We investigate the onset of irreversibility in a dense granular medium subjected to cyclic shear in a split-bottom geometry. To probe the micro and mesoscale, we image bead trajectories in 3D throughout a series of shear strain oscillations. Though beads lose and regain contact with neighbors during a cycle, the global topology of the contact network exhibits reversible properties for small oscillation amplitudes. With increasing reversal amplitude a transition to an irreversible diffusive regime occurs.

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In situ diagnostic techniques provide a means of understanding the internal workings of fuel cells so that improved designs and operating regimes can be identified. Here, for the first time, a combined current density and temperature distributed measurement system is used to generate an electro-thermal performance map of an air-cooled, air-breathing polymer electrolyte fuel cell stack operating in an air / hydrogen cross-flow configuration. Analysis is performed in low-and high-current regimes and a complex relationship between localised current density, temperature and reactant supply is identified that describes the way in which the system enters limiting performance conditions. Spatiotemporal analysis was carried out to characterise transient operations in dead-ended anode / purge mode which revealed extensive current density and temperature gradients. KeywordsCurrent mapping; temperature mapping; air-breathing fuel cell; performance optimisation; spatiotemporal electro-thermal analysis. Current mapping was first introduced by Cleghorn and Derouin[1] and Brett et al. [2,3], using printed circuit board (PCB) technology to produce segmented current collector plates, along with Stumper et al.[4], using segmented membranes. This technique shows that significant current density gradients can be generated as a result of fuel and oxidant depletion. Other techniques have proven the value of localised current measurements, including segmented PCBs [1,3,5-9], segmented current collector plates [10-12], shunt resistors [13-15] and Hall effect sensors [16-21]. Measurements have also been performed at sub-millimetre resolution [22,23], investigating the gradients across the land and channel areas. Most studies highlighted uneven performance under high current density, uneven fuel consumptions [2,5,6,14], the crucial influence of the operating conditions [3,13,19], stoichiometric ratios [8,11,12,20], fuel orientations [6,13], water management [10], CO electro-oxidation [7] and CO poisoning [24,25] distribution.Temperature distribution has also been extensively studied, identifying areas of higher electrochemical activity, hot spot formation and fuel depletion. Thermocouples can provide a crude measure of temperature inside fuel cells [16,17,26,27] but have accuracy limited to ±1 o C and cannot provide high spatial resolution. Moreover, thermocouples need to be inserted inside the fuel cell, which often requires design modifications. In contrast, infrared thermal imaging can provide very high spatial and temperature resolution [28][29][30][31][32][33][34], yet it typically requires use of specially designed fuel cells with a window transparent to infrared radiation, or is otherwise confined to measurement of the outer surface of a cell or stack.(highlighted using neutron imaging [37,38]) and nitrogen accumulation (measured via off-gas analysis [37,42,43]).Meyer et al. [37] reported an increase of the overall temperature and ohmic resistance in dead-ended mode. Eventually, local current density could be much higher than the...

Flow-fields are key factors in determining the operation of fuel cells. While extensive work has been conducted to develop and optimise the reactant flow and current collection performance of polymer electrolyte membrane fuel cell (PEMFC) components, there is a factor that remains largely unaccounted for. Depending on how a membrane electrode assembly (MEA) is incorporated into a cell, there will often be a small gap between the edge of the gas diffusion layer (GDL) and the seal or bipolar plate. This gap acts as a 'secondary flow-field' (SFF) that can bypass or affect/augment the conventional or 'primary flow-field'. Understanding how this affects performance (either positively or adversely) is essential for holistic flow-field design. This paper describes the issues associated with the SFF, examines how cell compression affects its width due to lateral expansion of the GDL and discusses the results of a 3-D computational model that investigates the effect of the SFF during dead-ended anode (DEA) operation for a fuel cell without a macroscopic (conventional) anode flowfield.