Flame speed predictions in planar micro/mesoscale combustors with conjugate heat transfer

Veeraragavan, Ananthanarayanan; Cadou, Christopher P.

doi:10.1016/j.combustflame.2011.04.006

Cited by 88 publications

(45 citation statements)

References 21 publications

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“…This phenomena may occur due to several reasons such as heat loss variations, heat transfer from post-flame to pre-flame zone (Hua et al, 2005;Leach and Cadou, 2005), the increment of reactive mixture residence time (Norton and Vlachos, 2003) and enhancement of mixing process, respectively. Also, some chemical and physical properties of the micro-combustor wall and the inlet mixture such as fuel type (Norton and Vlachos, 2003), wall thermal conductivity (Baigmohammadi et al, 2013;Vlachos, 2003, 2004;Raimondeau et al, 2002;Rana et al, 2014;Veeraragavan and Cadou, 2011;Zarvandi et al, 2012;Zhou et al, 2009), equivalence ratio and inlet velocity can influence combustion process in micro-combustors. Therefore, variation of the wall thermal conductivity and its physical properties can govern the combustion by proper pre-heating of fresh incoming mixture (Raimondeau et al, 2002) or thermal and radical species quenching processes .…”

Section: Frontiers In Heat and Mass Transfermentioning

confidence: 99%

Effect of Wall Thermal Conductivity on Hydrogen-Assisted Catalytic Ignition Characteristics of Propane-Air at Micro-Scales in Different Feeding Modes

Chen¹,

Gao²,

Xu³

2015

Frontiers in Heat and Mass Transfer

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Effect of wall thermal conductivity on hydrogen self-ignition and hydrogen-assisted ignition of propane-air mixtures in different feeding modes from ambient cold-start conditions were investigated numerically with chemical kinetic model in Pt/γ-Al2O3 catalytic micro-combustors. For the steady and transient state, effect of wall thermal conductivity on self-ignition characteristics of lean hydrogen-air mixtures was presented, and hydrogenassisted combustion of propane-air mixtures was investigated numerically in the co-feed mode and the sequential feed mode. The computational results indicate the large thermal inertia of the micro-combustor solid structure leads to slow temperature dynamics, and transient response is dominated by the thermal inertia. The heat localization in poorly conducting walls leads to fast ignition and shorter steady state time. In general, the concentration of hydrogen required for propane ignition increased with increasing wall thermal conductivity, decreasing inlet velocity, and decreasing inlet equivalence ratio of propane-air mixtures. In the co-feed mode, the combustion characteristics of hydrogen-assisted propane qualitatively resemble the selectively preheating initial portion of the combustion chamber wall. In the sequential feed mode, the time taken to reach steady state, the hydrogen cut-off time, the propane ignition time and the cumulative propane emissions increased with increasing wall thermal conductivity; the ignition characteristics are similar to partially preheating the initial segment for low and moderate wall thermal conductivity values (0.5 and 20 W/m· K); however, the ignition characteristics are close to completely heating the micro-combustor wall for high wall thermal conductivity values (200 W/m· K).

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Section: Frontiers In Heat and Mass Transfermentioning

confidence: 99%

Effect of Wall Thermal Conductivity on Hydrogen-Assisted Catalytic Ignition Characteristics of Propane-Air at Micro-Scales in Different Feeding Modes

Chen¹,

Gao²,

Xu³

2015

Frontiers in Heat and Mass Transfer

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“…As for the influence of heat recirculation on flame stability and speed in micro-combustors, Veeraragavan et al [43] have developed an analytical model for flame stabilization in meso-scale channels and categorically shown the influence of heat recirculation on the flame stability and speed. 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.…”

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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“…Veeraragavan [45,46] developed a two-dimensional analytical model for flame stabilisation in a mesoscale channel involving species transport in the gas, heat transfer in both the gas and structure. Since the conjugate heat transfer was accounted for, there was no need of assuming the values of the Nusselt number in the pre and post-flame regions.…”

Section: Fundamental Studiesmentioning

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%

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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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Flame speed predictions in planar micro/mesoscale combustors with conjugate heat transfer

Cited by 88 publications

References 21 publications

Effect of Wall Thermal Conductivity on Hydrogen-Assisted Catalytic Ignition Characteristics of Propane-Air at Micro-Scales in Different Feeding Modes

Effect of Wall Thermal Conductivity on Hydrogen-Assisted Catalytic Ignition Characteristics of Propane-Air at Micro-Scales in Different Feeding Modes

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

Micro/mesoscale combustion based portable power generating system

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