CAREER: Bottom-Up Understanding of Liquid Breakup at Supercritical Conditions

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The design of modern liquid-fueled engines is shifting toward higher pressures exceeding the fuel critical point (supercritical) to improve fuel-air mixing, enhance combustion efficiency, and reduce engine emissions. Liquid fuel injection generally entails liquid jet breakup into droplets, forming a spray. At supercritical conditions, however, spray formation transitions into a gas-like mixing behavior. The underlying mechanism of this transition, i.e., trans-critical breakup is elusive. Trans-critical breakup is linked to dramatic changes in fluid properties and reduced surface tension due to weakened intermolecular forces. However, the effect of molecular-level interactions on the breakup of microscopic droplets is not understood. This grant supports fundamental research to elucidate the mechanisms underlying liquid breakup at supercritical conditions from molecular interactions to higher scales to advance supercritical combustion. The results will enable new predictive capabilities in controlling supercritical mixing before combustion over multiple scales. This knowledge will promote the next generation of high-speed liquid-fueled propulsion systems for supersonic/hypersonic air and space transportation and supercritical power generation cycles. These benefits will promote U.S. clean energy initiatives and strengthen national security, defense, and economic competitiveness. The educational activities will cultivate an inclusive learning environment in multiphase flows and foster sustained mentorship for under-represented minorities and women to diversify the pipeline of future STEM leaders. Training teachers and informing students and parents at school’s STEM events will enhance public literacy on fluid mixing to promote clean combustion.This project intends to fundamentally understand the breakup of an isolated liquid droplet at supercritical conditions in both low-speed and shock-laden flows where shockwave interaction with droplets promotes breakup. The trans-critical shock-driven breakup mechanism is not known, as experimental diagnostics are not adequate for such extreme conditions, and models are decoupled from the molecular interfacial behavior that dictates droplet breakup. This project will address these knowledge gaps and generates new knowledge on the relationship between surface tension and phase change at supercritical conditions and its effect on droplet breakup. Three research objectives will be to (1) Identify the molecular interfacial behavior of a trans-critical droplet from molecular- to microscale; (2) Understand the breakup mechanisms of a trans-critical droplet in low-speed crossflow, and (3) Determine the shock-driven breakup mechanisms of a trans-critical droplet. The technical approach involves a bottom-up approach based on first principles involving coupled Molecular Dynamics-Direct Numerical Simulations and high-speed experimental measurements. The generated knowledge is critical for controlling fuel-air mixing in high-pressure liquid injection systems in diesel, rocket, gas turbine, scramjet, and rotating detonation engines.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.