R Mariani

Imaging gas and plasma interactions in the surface-chemical modification of polymers using micro-plasma jets

This paper reports on the correlation between gas flow and plasma behaviour in the outflow of a micro-atmospheric pressure plasma jet operating in helium using both 2D optical imaging and Schlieren photography. Schlieren photography shows that the helium outflow changes from laminar to turbulent conditions after distances between 20 and 50 mm from the nozzle. Above a flow rate of 1.4 slm, the length of the laminar region decreases with increasing flow rate. However, by contrast the visible plasma plume increases in length with increasing flow rate until its extension just exceeds that of the laminar region. At this point, the plasma becomes turbulent and its length decreases. Exposing polystyrene (PS) samples to the plasma jet significantly alters the water contact angle in a defined area, with the hydrophobic PS surface becoming more hydrophilic. This modification occurs both with and without direct contact of the visible glow on the surface. The radius of the treated area is much larger than the width of the visible jet but much smaller than the area of the turbulence on the surface. The treated area reduces with increasing nozzle–substrate distance.

Schlieren Photography of the Outflow From a Plasma Jet

Using Schlieren photography, the helium outflow configuration from a fine capillary-based microplasma jet discharge has been captured for free-stream conditions. The transition from laminar to turbulent flow is clearly identified with and without operation of the plasma. At a flow rate of 2.3 L·min-1 with no plasma operating, this transition occurs 54 mm from the nozzle; however, with plasma struck (peak voltages of 8 kVp-p), this reduces to 40 mm.

Experimental Studies on Co-Axial Vortex Loops

Vortex rings embody various essential characteristics of vortical motion, and the understanding of their interaction with shock waves has always been of great interest to scientists and engineers alike. The flow features of single compressible vortex loops have been studied theoretically, numerically, and experimentally, but much still remains to be investigated on the topic of the interaction of multiple co-axial compressible vortex rings. In particular leapfrogging, the phenomenon in which two vortex loops pass through each other’s core, has never been addressed experimentally in compressible flow. The present study aimed at experimentally studying the leapfrogging phenomenon of two, or more, vortex rings in compressible flow and to evaluate the flow physics of the induced interactions. Qualitative data was collected for pressure ratios P4/P1 of 4, 8, and 12 while preliminary quantitative PIV data were obtained only at pressure ratio of 4. Results confirmed that leapfrogging is possible, demonstrating the inverse proportionality between the vortex rings diameter and the propagation velocity

Effects of exit nozzle diameter on compressible vortex rings flow structure

An experimental study was conducted to assess the effects of the variation of nozzle exit diameter on the flow behavior and characteristics of compressible vortex rings. The experiments used a 15mm converging nozzle, a 30mm straight nozzle, a 45mm and 60mm diverging nozzle. The driver and driven gas was air. A vortex ring was generated by the impulse of the shock wave emerging from the open end of a shock-tube. High speed schlieren photography was employed to visualize the flow and quantitative data was extrapolated from the schlieren images. Three driver pressures were used, 4bar, 8bar, and 12bar, with the driven gas at ambient conditions. The shock wave Mach numbers generated inside the tube were 1.34, 1.54, and 1.66.

Compressible Vortex Loops in a Shock Tube with Helium Driver

An experimental study has been conducted on the generation and propagation of compressible vortex loops using helium as a driver gas. The study aimed at evaluating the flow characteristics of a compressible vortex loop generated using a lighter then air gas into air at ambient conditions. The advantage of such system when compared to a constant gas system based on ambient air is to effectively increase the Mach number while keeping the pressure ratio constant. Three driver pressures were used (4, 8, and 12bar) generating a theoretical Mach number value of 1.53, 1.89, and 2.12 respectively. Qualitative and quantitatve analysis were conducted. The generation of secondary vortex rings ahead of the main vortex ring were witnessed at all driver pressure. The structure of the trailing jet behind the vortex ring is shown to transition from a regular to a Mach reflection with increasing Mach disk size. The presence of the Mach disk results in the generation of vortical flow inside the main trailing jet with opposite circulation with respect to the outer jet boundary.