Experimental Investigation of Turbulent Gaseous Jets in Two-stroke Marine Engine

This thesis addresses the need to reduce greenhouse gas emissions in the maritime industry by investigating the optimization of gaseous fuel use for large two-stroke diesel engines such as methane and ammonia. However, the transition to gaseous fuels presents challenges, particularly in achieving efficient mixing within the confined space of an engine cylinder. The research focuses on providing experimental data to optimize Computational Fluid Dynamics (CFD) models in the investigation of gas mixing to minimize fuel slip.

It is proposed to use Planar Laser Mie Scattering (PLMS) for concentration measurements in transient jet flows. Compared with other research, here it is investigated to use the method of seeding the ambient air with particles instead of the flow itself to control the particle distribution.

A chapter is dedicated to the theoretical dimensioning of a scaled engine experiment. This involves determining the appropriate scaling factors and parameters to simulate gas injection in a large marine engine. The chapter outlines the necessary calculations and considerations for designing such an experiment, including the scaling of nozzle and cylinder diameters, injection pressures, and flow velocities. However, since the experimental method is not well known, a simple experiment had to test the feasibility of the PLMS method.

This experiment is performed in a specially designed box that includes a pulsed laser setup, a high-speed camera, a pressurized air system, a modular jet with interchangeable nozzles, and a particle generation system. The box is designed to conduct the experiments in a controlled environment with no external effects to ensure accurate and repeatable measurements.

The results demonstrate that the PLMS method can accurately measure concentration fields in transient jets with high Reynolds number. The study addresses the challenges of light scattering and attenuation. A correction model to account for these effects is proposed, providing reliable concentration estimates.

The impact of swirl on the mixing characteristics of a subsonic jet was further investigated. Using various swirl generators with distinct swirl numbers, it was found that higher swirl numbers significantly enhance jet growth and entrainment rates, leading to a more rapid decay of concentration along the centerline. The results indicate that, despite initial differences in the near-field, the centerline data for all swirl cases converge towards a common self-similar condition in the far-field. However, the presence of large-scale flow structures and associated fluctuations suggests that the assumption of self-similarity in the shear layer may not hold for all initial conditions.

It is concluded that the PLMS method, combined with the proposed correction model, is a viable technique to measure concentration fields in transient jet flows. The introduction of swirl significantly enhances mixing, providing valuable insights for the design of more efficient combustion systems in the maritime industry. The findings emphasize the importance of considering swirl characteristics and their impact on mixing efficiency, particularly in the near- and intermediate-fields, together with a new use of a well-known method for time-resolved concentration measurements not seen before.