Investigation of Plug Nozzle Flow Field
Plug nozzle, a passive altitude adaptive nozzle, for futuristic SSTO applications, exhibits greater efﬁciency as compared to conventional nozzles over a wide range of altitudes. The plug nozzle comprises of a primary nozzle and a contoured plug; an under–expanded jet exiting the primary nozzle is allowed to further expand over the plug surface for altitude adaptation. At design condition the ﬂow expands correctly to the ambient conditions on the full length plug surface, while at off design conditions the ﬂow adapts to the ambient conditions through wave interactions within the nozzle core jet. Based on thrust to weight considerations, the full length plug is truncated and this results in a base ﬂow rich in ﬂow physics. In addition, the base ﬂow exhibits an interesting transitional behaviour from open wake to a closed wake because of the wave interactions within the nozzle core jet. The plug surface ﬂow can further exhibit ﬂow complexities because of wave interactions resulting from the shear layer emanating from the splitter plates, in case of clustered plug ﬂows. Considering these ﬂow complexities, the design of the plug nozzles and analysing the associated ﬂows can be a challenge to the aerodynamic community. An attempt has been made in understanding this class of ﬂows in this thesis. This objective has been accomplished using both experimental and computational tools. In the present work, both the linear and annular plug nozzle geometries have been analysed for a wide range of pressure ratios spanning from 5to 80. The linear and annular nozzles have been designed for similar ﬂow conditions and their respective design pressure ratios are 60and 66. From the experimental and computational results, it has been shown that the computational solver performs well in predicting the wave interactions on the plug surface. In addition the limitations of the computational solver in predicting the plug base ﬂows in general has been brought out. This limitation in itself need not be considered as a serious handicap in the design and analysis of plug nozzle ﬂows; this is because the plug base contribution to the thrust is very minimal, as has been brought out in this thesis. Apart from this the high quality experimental data generated is also of immense value to the CFD community as this also serves as a valuable data base for CFD code validation. For analysis, the plug ﬂow ﬁeld has been categorized into three different regimes based on the primary nozzle lip expansion fan extent. The ﬂow ﬁeld is categorised based on the reﬂection of the primary nozzle lip expansion fan from plug surface, base region shear layer and symmetry line downstream of the base region recirculation bubble. This ﬂow division is particularly helpful in understanding the base wake characteristics with increasing pressure ratio. The base lip pressure and the base pressure variation have been discussed with respect to the primary nozzle lip expansion fan extent. In the open wake regime (or for low pressure ratios) the wave interactions within the core jet ﬂow impinge on the base region shear layer. Because of these interactions it is difﬁcult to propose an empirical model for open wake base pressure. In the closed wake regime (for higher pressure ratios), the base region recirculation bubble is completely under the shower of primary nozzle lip expansion fan. Hence the base lip pressure and base pressure are frozen with respect to stagnation conditions. Based on these insights it was possible to propose empirical models for linear and annular closed wake base pressure. Along with these, a mathematical model deﬁning a reference pressure ratio PR∗, beyond which the closed wake base pressure is expected to be more than the ambient pressure has also been proposed. This is expected to serve as a good design parameter. In case of linear plug ﬂows, this also serves the purpose of base wake transition, for the cases considered in this thesis. The ﬂow expansion process or the primary nozzle lip expansion fan extent was also useful in understanding the differences between the linear and annular plug nozzle ﬂow ﬁelds. In a linear plug nozzle, the ﬂow expands only in the streamwise direction while in an annular plug nozzle the ﬂow expands both along the streamwise and azimuthal directions. The ﬂow expands at a faster rate in case of annular nozzle as against linear nozzle. Hence differences are observed between the linear and annular nozzle on plug and base surfaces. On the annular plug surface more wave interactions are observed because of faster expansion. With regard to base characteristics, faster expansion in annular plug nozzle, with respect to linear nozzle, results in a lower base lip pressure, lower base pressure and higher wake transition pressure ratio. The realistic cluster plug conﬁgurations have also been considered for the present studies. The effects of clustering on the plug nozzle ﬂow ﬁeld have been brought out by considering two different linear cluster nozzles and one annular cluster nozzle. The differences in the ﬂow ﬁeld of a simple and cluster plug nozzle has been discussed. In case of simple plug nozzle wave interactions are observed only in the stream wise direction, while in case of cluster plug nozzle three dimensional wave interactions are observed because of the splitter plates. Along the splitter plate differential end conditions introduce a curved recompression shock on the plug surface. This recompression shock in turn induces a streamwise vortex and also a secondary shock. It has been observed that differences between the simple and cluster plug surface pressure ﬁeld are because of three dimensional wave interactions. Regarding the base pressure, differences between the simple and cluster geometries were observed for shorter truncation plug lengths (20% length plug). While for longer plug lengths (more than 34% length) the effects of clustering were reduced on the base pressure. Regarding the transition pressure ratio, differences were observed between simple and clustered plug nozzles for all the plug lengths considered. In addition, the performance of the plug nozzles has been carried out. From the analysis it was found that the primary nozzle and plug surface are major contributors towards thrust. The base surface contributes only about 2– 3% of the thrust at design condition. Hence, from a design point of view, a computational solver can be a useful tool considering its efﬁcacy on the plug surface and in the primary nozzle.
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