The factors which affect the sound production of a vortex as it passes through a nozzle are investigated at both low and high Mach numbers using time-accurate inviscid-flow computations. Vortex circulation, initial position, and mean-flow Mach number are shown to be the primary factors which influence the amplitude and phase of the sound produced. Nozzle geometry and distribution of vorticity are also shown to play significant roles in determining the detailed form of the signal. Additionally, it is shown that solution bifurcations are possible at sufficiently large values of vortex circulation. Comparisons are made between sound signals computed directly using a numerical method for the Euler equations and predictions obtained using a compressible vortex-sound analogy coupled with a compact-source assumption for the computation of vorticity dynamics. The results confirm that the latter approach is accurate for a range of problems with low mean-flow Mach numbers. At higher Mach numbers, however, the non-compactness of the source becomes apparent, resulting in significant changes to the character of the signal which cannot be predicted using the analogy-based approach. Implications for the construction of simplified models of vortex sound in solid-rocket nozzles are discussed.

Sound production of vortex-nozzle interactions

The Ariane 5 solid propellant booster, with a propellant mass of 230 tons and a thrust of 7500 kN, is the largest ever built in Europe. It is comprised of a hemispherical head, active only during the first part of the combustion, and two cylindrical segments, which are active throughout the entire cambustion time, and are separated from each other by means of restrietars or inhibitors. Inside the cambustion chamber of this motor, as well as other solid propellant rockets like the Titan IV and Space Shuttle boosters, flow instahilities have been observed. These instahilities cause thrust fluctuations of about 0.5%, and are believed to be the result of coupling between self-sustained vortex shedding from the inhibitors and chamber acoustics. A plane two-dimensional incompressible flow model implementing the Schwarz-Christoffel transformation, has been developed to study the vortexnozzle interaction. The chamber with nozzle is modelled as an infinitely long channel with a step. Calculations using the geometry of the 2-D cold flow experimental facility MICAT, predict the order of magnitude of the observed pressure fluctuations. Experiments have been conducted with an axial symmetrie cold flow model. First, acoustic characterisation without flow of three configurations, with chamber lengths of L = 231mm, L = 292.5mm and L = 405.5mm, gave the first and second harmonies associated with the lengths of the set-ups, with deviations from theoretically expected values being of the order of 5%. With a porous plate placed at the inlet of the test section, it was shown that sonic conditions could be obtained in the nozzle, tagether with a sufficient pressure drop across the porous plate, thus ensuring that the test section was acoustically closed. Finally, experiments were conducted to see if it was possible to identify acoustic coupling. This was done with three configurations, with totallengths L = 231mm, L = 405.5mm and L = 405.5mm, and a restrictorto-nozzle distance of l = 115.5mm, l = 115.5mm, and l = 290.0mm, respectively. Harmonie series were found in the latter two configurations. With L = 405.5mm and l = 115.5mm, the ground frequency of the harmonies was equal to the acoustic frequency, which is an indication of acoustic coupling.

Model of vortex-nozzle interaction and acoustic coupling in the Ariane 5 booster

An Investigation into the Effects of Swirl on the Performance and
 Emissions of an Opposed-Piston Two-Stroke Engine using Large Eddy
 Simulations

The opposed-piston two-stroke (OP2S) engine is a promising alternative powertrain technology that can offer thermal and brake efficiency improvements over conventional four-stroke engines. In recent years, many of the technical barriers of the OP2S engine architecture have been overcome. However, there is still a need for fundamental studies to provide insight into the scavenging and combustion processes of the OP2S engine to help design and operate the most efficient combustion systems using this engine architecture. This work aims to provide such insights by analyzing experimental data collected on a 3-cylinder, 4.9 L OP2S engine. Specifically, a scavenging efficiency sweep, an engine speed sweep with a constant scavenging efficiency, and an engine speed with a constant pressure differential across the engine were studied in detail. It was found that in diesel combustion, as the scavenging efficiency decreased, the increase in temperature of the hot, internal residuals resulted in a significant increase in heat transfer. Despite lowering pumping losses, this resulted in an overall decrease in brake efficiency. When the bulk thermodynamic conditions of two different scavenging efficiency cases (67% and 72%) were matched at port closing, the lower scavenging efficiency case still displayed a 0.7 percentage point penalty in net thermal efficiency and an increase in engine-out indicated specific emissions of NOx of 18% due to residual stratification in the cylinder. The results presented in this work show that the optimal breathing strategy in diesel combustion on an OP2S engine architecture is one that results in a slightly under-scavenged environment at port closing. However, the results did show potential system-level efficiency benefits of decreasing scavenging efficiency, meaning that alternative fuels and combustion strategies without the constraints of diesel combustion could achieve system-level efficiency gains by running significantly under-scavenged.

Experimental study of the impact of scavenging efficiency on diesel combustion in an opposed-piston two-stroke engine

Renewable fuels, such as the alcohols, ammonia, and hydrogen, have a high autoignition resistance. Therefore, to enable these fuels in compression ignition, some modifications to existing engine architectures is required, including increasing compression ratio, adding insulation, and/or using hot internal residuals. The opposed-piston two-stroke (OP2S) engine architecture is unique in that, unlike conventional four-stroke engines, the OP2S can control the amount of trapped residuals over a wide range through its scavenging process. As such, the OP2S engine architecture is well suited to achieve compression ignition of high autoignition resistance fuels. In this work, compression ignition with wet ethanol 80 (80% ethanol, 20% water by mass) on a 3-cylinder OP2S engine is experimentally demonstrated. A load sweep is performed from idle to nearly full load of the engine, with comparisons made to diesel at each operating condition. These results indicate that on the OP2S architecture, wet ethanol 80 produces near-zero soot and reduces engine-out NOx emissions by a factor of 3-5. Due to the combustion chamber geometry, which is optimized for diesel combustion, most of the fuel was injected near top dead center for a diffusion-style heat release process. Therefore, there is 1-3 percentage point thermal efficiency penalty associated with wet ethanol 80’s longer diffusion heat release process, since the nozzle hole size of the injector was not increased, and from evaporation-driven heat removal near top dead center. However, further optimization of the injectors and combustion chamber geometry could mitigate or eliminate this efficiency penalty by enabling a larger fraction of the total fuel to be injected earlier in the compression stroke.

R.R. Hessel

Papers

Effects of Port Angle on Scavenging of an Opposed Piston Two-Stroke Engine

Vortex-Acoustic Interaction in Internal Flows