Theoretical And Experimental Research On Energy Recovery Turbine In Reverse Cycle

The increasing application has driven the refrigeration and heat-pump industriesto enhance the energy efficiency of the system. The throttling process generally causeslarge energy loss in traditional vapor compression refrigeration systems. Applicationof an expander to replace the throttle valve will give higher refrigeration effect andobtain useful expansion power. The employment of this measure in large scale chillersand heat pumps will bring huge energy saving benefits. In this thesis, technologies ofthe expansion work recovery turbine, which are used in conventional refrigerant vaporcompression refrigeration systems, are studied by combining theoretical analysis,numerical simulation and experimental test.The thermodynamic analysis of the refrigeration cycle with an expansion workrecovery device is carried out. The results show that an internal heat exchangeraddition can bring the increase of the cooling effect and the power consumption, andcause the reduction of isentropic expansion work. Thus it does not always improve thesystem performance. Whether the energy efficiency of the cycle can be improved bythe addition of an internal heat exchanger depends on the thermodynamic propertiesof the refrigerant and the isentropic efficiency level of the expansion work recoverydevice. For the systems with R22as a refrigerant, an internal heat exchanger additionalways lowers the cycle coefficient of performance. For the systems with R134a as arefrigerant, the utilization of an internal heat exchanger will lower the cyclecoefficient of performance if the expander isentropic efficiency is higher than30%.The refrigerant flashing expansion process in the nozzle of turbine is investigatedby theoretical and experimental analyses. It is found that the non-equilibriumthermodynamics and the critical flow exist in this process. The two-phase refrigerantsound speed is discussed. The refrigerant velocity at the exit of the nozzle is found toachieve a supersonic flow. The configuration for a converging-diverging nozzle isproposed. Based on the theoretical analysis, two experimental nozzles are designedand fabricated. The pressure profile, the critical flow, the jet velocity and theconversion efficiency are investigated. The results show that there is a suddenpressure drop at the throat of the nozzle, being about0.7~0.9MPa. The refrigerantcritical mass flow rate is19800~24000kg/(s m2). Comparisons between thetheoretical predictions and the experimental results show that the relative deviation ofHomogeneous Equilibrium Model prediction is between35%and5%, the relativedeviation of Henry-Fauske model prediction is between15%and35%. Theexperimental nozzle jet velocity reaches a maximum of78m/s. The experimentalnozzle internal efficiency has an optimum value of96%. When the downstreampressure is higher than the critical pressure, the critical flow phenomenon disappears. The weak shock wave appears inside the nozzle. The jet speed and the efficiencydecline significantly. Under the same conditions, the larger the divergence angle, thehigher the nozzle jet speed and the energy conversion efficiency.The mathematical model of refrigerant two-phase flow in the rotor of the turbineis established. The irreversible loss in the rotor is analyzed. It is found that the higherthe refrigerating capacity of the chiller, the higher the efficiency of the rotor and thewider the high efficiency rotating speed range. There is an optimum rotating speedwhere the rotor efficiency peaks. The reduction of the rotor diameter results in higherrotor efficiency to some extent, but causes the significant increase of the optimumrotating speed. There is an optimum velocity ratio where the rotor efficiency peaks.The optimum velocity ratio increases with the increase of the chiller capacity and therefrigerant quality in the rotor. When the chiller capacity is more than200kW, theoptimum rotor speed ranges from6500rpm to8000rpm and the optimum rotorefficiency can surpass60%. When the chiller capacity is lower than50kW, the turbineefficiency is extremely low and even it can’t work.The turbine prototype and the corresponding mechanical dynamometer deviceare developed. The test results show that the height and the density of rotor bladeshave a great influence on the turbine efficiency. The no-load rotating speed range ofthe Prototype2is1300rpm~3000rpm. The torque, the recovery work and the turbineefficiency reach a maximum level at a certain rotor speed. The optimum value of therotating speed is about2000rpm.The peak value of the specific recovery work in theexperiments is254.8J/kg. A maximum isentropic efficiency of10.4%is achievedunder the experimental conditions. The improvement measures of the turbineprototype are discussed. The efficiency for the improved turbine is anticipated toincrease to39%~59%.The two-phase turbine in the refrigeration cycle starts from scratch. Somevaluable experience is obtained during the test. The work of this thesis will layfoundation for further studies.