IVAN KORKISCHKO
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Resumo IPEN-doc 27957 Geometric parameter optimization of a liquid jet liquid ejector2019 - KORKISCHKO, IVAN; MAFFEI, FELIPE S.; GIORIA, RAFAEL dos S.; MENEGHINI, JULIO R.Ejectors are devices employed as pumps or compressors, which work transferring momentum from a primary fluid (high pressure) to a secondary fluid (low pressure). On the one hand, their main advantages over standard pumps and compressors are no moving parts, no need of lubricants and seals, and low noise and maintenance. On the other hand, ejectors have low efficiency compared to other devices and a very narrow region of optimal operation. Thus, ejectors certainly benefit from optimization studies. This investigation was based on a CFD model of a liquid jet liquid (LJL) ejector. The finite element method was used, coupled with the k-epsilon turbulence model. The optimization study had three steps. First, the constants of the turbulence model were recalibrated to minimize the difference between the numerical and experimental efficiency curves. Second, using the main geometric parameters as control variables, the peak efficiency was maximized. Finally, the optimized geometry was further improved, considering the transitions between the different ejector components, which were originally sharp corners. The optimized round corners increased the ejector efficiency.Resumo IPEN-doc 26701 A hybrid serpentine-interdigitated flow channel geometry for fuel cells2019 - BERUSKI, OTAVIO; KORKISCHKO, IVAN; LOPES, THIAGO; FONSECA, FABIO C.; PEREZ, JOELMAFuel cells have impressive potential for decarbonization and as high efficiency power sources, however many challenges have yet to be addressed for large scale deployment and uptake. Among the many noteworthy lines of research underway, investigating the best flow field in a given device has been carried a number of times, with perhaps limited success regarding performance improvement. As a possible final attempt to look over such matters individually, from the component point of view, we propose yet another flow channel geometry for small-scale fuel cells, in particular polymer electrolyte fuel cells (PEFCs). The proposed geometry incorporates elements from the two most studied geometries, namely single serpentine and interdigitated. The rationale is that serpentine channels have large pressure drop, thus aiding in water removal, while interdigitated promises to deliver large quantities of reactants to the catalyst. However both seem to fail where the other excels, and thus devices are left to compromises. The new geometry, as well as its inspirations, are simulated in a previously validated computational model, further improved and with high spatial resolution, of a prototype PEFC cathode. The model is isothermic, non-electrochemical and disregards water, as the experimental system. However it has been shown to be useful when studying PEFCs, and a secondary goal of this work is to corroborate this. Comparing simulation results between geometries, it is seen that the hybrid geometry does inherit the characteristics of interest, i.e. high reactant utilization and pressure drop, suggesting it may be of use in real PEFCs. Finally, a niche application is proposed based on the reaction rate distribution of the hybrid geometry.Resumo IPEN-doc 26700 Unveiling fundamental transport phenomena in fuel cells2019 - LOPES, THIAGO; BERUSKI, OTAVIO; KORKISCHKO, IVAN; MANTHANWAR, AMIT M.; PISTIKOPOULOS, EFSTRATIOS N.; FONSECA, FABIO C.; MENEGHINI, JULIO R.; KUCERNAK, ANTHONY R.In situ and ex situ spatially-resolved techniques are employed to investigate reactant distribution and its impacts in a polymer electrolyte fuel cell. Temperature distribution data provides further evidence for secondary flows inferred from reactant imaging data, highlighting the contribution of convection in heat as well as reactant distribution. Water build-up from neutron tomography is linked to component degradation, matching the pattern seen in the reactant distribution and thus suggesting that high, nonuniform local current densities shape degradation patterns in fuel cells. The correlations shown between different techniques confirm the use of the versatile reactant imaging technique, which is used to compare commonly used flow field designs. Among serpentine-type designs, the single serpentine is superior in both equivalent current density and reactant distribution, showing large contributions from convective flow. On the other hand, the interdigitated design is shown to produce larger equivalent current densities, while showing a somewhat poorer reactant distribution. Considering the correlations drawn between the techniques, this suggests that the interdigitated design compromises durability in favour of power output. The results highlight how established techniques provide a robust background for the use of a new and flexible imaging technique toward designing advanced flow fields for practical fuel cell applications.Resumo IPEN-doc 25569 Estudos numéricos dos processos de distribuição de água durante ensaios de durabilidade em células a combustível do tipo PEM2018 - SANTIAGO, ELISABETE I.; SPRANDEL, LEOPOLDO; ANDREA, VINICIUS; CUNHA, EDGAR F.; ROBALINHO, ERIC; LINARDI, MARCELO; KORKISCHKO, IVANAs células a combustível estão entre as tecnologias mais promissoras, visto que estes dispositivos podem produzir energia elétrica com baixa emissão de poluentes e de forma muito eficiente [1]. O entendimento dos mecanismos de transporte de água em células a combustível do tipo PEM é um ponto chave para a definição de estratégias de gerenciamento de água e aumento de desempenho. Quantidade suficiente de água deve estar presente na célula para manter a condutividade protônica da membrana polimérica, no entanto, o excesso de água nas camadas porosas são as principais causas de perdas reversíveis [2]. Este estudo tem como objetivo a compreensão dos fenômenos envolvidos no transporte de água em células a combustível do tipo PEM em função das condições de operação da célula e propor condições de operação que otimizem o fluxo de água, permitindo maior durabilidade desse tipo de célula a combustível. Nesse sentido, foi realizado um estudo de modelagem e simulação numérica de uma célula a combustível unitária envolvendo os fenômenos de transporte e os processos eletroquímicos. A simulação da dinâmica da água foi realizada considerando um sistema bifásico (modelos de misturas) em diferentes condições operacionais, a saber temperatura e fluxo de gases. A análise dos resultados mostrou que a 0,6V, temperatura da célula de 75oC e vazão de H2 próximo a três vezes o valor estequiométrico, a temperatura ideal de umidificação é entre 80oC e 85oC. Temperaturas de umidificação maiores podem resultar em inundações dos canais de distribuição e camadas porosas enquanto temperaturas menores podem ocasionar a desidratação da membrana. A mesma temperatura de umidificação dos gases reagentes e da célula a combustível pode ser adotada desde que o fluxo de gases reagentes seja alto o suficiente para garantir a hidratação da membrana, neste caso, seis vezes o valor estequiométrico. Os resultados confirmam a importância do arraste eletro-osmótico no balanço de água e a importância de análises bifásicas em estudos de otimização de células a combustível. O balanço de água ideal pode ser alcançado definindo adequadamente os fluxos e temperaturas de umidificação dos gases reagentes em função do potencial de operação com auxílio de modelos numéricos.Resumo IPEN-doc 25562 Modeling, simulation and shape optimization of a proton exchange membrane fuel cell using computational fluid dynamics2018 - KORKISCHKO, IVAN; SANTIAGO, ELISABETE I.; CARMO, BRUNO S.; FONSECA, FABIO C.This paper presents the modeling, simulation and optimization of a single channel proton exchange membrane fuel cell (PEMFC) using computational fluid dynamics methods. The shape optimization of the cross section of the flow channels was employed to improve the electrical performance of the fuel cell. The minimization of the standard deviation of the current density on the longitudinal mid-plane of the membrane was the objective function of the single-objective optimization problem, the upper and lower widths of the flow channels were the control variables and a cross-section area restriction was imposed. The optimized flow-channel PEMFC presented improved electrical performance, with higher current and power densities and a more uniform current density distribution than the rectangular flow channel. It is also expected that a more uniform current distribution improves the durability and water management of the fuel cell.