# Experimental Comparison of Diesel and Crude Rapeseed Oil Combustion in a Swirl Burner
## 2 Materials and Methods
### Experimental Setup  
Figure 1 shows the atmospheric combustion test rig. The combustion air flow rate was controlled by a continuously adjustable frequency converter, determining primary \(\lambda\), which is presented through the paper. The air flow temperature was increased to 400 \({}^{\circ}\)C by heater filaments before entering the burner. Standard diesel oil (D, EN590:2014) and crude rapeseed oil (R) were both pumped from their respective tanks to the fuel manifold. Since R has excessive viscosity at 20 \({}^{\circ}\)C, a liquid preheater increased its temperature to 150 \({}^{\circ}\)C to facilitate atomization and hence vaporization. The atomizing air was taken at 5 bar and 20 \({}^{\circ}\)C from the high-pressure air network of the laboratory. The combustion power was set to 15 kW in all cases. To enhance flame stability, quarks can be effectively used for swirl combustion [52]. In the present case, a 15\({}^{\circ}\) quarl with a 16 mm slant height was installed on the burner lip to extend the blowout limitation. However, it was used for comparison only in the case of diesel oil combustion, since crude rapeseed oil combustion was principally limited by low fuel evaporation through the swirl number, \(S\)[41]. The calculation of \(S\) is detailed in reference [46] for the present setup, while its definition is shown by Equation (1)  
\[S=G_{\varphi}/(G_{\chi}\cdot R). \tag{1}\]  
where \(G_{\varphi}\) is the axial flux of the angular momentum, \(G_{\chi}\) is the axial thrust, and \(R\) is the inner radius of the burner. In the following, the burner without the quarl is further referred to as baseline (BL). Hence, diesel and rapeseed oil combustion in this configuration is noted as BL-D and BL-R. The setup with the 15\({}^{\circ}\) quarl is called Q15-D since the quarl was only effective in the case of diesel oil combustion. The reason to evaluate this case is the demonstration of the sensitivity of the combustion noise to the burner geometry. An SVAN 971 noise analyzer (Svantek, Warsaw, Poland) was placed at the height of the burner lip, 1 m away from the burner centerline. Its sensitivity was 28.74 mV/Pa, and the sampling rate was 12 kHz at 16 bit. The combined expanded uncertainty of both \(S\) and \(\lambda\) was below 5% at 95% level of significance in all the cases. A Testo 350 analyzer (Testo AG, Titisee-Neustadt, Germany) was used to measure the O\({}_{2}\), NOx, and CO concentrations in the flue gas. The uncertainty of O\({}_{2}\) measurement was 0.1 \(\
u\)/\(\
u\)%, while those of NOx and CO were below 10 ppm, also considering that all the pollutants were corrected to 15% O\({}_{2}\) level. The lean premixed prevaporized burner, which was used in a Capstone C-30 micro gas turbine (Capstone Turbine Corporation, Los Angeles, CA, USA) earlier, is shown in Figure 2. Its mixing tube is 75.5 mm long from the atomizer nozzle tip to the burner lip, and its inner diameter is 26.8 mm. The diameters of the atomizing air annulus are 0.8 mm inside and 1.6 mm outside. The inner diameter of the central fuel pipe is 0.4 mm. The lowest value of the atomizing gauge pressure, \(p_{g}\), of the plain-jet airblast atomizer was 0.3 bar, which was enough to avoid droplet combustion, and hence excessive pollutant emission. The elevated atomizing free jet is also reducing \(S\) through increasing \(G_{x}\) in Equation (1) besides generating a finer spray. The spray characteristics are discussed in detail in Section 2.2. Figure 1: The combustion test rig. Figure 2: The investigated burner and the used quarl. The burner features fifteen 45\({}^{\circ}\), fixed swirl vanes; their cross-section is shown on the left side of Figure 2. Due to the fixed geometry, the swirl number increased with the combustion air flow rate. To avoid the accumulation of fuel droplets near the nozzle, hence the coking [53], four circular slots are located below the swirl vanes; they reduce \(S\) since they do not swirl the air.