Using blends of hydrogen and methane as fuels in internal combustion engines is a promising strategy for reducing carbon dioxide emissions. However, the undesirable phenomenon of super-knock, which can severely and instantaneously damage an SI engine, limits its widespread adoption.
Unlike normal knock, super-knock is caused by a detonation wave due to a feedback loop between the heat release associated with the flame and pressure inside an engine cylinder. Hydrogen is at higher risk of super-knock than other fuels because of the ways engines must operate to run efficiently on hydrogen. One solution is to use a mix of hydrogen and methane; adding methane to hydrogen fuel can smooth combustion and reduce some types of emissions.
One concern when burning these fuels in spark-ignited engines is the transition from a “desirable” mechanism of combustion to the formation of a detonation wave. In desirable combustion, the wave of fuel combustion (called deflagration) propagates away from the spark ignition source. Under the wrong conditions, this combustion can instead lead to the formation of a detonation wave. This wave rapidly consumes all the fuel and results in the strong pressure spike called super-knock.
Researchers from Sandia National Laboratories, Argonne National Laboratory and the University of Connecticut, Storrs have now analyzed detonation formation in hydrogen/methane air mixtures, quantifying the effect of non-thermal reactions on the mechanism of detonation. The results are published in the journal Combustion and Flame.
The researchers investigated the impact of non-thermal reaction chemistry on the propagation of combustion fronts for a H2-CH4 fuel mixture burning in air within a bounded domain, representing an idealized engine cylinder.
Previous work has shown that in some combustion environments, the H + CH3 and H + OH radical-radical recombination and the H + O2 radical-molecule association reactions can form long-lived excited-state intermediates (such as CH4, H2O, HO2*) that can undergo subsequent reactions with H, O, OH, and O2 before undergoing collisional stabilization (to CH4, H2O, HO2).
The team included non-thermal “termolecular” reactions in the model, studying in particular the effects of such radical-radical recombination and radical-molecule association reactions. (Termolecular refers to the simultaneous collision of any combination of three molecules, ions, or atoms.)
The team used the S3D direct numerical simulation (DNS) code, with 1 micrometer spatial resolution, showing that inclusion of non-thermalized reaction chemistry influences chemical reaction fluxes during high-pressure H2-CH4 combustion and the transition of deflagration fronts to high-pressure, fast-moving detonation fronts.
On the other hand, the inclusion of chemical explosive mode analysis (CEMA), a reliable computational flame diagnostic tool to systematically detect important species and reactions formed during combustion, indicated that, irrespective of the presence of non-thermal reactivity, temperature and oxygen concentration remain the two most dominant variables affecting detonation formation in H2/CH4-air mixtures under engine relevant conditions.
The researchers note that this particular observation might change with different H2/CH4 blending ratios. The researchers’ proposed model appears in the image below.
The flame front at its formation (a and b) and during normal knock (c and d) or super-knock (e and f). The Route 1 mechanism favors knock, while the Route 2 mechanism leads to super-knock. Image courtesy of Sandia National Laboratories
First, a spark-triggered flame propagates outwards at speed ‘Sf’ accompanied by a pressure wave travelling at speed ‘a,’ where a is much larger than Sf (a >> Sf). Without termolecular reactions, the unburned gas near the cylinder wall spontaneously ignites before being consumed by the spark-triggered flame.
Subsequently, the emerging ignition front propagates outward with speed ‘SSp’ such that it remains decoupled from the pressure wave (SSp >> a), thereby leading to the formation of normal knock.
However, in the presence of termolecular reactions, coalescence between the spark-triggered flame front and the pressure wave occurs with ‘a’ being approximately as large as ‘Sf.’ The perfect synchronization between the pressure wave and the spark-triggered flame front results in the deflagration-to-detonation transition i.e., super-knock without any spontaneous ignition in the unburned end-gas.
The researchers propose that their non-thermal reaction chemistry needs to be included when modelling the combustion of H2-CH4 mixtures to accurately predict important aspects of flame behavior.
Swapnil Desai, Yujie Tao, Raghu Sivaramakrishnan, Yunchao Wu, Tianfeng Lu, Jacqueline H. Chen (2022) “Effects of non-thermal termolecular reactions on detonation development in hydrogen (H2)/methane (CH4) - air mixtures” Combustion and Flame doi: 10.1016/j.combustflame.2022.112277.