Characterisation of Multiple Concentric Vortices in Hybrid Rocket Combustion Chambers

Recent developments in hybrid rocket technology involve producing a coaxial bidirectional vortex flow field through use of tangential oxidiser injection at the base of the combustion chamber. This is found to significantly increase engine performance by providing enhanced thermal transfer at the fuel surface, resulting in increased fuel regression rates in addition to more efficient combustion. The double helical path of the flow results in reduced reactant loss at the chamber outlet whilst confining combustion to a high temperature core region defined by the inner vortex. This also results in enhanced thermal shielding as a large radial thermal gradient is established, which affords lightweight materials to used in the construction of the combustion chamber making it suitable for weight sensitive applications such as satellite propulsion. Analytical treatment of the coaxial bidirectional vortex has repeatedly found that it is theoretically possible to induce a vortex flow field consisting of multiple concentric vortices, which would further increase the benefits associated with the coaxial bidirectional vortex. Each additional vortex would further increase the length of the helical trajectory of the flow and allow for more compact combustion chambers to be designed which are both lightweight and highly efficient. However, the existence of multiple concentric vortices has yet to be confirmed and the parameters involved in producing such phenomenon are unknown, as are the parameters required to manipulate their behaviour. Therefore, the focus of this study is to investigate whether it is possible to induce multiple concentric vortices and affect their behaviour through parametric variation of geometrical constraints which are found to significantly influence vortex characteristics in similar cyclonic devices. A twofold approach is employed to investigate a range of cyclone chamber configurations both experimentally and numerically, in order to ascertain the effectiveness of each method with regards to resolving complex vortical flows and to observe and similarities in the results obtained. Due to experimental constraints a hydrocyclone was used in place of a gas cyclone which is commonly used to characterise the behaviour of the coaxial bidirectional vortex. The numerical analysis was performed using CFD, where 3D simulations were necessary to adequately resolve the spatio-temporal behaviour of the flow, with specific consideration given to solver settings applicable to the resolution of intense swirling flow. While the experimental analysis was conducted using a 2D time resolved PIV method, this was applied to the meridional and azimuthal planes of the cyclone chamber to enable comparison of the number of concentric vortices and the general characteristics of the vortex. Despite the limitations of the techniques applied it was found that it is possible to produce a confined vortex consisting of multiple concentric vortices, although the resultant flow structures are considerably more complex than initially thought. However, it was also found that the geometry of the cyclone chamber has a significant influence upon the structure of the vortex, with small chamber aspect ratios and large contraction ratios producing intense vortices which are associated with multiple concentric vortices. The position of the tangential inlets is also found to have a significant impact upon the structure of the flow, and several configurations of multiple concentric vortices were observed that are not accounted for by the analytical solutions. Another important result that was consistently observed is that the locus of zero axial velocity which defines the number of concentric vortices takes the form of modal structures, which appear to evolve in response to threshold geometrical parameters. It is thought that the underlying fluid mechanics of multiple concentric vortices are inherently linked to the harmonic characteristics of the cyclone chamber geometry, as several mechanisms have been identified which may allow for direct control over the structure of the vortex.