truefalse2016-03-16T23:02:09

This study optimizes numerically the relative sizes, spacings (internal structure), and aspect ratios (external configuration) of a single alkaline membrane fuel cell for maximum net power. The alkaline membrane fuel cell (AMFC) cellulosic membrane brings new light to the possibility of having alkaline fuel cells that are nontoxic and asbestos free as compared with static electrolyte cells that use an asbestos separator and ammonium-based alkaline anion-exchange membranes. A dimensionless dynamic mathematical model is utilized in the process, and the results are presented in normalized charts for generality. Two degrees of freedom are considered as follows: (i) the relative thicknesses of two reaction and diffusion layers and the membrane space (internal structure); and (ii) the external aspect ratios of a square section plate that contains all single alkaline membrane fuel cell components (external configuration). The optimized internal and external configurations result from the optimal balance between electrical power output and pumping power to supply fuel and oxidant to the AMFC. A third level of optimization is found, that is, the KOH mass fraction in the electrolyte that leads to a 3-way-maximized net power output. A sixfold variation in AMFC net power output is observed as the internal and external configurations, and KOH mass fraction are changed. Such effect stresses the importance of pinpointing the optimal AMFC configuration in order to avoid poor performance. New algebraic correlations are derived to indicate in dimensionless form, the optimal configurations for the internal and external structure, and resulting maximum net power output, which are important for scaling up and down the AMFC design with ease, without having to perform new simulations.

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The architecture of heat exchangers is a classical subject that has been studied extensively in the past. In this paper, we address the fundamental question of what the size of the heat exchanger should be, in addition to what architectural features it should have. The answer to the size question follows from the tradeoff between (1), the useful power lost because of heat transfer and fluid flow and (2), the power destroyed during transportation, manufacturing, and maintenance. Changes in heat exchanger size induce changes in the opposite sign in the power requirements (1), and (2). This fundamental tradeoff regarding size is illustrated by considering one side of a heat exchanger (one flow passage) in laminar flow and in fully rough turbulent flow, with several duct cross sectional shapes and arrays of channels in parallel. The size tradeoff is present in heat exchanger applications across the board, from vehicles to stationary power plants.

In this paper the results of experimental tests on turbulent boundary layer on a plate with air blowing through permeable porous wall are presented. Tests were performed in an aerodynamic tunnel varying the blowing rate within the range of 0 to 2.5% of the air flow velocity in the tunnel, using thermoanemometers of constant temperature. It has been determined that the blowing increases the boundary layer thickness and strongly transforms the boundary layer inner part and turbulent nucleus, while the outer part remains not deformed. An increase in blowing causes an abrupt viscous stress decrease, also decreasing skin friction. Thick boundary layers (up to 120 mm) allowed a profound analysis of Reynolds' turbulent stresses. It has been found a general distribution of turbulent stress in the turbulent layer which is independent of blowing velocity. It has been established for the turbulent characteristics that the determining coordinate is the relative horizontal velocity, not the distance from the wall. With a turbulent stress database obtained from turbulent stresses direct measurements, it has been proposed a flow model for boundary layer on a permeable porous plate. It has also been demonstrated that in high blowing conditions the horizontal velocity profile in the boundary layer becomes equal to the velocity profile in a plane jet.

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This paper presents a three-dimensional geometric optimisation of conjugate cooling channels in forced convection with internal heat generation within the solid. Pentagonal cooling channels configuration are considered. The main objective is to optimise the configuration in such a way that the peak temperature is minimised subject to the constraint of fixed global volume of solid material. The cooling fluid is driven through the channels by the pressure difference across the channel. The elemental volume of the structure and hydraulic diameter of the cooling channel were considered as design variables. The shape of the channel is allowed to morph to determine the best configuration that gives the lowest thermal resistance. A gradient-based mathematic optimisation algorithm (Dynamic-Q) is applied in order to search for the best optimal geometric configuration that improves thermal performance by minimising thermal resistance for a wide range of dimensionless pressure difference. Results obtained show that there are unique optimal geometry for a given pressure difference. Also, the results show that the effects of dimensionless pressure drop on minimum thermal resistance and hydraulic diameter of the channel are consistent with those obtained in the open literature.