Mixing Characteristics of Supersonic Jet from Bevelled Nozzles

The present study focuses on the effect of nozzle exit inclination on the mixing characteristics of Mach 2.17 overexpanded jets at the NPR 5, NPR 6 and NPR 7, using commercial software package ANSYS Fluent. The convergent-divergent nozzles, investigated are circular nozzle and bevel nozzle with bevel angle 300, and bevel angel 450. The nozzles are constructed with equal throat-to-exit area ratio, in order to maintain uniform Mach number at the nozzle exit. From the results, it was found that, the bevelled nozzles effectively reduce the jet core as much as 46%, indicating enhanced jet mixing. It was also observed that at lower NPR, i.e., at NPR 5, the Bevel30 nozzle is found superior over Bevel45 and circular nozzle and at the intermediate NPR, both of the Bevel30 and Bevel45 nozzle reduces the jet core with the same rate. However, at highest NPR of the present study, the Bevel45 nozzle exhibits the highest mixing enhancement. An early axis switching is seen for the Bevel30 jet at NPR 5 and for the Bevel45 jet at NPR 7.

Axis-Switching Behavior of Liquid Jets Issued from Non-Circular Nozzles Under Low-Intermediate Pressure

HighlightsThe flow behavior of water jets discharged from different orifices was investigated.High-speed photography (HSP) was used to obtain surface structures and spread characteristics of water jets.The deformation process in axis switching related to the corner vortices effect of non-circular jets was researched by numerical simulation.The axis switching of non-circular jets enhances entrainment ability of the jet.ABSTRACT. Low-intermediate pressure sprinkler irrigation systems are important research topics in the field of water-saving irrigation. Non-circular nozzles improve spray uniformity at lower pressures and are key components of sprinkler irrigation systems. In this article, the behavior of discharged water jets from nozzles with circular, square, and equilateral triangular orifices designed with the same flow rate was investigated. High-speed photography (HSP) was used to capture jet characteristics in the near field (z<20D). The largest spread angle was obtained for the square jet, which was on average 37% larger than that of the circular jet. In addition, numerical simulations were performed to analyze the axis-switching process using the large-eddy simulation (LES) method and the coupled level-set and volume of fluid (CLSVOF) method. The axis-switching phenomenon was observed in non-circular jets, in which surrounding air mixed with the jet and promoted the formation of thin diaphragm structures. The deformation process that occurs in axis switching is described according to the simulated vorticity and velocity fields. The research results suggest the axis-switching phenomenon is induced by corner vortex motions produced by the polygonal orifice, which accelerate the decay of the axial velocity and increase the jet entrainment rate. Thus, the effect of corner vortices should be considered in the design of polygonal nozzles. Keywords: Axis switching, High-speed photography, Liquid jet, Low-intermediate pressure sprinkler irrigation, Non-circular nozzle, Numerical simulation.

Entrance length effects on the flow features of rectangular liquid jets

An experimental study was carried out to investigate the effects of entrance length on the main characteristics of rectangular liquid jets discharged into the stagnant atmosphere. Six rectangular nozzles, all with the same aspect ratio of 3 but with different entrance length ratios ranging from 3.3 to 60 were constructed. The physics of the fluid flows was visualized by the aid of backlight shadowgraph technique and high speed photography. Flow visualizations revealed that in the mid-range of Weber numbers, the perturbations induced over the liquid surface remarkably depended on the entrance length ratio. Moreover, the characteristics of the axis-switching instability of rectangular liquid jets were measured. It was found that axis-switching wavelength was independent of the entrance length, while the amplitude of axis-switching was directly influenced. For entrance length ratios smaller than 10, the amplitude was increased with increase of entrance length, whereas for entrance length ratios higher than 10, this trend was reversed. Measurements of breakup length also showed that the transition of flow regimes was not perceptibly affected by the entrance length.

The Effect of Elliptical Diesel Nozzles on Spray Liquid-Phase Penetration under Evaporative Conditions

Elliptical diesel nozzles affect the fuel–air mixing process, and thus combustion and exhaust emissions. Experiments were conducted to study biodiesel spray liquid-phase behaviors for elliptical and circular nozzles through the Mie-scattering method under evaporative conditions. Based on the measurements, the results show that the elliptical nozzle spray liquid-phase penetration is smaller than the circular one under steady-state conditions. The deformation and the axis-switching behaviors of the elliptical jet are helpful in accelerating the breakup of the liquid core. Moreover, the injection pressure has little impact on the penetration of the liquid-phase spray for either geometrical orifice. Additionally, increasing the ambient temperature can reduce the penetration of liquid-phase spray, because an increase in temperature increases the rate of evaporation. The differences in steady liquid-phase penetration between circular and elliptical sprays decrease as the ambient temperature increases. Additionally, increasing the backpressure can decrease the liquid-phase penetration. The differences in steady liquid-phase penetration between circular and elliptical nozzles are reduced with the increase in backpressure, probably due to the axis-switching and deformation behaviors of the elliptical jet being restrained under high-backpressure conditions. Finally, the application of an elliptical orifice is beneficial for decreasing the spray liquid-phase penetration, and thus avoiding the fuel impingement in small engine combustion chambers. The lower liquid-phase penetration for elliptical spray indicated higher fuel and air mixture quality, which is helpful for reducing the diesel engine exhaust soot emissions.