Vortex shedding controlled combustion of the wake flame of an n-heptane wetted porous sphere

Aiming at the combustion characteristics of the microscopic flying fuel drops in fuel sprays in engines, an n-heptane wetted porous sphere (d0 = 6 mm) was employed to simulate the combustion of the fuel drops according to the flow similarity indicated by the Reynolds number (Re). The hot airflow provided by a wind tunnel with an inner diameter of 40 mm varied in flow velocity from U = 0.6 to 7.1 m/s and from temperature T = 423 to 723 K, with Re ∈ [0, 800]. Experimental results show that flow velocity plays a dominant role in determining the morphologies and fluctuation frequencies of the wake flame. The flame length and suspension height have a trade-off relationship and approach a linear relationship with the increase in air temperature. With increasing Re, the yellow flame gradually vanishes, and the blue flame ratio in the total flame increases following a hyperbolic tangent function. In addition, the length of the flame outline increases linearly with Re. The fluctuations in the flames under both natural and forced convective conditions and various air temperatures are proved to be determined by the vortex shedding behind the sphere, which helps one understand the underlying physics of spray combustion better.