Liquid Shear Stress Research Articles

Transient Studies of G-Induced Capillary Flow

A transient, one-dimensional numerical code is developed to model liquid motion in a square groove. Transient body forces up to 0.51 m/s 2 are investigated. Axial variation in liquid level, shear stress and heat transfer between the groove wall and the liquid, evaporation, and body forces are accounted for in the model. Dryout and rewet of the groove are allowed; the front location is determined using conservation of mass. An experiment is presented that measures the depth of liquid in the groove and the dryout and rewet front locations during a transient. Within the uncertainty of the measurements, the code predicts the correct liquid distribution and front locations

On the use of the stratified momentum balance for the deduction of shear stress in horizontal gas–liquid pipe flow

The use of the stratified flow momentum balance for the deduction of interfacial and liquid wall shear stresses from experimental measurements is examined. A systematic error analysis is applied to the governing equations using the principle of maximum uncertainty. A series of air–water experiments were conducted in 50 and 80 mm diameter pipes, in which gas pressure drop, liquid height and gas wall shear stress were measured. A framework for the correlation of the deduced shear stresses is proposed from the experimental measurements. The uncertainty analysis is used to show that the definition of mean liquid height does not significantly influence the overall results. The development of empirical equations based on such methods may lead to total uncertainties of up to 40%, irrespective of accuracy of the experimental data or the appropriateness of the correlating technique. Comparisons with state-of-the-art correlations for the liquid wall and interfacial friction factor data showed even larger discrepancies between measurement and prediction.

LDA MEASUREMENTS OF LOCAL VELOCITIES INSIDE THE GAS PHASE IN HORIZONTAL STRATIFIED/ATOMIZATION TWO-PHASE FLOW

New data on local axial velocities, inside the gas phase in horizontal stratified/atomization air--water pipe flow are reported. Mean local velocities, RMS values and other statistical information have been obtained by analyzing the instantaneous velocity records. From the velocity profiles a secondary flow pattern is inferred with an upward motion at the pipe wall and a downward motion at the vertical pipe centerline. The new measurements are assessed in connection with already reported data of liquid layer characteristics and wall shear stress, obtained under the same flow conditions. The velocity spectra clearly exhibit the influence of the large waves on the gas phase, near the gas–liquid interface; they also suggest that this influence extends up to the top wall of the pipe, probably through the secondary motion inside the gas phase.

Strength characterisation of microbial granules

The strength of microbial granules is still poorly understood. The granule strength is defined as the resistance to attrition and/or breaking by a mechanical force or the liquid shear stress. The strength of various types of microbial granules were studied. The objective was to develop an in vitro strength characterisation test and to be able to predict granule behaviour in full scale systems. Abrasion experiments were conducted in stirred tanks and bubble columns, while the influence of viscosity and medium composition, shear rate and particle size were studied. The medium viscosity did not influence the relation between shear rate and abrasion. Measurements in demineralized water or media containing complexing agents resulted in lower granule strengths presumably the result of the removal of calcium from the granule matrix. In bubble columns attrition is the predominant abrasion process. In full scale reactors, e.g. UASB, attrition is apparently insignificant. In stirred tanks also granule breakage was observed. The abrasion rate is not directly related to the stirrer tip speed. Larger methanogenic granules showed more abrasion compared to small ones. In contrast larger nitrifying granules were stronger than small ones. A standardized strength characterization test was developed to be able to compare strengths of different microbial granules. The strength of slow growing systems appeared to be higher as compared to fast growing systems. From an ecological point of view, cells growing slowly in continuous operated systems gain advantage from attachment and shear resistance or strength.

Mechanisms of biofilm detachment in fluidized bed reactors

Biofilm detachment in liquid fluidized bed biological reactors was investigated to point out how different mechanisms influence the process. Erosion due to liquid shear and abrasion due to collisions of particles were considered as possible mechanisms of biomass detachment in liquid fluidized beds. A dimensional analysis technique allowed the identification of the significant parameters affecting the process. The influence of these parameters was established on a lab-scale reactor. An empirical model was proposed to correlate the experimental data and to analyze the effect of some characteristic quantities, such as particle Reynolds number, biomass fraction, liquid shear stress and solid concentration, on the detachment rate. Detachment rate strongly increased with fluid velocity while, owing to modifications in biofilm structure and morphology during the biological growth, it slightly decreased with liquid shear stress.

Effect of mechanical ventilation on regional variation of pleural liquid thickness in rabbits.

We studied the effect of ventilation on the regional distribution of pleural liquid thickness in anesthetized rabbits. Three transparent pleural windows were made between the second and eight intercostal space along the midaxillary line of the right chest. Fluorescein isothiocyanate-labeled dextran (1 ml) was injected into the pleural space through a rib capsule and allowed to mix with the pleural liquid. The light emitted from the pleural space beneath the windows was measured by fluorescence videomicroscopy at a constant tidal volume (20 ml) and two ventilation frequencies (20 and 40 breaths/min). Pleural liquid thickness was determined from the light measurements after in vitro calibration of pleural liquid collected postmortem. At 20 breaths/min, pleural liquid thickness increased with a cranial-caudal distance from 5 microns at the second to third intercostal space to 30 microns at the sixth through eighth intercostal space. At 40 breaths/min, pleural space thickness was unchanged at the second to third intercostal space but increased to 46 microns at the sixth through eighth intercostal space. To determine this effect on pleural liquid shear stress, we measured relative lung velocity from videomicroscopic images of the lung surface through the windows. Lung velocity amplitude increased with cranial-caudal distance and with ventilation frequency. Calculated shear stress amplitude was constant with cranial-caudal distance but increased with ventilation frequency. Thus, pleural liquid thickness is matched to the relative lung motion so as to maintain a spatially uniform shear stress amplitude in pleural liquid during mechanical ventilation.

Mechanisms of biofilm detachment in fluidized bed reactors

Biofilm detachment in liquid fluidized bed biological reactors was investigated to point out how different mechanisms influence the process. Erosion due to liquid shear and abrasion due to collisions of particles were considered as possible mechanisms of biomass detachment in liquid fluidized beds. A dimensional analysis technique allowed the identification of the significant parameters affecting the process. The influence of these parameters was established on a lab-scale reactor. An empirical model was proposed to correlate the experimental data and to analyze the effect of some characteristic quantities, such as particle Reynolds number, biomass fraction, liquid shear stress and solid concentration, on the detachment rate. Detachment rate strongly increased with fluid velocity while, owing to modifications in biofilm structure and morphology during the biological growth, it slightly decreased with liquid shear stress.

Strength characterisation of microbial granules

The strength of microbial granules is still poorly understood. The granule strength is defined as the resistance to attrition and/or breaking by a mechanical force or the liquid shear stress. The strength of various types of microbial granules were studied. The objective was to develop an in vitro strength characterisation test and to be able to predict granule behaviour in full scale systems. Abrasion experiments were conducted in stirred tanks and bubble columns, while the influence of viscosity and medium composition, shear rate and particle size were studied. The medium viscosity did not influence the relation between shear rate and abrasion. Measurements in demineralized water or media containing complexing agents resulted in lower granule strengths presumably the result of the removal of calcium from the granule matrix. In bubble columns attrition is the predominant abrasion process. In full scale reactors, e.g. UASB, attrition is apparently insignificant. In stirred tanks also granule breakage was observed. The abrasion rate is not directly related to the stirrer tip speed. Larger methanogenic granules showed more abrasion compared to small ones. In contrast larger nitrifying granules were stronger than small ones. A standardized strength characterization test was developed to be able to compare strengths of different microbial granules. The strength of slow growing systems appeared to be higher as compared to fast growing systems. From an ecological point of view, cells growing slowly in continuous operated systems gain advantage from attachment and shear resistance or strength.

Transfert thermique pour la condensation du R123, du R134a et de leurs mélanges, en écoulement forcé entre deux plaques planes horizontales. Etude numérique.

Heat transfer for forced convection condensation of R123, R134a and their mixtures flowing between two horizontal parallel plates. Numerical study. Film condensation of pure and binary mixtures flowing between parallel plates is treated numerically. The coupled equations of mass, momentum, species and energy conservations for the two phases are solved with an implicite scheme. In this study, we retained the pressure forces, the liquid and vapor interfacial shear stress, the Dufour effect, the inertia and enthalpy convection terms, the turbulence in the two phases and the variation of the physical properties with the temperature and concentration. The results obtained for the condensation of refrigerants R123 and R134a, show strong influence of the composition of mixture on the mean heat transfer coefficient and the total pressure loss. The calculated mean Nusselt number is in good agreement with the experimental correlations of Mochizuki and Inoué [6] and Akers and Rosson [24]. A new correlation for the mean heat transfer for forced convection condensation of pure refrigerants R123 and R134a and their mixtures between horizontal flate plates is proposed.

Gas-Liquid Bubbly Flow in Vertical Pipes

Gas-liquid bubbly flow was investigated in vertical pipes for different flow conditions: fully developed turbulent downward flow in a 42.3 mm diameter pipe and upward flow in a 14.8 mm diameter pipe with liquid of elevated viscosity. Wall shear stress, local void fraction, and liquid velocity profiles, shear stress, and velocity fluctuations were measured using an electrodiffusional method. Results obtained demonstrate the existence of “universal” near-wall velocity distribution in a downward bubbly flow. The reduction of turbulent fluctuations is observed in downward flow as compared to a single-phase turbulent flow. The development of bubble-induced liquid velocity fluctuations in a “laminar” bubbly flow was studied.

A microfabricated floating-element shear stress sensor using wafer-bonding technology

A microfabricated floating-element (120 mu m*140 mu m*5 mu m) liquid shear stress sensor has been developed using wafer-bonding technology. The sensor has been designed for high shear stresses (1-100 kPa) and high-pressure environments (up to 6600 psi) and utilizes a piezoresistive transduction scheme. Analytical and finite-element method (FEM) modeling have been performed to predict the sensor response. The sensor has been tested for both its mechanical integrity in high-pressure environments and its output response in the controlled environment of a cone and plate viscometer. The processing steps in the fabrication of the sensor, the analytical and FEM modeling, the experimental procedures, and the results of the experiments are described. >

Mechanism of liquid—solid mass transfer and shear stress in three-phase fluidized beds

Abstract Local solid—liquid mass transfer and shear stress in gas—liquid—solid fluidized beds are studied experimentally to clarify the influence of gas flow. The electrochemical method of local mass transfer coefficient measurements was used. The effect of gas velocity on mass transfer is essentially due to a change in the velocity gradient close to the particle surface. In this case, each sphere is surrounded by a gas—liquid mixture, which has a lower homogeneous density, thus increasing the particle terminal velocity, which in turn affects positively the mass transfer coefficients. The turbulence affects the mass transfer in two different conflicting ways. At mean relative velocity, turbulence increases mass transfer, especially at high Re. In the subcritical domain, turbulence increases the drag coefficient and therefore decreases the particle terminal velocity. This latter effect could have a negative influence on mass transfer.

Biofilm detachment mechanisms in a liquid‐fluidized bed

Bed fluidization offers the possibility of gaining the advantages of fixed-film biological processes without the disadvantage of pore clogging. However, the biofilm detachment rate, due to hydrodynamics and particle-to-particle attrition, is very poorly understood for fluidized-bed biofilm processes. In this work, a two-phase fluidized-bed biofilm was operated under a constant surface loading (0.09 mg total organic carbon/cm(2) day) and with a range of bed height (H), fluid velocities (U), and support-particle concentrations (C(p)). Direct measurements were made for the specific biofilm loss rate coefficient (b(s))and the total biofilm accumulation (X(f)L(f)). A hydrodynamic model allowed independent determination of the biofilm density (X(f)), biofilm thickness (L(f)), liquid shear stress (tau), and Reynolds number (Re). Multiple regression analysis of the results showed that increased particle-to-particle attrition, proportional to C(p) and increased turbulence, described by Re, caused the biofilms to be denser and thinner. The specific detachment rate coefficient (b(s)) increased as C(p) and Re increased. Almost all of the 6, values were larger than predicted by a previous model derived for smooth biofilms on a nonfluidized surface. Therefore, the turbulence and attrition of bed fluidization appear to be dominant detachment mechanisms.

Growth characteristics of a Bombyx mori insect cell line in stationary and suspension cultures

AbstractThe growth characteristics of the BM‐5 insect cell line of Bombyx mori (silkworm) have been experimentally investigated in order to develop optimal growth protocols when these cells are used to produce large quantities of biopesticides or human proteins by recombinant baculoviruses. Experiments were performed in 2 mL wells and 200 mL spinner flasks. Spinner flasks were operated at 80 rpm with 0.3% methyl cellulose (MCL) added to the medium in order to protect the cells from liquid shear stress. In addition to the effect of agitation rate and amount of MCL added to the medium, the cell response during the adaptation to growth in suspension from stationary cultures is reported. Exposure of the cells to varying nutrient and metabolite concentrations is accomplished through batch and repeated‐batch modes in 2 mL wells. The results imply that glutamine is a limiting nutrient and lactate has an inhibitory effect on cell growth. Ammonia depletion from the medium was accompanied by uric acid accumulation, suggesting that ammonia is converted to this metabolic product by the “uricotelic” and “nucleicolytic” metabolic pathways.

Numerical Analysis of Heat and Mass Transfer From Horizontal Cylinders in Downward Flow of Air-Water Mist

A numerical analysis is made of heat and mass transfer from horizontal circular cylinders in a downward flow of air/water mist of polydisperse droplets, taking into account the far-upstream droplet size distribution and the blockage effect of the gas phase flow. The effects of the droplet size distribution, temperatures, and liquid-to-gas mass flow ratio upon the liquid film thickness and wall shear stress, velocity, and temperature of the air-water interface, two-phase Nusselt numbers, etc., are examined.

Non-newtonian liquid-air stratified flow through horizontal tubes—ii

Non-Newtonian liquid gas stratified flow data were obtained using 0.052 and 0.025 m dia horizontal circular ducts. Unless the liquid velocity was very low, the flow pattern generally observed was non-uniform stratified flow having an interfacial level gradient between the two phases. The Heywood-Charles model is valid for predicting the pressure drop and liquid holdup in pseudoplastic (shear thinning) non-Newtonian liquid-gas uniform stratified flow. Two-phase drag reduction, which is predicted by the Heywood-Charles model did not occur because there was a transition to semi-slug flow before the model criteria were reached. Interfacial liquid and gas shear stresses were compared.

Stability of Slag in Turbulent MHD Boundary Layers

The general problem of liquid layer stability is considered in the presence of body forces, and the pressure and shear perturbations generated by an external gas flow. In particular, linear stability analyses are applied to a liquid slag layer driven by a turbulent gaseous boundary layer in the presence of electromotive forces in an MHD channel. It is shown that entrainment of slag could occur in subsonic flows but that the slag layer is stable in supersonic flows. Typical results exhibit disturbance growth rates, phase velocities and disturbance frequencies as functions of liquid Reynolds number and interface shear stress. Within the restrictive assumptions of the analysis, the results show reasonable agreement with available experimental data.

Ultrafiltration of macromolecular solutions with high‐flux membranes

AbstractBatch and flow recirculation cells were used to study the properties of high‐flux ultrafiltration membranes with different macromolecular solutions. At low pressures, solutions of completely retained macromolecular solutes have a flux which is approximately the same as the flux of pure solvent. At higher pressures, the solution flux levels off. The flux, at the leveling‐off period, is approximately inversely proportional to the solution concentration. In this plateau region the flux increases with temperature and agitation of the solution but decreases with time. These results are explained by the formation of a gel layer on the membrane surface during the filtration of macromolecular solutions. In ultrafiltration, in contrast to dialysis and GPC, a linear polymer penetrates the selective barrier more readily than does a globular protein of the same molecular weight. The difference may arise from the liquid shear stresses within the barrier medium due to the movement of fluid relative to the pore walls, which is large only in ultrafiltration. Also, retention of polymers was found to decrease with pressure and to increase with agitation of the solution.

Dropwise condensation of steam at atmospheric and above atmospheric pressures

The results of the initial phases of a study concerned with the mechanism of the dropwise condensation of steam are presented. In the first phase of this work, equivalent heat transfer coefficients at atmospheric pressure for the dropwise condensation of steam on a vertical surface were determined experimentally. The coefficients ranged from 1140 to 37,000 B.t.u./hr-ft 2-°F for heat fluxes of 29,900 up to 167,000 B.t.u./hr-ft 2 with surface temperature differences varying from 45 to 2°F, respectively. Vapor velocities varied from 1·75 to 7·52 ft/sec with the flow directed down the vertical condensing surface. Observations indicated that the vapor velocity across the condensing surface has a significant effect on the equivalent transfer corfficient with the coefficient exhibiting a maximum with increasing vapor velocity. It is believed that this maximum reflects the transition between dropwise and mixed condensation resulting from the greater vapor—liquid interfacial shear stress developed at the higher vapor velocities. Visual observations of the condensation phenomenon were also made at pressures ranging from atmospheric to 200 psig. These observations showed that, as the pressure increases, a transition from dropwise to mixed condensation occurs between 25 to 50 psig. This transition is posited to be associated with the decreased surface tension of the condensed phase at the higher saturation temperatures. A theoretical analysis is presented which is consistent with the experimental observations.