Spray ignition enhancement for multifuel propulsion

Motily, Austen H.

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https://hdl.handle.net/2142/121451 Copy

Description

Title

Spray ignition enhancement for multifuel propulsion

Author(s)

Motily, Austen H.

Issue Date

2023-07-10

Director of Research (if dissertation) or Advisor (if thesis)

Lee, Tonghun

Doctoral Committee Chair(s)

Lee, Tonghun

Committee Member(s)

• Kriven, Waltraud M.
• Matalon, Moshe
• Kim, Kenneth S.

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• energy
• combustion
• heat transfer
• ceramics
• optics
• diagnostics
• fuel spray
• ignition
• enhancement
• multifuel
• propulsion
• fuel flexibility
• engine
• vehicle
• aircraft

Abstract

Since the dawn of the 21st century, alternative fuel integration has become a priority for the United States energy and transportation sectors. Vehicle propulsion has long relied on standard fuel processing routes from crude oil to refined petroleum products; however, novel methods to produce energy dense hydrocarbons present unique opportunities and new challenges for engine operation. Chief among these concerns is enabling propulsion systems that are resilient to fuel property variation and capable of reliable performance with multiple fuel types. The compression ignition (CI) engine platform is a prime candidate for fuel-flexible operation, but can exhibit several issues when utilizing low reactivity fuels. The present challenge for combustion scientists lies in developing technologies to enhance the spray ignition process and promote multifuel engine functionality. There are several potential strategies that can promote ignition enhancement; nevertheless, the most feasible method for near-term implementation is a hot surface energy addition device. High-temperature ceramic heaters are currently utilized in CI engines to aid startup in cold weather environments, but these devices are not designed for continuous activation, and they experience rapid degradation at elevated temperatures. Furthermore, fundamental experiments that investigate the enhanced spray ignition process, which can shed light on how to optimize engine performance, have yet to be conducted. This work examines several aspects related to practical implementation of hot surface ignition devices for multifuel propulsion. First, experiments are conducted in a rapid compression machine (RCM) to analyze the influence of a commercial ceramic heater on fuel spray ignition behavior. High-speed imaging indicates that the hot surface induces local ignition in the thermal boundary layer of the device, after which this flame kernel propagates at high velocities (23 m/s) to the flame lift-off length, where global heat release is achieved. Results demonstrate that above a critical surface temperature of 1250 K, robust ignition is consistently attained. At these conditions, ignition delay times are reduced to around 1 ms and the device mitigates the effects of chemical reactivity for a range of fuel cetane numbers from 30 to 48. Next, a thermal analysis of the state-of-the-art heater is performed to quantify device limitations. Infrared imaging of the device conducted at benchtop conditions shows that maximum surface temperatures can exceed 1600 K, whereas forced convection experiments demonstrate that heater surface temperatures can decrease by more than 400 K when subjected to engine-relevant flows. An analytical heat transfer model is developed which indicates that conduction can account for more than 30 % of the total input power, suggesting that heater design can be further refined to optimize energy deposition. Using the information gained from the ignition and thermal analysis studies, a prototype device is produced to demonstrate ceramic fabrication and material optimization capabilities at the university laboratory scale. A wet-bag cold isostatic press method is developed to fabricate refractory ceramics in an enclosed protective sheath geometry with high density (> 95 %) and closed surface porosity after pressureless sintering. Reactive fuel spray experiments with this prototype, conducted in the RCM, demonstrate that the device can accelerate ignition delays of F-24 fuel sprays to similar timescales (~1 ms) as the commercial heater. This effort looks to establish baseline capabilities with which to engage industry partners for prototype development. Fuel-flexible propulsion extends beyond the CI engine however, where ignition enhancement can also be advantageous for gas turbine engines, particularly in the scenario of altitude relight. The final investigation of the present work studies the physics behind hot-surface-assisted fuel spray ignition in the Army Research Combustor – Midsize 1, a single-swirl-cup experimental apparatus with optical access. At airflow temperatures relevant to flight at 10,000 feet (-5 °C), results demonstrate that a commercial heater can significantly improve ignition behavior, achieving a 50 % ignition probability performance metric with an equivalence ratio reduction of 15 %. These improved operational limits are also observed at higher fuel flowrates, where “assisted autoignition” of the spray is observed above equivalence ratios of 0.35 without activating the spark igniter. Combined with the investigations into CI engine ignition enhancement, these measurements provide a foundation for integrating multifuel capability in current and future propulsion systems.

Graduation Semester

2023-08

Type of Resource

Thesis

Copyright and License Information

Copyright 2023 Austen H. Motily

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