Mariko Watanabe

With the decarbonization of internal combustion engines, alternative fuels have gained increasing attention. When using fuels with low combustibility, such as ammonia, detailed analysis of the intake system and in-cylinder flow is essential for improving combustion efficiency. Proper orthogonal decomposition (POD) has been widely used to extract coherent structures in flow fields within internal combustion engines. However, most previous studies have focused on analyzing cycle-to-cycle variations in gasoline engines, while time-resolved analysis within a single cycle of diesel engines has rarely been conducted. In this study, the effect of tangential port opening on in-cylinder flow characteristics was investigated using an optical single-cylinder diesel engine equipped with two intake ports and two exhaust ports. The opening area of the tangential port was varied under five conditions using different gaskets, and in-cylinder velocities were measured using particle image velocimetry. POD was applied to the acquired velocity data to evaluate the flow structures of the higher modes and their correlations with the mean flow and turbulence intensity. The results showed that in POD mode 1, a swirl flow was formed during the compression stroke when the tangential port opening exceeded 25%. Evaluation of the correlation between POD mode 1 and the ensemble-averaged flow using the relevance index revealed a strong correlation during the compression stroke. In POD mode 2, complex flows were observed during the intake stroke, and structures different from the mean flow were also confirmed during the compression stroke. A moderate correlation was observed between POD mode 2 and turbulence intensity under all conditions. Energy contribution analysis indicated that in the early intake stroke, the variation in mode 1 was large, and the flows represented by mode 2 and higher modes were dominant, whereas in the late compression stroke, mode 1 consistently accounted for a higher proportion.

This study focused on the unsteady behavior of fire whirls. A laboratory‐scale fire whirl was generated, and temporal variations in flame height were measured from images taken by a high‐speed camera and subjected to frequency analysis. The flame height fluctuations of the fire whirl also showed intermittent behavior, such as the puffing of a pool flame. However, the period and amplitude were irregular compared to the pool flame. In addition, the fire whirl exhibited a greater amplitude spectrum at higher frequencies than the pool flame. To investigate the velocity distribution in the horizontal plane, particle image velocimetry (PIV) was employed. The results demonstrated that the mean velocity increased from the outer radial direction toward the inner radial direction, peaked, and decreased. Conversely, the coefficient of velocity variation decreased from the outer to the inner radial direction, exhibited a minimum, and then increased. Finally, the flame was photographed from horizontal and vertical directions under two conditions with different flow velocities from the fan to generate the fire whirl. Image analysis was employed to investigate the relationship between the center position of the flame and the flame height. The results demonstrated that under conditions where the flow velocity from the fan was low, the fire whirl was intermittent and moved following the circular path drawn by the swirling flow, exhibiting unstable behavior. Furthermore, the flame height was lower when the center of the flame was further from the liquid fuel pool.