Flows noninvasive measurements

The direct experimental verification of the computed flow field requires noninvasive measurements of velocity components inside the packed tube. One very promising technique for this is magnetic resonance (MR) as described in a recent review by Gladden (2003). She showed pictures of 3D MR visualization of axial velocity for flow of water in packings of spheres, and her group has used MR to connect the 3D structure of a packed bed to the transport phenomena in... 

The validation of CFD codes by comparison to reliable experiments is of the highest importance. Especially promising is the use of MRI methods to non-invasively provide flow fields and dispersion data. Major challenges will be to extend MRI and similar methods such as LDV and particle tracking to a wider range of conditions, and to develop noninvasive measurements of temperature to improve verification of heat transfer simulations. 

Experimental validation of CFD results is considered a prerequisite to paving the road for widespread acceptance of CFD in the chemical engineering community, especially in connection with multiphase flow applications. The authors do not intend to give here a complete review on available measuring techniques for single-phase and multiphase flows only the more advanced techniques are briefly discussed. For an overview of the latest advances realized in noninvasive measurement of multiphase systems the interested reader is referred to Chaouki et al (1997). The available experimental techniques can be classified according to the following aspects ... 

Hilty et al. have used remote NMR with microfluidic chips to obtain profiles of gas flow in these devices . Remote detection of the NMR signal both overcomes the sensitivity limitation of NMR and enables noninvasive measurement of microfluidic flow. Although used for gases, it can be applied to liquids also. 

Glaab T, Hoymann HG, Hohlfeld JM, Korolewitz R, Hecht M, Alarie Y, Fabel H (2002) Noninvasive measurement of midexpiratory flow indicates bronchoconstrictirai in allergic... 

M. Nakagawa, S.A. Altobelli, A. Caprihan, E. Fukushima, and E.-K. Jeong. Noninvasive measurements of granular flows by magnetic resonance imaging. Experiments in Fluids, 16(1) 54—60, 1993. 

Acoustic chemometrics has its greatest benefits in cases where haditional sensors and measurement techniques, such as flow, temperature and pressure transmitters cannot be used. In many processes it is preferable to use noninvasive sensors because invasive sensors may cause disturbances, for example fouling and clogging inside the process equipment such as pipelines, reactors cyclones, etc. In this chapter we concentrate mainly on new industrial applications for acoustic chemomehics, and only discuss the necessary elements of the more technical aspects of the enabling technology below - details can be found in the extensive background literature [3-5],... 

The most commonly utilized embedded sensor for temperature distribution mapping is the thermocouple. Wilkinson et al.130 developed a simple, in-situ, and noninvasive method of measuring the temperature distribution of a fuel cell with micro-thermocouples. In this study, thermocouples were located in the landing area of the flow field plates (in contact with the GDL of the MEA) of a fuel cell. The temperature data taken at different locations along the flow channel was then used to find each temperature slope, which in turn were related through mathematical equations to the local current density of each location. Thus, the current density distribution in the fuel cell was determined by simple temperature measurements. The results of this approach are discussed in more detail in Section... 

In an attempt to further simplify the caffeine phenotyping test, a trait measure based on the plasma or salivary paraxanthine caffeine concentration ratio between three hours and seven hours after administration of the probe has been suggested (56). High linear correlations (>0.89) have been observed between this trait value and caffeine s oral clearance, and if necessary, a predicted caffeine clearance value may be calculated from the ratio (56). Currently, this phenotyping approach appears to be the simplest and most noninvasive means of readily assessing CYP1A2 activity using caffeine as a probe in addition, the method is reproducible and appears to be robust (56), despite the theoretical dependency of the trait value on the urine flow rate (51). 

The analytical methods used in determining composition include separation, spectroscopic and radiant energy techniques, the measurement of electrochemical properties, and many others. The analyzers used can be intermittent or continuous, can depend on samples, and can be inserted into the flowing streams (probes) or be noninvasive. 

Such noninvasive designs are suitable for the measurement of the flow of solids and two-phase flows, including heavy slurries, and very corrosive and difficult measurement applications. Their disadvantages include high cost, a... 

Magnetic resonance imaging permitted direct observation of the liquid hold-up in monolith channels in a noninvasive manner. As shown in Fig. 8.14, the film thickness - and therefore the wetting of the channel wall and the liquid hold-up -increase nonlinearly with the flow rate. This is in agreement with a hydrodynamic model, based on the Navier-Stokes equations for laminar flow and full-slip assumption at the gas-liquid interface. Even at superficial velocities of 4 cm s-1, the liquid occupies not more than 15 % of the free channel cross-sectional area. This relates to about 10 % of the total reactor volume. Van Baten, Ellenberger and Krishna [21] measured the liquid hold-up of katapak-S . Due to the capillary forces, the liquid almost completely fills the volume between the catalyst particles in the tea bags (about 20 % of the total reactor volume) even at liquid flow rates of 0.2 cm s-1 (Fig. 8.15). The formation of films and rivulets in the open channels of the structure cause the further slight increase of the hold-up. 

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