Fig. 10b shows the relationship between the dimensionless mass flow rate of smoke, m m ̇ ̇ s s ref , (ṁ s re , f is the reference mass flow rate with V = 3 m/s), and the dimensionless wind speed, V gh , where h is the shaft height and g is the gravity acceleration. For cases with different HRRs,the scatters have similar trends. These two dimensionless parameters follow a good exponential relationship as 1.5e m m ̇ ̇ s s ref V gh , = − (0.4 < V gh < 1.3).