This study is about optimal design of ejector for hydrogen and operation test at recycle mode in polymer electrolyte fuel cells. Ejector has a simple structure, and does not need electricity. Because of these advantages, ejectors have been applied to a variety of industrial fields such as refrigerat...
This study is about optimal design of ejector for hydrogen and operation test at recycle mode in polymer electrolyte fuel cells. Ejector has a simple structure, and does not need electricity. Because of these advantages, ejectors have been applied to a variety of industrial fields such as refrigerators and so on. However, the existing commercial ejectors do not meet the practical operating requirements of the PEFCs system with the humidified hydrogen recycle. In this work, CFD is used to design an ejector applicable to 10 kW PEFCs. The structure of the ejector is divided into four large parameters(mixing chamber diameter, mixing chamber length, diffuser angle, nozzle exit positon) and the factors affecting performance are identified through the velocity vector. CFD models such as SST k-ω turbulence model, compressible and energy equation were applied in the CFD analysis. The optimum parameter of the mixing chamber diameter is 4.5 mm. As a result of the velocity vector, a wide range of vortex was observed in the mixing chamber at 6 mm compared with 4.5 mm. The optimum diffusion angle is 3 degrees. The vortex was formed at a wider angle than the optimal angle at the wall, and it was due to the flow separation. The dimension of the optimum nozzle exit position is 2 mm. When the nozzle exit position became long, the vortex moved to the suction chamber and interfered with the suction. The optimum mixing chamber length is 18 mm. there will be a drop of pressure when longer than optimum diameter due to friction flow. The optimal ejector is manufactured based on the CFD results. The optimal ejector was verified by mock-up bed test. The experiment was conducted in a secondary fluid dry and humidified. The first experiment was of the initial ejector. In the dP = 0.025 bara, the entrainment ratio of drying experiment was 1.18 and humidification experiment was 0.643. Optimal ejector was 1.36 at dry, 0.75 in the humidification experiment. In the humidification condition, water droplets are discharged to the ejector outlet. Condensation of water vapor due to temperature drop in the mixing chamber reduces the area of the mixing chamber and changes the flow to make the entrainment ratio lower than the drying condition. The second experiments were of ejectors with changed nozzle throat. The diameters was 1.4, 1.5 and 1.6 mm. 1.6 mm was lower entrainment ratio than other dimensions. At 1.4mm, it showed the same performance as the optimal nozzle throat, which shows that the secondary fluid reaches the choking at 1.5mm. Choking occurs when the mass flow through a duct is limited by the sonic condition. These results prove that 1.5 mm is the optimal dimension. The final experiment in the mock-up tests were of ejectors with changed mixing chamber diameter of 3, 4.5, and 6 mm. As a result, ejector operating range has changed according to the diameter. The operating range is 0.02 to 0.08 bara. This is a wide range, but the volume of the mixing chamber is not sufficient and the rising width is narrow. At 4.5mm, it showed wide operating range (0.03 ~ 0.08 bara) and high entrainment ratio.
The mock-up test proved that manufactured ejector has optimal dimension. Next, optimal ejector was tested on the 10 kW PEFCs system. 10 kW PEFCs has 75 cells and active area is 460cm2. Experiments were conducted at 150, 180, and 200 A in dead-end, recycle mode. During dead-end operation, the number of purge times is 18, 21, 23 for 1800 sec. and recycle operation, the number of purges reduced by about 45%, which is 8, 9, 10 times. Under the assumption of the adiabatic process, energy conservation law was applied and the secondary flow rate was determined by Q_pri+Q_sec=Q_out. Secondary Flow rates were 22.74, 28.78, 31.32 nlpm and entrainment ratio was 0.291, 0.306 and 0.301, which were low to those of the mock-up test (@0.06 bara, entrainment ratio =0.34). In conclusion, the ejector performance in the CFD was much different from the actual experiment. However, finding the optimal parameter was successful and verified through mock up test. Through the stack test, we confirmed that the ejector operates at good condition Performance estimates were inaccurate, but optimal dimensions could be determined, and CFD needs to be improved for more accurate estimation.
This study is about optimal design of ejector for hydrogen and operation test at recycle mode in polymer electrolyte fuel cells. Ejector has a simple structure, and does not need electricity. Because of these advantages, ejectors have been applied to a variety of industrial fields such as refrigerators and so on. However, the existing commercial ejectors do not meet the practical operating requirements of the PEFCs system with the humidified hydrogen recycle. In this work, CFD is used to design an ejector applicable to 10 kW PEFCs. The structure of the ejector is divided into four large parameters(mixing chamber diameter, mixing chamber length, diffuser angle, nozzle exit positon) and the factors affecting performance are identified through the velocity vector. CFD models such as SST k-ω turbulence model, compressible and energy equation were applied in the CFD analysis. The optimum parameter of the mixing chamber diameter is 4.5 mm. As a result of the velocity vector, a wide range of vortex was observed in the mixing chamber at 6 mm compared with 4.5 mm. The optimum diffusion angle is 3 degrees. The vortex was formed at a wider angle than the optimal angle at the wall, and it was due to the flow separation. The dimension of the optimum nozzle exit position is 2 mm. When the nozzle exit position became long, the vortex moved to the suction chamber and interfered with the suction. The optimum mixing chamber length is 18 mm. there will be a drop of pressure when longer than optimum diameter due to friction flow. The optimal ejector is manufactured based on the CFD results. The optimal ejector was verified by mock-up bed test. The experiment was conducted in a secondary fluid dry and humidified. The first experiment was of the initial ejector. In the dP = 0.025 bara, the entrainment ratio of drying experiment was 1.18 and humidification experiment was 0.643. Optimal ejector was 1.36 at dry, 0.75 in the humidification experiment. In the humidification condition, water droplets are discharged to the ejector outlet. Condensation of water vapor due to temperature drop in the mixing chamber reduces the area of the mixing chamber and changes the flow to make the entrainment ratio lower than the drying condition. The second experiments were of ejectors with changed nozzle throat. The diameters was 1.4, 1.5 and 1.6 mm. 1.6 mm was lower entrainment ratio than other dimensions. At 1.4mm, it showed the same performance as the optimal nozzle throat, which shows that the secondary fluid reaches the choking at 1.5mm. Choking occurs when the mass flow through a duct is limited by the sonic condition. These results prove that 1.5 mm is the optimal dimension. The final experiment in the mock-up tests were of ejectors with changed mixing chamber diameter of 3, 4.5, and 6 mm. As a result, ejector operating range has changed according to the diameter. The operating range is 0.02 to 0.08 bara. This is a wide range, but the volume of the mixing chamber is not sufficient and the rising width is narrow. At 4.5mm, it showed wide operating range (0.03 ~ 0.08 bara) and high entrainment ratio.
The mock-up test proved that manufactured ejector has optimal dimension. Next, optimal ejector was tested on the 10 kW PEFCs system. 10 kW PEFCs has 75 cells and active area is 460cm2. Experiments were conducted at 150, 180, and 200 A in dead-end, recycle mode. During dead-end operation, the number of purge times is 18, 21, 23 for 1800 sec. and recycle operation, the number of purges reduced by about 45%, which is 8, 9, 10 times. Under the assumption of the adiabatic process, energy conservation law was applied and the secondary flow rate was determined by Q_pri+Q_sec=Q_out. Secondary Flow rates were 22.74, 28.78, 31.32 nlpm and entrainment ratio was 0.291, 0.306 and 0.301, which were low to those of the mock-up test (@0.06 bara, entrainment ratio =0.34). In conclusion, the ejector performance in the CFD was much different from the actual experiment. However, finding the optimal parameter was successful and verified through mock up test. Through the stack test, we confirmed that the ejector operates at good condition Performance estimates were inaccurate, but optimal dimensions could be determined, and CFD needs to be improved for more accurate estimation.
주제어
#PEFCs Ejector CFD Operating test Hydrogen recycle
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