## 1 Introduction  
In recent years, the planar laser-induced fluorescence (PLIF) technique, a modern optical diagnostic method with high spatial and temporal resolution, has developed rapidly in the field of combustion diagnosis [1; 2; 3]. Many experimental results can be found in the literature regarding both qualitative characterizations of flame structure and quantitative measurements of interesting features, such as concentration, temperature, and velocity [4; 5; 6; 7; 8]. However, most successful applications are confined to the ideal environment of the laboratory, regardless of the impact of the external environment, i.e., variables such as temperature, humidity, and vibration on the PLIF system. Besides, the subjects are mostly laboratory flames [9; 10; 11], for example Bunsen flames. Small flame scales and an ideal environment provide convenience for the study of the mechanism, but if a harsh experiment environment and larger flame scale exist, this poses a great challenge to the PLIF system. A high-speed OH-PLIF system is capable of imaging flame structures and demonstrating dynamic evolution [12; 13], but when faced with bench tests, terrible problems come with it. Due to low laser energy output and the high requirements of the working environment, the commercial high-speedPLIF system cannot be applied to the complex environment of the bench test. For instance, in the experimental factory, the temperature fluctuates widely, and the humidity may reach 90%, greatly weakening the detection capability of the high-speed OH-PLIF system, thus limiting the engineering application of the PLIF technique. Actual industrial combustors, such as boilers and gas turbines, operate in extremely complex conditions. To our knowledge, in view of the performance evaluation of industrial-type burners, at present, limited exhaust pollutants and temperature have been investigated [14; 15]. However, flame growth during the combustion process is also important for assessing the quality of the burner. In order to improve the environmental adaptability of PLIF systems, and extend this technique to engineering applications, the main task is to solve the problem of temperature and humidity. Modular design is of great significance. The laser system needs special treatment, i.e., in which temperature control and moisture proofing have been employed. The specific measures will be discussed in the experiment details section. In the context of energy saving and emissions reduction, heavy oil, which has the advantages of abundant reserves and low costs, has been the focus of much attention [16]. Oxy-fuel combustion of heavy oil was applied to this experiment in order to reduce carbon dioxide emissions while offering a more efficient use of heavy oil resources [17]. However, at present, oxy-fuel technology faces huge safety issues due to both its enlarged flammability range and enhanced explosion severity with respect to air, which limits the practical application of this technology [18]. The heavy oil boiler, an industrial-type burner, operates with an oil spray involving a two-phase combustion process. The physical processes such as spray atomisation and transport, turbulence, and chemistry interact with each other, which makes it hard to predict combustion state by numerical simulations [19; 20]. The combustion characteristics of heavy oil under different conditions, including temperature and pollutant emission characteristics, have been investigated [17]. However, experimental studies on the flame structure of heavy oil combustion are still rare in the literature. The process of flame evolution is significant in the study of combustion characteristics and the evaluation of the performance of the burner. For this reason, it is imperative to describe the two-dimensional flame structure by OH-PLIF images. The aim of this paper is to report the successful application of PLIF technique to an industrial burner, which is beneficial to promoting development of the PLIF technique, and solving some engineering problems.