Experimental and numerical investigation of jet flow and flames with acoustic modulation Shuhn-Shyurng Hou a , De-Hua Chung b , Ta-Hui Lin b,c,⇑ a Department of Mechanical Engineering, Kun Shan University, Tainan 71003, Taiwan, ROC b Department of Mechanical Engineering, National Cheng Kung University, Tainan 70101, Taiwan, ROC c Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 70101, Taiwan, ROC article info Article history: Received 4 January 2014 Received in revised form 9 December 2014 Accepted 16 December 2014 Available online 7 January 2015 Keywords: Acoustic modulation Jet diffusion flame Natural flickering frequency Acoustic resonant frequency Schiliren contour Flow mixing abstract Ethylene jet diffusion flames under the influence of acoustic modulation from a loudspeaker at the nat- ural flickering frequency (f = 10 Hz) and the acoustic resonant frequency (f = 66 Hz) are experimentally and numerically studied. Experimental measurements (using hot-wire anemometry), numerical simula- tion results, and theoretical analysis results are consistent at the burner exit for cold jet flow without acoustic modulation. For cold jet flow with acoustic modulation, the maximum value of axial velocity measured at the axisymmetric axis, which occurs at a phase angle of / ¼ 90  , is compared to that obtained using numerical simulation. The axial velocities at the burner exit predicted by the numerical simulation for f = 10 and 66 Hz show similar characteristics, where the maximum and minimum values occur at / ¼ 90  and / ¼ 270  , respectively. Additionally, the maximum and minimum values are several times the steady state value (0.4 m/s). For f = 10 Hz, the shape of the Schiliren contours surrounding the flame changes from cap-like, to semicircle-like, to shuttle-like. The shape of the Schiliren contours inside the flame is mushroom-like for 45  6 / 6 225  . For f = 66 Hz, different from the case of f = 10 Hz, the Schiliren contours keep a separation distance surrounding the flame because the resonant coupling effect mainly governs the fuel line. The Schiliren structure inside the flame is found only in the luminous root for 180  6 / 6 225  due to the small difference in the refractive index for the other phase angles. The streamlines, iso-contours of axial velocity, or iso-contours of ethylene concentration can be used to quan- titatively determine the puff flame at both 10 and 66 Hz. For the flow field at f = 10 Hz, there is strong flow interaction between the gas flow and ambient surroundings. At 66 Hz, however, interaction between the sound wave and the fuel line occurs, i.e., acoustic resonance. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Flame puffing was first noticed by Higgins [1]. Since then, com- bustion with flow oscillation has been investigated by many researchers. Strawa and Cantwell [2] observed a methane diffusion flame modulated with a loudspeaker by flow visualization tech- niques. Gore et al. [3] repeated the former experiment and analyzed the results at the resonant frequency. Kim et al. [4,5] investigated the mechanism of mixing between the fuel and air in acoustically modulated jet diffusion flames. They measured the distribution of axial velocities near the burner exit and obtained instantaneous images at resonant and non-resonant frequencies [4]. Images obtained using OH laser-induced fluores- cence and soot scattering of flickering methane, propane, and eth- ylene flames (oscillating frequency = 10 Hz) showed turbulent flow [6]. The numerical simulation of CH 4 /air chemical reactions was reported by Kaplan et al. [7]. Previous investigations of acoustically modulated jet diffusion flames focused mostly on the improvement of combustion effi- ciency by increasing the mixing of fuel and oxygen, which increases combustion intensity and reduces fuel cost [8]. In indus- trial applications, stable lift-off flames can be formed using acous- tic modulation [9]. This reduces the large heat conduction to the burner exit and leads to lower NOx emission [10]. It was also found that the soot production under a forcing condition is four times greater than that of a steady flame burning with the same mean fuel flow velocity [6]. Numerical simulation also suggested a four-fold soot generation increase in this type of flame [7]. Recently, the effect of acoustic modulation on the mixing of fuel http://dx.doi.org/10.1016/j.ijheatmasstransfer.2014.12.047 0017-9310/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Department of Mechanical Engineering, National Cheng Kung University, Tainan 70101, Taiwan, ROC. Tel.: +886 6 2757575x62167; fax: +886 6 2352973. E-mail address: thlin@mail.ncku.edu.tw (T.-H. Lin). International Journal of Heat and Mass Transfer 83 (2015) 562–574 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt