Elevated Initial Temperatures Research Articles

Detonation structure and detonability of C $_2$ H $_2$ -O $_2$ mixtures at elevated initial temperature

We have studied the influence of initial pressure (\(P_0=0.05\)–1 bar) and temperature (\(T_0= 300\)–500 K) on the detonation structure of three acetylene-oxygen based mixtures. Two were stoichiometric, C\(_2\)H\(_2+2.5\) O\(_2\) and C\(_2\)H\(_2+2.5\) O\(_2+3.5\) Ar and one was lean, C\(_2\)H\(_2+10\) O\(_2\). The gaseous mixtures were heated by the detonation tube itself which was electrically heated by Joule effect at a rate of about 100 K per minute. Experimental results show that the detonation cell size \(\lambda \) of the stoichiometric mixtures increases with \(T_0\) and is nearly independent of this parameter for the lean one. The evolution of the cell size with respect to the initial temperature and pressure is reasonably well predicted through the ZND model using global or detailed kinetics. Calculations show that the influence of \(T_0\) on cell size depends strongly on the value of the reduced global activation energy Ea/RT\(_{{\rm ZND}}\) of the given mixture.

Adiabatic shear in annealed and shock-hardened iron and in quenched and tempered 4340 steel

Adiabatic shear localization is a catastrophic failure mechanism which can occur in ductile metals under high strain rate loading. This mechanism is driven by a thermal instability process in which rapid temperature rise due to plastic work couples with thermal softening to cause uniform deformation to collapse into narrow bands of intense shear within which material ductility is exhausted. Adiabatic shear localization is studied in three ferrous metals: annealed Armco and as-received Remco iron, both of which are high purity alpha iron; shock-hardened Remco iron; and 4340 steel quenched and tempered to a range of hardness levels. Using a compressive split-Hopkinson bar, punching-shear experiments were performed at room and elevated initial temperatures at shear strain rates of up to 45 000 s−1. Optical and scanning electron microscopy was performed on the deformed shear specimens to determine the extent of shear localization and mode of failure. Experimental evidence showed that the tempered 4340 steels were susceptible to localization through adiabatic shear banding; however, as-received and shock-hardened Remco iron and annealed Armco iron were not. Finite element simulations of the experiments were performed utilizing a user material subroutine developed as part of this research. This constitutive routine incorporates two adiabatic shear failure criteria, namely (i) maximum shear stress with a minimum critical shear strain rate and (ii) flow localization. These criteria proved to be capable of predicting the onset of an instability; however, the deformation which follows the instability was not predicted well.

Rich flammability limits of fuel mixtures involving hydrogen at elevated temperatures

The rich flammability limits of binary fuel mixtures of hydrogen with methane, ethylene and propane in air were determined experimentally for upward vertical flame propagation at elevated initial mixture temperatures up to 350°C at atmospheric pressure. A conventional stainless steel flammability test tube apparatus was used. Generally, the flammability limits of hydrogen–methane and hydrogen–propane mixtures obeyed Le Chatelier’s Rule at elevated temperatures reasonably well (when the corresponding limits of pure fuels were used) if the hydrogen concentration in the mixture was less than 70%. However, the limits of hydrogen–ethylene mixtures deviated very significantly from those calculated using the Rule. It was also found that the rich flammability limit of hydrogen–methane mixtures at initial temperatures higher than 200°C is a function of the residence time. The longer this time and the higher hydrogen concentration in the fuel mixture the smaller was the value of the rich limit. It was suggested that the narrowing of the rich limit is due to surface reactions on the stainless steel wall during the waiting time that tends to change the mixture composition just prior to spark ignition. The flammability limits of these mixtures exposed to longer residence time do not obey Le Chatelier’s Rule.

Detonation structure of C $_2$ H $_4$ -O $_2$ -Ar mixtures at elevated initial temperature

The influence of initial pressure and temperature on the detonation structure of stoichiometric ethylene-oxygen-argon mixtures was studied. The detonation tube (53 mm i.d. and 7.2 m long) was electrically heated by Joule effect at a rate of about 100 K per minute. For initial pressures ranging from 0.1 to 1 bar, experimental results show that the detonation cell size increases with the initial temperature in the range of 300–600 K. The detonation cell size data were correlated with the chemical induction length calculated using the ZND model for the detonation wave and global chemical reaction kinetics. Due to the low value of the reduced global activation energy of the chemical kinetics $(E_a/{R{{\rm T}_{ZND}}} \simeq 5)$ , the detonation cell size varies inversely with the initial density. Assuming that at elevated initial temperature the classical detonability criteria based on cell size are still valid, we find that the detonability of this mixture decreases when increasing initial temperature.

The Effect of Time of Exposure to Elevated Temperatures on the Flammability Limits of Some Common Gaseous Fuels in Air

The flammability limits of methane, ethylene, propane, and hydrogen were experimentally determined at elevated initial mixture temperatures up to 350°C at atmospheric pressure for upward flame propagation in a steel test tube apparatus. The existence of preignition reactions at these levels of temperatures that may influence the value of the flammability limits was also investigated. The fuel-air mixtures were exposed to elevated temperatures over different periods of time before spark ignition (up to 2 h). It was shown that the flammability limits for methane widened approximately linearly with an increase in the initial mixture temperature over the entire range of temperatures tested and were not affected by the length of the exposure time to these temperatures before spark ignition. However, different behaviour was observed for the flammability limits of the other tested fuels—ethylene, propane, and hydrogen. At higher temperatures the flammability limits narrowed and were very significantly affected by the exposure time. The longer was the exposure time of fuel-air mixtures to the elevated temperatures, the narrower were their flammability limits.

The behaviour of concrete in hot climates

The paper, after very briefly discussing the classification of world climates, considers hot climates before taking up a review of papers dealing with the effects of hot climates on the properties of both fresh and hardened concrete. Raw materials for concrete are discussed, where the adverse and positive effects of C3A and gypsum contents of Portland cement, thermal movement of concrete and sulphate and chloride contents of aggregate and mixing water in hot climates are pointed out. The effect of elevated temperature, ambient and initial paste temperature, ambient relative humidity, solar radiation, and water/cement ratio on the rate of Portland cement hydration and hydration product structure are discussed. The behaviours of both fresh and hardened concrete are taken up. The effect of evaporation from a fresh concrete surface on the behaviour of hardened concrete is discussed. The paper concludes that the presently published works on the properties of concrete in hot climates are fragmentary, uncoordinated and at times contradictory. It points out that systematic studies of the properties of concrete cast and continuously exposed to either hot-humid or hot-dry climates remain to be done.

Domains of flammability and thermal ignitability for pulverized coals and other dusts: Particle size dependences and microscopic residue analyses

The particle size dependence for the lean limits of flammability in air of Pocahontas coal dust (16% as received volatility, by ASTM proximate analysis), Pittsburgh coal dust (35% volatility), and polyethylene powder (100% volatility) were measured using narrow size distributions with average diameters ranging from 2 μm to over 400 μm. In all cases, the lean limits measured in an 8-liter chamber became insensitive to particle size below some characteristic diameter. Above those characteristic diameters, the lean limit concentrations increased significantly with increasing particle size until a critical size was reached, above which the dust was nonflammable for any concentration at ambient temperature and pressure. Both the characteristic diameter and the coarse size limit of flammability (critical diameter) were observed to increase monotonically with increasing dust volatility and increasing oxygen content of the dispersing gas. The same narrow size distributions were also used to measure the particle size dependenceof the thermal ignitability of those same dusts dispersed into preheated air in a 1-liter furnace. The minimum autoignition temperatures (AIT's) display behavior similar to that of the lean limit concentrations, except that heating of the mixture to the elevated temperature required for such thermal ignitability studies markedly increases its flammability range. At those elevated initial temperatures, the characteristic sizes are shifted to larger diameters and no critical sizes were observed for some of the AIT measurements even at the coarsest sizes studied. Also presented are scanning electron microscope (SEM) data that reveal the structuralchanges in coal particles that resulted from their participation in dust explosions. Char residues that are pockmarked with blow holes are the typical remnants from all dust explosions of bituminous coals. At high dust concentrations, those char residues are fused into large, agglomerated masses. Preignition and postignition residues sampled from the thermal ignitability furnace show the same pockmarked appearance for their char residues. In another experiment, using a high power, pulsed CO 2 laser beam, it was possible to carefully control the pyrolysis exposure time and to obtain SEM photomicrographs during the early stages of the pyrolysis-devolatilization process. A theory is presented which isolates the important physical processes in their simplestform. The processes are: (1) heating and devolatilization of the dust particles, (2) mixing of those volatiles with air, and (3) gas-phase combustion of the volatiles-air mixture. There are two loss processes that compete with that propagation sequence and can therefore quench propagation: (a) natural convection or buoyancy, operating through the mechanism of flame stretch, and (b) conductive-convective heat losses to dust particles, which, in addition to being sources of needed fuel, are also heat sinks. The measured insensitivity of the lean limit concentrations to particle size for the finer dusts reflects propagation under rate control by process 3 and quenching by loss process a. A particle size dependence appears at the characteristic diameter because rate control of propagation is shifting from process 3 to process 1, while quenching is still by loss process a. The dust mixture is nonflammable above some critical diameter when the propagation process is entirely rate controlled by process 1 and quenching occurs by a combination of loss processes a and b.

Effect of Initial Temperature on Flash Back of Laminar and Turbulent Burner Flames

The effect of elevated initial temperature on the critical boundary velocity gradient for the flashback of laminar and turbulent flames has been measured for flames of hydrogen and ethylene with air and propane and ethylene with mixtures of 50 percent oxygen and 50 percent nitrogen by volume. For each fuel-oxidant system measurements have been made at a single equivalence ratio close to that giving the maximum gradient. For each system the dependence of the critical boundary velocity gradient on initial temperature is the same for laminar and turbulent conditions. This is consistent with the concept that a turbulent burner flame near flashback is stabilized in a near-laminar portion of the boundary layer. The extent of variation of the critical gradient with initial temperature differs markedly among the several systems studied.