Mineral admixtures in mortars: Effect of inert materials on short-term hydration

This work is the first part of an overall project the aim of which is the development of general mix design rules for concrete containing different kinds of mineral admixtures. The separation of the different physical effects responsible for the modification of cement hydration, when chemically inert quartz powders are used in mortars, is presented. The phenomenological approach, based on semi-adiabatic calorimetry, is only associated with first-order phenomena, and the study excludes the complex physicochemical details involved in the chemistry of cement. The results, obtained for a wide range of fineness (between 180 and 2000 m²/kg) and replacement rates (up to 75%), show that short-term degrees of hydration in mortars containing a chemically inert mineral admixture (quartz) are always higher than for a reference mortar. This study confirms that cement hydration is enhanced by inert mineral admixtures. The two main physical effects responsible for the modification of the hydration of cement are identified as the dilution effect and heterogeneous nucleation. The dilution effect of the cement is highlighted with a coarse mineral admixture. The heterogeneous nucleation effect, although it increases with fineness of mineral admixtures, presents an optimum depending on the replacement rate. In the following part of this work, these results will be used for the development of an empirical model allowing us to quantify both physical effects.     

On the problem of laminar flame propagation in a gas with an inert dust

This paper presents a mathematical model and the results of calculation of the velocity of propagation of the flame front in a fuel gas with suspended inert particles taking into account the thermal expansion of the gas and the dynamic relaxation of particles. Dependences of the steady-state flame velocity on the particle size and mass concentration are obtained.     

Synthesis of nanooxides in two-phase laminar flames

This paper deals with a method for production of nanopowders of high-melting metal oxides by burning the starting metal powders in a laminar disperse flame. The method is called gas-dispersed synthesis. The combustion-zone structure for a laminar diffusion dispersed flame and for a laminar flame of premixed fuel and oxidizer was studied experimentally. Information was obtained on the temperature of combustion gases and its spatial distribution, the temperatures of the burning particles and of the condensed combustion products, and the combustion regimes of metal particles. The dependence of the properties of the resulting oxides on the flame and combustion-zone parameters and the mechanism of particle combustion is studied. Based on the results, an attempt is undertaken to reproduce the mechanisms of formation and growth of the condensed phase under conditions of laminar diffusion flames. The mass-averaged particle size of the oxide powders is estimated.Translated from Fizika Goreniya i Vzryva, Vol. 32, No. 3, pp. 24–33, May–June, 1996.     

Effect of the equivalence ratio on the effectiveness of inhibition of laminar premixed hydrogen-air and hydrocarbon-air flames by trimethylphosphate

This paper reports results of an experimental study and numerical simulation of the effect of the equivalence ratio (φ = 0.6–1.6) on the burning velocity of laminar, premixed atmospheric methane-air and propane-air flames without additives and with 0.06% trimethylphosphate (TMP). The effect of the equivalence ratio (φ = 0.7–4.5) on the burning velocity of hydrogen-air flames without additives and with 0.1% TMP was studied by simulation. The experimental and simulation results show that, in hydrocarbon flames doped with TMP, the inhibition effectiveness decreases sharply with a growth in φ from 1.2–1.3 to 1.4–1.6 and in hydrogen-air flames, the inhibition effectiveness increases with a rise in φ from 1.5 to 4.5. The reactions determining the dependence of the inhibition effectiveness on the equivalence ratio were found by analyzing the flame velocity sensitivity coefficients to changes in reaction rate constants. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 2, pp. 14–22, March–April, 2008.     

Effect of velocity nonequilibrium on the laminar flame propagation characteristics in an air-dispersed medium

Moscow. Translated from Fizika Goreniya i Vzryva, Vol. 28, No. 5, pp. 38–44, September–October, 1992.     

An investigation into the stability of enclosed laminar diffusion flames

Experiments on enclosed laminar diffusion flames have revealed the existence of a critical relative velocity of air and fuel which is associated with the threshold of low frequency flame vibration. Starting from the hypothesis that these vibrations are triggered by a selectively amplified disturbance arising in the free shear layers near the base of the flame, the authors have obtained evidence of the nature of the disturbance. Further systematic experiments have enabled them to account for the observed behavior of low frequency vibrating diffusion flames in the light of their hypothesis and in relation to the acoustical properties of their combustion system.     

Experimental study on self-excitations in nitrogen-diluted laminar lifted butane flames

Laminar lifted butane flames diluted with nitrogen have been investigated experimentally to determine distinctive self-excitation regimes in the flame stability maps and also to elucidate the individual self-excitation characteristics. Self-excitations of lift-off height are classified into five regimes in laminar free-jet lift-off butane flames diluted with nitrogen: a stationary lifted regime (regime I), a heat-loss-induced self-excitation (regime II), a buoyancy-induced self-excitation due to flame flicker as well as a heat-loss-induced self-excitation (III), a combined form of an oscillation prior to blow-out and a heat-loss-induced oscillation (regime IV), and a combined form of an buoyancy-induced self-excitation and a heat-loss-induced oscillation as well as an additional buoyancy-driven self-excitation due to flame flicker (regime V). Extremely low-frequency (<0.1 Hz) self-excitation is caused by conductive heat loss from the premixed wings to the trailing diffusion flame and can be explained by a proposed mechanism. It is also found that the flame oscillation prior to flame blow-out is also caused by buoyancy and also significantly affected by the conductive heat loss from the premixed wings to the trailing diffusion flame, thereby showing that the frequency with nozzle exit velocity increases in the triple-flame propagation mode and then decreases in the flame-front propagation mode. Characterization of the individual self-excitation mode is presented and also discussed with Strouhal numbers and its relevant parameters through the analysis of power spectrum for temporal variation of lift-off height.     

Effect of an inert high-modulus ceramic wall on detonation propagation in solid explosive charges

The effect of an intert high-modulus ceramic wall on detonation propagation in charges of a solid heterogeneous explosive was investigated experimentally and numerically. Subdetonation pressures occurred at the boundary between the wall and the explosive for the conditions investigated. Here the detonation velocity increased, and the mass velocity and the pressure at the detonation front decreased, which is explained by the indirect effect of an overtaking wave into the unreacted explosive and the chemical reaction zone. Transverse waves, which affect the detonation parameters, propagate perpendicular to the detonation front with a velocity of ∼6 km/sec. The initial decomposition rate of the explosive directly after the compression shock determines the degree of the transverse-wave effect. Novosibirsk State Technical University. Translated from Fizika Goreniya i Vzryva, Vol. 30, No. 5, pp. 107–114, September–October, 1994.     

Experimental and numerical study of transient laminar counterflow diffusion flames

In the present article, we analyze the nonsteady behavior of counterflow diffusion flames subjected to a time-dependent injection velocity in the case of hydrogen—air flames. The numerical study, using a finite-difference implicit linear multistep method and employing complex kinetics, is done for sinusoidal injection velocity variations, for moderate values and also for values near the extinction limit. The frequency response is obtained in these different cases. The experimental setup comprises two axisymmetric nozzles. In order to change the inlet velocities periodically, we use a vibrating mechanism in each burner. The amplitude and the frequency of injection velocities of the opposed nozzles are synchronously modified. The OH radical emission intensity is used to measure the flame response to velocity fluctuations. The experimental results are compared to numerical calculations. Good agreement is obtained in this comparison.Laboratoire EM2C, CNRS, Ecole Centrale Paris, 92295 Chatenay-Malabry cedex, France. Published in Fizika Goreniya i Vzryva, Vol. 29, No. 3, pp. 61–66, May–June, 1993.     

Effect of admixtures on hot-pressed cements

Types I and III portland cements were hot pressed at 25, 000 psi (170 MPa) pressure and 150°C for $\text{1}\text{2}$ hour with and without admixtures and further hydrated for different periods in de-ionized water. Results showed that strength and microhardness increase with hydration. The previously described relationship between strength and porosity [P = P_o exp (?KS)] si followed in this study as well. Microhardness promises to be a useful non-destructive test. X-ray diffraction analysis, DTA and SEM were used for characterization of final products, for correlation with the other properties. Strength increases were observed due to the action of some admixtures on type III cement, but apparently produced a negative action on type I pastes.     

Effect of added reactive agents on the flame propagation velocity in rich hydrogen-air mixtures

An approximate analytical method for evaluating the efficiency of the action of an inhibitor on the velocity and propagation limits of a flame in rich hydrogen-air mixtures with small amounts of added propylene and isobutylene inhibitors is proposed. The method is based on the model of a narrow reaction zone and the distinct features of the branching chain mechanism of hydrogen oxidation reactions. Using this method, it is shown that the occurrence of flame propagation limits at higher concentrations of added reactive agents (inhibitors) is caused by the existence of a positive feedback between the flame front velocity and the number of active combustion sites, which break down in the inhibitor-added reaction. According to this feedback, the action of the inhibitor decreases the flame temperature and flame velocity.     

Soot aggregates,superaggregates and gel-like networks in laminar diffusion flames

We show that soot formed in laminar diffusion flames of heavily sooting fuels evolves through four distinct growth stages which give rise to four distinct aggregate fractal morphologies. Each successive stage grows upon the previous stage; hence aggregates of one morphology, parameterized by a fractal dimension, form larger superaggregates or gels of a different morphology with a different fractal dimension. These results were inferred from large and small angle static light scattering from the flames, microphotography of the flames, and analysis of soot sampled from the flames. These results and the analysis were substantiated by comparison to computer simulations. The growth stages occur approximately over four successive orders of magnitude in aggregate size. Each growth stage is a fundamental physical process: Stages 1 and 3 are diffusion limited cluster aggregation in three and two dimensions, respectively; Stages 2 and 4 are percolation in three and two dimension, respectively.