Experimental Study of Turbulent Counterflow Diffusion Flames.

An experimental investigation is presented on the design, development and analysis of a counterflow system to stabilize highly turbulent nonpremixed flames under conditions rnging from stable to near extinction flames. The experimental effort consisted of three major components. First, a systematic study of turbulence generation scheme was undertaken by hot-wire anemometry. 'this study resulted in a novel turbulent generation scheme based on the use of high-blockage plates with noncircular openings, mounted upstream of a converging nozzle. The proposed design increased the turbulent Reynolds numbers (based on the integral length scale) to values on the order of one thousand. Second, the novel turbulent generation scheme was successfully applied to the opposed jet configuration under non-burning conditions, resulting in a flow field with turbulent Re almost one order of magnitude larger than any existing counterflow configuration. The flow field was analyzed in detail by PIV, using different data reduction techniques. Large scale oscillations of the stagnation surface, resulting from the coupling of intense turbulence and a counterflow intrinsic hydrodynamic instability, mask the truly turbulent fluctuations. A novel filtering technique, the Stagnation Surface Conditional Statistics (SFCS), was developed and successfully applied to analyze the flow field and the results compared to the more standard Proper Orthogonal Decomposition (POD). POD and SFCS were shown to be valuable tools to separate the truly turbulent fluctuations from instability-related oscillations. Third, the turbulence generation scheme was applied under nonpremixed burning conditions and SFCS, as well as POD, were applied to the flow field resulting from PIV measurements. Results were compared to the corresponding cold flow field, showing that heat release effects enhance the radial velocity fluctuations and the radial velocity gradients. The eddy turnover time is smaller than the residence time, therefore the 'young' turbulence limit has been greatly improved, as compared to the existing counterflow configurations. Most importantly, the present burner-turbulence generation scheme allows, for the first time, turbulent counterflow flames that operate in a turbulent Re regime of relevance to practical systems (Re t∼1000) such as gas turbines and internal combustion engines, under conditions of both vigorous burning and local extinction. The compactness of the flow domain, along with the short residence time (∼1 ms) make the present configuration a valuable benchmark for the validation of DNS and other computational models.