| In the context of the worsening global energy crisis,hydrogen energy and related industries are facing a period of large-scale development.Proton exchange membrane fuel cell(PEMFC)has attracted much attention because of its advantages such as large specific power,high power generation efficiency and rapid response to load changes.As an important circulating part of PEMFC hydrogen fuel supply and recovery subsystem,ejector has gradually became one of the research hotspots in PEMFC field.Aiming at the problem that the power range of a single traditional ejector is narrow,a new design and structure improvement method of the ejector is proposed in this paper:the optimal size is determined with the maximum entrainment ratio in the low power segment as the objective during structural optimization,and then the available hydrogen potential energy in the high power condition is released by adding bypass inlet,so as to ensure the efficient operation of the ejector in the wide power condition range.At the same time,the internal flow mechanism of the ejector is analyzed in detail combined with the power condition of the reactor and the structural parameters of the ejector.The main research contents are as follows:(1)The steady-state electrochemical model of PEMFC and the critical circular size design model of the ejector were established based on MATLAB software.Combined with the power condition of the 9 kW air-cooled reactor and relevant empirical formulas,the complete structural parameters of the circulating ejector were obtained.The two dimensional numerical simulation model of the ejector is established by ANSYS software,and the reliability of the model is verified by comparing with the experimental data.Through the simulation of the size of the designed ejector,the complete operating conditions of the ejector pressure in the range of 150～700 kPa hydrogen supply pressure were obtained.(2)The size optimization study of the ejector was carried out in combination with the power condition of the reactor.The internal flow rule and ejector performance changes under the structural changes such as area ratio and nozzle distance were respectively explored,and the optimal value of each parameter was obtained with the stable operation of the full power range as the goal.The results show that the area ratio is the most important structural factor affecting the performance of the ejector,which determines the flow space of the dynamic fluid and the ejector fluid inside the ejector.In order to ensure the stable operation of the ejector under low power conditions,the optimal area ratio is 6.25.Mixing chamber length and nozzle distance jointly affect the mixing process of driving fluid and dynamic fluid.The diffuser angle and diffuser chamber length determine the pressure boost process of the mixing fluid.Considering the performance of the ejector at full power of the reactor,the distance between the nozzle is 2.1mm,the mixing chamber length is 8.4mm,the diffuser chamber length is 14mm,and the diffuser angle is 9°.(3)Based on the optimized traditional ejector,the flow field characteristics were analyzed,and the appropriate bypass inlet location was found to solve the problem of waste of available hydrogen potential energy under high power conditions.The influence analysis of length ratio and bypass inlet diameter was carried out respectively,and the flow mechanism inside the bypass ejector was revealed combined with different working conditions.The results show that the position of the bypass inlet determines the power source of the ejecting bypass fluid,and the diameter of the bypass inlet determines the low pressure range covered by the bypass inlet.Considering the performance of the bypass ejector in the power range,the best length ratio is 1.1 and the best inlet diameter is 2.4 mm.Due to the limitation of hydrogen supply pressure,the bypass injector could not work normally under the condition of 2.37kW,and the corresponding ejector performance under other conditions was greatly improved,especially under the condition of 9.04kW,compared with the traditional ejector entrainment ratio increased by 92%.(4)The flow characteristics of the two ejectors were compared,and the influence of humidity of the ejecting gas and outlet back pressure was analyzed.The results show that the ejector performance is improved at high relative humidity due to the difference in the density of hydrogen and water vapor,but the corresponding hydrogen circulation is reduced.The conventional ejectors have obvious critical back pressure,and the larger the reactor output power,the wider the corresponding critical operating range.But the bypass ejector changes the internal low pressure distribution when the outlet back pressure increases,and its corresponding critical back pressure becomes difficult to define.The design method of bypass ejector proposed in this paper is suitable for the wide power condition of the galvanic pile and the related structure improvement strategy is easy to implement,which can provide reference value for the research and application in related fields. |
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