Optical investigations of nozzle geometrical and dynamic factors on formation and development characteristics of string cavitation with large-scale diesel tapered-hole nozzles

Cavitation is known to be an essential physical phenomenon to induce the fuel primary breakup process, which further influences subsequent secondary atomization and combustion in diesel engines. Different from normal geometry-induced cavitation, the special vortex-induced cavitation, which may influence strongly spray characteristics, is seldom investigated comparatively. In this paper, formations and developments of string cavitation are captured in scaled-up transparent replicas of diesel tapered-hole nozzles by high-speed imaging technology. The ensemble average images were post-processed by MATLAB code for characterizing the string cavitation at fixed needle lifts. The results indicate that string cavitation tends to occur initially in the middle of the nozzle holes. Besides, shedding bubbles from geometry-induced cavitation may stimulate the string cavitation inception as an inducing factor. The morphology and duration of both two types of string cavitation are largely influenced by needle lifts and cavitation number. Moreover, it is concluded that string cavitation hardly occurs in nozzles with low hole entrance location, and the hole entrance rounding structure changes the distribution characteristics of string cavitation. Finally, it is ascertained that the variation of magnification ratio changes little or nothing about development and distribution characteristics of string cavitation.

Numerical Investigation on Static and Rotor-Dynamic Characteristics of Convergent-Tapered and Divergent-Tapered Hole-Pattern Gas Damper Seals

To study the influence of taper seal clearance on the static and rotor-dynamic characteristics of hole-pattern damper seals, this paper develops three-dimensional transient computational fluid dynamic methods, which comprise single-frequency and multi-frequency elliptical orbit whirl model, by the transient solution combined with a mesh deformation technique. Through the investigations, it is illustrated that: (1) In the present paper, the leakage rates of convergent-tapered hole-pattern damper seals are less than divergent-tapered hole-pattern damper seals for the same average seal clearance, and the maximum relative variation reaches 16%; (2) Compared with a constant clearance hole-pattern damper seal, the maximum relative variation of the rotor-dynamic coefficients is 1,865% for nine taper degrees in this paper; (3) Convergent-tapered hole-pattern damper seals have smaller reaction forces and effective damping coefficient, larger cross-over frequency, and direct stiffness coefficient, while divergent-tapered damper seals have the opposite effects; (4) Divergent-tapered hole-pattern damper seals alleviate the rotor whirl because of a larger effective damping coefficient when the rotor system has large natural frequency and small eccentricity. Convergent-tapered damper seals provide both sealing and journal bearing capabilities at the same time, and are more advantageous to the stability of the rotor system when rotor eccentricity is the main cause of rotor instability.

Experimental Verification of the Pumping Effect Caused by the Micro-Tapered Hole in a Piezoelectric Atomizer

In this study, we examined the use of a dynamic micro-tapered hole as a micro-scale tapered flow tube valveless piezoelectric pump. Firstly, we obtained photographs of a micro-tapered hole by using an environmental scanning electron microscope (ESEM). Then, we explained the pump effect of the micro-tapered hole, and derived the atomization rate equation. Furthermore, we reported an atomization rate measurement experiment that eliminated the atomization caused by a pressure increase, and demonstrated that a change in the volume of a micro-tapered hole could produce atomization. The experimental results indicate that, under the same voltage, the forward atomization rate is much higher than the reverse atomization rate and that the atomization rate increases with the micro-tapered hole volume. The experimental results show that the atomization of the micro-tapered aperture atomizer is caused by its pumping effect. Moreover, the flow resistance and volume of the micro-tapered hole can affect the atomization rate.