5.1. Flow Pattern and Gas Velocity
The visualization of flow in examined cyclones was conducted, with so-called
Q-criterion, defined as [
57]:
where
is the vorticity tensor and
S is the rate-of-strain tensor:
This criterion is often used to identify the cores of vortices, their location and scale, that are formed in various equipment of process engineering, including energy production apparatuses and dust separators [
38,
40].
Example visualizations of vortices formed in cyclones are presented in
Figure 7. To enable comparisons of cyclones, visualizations were made with fixed, appropriate values of
Q-criterion iso-surfaces, colored by velocity magnitude.
The images show two different types of vortices, forming in cyclones. The main (outer) vortex, with scale (diameter) comparable to the cyclone barrel (
Figure 7 top raw), participating in the dust removal process, where particles are separated, and a smaller-scale (inner) vortex, where a stream of purified gas leaving the cyclone (
Figure 7, bottom raw). Its diameter is close to that of the vortex finder and it is forming in the cyclone center, close to cyclone vertical axis.
For each of cyclones, the scale and location of the main vortex are similar, but the range of the first spiral, generated immediately after the inlet changes. The smallest height of the first spiral (its shortening) is observed for the cyclone with the shortest vortex finder (cyclone III), therefore it can be assumed that shortening this length will generate just such an effect. Moreover, at this inlet velocity in all tested cyclones, a slight gas flow is generated in the dust hopper, which may cause re-entrainment of the smallest, previously deposited particles.
As might be expected, the inner vortex shape, scale and location are dependent on the diameter of the vortex finder. In each of cyclones, the scale of the inner vortex in their lower parts is similar, but it changes close to the tip of the vortex finder, from the smallest in the case of the cyclone V (with the smallest outlet diameter) to the largest in the cyclone IV (with the largest outlet diameter).
In a cyclone with a larger diameter of an outlet (cyclone IV), a disturbance in the flow inside the vortex finder is also observed. In other cyclones the flow in this zone is relatively stable. This is also confirmed by vector maps of flow, shown in
Figure 8.
In cyclone IV, with the largest vortex finder diameter a gas swirls inside the center of the vortex finder and even the gas reverse flow was observed. On the other hand, in a cyclone with an outlet of smallest diameter (cyclone V), as a result of reducing this diameter, there was a significant increase in gas velocity, significantly different from values obtained for other models.
It is also interesting that in all cyclones with standard-length outlet (cyclones I, IV and V), a formed swirl of the gas inlet stream was observed near the tip of the vortex finder. It may trigger a part of the dusty gas to pass directly from the inlet to the outlet. This may reduce the collection efficiency of these cyclones. Similar phenomena were also reported and described in [
58]. The lengthening or shortening of the vortex finder can largely eliminate this phenomenon.
The above-described flow pattern is also confirmed by contour maps and graphs, showing predicted distribution of resultant gas velocity (velocity magnitude)—
Figure 9 and
Figure 10.
In the first three cyclones (cyclones I, II and III) with outlet of the same diameter, the gas flow is qualitatively similar. However, quantitative differences are predicted for the gas velocity inside vortex finders. In cyclones with an elongated (cyclone II) or shortened (cyclone III) outlet in relation to the basic model, lower gas velocities are observed inside the vortex finder, with a local minimum in the cyclone axis. These minimum values can be even twice lower than those in the basic model (
Figure 10a).
Another flow pattern in this top zone is seen for cyclones with a larger (cyclone IV) or smaller (cyclone V) outlet diameter (
Figure 10b). In the first of them, a distinct zone of relatively low velocities (u = 0 ÷ 4 m/s) is observed. It is located close to the vertical axis of the cyclone and covers practically the entire height of cyclone, including the vortex finder. In the second of them, with the smallest diameter of the outlet, the flow is completely different. The velocity distribution inside the apparatus is more uniform, and—as a result of the reduction of the vortex finder diameter—there is a significant increase in the gas flow velocity, inside it. The gas flow velocity in the outlet axis is even 2.5 times higher than that of the basic model.
Curves of velocity in cross-section plane z
(
Figure 10c,d) for all cyclones are similar, and have the so-called inverted W-shaped profile [
7,
8]. Two, almost symmetrical local maximums are observed close to cyclone wall and minimum in the cyclone axis. For cyclones with the same vortex finder diameter (cyclones I ÷ III,
Figure 10c) values of maximum are very close. However, the minimum values differ-from the largest for cyclone with maximum outlet length to about twice lower when the outlet was the shortest. It can therefore be concluded that the elongation of the outlet intensifies the flow in the exhaust gas stream in this zone.
It is also interesting to compare the velocity profiles for cyclones with the largest (cyclone IV) and smallest (cyclone V) diameter of the vortex finder (
Figure 10d). In the analyzed zone of cyclones, the maximum velocity values both in terms of location and values are similar to those in other cyclones, while the minimum values, especially in the case of a cyclone with the largest outlet diameter, differ even by about two times. Thus, it can be seen that increasing the diameter of the vortex finder to the proposed dimensions clearly weakens the flow in the gas stream leaving the cyclone.
Numerical simulations also showed in all cyclones an interesting phenomenon of high gas flow velocities (even higher than the gas inlet velocity) at the connection of the inlet with the barrel (
Figure 9, cross-section
). It was also reported earlier in or paper [
40]. The maximum values of gas flow velocity in this zone for cyclones I ÷ V are respectively:
u = 19.1, 19.2, 19.1, 19, 18.8 m/s, which are values of about 25 ÷ 28% greater than the inlet velocity. The reason for such an increase in velocity may be a sudden narrowing of the gas stream at the start of its rotation and a significant increase of the tangential component of velocity, crucial for a dust removal process. This tendency was also observed and reported in [
9], where the research was focused on classic, Stairmand [
12] cyclone and optimized, new design cyclone.
5.2. Pressure Distribution and Pressure Drop
Pressure drop is a key parameter for a cyclone separator, which-in connection with the collection efficiency-proves the effectiveness of its operation and the profitability of its application. The basis for determining of the pressure drop for examined cyclones were total pressure distributions, shown in
Figure 11 and
Figure 12.
It is seen, that total pressure and its distribution depend on geometry of the vortex finder. The maximum vales of the total pressure were observed for the cyclone V, the minimum ones in cyclone IV. This tendency is rather obvious and it is related to the diameter of the outlet in these cyclones, respectively the largest and the smallest. It can also be seen that the total pressure distribution in all cyclones has the highest values at the top of separator and low or even underpressure (cyclone IV with the biggest outlet diameter) in a zone close to the cyclone axis.
All curves presented in
Figure 12 confirm this quantitatively, profiles have s similar shape (U-shaped profile), with maximum values close to cyclone walls and a local minimum in cyclone axis. The differences between individual models are smaller than those for velocity. For cyclones with the same vortex finder diameter but different lengths (cyclones I ÷ III,
Figure 12a,c) curves practically coincide and differences are negligible. Slight discrepancies are observed when outlet diameters are changed (cyclones IV and cyclone V,
Figure 12b,d). Minimum values of total pressure in a cyclone axis for these models differ by about 10 times, with even underpressure for cyclone V.
It can be concluded that for tested cyclones the impact of the length of the vortex finder on the total pressure distribution is less significant than an influence of its diameter. This is also confirmed by the quantitative data shown in
Table 5, determined on the basis of simulations, measurements and empirical correlation.
In simulations the pressure drop for cyclones was determined on the basis of the difference of averaged values of the total pressure at the entire cross-sectional area of an inlet and the vortex finder (similarly to e.g., in [
22,
23]). This is a different, but more accurate method, than that used in our earlier work [
40], where pressure difference was determined on the basis of point values. In addition—for cyclone I (basic model)—results of simulations were compared to measurement data. The pressure drop for all cyclones was also calculated from empirical Coker formula [
59], which—as shown in previous research [
40]—describes well the pressure drop in examined cyclones. This formula is typical-it takes into account an inlet gas velocity
, gas density
and cyclone geometry:
were
coefficient is determined from the following relationship, taking into account dimensions of the cyclone inlet and outlet:
As shown by the data presented in
Table 5, the proposed numerical models of cyclones describe both pressure distributions and the pressure drop relatively well.
The tendency for pressure drop is correct and consistent with engineering practice—small differences in the case of the same outlet diameter and more significant when changing it. Smaller pressure drop values are observed when vortex finder diameter is the largest (cyclone IV) and the largest values for the smallest vortex finder diameter (cyclone V). It is related to smaller and bigger gas flow resistance, respectively.
The pressure drop values, predicted with CFD simulations and measurements for the basic model are practically identical, while in the comparison with values calculated from the Coker correlation the relative error is in the range of 16 ÷ 18%. For cyclones with the same vortex finder diameter but different length (cyclone I ÷ III) and cyclone with the largest vortex finder diameter (cyclone IV) predicted values are slightly overestimated in comparison with calculated ones (in the last model relative error is greater 31%). For the cyclone with the smallest diameter of vortex finder (cyclone V) the predicted value is slightly underestimated (relative error 14%).
It should be noted, however, that the diameters of outlets in cyclones IV and V were selected as the extreme values within the range of values recommended in the literature, which may be the cause of the described errors and discrepancies.