Experimental study of mixing enhancement of viscous liquids in confined impinging jets reactor at low jet Reynolds numbers

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Experimental study of mixing enhancement of viscous liquids in confined impinging jets reactor at low jet Reynolds numbers Zhe-hang Shi, Wei-feng Li, Ke-jiang Du, Haifeng Liu, Fu-chen Wang

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S0009-2509(15)00554-0 http://dx.doi.org/10.1016/j.ces.2015.08.014 CES12541

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Received date: 10 June 2015 Revised date: 4 August 2015 Accepted date: 8 August 2015 Cite this article as: Zhe-hang Shi, Wei-feng Li, Ke-jiang Du, Hai-feng Liu, Fuchen Wang, Experimental study of mixing enhancement of viscous liquids in confined impinging jets reactor at low jet Reynolds numbers, Chemical Engineering Science, http://dx.doi.org/10.1016/j.ces.2015.08.014 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Experimental study of mixing enhancement of viscous liquids in confined impinging jets reactor at low jet Reynolds numbers

Zhe-hang Shia,b, Wei-feng Lia,b,*, Ke-jiang Dua,b, Hai-feng Liua,b, Fu-chen Wanga,b a

Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China

University of Science and Technology, Shanghai 200237, China b

Shanghai Engineering Research Center of Coal Gasification, East China University of Science and Technology,

Shanghai 200237, China

*

Corresponding author. Tel.: +86 21 64251418; fax: +86 21 64251312

E-mail address: [email protected] (W. Li).

Abstract Mixing performance of water and glycerol-water solution in a confined impinging jets reactor (CIJR) with and without excitation was experimentally studied using planar laser induced fluorescence (PLIF) at 100≤Re≤500. The effects of the jet Reynolds number (Re), fluid viscosity and excitation on the oscillation behaviors and mixing performance in CIJR have been qualitatively and quantitatively investigated. Results show that for Re≤100 the flow is segregated with poor mixing; for Re≥150 the flow evolves to an oscillation regime with strong mixing. Compared with the Reynolds number, the fluid viscosity has an insignificant effect on the mixing in CIJR. The mixing in CIJR at low jet Reynolds number can be effectively enhanced by excitation with low frequency, as a periodic oscillation is induced by the pulsed inflow, which further causes the continuous folding and stretching of the impingement plane. The 1

enhancement of mixing is strengthened with the increase of excitation amplitude and is weakened with the increase of excitation frequency.

Highlights •

The mixing performance in a CIJR is studied with PLIF.

•

Influences of viscosity and excitation on mixing enhancement are investigated.

•

The mixing at low Re can be effectively enhanced by excitation.

•

Compared with Re, the fluid viscosity has an insignificant impact on the mixing.

Keywords Confined impinging jets reactors, viscous fluid, mixing, excitation, oscillation, PLIF

1. Introduction Mixing is one of the most common unit operations in chemical industrial processes, which leads to the homogenization of different concentration of miscibility materials and promotes the heat transfer, mass transfer and chemical reaction of different materials or phases. Several conventional mixing methods in engineering include mechanical stirring, jet mixing and static mixing, etc. As impinging jets can effectively intensify mixing process, they have been widely used in industrial processes such as rapid reaction, the polymer or nanoparticles synthesis, combustion and gasification (Tamir, 1994; Kolodziej et al., 1982; Johnson and Prud’homme, 2003; Santos and Sultan, 2013). Especially, the confined impinging jets reactors (CIJR) and T-jets reactors, as typical impinging jets reactors, attract increasing attention of many researchers (Wood et al., 1991; Teixeira et al., 2005; Santos et al., 2002; Thomas and Ameel, 2010; Icardi et al., 2011; Sultan et al., 2012, 2013; Tu et al., 2014, 2015). 2

In recent three decades, the effects of operational conditions, flow boundaries and geometrical parameters on the flow and mixing in CIJR have been investigated by flow visualization techniques with passive tracers and numerical simulations. The results in the literature have indicated that for Re≤100 a segregated steady flow regime occurs and at higher Reynolds numbers the flow is observed to transform to a dynamic chaotic flow regime in CIJR (Tucker and Suh, 1980; Mahajan and Kirwan, 1996; Johnson and Wood, 2000; Santos et al., 2005, 2008, 2009; Li et al., 2014). Up to now the flow regime in CIJR has been intensively investigated, but the internal relationship between the oscillation behaviors and mixing performance has not been clearly revealed yet. The mixing performance in CIJR is the main concern of numerous researches. Tucker and Suh (1980) investigated the mixing quality in reaction injection molding (RIM) by flow visualization at 40500. Santos et al. (2008) used the particle image velocimetry (PIV) technique to characterize the flow field in RIM, and results indicate that the mixing in RIM occurs for Re>120. Fonte et al. (2015) studied the flow regimes and mixing performance in a confined impinging jets (CIJ) mixer with PLIF at 50
number is low. The previous studies show that for low jet Reynolds number, the flow in CIJR displays a stable separated regime with poor mixing quality (Tucker and Suh, 1980; Unger et al., 1999; Santos et al., 2008; Fonte et al., 2015). As a result, how to improve the mixing in CIJR under low jet Reynolds numbers is the key issue. Pulsation techniques have been used to cause the onset of convective mixing in some mixers. Ito and Komori (2006) experimentally investigated the promotion of the mixing efficiency in a micro-channel by a vibration technique. Their results show that the mixing quality is improved with increasing frequency of the mechanical vibration, and almost complete mixing can be accomplished as the frequency exceeds 90 Hz. Ian and Aubru (2003) numerically demonstrated that periodic pulsating fluid could effectively intensify the physical mixing of two aqueous reagents in a T-shape channel. They have found that the best mixing quality occurs when two inlets are pulsed with 180 degree phase difference. Niu et al. (2006) used pulsation flow to enhance the mixing in a microfluidic channel, and have reported that good mixing can be achieved at an optimal pulsation frequency range, beyond which the pulsation becomes ineffective. Erkoc et al. (2007) performed numerical simulations to study the effect of pulsation on the flow dynamics of a 2D laminar RIM mixer, and have found that the effect of pulsation is enhanced with increasing pulsation amplitudes. Above studies show that pulsed inflow can improve the mixing in mixers, but the effects of oscillation behaviors modulated by the pulsed inflow on the mixing performance have been unclearly revealed yet. Moreover, most studies about pulsation techniques are on the T-mixers or microfluidic channels, while the experimental study on the mixing enhancement in CIJR by active pulsation at low Reynolds number is very rare. In our recent study (Li et al., 2014), a segregated flow regime, a self-sustained deflective oscillation and a combination regime of vortex shedding and axial 4

instability in CIJR have been clearly characterized at 100≤Re≤2000. Moreover, our team (Li et al., 2015) has successfully used the excitation of pulsation inflows of the opposed jets to modulate the flow dynamics in CIJR. It is observed that for low Reynolds number the flow in CIJR under excitation is characterized by the formation of oscillation and vortices in the impingement plane. The work has pointed out that the mixing in CIJR may be enhanced by excitation, but the fluid in that work is air and the mixing quality has not been quantitatively assessed. In order to further investigate the effects of the excitation on the mixing performance in CIJR, we successfully used the PLIF to obtain 2D concentration field of water and glycerol-water solution in CIJR at 100≤Re≤500. We aim to reveal the mixing mechanism and rules of the mixing enhancement in CIJR under excitation, and exploit a method to intensify the mixing of impinging jets at low jet Reynolds numbers in CIJR for engineering applications.

2. Experimental setup and methods 2.1. Experimental setup The schematic setup is shown in Fig. 1. Two streams of liquids (indicated by Stream A and Stream B) were pumped into the reactor and then impinged on each other. The pumps were placed 6 meters above the CIJR, and the inflow pipes were long enough to eliminate the pulsation caused by pumping. Pure water and glycerol-water solution with concentration of 52.8% (w/w) and viscosity of 11.4 mPa·s were experimentally studied, respectively. The working fluids’ temperature was at 10±1 ℃. The pumps shown in Fig. 1 were mute micro-pumps with maximum flux of 166 ml/s, and their working frequencies were in the range of 50-60 Hz. The respective measurement ranges of the rotameters in Fig. 1 were 0.0067-0.067 ml/s and 5

0.042-0.42 ml/s, and the accuracies were ±2.5% of full scale deflection. The schematic diagram of the CIJR is shown in Fig. 2, and its dimension with precision of ±0.02 mm is listed in Table 1. The original point o is the intersection of the axes of nozzles and chamber. The reactor had a flat top and a conical constriction at the outlet and was made of plexiglas material. In addition, to reduce the optical distortion caused by the round chamber, a rectangular outer surface of the CIJR was processed. Stream A was dissolved with the rhodamine 6G fluorescent dye, while Stream B was non-dyed. Rhodamine 6G can emit fluorescence at a wavelength of 560 nm under the excitation of the laser at a wavelength of about 532 nm, and then the fluorescence was captured with the CCD (FlowSense, EO 4M camera) camera of the PLIF system. The digital signal was processed by the data acquisition system, and then the computer screen displays an instantaneous image of the dynamic flow field. As the gray value of the captured image is proportional to the concentration value of the solution, the instantaneous images of flow field can be converted to the concentration distribution images of the tracer by the software (Dynamic Studio) of the PLIF system. The laser source was a Nano Nd:YAG solid-state continuous laser (Dantec), and a thin slice laser source had a thickness of 0.5 mm. An orange glass filter was mounted to the CCD camera lens to filter possible reflections of the green laser. The camera resolution was 1392×1040 pixels, and the spatial resolution was 40-60 µm, and the images were acquired at the x-z plane in CIJR with a frequency of 10 Hz. Fig. 1. Schematic diagram of experimental apparatus. Fig. 2. Cross-sectional of confined impinging jets reactor.

As shown in Fig. 1, the pulsed inflow from another sink C was added to the base flow of Stream B, and was controlled by the periodic open and close of an electromagnetic valve with a very short response time (less than 5 ms). Fig. 3 shows 6

the schematic of the flux controlling of the opposed jets in CIJR with excitation, and the momentum balance of the opposed jets was adjusted according to the relationship of QA = QB + QC . The excitation amplitude (k) was defined as the ratio of the pulsed flow (QC) to the base flow (QA). The detailed excitation method can be found in our previous work (Li et al., 2015). Fig. 3. Schematic of the flux controlling of the opposed jets in CIJR with excitation.

The summary of the experimental cases of the flow visualization is listed in Table 1. In the experiment, the jet Reynolds number is defined as Re =

4 ρ QA πµ d

(1)

where QA is the volume flux of Stream A, d is the nozzle exit diameter, ρ is the liquid density and µ is the liquid dynamic viscosity. Table 1 Summary of experimental parameters.

2.2. PLIF calibration method and mixing quantification For a given tracer, the fluorescence intensity has a linear relationship with the local concentration when the tracer concentration is sufficiently low. In this experiment, several measurements were performed at different concentrations of rhodamine 6G in standard homogeneous solutions to find the linear region of the tracer concentration. For every concentration calibration, the focal length and exposure time of the CCD camera were kept constant, and 150 images were captured to calculate the time average value of fluorescence intensity of the standard homogeneous concentration. It turned out that the standard deviation of each pixel’s gray level in the whole field was less than 5%. The calibrated linear relationships between the fluorescence intensity and the rhodamine 6G concentration are shown in

7

Fig. 4. It can be seen that the linear relationships of water and glycerol-water solution are good with the concentration of rhodamine 6G below 0.20 mg/L. Hence, the adopted concentration of rhodamine 6G in this experiment was 0.15 mg/L, at which the standard deviation of the gray level was less than 2%. Fig. 4. Relationships of concentration of rhodamine 6G and fluorescence intensity.

In present work, the definition of Intensity of Segregation (IOS) was used to quantitatively assess the mixing degree in CIJR. Danckwerts (1952) defined the Intensity of Segregation as

IOS =

C ′2

(

C 1− C

(2)

)

C = C + C′

(3)

where C is the local concentration of the rhodamine 6G at each point, C is the average value of concentration, C ′ is the fluctuation value of concentration, and

C ′2 is the variance value of concentration. The IOS reflects the extent to which the concentration in the clumps departs from the mean. It has the value of 1 when segregation is complete, that is no mixing occurs, and the value of 0 when the concentration is uniform, that is perfect mixing is achieved. Each IOS value in this paper is the time average value obtained from the 100 images, and the error bars indicate the standard deviation (σ) of IOS of 100 measurements, which is defined as 2

σ IOS =

1 100 ∑ ( IOSi − IOS ) . 100 i =1

8

(4)

3. Results and discussion 3.1. Flow visualization Fig. 5 shows the typical PLIF images of water and glycerol-water solution in CIJR without excitation at 100≤Re≤500. For Re=100, the flow field in CIJR exhibits a stable segregated flow regime. The left and right fluids in the chamber are segregated uniformly, which indicates that the mixing quality of two jets is poor as mixing only occurs in the contact surface caused by molecular diffusion. It can also be observed that the concentration distribution in the upper part of the reactor is slightly homogeneous. Since vortices are formed above the impingement point by the effect of reactor dome, the mixing occurring in the contact surface of vortices is improved fractionally. For Re=150, the impingement plane in CIJR displays a deflective oscillation with a regular period and forms large scale vortices downstream of the impingement point. The concentration distribution at the outlet of CIJR becomes homogeneous because of the continuous folding and stretching of the impingement plane, which shows that the dynamics of mixing is enhanced with oscillation regime. At Re≥200, the mixing scales of the vortex and lamellar striation are reduced with increasing Reynolds numbers, and the concentration distribution in the chamber becomes more homogeneous. At high Reynolds numbers, the impinging region with high turbulent energy dissipation accelerates the breakup of the impingement plane into small lamellar structure, and good mixing quality can be achieved. The flow visualization results of water and glycerol-water solution in CIJR can be compared with those of air reported in our previous study (Li et al., 2014). Results show that for Re≤100 the flows of the three fluids exhibit a stable segregated flow regime; for 150≤Re<300 the flows display a self-sustained radial deflective oscillation 9

flow regime; for 300≤Re≤500 the flows behave a hybrid regime of radial deflective oscillation and axial oscillation. Although the viscosity of water is about 72.5 times of air, and viscosity of glycerol-water solution is about 8.7 times of water, the evolution rules of flow patterns with Reynolds numbers in CIJR are basically the same. It implies that the fluid viscosity has little impact on the flow pattern in CIJR compared with the Reynolds number. Fig. 5. PLIF images of water and glycerol-water solution in CIJR without excitation at various Reynolds numbers.

In order to improve the mixing quality at low jet Reynolds number, the concentration field of water and glycerol-water solution in CIJR under excitation was investigated by PLIF. Fig. 6 presents the typical PLIF images of water in CIJR under excitation at Re=100 and 150. It can be observed that the flow in CIJR with excitation exhibits a combination of a periodic axial oscillation and radial deflective oscillation, and forms an orderly vortex sheet downstream the impingement plane. The size of vortex structure increases with increasing excitation amplitudes, but decreases with increasing excitation frequencies. At fe≥5.0 Hz, the excitation has an insignificant impact on the flow in CIJR. At Re=100 and low excitation frequency, the concentration distribution also shows that the mixing occurs throughout the chamber. The mixing is improved remarkably at low excitation frequency by the vortex and lamellar striation, and no longer merely relies on molecular diffusion, but is highly intensified by the turbulent diffusion. The typical PLIF images of glycerol-water solution in CIJR under excitation at Re=100 and 150 are shown in Fig. 7. For Re=100 and fe=5.0 Hz, small amplitude of the axial oscillation and small scale of vortex sheet in the impingement plane can still be observed, which indicates that the range of response frequency of glycerol-water 10

solution to the excitation is extended. While at Re=150 and k=20%, the orderly vortex sheet has not been observed, which suggests the glycerol-water solution in CIJR with a self-sustained radial deflective oscillation becomes harder to be modulated than water. Fig. 6. PLIF images of water in CIJR under excitation at Re=100 and 150. Fig. 7. PLIF images of glycerol-water solution in CIJR under excitation at Re=100 and 150.

The time series of the positions of the impingement point during the excited axial oscillation of water and glycerol-water solution in CIJR at Re=100 and k=20% are presented in Fig. 8. Results show that the impingement point almost remains stable in the absence of excitation. With the pulsed inflow, the movement of the impingement plane presents a periodic axial oscillation, whose frequency is nearly equal to the excitation frequency. Fig. 8. Time series of impingement point on the axis during the excited oscillation of water and glycerol-water solution in CIJR at Re=100 and k=20%.

Fig. 9 shows the oscillation amplitudes (A) of the excited axial oscillation of the impingement plane of water and glycerol-water solution versus excitation frequency at Re=100 and different excitation amplitudes, in which each value is the average analyzed from 10 oscillation periods, and the error bars mark the standard deviations. It can be observed that the excited axial oscillation amplitude decreases with the increase of excitation frequency and rises with the increase of excitation amplitude. The excited axial oscillation amplitude of glycerol-water solution is larger than that of water under the same excitation condition. Fig. 9. Amplitudes of the excited axial oscillation of the impingement plane in CIJR at Re=100.

In our previous study, Li et al. (2015) have proposed a semiempirical formula to predict the axial oscillation amplitude of the impingement plane in CIJR with 11

excitation as follows

A 1 = a⋅ 1 2 fe k ⋅ u0 2 where

(5)

1 k ⋅ u0 is the instantaneous velocity difference of the opposed jets in CIJR 2

under excitation, and a is the ratio of the movement speed of the impingement plane to the instantaneous velocity difference of the opposed jets. It can be deduced from Eq. 5 that the amplitude of the axial oscillation caused by excitation in CIJR is inversely proportional to the excitation frequency, but is proportional to the jet velocity and the excitation amplitude, which can explain the results in Fig. 9. The excited axial oscillation amplitude of glycerol-water solution is greater than that of water, because the jet velocity increases with the fluid viscosity at identical Reynolds numbers. The axial oscillation amplitudes of water, glycerol-water solution and air under different excitation conditions are normalized by

1 k ⋅ u0 , and the normalized 2

amplitudes are fitted by Eq. 5, as shown in Fig. 10. Results reveal that the normalized excited axial oscillation amplitudes of three kinds of fluids are overlapped to three curves. The fitted value of constant a of water is 0.7750, and is 0.2024 for glycerol-water solution, which indicates that the excited axial oscillation amplitude of glycerol-water solution is smaller than that of water under the identical velocity difference of the opposed jets and excitation conditions. Since the movement resistance of impingement plane with around fluid in reactor during the axial oscillation increases with the fluid viscosity, the normalized excited axial oscillation amplitude of glycerol-water solution decreases consequently. Fig. 10. Normalized amplitudes of the excited axial oscillation of the impingement plane in CIJR at Re=100.

12

3.2. Mixing quantification For the water and glycerol-water solution in CIJR without excitation, the time average IOS values in the line of z/D=-1.5 and y/D=0 of the chamber are shown in Fig. 11. It can be seen that for Re=100 the IOS values of water and glycerol-water solution are all greater than 0.5, so the mixing quality in CIJR with a segregated flow regime is poor. When Re is increased to 150, the IOS values of water and glycerol-water solution both sharply decrease to about 0.02, which shows that good mixing is achieved. This sharp decrease of IOS is associated with the flow regime transition from a steady segregated flow to an oscillation regime, with which the mixing structures of vortex and lamellar striation generate in the chamber, as seen in Fig. 5. For higher Reynolds numbers, the IOS values decrease slowly. These results indicate that no matter for the water or glycerol-water solution, for Re≥150 the high efficiency of mixing quality in CIJR is obtained just depending on the hydrodynamics of the impinging jets. While for Re≤100, satisfying mixing quality in CIJR can’t be achieved simply relying on the hydrodynamics, so it is quite necessary to develop an effective mean to promote the mixing at low jet Reynolds number. Fig. 11. Time average Intensity of Segregation in the line of z/D=-1.5 and y/D=0 in CIJR.

It is generally acknowledged that the mixing degree becomes worse with the increase of fluid viscosity. However, although the viscosity of glycerol-water solution is considerably larger than water, it can be observed from the IOS values in Fig. 11 that the mixing quality of water and glycerol-water solution is roughly equal. Results indicate that, compared with the Reynolds number, the fluid viscosity has an insignificant effect on the mixing in CIJR. It should be pointed out that for Re=150 and 200, the IOS values of glycerol-water solution are slightly smaller than that of 13

water, and the reason is maybe related to the oscillation frequency in CIJR. Experimental results reveal that the self-sustained deflective oscillation frequencies of glycerol-water solution are 2.3 Hz and 3.6 Hz, but the frequencies of water are 0.6 Hz and 1.1 Hz. The mixing process in CIJR can be accelerated and intensified by faster oscillation. It should also be noted that the curve of glycerol-water solution presents a minimum at Re=200. However, the IOS values at Re=200, 300 and 500 and the differences between them are very small, which indicates that the minimum at Re=200 results from the statistical error. Fig. 12 shows for Re=100 the dimensionless concentration distribution of water and glycerol-water solution at the x-axis and the line of z/D=-1.5 and y/D=0 of the chamber. For water and glycerol-water solution without excitation, the dimensionless concentration distribution at the inlet and outlet does not change basically, while the concentration distribution at the outlet becomes relatively uniform for excitation of k=20%, fe=0.5 Hz. The evolution of the time average IOS values of water and glycerol-water solution along the z-axis of mixing chamber is shown in Fig. 13. It can be observed that the maximum IOS value appears close to the impingement point. At Re=100, the IOS values in the absence of excitation are almost larger than 0.5 after the jets impingement point, while decrease remarkably under excitation. At Re=150, the IOS values with or without excitation decrease steeply after the impingement point. It should also be noticed that the IOS values under excitation are smaller than those without excitation at z/D≥-0.8, which shows the mixing becomes more rapid due to the excitation. Fig. 12. Dimensionless concentration distribution at the x-axis and the line of z/D=-1.5 and y/D=0 in CIJR with Re=100. (a) Water (b) Glycerol-water solution

14

Fig. 13. Evolution of the time average Intensity of Segregation along the z-axis of the mixing chamber.

The time average IOS values versus excitation frequency at z/D=-1.5, y/D=0 and k=20% are shown in Fig. 14. For Re=100 and fe=0.5Hz, the IOS values of water and glycerol-water solution both sharply decrease from approximate 0.5 to about 0.03 and 0.05 respectively, while with the increase of excitation frequency, the IOS values of water and glycerol-water solution increase first and then reach a level roughly equal to those without excitation. One can also be observed from Fig. 14 that for Re=100, the extent of mixing improvement of glycerol is slightly smaller than that of water. This result reveals that the effect of excitation will be slightly weakened as the fluid viscosity increases. For Re=150 and 200, the IOS values in CIJR with or without excitation are all less than 0.03, which indicates that the excitation has insignificant effect on the ultimate mixing degree. Fig. 14. Time average Intensity of Segregation at z/D=-1.5, y/D=0 and k=20%.

Fig. 15 shows the time average IOS values at z/D=-1.5, y/D=0, Re=100 and various excitation amplitudes. It can be seen that the IOS values of water and glycerol-water solution both decrease with increasing excitation amplitudes. However, for fe≥5.0 Hz the decrease of IOS values is not remarkable and the ultimate mixing quality is unsatisfactory. (a) Water (b) Glycerol-water solution Fig. 15. Time average Intensity of Segregation at z/D=-1.5, y/D=0 and Re=100.

Fig. 16 shows the dimensionless size of large-scale vortex structure of glycerol-water solution in CIJR with excitation and the corresponding IOS values at z/D=-1.5, y/D=0 and Re=100 versus excitation frequency. The definition of the size of vortex structure (L) is shown in Fig. 16. It can be observed that the size of vortex 15

structure increases and the IOS values decrease with decreasing excitation frequencies or increasing excitation amplitudes. Above results demonstrate that the mixing performance can be effectively promoted by the large-scale vortex structure produced in CIJR under excitation. Fig. 16. Dimensionless size of large-scale vortex structure of glycerol-water solution in CIJR with excitation and the corresponding IOS at z/D=-1.5, y/D=0 and Re=100.

Results indicate that the mixing in CIJR can be effectively enhanced at low excitation frequency and jet Reynolds number, as a combination of a periodic axial oscillation and radial deflective oscillation dynamic behavior is induced by excitation. The oscillation further causes the continuous folding and stretching of the impingement plane and the mixing structures of vortex and lamellar striation. It can be concluded that the improvement of mixing in CIJR is strengthened with the increase of excitation amplitude, and is weakened since the amplitude of the excited axial oscillation and the size of the vortex structure decrease with the increase of excitation frequency. Due to the difference of experimental conditions and pulsation methods, a direct comparison of current study with the very limited results in the literature is difficult. Niu et al. (2006) also have observed that the mixing quality in a microfluidic channel is increased with pulsation amplitudes, and high-efficiency mixing can be accomplished at an optimal pulsation frequency range, beyond which poor mixing occurs. In current work, better mixing in CIJR is accomplished at low excitation frequency, which is different from the result reported by Ito and Komori (2006) that good mixing in a microchannel can be achieved at high mechanical vibration frequency. This discrepancy is maybe due to the difference of the excitation method and vibration technique, for the flow in the microchannel in the literature is disturbed more intensely with the increase of vibration frequency, while the excited oscillation 16

amplitude in CIJR decreases with the increase of excitation frequency in present work.

4. Conclusions In current study, the mixing performance in CIJR under excitation for 100≤Re≤500 was experimentally investigated using PLIF. The effects of the jet Reynolds number, fluid viscosity and excitation on the mixing performance in CIJR were qualitatively and quantitatively analyzed. Although the viscosities of the air, water and glycerol differ remarkably, the evolution rules of flow patterns with Reynolds numbers in CIJR are basically the same. The oscillation regime in CIJR determined by the jet Reynolds number is a key factor to the mixing quality. The fluid viscosity has an insignificant effect on the mixing in CIJR compared with the jet Reynolds number. The mixing will not be hindered with the increase of the inflow fluid viscosity at identical Reynolds numbers. The mixing in CIJR at low jet Reynolds number can be effectively improved by excitation. The rules of the mixing enhancement and mixing mechanism in CIJR under excitation are revealed. As a periodic oscillation behavior in CIJR is induced under excitation, the impingement plane folds and stretches, the mixing scales reduce, and then the contact surface for molecular diffusion increases. Current paper demonstrates an excitation method of mixing intensification for the impinging jets in CIJR at low Reynolds number, and also provides some principles to enhance the mixing in CIJR for engineering applications.

17

Acknowledgments This study was supported by the National Natural Science Foundation of China (91434130).

Nomenclature Roman letters a

ratio of the movement speed of the impingement plane to the instantaneous velocity difference of the opposed jets [dimensionless]

A

oscillation amplitude [mm]

C

local concentration of the rhodamine 6G at each point [mg/L]

C0 original concentration of rhodamine 6G [mg/L] d

nozzle diameter [mm]

D

reactor chamber diameter [mm]

fe

excitation frequency [Hz]

h

nozzle center location [mm]

H

reactor chamber height [mm]

i

the number of images

I

fluorescence intensity

k

excitation amplitude [dimensionless]

L

size of vortex structure [mm]

QA

base flow rate of Stream A [m3/s]

QB

base flow rate of Stream B [m3/s]

18

QC

pulsed flow rate [m3/s]

Re

Reynolds number [dimensionless]

t

time [s]

u0

bulk mean exit velocity of the nozzle [m/s]

w

weight of working fluid [g]

x, y, z three-dimensional coordinates Greek letters

µ

dynamic viscosity [Pa·s]

ρ

density [kg/m3]

σ

standard deviation

Abbreviation CIJR

confined impinging jets reactor

CIJ

confined impinging jets

IOS

Intensity of Segregation

PIV particle image velocimetry PLIF

planar laser induced fluorescence

RIM

reaction injection molding

Superscripts ´

￣

fluctuating term average

19

References Danckwerts, P.V., 1952. The definition and measurement of some characteristics of mixtures. Appl. Sci. Res. 3, 279–296. Erkoç, E., Santos, R.J., Dias, M.M., Lopes, J.C.B., 2007. Enhancing the RIM process with pulsation technology: CFD study. Proceedings of European congress of chemical engineering (ECCE-6) Copenhagen, September, pp. 16–20. Fonte, C.P., Sultan, M.A., Santos, R.J., Dias, M.M., Lopes, J.C.B., 2015. Flow imbalance and Reynolds number impact on mixing in Confined Impinging Jets. Chem. Eng. J. 260, 316-330.

Ian, G., Aubru, N., 2003. Enhancement of microfluidic mixing using time pulsing. Lab on a Chip. 3, 114-120. Icardi, M., Gavi, E., Marchisio, D.L., Barresi, A.A., Olsen, M.G., Fox, R.O., Lakehal, D., 2011. Investigation of the flow field in a three-dimensional Confined Impinging Jets Reactor by means of microPIV and DNS. Chem. Eng. J. 166, 294–305. Ito, Y., Komori, S., 2006. A vibration technique for promoting liquid mixing and reaction in a microchannel. AIChE J. 52, 3011–3017. Johnson, B.K., Prud’homme, R.K., 2003. Chemical processing and micromixing in confined impinging jets. AIChE J. 49, 2264–2282. Johnson, D.A., Wood, P., 2000. Self-sustained oscillations in opposed impinging jets in an enclosure. Can. J. Chem. Eng. 78, 867–875. Kolodziej, P., Macosko, C.W., Ranz, W.E., 1982. The influence of impingement mixing on striation thickness distribution and properties in fast polyurethane polymerisation. Polym. Eng. Sci. 22, 388–392. Li, W.F., Du, K.J., Yu, G.S., Liu, H.F., Wang, F.C., 2014. Experimental study of flow 20

regimes in three-dimensional confined impinging jets reactor. AIChE J. 60, 3033–3045. Li, W.F., Qian, W.W., Yu, G.S., Liu, H.F., Wang, F.C., 2015. Experimental study of oscillation behaviors in confined impinging jets reactor under excitation. AIChE J. 61, 333-341. Macosko, C. W., 1989. RIM Fundamentals of Reaction Injection Molding. Hanser ed. Munich. Mahajan, A.J., Kirwan, D.J., 1996. Micromixng effects in a two-impinging-jets precipitator. AIChE J. 42, 1801-1814. Niu, X., Liu, L., Wen, W., Sheng, P., 2006. Hybrid approach to high-frequency microfluidic mixing. Phys. Rev. Lett. 97, 044501. Santos, R.J., Teixeira, A.M., Costa, M.R.P.F.N., Lopes, J.C.B., 2002. Operational and design study of RIM machines. Int. Polym. Proc. 17, 387–394. Santos, R.J., Teixeira, A.M., Lopes, J.C.B., 2005. Study of mixing and chemical reaction in RIM. Chem. Eng. Sci. 60, 2381–2398. Santos, R.J., Erkoç, E., Dias, M.M., Teixeira, A.M., Lopes, J.C.B., 2008. Hydrodynamics of the mixing chamber in RIM: PIV flow-field characterization. AIChE J. 54, 1153–1163. Santos, R.J., Erkoç, E., Dias, M.M., Lopes, J.C.B., 2009. Dynamic behavior of the flow field in a RIM machine mixing chamber. AIChE J. 55, 1338–1351. Santos, R.J., Sultan, M.A., 2013. State of the Art of Mini/Micro Jet Reactors. Chem. Eng. Technol. 36, 937–949. Sultan, M.A., Fonte, C.P., Dias, M.M., Lopes, J.C.B., Santos, R.J., 2012. Experimental study of flow regime and mixing in T-jets mixers. Chem. Eng. Sci. 73, 388–399. 21

Sultan, M.A., Krupa, K., Fonte, C.P., Nunes, M.I., Dias, M.M., Lopes, J.C.B., Santos, R.J., 2013. High-throughput T-jets mixers: an innovative scale-up concept. Chem. Eng. Technol. 36, 323–331. Tamir, A., 1994. Impinging Streams Reactors: Fundamentals and Applications. Amsterdam: Elsevier. Teixeira, A.M., Santos, R.J., Costa, M.R.P.F.N., Lopes, J.C.B., 2005. Hydrodynamics of the mixing head in RIM: LDA flow-field characterization. AIChE J. 51, 1608–1619. Thomas, S., Ameel, T.A., 2010. An experimental investigation of moderate Reynolds number flow in a T-Channel. Exp. Fluids 49, 1231–1245. Tucker, C.L., Suh, N.P., 1980. Mixing for reaction injection molding I: Impingement mixing of liquids. Polym. Eng. Sci. 20, 875-886. Tu, G.Y., Li, W.F., Du, K.J., Wang, F.C., 2014. Experimental investigation of deflecting oscillation in T-jets reactor. Chem. Eng. Sci. 116, 734–744. Tu, G.Y., Li, W.F., Qian, W.W., Shi, Z.H., Liu, H.F., Wang, F.C., 2015. Experimental study on oscillation behaviors in T-jets reactor with excitation. Chem. Eng. Sci. 134, 67–75. Unger, D.R., Muzzio, F.J., 1999. Laser-induced fluorescence technique for the quantification of mixing in impinging jets. AIChE J. 45, 2477–2486. Wood, P., Hrymak, A.N., Yeo, R., Johnson, D.A., Tyagi, A., 1991. Experimental and computational studies of the fluid mechanics in an opposed jet mixing head. Phys. Fluids A. 3, 1362–1368.

22

23

Fig. 1. Schematic diagram of experimental apparatus. Stream C

Sink C

Excitation unit

Water-pump B

Water-pump A

Rotameter A

Figures

d

h= 0.5D H= 2D

Fig. 2. Cross-sectional of confined impinging jets reactor.

24

Q QC

QA QB

0

t

Fig. 3. Schematic of the flux controlling of the opposed jets in CIJR with excitation.

6000

I (Gray value )

5000

Water Glycerol-water

4000 3000 2000 1000 0 0.00

0.05

0.10 C0 (mg/L)

0.15

0.20

Fig. 4. Relationships of concentration of rhodamine 6G and fluorescence intensity.

25

Re=100

Re=150

Re=200

Re=300

Re=500

Water

Glycerolwater

Fig. 5. PLIF images of water and glycerol-water solution in CIJR without excitation at various Reynolds numbers.

26

Re=100 fe (Hz)

0.5

1.0

1.7

2.5

5.0

10.0

20.0

k=10%

k=20%

k=40%

Re=150

k=20%

Fig. 6. PLIF images of water in CIJR under excitation at Re=100 and 150.

27

Re=100 fe (Hz)

0.5

1.0

1.7

2.5

5.0

10.0

20.0

k=10%

k=20%

k=40%

Re=150

k=20%

Fig. 7. PLIF images of glycerol-water solution in CIJR under excitation at Re=100 and 150.

28

fe

Water

Glycerol-water

(Hz)

0

0.5

1.0

1.7

2.5

Fig. 8. Time series of impingement point on the axis during the excited oscillation of water and glycerol-water solution in CIJR at Re=100 and k=20%.

29

0.7 Water, k=10% Water, k=20% Water, k=40% Glycerol-water, k=10% Glycerol-water, k=20% Glycerol-water, k=40%

0.6 0.5

A/D

0.4 0.3 0.2 0.1 0.0

0

1

2

3 fe (Hz)

4

5

6

Fig. 9. Amplitudes of the excited axial oscillation of the impingement plane in CIJR at Re=100.

30

1.0 A 1 = 0.3875 1 fe u0 ⋅ k 2

0.8

A 1 u0 ⋅ k 2

0.6

A 1 = 0.3362 1 fe u0 ⋅ k 2

0.4

A 1 = 0.1012 1 fe u ⋅k 2 0

0.2

0.0

Water,k =10% Water,k =20% Water,k =40% Glycerol-water, k=10% Glycerol-water, k=20% Glycerol-water, k=40% Air, k =10% Air, k =20% Li(2014) Air, k =40%

0

1

2

3 fe (Hz)

4

5

6

Fig. 10. Normalized amplitudes of the excited axial oscillation of the impingement plane in CIJR at Re=100.

Fig. 11. Time average Intensity of Segregation in the line of z/D=-1.5 and y/D=0 in CIJR.

31

C/C0 Fig. 12. Dimensionless concentration distribution at the x-axis and the line of z/D=-1.5 and y/D=0 in CIJR with Re=100.

32

(a) Water

(b) Glycerol-water solution Fig. 13. Evolution of the time average Intensity of Segregation along the z-axis of the mixing chamber.

33

Fig. 14. Time average Intensity of Segregation at z/D=-1.5, y/D=0 and k=20%.

34

(a) Water

(b) Glycerol-water solution Fig. 15. Time average Intensity of Segregation at z/D=-1.5, y/D=0 and Re=100.

35

1.0

0.7

0.9

0.6

0.8

0.5

L/D

0.4

IOS

0.7

L

0.6

0.3

0.5

0.2

0.4

0.1

0.3

0.0 0

5

10

fe (Hz)

0.2

15

20 k=10% k=20% k=40%

0.1 0.0 0

5

10

15

20

fe (Hz) Fig. 16. Dimensionless size of large-scale vortex structure of glycerol-water solution in CIJR with excitation and the corresponding IOS at z/D=-1.5, y/D=0 and Re=100.

Table Table 1 Summary of experimental parameters.

Fluid media Water Glycerol-water

d (mm)

Dimensionless chamber diameter D/d

Jet Reynolds number Re

4.20

4.78

100-500

Nozzle diameter

36

Excitation frequency

Excitation amplitude

fe (Hz)

k (%)

0-20

0-40