Numerical Simulation On Evaporation Process And The Development Of CAD System For Thin Film Evaporator

The Mechanical Agitated Thin-Film Evaporator (TFE for short) is a new-type highly efficient evaporating equipment. It utilizes the rotating blade to disperse the fluid as uniform thin film so as to concentrate the material. Compared with the other TFE, the formation of liquid film depends on the mechanical function rather than the application of secondary steam. The evaporator can be operated in a vacuum and high heat-transfer coefficient & large evaporation intensity can be obtained. It is special suitable for the evaporation and concentration of the fluids which are thermal decomposition or have the high boiling points or high viscosity. At present, some progress has been made for the research and extended application of TFE, but the foundational data for design of TFE are still limited due to the complication of liquid film formed by rotor blade and the boiling heat transfer process. Many designs and products are obtained by the estimation of engineering magnification. There isn't a set of domestic integrated design calculation method for TFE. Also, the construction drawing is finished by directly calling the drawing command under AutoCAD environment. This leads to great amount of work, low efficiency and the delay of design and exploitation cycle of products. On the basis of the developed 0.4m2 TFE test equipment, the flow and heat transfer performance of different liquids of TFE was investigated by numerical simulation and experimental study. Based on the TFE design status and the software project ideas, The CAD application system of TFE was developed, thus, the design calculation and construction drawing plotting for TFE can be achieved automatically. The main work and conclusions are as follows: 1) The flow model of TFE for different fluids was developed. A calculation method of liquid film thickness for high viscosity fluid was put forward. The flow of water and viscous fluid was simulated using CFX4.4 software and the velocity distribution was obtained.The results show that the radial mixing of fluid can be promoted and the material exchange rate between the fillet and film can be increased by increasing the rotor speed.There was one common optimum feed rate under different rotor speed for different fluids.For water, the ratio of flow boundary layer thickness to film thickness δf/δwas the minimum under the optimum feed rate.The flow status was affected seriously by fluid viscosity. Difference existed for the axial velocity distribution between viscous fluid and water,and no obvious flow boundary layer was formed for viscous fluid. 2) The heat transfer calculation model of TFE for different fluids was developed.The mean temperature distribution along axial and film thickness direction was obtained, and the film heat transfer coefficients αin heating section under different conditions were calculated.The results show that there was one common optimum feed rate under different operation conditions for different fluids.At this feed rate, the mean velocity in fillet section u sreached the maximum and so did the film heat transfer coefficients α.For water, both the ratio of flow boundary layer thickness to film thickness δf/δand temperature boundary layer thickness to film thickness δt/δreached the minimum under high rotor speed or the optimum feed rate.There existed some inherent relation between the flow boundary layer and temperature boundary layer, the film temperature distribution and heat transfer coefficient were affected seriously by the thickness of transfer boundary layers. For viscous fluid, no obvious temperature boundary layer was formed under the calculation parameters. 3) The evaporation experiments on water were made, the variation curves of the overall heat transfer coefficient K and evaporation intensity EI of TFE were obtained. It was found that the parabolic relation existed between K and feed rate, there was an optimum feed rate which was similar to the numerical simulation value. 4) The evaporation experiments on caustic soda were also made by changing the operation parameters. The results show that the variation trends of the overall heat transfer coefficient K and evaporation intensity EI were the same as those of water. K was decreased and EI was increased with the increase of fluid viscosity, which was coincident with the numerical simulation results. The numerical simulation and experimental results show that TFE is most suitable for the evaporation and concentration of high viscosity fluids in point of evaporation intensity. 5) The CAD application system of TFE was developed by Visual Basic 6.0 exploiting technology on Windows 2000 operation system and AutoCAD 2000 drawing environment. The parametric design technology, ADO Data technology and Microsoft Office software wereused in the system. Four modules such as design calculation, parametric drawing, results output and management system are included in the system. The dimension of the thin film evaporator is determined synthetically by the requirement of "Chemical Engineering Handbook"and the optimum feed rate. 6) In terms of the structure characteristics of TFE, the whole drawing is separated as the basic part, component part and the integral three levels through classification parametric drawing strategy. The basic feature parameters of the TFE parts were extracted, the parametric design of drawing was realized by program, the component drawing and the assembly drawing were obtained through assembling. The trimming and hiding process of drawing was finished by the system. 7) An effective trimming and hiding method of plane drawing on Visual Basic and AutoCAD 2000 exploiting environment was realized. In this method, the intersections solving, operation of judging and trimming were finished by AutoCAD. It proves that the method is convenient, effective and reliable. It is also of universal use.