查看更多>>摘要：The focus of this study is to evaluate the effects of porous media inside the gas flow channel of Proton Exchange Membrane Fuel Cells (PEMFC) on four different output parameters of voltage, power density, pressure drop, and Nusselt number (Nu) considering the impacts of its thickness, viscous resistance, and current density. Although it is proved that the new design will improve the convective heat transfer, there have not been studies to evaluate the effects of this porous layer on the electrochemical performance and heat transfer simultaneously. The results showed that viscous resistance has by far the highest impact on the power densities in high current densities while thicker inserted porous layer improves the performance. Results also demonstrate that a parameter is needed to consider all these output parameters at the same time, hence the Evaluation Criterion of Proton Exchange Membrane (ECPEM) is defined using artificial neural network (ANN) modeling. Single-objective optimization of the ECPEM is developed using the ANN models to produce 250,000 data. The optimum value of ECPEM was obtained 78.88 in the thickness of 500 mu m and the viscous resistance of 2,111,000 (1/m(2)) while the current density is equal to 0.65 (A/cm(2)).
查看更多>>摘要：This study proposes a comprehensive evaluation of the actual greenhouse effect related to the operation of ORC systems in the kW scale. The method is derived from the TEWI (total equivalent warming impact) concept for refrigeration systems, since it includes both direct and indirect contributions to the greenhouse gas emission related to the ORC system. A comparison between traditional HFC-134a (R134a) and some of its low-GWP replacements has been performed, accounting for the effect of the operating fluid leakage during system operation, but also for the indirect contribution associated to the lower performance that can be achieved using more sustainable working fluids, such as hydrofluoroolefins (HFO). Alternative fluids that have been tested are two pure compounds (R1234yf and R1234ze(E)), and four mixtures (R134a-R123434; R-134a-R1234ze(E); R515A; R430A). A semi-empirical lumped-parameters model has been employed for simulating the behavior of the ORC system. For the model validation, the experimental data collected on a reference 2-kW ORC test bench with R134a have been used. The model was then applied to investigate the performance of the system working with alternative fluids. The results show that the indirect emissions associated to HFOs may lead to higher values of total equivalent CO2 emissions, with respect to the employment of R134a as working fluid. The main factors affecting the environmental evaluation, such as emission factors, fluid leak rate and R134a concentration in the mixture, can be decisive and are discussed in this work.
查看更多>>摘要：Since the early 1990s, the pulsating heat pipe (PHP) has emerged as one of the most innovative, effective and potentially more convenient passive two-phase heat transfer systems, thanks to its good performance, versatility, and construction simplicity. On the other hand, the PHP is characterized by complex thermohydraulic behaviour that still presents a true challenge to designers, which has led to significant interest by a growing number of researchers. The technological readiness level (TLR) of this technology is quite broad depending on the application: for instance, the industrial community is starting to consider the PHP as a reliable solution for electronic cooling in ground conditions, while implementations in the cryogenic temperature range and in space environments is also being extensively explored. This vision paper aims at shedding light on the current knowledge and prediction capability of PHP numerical models, on unsolved phenomenological issues, on the current technological challenges and the future perspectives of this fascinating heat transfer device. Specifically, after a general introduction and a brief overview of the current knowledge and the open issues of PHPs, special focus is devoted to the following topics: flat-plate PHP assessments; advancements in PHP modelling and simulation; flow stabilization techniques; non-conventional fluids subdivided into fluid mixtures, self-rewetting fluids, nanofluids; cryogenic applications, space applications, and finally the newest frontiers of flexible PHPs. Each section is accompanied by a brief roadmap providing directions for future research based on key challenges, which are also gathered and summarized in the final outlook section.
NSTL
Elsevier