Numerical Simulation And Structure Optimization Of Steam Ejector

Waste heat utilization is not only an important way to improve the energy efficiencyof combined heat and power(CHP)plant,but also an essential way to achieve clean and low-carbon urban heating.At present,among the commonly used technologies of waste heat recovery and utilization,the CHP with the high back-pressure turbine technology is widely concerned and applied.By properly raising the back-pressure of steam turbine,the waste heat can be directly recovered and utilized,which greatly reduces the heat loss of the CHP plant and has remarkable effect of energy-saving and emission reduction.However,the parameters of thermal heating network have great influence in high-back pressure heating system.For example,the recovery rate of exhaust steam will decrease when the temperature of supply/return water is high.In view of the above problems,this paper proposes a high back-pressure waste heat cascade heating system coupled with the steam ejector.The heating source of this system are the high back-pressure exhaust steam,the steam ejector exhaust steam,and the extraction steam,which compose a three-stage heating transfer process for heating the supply water.Among the heating process,the steam ejector uses the extraction steam to inject the exhaust steam and improve the energy grade of the exhaust steam to recover the waste heat for heating,which improve the utilization rate of the exhaust steam and further reduce the energy consumption of the heating system.Firstly,for the high back-pressure waste heat cascade heating system,the EBSILON software is used to establish the thermal system model of the CHP plant and carry out the simulation of off-design conditions.The thermodynamic performances the high back-pressure waste heat heating system is calculated and analyzed.Secondly,in order to improve the exhaust steam utilization rate of CHP plant,a cascade heating system coupled with steam ejector is proposed,and the steam ejector is designed and optimized:(1)combined with the design and calculation model of steam ejector based on gas dynamic function method and the thermodynamic performance of CHP plant,a method to determine the boundary conditions of steam ejector with the goal of maximizing the exhaust steam recovery rate of the system is proposed.(2)the single parameter analysis of each structure of the steam ejector is carried out by using numerical simulation method.It is found that the diameter of the mixing chamber with constant cross-section,the distance between the nozzle outlet and the mixing chamber inlet,and the inclination angle of the mixing chamber inlet all have an optimal value in the design condition,which makes the operation performance of the ejector reach the best,while the length of the mixing chamber with constant cross-section has almost no change.(3)According to the results of the orthogonal experiment,the structure of the steam ejector is optimized,and the numerical simulation results show that the injection coefficient reaches 0.9444,with an increase of 1 8.6%.Finally,the thermodynamic performance of high back-pressure heating system coupled with steam ejector is compared and analyzed,and the system sensitivity is analyzed.The results show that:under the design conditions,the exhaust steam recovery rate of the novel system coupled with steam ejector can reach 100%,compared with the original heating system,the heating capacity increases by 30.72MW,and the standard coal consumption for power generation decreases by 8.63g/(kW·h).The influence of back-pressure of steam turbine change on the system thermodynamic performance is analyzed.When the back-pressure of steam turbine is 34～54kPa,the exhaust steam recovery rate of the case system is 8.7%～35.7%higher than that of the original heating system,greatly improving the maximum heating capacity of the novel system.By analyzing the influence of heating load and power generation load change on the thermal economy of the system,it is found that the novel system adapts well to the change of electric heating load of the CHP plant and improves the recovery rate of exhaust steam,except that exhaust steam can be directly used for heating at the beginning and end of heating.Based on the thermal economic analysis of the whole heating season,the average standard coal consumption of power generation the novel system is 8.12g/(kW·h)lower than that of the original heating system,saving about 15600 tons of standard coal per year,and the effect of energy saving and emission reduction is remarkable.