Showing papers on "Air entrainment published in 1994"

Hydrodynamic assist of dynamic wetting

Abstract: Dynamic wetting speeds are limited by the gross entrainment of air between the liquid and the moving substrate. We present experimental data for the curtaincoating method, in which liquid impinges at high speed on the substrate. We also show that air entrainment is strongly affected by macroscopic hydrodynamics and is subject to hysteresis. Using boundary-layer theory, a simple hydrodynamic model is developed for the flow field in the impingement zone away from the dynamic wetting line. The model approximately accounts for the shear thinning of polymer solutions and for the influence of surface tension. We apply the molecular kinetic theory of dynamic wetting, modified to account for hydrodynamic stress, to the immediate vicinity of the wetting line. The main result is a correlation for airentrainment data. The experimentally calibrated model predicts that the hydrodynamic assist of wetting is greatest when the dynamic wetting line is located beneath the impinging curtain. Flow visualization supports this physical picture.

Drag reduction in open channel flow by aeration and suspended load

Abstract: In supercritical open channel flows, air is entrained at the free surface. Such air-water flows, called selfaerated flows, exhibit smaller friction losses than non-aerated flows. New data on drag reduction in selfaerated flows are presented. It is shown that the drag reduction process is linked with the presence of an air concentration boundary layer next to the channel bottom. An analogy with dilute polymer solutions and micro bubble modified boundary layers is developed and it is suggested that the presence of air next to the bottom increases the effective viscosity of the mixture and the sublayer thickness. A parallel with sediment laden flows is also developed. Although the distribution of suspended sediments differs from the distribution of air bubbles, it is suggested that the mechanisms of drag reduction observed in suspended sediment flows are similar to those in self-aerated flows.

Apparatus and system for producing foamed cementitious products

Abstract: An apparatus and method for introducing a metered volume of air into a continuously generated and flowing slurry composed of metered dry cementitious material, metered water and metered air entraining (or foaming) agent and for homogenizing this air into bubbles or cells of relatively small size to produce a uniform cellular product. The apparatus comprises a positive displacement progressive cavity slurry pump driven by a variable speed motor. By driving the pump at a speed in excess of the speed required to accommodate the volume of slurry being generated, the pump will draw air proportionately to this excess of speed. Further elements of the apparatus comprise a closed conduit system for the slurry discharging from the pump, this system communicating with a cylindrical homogenizing apparatus wherein rotating elements homogenize the metered air and slurry with the cooperation of the air entraining agent into a foamed material having relatively small air cells of uniform size. The homogenized slurry material is propelled through and beyond the homogenizing apparatus through a communicating conduit to the point of deposit.

Method of producing concrete structures with a surface protection and a concrete structure produced in accordance with the method

Abstract: Method of producing concrete structures with a surface protection layer and a concrete layer. The method includes moulding the concrete layer and the surface protection layer using a "wet in wet" process. The surface protection layer preferably consists of a cement allied mortar having a low water binding average ratio and a binding agent strength above 70 Mpa. The concrete layer which preferably consists of a concrete having a lower binding agent strength than the surface protection layer, is preferably an air entrained ballast concrete with dense structure and a strength in the range of 10-20 Mpa. The surface protection layer is cast in the bottom of a mold, and the concrete layer is cast above the surface protection layer. During desiccation of the material, the surface protection layer shrinks less than the concrete layer, so that pressure strains are created through deformation differences in the surface protection layer and concrete layer.

Numerical modelling and computation of fluid pressure transients with air entrainment in pumping installations

Abstract: In pumping installations such as sewage pumping stations, where gas content and air entrainment exist, the computation of fluid pressure transients in pipelines becomes grossly inaccurate when a constant wave speed is assumed. An accurate numerical model with gas release and absorption has been developed in this paper and used to compute the fluid pressure transients in the pumping mains of selected pumping installations. Free and dissolved gases in the transported fluid and cavitation at vapour pressure are also modelled. When compared with the gas-free case, computations show that entrained, entrapped or released gases amplify the positive pressure peak, increase surge damping and produce asymmetric pressure surges. While the upsurge with air entrainment in the pipelines was considerably amplified, the downsurge was only marginally reduced. The computed results show good agreement with the data available.

Permittivity of Portland cement concrete at low RF frequencies

Abstract: Portland cement concrete (PCC) specimens were cast and most cured for 28 days. Their complex permittivity was measure dover a frequency ranger of 100 kHz--40 MHz as part of an ongoing study intended to develop a database of electromagnetic properties of PCC. Such information would allow the improvement of current nondestructive evaluation (NDE) methods used to evaluate concrete structures. Low RF (radio frequency) measurements were carried out using a parallel plate capacitor setup connected to an HP 4195A Network/Spectrum Analyzer, Using a complex capacitance model for the capacitor, the complex permittivity of twelve different PCC mixes was evaluated; measurements were performed on four specimens per batch. The selected mixes included three water-to-cement (w/c) ratios, two aggregate types, and two levels of entrained air content; Type 1 portland cement was used. Preliminary measurements showed that the dielectric constant of the concrete specimens varied over frequency from nearly 150 at 1 MHz to around 6 at 40 MHz after 28 days of moist curing. Measured loss tangent was in the range of 0.86--1.8 at 1 MHz and 1.0--2.9 at 40 MHz. A significant effect of w/c ratio, type of aggregate, air entrainment, and the curing state of specimens was observed.

Canadian experience with air-entrained high-performance concrete

Abstract: This article details the experiences of Canadian bridge builders in the use of high-performance concretes High-performance concretes have a compressive strength between 50 and 100 MPa They can be difficult to work with due to air-entrainment specifications and maximum and minimum concrete temperatures The Highway 50 Bridge and the Portneuf Bridge in Quebec illustrate the successful use of these concretes Both bridges had a compressive strength of 60 MPa at 28 days The Highway 50 Bridge was placed in June at an air temperature of 278 deg C (82 deg F) and the Portneuf Bridge was placed in late October at an air temperature of 3 deg C (37 deg F) Discussions on concrete composition, curing, and quality are included Proper materials selection and proportioning are key to concreting success

Atomization and air-entrainment characteristics of unsteady dense sprays

Abstract: Sauter mean diameter and air-entrainment characteristics of nonevaporating unsteady dense sprays are measured by means of an image analysis technique which uses an instantaneous shadow picture of the spray and amount of injected fuel. Influences of injection pressure and ambient gas density on Sauter mean diameter and air entrainment are investigated parametrically. An empirical equation for the Sauter mean diameter is proposed based on a dimensionless analysis of the experimental results in which Sauter mean diameter decreases with an increase in injection pressure and a decrease in ambient gas density. It is also shown that the air-entrainment characteristics can be predicted from the quasi-steady jet theory.

Plunge-Pool Scour by Inclined Jets

Abstract: A series of experiments were performed in a sand-bed plunge pool with circular jets produced by nozzles at different angles and elevations and conveying different flow discharges. The effects of air entrainment, jet inclination angle, jet velocity, and tailwater depth on the depth of scour were analyzed. It was found that air entrainment tends to reduce the amount of scour due to increased dissipation of jet energy by the formation of air bubbles in the plunge pool. Dimensional analysis was used to obtain a relationship between the scour depth and related flow variables.

On the Air-Entrainment Characteristics of Diesel Sprays and Flames in a Quiescent Atmosphere

Abstract: Air-entrainment characteristics of non-evaporating sprays and flames are measured by means of high speed photography including ordinary shadowgraphy of sprays, backscattered light illumination photography and laser shadow photography of flames. Effects of injection pressure and nozzle orifice diameter on air entrainment characteristics are investigated. Using a two-zone thermo-dynamic model, the amounts of air entrained into flames are calculated from the data obtained from the photographs and the pressure measurement in the combustion chamber. The air-entrainment characteristics of flames are compared with those of the corresponding sprays. It has shown that immediately after the start of ignition, the air entrainment into a flame increases more rapidly as compared with the corresponding spray and then, with the development of diffusion combustion, the air entrainment gradually approaches that of the spray.

Waste air-entrained concrete muck as wall material

Abstract: Wall material is made of waste air-entrained concret muck, fibre, early-strength cement, water-soluble adhesive and other industrial wastes, and features light weight, high strength, and excellent thermal insulation performance, and the ratio of industrial wastes may be up to 80% of raw materials

Production of fly ash having low separability and production of concrete having low separability

Abstract: PURPOSE:To reduce an amount of a separation-reducing agent used in obtaining a compaction-free concrete. CONSTITUTION:In a method for producing concrete by kneading cement, aggregate, water reducing agent and water, cement having low separability obtained by mixing a cement clinker with a separation-reducing agent and crushing the mixture is used as the cement. Thereby, the separation-reducing agent can more effectively be dispersed into the cement and sufficient separation reducing effect can be obtained in a small amount of the separation reducing agent used. Since an amount of the separation reducing agent used is reduced, catching of air in kneading is reduced and a defoaming agent is made unnecessary. The producing method is free from problems of lowering of strength due to air entrainment. Since the producing method need not the addition of many kinds of admixtures and additives, blending and concrete kneading operation are reduced and the production is made inexpensive. Poor execution, oscillation in forming, noise pollution and shortage of man power are solved by making compaction unnecessary.

Air Entrainment and Bubble Behavior in Plunge Pools

Abstract: This study analyzes air entrainment by falling nappes and bubble behavior in plunge pools, using an experimental-analytical-numerical method. The experimental work was done at the Tennessee Valley Authority (TVA) Engineering Laboratory. An air-water concentration probe (originally proposed by Lamb and Killen, 1950) constructed by TVA was calibrated and used to determine contours of air concentration in the plunge pool of an overflow nappe. High-speed photography was used to determine representative bubble sizes. The air entrainment was computed analytically by integrating the momentum and continuity equations over the air boundary layer of the nappe. A finite difference, fluid flow solver was used to compute a representative water flow field in the plunge pool. Using that water flow field, the trajectory and residence time of bubbles in the plunge pool were computed by solving the two-dimensional Lagrangian equations of motion. The analytical results show good agreement with experimental data collected by the author and other findings documented in the literature.

Self-Aerated Flow on Corps of Engineers Spillways

Abstract: : Air entrainment in free-surface spillway flows is described with two concepts: 'entrained air,' which is air being transported by the flow as bubbles, and 'entrapped air,' which is the air transported with the flow in the roughness of the water surface. Results from flume experiments are used to develop a mathematical description of entrained and entrapped air for flow along a spillway face. Observations from a full-scale spillway validate the procedure. The theory of gas transfer shows how entrained air affects gas transfer with large increases in the interfacial area. A bubble-size distribution, based on photographic analyses, was used to estimate the surface area in self-aerated flow. Previous experimental work defined the minimum air concentration to prevent cavitation damage at approximately 8 percent. The location along the spillway now can be estimated where entrained air at the spillway surface reaches this concentration. Air entrainment, Gas transfer, Cavitation, Spillway, Free surface

Special Issue on Structural Materials. Freeze-Thaw Resistance of Concrete Incorporating Ferro-Nickel Slag Fine Aggregates.

Abstract: A total of five ferro-nickel slag fine aggregates were examined by blending natural fine aggregates to investigate the freeze-thaw resistance of concrete with these fine aggregates. The volume blending ratio of slag fine aggregates in excess of 60% resulted in poor freeze-thaw resistance of concrete when the water cement ratio exceeded 55% and the air content less than 5%. The inherent freeze-thaw resistance of slag sand particles was superior to that of natural fine aggregates. The poor resistance of concrete with slag fine aggregates couldn't be well explained by the air content and spacing factor. The major cause to impair the resistance was guessed the excess bleeding and it was found that the larger the amount of bleeding, the poorer the freeze-thaw resistance of concrete.It was concluded that the durable concrete could be produced by suppressing the bleeding less than 0.4cm3/cm2 together with the suitable air entrainment.

Loss of freeze-thaw durability of concrete containing accelerating admixtures

Abstract: Rapid freeze–thaw durability tests on air entrained concrete mixes containing a proprietary nonchloride accelerating admixture or CaCl2 show that although early age compressive strength acceleratio...

Development of a water-cement ratio meter. final report - phase ii

Abstract: The objective of this project was to develop a water-cement ratio measurement meter for easy, rapid and reliable use in the field. The goal was that the equipment be portable, easy to use, low cost and low maintenance, and be able to provide consistent and accurate results. A prototype device has been developed to measure the water-cement ratio of a plastic concrete mix. The method is based on the turbidity of water-cement slurry separated from a concrete mixture by "pressure sieving". Consistent results were obtained for both air entrained and non-air entrained concrete. Statistical analyses of the test results have shown that the Kansas Water-Cement Ratio Meter can measure the water-cement ratio of fresh concrete with an accuracy of +/-0.01 with 90% confidence. The test time is less than 10 minutes which includes cleaning the equipment. Further evaluation with admixtures, and under field conditions are planned.