Simulation On Flow Field Of Turboexpander For Low-temperature Geothermal ORC Power System

In the situation of global fossil energy shortages and deteriorating environmental,new energy development and waste heat utilization have became necessary initiatives to solve the current deteriorating energy situation. Organic Rankine Cycle(ORC)technology has been efficiently used for geothermal energy, it will ease the current energy crisis situation greatly. Turboexpander is the core component of Organic Rankine Cycle system, it has good prospects for the low-temperature heat recovery and utilization occasions, the use of turboexpander can achieve higher conversion efficiency.In this paper, mechanism analysis for low-temperature geothermal energy was conducted using Organic Rankine Cycle technology. Thermodynamic analysis for Organic Rankine Cycle power system was completed. Evaporating temperature,overheating temperature and other major system parameters were analyzed. For basic Organic Rankine Cycle system, cycle thermal efficiency and output power of expander obviously increased with the rising of evaporating temperature. An increase in temperature difference between the import and export in expander can significantly improve system thermal efficiency. The effect of overheating temperature on the ORC system was different from the general Rankine cycle, it was related to thermal properties of organic working fluid and systems conditions.Thermodynamic calculation of 50 kW turboexpander was completed based on the system parameters. The 1D design of the turboexpander is presented in details.Furthermore, commercially-available software Ansys-CFX is used to perform preliminary steady-state CFD simulations of the R245 fa turboexpander. Finally, the optimization of turboexpander impeller was completed using uniform design method.The conclusions are as follows: System thermal efficiency was first increased and then decreased with the increase of overheating temperature, and there have a maximum cycle thermal efficiency and optimum overheating temperature.There exist some impact and wake losses in the area of the leading and the trailing edge of impeller blade, a partial area of low pressure has generated due to the excessive expansion at30% relative chord in impeller, while there has a secondary stream flows from hub to the wheel rim.A smaller wheel diameter ratio within reasonable range of values can reduce flow loss. In about 30% relative chord length near the rim area, the working fluid flow direction has a greater turning direction, and there has a big produced entropy region by the local low-energy group within rim wall. The loss was further increase at the trailing edge of the impeller exit, there are several reasons to cause this phenomenon, such as the loss of low velocity flow stream, the main fluid mixing effect and part of the mechanical energy is converted to heat the working fluid and so on.There has a significantly increased entropy region compared with intermediate due to the presence of the boundary layer over the rim and the hub side. A larger meridional flow channel width within the reasonable range of values can increase the effects of flow control on impeller outlet. And it can also helps to reduce flow loss and improve the efficiency of the expander.