Quantitative Analysis of Flame Kinematics in Premixed Hydrogen-Air-DNG Jet Flames using PIV Measurements and Flame Front Segmentation

Abstract

Today's electricity supply falls short of current demands, leading to the utilization of gas turbines in both ground based and avionic infrastructures. Nevertheless, these often rely on carbon-based fuels, resulting in escalating CO2 emissions. However, adopting hydrogen as a fuel eliminates carbon emissions. Aside from zero carbon emissions, hydrogen has a higher energy density by weight compared to conventional fuels. This makes it an distinct option for applications requiring efficient energy storage and delivery. Due to its wide flammability range and low ignition energy, hydrogen can combust in scenarios where traditional fuels might not. This unique characteristic, while advantageous in certain contexts, requires detailed study to ensure safe and efficient combustion in gas turbines. However, the combustion of hydrogen inherently results in elevated flame temperatures, thereby generating increased NOx levels. Furthermore, hydrogen's high mass diffusivity translates to a reduced Lewis number. Consequently, it becomes vital to grasp the local dynamic characteristics of the flames, particularly at stable and flashback points. Understanding the thermo-physical behavior of hydrogen flames, especially at stable and flashback points, therefore, requires experimental studies to reflect real life complexities. These tools can offer insights into turbulence-flame interactions, flame stabilization, and emission formation mechanisms.


In this research, an exploration was undertaken to understand the local kinematics and dynamics of Hydrogen and DNG flames, with emphasis on stable states and those approaching flashback conditions. Particle Image Velocimetry (PIV) experiments were employed on a Bunsen burner setup, facilitating the capture of the jet flames' velocity fields. Both low-speed and high-speed recordings were captured by high-speed camera, providing distinct insights into flame dynamics. Flame front detection was achieved using Mie-scattering, capitalizing on the differential seeding particle densities between the unburnt and burnt regions. Intensity differences between these regions were meticulously captured with a bilateral filter, leading to the successful extraction of the flame front. This extracted front was subsequently distinguished via segmentation and superimposed onto the velocity field. Low-speed recordings offered a generalized perspective on flame turbulence characteristics through cold flow validation, while high-speed recordings unveiled specific dynamics, inclusive of flame curvature, local flame and displacement speeds, and both normal and tangential velocities and stretches. It consistently holds the 1-D unstretched flame speed, even as the Reynolds number increases, aligning with the respective flashback points and stable conditions of the flames. As a result, in-depth comparison of DNG and $H_2$ fuels in terms of flame dynamics and kinematics were discerned.