3.1. Effect of voids on superficial air velocity in the
downcomer Fig. 2shows the superficial velocity in the downcomer against the inlet air velocity for three agricultural materials. The amount of air percolating through the downcomer linearly increases with increase of inlet air velocity. In addition, it depends upon the material type through which the air is passing. Thus, highest superficial air velocity occurs in the bed of paddy followed by corn and soybean. The proportion of the air in the downcomer was approximately 28% of total airflow for paddy and 21% for corn and soybean. The larger amount of air flowing through downcomer can be attributed to the larger proportion voids in the material, thus reducing the resistance to airflow and the corresponding pressure drop. The relations between velocities at the downcomer and at the inlet as well as voids can be mathematically described by
Ud
Uin
¼0.0435e
0.3
d
ð7Þ
withR
2
of 0.93. Eq.(7) was statistically fitted with the
data in the range of voids between 0.36 and 0.56 and
inlet velocities between 37 and 52 m/s.