断变薄;(3)温度分布和传热传质效果都存在由于波纹产生的波动趋势;(4)在水平方向上,温度都是逐渐降低的,并且在液相和气相中,其变化梯度都先大后小的,两相相界面附近的温度在变化,但温度变化速率不连续;(5)提高壁面热通量、降低空气相对湿度、提高空气进口速度、增加液相进口温度与流量都可以促进液蒸发效应,但其蒸发速率的提高并不一定能说明其传热性能提高了。通过以上研究初步模拟研究影响板外降膜蒸发流动和传热传质性能的各项因素,加深对实际降膜蒸发设备中的液膜流动以及传热过程的认识,并对实际工业应用中改善提升传热效果的优化措施提供理论指导。更多还原

【Abstract】 Plate falling film evaporator is a kind of new highly efficient heat exchange equipment,By using the principle of falling film evaporation mass transfer to drive the heat transfer to improve the heat transfer effect,and has broad prospects in the fields of petroleum,chemical industry and sea water desalination.Because the outer plate falling film evaporation is a complicated heat and mass transfer process,and the lack of the in-depth understanding on the process of falling film evaporation.Therefore,study on the heat and mass transfer characteristics have very stronger necessity and practical significance.Based on the basic theory of fluid mechanics the and the numerical simulation method,this paper using the VOF model in the fluid mechanics simulation software FLUENT。First of all,the use of user defined function(UDF)method to define the liquid phase at the interface of the evaporation effect establish plate type falling film evaporation 2D simulation model,Preliminary study on the effects of different conditions on the heat transfer performance,by numerical simulation to obtain the main research contents:(1)improve the reasonable temperature and liquid phase entrance wall heat flux,can improve the corresponding interface temperature,enhanced evaporation effect,improve heat transfer performance;(2)increase the inlet air flow rate and reduce the liquid inlet flow rate can be enhanced evaporation heat transfer,improve the heat transfer efficiency of the evaporator,but choose the suitable liquid inlet flow rate and air inlet velocity.Then on this basis to define the flow and heat transfer characteristics of gas relative film shearing force and surface tension effect of the source to established two-dimensional corrugated plate falling film evaporation simulation model,get the following conclusion:(1)corrugated siding face greater influence of flow field in corrugated depressions gas velocity is relatively small,The velocity of liquid phase is more and more downward velocity under the action of gravity;(2)in the process of liquid film flowing down,the liquid phase of the inlet part of the liquid film thickness is larger,and with the flow,the thickness of the liquid film decreases;(3)the temperature distribution and the heat and mass transfer effect are all due to the fluctuation of the ripple;(4)in the horizontal direction,the temperature is gradually reduced,and in the liquid phase and gas phase,the change of gradient is first large and small,and the temperature near the interface changes,but the temperature change rate is not continuous;(5)improve the wall heat flux and reduce the air relative humidity,improve the air inlet velocity,the increase of liquid phase inlet temperature and flow rate can promote liquid evaporation effect,but improve the evaporation rate does not necessarily indicates that its heat transfer performance is improved.Based on the above research,Preliminary simulation study on the factors affecting the flow and heat and mass transfer performance of the falling film evaporator,deepen the understanding of the liquid film flow and heat transfer process in the actual falling film evaporation equipment,and to provide theoretical guidance for improving the heat transfer efficiency in practical industrial applications.更多还原