3. Experimental results
3.1. Compressed air injection method
In order to increase particle collection efficiency, it is important
to increase the rotational air velocity in the upper part of the cylindrical
wall region. The previous report examined the increase of
particle collection efficiency using the single air injection method.
However, the particle separation performance can be increased by
use of a multi air injection ring as shown in Fig. 3. The collection
efficiency of a cyclone operating on a single air inlet and with a secondary
air inlet at the same air flow rate was reported in our previous
paper [5]. The experimental result of the collection efficiency
using the secondary air inlet is higher than that of only the single
air inlet case under the same air flow rate condition.
Fig. 4 shows the effect of the number of injection air jets on particle
separation efficiency. The main flow rate was 500 l/min and
the injection air jet flow rate was 200 l/min. The 50% cut size decreases
as the number of injection air jets increases. Among the
three cases, the minimum 50% cut size was obtained with four
air jets, because ideal rotational flow field is formed under these
conditions. Fig. 5 shows the effect of the injection air jet flow rate
on partial separation efficiency with four injection jets. The 50% cut
size decreases with the increase of the injection air jet flow rate.
Because the rotational fluid velocity near the upper cylindrical wall
region increases under the increased injection flow rate, the centrifugal
effect on the particle increases and causes a decrease in
the 50% cut size. The 50% cut size of approximately 0.5 lm was obtained
with the four injection jets. The compressed air injection
method shown in Figs. 4 and 5 is effective in increasing particle
collection efficiency, but the required energy increases in the case
of large diameter cyclones due to the compressed air. The next 3.2. Free air inflow method
In order to increase rotational fluid velocity near the upper
cylindrical region, the new method shown in Fig. 6a is proposed
and its particle separation performance was examined. The upper
part of the new cyclone uses special rings where the inflow of free
air is introduced tangentially by a slit pipe. Type A is the standard
cyclone without the slit and type B to type E are the cyclones with
special rings. The total slit area A increases with the increase of slit
number n. The experiments were carried out using each type of cyclone
under a constant static pressure difference at the outlet pipe.
In order to calculate the separation performance, the flow rate of
main air and free air inflow were estimated using the following
method. For the constant pressure drop difference conditions with
and without free air inflow, the following equation is obtained at
the inlet of the cyclone as shown in Fig. 6b.