Impacts of inner/outer reactor heat recirculation on the characteristic of micro-scale combustion system Ghobad Bagheri a,⇑ , Seyed Ehsan Hosseini b a Young Researchers and Elite Club, Kermanshah Branch, Islamic Azad University, Kermanshah, Iran b High-Speed Reacting Flow Laboratory, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia article info Article history: Received 30 April 2015 Accepted 22 July 2015 Keywords: Micro-combustion Heat recirculation Flame velocity Flame thickness Excess enthalpy abstract Flame stability and thermal performance of two different heat recirculation micro-combustors (inner reactor heat recirculation (IHR) and outer reactor heat recirculation (OHR)) are investigated using computational ﬂuid dynamics (CFD) and compared together. A two-dimensional steady state CFD model including temperature dependent properties, laminar ﬂow and transport, one step chemical reaction, surface-to-surface radiation, and heat conduction within solid walls has been carried out to assess ﬂame propagation velocity, ﬂame thickness, excess enthalpy, heat loss, and emitter efﬁciency. It is observed that both cases signiﬁcantly extend ﬂammability and blow-off limits due to preheating of the reactive mixture. The maximum ﬂame propagation velocities of IHR and OHR in stoichiometric mixture are predicted 160.2 cm/s and 126.1 cm/s, respectively. It is found that super-adiabatic ﬂame temperature takes place when dimensionless excess enthalpy is positive and it is maximum in the stoichiometric equivalence ratio. Heat loss can be varied from 245.8 to 248.6 W for IHR and from 249.6 to 254 W for OHR conﬁguration. Therefore, there is a relative improvement in the Thermal quenching limit of IHR. It is concluded that IHR micro-combustor profoundly affects ﬂame characteristic and stability, but OHR presents a higher range of emitter efﬁciency. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction It is evident that hydrocarbon fuels have a capacity for  100 times more energy–density than the most advanced batteries. Thus devices generating electricity from fuel at > 1% energy conversion efﬁciency demonstrate improvements over batteries for micro-scale systems [1,2]. These aspects of combustion based micro-power devices are deemed to ﬁnd applications to fabricate compact, portable, and lightweight decentralized power genera- tors [3,4]. Homogeneous combustion is a challenging issue at the micro-scale combustors due to the higher surface-to-volume ratio, thermal and radical quenching [5]. It was reported that the quenching distance is proportional to the ﬂame thickness, that the ﬂame thickness is inversely related to the burning velocity [6]. Therefore, to decrease the size scale of the combustor, the ﬂame thickness should be reduced ﬁrst. Several approaches can be employed for ﬂame thickness reduction such as, reducing the molecular distance by increasing the pressure, the application of speciﬁc oxidants or fuels with characteristic of higher burning velocity, and the catalytic reaction combustors to preserve the chemical chain reaction of termination [7–10]. However, the men- tioned methods are not directly associated with combustor conﬁg- uration design. They can be applied to various combustors after determining the best conﬁguration design. Consequently, several conﬁgurations have been suggested to overcome the ordinary quenching limits such as; vortex combustor [11,12], bluff-body [13–15], micro ﬂameless combustor [16], and heat recirculating or excess enthalpy combustors [1,17,18]. In the heat recirculating combustor, because of the thermal energy transferring from combustion products to the reactants without mass transfer, the total enthalpy of reactant can reach a higher level than incoming cold reactants. In other words ﬂame propagation would sustain under the conditions of low heating value fuel, lean mixture, and large heat losses that would quench without heat recirculation [19,17]. On the application side, Kim et al. [20] experimentally analyzed the propane–air mixture combustion in various conﬁgurations of swiss-roll combustor and different conditions of wall heat transfer. They pointed out that ﬂames can be successfully propagated for a considerable extent of ﬂow rates and mixture equivalence ratios by transferring heat of products to preheat the incoming cold reactants across solid walls. Several studies on combustion http://dx.doi.org/10.1016/j.enconman.2015.07.056 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +98 9335246112. E-mail address: ghobad.bagheri@gmail.com (G. Bagheri). Energy Conversion and Management 105 (2015) 45–53 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman