Spray combustion increases the surface area of liquid fuel so that heat and mass transfer occur quickly and efficiently. This combustion method provides power for engines such as those in jets and rockets. The periodically vibrating heat release in the combustion chamber causes combustion instability within the engine that may result in fatal malfunctions and even explosions.

  While burning micro-droplets within the high-pressure spray perform a crucial role in combustion, determining their effects on uncontrolled engine disintegration has proven unsuccessful to-date.

  Following the scaling laws of fluid dynamics, Zhang et al. simulated flying fuel droplets using a fuel-wetted porous sphere in a wind tunnel. The morphologies and flicker characteristics of the sphere’s wake flames were investigated under the conditions of convective airflow velocity.

“The combustion of droplets in a convective environment is complex and difficult to quantitatively study,” said author Yanju Wei. “It is a highly coupled process of air flow, evaporation, heat and mass diffusion, combustion, flame propagation, and radiative heat transfer.”

The researchers confirmed that higher temperatures will likely stabilize the wake flame. However, air flow velocity controls the flame’s morphology and fluctuation frequency. Both natural and forced convective conditions indicate that the fluctuations are determined by Karman vortex shedding behind the sphere.

  Results from the study may significantly simplify the treatment of spray combustion, as well as improve the design of the combustion chambers of various liquid fuel engines.

  “This work may lead to a new upsurge in droplet combustion research and provide clues to solve combustion instability,” said Wei.

Source: “Vortex shedding controlled combustion of the wake flame of an n-heptane wetted porous sphere,” by Yajie Zhang, Yajing Yang, Yanju Wei, and Shenghua Liu, AIP Advances (2022). The article can be accessed at https://doi.org/10.1063/5.0095714.