On the stability of a turbulent non-premixed biogas flame: effect of swirl strength and fuel nozzle geometry
Biogas is a renewable gaseous fuel with low calorific value and a low burning velocity. This burning characteristic makes stabilizing biogas flame difficult especially in high flow velocity applications, and hence presenting a real challenge for power generation systems. This thesis presents an experimental investigation of the effect of burner geometry (i.e., fuel nozzle geometry and swirl strength of the co-airflow) on the stability limits of a turbulent non-premixed biogas surrogate flame. Three different co-airflow swirl strengths (S = 0, 0.31, 0.79) were implemented using swirl generators with vane angle of 0º, 25º and 50º, respectively. Six different fuel nozzle geometries were used in order to study the effect of fuel jet centerline velocity on the stability limits of a low swirling (i.e., 25º) non-premixed biogas flame. Moreover, the biogas surrogate fuel composition was kept constant (60% CH4 and 40% CO2 by volume) using a mixture of pure methane and carbon dioxide gases. The results of the effect of co-airflow swirl strength on the stability limits of biogas flame revealed that the swirl plays an important role on both the flame mode and its stability limits for both attached and lifted flames. The experimental results revealed that at low swirl strength the attached flame lifts off and stabilizes at a distance above the burner, while at high swirl strength the flame remains attached but shortens and burns blue. Overall, the high swirl attached flame was found to stabilize over a wider range of flow conditions in comparison to the attached and lifted flame produced by low swirl. Importantly, the central fuel jet characteristics (induced by varying the fuel nozzle geometry) were found to drastically influence the upper and lower blowout limits of the low swirl biogas lifted flame, while multi-hole fuel nozzle geometry was found to significantly enhance the stability ranges. 2D PIV data was used to explain the stability limits and the experimental flame results were used to propose semi-empirical correlations capable of describing the turbulent biogas blowout stability limits.