Experimental and Computational Studies on Deflagration-to-Detonation Transition and its Effect on the Performance of PDE
Bhat, Abhishek R
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This thesis is concerned with experimental and computational studies on pulse detonation engine (PDE) that has been envisioned as a new concept engine. These engines use the high pressure generated by detonation wave for propulsion. The cycle efficiency of PDE is either higher in comparison to conventional jet engines or at least has similar high performance with much greater simplicity in terms of components. The first part of the work consists of an experimental study of the performance of PDE under choked flame and partial fill conditions. Detonations used in classical PDEs create conditions of Mach numbers of 4-6 and choked flames create conditions in which flame achieves Mach numbers near-half of detonation wave. While classical concepts on PDE's utilize deflagration-to-detonation transition and are more intensively studied, the working of PDE under choked regime has received inadequate attention in the literature and much remains to be explored. Most of the earlier studies claim transition to detonation as success in the working of the PDE and non-transition as failure. After exploring both these regimes, the current work brings out that impulse obtained from the wave traveling near the choked flame velocity conditions is comparable to detonation regime. This is consistent with the understanding from the literature that CJ detonation may not be the optimum condition for maximum specific impulse. The present study examines the details of working of PDE close to the choked regime for different experimental conditions, in comparison with other aspects of PDEs. The study also examines transmission of fast flames from small diameter pipe into larger ducts. This approach in the smaller pipe for flame acceleration also leading to decrease in the time and length of transition process. The second part of the study aims at elucidating the features of deflagration-to-detonation transition with direct numerical simulation (DNS) accounting for and the choice of full chemistry and DNS is based on two features: (a) the induction time estimation at the conditions of varying high pressure and temperature behind the shock can only be obtained through the use of full chemistry, and (b) the complex effects of fine scale of turbulence that have sometimes been argued to influence the acceleration phase in the DDT cannot be captured otherwise. Turbulence in the early stages causes flame wrinkling and helps flame acceleration process. The study of flame propagation showed that the wrinkling of flame has major effect on the final transition phase as flame accelerates through the channel. Further, flame becomes corrugated prior to transition. This feature was investigated using non-uniform initial conditions. Under these conditions the pressure waves emanating from corrugated flame interact with the shock moving ahead and transition occurs in between the flame and the forward propagating shock wave. The primary contributions of this thesis are: (a) Elucidating the phenomenology of choked flames, demonstrating that under partial fill conditions, the specific impulse can be superior to detonations and hence, allowing for the possibility of choked flames as a more appropriate choice for propulsive purposes instead of full detonations, (b) The use of smaller tube to enhance the flame acceleration and transition to detonation. The comparison with earlier experiments clearly shows the enhancements achieved using this method, and (c) The importance of the interaction between pressure waves emanating from the flame front with the shock wave which leads to formation of hot spots finally transitioning to detonation wave.