Weiwei Yang

A microfluidic-structured flow field for passive direct methanol fuel cells operating with highly concentrated fuels

Conventional direct methanol fuel cells (DMFCs) have to operate with excessively diluted methanol solutions to limit methanol crossover and its detrimental consequences. Operation with such diluted methanol solutions not only results in a significant penalty in the specific energy of the power pack, limiting the runtime of this type of fuel cell, but also lowers the cell performance and operating stability. In this paper, a microfluidic-structured anode flow field for passive DMFCs with neither liquid pumps nor gas compressors/blowers is developed. This flow field consists of plural micro flow passages. Taking advantage of the liquid methanol and gas CO2 two-phase counter flow, the unique fluidic structure enables the formation of a liquid–gas meniscus in each flow passage. The evaporation from the small meniscus in each flow passage can lead to an extremely large interfacial mass-transfer resistance, creating a bottleneck of methanol delivery to the anode CL. The fuel cell tests show that the innovative flow field allows passive DMFCs to achieve good cell performance with a methanol concentration as high as 18.0 M, increasing the specific energy of the DMFC system by about five times compared with conventional designs.

Three-Dimensional Numerical Study of Natural Convective Heat Transfer of Liquid in a Cubic Enclosure

ABSTRACT Steady-state laminar natural convection in a cubic enclosure with a cold vertical wall and two hot square heaters with constant temperature on the opposite wall is studied numerically. The enclosure is filled with various liquids. Three-dimensional Navier–Stokes Equations are solved by employing the SIMPLE algorithm. Computations are performed for a range of Rayleigh number from 103 to 107 while enclosure aspect ratio varies from 0.05 to 1.6. The effects of Rayleigh number, enclosure aspect ratio, and Prandtl number on heat transfer characteristics are studied in detail. The results show that the flow field is very complex and heat transfer from the two heaters is not the same. The effects of Prandtl number are negligible in the range from 5 to 140 with other parameters kept constant. This allows the use of liquids such as water for studying other dielectric liquids, provided the flow geometry and other nondimensional parameters are similar. The overall Nusselt number increases markedly with Rayleigh number. It is also affected by enclosure aspect ratio. It attains the maximum value when aspect ratio is in the range of 0.1–0.2 and decreases as enclosure aspect ratio varies from 0.2 to 1.6. Also, various settings of cooling face and arrangement of heaters are investigated, and the results show that they have considerable effects on heat transfer of both heaters.

Parametric Optimization of Zeotropic Mixtures Used in Low-Temperature Organic Rankine Cycle for Power Generation

In the design and optimization of the ORC system, the selection of working fluid is one of the most important factors that should be considered. In this work, considering different heat sources with their temperatures ranging from 80 to 120 °C, 8 different zeotropic mixtures were proposed and their thermodynamic and economic performance for two types of traditional ORC systems (i.e. basic organic Rankine cycle, BORC and organic Rankine cycle with internal heat exchanger, IHORC) were investigated. Firstly, economic analysis were performed for both systems; Secondly, genetic algorithm (GA) was then introduced to determine the optimal fractions and other operation parameters for zeotropic mixtures under different working conditions and systems, the algorithm implementation process was described. Thirdly, the optimization studies were performed by using annual cash flow as the objective function. The optimal thermodynamic performance of different zeotropic mixtures and their components were both calculated and compared. For the different heat sources temperatures, the optimal zeotropic mixtures and their optimal fraction were recommended according to the calculated results.Copyright

Fuel Cells: Direct Alcohol Fuel Cells: Modeling

Mathematical modeling plays an important role not only in helping us to understand the physiochemical phenomena occurring in fuel cells, but also to design and develop fuel cell systems. This article presents an overview of the mathematical modeling of direct alcohol fuel cells (DAFCs). It starts with an introduction to an empirical model that helps in interpreting experimental data, determining kinetic parameters, and identifying various types of voltage loss. The article then focuses on discussing a steady-state isothermal two-phase mass transport model that considers conservation equations of mass, species, heat, and current transport in various regions of a single DAFC cell.