TERMOTEHNICA 2/2012 46

A COMPARATIVE STUDY BETWEEN A FIXED AND A MOBILE FINE BUBBLE GENERATOR

Prof., PhD Eng. Nicolae BĂRAN, Prof., PhD Eng. Octavian DONŢU , PhD Stud., Eng. George Lucian IONESCU, PhD Stud., Eng. Ionela-Mihaela CĂLUŞARU

„POLITEHNICA“ UNIVERSITY OF BUCHAREST, ROMANIA

Rezumat. Prima parte a lucrării prezintă soluția constructivă pentru un generator de bule fine, precum și standul experimental construit pentru testarea acestuia. Sunt prezentate rezultatele experimentale privind creșterea concentrației de oxigen dizolvat în apă până la o valoare specificată. Același tip de generator de bule fine este, ulterior deplasat cu o viteză de 0,2 m/s într-un rezervor de apă; se poate remarca faptul că timpul necesar pentru atingerea aceleiași concentrație de oxigen dizolvat în apă este de patru ori mai redus. Cuvinte cheie: oxigenarea apei, generator de bule fine..

Abstract. The first part of this paper presents the constructive solution of a fine bubble generator as well as the experimental stand constructed for its testing. The experimental results regarding the increase of the concentration of oxygen dissolved in water up to a specified value are presented. The same type of fine bubble generator is subsequently displaced with a speed of 0.2 m/s, in a water tank ; it can be remarked that the time to reach the same concentration of oxygen dissolved into water is four times reduced. Keywords: water oxygenation, fine bubble generator.

1. INTRODUCTION

The oxygenation of waters is a fundamental pro-cess of thermodynamics and specifically it represents a mass transfer process between air and water. This process is based on the oxygen transfer from air into water or the transfer of the pure oxygen in a volume of water.

The water is subjected to an aeration process in a tank, lake or any other water with an open surface, as long as this water has an oxygen deficit, „greater or smaller”, compared to the saturation state.

The pneumatic aeration has clearly superior performances compared to the other „mechanical” aeration systems. These performances (the aeration process performance, the efficiency of aeration) are influenced by the constructive solution of the devices that disperse air into water. The devices that generate fine bubbles are the porous distri-butors endowed with porous or rubber membranes etc.

These devices have the following disadvantages: – they do not assure a uniform distribution of

the nozzles trough which the air can enter in water; – the nozzles have not the same diameter. These disadvantages are eliminated by the

construction of fine bubble generators using non-conventional technologies, namely electro-erosion

[1]. Using this technology nozzle with the diameters 0.2; 0.3; 0.4; 0.5 mm was obtained. In this paper fine bubble generators (FBG) with nozzles of 0.3 mm are presented, obtained by another technology, namely the « micro perforation » [2].

Both fine bubble generators, the fixed and the mobile one, have the same number of nozzles (n = 17), and the conditions for the experimental research have common elements:

– the height of the water layer: H = 500 mm; – the initial concentration of the oxygen dis-

solved in water: Co = 5.4 mg/l;

– the input air flow: 3600 dm /hV ;

– water temperature: t = 24oC; – the final concentration of the dissolved oxygen

that must be reached Cf = 8.3 mg/l [3]. The aim of the experimental researches is to prove

that the aeration plants with mobile fine bubble generators are more efficient than the classical fixed fine bubble generators. The aeration with mobile fine bubble generators is necessary especially in the places where zones with a low level of oxygen dis-solved are formed in a large aquatic tank (lake, pond etc.), and a fixed FBG plant would lead to additional exploitation costs. Also the experimental researches are done to sustain the theory that asserts that an aeration system composed by mobile fine bubble

A COMPARATIVE STUDY BETWEEN A FIXED AND A MOBILE FINE BUBBLE GENERATOR

TERMOTEHNICA 2/2012 47

generators is more efficient than a fixed one, because the bubbles have a larger trajectory in the water mass. The contact between the air bubble and the water mass around the bubble is increased, leading to the increase of the performances of the aeration system.

2. THE PRESENTATION OF THE EXPERIMENTAL SETUP FOR RESEARCHES ON THE FIXED FINE BUBBLE GENERATORS

2.1. The constructive solution of the fine bubble generator

The principal element of the setup is the fine bubble generator (FBG) constructed on a parallele-piped shape, on the upper part being covered by a nozzle plate (Fig.1).

80

240

88 8 88

Ø0,4

a

b

Fig. 1. Fine bubble generator: a – the nozzle plate; b – the body of the fine bubble generator.

The nozzle plate (a) is made from transparent

plexiglass and has 17 Ø0.3 mm nozzles; it is sealed on the upper part of the body of the FBG, which is fed with compressed air on the two lateral sides (b).

2.2. Presentation of the experimental setup

The fine bubble generator is fixed in the upper part of a water tank (Fig. 2.). The concentration of oxygen dissolved in water (Co) is measured at the beginning of the experiment and subsequently from 15’ to 15’. The saturation concentration is reached after about 120'. The technical specifications of the oxygen meter require that the probe be displaced in water with a speed superior to 0.3 m/s; this condition

is fulfilled using a mechanism (6) that is displaced following a trajectory generated by computation software [4].

Fig. 2. The sketch of the experimental plant: 1 – control electronics: a – supply unit; b – switch; c – control

element; 2 – measuring instruments panel; 3 – pipe for the transport of the compressed air; 4 – thermometer joint; 5 –

manometer joint; 6 – mechanism for probe driving; 7 – water tank; 8 – oxygen meter probe; 9 – fine bubble generator.

A working FBG can be noticed in Figure 3; it

works in “dynamical” conditions [5]. During the experimental research, the pressure

(p = 126 mbar = 1285.2 mmH2O) and the flow rate at the entrance of the FBG were kept constant.

Fig. 3. Fine bubble generator with 17 holes (Ø 0.3 mm) in function.

2.3. Obtained experimental results

At the beginning of the experiment the con-centration of oxygen dissolved in water was of 5.46 mg/l. After 120’ it was of 8.31 mg/h (see Fig. 4).

Nicolae BĂRAN, Octavian DONŢU, George Lucian IONESCU, Ionela-Mihaela CĂLUŞARU

48 TERMOTEHNICA 2/2012

Fig .4. The variation of the O2

concentration in water function of time.

In order to compare experimental data obtained

as results of the measurements performed on a FBG with 17 nozzles of 0.3 mm diameter and rectangular shape, with the theoretical data, a computation software was elaborated, for the numerical integration of the equation of the transfer speed of the oxygen towards water [6] [7]:

d

d L s

Ca K C C

where: d

d

C

– the transfer speed of the dissolved

oxygen [kg O2/m3 s]; aKL – volumetric mass

transfer coefficient; Cs – concentration of oxygen dissolved in water at saturation [mg/l]; C – current concentration of oxygen dissolved in water, measured value [mg/l].

Numerical integration using 4th order Runge-Kutta method [9] allowed obtaining the data graphically represented in Figure 4.

Good coincidence between theoretic and experimental data can be remarked from figure 4. The graphs presented above are validated by data existing in specialty literature [10].

3. THE PRESENTATION OF THE EXPERIMENTAL SETUP FOR RESEARCHES ON THE MOBILE FINE BUBBLE GENERATORS

The fine bubble generator has the same

construction as the one presented in paragraph 2.1. The tank in which the fixed FBG is introduced is presented in figure 6.a. The water from the tank was oxygenated during two hours with an air flow

rate 3600 dm /hV and H = 500mm – height of the water column.

Fig. 5. Modification of the concentration of O2 dissolved in water function of time: 1 – graph built using experimental data; 2 – graph built using calculated data.

A COMPARATIVE STUDY BETWEEN A FIXED AND A MOBILE FINE BUBBLE GENERATOR

TERMOTEHNICA 2/2012 49

500

500

4000

500

a

b

x

z

y

y

z

x

Fig. 6. Sketch of the aeration tanks: a – aeration tank for a fixed FBG; b – aeration tank for a mobile FBG.

In this case, the concentration of O2 dissolved in

water increases from C0 = 5.46 mg/l to C120 = = 8.31 mg/l. If we increase the tank eight times along the Ox axis we obtain (Fig. 9.1.b), of volume

3 38 0.125 m =1 m . If a sole fixed FBG is installed in this tank, a greater time is needed in order to reach from C0 to C120, theoretically equal to 8 2 h = 16 h . What happens if a mobile FBG with the same cha-racteristics is introduced in the large tank and aerated using the same flow rate as the one used for the fixed FBG?

In Figure 7 the sketch of the plant for mobile fine bubble generators is presented:

The running path (2) on which the platform (3) is transported is located in the upper part of the

water tank (1); on the platform there is an electrically driven mechanism that provides:

– the horizontal displacement of the platform with a speed wFBG=0.2 m/s;

– the 180o rotation of the FBG (4) in positions A and B (Fig. 7) for which the displacement direction of the platform is inversed.

The initial data (C0, H etc.) are the same as for the experiments involving the fixed FBG. The volume of water being 8 times larger, theoretically the oxygenation time should be of 8·2 h = 16 h. The experimental data obtained for a fixed FBG and for a mobile FBG are graphically presented in Figure 8.

Fig. 7. Sketch for the measurement of the concentration of dissolved oxygen for mobile FBG.

Nicolae BĂRAN, Octavian DONŢU, George Lucian IONESCU, Ionela-Mihaela CĂLUŞARU

50 TERMOTEHNICA 2/2012

Fig. 8. The variation of the concentration of O2 dissolved in water function of time:

1 – experimental results for a mobile FBG; 2 – experimental results for a fixed FBG. 1-

Hence, as one can remark in Figure 8, the aera-tion using mobile fine bubble generators is more efficient, namely the air bubbles transfer more oxy-gen in water. If for a volume of water of 0.125 m3 the value of 31 mg/l dissolved O2 was reached in 120 min in the case of a fixed FBG (Fig. 8), using the mobile FBG the same value was reached after only 210 min for a water volume eight times larger, (1 m3), reaching the value of 8.40 mg/l O2 dissolved in 240 min and not in 960 min, as it would have been expected as given by the similarity constant. There-fore the oxygenation time is reduced four times.

4. CONCLUSIONS

● Performed experimental research proved that in order to reach the same concentration „C120” starting from C0, 16 hours are not anymore necessary, but only four hours. Hence one of the advantages of using the mobile FBG is the fact that it shortens the oxygenation duration four times.

● The turbulence degree of the water increases by using a mobile FBG, leading to an increase of the transfer speed of oxygen in water.

BIBLIOGRAPHY

[1] Besnea D., Băran N., Mateescu G. Using Non-Conventional Technologies In Order to Build Fine Bubbles Generators, Romanian Review Precision

Mechanics Optics and Mechatronics, Nr.36/2009, pg. 31-37, ISSN 1584-5982.

[2] Alexandru Pătulea, “Influence of functional parameters and of architecture of fine bubble generators on the efficiency of the aeration plants” (in Romanian), PhD Thesis, POLITEHNICA University Bucharest, 2012.

[3] Rubin Battina, Timothi Rettich and Toshihira Tominaga, The solubility of oxigen and azot in liquids, J.Phys.Chem.Ref.Data vol 12. no.2.1985.

[4] Nicolae Băran, Daniel Besnea, Despina Duminică, Iulian Avarvarei, Alexandru Pătulea, Mechanism For The Displacement Of An Oxygen Meter Probe In Stationary Waters, 3 rd International Conference an Electronics Computer Technology (IECT 2011), vol I IEEE Press, April, Kanyakumari, India 2011, pp.283-286.

[5] Gabriela Oprina, Irina Pincovschi, Gheorge Băran, Hydro-gas-dynamics of aeration systems equipped with fine bubble generators (in Romanian), POLITEHNICA PRESS Publishing House, Bucharest 2009.

[6] Dan Robescu, Diana Robescu, Attila Verestoy, Reliability of processes, plants and equipments for water treatment and purification (in Romanian), Technical Publishing House, Bucharest, 2002.

[7] Irina Pincovschi, Hydrodynamics of disperse gas-liquid system (in Romanian), PhD Thesis, POLITHENICA University of Bucharest, 1999.

[8] Gh. Jinescu, Vasilescu P, Jinescu C, The Dynamic of the real fluids in process instalations (in Romanian), Semne Publishing House, Bucharest 2001.

[9] Corneliu Berbente, Sorin Mitran, Numerical methods, Technical Publishing House, Bucharest, 1997.

[10] Ionela Căluşaru, Nicolae Băran, Alexandru Pătulea, Determination of Dissolved Oxygen Concentration in Stationary Water, Romanian Journal of Chemistry, vol 63. no. 12/2012. Bucureşti, pp.1312-1315.