# Experimental Comparison of Diesel and Crude Rapeseed Oil Combustion in a Swirl Burner
## 2 Materials and Methods
### Atomization Characteristics  
The purpose of atomization is to generate fine droplets that evaporate inside the mixing tube to supply the flame front with homogeneous fuel vapor-air mixture. Atomization is sensitive to both fluid dynamical and fuel properties. Hence, the relevant physical parameters are summarized in Table 3 at the respective fuel inlet temperatures. Rapeseed oil was preheated to 150 \({}^{\circ}\)C, principally, to reduce its viscosity to a similar value to that of diesel oil. Since the thermal power was identical and R is an oxygenated fuel, the required mass flow rate was slightly higher.  
Estimation of the volume-to-surface diameter, \(SMD\), requires the following non-dimensional numbers for airblast atomization [56]. The Ohnesorge number, Oh, is calculated from material properties and nozzle geometry as  
\[\text{Oh}=\mu_{F}/(\sigma\cdot d_{0}\cdot\rho_{F})^{0.5} \tag{2}\]  
where \(\mu_{F}\) is the dynamic viscosity of the fuel, \(\sigma\) is the surface tension, \(d_{0}\) is the inner diameter of the fuel pipe, and \(\rho_{F}\) is the density of the fuel. Oh was 0.0358 and 0.0367 for D and R, respectively. The fluid dynamical conditions are represented by the Weber number  
\[\text{We}=w_{R}^{2}\cdot d_{0}\cdot\rho_{A}/\sigma \tag{3}\]  
where \(w_{R}\) is the relative velocity between the air and fuel, and subscript \(A\) represents air. Both \(w_{R}\) and \(\rho_{A}\) were calculated by assuming an adiabatic expansion [36]. The last required non-dimensional number for \(SMD\) estimation is the air-to-fuel mass flow ratio, \(AFR\). Now, \(SMD\) can be estimated as  
\[\frac{SMD}{d_{0}}=\left(A\cdot\text{We}^{-0.5}+B\cdot\frac{(\mu_{F}/\rho_{F})_ {25}\,^{\circ}\text{C}}{(\mu_{F}/\rho_{F})_{150}\,^{\circ}\text{C}}\cdot \text{Oh}\right)\cdot\left(1+\frac{1}{AFR}\right) \tag{4}\]  
where \(A=0.477\) and \(B=0.35\) for D and 0.2 for R are empirical constants determined for this atomizer. Nevertheless, it is envisioned that these parameters hold for other airblast atomizers as well [31].  
Tables 4 and 5 show the estimated \(SMD\) values and We and \(AFR\) for both fuels. Since the preheating of R decreased both the viscosity and surface tension below these parameters of D, and the densities are similar, \(SMD\) of R is slightly smaller than that of D at all \(p_{g}\) values. As a consequence, their evaporation characteristics are also similar [36] if crust formation can be omitted, providing similar conditions for combustion. The slightly higher We of R compared to D is due to the lower \(\sigma\)  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Case No.** & \(p_{g}\) [Bar] & \(S\) [–] & \(A\) [–] & **Fuel** & **Geometry** \\ \hline Case 18 & 1.6 & 0.29 & 1.30 & D & BL \\ Case 19 & 0.45 & 0.29 & 0.73 & D & Q15 \\ Case 20 & 0.8 & 0.24 & 0.86 & D & Q15 \\ Case 21 & 1.1 & 0.24 & 1.01 & D & Q15 \\ Case 22 & 1.6 & 0.29 & 1.30 & D & Q15 \\ \hline \hline \end{tabular}
\end{table}
Table 2: _Cont._However, \(AFR\) decreases as identical thermal power requires a higher fuel flow rate of R due to the difference in their lower heating value.