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Quenching-based techniques
The first technique is an extension of the LIF technique based on the quenching phenomena of the fluorescence. The combination of fluorescence intensity measurements with the effect of fluorescence quenching for gas concentration measurements in liquids was first introduced by (Vaughn and Weber, 1970). As oxygen has been known as an excellent quencher (Jimenez et al., 2013), Wolff et al. (Wolff et al., 1991) introduced primarily LIF quenching technique to visualize O2
concentration near a gas-liquid interface. Many following researchers (Asher and Litchendorf, 2009; François et al., 2011a; Herlina and Jirka, 2004; Kapoustina et al., 2015) have continued to work on the same subject by studying the absorption of O2. In a quenching experiment, the ability of some molecules called “quenchers” to inhibit the fluorescence dye is considered. The quenching reaction can be depicted as the following figure:In the absence of quencher, the fluorophore A, which is excited by absorption of a photo (e.g. from a laser source), will jump to a higher electronic energy level A*. Then A* can return to the ground state by the non-radiative way with a rate constant and
by emitting a photon (fluorescence) with the radiative rate . The fluorescence 0 lifetime is defined as: 1 (1-36)
Rheological measurement: principle of rheometer
In order to measure the rheological properties of fluids, a viscometer or a rheometer is used which can be grouped into two main types: tubular rheometer and rotational rheometer. Compared with a tubular rheometer, the rotational rheometer is popular since regarding its advantages like wider shear range, small volume sample required, operation with continuous rotation and rotational oscillation, capability for viscoelasticity measurement etc.
In rotational rheometer, a sample of fluid is filled into the narrow gap of the sensor system which always consists of a rotator called geometry and a stationary plate or container. The sensor systems can be classified into three categories: cylinder, cone-plate, and plate-plate (see Figure 1-13). Moreover, for the same sensor system, commercial rational rheometers can also employ a wide range of geometries for viscosity and flow behavior measurement (see Figure 1-14).
The measurement with rotational rheometers can be carried out in one of two operation modes:
1. Rotation with controlled shear rate (CSR): A rational speed (angular velocity) is preset which in the sensor system filled with a sample causes a shear rate. The torque required for achieving and maintaining the desired shear rate is the viscosity-proportional parameter.
2. Rotation with controlled shear stress (CSS): According to the reversed principle, here a torque (shear stress) is preset and the resulting movement (deformation) i.e. the resulting angular velocity (shear rate) is measured.
Conversion of torque into the shear stress and rotational speed into the shear rate is via conversion factors which depend on the geometries. The shear stress is proportional to the torque [N•m] and to a characteristic geometry factor identified as(shear High torque means also high shear stress. Large values of stand for small sensors.
Table 1-4 Conversion factors of torque to shear stress KSS and rotational speed to shear rate KSR for different sensor systems.
Rheological behavior
Viscosity values are not constant values as they are affected by many conditions. The fluids have different flow behavior under shear even at constant temperature. We can use a rheogram or flow curves to represent these rheological behaviors. There are two ways to plot the flow curves:
1. Shear stress as a function of shear rate ( vs ̇);
2. Viscosity as a function of shear rate ( vs ̇), applying the law of viscosity, each measuring viscosity is calculated with Eq. (1-53), which is called apparent viscosity.
We can classify the fluids into different types depending on theological behavior as follows and their flow curves plotted in Figure 1-15.
Figure 1-15 Representation of different rheological behaviors (shear thinning,
Newtonian and shear thickening) in a rheogram (I: vs ̇ vs ̇ ; II: )
Newtonian fluids
Newtonian fluids (or: Ideally viscous fluids) is a fluid in which that the measured viscosity of the fluids is independent of the shear rate (Figure 5.2, line 1). That means the viscosity is a constant and the shear stress are linearly proportional to the shear rate. Typical materials from this group include water, mineral oil, salad oil, solvents such as acetone, glycerol, benzene, etc. All the other fluids are called the non-Newtonian fluids since their viscosity depends on the shear rate.
Shear thinning
Shear-thinning behavior (or: pseudoplasticity) is characterized by decreasing viscosity with increasing shear rates (Figure 5.2, line 2). Typical materials that show this behavior are coatings, glues, shampoos, polymer solutions, and polymer melts. Since viscosity is shear-dependent, it should always be given with the shear condition. The shear thinning phenomenon is generally considered to be the result of microscale structural rearrangements within the fluid, or the organization of microspheres into hexagonally packed structures that slide over each other more easily (i.e. exhibit lower viscosity, a schematic explanation for polymers is given in Figure 1-16) than at lower shears where randomly organized particles collide frequently
Shear-thickening
Shear-thickening (or: dilatant flow behavior) means increasing viscosity with increasing shear rates (Figure 5.2, line 3). Materials that typically display such behavior include highly filled dispersions, such as ceramic suspensions (casting slurries), starch dispersions, plastisol pastes that lack a sufficient amount of plasticizer, dental filling masses (dental composites) as well as special composite materials for protective clothing (Mezger, 2014). This behavior is rare and occurs in concentrated slurries when the increment of the shear rate leads to the formation of clusters that rigidify the suspension structure. If the suspension is composed of associative polymers, an increase in shear forces favors the polymer interactions resulting in a more solid structure.
Table of contents :
Chapter 1 Fundamental principles
1.1 Principles of gas-liquid mass transfer
1.1.1 Diffusion coefficient
1.1.2 Mass transfer coefficient
1.1.3 Theories of mass transfer
1.2 Gas-liquid mass transfer measurement
1.2.1 Classic method
1.2.2 Visualization techniques
1.3 Rheology principle
1.3.1 Definition of shear stress, shear rate and viscosity
1.3.2 Rheological measurement: principle of rheometer
1.3.3 Rheological behavior
1.3.4 Viscosity of polymer systems
1.3.5 Rheological models
1.4 Conclusions
Chapter 2 Material and Methods
2.1 Material
2.1.1 Polymers to study
2.1.2 Oxygen indicator (dye)
2.2 Experimental setup
2.2.1 Column
2.2.2 Syringe pump
2.2.3 Nd:YAG laser
2.2.4 CCD camera
2.2.5 High-speed camera
2.2.6 Acquisition system
2.2.7 Oxygen probe
2.3 Experimental protocol
2.4 Methods on bubble hydrodynamics
2.4.1 Bubble velocity and trajectory
2.4.2 Bubble shape and size
2.5 Methods on mass transfer quantification
2.5.1 Calibration of gray level and dissolved concentration
2.5.2 Image processing
2.5.3 Determination of the diffusion coefficient
2.5.4 Determination of the liquid side mass transfer coefficient
2.5.5 Error analysis
2.6 Conclusion
Chapter 3 Comparison of different visualization techniques (PLIF, PLIF-I and colorimetric techniques)
3.1 State of the arts
3.1.1 Free bubbly flows
3.1.2 Plane interface flows
3.1.3 Taylor flows and confined flows
3.2 Experiment under controlled conditions
3.3 Visualization result
3.3.1 Image of side view
3.3.2 Image of bottom view
3.4 Quantification of mass transfer
3.4.1 Calibration result
3.4.2 Mass flux
3.4.3 Diffusion coefficient
3.4.4 Mass transfer coefficient
3.5 Summary of advantage and limitation
3.6 Conclusion
Chapter 4 Hydrodynamics of the Bubble: Characterization of bubble shapes by parametric equations
4.1 State of the art
4.1.1 Bubble shape
4.1.2 Bubble velocity and drag coefficient
4.2 Bubble shape characterization and results
4.2.1 Bubble shape regimes in different solutions
4.2.2 Characterization with aspect ratio
4.2.3 Characterization with parametric equations
4.2.4 Result of shape parameters
4.2.5 Prediction of bubble shape
4.3 Result of bubble velocity and drag
4.3.1 Bubble velocity
4.3.2 Drag coefficient
4.4 Trajectory characterization and result
4.4.1 Trajectory fitting model
4.4.2 Trajectory result
4.5 Conclusion
Chapter 5 Mass transfer in polymer fluids
5.1 State of the art
5.1.1 Mass transfer quantification by visualization techniques
5.2 Mass transfer visualization and characterization
5.2.1 Hydrodynamic result
5.2.2 Oxygen concentration field in the bubble wake
5.2.3 Characterization with Gaussian equation
5.3 Result of mass transfer quantification
5.3.1 Mass flux
5.3.2 Diffusion coefficient
5.3.3 Mass transfer coefficient
5.4 Extension to mass transfer from big bubbles
5.5 Conclusion
References
