Study On Flow Field Characteristics Of Geothermal Energy ORC Expander

With the continuous reduction of global fossil fuels and increased demand,the development of geothermal energy and other renewable energy has been more and more attention.China's geothermal reserves are huge,mainly in the low-temperature geothermal resources,these low-grade geothermal energy is difficult to promote the water vapor Longken cycle power generation.As the Organic Rankine Cycle(ORC)uses a relatively low boiling point of the working fluid,it can be used to drive the turboexpander at low temperatures.Therefore,Organic Rankine Cycle system is considered as an effective power generation mode under medium and low temperature heat conditions.The turboexpander has the characteristics of high speed,high single-stage expansion ratio and wide running range.It is often used as a thermal power conversion device for low power Organic Rankine Cycle system.In this paper,the low temperature geothermal energy of 90? was studied.Firstly,select R245 fa as organic working fluid.Then the thermodynamic analysis and design of the Organic Rankine Cycle system were carried out,and the optimization of the cyclic parameters was carried out.The results show that the increase of the condensing temperature reduces the exergy efficiency of the system and the output of the turboexpander,increases the exergy loss of the system and weakens the economic performance of the system,so the lower the cold temperature means higher efficiency and economic performance.The supercooled temperature does not affect the output power of the turboexpander,but will reduce the exergy efficiency of the system and increase the exergy loss of the evaporator.A higher heat source temperature can reduce the heat transfer area of the evaporator and improve the economy of the system.One-dimensional thermal design of a 200 k W turboexpander and three-dimensional modeling of the volute,guide vane and impeller blade were carried out.Then through the Turbogrid meshing,using CFX for numerical simulation,the flow field characteristics of the turboexpander were analyzed.The results show that the working fluid is evenly distributed in the volute,and there is no swirling flow or secondary flow.The pressure and temperature are higher on the outer side of the volute and lower in the inner exit.Because the structure of the guide vane is constant in the leaf height direction,the flow field characteristics change little with the direction of the leaf height.In the guide vane,before 30% relative chord the working fluid pressure change little.At the exit of the trailing edge of the guide vane,the pressure of the working fluid is significantly reduced due to the expansion of the gas at the beveled portion.The pressure contours in the impeller blade are relatively sparse near the leading edge and are denser in the vicinity of the trailing edge.The expansion of the organic working fluid in the impeller blade occurs mainly in the second half of the impeller blade.There is a shock waves in the leading edge of the blade due to the cause of the horn.At the trailing edge of the impeller blade due to the bending of the impeller,the rate of gas expansion is faster.The tip gap creates a leakage of fluid between the pressure surface and the suction surface,causing interference to the main stream,resulting in a lower efficiency of the expander.The vortices caused by the tip clearance increase with the increase of the tip clearance,and the vortex flows throu gh the entire impeller flow field.The study of the variable condition shows that the efficiency of the expander increases first and then decreases with the increase of the speed,and there is a peak that shows that if the speed of the expander is running beyond the rated speed,the efficiency of the expander will fall.So in actual operation,should prevent the expansion machine over-speeding.