Chemical Engineering and Processing 79 (2014) 7–13 Contents lists available at ScienceDirect Chemical Engineering and Processing: Process Intensification j ourna l h o mepage: www.elsevier.com/locate/cep Numerical investigation of CH 4 /O 2 mixing in Y-shaped mesoscale combustors with/without porous media Yi Liu, Jingyi Zhang, Aiwu Fan ∗ , Jianlong Wan, Hong Yao, Wei Liu State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China a r t i c l e i n f o Article history: Received 22 November 2013 Received in revised form 20 January 2014 Accepted 6 March 2014 Available online 15 March 2014 Keywords: Mesoscale combustor Y-shaped channel Porous media Mixing enhancement Mass dispersion effect a b s t r a c t A good mixing of reactants is important for non-premixed combustion in miniature combustors. In this paper, mixings of methane and oxygen in Y-shaped mesoscale combustors with and without porous media were compared numerically. The results show that when there is no porous media in the horizontal channel, the mixing becomes worse with the decrease of the included angle between two inlets, or with the increase of inlet velocity. The reason is that for the case without porous media, the dominant mixing mechanism is molecular diffusion under concentration gradients. In contrast, for the case with porous media, due to the mass dispersion effect which becomes more significant with the decrease of channel width, satisfactory mixing can always be attained in the Y-shaped mesoscale combustor. Moreover, fairly good mixing can still be achieved in the horizontal channel of shorter length. All these demonstrate that the porous media greatly promotes the methane/oxygen mixing in the Y-shaped mesoscale combustor, which is beneficial for flame stabilization. Meanwhile, the combustor dimension can be further scaled down because good mixing is possible in the channel with even smaller included angle and shorter length. This is very important for the application of miniature power generation system. © 2014 Elsevier B.V. All rights reserved. 1. Introduction With the rapid development of micro-electromechanical sys- tems (MEMS), the demands for appropriate portable power generators are growing. The high energy densities of hydrocarbon fuels make it possible for combustion-based micro power genera- tion systems to drive the MEMS in the future [1–3]. Therefore, micro combustion has drawn extensive attention in the past decades. However, there are some critical challenges to sustain a sta- ble combustion in miniature combustors. Those adversities include the increased heat losses due to large surface area-to-volume ratio of the combustor chamber and the reduced residence time of the fuel/oxidant mixture. To achieve a complete reaction in micro- and meso-scale combustors, premixed combustion is superior to non- premixed combustion. Thus, premixed combustion under small scale has been widely investigated experimentally and numeri- cally [4–22]. However, although premixed combustion is beneficial for a complete conversion in miniature combustors, it still has some shortcomings. First, premixed combustion needs a separate ∗ Corresponding author at: State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, China. Tel.: +86 27 87542618; fax: +86 27 87540724. E-mail addresses: faw 73@163.com, faw@hust.edu.cn (A. Fan). fuel/oxidizer mixer which makes the whole system more compli- cated. In addition, premixed flame is prone to flashback which gives rise to a potential safety problem for users of portable micro power generators. In contrast, non-premixed combustion does not have such demerits. Therefore, non-premixed combustion also gained extensive attentions from researchers. As the Reynolds number in micro- and meso-scale combustors is relatively low, the molecular diffusion is the dominant mecha- nism for fuel/oxidizer mixing, which is always unable to achieve a good mixing for a continuous flame and a complete reaction. For instance, Miesse et al. [23–25] experimentally investigated the diffusion flames in a Y-shaped micro-burner and they observed multiple isolated flame cells. Since the fuel conversion degree when the flame cells are present is poor, it represents an undesirable mode of combustion in terms of efficient micro-burner operation. To reveal the mechanisms responsible for these discrete flamelets, Xu and Ju [26] studied the non-premixed flames in a heated rectan- gular channel with a gap distance of 6 mm and a width of 100 mm. Fuel and air were fed into the channel via two horizontally parallel inlet ports. By controlling the flow velocity and wall temperature, a ‘flame street’ that consists of multiple flamelets inside the mixing layer was observed, which is similar to that reported by Miesse et al. [23–25]. They pointed out that onset of the ‘flame street’ was due to the process of diffusion and re-ignition. Therefore, for high combustion efficiency, it is critical to promote the mixing http://dx.doi.org/10.1016/j.cep.2014.03.002 0255-2701/© 2014 Elsevier B.V. All rights reserved.