Very large-scale simulation of droplet group evaporation and studying the effect of mutual interference of droplets

The evaporation of fuel droplet groups plays a critical role in spray combustion processes. Various factors such as temperature, droplet size, airflow, fuel properties, ambient pressure, spray characteristics, and mixing/turbulence influence the evaporation process. Achieving high performance in gas turbine engines requires effective control of spray combustion, and accurate prediction of this process is crucial for engine design. However, existing models for spray combustion are insufficient in terms of precision and cost-efficiency. The complexity of spray combustion processes, which involve fuel atomization, droplet dispersion, evaporation, and chemical reactions, makes theoretical modeling challenging. Conducting experiments is difficult and costly due to the transient nature of the processes and the small spatial scales involved. Therefore, numerical analysis is seen as a valuable complement to theoretical and experimental modeling.

To improve the accuracy of numerical simulations and contribute to the development of more efficient combustion systems, Dr. Taisuke Nambu and Dr. Yasuhiro Mizobuchi from the Japan Aerospace Exploration Agency conducted a new study published in the peer-reviewed Journal Combustion and Flame. They focused on investigating the effect of mutual droplet interference on the evaporation rate of droplets in groups. The authors used detailed gas-liquid interface-resolved simulation to analyze a single droplet, more importantly this is the first time that multiple droplets have been successfully analyzed. According to the authors the results of this analysis will be very useful for future group combustion modeling.

Evaporation from the gas-liquid interface was calculated directly from physical quantities near the gas-liquid interface. In their numerical analysis, the researchers used the Navier-Stokes equations to analyze a gas-liquid flow system. They employed a fractional step method with second-order spatial accuracy schemes to enhance solution accuracy. The Poisson equation for pressure distribution was solved using a preconditioned Bi-CGSTAB method with a four-level V-cycle multigrid method. The gas-liquid interface condition was incorporated using the Ghost fluid method, while the position of the interface was accurately determined by the coupled volume of fluid with level set method.

To validate their numerical method, the authors compared it with theoretical analyses and experimental data for single droplet evaporation. The accuracy of the method was confirmed through agreement in evaporation rate, wet-bulb temperature, and comparison with microgravity experiments. Minor discrepancies were observed, primarily due to factors such as radiation and natural convection. Moving forward, the researchers presented simulations and models of droplet group evaporation based on numerical computations. Two cases with different initial diameter and droplet number were analyzed using the interface-resolved simulation method. The results revealed slower evaporation rates in droplet groups due to mutual interference. They proposed a coefficient of evaporation rate interference to model the decrease in evaporation rate caused by droplet interference. Multiple analyses were conducted to validate the model, and a correction to the evaporation state parameter was implemented.

In summary, the investigation and modeling of fuel droplet group evaporation by Dr. Taisuke Nambu and Dr. Yasuhiro Mizobuchi and the analysis was conducted on a scale that has never been performed before, using very large computational resources will contribute to advancing knowledge and predictive capabilities in the field of spray combustion. The validation of their numerical analysis method provided insights into droplet group behavior and the effects of interference. The proposed evaporation rate interference coefficient enhanced the precision and cost-effectiveness of spray combustion process forecasting. The new study holds significant importance for improving combustion system design, enhancing numerical simulations, accurately representing droplet interference, offering cost-effective approaches, and paving the way for future advancements and applications in spray combustion. Future research in this area may focus on identifying and refining factors contributing to discrepancies between numerical simulations and experiments. Additionally, expanding the analysis to include complex geometries and real-world conditions would further enhance the understanding and application of spray combustion processes in practical settings.