Turbulence measurements

H. Kobayashi, T. Nakashima, T. Tamura, K. Maruta, and T. Niioka. Turbulence measurements and observations of turbulent premixed flames at elevated pressures up to 3.0MPa, Combust. Flame 108 104-117. 

Stress on particles occurs in the velocity and turbulence fields of reactors. Therefore, for the initial estimate of stress according to Eqs. (1) and (2), the theoretically derived basic Eqs. of velocity fields can be used in the case of laminar flow, and the results of turbulence measurements in the case of turbulent flow. 

According to current turbulence measurements in stirred tanks, there is a very considerable difference between the maximum energy density and the mean energy dissipation. Various authors (see e.g. [24] -[26] conclude that the... 

According to turbulence measurements performed by Mockel [24] the maximum energy dissipation occurs in this zone with E = 0.13. 

Since the results demonstrate the important role of the impeller size, the first approach adopted for the correlation of the test results was to try to use the generalised formula of Eq. (9) for the maximum energy dissipation on the basis of the turbulence measurements. 

Where the Reynolds stress formula (2) and the universal law of the theory of isotropic turbulence apply to the turbulent velocity fluctuations (4), the relationship (20) for the description of the maximum energy dissipation can be derived from the correlation of the particle diameter (see Fig. 9). It includes the geometrical function F and thus provides a detailed description of the stirrer geometry in the investigated range of impeller and reactor geometry 0.225derived from many turbulence measurements, correlation (9). 

The as yet unpubhshed results of more recent turbulence measurements confirm the geometrical dependence of Eq. (20) as regards the ratio h/D and blade number z for radial stirrers. For the influence of the diameter ratio d/D, the relationship approximating to Eq. (20) Cm/eoc (D/d) was derived from these tests, in agreement with Zhou and Kesta [64]. However, turbulence measurements in axial stirrers resulted in substantially lower energy dissipation [64]. 

The turbulent fluctuation frequency can be estimated by means of turbulent measurements. Mockel 124] found that the wave number k = 2Tlft/u in the interesting dissipation range is k>ko with the limiting value ko= (0.1. ..0.2)qL-From this becomes the frequency to ft> (0.016...0.032)u/qL. An important measure should be the related number of turbulent fluctuation z/zp which occur during the residence time of particles ti=Vi/qp inside the fictive impeller volume Vj at one circulation. It follows to ... 

Figure 28 (Y an, Yao and Liu, 1982) presents the results of turbulence measurement above a distributor plate by means of a hot-wire anemometer, indicating that turbulence intensity increases sharply near the distributor, especially for the perforated plates and the cap plate, and then diminishes beyond a certain distance (40 mm in the present case) to some nearly constant low values.

R. W. McMillan, R. A. Bohlander, G. R. Ochs, R. J. Hill, S. F. Clifford. Millimeter Wave Atmospheric Turbulence Measurements Preliminary Results and Instrumentation for Future Measurements , Optical Engineering, Vol. 22, No. 1, pp. 32-39, January/February 1983. 

Vaezi, V., E. S. Oh, and R. C. Aldredge. 1997. High-intensity turbulence measurements in a Taylor-Couette flow reactor. J. Experimental Thermal Fluid Science 15 424-31. 

The hot-wire anemometer can, with suitable cahbration, accurately measure velocities from about 0.15 m/s (0.5 fl/s) to supersonic velocities and detect velocity fluctuations with frequencies up to 200,000 Hz. Eairly rugged, inexpensive units can be built for the measurement of mean velocities in the range of 0.15 to 30 m/s (about 0.5 to 100 ft/s). More elaborate, compensated units are commercially available for use in unsteady flow and turbulence measurements. In cahbrating a hotwire anemometer, it is preferable to use the same gas, temperature, and pressure as will be encountered in the intended apphcation. In this case the quantity I RJAt can be plotted against /v, where I = hot-wire current, = hot-wire resistance. At = difference between the wire temperature and the gas bulk temperature, and V = mean local velocity. A procedure is given by Wasan and Raid [Am. Inst. Chem. Eng. J., 17, 729-731 (1971)] for use when it is impractical to calibrate with the same gas composition or conditions of temperature and pressure. Andrews, Rradley, and Hundy [Int. J. Heat Mass Transfer, 15, 1765-1786 (1972)] give a cahbration correlation for measurement... 

The Kolmogorov relationships are not valid in immediate proximity of the wall, as the turbulence is strongly anisotropic in this region. For a first assessment of the order of size of the turbulence measurements, the point of maximum dissipation is chosen. 

Karlovitz B., et al. (1953). A. Flame propagation across velocity gradients. B. Turbulence measurement in flames. Fourth Symposium (International) on Combustion, Williams and Wilkins, Baltimore, p. 613. 

Sommerfeld, M. (1993), Reviews in numerical modeling of dispersed two phase flows. Proceedings of 5th Int. Symp. on Refined Flow Modeling and Turbulence Measurements, Paris. 

Grimmond, C.S.B., Salmond, J.A., Oke, T.R., OfFerle, B., and Lemonsu, A. (2004) Flux and turbulence measurements at a densely built-up site in Marseille Heat, mass (water and carbon dioxide), and momentum,./. Geophys Res. 109, D24101. 

Chara, Z. Zakin, J.L. Severa, M. Myska, J. Turbulence measurements of drag reducing surfactant systems. Exp. Fluids 1993, 16, 36-41. 

Lesieur M (1993) Advance and State of the Art on Large-Eddy Simulations. 5th Int Symp on Refined Flow Modehng and Turbulence Measurements, Sep. 7-10, Paris... 

Balzer G, Simonin O (1993) Extensions of Eulerian gas-solid flow modelling to dense fluidized bed prediction. In Proc 5th Int Symp on refined flow modelhng and turbulence measurements, ed Viollet PL, Paris, pp 417-424... 

In this method correlations between various velocity fluctuations are used to determine the turbulent energy spectrum, E k), and several turbulence length scales. The correlations mentioned contain information about how velocities and other flow properties are statistically related in the turbulent flow. Turbulence measured at a fixed point can be described as a fluctuating waveform. If two instantaneous waveforms appear to have a corresponding behavior, they are said to be correlated. Equation (1.311) shows how velocity fluctuations at two points can be statistically correlated if the distance between the two points are small. 

Although the time average given above is shown as the limit as times goes to infinity, in practice it is only necessary to make measurements over a period which is long compared with the frequency of the fluctuations. Thus, for turbulence measurements in pipes, the average value found by Eq. 16.2 for t equal to a few seconds is numerically equal to that found for any longer period. 

Historically, most turbulence measurements have been made with hot-wire anemometers. In recent years laser-Doppler anemometers have been widely used as well. 

Wall shear stresses on the order of 1750 dynes/cm were observed on the sinus wall towards which the high flow was directed. Turbulent measurements with both models indicated -turbulent shear stresses on the order of 500 to 2000 dynes/cm immediately downstream (3 to 15 mm) from the valve. 

Flow patterns 145 Flow rate-head concepts 147 Turbulence measurements 148 Mixing time 149... 

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