**3.1 Pressure drop**

The pressure drop for the clean gas simulation that was run before injecting the particles amounted to 1398.01 Pa but this value increased on injection of particles. Whereas, Fassani and Goldstein [3] and Hoffmann et al. [6] reported the pressure drop to decrease on the introduction of dust particles. Moreover, this reduction was more pronounced at higher inlet velocities. The increase in pressure drop can be attributed to an increase in surface roughness of the cyclone separator walls as particles tend to accumulate towards the walls. **Table 1** shows that the increase in pressure drop for one way coupled simulations is almost same for all the mass loadings with only slight deviations. Whereas for two-way coupling, the pressure drop values are seen to rise marginally with mass loadings. This suggests that due to the increase in mass loadings, the force imparted on the fluid by the particles has a considerable effect on the pressure field inside the cyclone separator. Similar observations were made by Huang et al. [5] and the increased pressure drop was attributed to decrease in gas pressure due to increased wall friction. However, contradictory findings with respect to pressure drop have been reported in the literature [1, 3, 6], which is consistent with the present work for mass loading up to 0.1 kg/kg of gas in


#### **Table 1.** *Pressure drop and separation efficiency values at t = 7.45 s.*

one-way coupled simulation. Particle-particle interaction may be the prime reason for reduction in pressure drop for the higher mass loading. In the proposed work mass loading is limited to 0.3 kg/kg of gas for which particle-particle interactions have not been considered. The pressure drop obtained in the proposed work for the considered mass loading ranges matches with Baskakov's model [23].

#### **3.2 Particle motion**

Four mass loadings were simulated for the SHEC, The particles were injected into the cyclone after 2.87 s of the start of the fluid flow as it is quite enough for the flow to become steady. Nine particle sizes (8.05 × 10−7, 1.04 × 10−6, 1.34 × 10−6, 1.74 × 10−6, 2.23 × 10−6, 2.9 × 10−6, 3.75 × 10−6, 4.88 × 10−6, 6.24 × 10−6 m) were injected into the cyclone and the simulations were run till *t* = 6.95 s. After which particles still remained in the separation space of the cyclone separator, but were few in number and mainly smaller in diameter. **Figure 3** shows the radial profiles of tangential velocity at three axial locations for mass loadings 0.1 and 0.3. The figure shows that for the mass loadings of 0.1, the tangential velocity distribution for one way and two-way coupled simulations is somewhat similar. This signifies that for such low mass loadings there is hardly any difference between the two simulation strategies thereby confirming the similar pressure drop and collection efficiency values obtained from the two simulation methodologies. But for mass loading of 0.3, the peak tangential velocities for the two-way coupled simulation is seen to decrease this decrease in tangential velocity indicates a change in the flow field and also a decrease in the swirl. It is also observed that this decrease in tangential velocity is more prominent in the free vortex

#### **Figure 3.**

*Radial profiles of tangential velocity top row: mass loading 0.1, bottom row: mass loading 0.3; from left to right: axial location 0.75D, 2D, 2.5D from the cyclone roof.*

### *Effects of Mass-Loading on Performance of the Cyclone Separators DOI: http://dx.doi.org/10.5772/intechopen.102090*

region where the particles tend to accumulate. Such a behavior has also been observed by Fassani and Goldstein [3]. An interesting observation is that although the swirl reduces, the pressure drop increases though slightly for two-way coupling. This increase in pressure drop can be explained as a result of dominance of increase in wall friction due to presence of a high concentration of particles near the wall as compared to reduction in pressure drop which occurs due to reduction in tangential velocity.

The particles inside the cyclone separator for one way coupling at five instances of time for mass loadings 0.1, and 0.3 are shown in **Figures 4** and **5**. It can be clearly seen that at a later stage in the separation process, only smaller diameter particles are left out in the cyclone and the number of particles left in the cyclone after this time decreases on increasing the mass loadings. Three particle diameters at *t* = 4.5 s for all the mass loadings are shown in **Figures 6** and **7**. The figures confirm that the fate of larger particles is decided very early in the separation process and also that increasing the mass loadings decrease the number of smaller particles left out in the cyclone separator at a later stage. An interesting point seen in **Figures 6** and **7** is that some larger diameter particles are also found stuck in recirculation zones along the annular part of the roof of the cyclone. This gives an idea of the strength of the recirculation zones operating inside the cyclone separator. Comparison of **Figures 6** and **7** shows that at the same time, the number of particles left in the cyclone separator is less for two-way coupled simulations this suggests that the change in flow field brought about by the two-way coupled simulations not only increases the collection efficiency but also decides the fate of particles sooner than that of one way coupling. This was not observed at lower mass loadings than 0.1 but is significantly observed at only mass loadings of 0.3. Two way coupled simulations [5] also showed that the influence of particles on the gas flow field was relatively small at lower mass loadings. This gives an idea of how closely two-way coupling can approximate the real phenomena occurring inside the cyclone separator as compared to one way coupling.

**Figure 4.** *Particles inside the cyclone from left to right at t = 3.2 s, 3.8 s, 4.5 s, 5.6 s, 6.9 s at mass loadings 0.1.*

#### *Applications of Computational Fluid Dynamics Simulation and Modeling*

#### **Figure 6.**

*Single realization of particle sizes 8.05 × 10−7, 2.23 × 10−6, 6.24 × 10−6 at t = 4.5 s, for one way coupling at mass loadings 0.3.*

*Effects of Mass-Loading on Performance of the Cyclone Separators DOI: http://dx.doi.org/10.5772/intechopen.102090*

**Figure 7.**

*Single realization of particle sizes 8.05 × 10−7, 2.23 × 10−6, 6.24 × 10−6 at t = 4.5 s, for two-way coupling at mass loadings 0.3.*

#### **3.3 Collection efficiency**

The collection efficiency with respect to mass loadings and the simulation methodology applied at the four mass loadings are given in **Table 1**. **Figure 8** presents both the pressure drop and collection efficiency of the cyclone model at

#### **Figure 8.**

*Pressure drop and collection efficiency obtained at the four solid loadings.*

the four solid loadings tested, it is seen that for one way coupled simulations, the pressure drop increases slightly whereas the collection efficiency is seen to decrease marginally. On the other hand, the two way coupled simulations show a marginal increase in the pressure drop and a considerable increase in collection efficiency after the solid loadings increases beyond 0.1. It is observed that one way coupled simulations predicts similar separation capability of cyclone separators at different mass loadings. A similar collection efficiency of the three cases explains the incapability of one way coupled simulations to predict the improvements pointed out by [1]. Therefore the efficiency obtained on considering one way coupled simulations can lead to incorrect estimation of the separation capability at higher mass loadings. Whereas the separation efficiency for two-way coupling can be seen to rise with mass loadings suggesting that the simulation strategy is able to account for the sweeping effect caused by larger particles. Moreover, it is observed that the two strategies of simulation provides similar results till mass loadings of 0.1 but beyond this point, differences in the pressure drop and separation efficiency for both the simulation strategies can be observed. The efficiency obtained by conducting two-way coupled simulations at higher mass loadings is more reliable than one way coupled as the latter does not account for the forces imparted on the continuous phase by the dispersed phase. The fact that any increase in mass of the particles does not have an effect back on the flow and therefore is unable to reduce the swirl.
