On the application of colour schlieren on supersonic jets

A conical convergent-divergent (CD) nozzle of design exit Mach 1.5 and diameter 5mm was designed and manufactured. The supersonic flow produced by the nozzle was experimentally investigated using the Z-type colour schlieren technique at three nozzle pressure ratios (NPR) of NPR= 6.44, NPR= 5.08 and NPR= 3.72. The supersonic flow was tested at three conditions and the conditions were namely freely exhausting jet, impinging jet on a flat plate and impinging jet on an orifice. Two separation distances were tested in the conditions of impinging jet on a flat plate and impinging jet on an orifice to investigate the effects of varying separation distance. The resultant colour schlieren images obtained experimentally were analysed qualitatively and an Abel transform inversion algorithm was later applied to obtain quantitative measurements of the density gradient in the images. The NPR and separation distance were found to be the determining factor for the formation of shock diamonds and the density gradient in the flow. The operating NPR determines whether the jet is under expanded, perfectly expanded or over expanded and alter the shock diamonds of the flow accordingly. The freely exhausting jet was first experimentally investigated and the colour schlieren images match theoretical models of under expanded and highly over expanded jets [3,4,7]. The density gradient of the incident shock was found to be dependent on the proximity between the jet operating condition and the perfectly expanded jet condition. The density gradient of the subsequent shock cells is fully dependent on the NPR whereby a higher NPR yields a higher gradient and vice versa. The length of the first and second shock cell length also increases with increments of NPR. Impinging jet on a flat plate was investigated next and it largely matches the proposed schematic of an impinging jet on a flat plate by Belov et al (1973) with the additional presence of a stagnation bubble [6,11]. The initial shock varies with the NPR as per the freely exhausting jet but the shock subsequently gets disrupted by reflected flow from the impinging plate. Higher NPR or lower separation distance strengthens the initial shock and reduces the disruptions due to the reflected flow. The magnitude of the density gradient also increases as the shock is strengthened. Impinging jet on an orifice was last investigated and the flow was distinctly segregated into two sections. The first section is the flow before impinging on the orifice and the second section is the flow after exiting the orifice. The flow in the first section largely resembles the condition of impinging jet on a flat plate without the stagnation bubble due to the orifice allowing flow to pass through and not recirculate. Minor disturbance in the flow dependent on the NPR and separation distance are also observed due to reflected flow. The orifice compresses the flow and causes the flow to exit as an expansion wave and proceeds to form recurring shock cells regardless of the input flow conditions. However, the magnitude of the density gradient and length of shock cells exiting from the orifice is dependent on the NPR and separation distance. Increasing the NPR or reducing separation distance increases the magnitude of density gradient and length of shock cells and vice versa.Bachelor of Engineering (Aerospace Engineering