Chung Law and Yiguang Ju have carried out fundamental research on hydrogen and dimethyl ether combustion to pave the way for use of alternative fuels in the transportation sector. Chung Law also addressed the safety issues in handling and storage of hydrogen gas by analyzing the explosion hazards associated with the sudden release of high pressure a hydrogen jet into air.

 


Hydrogen Combustion

Proponents of a hydrogen economy envision using hydrogen gas as an energy carrier to be used in mobile applications, such as cars and trucks. Although hydrogen use in fuel cell technology is envisaged in the long run, a more immediate environmental friendly alternative is the use of hydrogen gas in conventional internal combustion engines (ICE’s), which would be adapted to the new gas characteristics.

Chung Law and colleagues have carried out detailed studies of the characteristics of hydrogen gas ignition and flame properties at high initial pressures, corresponding to realistic engine conditions. Because hydrogen gas is light and has a low initial density at ambient pressure, hydrogen ICE’s are usually operated at high pressures (i.e., supercharged conditions) in order to obtain greater power per unit volume. In the early years of the grant, the team determined the conditions for hydrogen ignition and hydrogen flame propagation. They found that the flame surface at higher pressures becomes unstable, leading to cellular pulsations. The effect was found to increase the flame speed and burning rate of fuel, a beneficial aspect of hydrogen use in ICE’s promoting the power of the engine. Later efforts of the group focused on formulating theoretical models for the flame surface instability aimed at capturing their observations.

Hydrogen gas can sometimes be too easily ignited under supercharged conditions, creating undesirable high pressure shock waves in a phenomenon known as engine knock. Chung Law and his group proposed a new strategy to limit knock while maintaining the desirable aspects of hydrogen combustion. They showed that dilution of the hydrogen gas with a heavier fuel such propane may eliminate the propensity of the flame to be destabilized by reducing the flame sensitivity. This finding implies that use of hydrogen/propane mixtures could reduce the need for supercharging required in an engine, reduce the tendencies for the detrimental events of knock and pre-ignition, and lower the potential for explosion of hydrogen in storage.

The team also developed a new approximate model for the flame stability by simultaneously considering the two different stability-controlling mechanisms: the hydrodynamic mechanism of instability, which is enhanced at higher pressures, and the thermal-diffusive mechanism, which is controlled by the disparity of the heat diffusivity with respect to the mass diffusivity of the fuel. The model was found to be in good agreement with experimental observations of the onset of instability in outward growing spherical flames.

To investigate the consequences of the coupled hydrodynamic and diffusive instabilities on the flame dynamics beyond the stability boundary predicted by their model, the team carried detailed numerical simulations of the dynamics of flame evolution. The key parameters were identified to be the ratio of fuel to heat diffusivities and the sensitivity of chemical reactions to temperature fluctuations. At high fuel diffusivities (i.e., large hydrogen to propane ratio), results show that the interaction between these two modes of instabilities yields distinct evolutions of cell splitting, merging, growth, lo cal extinction, and lateral motion. These elemental processes also dramatically increase the burning rate through hydrodynamic fluctuations and increase of flame surface area. Thus the extent of hydrodynamic instability could control the burning rate and improve fuel efficiency.

The group further studied the flame dynamics corresponding to low fuel diffusivity encountered when the proportions of propane are considerable. The non-linear evolution of the flame captured numerically was found to consist of regimes of stable cell propagation, periodic pulsating cellular flames, and irregular pulsating cellular flames as the chemical sensitivity is increased. It was also found that unsteady pulsating flames can propagate faster than the ideal laminar flame. This implies that fast burning rates can also be obtained with significant propane addition, but without introducing the propensity to fast ignition and knock obtained at lower propane addition. This extended the range of chemical control of the flame dynamics considered previously.

An important aspect of the hydrogen economy is the safety associated with storage and handling of a light and highly reactive gas. The accidental scenario of a high pressure jet release from a punctured storage tank or high pressure line was investigated. The release of high pressure hydrogen, characterized by a low molecular weight and high sound speed, yields a very strong shock transmitted into the surrounding air. Dilution with propane also helps reduce this hazard. In the pure hydrogen case, the shock heats the air by several thousands of degrees, depending on the hydrogen storage pressure. The contact and mixing between the discharging hydrogen and hot air creates a serious ignition potential. The initial work of Law’s group aimed at characterizing the strengths of the jet-driven shock waves, in order to determine if the local temperature is conducive to ignition. Time evolution of the jet discharged flowfield was also obtained theoretically and verified by detailed numerical simulations. The knowledge of the temperature field evolution permits future assessment of the ignition of the hydrogen-air mixing layer via direct numerical simulations and/or analytical tools.

 


Dimethyl Ether (DME)

In a related project, Yiguang Ju has led an effort to study the combustion of dimethyl ether (DME), a synthetic liquid fuel under consideration in the Capture Group’s coal synfuels generation work. The research measured the flame speed data of DME at normal and elevated pressures and demonstrated that the previous DME kinetic models were not able to reproduce the experimental results. These experimental data contributed to a successful new DME mechanism development in Prof. Dryer’s group. In addition, the group also found that a small amount of DME addition in methane and natural gas can significantly shorten the ignition delay time. This conclusion suggests that natural gas can also be used in homogeneously charged compression ignition engines (HCCI) by doping with a small amount of DME.

The group’s simulation also demonstrated that a lean DME-air mixture behaves much differently than mixtures of other large hydrocarbon fuels. They found that the combined effect of radiation and flow stretch results in many new flame regimes, which significantly modify the actual burning limit of DME. Moreover, the experimental study of a DME jet diffusion flame showed that, contrary to the prevailing theory, a DME flame cannot be lifted even at a Schmidt number larger than unity. The experimental observation and theoretical analysis led to a general conclusion that the unique DME liftoff property also applies to other oxygenated fuels.