Study on Mixing Characteristics of Ventilated Central Tornado Flow in

Stirring Tank

Zhai Tian 1, a, Hao Huidi 1,b and Lei Jianyong 1,c 1 School of Chemical Engineering, Northwest University, Xi’an, 710069, China

aruiruiruitang@126.com,

bhaohuidi@yahoo.com.cn,

c464889308@qq.com

Keywords: Compound Processes, Stirring Tank, Single Ventilated Pipe, Dual Ventilated Pipe; Numerical Simulation Machining.

Abstract. The research of the flow simulation in the inner stirring tank with the single pipe and dual

pipe was studied by CFD, which has demonstrated the fluid simulation effect of the single pipe and

dual pipe. It has found that the effect is better in the single pipe stirring tank than the dual pipe one.

Basing on this result, the flow simulation effect of dual pipe is better than the single one, and the result

has a certain practical significance effect on industry.

Introduction

Gas and liquid system are widely used in petrochemical, energy, chemical industry, biochemical

industry, environmental engineering and food engineering, and so on. The purpose of the dispersion

gas and liquid mixing operation is to disperse the gas which is put into the stirred tank, and increases

the two phase contact area with the unit volume of liquid, so that which can improve the mass transfer

rate, and speed up the reaction[1]. With the computational fluid dynamics (CFD) is developing

rapidly, the numerical analysis method has became one of the important methods of research in

chemical equipment industry. There have been many scholars at home and abroad have carried on the

numerical research[2-4]about agitator fluid mechanics performance and heat transfer performance.

This paper is used CFX fluid analysis and simulation method of ANSYS software to simulate the

two phase gas and liquid flow mixing characteristics of the ventilation type center dragon volume

flow mixing groove. And it qualitatively explaining the form and numbers of opened hole to effect the

mixed flow. The meaning of the design and industrial amplification application of the gas-liquid

mixing tank have a certain practical significance.

Building mode

The bottom plate of central ascending screw flow mixing tank has logarithmic spiral line, and the

main size is shown in Table 1.The type of the blade is parabola. And the diameter of single vent and

double vent pipe are all 5 mm, which is shown as Fig. 1.The physical model of the parabolic blade is

shown as Fig. 2. The physical model of logarithmic spiral line is shown as Fig. 3.

ANSYS is a general finite element analysis software. And mesh dividing is an important part of

finite element analysis with ANSYS, which is used "decomposition and overcome " strategy. It

mainly including: Automatically divided mesh method; Tetrahedral mesh method; Sweeping method;

Multi-domain method; Hex Dominant method. Thinking about the computation, mesh generation and

convergence difficulty, we use the tetrahedral mesh method to optimize the mesh number and get

unrelated solution. Dividing blade and deflector can better capture the flow characteristic. And all the

wall surface apply "Inflation" [5-7]

,which characteristics can ensure that the velocity gradient on the

wall will not change largely. The blade mesh is shown as Fig. 4 and the deflector mesh is shown as

Fig. 5.

Table 1 The main size

Stirring

tank

The

bottom plate

diameter

The bottom

plate

thickness

Shell

thickness

Shell

height

Vent pipe internal

diameter

Size（mm） 300 5 5 450 5

Advanced Materials Research Vols. 821-822 (2013) pp 1381-1385Online available since 2013/Sep/18 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.821-822.1381

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 128.173.127.127, Virginia Tech University, University Libraries, Blacksburg, USA-17/08/14,15:52:25)

(a) The single pipe (b) The dual pipe Fig.2 Parabolic impeller

Fig.1 The model diagram of mixing tank

Fig.3 Logarithmic spiral line Fig.4 Parabolic impeller mesh Fig.5 Logarithmic spiral line mesh

According to structure and fluid flow feature about the stirring tank, the boundary conditions

definition is as follows:

(1) Because the gas enter from the pipe and flow out from the Opening , the type of Opening form set

as Outlet.

(2)Stirring shaft, impeller, the surface of tank, and the baffle are all used solid wall boundary

conditions, which are all set as "WALL".

(3)For liquid use no slip boundary, and gas phase use free slip boundary conditions [8-10].

(4)The speed of stirring shaft sets as 300 r/min, and the gas velocity sets as 5 m/s

(5)The gas phase volume fraction is 1, and liquid phase volume fraction is 0.

(6)Convergence conditions: convergence residual < 10-4

,which is shown as Fig.6.

The numerical simulation

Simulation process uses control volume method, and use Rhie - Chow algorithm to avoid shaking.

The coupling of Pressure-velocity is used SIMPLEC algorithm, and the discrete convection is used

mixed - weather difference format. Convergence criterion is the RMS, which definite the mass,

momentum and turbulence equation convergence residual as 1 x 10-4

, and set up certain iteration steps

to start iterative calculation until the mass, momentum and turbulence equation convergence. This

paper adopts multiple reference frame method (MRF) to solve interaction between the movement

blade and static deflector. MRF divide the solution domain into internal rotation domain and external

static field. Internal rotation field include the movement blade and around its fluid; and solve

continuity equation and momentum equation in the rotation coordinate system ; the external static

field include the static deflector ,and shell and the pipe shell putting the body part, and in the static

coordinate system solve the continuity equation and momentum equation. The interface of the two

domain through the interpolation to realize the mass, momentum and energy exchange. Through the

simulation we can see that the given initial conditions are suitable for gas-liquid mixing

characteristics, and the convergence residual < 10-4

.

1382 Advances in Textile Engineering and Materials III

Fig.6 The result of calculation

Single vent numerical simulation and analysis.From the gas velocity vector diagram that Fig. 7we

can see, gas enter into the single passing tube at the speed of 5 m/s , flow below the blade and then

flow up along the wall, when reach the top surface, flow down along the stirring shaft. When liquid

which has a given speed for 300 r/min in parabolic blade agitation tank is in a high speed rotates, the

blade produce extrusion to the nearby fluid ,in gravity, the axial flow which flow along the axis up and

down is stronger in blade area .From the Fig. 8 we can see, the liquid form two whirlpool in the upper

and lower blade area.

Fig.7 The velocity vector of air Fig.8 The velocity vector of fluid

with the single pipe with the single pipe

Double vent numerical simulation and analysis.

In two bend place, gas also enter into at the speed of 5 m/s , from the Fig. 9 gas velocity vector

diagram. We can see, gas flows from down to up along the two bend, along the wall, when reaching

the top, then flows down along the stirring shaft and form two whirlpools.

Also given parabolic blade speed for 300 r/min, we can see from the Fig. 10 liquid velocity vector

diagram that under the high-speed rotation of the blade. the liquid form two obvious whirlpools at the

top of the blade; Also in the nearby area of the guide plate, liquid flow upward and mix with the fluid

near the blade, and finally form a certain eddy current under the blade.

Single cleaning and double vent comparison.

From the simulation results and the above analysis, we can see that for center dragon volume flow

type agitation tank, mainly axial flow is given priority , the whole fluid field is in the influence scope

of the circular flow basically, the velocity is the largest in the blade on edge blade tip place, and with

a strong axial component, the back mixing of full tank is in good condition, speed is basically uniform

and can realize a good mixing effect, be helpful to material for mixing or suspended.

Advanced Materials Research Vols. 821-822 1383

Fig.9 The velocity vector of air Fig.10 The velocity vector of fluid

with the dual pipe with the dual pipe

In the same speed of gas and blade rotation, the simulation effect and the gas-liquid mixing effect of

the double vent agitation tank are better than the single one. And under the drive of the guide plate, the

formation of vortex of the double vent is better, so that can make gas and liquid mixed more fully.

Conclutions

According to adopting the RNG k - ε turbulence model, multiple reference frame method (MRF) and

doing a numerical simulation to the mixing slot unidirectional flow, we could obtain:

(1)Center dragon volume flow mixing tank, under the action of the guidance of the deflector, forming

radial flow in tank bottom surface, and then, forming spiral dragon volume flow in the tank central

part, and then going back to the impeller area, thus forming a single cycle flow in the full tank .

(2)Single cleaning mixing tank, flow to the top and back into near the stirring shaft along the tank

wall after passing into the gas, form vortex, the liquid form vortex near the stirring shaft.

(3) Double vent mixing tank, flow upward along the tank wall after passing into gas, return to the

nearby area of the blade. Liquid produce radial flow near the stirring shaft and form two obvious

vortex.

(4) The mixing effect of the double vent agitation tank is better than the single ‘s, it can make the gas

and liquid fully mixed and has a certain practical significance to the agitation tank optimization

design and industrial amplification effect .

References

[1] Kant arci N, Borak F, Ul gen K O. Bubble column reactors[J] . Process Biochemistry, 2005, 40:

2263-2283.

[3] SCHUGREL K,TODT J,LUCKE J，et al. Gas holdup and longitudinal dispersion in different

types of multiphase reactors and their possible application for microbial processes[J].Chem Eng

Sci,1977,32( 4) : 369-375．

[4] DOSHI Y K,PANDIT A B. Effect of internals and sparger design on mixing behavior in

sectionalized bubble column[J].Chem Eng J,2005,112( 1 /2) : 117-129．

[5] ALVAR J,AL-DAHHAN MH. Liquid phase mixing in trayed bubble column reactors[J]． Chem

Eng Sci，2006，61( 6) : 1819-1835．

[6] DREHER A J,KRISHNA R. Liquid-phase backmixing in bubble columns， structured by

introduction of partition plates[J].Catalysis Today,2001，69( 1/2/3/4) : 165-170．

[7] RUBIO F C,MIR NAS,CER NGAR AMC, et al.Mixing in bubble columns: a new approach for

characterizing dispersion coefficients[J].Chemical Engineering Science，2004， 59( 20) :

4369-4376．

1384 Advances in Textile Engineering and Materials III

[8]WU Y X, ONG B C,ALDAHHAN M H. Predictions of radial gas holdup profiles in bubble

column reactors [J]. Chemical Engineering Science.2001, 56(3): 1207-1210.

[9]Yu Wang,Wei Fan, Ying Liu, Zhiyong Zeng, Xu Haoa,Ming Changa.Modeling of the

Fischer–Tropsch synthesis in slurry bubble column reactors[A].Chemical Engineering and

Processing,2008,47:222-228.

[10]Guo H, Zhao J F, Lv C P, et al. Experimental study of fuel cells performance in

short term microgravity condition (in Chinese). J Eng Thermophys, 2008, 29(5): 865—867

Advanced Materials Research Vols. 821-822 1385

Advances in Textile Engineering and Materials III 10.4028/www.scientific.net/AMR.821-822 Study on Mixing Characteristics of Ventilated Central Tornado Flow in Stirring Tank 10.4028/www.scientific.net/AMR.821-822.1381

DOI References

[8] WU Y X, ONG B C, ALDAHHAN M H. Predictions of radial gas holdup profiles in bubble column

reactors [J]. Chemical Engineering Science. 2001, 56(3): 1207-1210.

http://dx.doi.org/10.1016/S0009-2509(00)00341-9