Hydrogen and Hydrogen/Hydrocarbon Mixtures
Chung Law and colleagues are carrying out simulations and experiments to find solutions for hydrogen combustion and safety problems. The team has previously shown that mixing propane with hydrogen decreases the reaction intensity of combustion based on pure hydrogen, which in turn would imply that use of these mixtures could reduce the need for supercharging required in an engine, as well as reduce the tendencies for the detrimental events of knock and pre-ignition.
This year, the group carried out both experimental and theoretical investigations of the onset of flame wrinkling in mixtures of hydrogen and propane in air, which is directly related to the burning intensities of the flames. In the lab, the team measured and mapped out critical conditions for the onset of instability over a range of pressures and mixture compositions. For comparison, a new analytical model was developed to apply asymptotic theory to instability development on flames in hydrogen/propane mixtures. Values of the critical flame size at the onset of flame wrinkling predicted by the model were shown to compare favorably with the experimentally measured values, allowing quick prediction of stable and unstable flame regimes.
On another front, the team carried out numerical simulations of the dynamics of flame cell evolution to investigate discrepancies between observed and predicted combustion properties. The results show that for lean hydrogen/air flames, the interaction between hydrodynamic and diffusional-thermal instabilities causes distinct evolutions of cell splitting, merging, growth, local extinction, and lateral motion. These elemental processes can dramatically increase the burning rate and cause fluctuations of the flow and species concentrations through increases in the flame surface area. At the same time, the interaction reduces the combustion efficiency through local reactant leakage. Thus controlling the extent of hydronamic instability could facilitate the burning rate and improve fuel efficiency.
Dimethyl Ether (DME)
In a related project, Prof. Yiguang Ju is leading an effort to study the combustion characteristics of dimethyl ether, a synthetic liquid fuel, through both laboratory experiments and numerical simulations. In the lab, laminar burning velocities of dimethyl ether (DME) and air premixed flames at elevated pressures up to 10 atm were measured using a spherical bomb. The group’s work shows that flame speed decreases considerably with the increase of pressure. In addition, experiments showed that at high pressures the rich DME-air flames are strongly affected by the hydrodynamic and thermal-diffusive instabilities. The group is now compiling measurements of chemical kinetics for DME-air combustion to improve the accuracy of numerical modeling for industrial application.
The group’s numerical simulations also demonstrate that lean DME-air mixtures are much different from mixtures for other large hydrocarbon fuels. They find that the combined effect of radiation and flow stretch results in a new flame bifurcation and multiple flame regimes. Experimentally, CO2 dilution in air will lift the DME-air flame and significantly reduce soot emissions. The above results provide important fundamental data and information for DME combustor design.