3차원 PTV기술을 이용한 고-액  Dispersion 현상에 관한 연구

함승주, Robert S. Brodkey

연세대학교 화학공학과, The Ohio State University

The Research on the Solid-Liquid Dispersion System Using Particle Tracking Velocimetry (PTV) Technique

Seungjoo Haam, Robert S. Brodkey

Department of Chemical Engineering, Yonsei University,

Department of Chemical Engineering, The Ohio State University

Experiment

The turbulent velocities of the two phases during solid-liquid dispersion in a vertical pipe flow system was studied. The level of turbulence in the fluid phase in the presence of bead is different from that of a pure fluid since the motion of solid beads affects the liquid phase. Also, for the solid phase, the solid particles fluctuated with the motion of the liquid. for fluid-solid flow system, we need to develop a suitable model to describe the interactive motions of both phases in a turbulence flow field. Turbulent liquid velocity data are essential to check the validity of existing theoretical models. A study of motions of solid beads is also necessary for the same reason, Modeling of two-phase local fluctuations is less important under steady state laminar flow conditions, but is very important either during unsteady operation or when the flow conditions, The relative local fluctuations of the solid and fluid phases play an important role for surface controlled operations such as mass transfer, kinetics and drop breakup. Once a model is acquired, a wide variety of practical situations can be studied. For example, applications to crystallization, fluid bed combustion, solid transport, jet combustion with solid fuels and gasification, are all possible.

Large spherical molded polymethylmethacrylate (PMMA) beads(dp= 4.765mm) were used as the solid particles or beads in p-cymene for the fluid phase for index of refraction matching. Since it is very difficult to measure accurately the fluctuations of large nonbuoyant solids together with measurements of the fluctuations of the fluid in presence of such solids, the measurements for fluid and solid phases were carried out independently but under the same conditions. Laser Doppler Anemometry (LDA) was used to measure the velocity of the liquid flow field. The two-dimensional velocity field(axial and radial) components were measured mainly at the center of the column, away from the wall. There was an excellent index of refraction match between the fluid, para-cymene, and the dispersed solid, PMMA beads, which allowed adequate measurements of the turbulent fluid velocities without generating noise signals from the solid beads. figure.1 shows the experimental apparatus of or dispersion system. Statistical and ensemble averaged evaluation of turbulent velocity data was analyzed based on several independent times as more information on the solids concentration was attained and as we developed more refined means of analysis. The Particle Tracking Velocimetry (PTV) technique was used to measure the three dimensional components of the velocity of the solid phase. The PTV technique adopts an visualization and image processing methodology to analyse the motion of solid beads during dispersion. Figures 2 and 3 shows the steps of PTV and Raw image of solid beads. To extract the velocity data from high solid concentration, only 15% of the beads were dyed and used as tracers and the rest remained clear and index of refraction matched with the fluid. With data for the relative local fluctuations of solid and fluid phases measured from LDA and PTV techniques, our final goal is to find the most appropriate theories that could predict adequately the relative motions of two-phase dispersive flow.

Result and discussion

The use of p-cymene as the index of refraction matching fluid for the po;ymethylmethacrylate (PMMA) beads proved to be an excellent choice. In the visualization experiments, the IR matched clear beads were not visible in the p-cymene liquid. Under the green laser light (□=514.5 nm), the index proved to be quite adequate. As a worst estimates, the beads themselves could contribute as much as 5 or 20% of the signal; however ,it is believed to be much less. The data obtained from LDA measurement of the axial velocity in the dispersion column with beads clearly showed that the presence of the beads caused a significant lowering of the velocity. The beads had a density greater than the fluid and created wakes behind the beads, which caused a lowering of the velocity. The lower velocities caused by the presence of the beads had long negative tails in the histograms and these tails contributed to the higher turbulence level of the fluid. The total fluctuation energy, RMS velocity and turbulence intensity data from LDA measurements were compared between the period both with and without the presence of the beads . The results showed that the presence of the beads extracted energy from the mean flow and provided that energy to the fluctuation turbulence level. The increase of turbulent intensity was impressive along the centerline of the dispersion column where the measurement was made. Table 1 show the experimental results at corresponding bead concentration. The axial turbulent intensity of the fluid without beads was 12% and with beads, the level was increased to 49% for 3/16 inch beads. RMS velocity was used for the radial components since the use of intensity of turbulence was not suitable since the radial mean velocity was close to zero. The presence of beads had a greater effect on the axial velocities rather than radial velocities. However, the enhancements on the radial direction were not small. The PTV system, accompanied with an image enhancement algorithm, proved to be the best choice for studying solid movement. It is shown that for PTV system, particle overlapping was a major limitation especially for the high concentration period. An additional image enhancement algorithm including particle decomposition process were necessary to provide the optimal image input for the PTV system. In our research, we were able to decompose the overlapped particles up to 95% of the total particles in a single image and obtained the accurate velocity information with high accuracy. The use of the PTV system also provided the concentration measurement. The PTV algorithm produced very accurate particle counts (concentration) in the concentration level from low to moderate periods. Even for high concentration period, the PTV algorithm were able to measure the concentration with an accuracy of 95%. Application of our experimental data using large particles to the classical two-phase models for small particles proposed by Soo[ ] and Hinze[ ] showed signigicant deviations as expected. However, the4/9 power law model for large particle proposed by Hestroni[ ], which is believed to be the only reliable theory for the large particle system, was fairly well matched with the experimental data for the axial fluctuations. the radial fluctuations of the carrier fluid showed more scatter, but the data were still in the 4/9 power law range.

Bead Collector

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ACP Tank      Square          ∵∴

Column          ∴∴

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Cylindrical      ∴∴

Pump  Column        ∴∴

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Storage                                              ∴∴

Tank                               Lead Balls       ∴∴

Figure.1: Operating system of dispersion

Figure.2: PTV system

unit[cm/sec]	Fluid Phase					Solid Phase(Bead)
	Axial			Radial		Axial		Radial
	Mean	RMS	Turbulent Intensity(%)	Mean	RMS	Mean	RMS	Mean	RMS
Bead conc.=3%	21.88	7.89	36.06	1.37	4.49	13.10	4.77	0.26	4.03
Bead conc.=7%	19.62	9.61	48.78	1.68	4.51	10.42	5.54	-0.02	3.98
Bead conc.=6%	22.73	9.24	40.65	1.25	3.44	7.89	6.16	0.06	3.81
Bead conc.=3.5%	27.70	6.63	23.94	0.71	3.08	8.09	5.55	-0.10	3.43
Bead conc.=2%	28.43	4.41	15.51	0.81	2.84	7.46	4.87	0.17	3.32
Pure Liquid	28.38	3.29	11.59	0.74	3.05	.	.	.	.

Figure 3: Raw image containing   Table.1: Overall Comparisons of Measured            stereo views       R.M.S for Fluid and Solid                                                                  phases(3/16"beads)

Figure.4:Dependence of the carrier fluid fluctuation velocity (axial) on the complex{□(CD3/2 /(□f/□p)}4/9 using Ur=Uc-Up(from PTV),  □=(Mf/Mp),   U'=instantaneous velocity