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Modern diesel engines employ higher injection pressure in the common-rail injection system that utilizes high-speed fuel jet to achieve rapid fuel-air mixing for spray atomization improvement. However, the spray characteristics under super high pressure are not well understood. To provide more information on spray characteristics, both the macro- and micro-characteristics of spray have been investigated under super high pressures (up to 300 MPa) via an experimental method. The spray penetration and spray cone angle were measured using a high speed camera and the microscopic characteristic of Sauter mean diameter (SMD) was obtained via Malvern Spray Tec to analyze the influence of super high injection pressure on diesel spray atomization. Induced shock waves were captured using a Schlieren system when the spray tip velocity was greater than the local sound velocity accompanied by diesel spray. The results obtained show that higher injection pressures lead to larger spray velocities and a smaller shock wave angle if it exists, with connection between spray and shock waves, and generates a smaller SMD with less decreasing amplitude. Additionally, two different types of leading shock waves—bow and oblique shock wave appeared sequentially with increasing injection pressure. [ABSTRACT FROM AUTHOR]

The present investigation numerically studies the evolutions of decomposition and combustion within an ADN-based thruster, and the effects of the catalyst-bed’s three structure parameters (length, diameter, and wall thickness) on the general performance of ADN-based thruster have been systematically investigated. Based upon the calculated results, it can be known that the distribution of temperature gives a Gaussian manner at the exits of the catalyst-bed and the combustion chamber, and the temperature can be obviously effected by each the three structure parameters of the catalyst-bed. With the rise of each the three structure parameter, the temperature will first increases and decreases, and there exists an optimal design value making the temperature be the highest. Via the comparison on the maximal temperature at combustion chamber’s exit and the specific impulse, it can be obtained that the wall thickness plays an important role in the influences on the general performance of ADN-based thruster while the catalyst-bed’s length has the weak effects on the general performance among the three structure parameters. [ABSTRACT FROM AUTHOR]

An experimental study is performed to study the effect of inner and outer injection pressure on spray characteristics and the detailed structure of interacted liquid sheets of a coaxial swirl injector. A high speed camera is used to visualize the transition merging process of the coaxial swirling liquid sheet. A laser reflection system is set up to analyze the wave frequency of liquid sheet. A Malvern RTSizer is used to measure the droplet size of the coaxial spray. It is found that the structure of the interacted spray is considerably different from the individual spray, the interaction between the inner and outer liquid film promoted the breakup. At the same injection pressure, the temporal instability of a liquid sheet does not change spatially. For both individual and coaxial spray, the wave frequency increase as the injection pressure increase. The individual inner spray is more unstable and easier to breakup. Furthermore, the relationship between the surface instability and the droplet size distribution of the coaxial liquid spray is investigated quantitatively. For both individual spray and coaxial sprays, the Sauter Mean Diameter (SMD) decrease as the inner injection pressure increase. The inner spray has a higher weighing factor that influences the SMD of the coaxial spray. For coaxial spray at low inner injection pressure, the liquid film thickness would be the dominate factor to the SMD. [ABSTRACT FROM AUTHOR]

Spray atomization process of a liquid-liquid coaxial swirl injector in bipropellant thruster has been investigated using volume of fluid (VOF) method coupled with large eddy simulation methodology. With fine grid resolution, detailed flow field of interacted liquid sheet has been captured and analyzed. For coaxial swirling jet, static pressure drop in the region between the liquid sheets makes two liquid sheets to approach each other and merge. A strong pressure, velocity and turbulent fluctuations are calculated near the contact position of two coaxial jets. Simulation results indicate that additional perturbations are generated due to strong radial and axial shear effects between coaxial jets. Observation of droplet formation process reveals that the Rayleigh mode instability dominates the breakup of the ligament. Droplet diameter and distribution have been investigated quantitatively. The mean diameter of the coaxial jets is between that of the inner and the outer jets. Compared with the individual swirling jets, wider size distributions of droplets are produced in the coaxial jets. [ABSTRACT FROM AUTHOR]

An experimental study was carried out to investigate the effects of ultra-high injection pressure on the penetration characteristics of diesel sprays and leading edge shock waves generated around the sprays that propagate in two modes. The visualization of the spray field using high-speed photography was carried out in the ultra-high pressure fuel injection system equipped with a diaphragm rupture atomizer. Schlieren imaging was used to visualize the shock waves. The experimental results indicated that the spray jet had stronger penetration momentum at higher pressures, but the penetration spacing diminished with increasing pressure between 200 MPa and 400 MPa (i.e., at ultra-high injection pressure). Furthermore, the penetration model proposed by Hiroyasu and Arai retained its predictive power at ultra-high pressures. Regarding the propagation behavior of the induced leading edge shock wave, two modes were observed: that of “spherical” and “oblique” shock waves—these can be characterized by different generation mechanisms and velocity distributions. Additionally, observing the effects of injection pressure on the penetration characteristics of the two modes yielded insight into the duration of the rapid ascent of spherical velocity and the correlations between the spray front angle θ , oblique shock angle β , and spray tip Mach number M in the oblique shock. [ABSTRACT FROM AUTHOR]

The phenomenon of shock waves can be commonly seen in fuel spray under ultra-high injection pressure conditions, where the spray jet penetrates at speeds greater than Mach 1. This paper focuses on the induction of shock waves and their propagation characteristics generated by a supersonic fuel jet at 400 MPa, and simultaneously considers the impact of jet atomization. A specially designed atomizer, utilizing diaphragm rupture, was adopted to replace the traditional electronic valves fuel system to meet the ultra-high injection pressure. Visualization of the spray field by using a high-speed camera was conducted, and a Schlieren apparatus was equipped for shock capturing based on a variable environmental density distribution. The experimental results indicated that the leading edge shock wave shows two forms in the early stage, and each penetrates in a unique way. Along with the spray development, multiple expansion waves were sequentially generated, followed by the leading shock wave. The evolution of wave velocity over time was calculated from the penetration results, and the results showed that the velocity first increased rapidly and then slowly decreased as a result of the effect of air resistance, with a gradual tendency of decreasing for the four peak velocities of the intensity weakened waves. In addition, the spray convex flow path had an inhibitive effect on the deceleration of the first expansion wave. These results give an insight into diesel spray and propagation process of induced shock waves at an under ultra-high injection pressure of 400 MPa. [ABSTRACT FROM AUTHOR]

In the present investigation, the transient developments of flow and cavitation within an injector’s nozzle during the oil drainage process have been studied by numerical method for a high-pressure common-rail DI diesel engine, both the variation regulations of macro parameters (indicating flow characteristics and cavitation characteristics) and the distribution manners of important physical fields (indicating the cavitation evolution in the micro) have been obtained and analyzed. The obtained numerical results indicate that, during the oil drainage process, both mass flow rate and flow coefficient monotonously increase with declining variation rates, both averaged flow velocity and averaged turbulent kinetic energy also monotonously increase; however, to the curve of TKE-needle lift, there exist certain points give abrupt increase. The difference in TKE curve compared to averaged flow velocity is mainly attributed to the sudden variation of cavitation. Based upon the numerical results, the cavitation bubble will not be formed until the needle lift has been raised to a certain position due to the lower flow velocity and the lack of low (even negative) pressure zones. As needle rises, the primary bubbles are formed near the lower corner after nozzle’s entrance; but as needle further rises, the positions at where bubbles are formed have been transferred to the upper corner and then being blow downwards orifice as the increase of flow velocity. [ABSTRACT FROM AUTHOR]

The combustion processes of Ammonium perchlorate/Hydroxyl-terminated polybutadiene (AP/HTPB) aerospace propellant are analytically studied to determine the effect of wall materials and thickness on the heat transfer characteristics within a solid micro thruster. In this study, the flow and combustion characteristic of the propellant are described in terms of a combustion rate equation associated with a chemical reaction kinetics model. The profiles describing changes in the combustion surface by the user-defined function (UDF) are used to obtain the distribution of temperature and content of propellant using a numerical method. The combustion time and thrust are simulated to validate the results by comparing them with the results from the literature. Moreover, we demonstrate that a smaller thermal conductivity and a larger heat capacity of the wall material can lead to improvement in the thruster performance. Finally, analyzing the significant effects of wall thickness the heat transfer characteristics within the thruster allowed us to determine an optimal wall thickness value. [ABSTRACT FROM AUTHOR]