A.A. Mouza

The influence of small tube diameter on falling film and flooding phenomena

Flooding experiments have been carried out in vertical small i.d. tubes (6, 7, 8 and 9 mm), using smooth inlet and outlet conditions, with air and two liquids (water and kerosene). Experimental data on free falling film characteristics have also been obtained, in the same test sections, which aid the interpretation of flooding phenomena. These new data suggest that the tube diameter strongly affects film flow development, possibly promoting wave interaction and damping. In turn, the wavy film evolution essentially determines flooding characteristics. At small liquid Reynolds numbers (ReL < 300) critical flooding velocities, UG, follow a trend already reported in the literature, i.e. decreasing with increasing liquid rate. However, at higher ReL the trend is reversed, i.e. increasing UG with increasing liquid rate. This has not been reported in the literature before and may be attributed to damping of waves. At still higher ReL, another region is evident in the flooding curves, characterized by nearly constant flooding velocity. The dominant mechanism in almost all cases is wave growth and upward dragging by the gas, initiated at the liquid exit. 2002 Elsevier Science Ltd. All rights reserved.

Incipient flooding in inclined tubes of small diameter

Abstract This is an experimental study on the effect of tube diameter, inclination angle and liquid properties on incipient flooding in inclined small diameter tubes, in gas–liquid counter-current flow. Flooding experiments are conducted with four relatively small i.d. tubes (6, 7, 8 and 9 mm) in the range of inclination angles 30–60° and smooth inlet/outlet, i.e. a porous tube segment for the liquid and tapered section for gas entry. The effect of liquid properties is examined by using two different liquids, namely water and kerosene. To facilitate the interpretation of flooding data, the free-flowing liquid layer characteristics are also investigated. The results indicate that under these conditions, the effect of tube i.d. and angle of inclination on incipient flooding is significant, but the influence of liquid physical properties is most pronounced.

Visual observations of flooding in narrow rectangular channels

Abstract New flooding data in a vertical rectangular channel with 5 and 10 mm gap between its main parallel plates are reported. Visual observations and fast recordings are made to determine conditions associated with the onset of flooding in the channel. For the 10 mm gap channel a combination of mechanisms, i.e., levitation and upward transportation of laterally coherent or isolated waves, and local wave bridging, appear to be responsible for triggering flooding. The critical flooding velocity is curiously nearly independent of liquid flow rate above film Reynolds number approximately 500. In the 5 mm gap channel and for small Re L , flooding occurs at the liquid entrance, whereas “massive” wave bridging, occurring above the middle of the test section, is the dominant flooding mechanism for relatively higher Re L . Predictions based on small amplitude stability analysis are in good agreement with flooding data from the latter case.

Characteristics of scales from the Milos geothermal plant

Samples of scales have been studied using X-ray diffraction, elemental analysis and scanning electron microscopy. Analyses have also been made of brine and steam samples. The scale consists of heavy metal sulfides and silicon compounds which account for 90-100% of the deposited mass. The composition of the scale depends on the location with respect to the fluid flashing point. Samples in the vicinity of this point largely consist of metal sulfides (PbS, ZnS and CuFeS/sub 2/), while the percentage of silica and possible of other silicon compounds tends to increase farther downstream. In all the samples, a significant part of the iron is not in the form of sulfide and is possibly bound into the silica matrix by some kind of ion-bridging. It is also observed that a silicon - rich layer adheres to the metal surface, even in places where sulfides are the main constituents of the scale.

CFD CODE APPLICATION TO WAVY STRATIFIED GAS-LIQUID FLOW

Acomputational e uid dynamics code was employed to obtain detailed characteristics of horizontal wavy stratie ed two-phase eow for two geometries (pipe and channel). By treating the gas and the liquid phases separately, the calculations are performed utilizing information concerning the pressure drop and the liquid e lm thickness, which can either be obtained experimentally or calculated using one of the available correlations. The basic convergence criterion is the continuity of the time-averaged interfacial velocity and shear stress between the two phases. A commercial CFD code (CFX 1 ) was used for the calculation of the velocity proe les of both phases and the distribution of the shear stresses, i.e. liquid-towall, interfacial. The calculated values are found to be in good agreement with those obtained experimentally.

Local Velocities Inside the Gas Phase During Counter-Current Two-Phase Flow in a Narrow Vertical Channel

New data are reported on local axial velocities inside the gas phase in a vertical rectangular channel with an 8-mm gap between its main parallel plates, during counter-current air/water flow. Such a flow is considered to simulate, though in a simplifled manner, a single flow ‘element’ of a compact condenser. Mean local velocities, velocity spectra, turbulent intensities and other statistical information have been obtained by analysing the instantaneous velocity records of the gas phase acquired in presence of the liquid film, for relatively low gas Reynolds numbers. A computational fiuid dynamics code (CFX®) was employed to obtain the velocity profiles and the values calculated are found to be in good agreement with those obtained experimentally.The data suggest that for gas Reynolds numbers in the range of 500–1300 the gas motion resembles the behaviour of turbulent flow.

UTILIZATION OF USED AUTO-CATALYTIC CONVERTERS IN SMALL COUNTRIES: THE GREEK PARADIGM

Abstract In this conceptual study the utilization of deactivated auto-catalytic converters in small countries, aiming at the recovery of platinum group metals (PGM) is discussed. Several recycling scenarios are examined in an attempt to reach a financially optimal solution. A sensitivity analysis is conducted in order to define the most significant parameters affecting the viability of such a recycling scheme. What is suggested is that for small countries this utilization should be limited to the collection and pre-treatment of used catalytic converters. leaving the precious metal recovery to companies already specializing in such activities. Moreover, only large-scale collection schemes, which can invest in equipment allowing sampling and chemical analysis, should be promoted.

Comparing the Mixing Performance of Common Types of Chaotic Micromixers: A Numerical Study

High-efficiency micromixers are of critical importance in several industrial applications. Implementing chaotic advection to passively enhance fluid mixing is a well-established technique. The purpose of the present study is to compare different configurations of passive mixers. Six different microchannel designs, commonly used in the majority of mixing applications, were numerically evaluated in the present study. Both two-dimensional and three-dimensional serpentine-type microchannel designs were investigated. For each design, the effect of geometrical parameters was studied using a commercial computational fluid dynamics code. Results are presented in terms of mixing efficiency and pressure drop. It is revealed that increasing the flow path by providing compact fluid passages is not the only way to achieve better mixing; inducing a helical flow along the flow length has an additional benefit in improving the mixing performance. The results of this study can be used as a basis for further improving the design of micromixers, while they can also provide the engineers with guiding principles, not for selecting the optimum set of geometrical parameters for a specific microchannel design but for screening out the least efficient micromixer configurations.

Designing and testing regenerative pulp treatment strategies: Modeling the transdentinal transport mechanisms

The need for simulation models to thoroughly test the inflammatory effects of dental materials and dentinogenic effects of specific signaling molecules has been well recognized in current dental research. The development of a model that simulates the transdentinal flow and the mass transfer mechanisms is of prime importance in terms of achieving the objectives of developing more effective treatment modalities in restorative dentistry. The present protocol study is part of an ongoing investigation on the development of a methodology that can calculate the transport rate of selected molecules inside a typical dentinal tubule. The transport rate of biological molecules has been investigated using a validated CFD code. In that framework we propose a simple algorithm that, given the type of molecules of the therapeutic agent and the maximum acceptable time for the drug concentration to attain a required value at the pulpal side of the tubules, can estimate the initial concentration to be imposed.

Liquid Layer Characteristics in Stratified Gas-Liquid Downflow: A Study of Transition to Wavy Flow

The purpose of this work was to study the transition from the smooth to the wavy stratified flow regime for various pipe inclination angles and liquid physical properties. The accurate characterization of both the structure of the gas-liquid interface and the flow field inside the liquid layer can improve our physical understanding of the mechanisms involved in the evolution of waves in stratified gas-liquid flow. To study the influence of liquid properties on the mechanisms promoting wave formation, several liquids are used (i.e., water, Tween®, and aqueous-glycerin solutions). The experiments are conducted in a 24 mm i.d. pipe for various inclination angles (1–9°) with respect to the horizontal position. Liquid layer thickness time records are acquired using a parallel-wire conductance technique from which mean layer thickness, rms, and power spectra of the fluctuations as well as wave celerities are calculated. Measurements of the axial velocity component in the liquid layer using laser-Doppler anemometry are also reported. Statistical analysis of such local liquid velocity data in conjunction with the liquid layer characteristics reveals a strong interplay between wave evolution at the interface and the flow field development inside the liquid layer. Finally, results of numerical calculations using a CFD code are obtained to facilitate data interpretation.