A Dimensional optimization study on cyclone performance under the oscillating boundary condition
Graphical abstract
Introduction
Although cyclones separators have been widely used since the 19th century, they are still popular devices in industries for separating particulate matter from fluid flow. This widespread use of cyclones is related to considerable advantages (simplicity of structure, economy cost of operation, and maintenance) against some partial disadvantages (corrosion and deposition). The separation of two-phase flow with different densities occurs through swirling turbulence, so the cyclone flow analysis is quite a bit complex. Most cyclones have a tangential inlet that causes the swirling flow to enter the main cylinder [1], [2], [3]. Based on this rotation, larger particles hit the wall and are trapped at the tip of the cone part. At the bottom of the cyclone, a reverse flow is induced in which the smaller particles escape with it through the inner cylinder (vortex finder) [4], [5], [6]. The boundary from which the larger particles come out is called the dirty outlet, and the boundary where the airflow exits with the smaller particles is called the clean outlet of the cyclone. The amount of static pressure in the central area of the cyclone is less than near the peripheral wall. It causes particles to escape from the areas along the wall to the clean outlet. If this pressure distribution in the central region can be changed by applying pressure fluctuations at the boundaries, the outflow of particles from the clean cyclone boundary can be reduced. The reverse must also be done in the area of the dirty outlet; the low-pressure zone should be created by applying oscillating boundaries to pull particles to these zones and finally trap them [7]. The cyclone separator's functional parameters are collection efficiency and pressure drop. Experimental and numerical methods have extensively studied the effect of various variables such as flow, geometry, fluid, particles and external stimulate on the cyclone functional parameters as follows:
Leith and Mehta [8] examined six different cyclone designs. As a result, cyclones with smaller inlet and outlet cross-sections have optimal performance. Kim and Lee [9] studied radial parameters, including cyclone diameter and vortex finder. Their results showed that reducing the diameter of the vortex finder improves performance. Xiang and Park [10] and Chuah et al. [11] also investigated the effect of cone diameter on separation efficiency. Reducing the cone base diameter increases the cyclone separation efficiency significantly. Kenny and Gussman [12] also showed that in shorter and smaller cyclones, the performance is affected by the parameter of the inlet and outlet cross-section size, and in lengthier and larger cyclones, it is affected by the cone size. In a numerical study, Elsayed and Lacor [13] examined nine cyclones with different diameters and lengths of vortex finder. In their study, reducing the diameter of the vortex finder by 40%, increases the maximum tangential velocity by 25%. Hsiao et al. [14] also studied changes in inlet dimensions, diameter, vortex finder and cone length. Their results show that reducing the cone diameter reduces the cut diameter and increases the pressure drop. In Venkatesh et al. [15] study, using experimental design methods, geometric parameters such as outlet diameter and length and width of the inlet section have been selected so that the performance of a new cyclone is optimized compared to the classical cyclone. Wasilewski et al. [16] examined fifteen vortex finder structures with five different diameters and three different insert lengths. This change in the structure of the vortex finder significantly reduces the pressure drop and can increase the separation efficiency by up to 15%.
Cyclone flow field studies are generally limited to reporting average flow velocity. The fluctuation component of velocity has been rarely investigated [17]. Investigation of velocity fluctuations requires unsteady flow solving, which increases computation costs over a steady flow. However, unsteady flow solving is inevitably needed in the present application. Therefore, most research has been done experimentally in this field. Vekteris et al. [18] used a wave generator in the cyclone, which increased the particle separation efficiency by 10%. Elsayed and Lacor [19] have investigated the effect of the inlet diameter on the tangential velocity, which increases the maximum tangential velocity and decreases the radial velocity with the increase of the inlet diameter. In an experimental study, Gu et al. [20] introduced two dominant frequencies for fluid flow and one lower frequency for particles flow. Jia et al. [21] also compared the strength and frequency of a cyclone's primary and secondary vortices. They showed that the secondary vortex has a higher frequency and lower strength. Gao et al. [22] also introduced two dominant frequencies for the primary vortex and one frequency for the secondary vortex. Muschelknautz [23] investigated multiclones based on swirl tube cells with those based on uniflow cyclone cells regarding their efficiency at a given pressure drop. The results indicate that, under the considered operation conditions, a uniflow multiclone is more efficient than its swirl tube counterpart if space is limited onto about the volume needed by the uniflow multiclone for its optimum performance and the available pressure drop is low. Sun et al. [24] also declared that the variations of flow oscillations in the radial direction are less than that of in the axial direction. In a study, Shastri and Brar [25] noted the effect of conical section variation on the tangential fluctuating velocity component is more than the average component. Wei et al. [26] investigated different vortex finder diameters and inlet dimensions in cyclones using the Reynolds Stress Model to determine the mechanism of stagnation of axial velocity. In Their result as vortex finder diameter and inlet dimension increased, stagnation worsened and If the pressure gradient at the centerline of cyclone separator is positive, stagnation of axial velocity appears in the separation space of cyclone. Dziubak [27] showed that removal (by suction) of the accumulated dust from the dust collector causes a noticeable increase in cyclone filtration efficiency and the effective suction flow must not exceed 10–15% of the outlet flow value from the cyclone. Dziubak and Boruta [28] studied the effect of mineral dust in the air sucked in by an engine on accelerated component wear and reduction in performance. Their results show that placing an air filter in the path of the air entering the engine causes an additional pressure drop (air filter resistance increase), which leads to engine power decrease and increased fuel consumption. In a study, Vaziri et al. [29] mentioned that applying flow oscillations to the cyclone strengthens the vortex and reduces the pressure drop and efficiency. Dziubak [30] performed cyclone geometrical changes in the symmetrical inlet, the cylindrical outlet tube, and the edges of the inlet opening and showed that the possibilities of increasing the separation efficiency or decreasing the pressure drop of a cyclone by changing the main dimensions of the cyclone are limited, because any arbitrary change in one of the dimensions of an already operating cyclone may cause the opposite effect. Khalili et al. [31] investigated the effect of the height variation of the cyclone components under non-uniform flow. Their results indicate that under non-uniform flow, a decrease in the vortex finder height and an increase in the height of the cyclone's cylinder and cone minimize the pressure drop. Besides, applying oscillations along with the 30% change in the vortex finder height increases vortices power in the cone, cylinder, and vortex finder zones by 4.23, 8.99, and 21.42 percent, respectively. Also, in another studying [32] a new design based on the minimum height, optimal efficiency and pressure drop for cyclones used in the air delivery system of internal combustion engines is presented. Their results show the optimized cyclone has 21.10% less Euler number and 51.54% shorter height. Additionally, on average, it has a 12.4% higher efficiency than the classic model. Chen et al. [33] declared that the 1D3D/w cyclone is optimal for the separation efficiency of push chain concrete spraying machine and the effect of cyclone inlet size on separation performance under periodic velocity is significant.
Previous studies have extensively investigated the effect of cyclone parts size change under uniform boundary conditions. Also, vortex fluctuations in cyclone fluid flow have been seldom studied. One of the more realistic ways to consider the flow in a cyclone, especially when used as a combustion air filter for internal combustion engines, is to consider the oscillation of the flow at the cyclone boundaries. According to the frequency and amplitude of the engine flow oscillations, the cyclone boundaries are subjected to non-uniform flow. The effect of the cyclone geometry variation (diameter of vortex finder, inlet section and cone base) on performance parameters, including pressure drop and efficiency under fluctuating flow conditions of boundaries, is an innovation of this research. Also, multi-objective optimization is conducted, and the best geometry for the cyclone under non-uniform flow is introduced. Furthermore, another innovation of this research is the investigation of vortices in different cyclone areas under the influence of oscillating flow to provide equations for cyclone performance such as pressure drop and efficiency. For this purpose, after expressing the theory and equations, generating geometry of different models, and grid generation, numerical solution validation is performed. Then the results of the numerical solution and the effects of geometry variation on the performance of the cyclone under the non-uniform boundary are represented as correlations. Finally, in order to have the best cyclone performance under non-uniform, the optimal dimensions are introduced using multi-objective optimization.
Section snippets
Equations and parameters
The fluid phase is analyzed from the Eulerian viewpoint. Correction coefficients have been used for this model, leading to problems such as instability and solution divergence [[34], [35], [36]]. Recent studies show that tangential velocity fluctuations affect cyclone performance, and the RSM cannot predict the fluctuations [37]. Also, the flow oscillations applied to the boundaries are a function of time, so the LES1 model is used for continuous phase flow [38]. Some
Geometry
In this research, a base model that is a classic Stairmand cyclone has been selected. The other six models are created by changing the diameter of the vortex finder, the cone base diameter, and the hydraulic diameter of the inlet section. The extent of variations in these dimensions has particular importance. In the research of Hsiao et al. [14], the cyclone outlet diameter has been decreased by 50% and increased by 30%. Cyclone inlet dimensions have also been investigated between a decrease of
Results
Flow field and performance parameters, including pressure drop and efficiency, are investigated under uniform and non-uniform flow conditions for the seven models introduced in sections 1.3.
Conclusion
In using cyclones as air filters for internal combustion engines, the flow cannot be considered ideally uniform, and flow oscillations must be applied to the boundaries. Also, optimizing the dimensions of the cyclone can provide the necessary volume for other uses in the engine space. Seven cyclone models have been created, including a base model that is a classic Stairmand cyclone. Models 1 and 2 with 50% smaller and larger vortex finder diameter, models 3 and 4 with 50% smaller and larger
Statement of novelty and significance
In the present manuscript, due to the application of cyclones in the inlet air filter of the internal combustion engine, an external excitation is applied to the boundaries of the cyclone. Flow field study, including frequency and amplitude of fluctuation of the cyclone's primary and secondary vortex and performance parameters like efficiency and pressure drop, is performed. Also, pressure drop correlation for non-uniform flow is introduced and then, applying multi-objective optimization, the
Declaration of Competing Interest
None.
Acknowledgments
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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