Shock Tube Fabrication and Ignition Delay Measurements of Ammonia-Air Mixtures

Climate change poses a significant threat to civilisation, driven largely by the combustion of hydrocarbon fuels used in power generation, industrial processes, and transportation. Recently, there has been a growing focus on green fuels like ethanol, biodiesel, and hydrogen carriers such as ammonia for powering engines and heating systems. Ammonia is particularly promising due to its higher energy density compared to liquid hydrogen. Additionally, its century-long use as a fertiliser and industrial chemical makes it safer and easier to store and transport. There is, however, a lack of information available on the combustion properties of ammonia, such as autoignition delay time and branched chemical reaction rates. In the work described herein, the design and fabrication of the RMIT University Shock Tube (RUST) is discussed. The goal of this research was to design and fabricate a shock tube suited for investigating auto-ignition delay times of ammonia at pressures and temperatures found in real engines. The design methodology and features of subsystems like gas distribution, control system, and data acquisition has been discussed. An in-house gas dynamics tool based on shock equations was programmed to predict test conditions when driver and driven gas mixtures are used. A MATLAB program was developed to process pressure data from high-frequency piezoelectric transducers, calculating ignition delay using the steepest gradient method. The program identifies the autoignition point by locating the intersection between the test pressure line and the steepest rate-of-change of pressure signals from the transducer nearest the endwall. Engine conditions for ammonia combustion were defined using isentropic relations, and a test matrix was created for a temperature range of 500-1100 K and a pressure range of 10-120 bar. Through the course of the project, it was discovered that the test time of the RUST (∼2 ms) was not sufficient to measure ignition delay of ammonia at engine conditions and higher temperature conditions (1600 to 2100 K) were studied instead. Techniques like gas tailoring and driver extensions to extend test times of the RUST to up to ∼9 ms were discussed along with X-T simulations. A reasonable agreement was found between measured data and literature. In addition to ignition delay measurements, novel theories for diaphragm deformation were proposed and compared with real experimental data. Two theories – one for elastic region and the other for plastic region were discussed and the deformation of polycarbonate diaphragms of varying thicknesses (0.015” to 0.040”) was recorded. The elastic theory over-estimated deflection in the plastic region while the plastic theory predicted deflection with a ± 9 % accuracy.