## 1 Introduction
There is limited literature on the effects of fuel mass flow rate oscillations on blowoff limits in swirling flames, and most studies focus on premixed flames only [25; 26; 29]. However, jet fuel is typically employed and sprayed into the combustion chamber in practical aero-engines. In most cases, the liquid oil cannot be fully diffused or perfectly premixed with air before entering the combustion zone due to the short injection length. Additionally, non-premixed flames never flashback except when partial premixing occurs [31; 32; 33]. Thus, a non-premixed flame was applied in our work since it is similar to the conditions in practical aero-engines. Various numerical and experimental investigations have shown that non-premixed flames show localized extinction progressively when approaching global blowoff [19; 34; 35; 36; 37; 38; 39; 40]. Our work aimed to study the effects of external acoustic force on combustion instability and blowoff characteristics in non-premixed swirling flames, elucidating the mechanism behind the blowoff events and offering new strategies to extend the blowoff limits. Many investigators have focused on detecting the blowoff events and are mainly interested in the mechanisms influencing the blowoff. Because the chemiluminescence (CL) intensity is directly related to chemical reaction rates and is proportional to the overall spontaneous emission of OH\({}^{\star}\) radicals [41; 42; 43], it has previously been used as a rough measure of heat-release rate and equivalence ratio [44; 45; 46; 47]. It has been demonstrated that OH\({}^{\star}\)-CL is a suitable indicator for the average shape and location of the reaction zone [46; 48]. In addition, it inherently has a short response time that provides fast detection of flame instability. Based on previous studies by [PERSON] et al. [47], OH\({}^{\star}\)-CL can be effectively employed as a precursor of blowoff and assess the proximity to blowoff, which is valid in both premixed and non-premixed combustors, whether with or without swirling. In this work, high-speed OH\({}^{\star}\)-CL imaging was used to determine the flame shape and global heat release qualitatively rather than as a quantitative measure of the reaction zone because the present flames are not fully premixed. This work focused on the precursor events to offer information on the proximity of the blowoff limit under different operating conditions. In this work, the response of non-premixed swirling methane flames to fuel mass pulsation induced by external forcing at conditions close to blowoff limits was inspected experimentally. The 2D heat release of the reacting flow field was visualized by 20 kHz OH\({}^{\star}\)-CL and the planar laser-induced fluorescence (PLIF) of the formaldehyde (CH\({}_{2}\)O) and the sulfur dioxide (SO\({}_{2}\)). CH\({}_{2}\)O-PLIF was used to identify the flame's early stage of fuel decomposition [49]. SO\({}_{2}\)-PLIF has been demonstrated as a reliable flame marker because the SO\({}_{2}\) fluorescence signal strongly depends on temperature [50; 51]. It was found that SO\({}_{2}\) appears closer to the sharp temperature gradient than OH radicals. In addition, SO\({}_{2}\)-PLIF can be stimulated using the fourth-harmonic generation of an Nd:YAG laser at 266 nm. It simplifies the experimental setup, especially when working at high repetition rates where dye bleaching is a concern. The combined CH\({}_{2}\)O- and SO\({}_{2}\)-PLIF was adopted to distinguish the low- and high-temperature regions of the reacting conditions approaching blowoff. Adopting the method of [PERSON] et al., when approaching the blowoff, the sudden drops of the OH*-CL intensity are interpreted as temporary extinction of flame and can be used as a promising indicator to provide the earliest detection of blowoff events [52; 53; 54; 55; 56]. The burner and experimental details are presented in the next section, followed by the results and discussion, and then closing with the conclusions.