Effect of air preheating temperature on the dynamic behaviour of a swirled spray flame

Abstract

The effects of the air inlet temperature on the dynamic behaviour of a spray flame are reported for different air flow rates. Dodecane droplets are generated through a pressurised nozzle and introduced in the combustion chamber fed with air via two co-rotating swirlers. The flow rate of dodecane is kept constant (1.24gs-1) while the air flow rate is changed from 28gs-1 to 35gs-1 and its temperature from 70°C to 220°C. Microphones are used to record the different dynamics observed while a camera and a Phase Doppler Anemometry system allow to monitor the flame and the fuel spray.For low flow rates (between 28gs-1 and 32gs-1, leading to an overall equivalence ratio ranging between 0.6 and 0.7), the pressure fluctuations start by increasing with an increase of the temperature until, for a given temperature, the pressure fluctuations suddenly drop. This threshold temperature depends on the air flow rate. When displaying the pressure fluctuation level as a function of the bulk velocity (taking both air flow rate and temperature into account), the curves collapse into a similar shape. The instability frequency monotonically increases with the bulk velocity. For high flow rates (between 32gs-1 and 35gs-1, leading to overall equivalence ratio ranging between 0.5 and 0.6), a hysteresis phenomenon is found. Pressure fluctuations are relatively high for constantly decreasing temperature conditions and have almost similar levels as those obtained for lower flow rates. When constantly increasing the temperature, the levels are much lower, showing a bi-stable behaviour for the pressure fluctuations at a given operating point. A simple analysis based on the acoustic energy balance equation allows to qualitatively retrieve the behaviour of the pressure fluctuations amplitude and frequency for the low flow rate cases.