Research On The Transition Process Of Variable Operating Conditions Of Tidal Bidirectional Tubular Turbines

Ocean energy is a clean and renewable energy source,of which tidal energy is one of the most industrially valuable forms.It is of great importance in reducing dependence on fossil fuels and reducing greenhouse gas emissions.The tidal bidirectional flow turbine with the function of forward and reverse operation is a key hydraulic mechanical equipment for the development of tidal energy.There are six operating conditions in operation:forward and reverse turbines,forward and reverse drainage,and forward and reverse water pumps.The evolution of flow patterns during the process of operating condition conversion will affect the stability and safety of the unit.Moreover,due to the relatively large number of downstream components of the impeller under reverse flow conditions and the complex spatial structure,more severe flow patterns will be induced during the process of operating condition conversion,which could seriously affect the stability of unit operation.Therefore,this paper carries out numerical simulation research on the two operating condition conversion processes of reverse power generation-reverse drainage and reverse drainage-forward water pump.The main work contents include the following points:(1)A three-dimensional model of the full flow path of a bidirectional flow turbine is established,and model calculation data and experimental resultss are benchmarked for forward and reverse turbines under operating conditions.The simulation results obtained are in good agreement with the experimental values,and the error is less than 2%,proving the feasibility of the established model and calculation boundary conditions for transient calculations of bidirectional flow turbines.The internal flow field and flow characteristics of each component under different flow conditions of the forward and reverse turbine conditions are compared and analyzed.It is concluded that the comprehensive level of hydraulic parameters during operation under forward turbine conditions is better than that under reverse turbine conditions,so research on variable operating condition transition processes during reverse operation of turbine units is carried out.(2)The transient process of changes in the flow state in the main flow area of the internal flow field with time during reverse turbine-reverse drainage conditions wasis analyzed.Based on the entropy production theory,the energy loss of each flow component in the process of working codition conversion is calculated.It is found that due to its complex structure and multiple flow components,unstable flow phenomena occurr in the outlet channel area,and the entropy production rate in the guide vane region and the outlet path region increases with time;there are negative pressure areas between the impeller and guide vanes,and such negative pressure areas are prone to cavitation,vortexes and other unstable phenomena that affect the unit operation.The analysis results show that during the transition process,the flow state at the outlet of the impeller is unstable,with blade tip leakage flow phenomenon occurring,which will affect the flow state of subsequent flow components and thus affect the unit operati on efficiency.An analysis of the correlation between the vortex structure and entropy production rate in the guide vane and outlet flow area is made,and the conclusion is drawn that:during the transition from reverse turbine to reverse drainage operation,the vortex structure and high entropy production rate area in the guide vane and outlet flow area continues to increase with time,and the flow stability weakens.(3)The numerical simulation is carried out for the reverse drainage and forward pump condition in which the angles of both blade and guide vane change.The transient process of changes in the flow state in the main flow area of the internal flow field with time during reverse turbine-reverse drainage conditions is analyzed.It is found that during this process,the unit experienced braking conditions-water pump conditions,in which the braking conditions is poor and flow stability is weak.At this time,the entropy production rate in the impeller region,guide vane region and outlet path region is high,and the hydraulic loss is large.With the unit entering the pump condition and running smoothly,the entropy production rate in each region of the unit decreases and the hydraulic loss decreases.Moreover,blade tip leakage still exists,and the influence on the flow state of the guide vane area’s flow state is gradually weakened as the braking condition ends.An analysis of the correlation between the vortex structure and entropy production rate in the guide vane and outlet flow area is made,and the conclusion is drawn that during the transition from reverse drainage to forward pump operation,the vortex structure and high entropy production rate area in the guide vane and outlet flow area increased first and then decreased with time,while the vortex structure at the tail of the bubble body tendeds to stabilize and does not disappear during the transition process.The influence of different water pump heads on operating status during reverse drainage-forward water pump variable operating condition transition process is explored.The external characteristic parameters and pressure coefficients of units operating under different head variable operating conditions are compared and analyzed.It is found that the fluctuation amplitude of external characteristic parameters and pressure coefficient under high initial head conditions is smaller than that under low initial head conditions...