# Experimental Comparison of Diesel and Crude Rapeseed Oil Combustion in a Swirl Burner
## 4 Conclusions  
The present paper discussed the experimental analysis of crude rapeseed oil flame, highlighting the differences and similarities with standard diesel oil combustion, facilitating the understanding of conditions where the two fuels are equivalent. The tests were performed in a 15 kW burner operated in the open atmosphere. To extend the blowout stability, a 15\({}^{\circ}\) quarl was installed on the burner lip in the case of diesel oil combustion. After discussing the atomization characteristics and the evaporation of heavy renewable fuels (HRF), an acoustic analysis was performed, using Wavelet transform. Then, the pollutant emission of the investigated cases is discussed. Based on the results, the following conclusions were derived:  
1. Swirl combustion is characterized by narrow-band frequencies at a few hundred Hz; however, high-frequency components can be located in a wider band. Therefore, Wavelet transform is an excellent tool to evaluate the temporal signal, as it is a multi-scale technique, properly fitting to the acoustic signal of a flame;
2. Frequencies of \(f_{1}=230\) Hz, \(f_{2}=500\) Hz, \(f_{3}=1600\) Hz, \(f_{4}=3200\) Hz were found to dominate the investigated spectra. The joint histogram of the wavelet coefficients and single histogram of the pressure signal was evaluated as the spectrum showed stochastic magnitude variation. Stable V-shaped flames, which were observed only in the case of diesel oil combustion, were governed by \(f_{1}\) and \(f_{2}\), while the characteristic frequencies of straight flames were \(f_{3}\) and \(f_{4}\). Flame blowout was accompanied by decreasing amplitudes of both pressure fluctuations and wavelet coefficients;
3. Combustion of rapeseed oil and diesel oil were found to be similar from an acoustic point of view. The joint histogram of the governing frequencies showed identical shape. However, flame acoustics differed notably when a 15\({}^{\circ}\) quarl was put on the burner lip, indicating that the analysis is highly sensitive to the burner geometry;
4. Pollutant emission of rapeseed oil combustion slightly exceeded that of diesel oil combustion. The quarl affected the pollutant emissions favorably in the case of diesel oil combustion since the rapeseed oil flame was blown out with a quarl. Rapeseed oil combustion might result in high NO\({}_{\text{X}}\) emission when the droplet size is small due to the possibility of crust formation, hence hindering the evaporation process.  
The conclusions of the present research can be directly used in steady-operating applications such as boilers, furnaces, and gas turbines, especially in rural areas where crude vegetable or other waste oils might be an economically favorable energy source. There are several opportunities for future research since the combustion of HRF types might require special attention. Lowering the viscosity through preheating is a traditional method; however, crust formation might only be relevant above a certain droplet size. Determining this limitation would facilitate the spreading of the utilization of various HRFs. Wavelet transform was proven to be a powerful tool to filter the acoustic signal to analyze the flame behavior, which might be useful for predicting flame blowout for HRF combustion. Pollutant emission of simple burners can be optimized by, e.g., using a properly designed quarl. Such a feature might offer a reduced NO\({}_{\text{X}}\) and CO emission. All the measurement data and evaluation Matlab code will be available upon request.  
This paper was supported by the National Research, Development and Innovation Fund of Hungary, project No. OTKA-FK 124704, BME NC TKP2020 Thematic Excellence Program, and the Janos Bolyai Research Scholarship of the Hungarian Academy of Sciences.  
The author declares no conflict of interest.