Investigation of flow structures in passive and reactive turbulent mixing

This investigation is aimed at the comparative study of scalar turbulent flow structures in passive and reactive mixing by means of the orthogonal cross beam schlieren system and also by numerical flow computation. A conventional k-e turbulence model is used with the aid of the mixing length hypothesis for starting computation. In the near zone, two approaches are introduced: a coherent scalar flux and an increased diffusivity representation are adopted to accommodate the effects of coherent structures for scalar passive and reactive mixing. A new radial-axial cross beam deflection system arrangement is introduced for detecting the unconditioned cross correlation functions associated with coherent flow structures. The method of the cross beam schlieren technique is investigated in more detail: in particular the system frequency response, the nonlinear response of the detecting system associated with beam thickness and property fluctuations, and the assumptions involved. It was found that the additional scalar flux in the near zone of the nozzle was an order of magnitude greater than turbulent flux determined by the conventional turbulent mixing model, and was required in the flow computations for both passive and reactive mixing flows. In the heated jet, vortex rings of roughly one and a half diameter streamwise spacing with ellipsoidal cross section density cores for which the major axis is pointed inwards and in the flow direction was found to be most consistent with the , results from radial-axial and axial-axial detecting systems, and with the coherent flux magnitude required to predict the mean flow development. A close relationship between the integral length scale observed from the cross beam detecting system arrangement with beams at 45° to the radius and the mixing length for computation of scalar mixing was found. The flow computation of the diffusion flames using a single reaction process based on bulk averaged experimental data indicates that the outer annular air jet influences the reaction by increasing the combustion intensity. It was found that integral scale and other turbulent properties of the turbulent diffusion flames tended to approach those of passive mixing flows as distance from the nozzle increased and the reaction intensity reduced. Initially large scales in diffusion flame did not grow rapidly with distance, and relative fluctuation levels reduced as a consequence of the progressive transition from intense combustion towards thermal mixing. In this respect the turbulent reaction zone undergoes progressive development of the underlying process from one of strong reaction towards one similar to other passive mixing flows without reaction