On flame propagation in narrow channels with enhanced wall thermal conduction

Veeraragavan, Ananthanarayanan

doi:10.1016/j.energy.2015.09.085

Cited by 79 publications

(18 citation statements)

References 39 publications

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“…They found that combustor design parameters (such as the wall thickness ratio, thermal conductivity ratio and heat loss to the environment) influence the flame speed through their influence on the total heat recirculation. Recently, the influence of orthotropic wall materials on the flame speed was investigated by Veeraragavan [44], whereby the effects of orthotropic wall thermal conductivities, heat loss, and wall thickness on the flame speed were explored, and the results indicated that total heat recirculation is the primary parameter that controls the flame speed. The first ever experimental demonstration of the use of orthotropic walls in microcombustors has been realized by Kang and Veeraragavan [45], who showed that the flammability limits can be enhanced due to the heat conduction through the plates is enhanced at large mixture flow rates.…”

Section: Introductionmentioning

confidence: 99%

The Influence of Slight Protuberances in a Micro-Tube Reactor on Methane/Moist Air Catalytic Combustion

Wang

Ran

et al. 2016

Energies

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Abstract:The combustion characteristics of methane/moist air in micro-tube reactors with different numbers and shapes of inner wall protuberances are investigated in this paper. The micro-reactor with one rectangular protuberance (six different sizes) was studied firstly, and it is shown that reactions near the protuberance are mainly controlled by diffusion, which has little effect on the outlet temperature and methane conversion rate. The formation of cavities and recirculation zones in the vicinity of protuberances leads to a significant increase of the Arrhenius reaction rate of CH 4 and gas velocity. Next, among the six different simulated conditions (0-5 rectangular protuberances), the micro-tube reactor with five rectangular protuberances shows the highest methane conversion rate. Finally, the effect of protuberance shape on methane/moist air catalytic combustion is confirmed, and it is found that the protuberance shape has a greater influence on methane conversion rate than the number of protuberances. The methane conversion rate in the micro-tube decreases progressively in the following order: five triangular slight protuberances > five rectangular protuberances > five trapezoidal protuberances > smooth tube. In all tests of methane/moist air combustion conditions, the micro-tube with five triangular protuberances has the peak efficiency and is therefore recommended for high efficiency reactors.

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Section: Introductionmentioning

confidence: 99%

The Influence of Slight Protuberances in a Micro-Tube Reactor on Methane/Moist Air Catalytic Combustion

Wang

Ran

et al. 2016

Energies

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“…However, experimental works have inherent difficulties in obtaining spatially resolved measurements at small scales. Similarly, analytical models which are used to understand the underlying microcombustor physics regarding heat recirculation [46,67], can only interpret their results in a qualitative sense since they use several simplifications and assumptions in order to develop a closed-form solution.…”

Section: Introductionmentioning

confidence: 99%

“…Similarly, theoretical models can also be used to understand the underlying microcombustor physics regarding heat recirculation [46,67], however, their results can only be interpreted in a qualitative sense as they use several simplifications and assumptions in order to develop a closed-form solution. On the other hand, numerical models do not make such assumptions and are capable of quantitatively revealing detailed physical features and can therefore be used as a reliable tool that develops the understanding of current configurations and supports the design of future systems.…”

Section: Introductionmentioning

confidence: 99%

“…This energy can be harnessed via combustion in the form of heat and converted to useful power. In order to achieve this, combustion in a narrow passage where the characteristic dimension is of the order of the flame thickness has received considerable attention in the past two decades and is termed microscale (dimension <1mm) or mesoscale (dimension close to flame thickness) combustion [5,46,66,67,70,73,91,106,[123][124][125]. The potential applications include portable power systems and propulsion systems for small scale rockets [4].…”

Section: Introductionmentioning

confidence: 99%

“…Micro/mesoscale combustion which can be viewed as combustion in narrow passages or ducts on the order of the flame thickness (dimension < 1 mm for microscale and > 1 mm for mesoscale), has drawn wide research interest in the past two decades [4,5,46,55,67,78,106,138,159,[164][165][166][167]. Compared to traditional lithium-ion or alkaline batteries, micro/mesoscale combustion using hydrogen or hydrocarbons as energy sources could offer considerably higher (tens of times) energy densities per unit mass and instant rechargeability, thereby leading to fewer logistical issues [1,168].…”

Section: Introductionmentioning

confidence: 99%

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Micro/mesoscale combustion based portable power generating system

Kang¹

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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...

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