08/11/2026
By Marley O'Neil

The Francis College of Engineering, Department of Mechanical Engineering, invites you to attend a doctoral dissertation proposal defense in mechanical engineering by Shubhra Kanti Das titled "Lean Spherically Expanding Flames: From Operational Limit Extension to Explosion Safety Assessment."

  • Date: Thursday, August 20, 2026
  • Time: 11 a.m. – 1 p.m.
  • Location: Southwick 240

Committee

  • Advisor: John Hunter Mack, professor, Department of Mechanical & Industrial Engineering, UMass Lowell
  • Noah Van Dam, associate professor, Department of Mechanical & Industrial Engineering, UMass Lowell
  • Juan Pablo Trelles, professor, Department of Mechanical & Industrial Engineering, UMass Lowell
  • Dimitris Assanis, associate professor, Department of Mechanical Engineering, Stony Brook University

Abstract

Combustion of hydrocarbon fuels remains the dominant energy-conversion approach in transportation and stationary power-generation systems. However, increasingly stringent emission regulations have intensified the need for cleaner and more sustainable combustion technologies. Lean combustion and exhaust gas recirculation (EGR) are widely used to suppress NOₓ formation by reducing peak combustion temperatures, while also improving thermal efficiency through a higher effective ratio of specific heats and reduced pump losses relative to stoichiometric throttled operation. Nevertheless, excessive dilution of the mixture and operation near the lean flammability limit can reduce the laminar burning velocity, weaken the early development of the flame-kernel, and increase the susceptibility to combustion instability, partial combustion, misfire, and incomplete combustion. Carbon-free fuels such as hydrogen (H₂) and ammonia (NH₃) are attractive alternatives because they can potentially be integrated into existing internal combustion engines and gas turbines with appropriate modifications. However, their practical implementation is complicated by their markedly different combustion characteristics, particularly under lean and EGR-diluted conditions. H₂ exhibits high diffusivity, low ignition energy, a wide flammability range, and rapid flame propagation, whereas NH₃ is characterized by low reactivity, high ignition-energy requirements, and slow flame propagation. Methane (CH₄) serves as a benchmark for assessing the combustion behavior of H₂ and NH₃ as potential replacements for conventional hydrocarbon fuels.

This research aims to investigate the flame-propagation, combustion, and explosion characteristics of spherically expanding laminar flames of conventional and carbon-free fuels under lean and diluted conditions in an optically accessible constant-volume combustion chamber (CVCC). Premixed flames of methane (CH₄)–air and ammonia (NH₃)–air are examined over a range of equivalence ratios. Particular emphasis is placed on comparing conventional spark ignition with alternating-current (AC) plasma-assisted ignition to determine their effectiveness in extending the lean flammability limit, improving early flame-kernel development, and promoting stable flame propagation in low-reactivity mixtures.

High-speed schlieren imaging is used to characterize differences in flame morphology, flame-front shape, flame transition, and laminar burning velocity between spark and AC plasma ignition. To investigate lean combustion from another perspective, this part of the study examines the combined effects of hydrogen (H₂) enrichment and CO₂-based EGR dilution on CH₄ combustion. Hydrogen addition is examined as a strategy to enhance mixture reactivity and accelerate flame propagation, whereas CO₂ dilution is used to represent EGR and moderate combustion intensity. Their competing effects are quantified through measurements of laminar burning velocity and heat-release rate.

Since H₂ is utilized as a fuel in this research, another perspective of the study focuses on explosion safety. In this part, hydrogen explosion characteristics are investigated using high-speed flame imaging and pressure measurements for CH₄ and H₂–CH₄ mixtures. The correlation between grayscale flame intensity and pressure-rise rate is evaluated as an image-based diagnostic method for explosion assessment when direct pressure measurements are unavailable.