Dynamics of Non-Premixed Bluff Body-Stabilized Flames in Heated Air Flow

Abstract

This paper describes a study of the fundamental flame dynamic processes that control bluff body-stabilized combustion of liquid fuel with low dilatation. Specifically, flame oscillations due to asymmetric vortex shedding downstream of a bluff body (i.e., the Be´nard/von-Ka´rma´n vortex street) were characterized in an effort to identify the fundamental processes that most affect the intensity of these oscillations. For this purpose, the spatial and temporal distributions of the combustion process heat release were characterized over a range of inlet velocities, temperatures, and overall fuel-air ratios in a single flame holder combustion channel with full optical access to the flame. A stream of hot preheated air was supplied to the bluff body using a preburner, and Jet-A fuel was injected across the heated gas stream from discrete fuel injectors integrated within the bluff body. The relative amplitudes, frequencies, and phase of the sinusoidal flame oscillations were characterized by Fourier analysis of high-speed movies of the flame. The amplitudes of the flame oscillations were generally found to increase with global equivalence ratio, reaching a maximum just before rich blowout. Comparison of the flame dynamics to the time-averaged spatial heat release distribution revealed that the intensity of the vortex shedding decreased as a larger fraction of the combustion process heat release occurred in the shear layers surrounding the recirculation zone of the bluff body. Furthermore, a complete transition of the vortex shedding and consequent flame stabilization from asymmetric to symmetric modes was clearly observed when the inlet temperature was reduced from 850°C to 400°C (and hence, significantly increasing the flame dilatation ratio from Tb/Tu ∼ 2.3 to 3.7).

A Study of Slender Bluff-Body Reacting Wakes Formed by Concurrent or Countercurrent Fuel Injection

“…Concurrent fuel injection from the cylinder trailing face into the primary vortex, resulting in a non-premixed configuration, is also documented under the same conditions, and the respective performances are compared. The above configuration lends itself to the examination of various forms of fuel injection and flame development under the action of a dynamically evolving mixing field (Bakrozis et al, 1999;Kim et al, 2010;Cross et al, 2010;Ohiwa et al, 1994;Shanbhouge et al, 2009), while it is also appropriate for model development and validation (Baudoin et al, 2009;Giacomazzi et al, 2004;Papailiou et al, 2000). Partial fuel premixing and stratification are currently being investigated as a core strategy for meeting the combined constraints of anticipated fuels, pollutant emissions, fuel consumption, and stability through optimum control of mixture placement (Tuttle et al, 2011).…”

“…However, studies of direct (Bakrozis et al, 1999;Kim et al, 2010;Ohiwa et al, 1994) or indirect (Cross et al, 2010) fuel injection into the wake of planar center-bodies have emphasized the suitability of the planar fuel-injected arrangement for highlighting the interactions between the large-scale vortex development, the fuel injection process, the spatial mixture variation, and their effects on stability, heat release, and emissions. This is also an important configuration for evaluating models and developing practical injectors from struts, such as in some stationary gas turbines, afterburners, furnaces, and utility or household burners.…”

“…Due to their practical relevance, bluff-body flows have been exploited as a research tool in relevant experimental (Bakrozis et al, 1999;Cross et al, 2010;Kim et al, 2010;Masri et al, 1998;Tuttle et al, 2011) and computational (Baudoin et al, 2009;Giacomazzi et al, 2004;Jenny et al, 2001;Nanduri et al, 2010;Papailiou et al, 2000) investigations. These efforts have increased insight into the local turbulent flame structure under such complex conditions and have established a database for the development of modeling procedures.…”

On the Influence of Fuel Distribution on the Flame Structure of Bluff-Body Stabilized Flames

“…The developed experimental setup uses a number of circular fuel injection orifices located just upstream of the flameholder's trailing edge that injected liquid fuel into the crossflow. Earlier studies [4,5] showed that the flame structure generated in this problem dramatically differed from that attained in the premixed case that used the same bluff-body to stabilize a combustible mixture. Eor the case of closely coupled fuel injection, high-speed imaging suggested that the reaction takes place in stable, stoichiometric flamelets, distributed over a relatively large reaction zone existing throughout the wake flow behind the bluff-body.…”

“…Figure 4 (taken from Ref. [4]) shows the flame images extracted from high-speed movies of premixed and nonpremixed bluff-body stabilized combustion processes. These images show the fundamental difference in flame structure between these flames.…”