Fluid Analysis Of Baffles And Nozzles Of A Scrubber For Fishing Vessels Based On D4AK-C Diesel Engine

Scrubber technology is widely used in industry,such as industrial coal-fired boilers,environmental monitoring and control,automobile exhaust treatment and so on.Exhaust gas scrubber for marine engines is a kind of equipment to reduce particulate matter,nitrogen oxides,sulfur oxides and soot emissions from exhaust gases produced by internal combustion engines through a series of chemical and physical reactions.In recent years,with the improvement of MARPOL's sulfur oxide emission standards for ships,many scholars and research institutions have done a lot of research work on the mechanism of scrubbers and how to improve the efficiency of dust removal and desulfurization of scrubbers.However,at present,there are few studies and reports on scrubbers used on small vessels in China and abroad.As a small and medium-sized hulls,offshore fishing vessels produce pollutants such as sulfur oxides,nitrogen oxides and soot produced by diesel engine combustion of heavy oil,which not only pollute the atmosphere,but cause harm to human health.Based on D4AK-C diesel engine,this thesis studies and designs a washing system for small fishing vessel,which is used for dust removal and desulfurization of fishing vessel tail gas in offshore fishing operations.Based on the analysis of the flow field distribution in the chamber of small vertical and horizontal scrubbers,the relationship between the efficiency of dust removal and desulfurization of scrubbers and the energy consumption of scrubbers and the parameters such as the baffle and nozzle of scrubbers was studied.The main research contents of this thesis include:1)Designing the washing system of small fishing vessel by using the mechanism of wet dust removal and desulfurization of semi-closed-loop freshwater method.The system mainly consists of diesel exhaust intake pipe,washing chamber,spray system,pump,waste liquid pool,exhaust pipe,drainage pipe,baffle and wave plate defogging device.It is suitable for the filtration and washing of pollutants such as sulfur oxides in diesel exhaust gas of small offshore fishing vessels.The equipment occupies small hull space,has low installation and maintenance cost.It also has obvious desulfurization and dust removal effect.Through the design of the intake pipe,the phenomenon of gas-liquid recirculation in the chamber of the scrubber is avoided,and the leakage of the scrubber is reduced.The exhaust gas of diesel engine will carry a large amount of detergent after being washed in the chamber of the scrubber,which is easy to cause corrosion to the pipelines and fans.The problem of small chamber of the scrubber dewatering is solved by dewatering treatment of the defogging device.Through the research and design of the small-scale washing system for fishing vessels,it plays a basic guiding role in the research of this type of scrubber.2)Based on the four working velocities of D4AK-C diesel engine,the tail gas flow rate of diesel engine at different power is calculated.The three-dimensional model of two kinds of scrubbing chambers was divided by structured grid in ICEM CFD.The flow field distribution in the chamber of small vertical/horizontal scrubbers for fishing vessels was simulated and analyzed by ANSYS CFX software module in computational fluid dynamics(CFD).The Reynolds Stress Model is used to describe the turbulent flow in the scrubber chamber.Pressure drop is one of the important parameters to measure the energy consumption of the scrubber,and the streamline distance is related to the time and distance of gas-liquid mixing collision in the scrubber.Through the study of the relationship between the pressure drop and the number and layers of the baffle in the scrubber,the influence of the number and layers of the baffle on the gas flow path in the scrubber chamber was tracked,and the relationship between the parameters of the baffle and the gas-liquid mixing collision effect and energy consumption in the small scrubber chamber was studied.Two optimum design schemes of the baffle in the chamber structure are analyzed.Through the analysis of single-phase flow simulation model of diversion plate,it is shown that the flow field distribution of scrubber is the best when two vertical baffles of 210 mm length are designed and installed in the chamber of small horizontal scrubber for fishing vessels with a volume of 60 liters,and that the flow field distribution of scrubber is the best when two horizontal baffles of 240 mm length are designed and installed in the chamber of small vertical scrubber for fishing vessels with a volume of 60 liters.3)Based on the study of the relationship between the baffle's parameters and the flow field distribution of the scrubber,the influence of the nozzle's parameters on the gas-liquid mixing effect and the flow field variation of the scrubber was studied by comparing the simulation results of single-phase flow and multi-phase flow.Tetrahedral structured mesh was used to divide the scrubber chamber model,Elsasser Particle Wall Collision Model was used to describe the collision process between droplets and the scrubber wall,Reynolds Stress Model and Particle Track Model were used to describe the turbulent motion caused by gas-liquid coupling in the scrubber chamber respectively,and to track the change of the droplet line trajectory.By changing the liquid-gas ratio,the changing trend of the relationship between the injection angle,direction and number of nozzles in the scrubber chamber and the pressure drop and streamline distance of the small vertical exhaust gas scrubber in the two-phase and single-phase flow models was analyzed,and the authenticity and reliability of the simulation results under different models were confirmed.The velocity distribution field,temperature distribution field,pressure distribution field,flow distribution field and the distribution of detergent in the chamber of scrubber of small fishing vessel are visualized by CFX simulation cloud images.The distribution and flow characteristics of the flow field in the chamber of small vertical exhaust gas scrubber are revealed,and the optimal design scheme of nozzle's parameters is determined.The analysis and comparison of simulation results of discrete two-phase flow and single-term flow of nozzles show that in the chamber of small vertical scrubbers for fishing vessels,the effect of scrubbers with horizontal injection of nozzles is better than that with vertical injection.When the number of nozzles is six and the injection angle is sixty degrees,the washing effect of small vertical scrubbers is the best and the energy consumption is the lowest.