Flow regime measurements

Flow regime identification is dependent on the geometry of the bioreactor. For example, flow regimes in bnbble columns will be different from those identified in stirred-tank bioreactors. In some cases, the experimental techniques used to identify flow regimes are system independent, while in other cases, the technique was developed for a particular geometry. The specific flow regime definitions in common bioreactor types are described in detail in their respective chapters. 

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Nonintrusive Instrumentation. Essential to quantitatively enlarging fundamental descriptions of flow patterns and flow regimes are localized nonintmsive measurements. Early investigators used time-averaged pressure traverses for holdups, and pilot tubes for velocity measurements. In the 1990s investigators use laser-Doppler and hot film anemometers, conductivity probes, and optical fibers to capture time-averaged turbulent fluctuations (39). 

In addition to flow regime, hold-up and pressure drop are two other important parameters in two-phase gas-liquid flows. Hold-up is defined as the relative portion of space occupied by a phase in the pipe. It can be expressed on a time or space average basis, with the actual method chosen depending on the intended use of the hold-up value, and the measurement method employed. There are numerous correlations in the literature for hold-up, but most are based upon a pressure drop-hold-up correlation. The following expression is a widely recognized empirical relationship between hold-up and pressure drop ... 

The above experimenters have used the technique described to obtain flow rate measurements of the liquid wall-film at various mass velocities, tube dimensions, etc., and some typical results from Staniforth and Stevens (S7) are shown in Fig. 7. Also shown are the values of burn-out heat flux obtained at the four different mass velocities indicated. It can be seen that the liquid-film flow rate decreases steadily with increasing heat flux until at burn-out the flow rate becomes zero or very close to zero. We thus have confirmation of a burn-out mechanism in the annular flow regime which postulates a liquid film on the heated wall diminishing under the combined effects of evaporation, entrainment, and deposition until at burn-out, the film has become so thin that it breaks up into rivulets which cause dry spots and consequent overheating. 

The annular flow regime is very extensive, and the above mechanism of burn-out is stated (S7) to be consistent with the film-flow measurement data over a range of exit qualities from 10 to 100% for uniformly heated round tubes. A summary of experimental observations on flow patterns produced... 

The above results show close agreement between the experimental and theoretical friction factor (solid line) in the limiting case of the continuum flow regime. The Knudsen number was varied to determine the influence of rarefaction on the friction factor with ks/H and Ma kept low. The data shows that for Kn < 0.01, the measured friction factor is accurately predicted by the incompressible value. As Kn increased above 0.01, the friction factor was seen to decrease (up to a 50% X as Kn approached 0.15). The experimental friction factor showed agreement within 5% with the first-order slip velocity model. 

The convective and nucleate boiling heat transfer coefficient was the subject of experiments by Grohmann (2005). The measurements were performed in microtubes of 250 and 500 pm in diameter. The nucleate boiling metastable flow regimes were observed. Heat transfer characteristics at the nucleate and convective boiling in micro-channels with different cross-sections were studied by Yen et al. (2006). Two types of micro-channels were tested a circular micro-tube with a 210 pm diameter, and a square micro-channel with a 214 pm hydraulic diameter. The heat transfer coefficient was higher for the square micro-channel because the corners acted as effective nucleation sites. 

A summary of the nine batch reactor emulsion polymerizations and fifteen tubular reactor emulsion polymerizations are presented in Tables III IV. Also, many tubular reactor pressure drop measurements were performed at different Reynolds numbers using distilled water to determined the laminar-turbulent transitional flow regime. 

Fig. 16. Effects of flow regime on cell viability measured by trypan blue. Exposure time 1 h [1]...

Separation layer mixers use either a miscible or non-miscible layer between the reacting solutions, in the first case most often identical with the solvent used [48]. By this measure, mixing is postponed to a further stage of process equipment. Accordingly, reactants are only fed to the reaction device, but in a defined, e.g. multi-lamination-pattem like, fluid-compartment architecture. A separation layer technique inevitably demands micro mixers, as it is only feasible in a laminar flow regime, otherwise turbulent convective flow will result in plugging close to the entrance of the mixer chamber. 

This condition can be met by choosing either a RHSE of a sufficiently large radius, or by maintaining a high speed of rotation. From the results of the turning moment measurements shown in Fig. 4, one may take Re = 200 as the lower limit where the boundary layer approximation is valid. Thus the useful flow regime for electrochemical application is ... 

Very refined measurements at various positions of the packed bed were made by Jolls and Hanratty (J6), who used an active sphere (electrode) in a packed bed consisting of 1-inch inert spheres. The overall mass-transfer data for the turbulent flow regime suggest a dependence of... 

There are 318 river gauging stations within the Ebro basin [80]. Around 60 of these monitor natural flow regimes and are typically located around the edges of the basin in the medium to higher reaches of the tributary rivers [8], Others are located on rivers whose streamflow has been altered by reservoirs (see [9] for a review of historical water policy in Spain). In total, 187 reservoirs impound 57% of the mean annual runoff [10]. As an example, annual discharge measured... 

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