Effect Of Structural And Operating Parameters On The Entrainment Ratio Of Ejectors

The hydrogen recirculation system can employ an ejector replaced by a traditional mechanical pump to optimize fuel usage and to extend service life of the proton exchange membrane fuel cell.Because of the technical features such as simple construction,compact size and low operation and maintenance costs,the ejector has a promising application in fuel cell hydrogen recirculation systems.The working fluid flowing inside the ejector has complex hydrodynamic phenomena,resulting in the flow field that is highly susceptible to disturbance by changes in the solid boundary or boundary conditions,which in turn affects the performance of the ejector.Changes in the structural parameters of the ejector lead to changes in the solid boundary of the flow field,and differences in the working parameters correspond to differences in the boundary conditions.For achieving superior ejector performance,it is critical to investigate the impact of structural and operational characteristics on ejector performance.In this study,the entrainment ratio is used as the evaluation index of the working performance of ejectors.The ejector’s entrainment ratios under various structural and functioning factors are determined through simulation experiments in order to get the entrainment ratio’s variation law.Overall nozzle exit position,mixing chamber diameter,and mixing chamber length-to-diameter ratio are used as essential structural elements of the ejector in the first part of the assessment.And the changes in the entrainment ratio under different values of structural parameters are analyzed to find the best combination of structural parameters for the ejector under design conditions.Following that,a wide variety working fluid inlet and outlet pressure is given based on the geometric model of the ejector with optimal structural parameters,and the simulation findings show how the entrainment ratio change with the working situation.Finally,by comparing the value of the entrainment ratio of different ejector models at various operating conditions,the optimal structural parameters of the ejector for each operating condition were found.The optimal structural parameters changing regularity with the different operating parameters is concluded.The study approach is based on computational fluid dynamics study,and establishes a threedimensional computational fluid dynamics model by using a hybrid mesh of the ejector.To confirm the accuracy of the ejector’s three-dimensional computational fluid dynamics model,the simulation results are compared with the experimental data.The conclusions can be drawn here:（1）Different structural geometry of the ejector has an effect on the entrainment ratio.Nozzle exit position largely determines the expansibility of the nozzle flow at the entrance of the mixing chamber.The distance between the nozzle exit position and the entrance of the mixing chamber must not be too near or too far away.For the mixing chamber,the diameter of the mixing chamber controls the mixture of the working fluid within the chamber.Too large a mixing chamber diameter causes excessive expansion and too small a mixing chamber diameter results in the impact of the jet body with the inner wall of the ejector,both reducing the ejecting performance.The mixing chamber length to diameter ratio is critical to guaranteeing complete mixing of the working fluid within the chamber.Further increases in the mixing chamber length to diameter ratio are not meaningful in terms of improving performance when the mixing process can be achieved.（2）The intake and outlet pressures impede the fluid flow inside the ejector,and thus affect the ejector’s performance.The mass flow rate of both the driving fluid and the ejected fluid will consequently change against the altering inlet pressure of the driving fluid,but the entrainment ratio has different trend for different geometric models and operating parameters.The ejected fluid’s entrance pressure is a linear consequence of the ejected fluid’s mass flow rate.The outlet pressure affects the expelled fluid mass flow rate by causing a change in the mixing pressure inside the mixing chamber.Apart from the inlet pressure of the drive fluid,alterations in the working fluid’s inlet and outlet pressure have almost no relevance on the drive fluid’s mass flow rate.（3）The optimal structural parameters of the ejector will also vary with the deviation of the actual operating conditions relative to the defined working conditions.Based on the optimum structural parameters at the set working conditions,with increasing driving fluid inlet pressure and decreasing outlet pressure,the ideal nozzle exit position remains constant.When the driving fluid inlet pressure drops,requiring closer contact to the mixing chamber inlet,or increasing the distance from the mixing chamber inlet when the output pressure exceeds the design pressure.The optimum mixing chamber diameter is influenced by changes in operating conditions and needs to be increased or decreased accordingly,considering deviations from the design operating conditions.The proper length to diameter ratio of the mixing chamber,allows for a high entrainment ratio to be maintained in a range of operating conditions.