Micro/mesoscale combustion based portable power generating system

Abstract

Micro/mesoscale combustion-driven power generation is a promising alternative to replace traditional batteries for powering small scale electronics owing to its much higher energy densities. However, the considerable heat losses from the combustor walls due to the dramatically increased surface-tovolume ratio, as well as the short flow residence time at small scales pose a technical challenge in stabilising the flame inside the micro/mesoscale combustor. On the other hand, the significantly enhanced flame-wall thermal coupling at small scales can also lead to some unique flame characteristics.Recently, solid state power conversion of the heat released in the micro/mesoscale combustor such as thermoelectric (TE) and thermophotovoltaic (TPV) has been favoured over traditional power conversion methods such as micro-turbines. This is due to the excessive mechanical and viscous losses that rotating machinery suffers from upon miniaturisation.One of the main aims of this thesis is to perform numerical simulations to investigate the fundamental aspects of micro/mesoscale combustion, such as micro-flame behaviours of propagation and stabilisation, the conditions under which flame dynamics occur and the choices that suppress them. A set of modelling techniques that gives guidelines and practices for performing the time-accurate micro/mesoscale combustion simulations has been developed through investigation. Knowledge on standard and well-accepted numerical methods in literature are collected in a cohesive document. The less well-established modelling choices have been thoroughly evaluated and discussed.Using the developed numerical models, the effects of hydrogen and carbon monoxide addition on premixed methane/air flame dynamics is investigated. Results show that the flame instabilities which exist in pure methane/air flames could be effectively suppressed via a small amount of additives. Deep understanding of the underlying physics and chemistry has been gained. The effects of wall temperature profiles on the simulated micro-flame behaviours are also studied. The role of the combustion heat release and the gas-solid heat transfer in determining the micro-flame propagation speed is identified.Experimental works are also carried out, towards a full system demonstration for micro-power generation. The focus is placed on the "wall thermal engineering" on the improvement of the combustor performance and power output. The thermally-orthotropic pyrolytic graphite for the combustor wall material is explored. Compared to an isotropic material of stainless steel, a widened flame stability limit with pyrolytic graphite is demonstrated. Moreover, a uniform and moderate temperature distribution over the combustor surface is achieved, which is favourable for thermoelectric power conversion applications. Meanwhile, a novel, compact mesoscale thermophotovoltaic power generating system using a combination of porous media combustion and a simple band-pass filtering method is developed. The use of the porous media effectively enh...

Numerical Modelling of Anisotropic Heat Recirculation in Microcombustors

“…In Section 3.2.3 Kang's [24] conclusions suggested the need for full CHT over prescribed wall temperatures in order to accurately simulate transient microcombustion with heat recirculation. Additionally it is clear that neither the combustor structure's influence on flame stability nor anisotropic heat conduction's effect can be observed using prescribed wall temperatures.…”

“…In Kang's study of microcombustion for power generation [24], wall temperature profiles were prescribed using a variety of step, linear and hyperbolic functions to simulate the effect of wall temperature on flame dynamics in Eilmer 3. In a similar manner to Kurdyumov et al [28], oscillatory flame repetitive extinction and ignition (FREI) behaviour was found to be closely linked to wall temperature profiles.…”

“…(a) Numerically simulated fluid temperature flow from Kang [24] (b) Prescribed temperature from Denman [5] Figure 3.6: Fluid flow temperatures…”

“…These were based on the parameters for an orthotropic graphene microcombustor and partially on Kang's [24] simulation parameters. Note that the x and 1 directions are aligned (as are the y and 2 directions) for convenience (and since this gives the highest resistance in the correct direction).…”

Simulating Micro Combustion with a Compressible Flow Solver

“…Micro channel combustion is one such process used to generate power. Hydrocarbon micro combustion is a current area of interest as it provides a promising alternative to lithium batteries for portable power applications (such as for small handheld devices) [4]. [1] This is due to the high energy density of hydrocarbon fuels.…”