Comparison of Different Hydrodynamic Characteristics of Air-Water System using Dissimilar Motionless Mixers

The hydrodynamic characteristics of mixing fluids are always the points to consider in improvement of their mixing quality especially using motionless mixers normally stated as “Static Mixers”. Motionless mixing technique was adopted for Air-Water system with the advantage of negligible power consumption over dynamic mixers. New static element “Baffle-Type Static Element” was introduced and kept under the experimentations with which different hydrodynamic characteristics were experimented and were compared to those of already used in recent studies. Dissolved Oxygen (DO) content, static mixer geometry (i.e., Baffle, Blade, Wheel, Plate and Needle), mixing fluids flow rates were chosen as variables and selected in this content as rate of mass transfer study which founds out to be significant using Baffle-Type static element. volumetric mass transfer was also achieved at higher scale which gives a clear indication of increase the mass transfer coefficient in between the comparison of Baffle-Type element and other mentioned elements. Pressure droplet and depletion in air bubble size across static elements were visually perceived using Hg-Manometer and still photography respectively. A mathematical model was also developed portraying the air bubble diameter at different flow rates for this system. Other hydrodynamics like higher DO Content, less power consumption were also found to be more advantageous for Baffle-Type static element. The novelty of this study was to introduce the advantages of using Baffle-Type static element as compared to the hydrodynamic characteristics of other elements available in the literature.


I. INTRODUCTION
Mixing hydrodynamics always comes along with the 'Power Consumption' as a control parameter which normally influencing the 'Efficiency of the System'. The most efficient system consumes the maximum internal kinetic energies of mixing fluids with very less use of external power source. Motionless mixers normally stated as 'Static Mixers' are counted under those systems which utilize the kinetic energy of mixing fluids. These mixers once used in replacement of power mixers becomes more economical in the process industries of blending, dispersion and many others which does not require any complex designing too. The capability of the usage of these static mixers at higher temperatures makes them more eligible to be used in many major applications of process industries like extruding, spinning etc. for the homogenization of polymers at elevated temperatures. The continuous advancement in static mixing methodology has been witnessed since 1965 but there is an exponential advancement in this field from 2007. In 2007 an author and researcher studied and experimented different design modifications to different static elements used for turbulent mixing of the fluids [1].
In the year 2009 a group of researchers studied and experimented Liquid-Liquid dispersion coefficient and pressure drop by mixing the fluids at higher Reynold's number using 'Sulzer SMX Static Mixer' [2]. An author along with his associates investigated the Gas particle dispersion and the effect of Gas particle velocity on it, the year was 2010. Overall, he presented the tabular form data in which different hydrodynamics characteristics for Air-Water system was tabulated using 'SMX Static Mixer' [3]. Experimental investigation was done by some researchers in 2011 to determine the volumetric mass transfer, pressure drop and interface area in a multi-scale micromixer with complex geometry [4]. A researcher Theron along with fellow companions in 2011 compared the emulsification characteristics of different static elements used for mixing the fluids under turbulent flow regime [5]. Researchers in 2013 introduced spacers for static mixers for spiral wound modules [6]. An author and researcher along with his associates in 2015 investigated the mass transfer and pressure drop with gas used as a mixing fluid for continuous phase and determines other hydrodynamic characteristics using static mixing methodology [7].

II. EXPERIMENTAL SETUP
This study was carried out in order to increase the mixing performance of Air-Water system. Hydrodynamic characteristics study was done on an experimental setup with the introduction of 'Baffle Type' static element inside the static mixing setup and same were compared by using the other static elements already in use in recent studies (i.e. Blade, Wheel, Plate and Needle) as shown in Figure 1. The experimental setup, as shown in Figure 2, consists of very basic elements of 'Static Mixing System' which contains the main pipe of Perspex factual of 80 mm diameter. Inside the main tube the static elements were placed at equal distance in order to study the different hydrodynamic parameters. Hg-Manometer was used in between the sampling points to measure the reduction in pressure across the static element. Circulation of water through the complete cycle was done by a centrifugal pump for which the flow rate was measured through a water rotameter (measuring capacity from 1000 to 4400 gal/hr). Compressed air from the leading edge of the tube was induced inside the system in the form of air bubbles for which the flowrate of the air was measured through an 'Air Flowmeter' (measuring capacity from 5 to 20 lit/min). A bubble visualizing cell installed at the tail end of the main tube was used in order to have the air bubble size depicting the content of mixing done inside the tube through static elements. The main element designed and used in this study was Baffle-Type static element, as shown in Figure 3, in which the 5 holes each of 12.7 mm, equidistant from each other over the circumference of the element was taken into experimentation for enhancing the hydrodynamic characteristics. And then was further compared for hydrodynamic characteristics with other geometry of elements as shown in Figure 1. The hole diameter was decided after the series of experimentation with variable diameters after which 12.7 mm diameter was finalized in which the maximum dispersion of air bubbles was recorded which depicts the rate of mass transfer. Through experimentations, below mentioned hydrodynamic characteristics of the Air-Water system were analyzed: i.
Pressure drop of mixing fluid across each element which indicates the extent of reduction in air bubble size inside water. ii.
Air bubbles sizes at different flow rates of fluid using different Static elements. "The bubbles were captured in the Bubble Visualizing Cell" as shown in figure 2 after which the average size of the bubble was measured against variating flow rates of the fluids. iii.
Mass Transfer Coefficient (KLa) of the system experimented at different velocities of the mixing fluids and generated the system equation of the KLa using the sets of data generated through experiments.

A. Pressure Reduction Across the Static Elements
One of the most influencing parameters considered for hydrodynamics is reduction in pressure of mixing fluid across the static elements. As shown in Figure 2, a Hg-Manometer installed in between the 1 st and 3 rd element was used to calculate the pressure drop across 3 static elements. From the series of experiment sets conducted, the direct influence of flow rate of stream was observed

Comparison of Different Hydrodynamic Characteristics of Air-Water System using Dissimilar Motionless Mixers
3 with the reduction in pressure. Figure 4 shows a comparative study of drop in pressure of the flow using Baffle-Type static element with the other systems profound in literature. From the results it is evident that the Baffle-Type element causes more pressure drop across its sides which clearly indicates more mass transfer on both of its sides due to its symmetrical geometry.

B. Dissolved Oxygen (DO) Content / Air Bubble Size:
The visual representation of mass transferred from secondary fluid to primary fluid is considered as the content of Dissolved Oxygen (DO). The oxygen content dissolved into the fluid was determined through the same as reported by author Turunen, and associates [11]. Fluid samples were collected through the sampling points as shown in Figure 2, and 'Chemical Analysis' technique was applied on them to determine the DO content of the flowing fluid. A comparative study was done between Baffle-Type static element with other static elements for DO content as shown in Figure 5. From Figure 5, it is evident that the DO content for Baffle-Type static element comes out to be far more than other elements used in this study. The higher DO content for Baffle-Type element is due to the rapid bubble breakage on both sides of the Baffle-Type element as the element is of symmetrical geometry in comparison with other elements. In addition to the symmetrical geometry, the other factors for higher DO content is because of more area available for air bubble to disperse into the water as the element contains 5 holes equally spaced from each other. As discussed before, the static elements were experimented by installing them at right angle (90 o ) with pipe diameter, however, the DO content of the system can be varied at different angles of the static element. For Baffle-Type static element, the DO content can be increased if the elements are installed at 72 o with the pipe diameter. Table 1 shows the optimal combinations of flow rates to attain the optimum DO content within the system specifications.  Likewise, the air bubble size diameters were measured, as the visual representation of higher DO content is the depletion in air bubble size. As the DO content increases in the system, the air bubble diameter tends to reduce. Air bubbles were captured in bubble visualizing cell installed at the tail end of the main pipe, as shown in Figure 2, from the data collected through the number of still images it was apparent that the air bubble diameter reduces as the flow rate of the fluids are increased. Figure 6 shows the same, in which the air bubble diameters are shown in reducing trend with the increase in flow rate of the fluids using Baffle-Type static element. The set of data of air bubble size using Baffle-Type element was compared with that of the system used by some researchers. Figure 7 showing the comparison of bubble size of the air using Baffle-Type static element with that of the system adopted by proposed system and depicting that the Baffle-Type element reduces the air bubble size more than that of other due to the higher DO content and mass transfer [12].

C. Mass Transfer Coefficient (KLa)
The extent of mixing of one fluid into another is normally categorized as Mass Transfer Coefficient or KLa. To determine the same for the system studied, by Turunen, and associates. The method was adopted in which secondary fluid was nitrogen with water as primary fluid. Generally, the reduction in air bubble diameter tends to the increase in mass transfer between the fluids. So, the general parameters of the system like velocities of water and air founds to be the impacting on the Coefficient Of Mass Transfer or KLa Eq. (1) [13].
K a = K x x (1) Figure 8: Comparative Study of Mass Transfer Coefficient (KLa) using Baffle-Type Element with that of proposed system [14] During the manipulation of the data collected through the series of experiments conducted with different velocities of water and air, the system model of KLa comes out be the following: K a = 12.20 * 10 −4 x 1.51 x −0.26 Likewise DO content and air bubble diameter, KLa was also compared with that of the system developed by Heyouni and associates as shown in Figure 8, which clearly shows the higher KLa as compared to that of proposed system [14] and [15].

D. Approximate Bubble Diameter-A Mathematical Hint
As discussed above also, the bubble size depends amongst various parameters on mass transfer in dimensionless number as proposed by Legrand and associates shown by the following equation: Where: Reynolds number = This standard equation (A) was used to generate the 'System Equation' of bubble diameter in which constants i.e., K, a and b were determined at different optimum Reynold's numbers and Weber's numbers at different velocities of air and water.
Taking " " on both sides: Where: , and are constants to be found from experiments. = Approximate diameter of bubble measured through experiments.
= Nozzle diameter through which air is flowing.
First set of data was set in which the flow rates of the water were set as 3000, 3500 and 3900 gal/hr. and air flow rates were set as 10, 15 and 20 lit/min. Using these flow rates of water and air, Weber numbers (We) and Reynolds numbers (Re) were calculated to generate the below equations Eq. [4], Eq. [5] and Eq. [6]. The bubble diameters "d" was taken the same as measured from the "Bubble Visualizing Cell" at these flow rates of water and air. Below are some mathematical calculations to determine the "System Following deductions were carried out from this study: Pressure reduction across the Baffle-Type element was observed more in comparison with other elements due to more mass transfer of air bubbles into the water. Due to symmetrical geometry of Baffle-Type static element, the reduction in air bubble size was more as compared with that of the use of other elements. This benefit of symmetrical geometry of baffle element leads towards the usage of less elements (3 elements of Baffle elements as compared to 42 elements in Lightnin Static Mixer). The rapid reduction in air bubble size clearly depicts the increase of DO content, for which, when the Baffle-Type element was compared with other static elements, it comes out to be significantly more (4.12 mg/lit) with optimal combination of flow rates of fluids i.e. 3900 gal/hr (for water) and 20 lit/min (for air). Mathematical models were developed for KLa and reduction in air bubble size diameter (d/dp). The higher rate of DO content with the use of Baffle-Type static element add one more benefit of this system i.e., higher rate of KLa as compared with that of the systems available in literature.

Acknowledgment
I would like to thanks almighty Allah first and then the management of Sharif College of Engineering and Technology, Lahore, Pakistan, for their support and their assistance throughout this study.

Authors Contributions
The author, Mazhar Hussain, confirms sole responsibility i.e., study conception and design, data collection, analysis and interpretation of results, manuscript preparation and technical implementation.

Conflict of Interest
The author declares no conflict of interest and confirms that this work is original and has not been plagiarized from any other source, whether electronic or print media. The information obtained from all of the sources is properly recognized and cited below.

Data Availability Statement
The testing data is available in this paper.

Funding
This research work is not funded by any agency/project.