Session: 12-06-03: Modeling of Nano- and Micro-Scale Phase Change Processes

Paper Number: 173898

Effects of Liquid-Gas Interfacial Heat and Mass Transfer on Evaporation of Diesel Fuel in a Supercritical Environment 

Advanced propulsion systems like scramjets, detonation engines, and rocket engines operate at conditions exceeding the critical pressure and temperature of injected liquid fuels. Further, liquid fuel injected into high-speed compressed air breaks into microscale droplets. Understanding evaporation of microscale liquid fuel droplets in supercritical environments is of great importance to the design and development of advanced propulsion systems such as scramjets, detonation engines, and rocket engines.

Different from the evaporation of larger millimeter-sized droplets, the evaporation of fuel microdroplets in supercritical environments could be affected by the liquid-gas interfacial heat and mass transport processes. Molecular dynamics (MD) simulations are ideally suited to study liquid-gas interfacial transport processes. The main objective of this work is to use MD simulations to evaluate the thermal and mass diffusion resistance at fuel-air interfaces when liquid fuel evaporates in supercritical environments, and study how the liquid-gas interfacial resistance affects the evaporation of fuel droplets.

In the MD model, we investigate the evaporation of liquid dodecane (a diesel surrogate) in air (approximated as a N₂ gas) at a temperature ranging from 600 K to 800 K and at a pressure ranging from 10 atm to 30 atm. There are two energy transport modes across the fuel-air interface. One is the heat conduction from the air flow to the fuel, which is mainly driven by the temperature difference between the hot air and the fuel droplet. The other is the fuel evaporation to the surrounding air. The heat conduction flux and evaporation heat flux are in opposite directions at the fuel-air interface. After a short initial heating process, an energy balance is established at the fuel-air interface, and subsequently the liquid fuel remains at a temperature well below critical temperature of dodecane even when the surrounding N₂ gas is much higher than the critical temperature. As a result, a clear liquid-gas interface can be identified in all cases at the quasi-steady state evaporation process and the liquid fuel temperature is higher than the wet-bulb temperature by ~ 10%. From the temperature change and vapor density change across the liquid-gas interface and the interfacial heat conduction and evaporation flux obtained from non-equilibrium MD simulations, we determined the interfacial thermal and mass diffusion resistance. Furthermore, we extract the liquid-gas interface properties including mass and thermal accommodation coefficients from MD simulations and incorporate them into relationships derived from kinetic theory of gases (KTG) to find the theoretical prediction of the interfacial thermal and mass diffusion resistance. Our analysis shows that the interfacial resistance can be well predicted by the KTG-based equations.

As the N₂ gas pressure and temperature exceeds the critical pressure and temperature of dodecane, the high collision rate of gas molecules on liquid fuel surfaces results in a high interfacial thermal conductance. Meanwhile, the mass accommodation coefficient, which strongly affects the interfacial mass diffusion resistance, was not dramatically reduced by the high pressure N₂ gas. Consequently, the equivalent length of interfacial thermal and mass diffusion resistance in supercritical environments is less than 20 nm. Our modeling results indicate that the liquid-gas interfacial resistance can be neglected in the analysis of liquid fuel droplet evaporation in supercritical environments for microscale fuel droplets.

Presenting Author: Wazih Tausif Missouri University of Science and Technology

Presenting Author Biography: Wazih Tausif is a Ph.D. student from the Department of Mechanical and Aerospace Engineering at Missouri University of Science and Technology.

Authors:

Wazih Tausif Missouri University of Science and Technology
Shyam Menon Louisiana State University
Zhi Liang Missori University of Science and Technology