Beneficial use of a cell coupling rheometry, conductimetry, and calorimetry to investigate the early age hydration of calcium sulfoaluminate cement

A specific cell was designed to monitor simultaneously the evolution of the viscoelastic properties, electrical conductivity, and temperature of a cement paste with ongoing hydration. Hydration of calcium sulfoaluminate cement by demineralised water or by a borated solution was then investigated as an example. Borate anions acted as set retarders but to a smaller extent than with ordinary Portland cement. The delay in cement hydration resulted from the precipitation of an amorphous or poorly crystallized calcium borate, which also caused a rapid stiffening (and thus a loss of workability) of the paste after mixing. The gypsum content of the CSA cement was shown to play a key role in the control of the cement reactivity.     

Elongational and shear flow in polymer-clay nanocomposites measured by on-line extensional and off-line shear rheometry

Rheological behaviour of polymer nanocomposites has been usually characterized by rotational as well as capillary rheometry, which are both time and cost consuming. We have already published that reinforcement in polymer-clay nanocomposites can be estimated very fast using extensional rheometer in combination with a capillary rheometer. It has been proven that the magnitude of melt strength can be correlated with that of tensile strength, i.e. 3D physical network made of layered silicate and polymer matrix, which is responsible for material reinforcement, can be monitored directly using extensional rheometry. Therefore, additional time for samples preparation by press or injection moulding as well for long measurements by tensile testing is not required any more. In this contribution, results of extensional rheometry measured directly during compounding process are presented. In this manner, further reduction in time required for material characterization has been achieved. The samples have been prepared by advanced compounding using a melt pump and special screw geometries. With the use of on-line extensional rheometry and off-line rotational rheometry, different nanocomposites have been tested and the effect of processing conditions (screw speed and geometry in the twin-screw extruder) on elongational and viscoelastic properties has been investigated. It has been found that the level of melt strength measured by extensional rheometry correlates with a high accuracy with dynamic rheological data measured by rotational rheometry. It was hereby confirmed that the network structure made of silicate platelets in polymer melt is reflected in both elongational and shear flow in the same way.     

Heat flux measurements in stagnation point methane/air flames with thermographic phosphors

Light-induced phosphorescence from thermographic phosphors was used to study the wall temperatures and heat fluxes from nearly one-dimensional flat premixed flames. The investigated flames were stoichiometric, lean and rich laminar methane/air flames with equivalence ratios of φ = 1, φ = 0.75 and φ = 1.25 at ambient pressure. The flames were burning in a stagnation point arrangement against a water-cooled plate. The central part of this plate was an alumina ceramic plate coated from both sides with chromium-doped alumina (ruby) and excited with a Nd:YAG laser or a green light-emitting diode (LED) array to measure the wall temperature from both sides and thus the heat flux rate from the flame. The outlet velocity of the gases was varied from 0.1 to 1.2 m/s. The burner to plate distance (H) ranged from 0.5 to 2 times the burner exit diameter (d = 30 mm). The measured heat flux rates indicate the change of the flame stabilization mechanism from a burner stabilized to a stagnation plate stabilized flame. The results were compared to modeling results of a one-dimensional stagnation point flow, with a detailed reaction mechanism. In order to prove the model, gas phase temperatures were measured by OH-LIF for a stoichiometric stagnation point flame. It turns out that the flame stabilization mechanism and with it the heat fluxes change from low to high mass fluxes. This geometry may be well suited for further studies of the elementary flame wall interaction.     

In situ LIF temperature measurements in aqueous ammonium chloride solution during uni-directional solidification

We present in situ whole-field measurements of the temperature field using laser-induced fluorescence in a study of bottom-chilled uni-directional solidification of aqueous ammonium chloride. We utilize a two-color, two-dye, ratiometric approach to address the significant spatial and temporal variations of laser sheet intensity field due to refractive index variations caused by the evolving concentration and temperature fields. In our work we take advantage of two temperature sensitive fluorescent dyes with opposite temperature sensitivities in order to increase the overall sensitivity and temperature resolution of the measurements. The resulting temperature sensitivity (about 4% K⁻¹) is more than a factor of two higher than the original work of Sakakibara and Adrian (Exp Fluids 26:7–15, 1999) with a sensitivity 1.7% K⁻¹. In situ measurements of the temperature field during solidification are presented, along with temperature characteristics of some of the complex flow features, such as plumes and fingers.     

Nonintrusive measurements of mixture concentration fields by analyzing diffraction image patterns (point spread function) of nanoparticles

The diffraction image patterns of small particles are referred to as their point spread function (PSF); these patterns vary distinctively with the refractive index (RI) of a transparent test medium when the particles are imaged through the medium. The RI correlates directly with the mixture concentration, so proper inversion of measured PSFs can provide full-field information on the mixture concentration field. In this study, fluorescent nanoparticles of 500 nm diameter are fixed on a glass surface by means of evaporative self-assembly, and the time-varying test mixture is placed in front of the glass surface. The time-varying and full-field PSF distributions are imaged and digitally analyzed to determine the local RI values as well as the local mixture concentrations. Both immiscible water/oil mixture and miscible water/glycerol mixture are imaged. The present method shows an RI measurement to have an uncertainty of ±5 × 10⁻³ RIU and the mixture concentration measurements to have uncertainty of approximately 4%.     

A multiple-reflection cell suited for absorption measurements in shock tubes

The improvement of the detection limit in optical concentration measurements by means of multiple-reflection cells (MRC) has become a well established method in absorption spectroscopy.To apply such an experimental method to shock tubes, several technical problems have to be solved. The use of special optical cells showing mechanical ruggedness is required.In this report we present such a MRC designed for a cylindrical shock tube with an inner diameter of 100 mm. By means of 8 mirrors (optical coated stainless steel) the absorption length has been increased by almost one order of magnitude (863 mm) while the optical path is kept in one plane. The use of fixed flat mirrors provides a very high mechanical stability, which facilitates the application to shock tubes.The first experimental results with this MRC were obtained by measuring the thermal decomposition of O₃ behind incident shock waves using an UV laser beam. A direct comparison to the signals simultaneously detected by the use of the conventional absorption length (i.e. the tube diameter of 100 mm) documents the suitability of this MRC for applications in chemical kinetics.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1.0 and the AMS fonts, developed by the American Mathematical Society.     

Birefringence measurements on polymer melts in an axisymmetric flow cell

 The stress-optical rule relates birefringence to stress. Consequently, measurement of flow birefringence provides a non-intrusive technique of measuring stresses in complex flows. In this investigation we explore the use of an axisymmetric geometry to create a uniaxial elongational flow in polymer melts. In axisymmetric flows both birefringence and orientation angle change continuously along the path of the propagating light. The cumulative influence of the material's optical properties along the light's integrated path makes determination of local birefringence in the melt impossible. One can nevertheless use birefringence measurements to compare with predictions from computer simulations as a means of evaluating the constitutive equations for the stress. More specifically, in this investigation we compare the light intensity transmitted through the experimental set-up vs entry position, with the theoretically calculated transmitted intensity distribution as a means of comparing experiment and simulation. The main complication in our experiments is the use of a flow cell that necessarily consists of materials of different refractive indices. This introduces refraction and reflection effects that must be modeled before experimental results can be correctly interpreted. We describe how these effects are taken into account and test the accuracy of predictions against experiments. In addition, the high temperatures required to investigate polymer melts mean that a further complication is introduced by thermal stresses present in the flow cell glass. We describe how these thermal-stresses are also incorporated in the simulations. Finally, we present some preliminary results and evaluate the success of the overall method. Received: 2 April 2001 Accepted: 27 August 2001     

Sources of error and other difficulties in extensional rheometry revisited: commenting and complementing a recent paper by T. Schweizer

In a recent paper by T. Schweizer (Schweizer 2000) a large collection of experimental difficulties associated with the measurement of uniaxial extensional properties of polymer melts in the Rheometrics RME extensional rheometer is described. The work covers topics such as sample preparation for different types of polymers (sensitive or not to moisture) supplied in different shapes (pellets or powder), the necessary corrections to the tensile force, and the ever-present problem of determining the true strain rates of the experiments. The aim of the present paper is to complement and expand the work of Schweizer by pointing out other experimental problems that are the cause of errors in extensional rheometry of polymer melts. The present analysis, however, is not exclusively dedicated to the RME, unlike that of Schweizer, being directed instead to a general class of apparatus that work according to the principle of stretching a constant length sample between pairs of counter-rotating rollers; for example, all the data shown was obtained with our own extensional rheometer (Maia et al. 1999). This work will focus on the importance of the correct choice of the supporting media used for sample heating and support, the importance of end-effects, and the influence that the griping surfaces can have in such measurements. Received: 23 February 2001 Accepted: 15 May 2001     

Chemical analysis by force measurements in electric fields

A method is presented measuring the force acting on a sample in the inhomogeneous part of the electric field of a condensor. The possibilities for using this method in automatic chemical analysis of liquids or in liquid chromatography are discussed.     

Crack detection in elastic beams by static measurements

This paper deals with the identification of a single crack in a beam based on the knowledge of the damage-induced variations in the static deflection of the beam. The crack is simulated by an equivalent linear spring connecting the two adjacent segments of the beam. Sufficient conditions on static measurements which allow for the unique identification of the crack are presented and discussed. The inverse analysis provides exact closed-form expressions of position and severity of the crack as functions of deflection measurements for different boundary conditions. The theoretical results are confirmed by a comparison with static measurements on steel beams with a crack. Extension of the presented analysis to multiple cracks is briefly discussed.     

Velocity measurements in a turbulent,dilute, two-phase jet

The results of an experimental investigation into the behavior of unconfined, steady, fully turbulent, two-phase jets are described. A round jet of 25.4 mm in diameter, exit velocity of 20 m/s and containing 80 m beads with a mass density of loading of 1.5% was examined. Mean velocity profiles at several stations, as well as the rms values and velocity cross-correlations for both phases were measured by laser-Doppler velocimetry. It was found that the particles lagged the fluid by 8% near the exit, but later, at 9 jet diameters downstream led it by about 7%. Also, the velocity profiles of the particles were flatter than those of the fluid.     

Velocity measurements in slow flow by the conductance-tracer method

The conductance tracer velocity measurement method, which is usually employed for determination of mean velocity, is extended to point velocity measurement of the order of a few millimeters per second with accuracies of the order of 10%. Velocity is determined by the time required for a conducting solution to travel between two measuring stations. At each station the conductance between two electrodes is sampled and the resulting conductance time trace is analysed numerically. Cross talk and electrolysis problems are overcome by AC current, alternate switching conductance measurements. The leading edge criteria, suggested in this work, seem to be best suited for travel time determination in such flows. The existence of shear does not seem to have an influence on the validity or the accuracy of the measurement.     

Velocity, density and transport measurements in rotating, stratified flows

A technique is presented for measuring velocity, density and scalar transport in a buoyant rotating gravity current. Existing methods for combined PIV and PLIF are modified for use in a stratified flow on a rotating table and strategies for beam alignment, index of refraction matching, surface tension matching and photobleaching correction are presented. In addition, the PIV–PLIF technique is modified to resolve the velocity and density fields in a cross-section of the current perpendicular to the mean flow direction, allowing the transport in this direction to be computed. This is done by rotating the plane of the laser sheet 15° to the horizontal. This sheet angle is high enough that the entire cross-section of the current is contained in the viewing area, but low enough that horizontal PIV particle displacements are resolved. The technique is used successfully to measure the transport of buoyant fluid in a non-rotating channel to within 5% of the prescribed flow. Results from a rotating gravity current experiment are then presented and compared with previous experiments.     

Errors in hot-wire X-probe measurements induced by unsteady velocity gradients

 Errors in hot-wire X-probe measurements due to unsteady velocity gradients are investigated by a comparison of hot-wire and laser Doppler velocimetry (LDV) measurements. The studied flow case is a laminar boundary layer subjected to high levels of free-stream turbulence, and the hot-wire data shows a local maximum in the wall-normal fluctuation velocity inside the boundary layer. The observed maximum is in agreement with existing hot-wire data, but in conflict with the present LDV measurements as well as existing results from numerical simulations. An explanation for the measurement error is suggested in the paper. Received: 16 October 2000 / Accepted: 23 July 2001     

Eduction of vortical structures by pressure measurements in noncircular jets

Vortical structures in the noncircular jets excited at the interaction mode were educed by measuring fluctuating static pressure, and their characteristics were discussed in relation to the mixing mechanism of the noncircular jets. The contours of phase-average fluctuating pressure show clearly the vortical structures of the noncircular jets, which compare reasonably with those observed by the flow visualization experiments. The evolution of the vortical structures is characterized by partial merging, stretching and splitting to smaller vortices. The effects of the noncircular vortical structures on mixing were also discussed based on the quantitative measurements of the velocity fields, the results suggesting that the interactions of vortical structures in the noncircular jets are very effective to enhance mixing.     

The spectrum of water waves produced by moving point sources, and a related inverse problem

Parameters of moving sources on the sea surface are recovered by spectral analysis of the induced surface waves. The method can be an alternative to the standard way of seeing the ship directly, in particular, when the direct observation is impossible.