# Experimental Comparison of Diesel and Crude Rapeseed Oil Combustion in a Swirl Burner
## 1 Introduction
The evaporation of the above-mentioned water-oil emulsion, exploiting micro-explosions, was studied by [PERSON] et al. [32], demonstrating the quicker evaporation of heavy fuel oil (HFO) sample compared to neat fuel utilization. The evaporation of heavy fuels is still less known, since the outer surface of the droplet might solidify even for kerosene gel [33]. Significant crust formation was similarly observed for HFO [34], featuring pores on the surface [35] where the oil inside left the droplet when evaporated. To overcome this issue, atomization with steam [30] might help, substituting air with an H\({}_{2}\)O atmosphere in the vicinity of the fuel nozzle. The drawback of this technology is that treated water is required, which is abundant in thermal power plants using steam turbines, but uneconomical for the majority of applications. Since the present study focuses on the combustion of crude rapeseed oil, its evaporation characteristics will not be covered in the latter part of this paper; it was published in an earlier study [36]. The purpose of this paragraph was to introduce the difficulties in heavy oil combustion, which usually lead to excessive emissions due to the inhomogeneous flame temperature field. HFO is much more viscous and less volatile than any HRF, nevertheless, the unsaturated bonds and the natural oxygen content both facilitate polymerization. Hence, a similar systematic study on HRF would lead to a better understanding of the proper conditions for their combustion. Consequently, the substitution of light and heavy fuels, even with proper preheating for favorable atomization characteristics, is far from trivial. Modern combustion chambers, in which flame radiation is less favorable, highly lean mixtures in swirl burners are combusted [37] since the NOx concentration nearly doubles with every 50 K increase in flame temperature [38]. Such an operation is limited by thermoacoustic instabilities [39] besides the theoretical limit in air-to-fuel equivalence ratio, \(\lambda\)-2 for most hydrocarbon fuels [40]. Hence, the operating point should approach the lean blowout limitation as much as possible by design. It was found in an earlier work that stable combustion of crude rapeseed oil is confined to the weak swirl region in atmospheric combustion [41]. However, the acoustic characteristics of liquid fuel combustion are rare in the public literature, which was the primary motivation of this study. The acoustic spectrum of V-shaped turbulent flames is confined below 1.5-2 kHz [42; 43]; the dominant frequencies are located at a few hundred Hz [44; 45]. However, with weak swirl, when the flame is straight, the combustion-related frequencies can also be found at higher frequencies [46]. In engineering practice, temporal signals are typically evaluated by using Fast Fourier Transform (FFT) [44]. Nevertheless, the spectrum contains significant noise besides fluctuating harmonic components. To overcome this issue, Wavelet transform (WT) can be used, which enables the use of various base functions [47]. The superiority of WT over FFT in combustion diagnostics was first published by [PERSON] and [PERSON] [48]. [PERSON] and [PERSON] [49] followed their motivation for lean, non-swirl flames, analyzing the frequency domain up to 200 Hz. [PERSON] et al. [50] used the WT of the CH\({}^{*}\) signal, sampled at 5 kHz, for the lean blowout prediction of a swirl-stabilized LPG flame. [PERSON] et al. [51] also investigated a swirling propane flame, however, their frequency range was limited to 500 Hz. Note that the majority of the above-cited papers all focus on gaseous fuel combustion, hence data on liquid fuel combustion are already a novelty. The fundamental and practical details of the comparison of FFT and WT will be discussed in Section 2.3. Substituting a conventional light fuel with HRF raises numerous practical questions.