Tran X. Phuoc

Laser spark ignition : experimental determination of laser-induced breakdown thresholds of combustion gases

Abstract This short paper reports preliminary results on experimental measurements of breakdown threshold laser intensities of air, O 2 , N 2 , H 2 , and CH 4 using a Q-switched Nd–Yag laser operating at 532 nm and 1064 nm and 5.5 ns pulse. The breakdown threshold intensities were measured for the range of pressure from 150 to about 3040 Torr. The results showed that the pressure dependence of breakdown threshold is I thr ∝ p − n which is in agreement with the inverse bremsstrahlung absorption process creating breakdown. The degree by which I thr depends on pressure was found to be stronger at 532 nm than at 1064 nm indicating the important effect of diffusion loss.

Pulsatile flow of blood using a modified second-grade fluid model

We study the unsteady pulsatile flow of blood in an artery, where the effects of body acceleration are included. The blood is modeled as a modified second-grade fluid where the viscosity and the normal stress coefficients depend on the shear rate. It is assumed that the blood near the wall behaves as a Newtonian fluid, and in the core as a non-Newtonian fluid. This phenomenon is also known as the Fahraeus-Lindqvist effect. The equations are made dimensionless and solved numerically.

An optical and spectroscopic study of laser-induced sparks to determine available ignition energy

This work conducts an optical and spectroscopic study of laser-induced sparks created in air. It is aimedat determining the minimum ignition energy associated with laser spark ignition. Gas breakdown was produced using a single-mode, Q-switched Nd:YAG laser. It produced a 0.6 cm diameter beam at a wavelength of 1064 nm with a 5.5 ns pulse duration. The beam deflection and the absolute two-line intensity ratio techniques were used for measuring the spark expansion and temperature. The results were used to determine the shock propagation energy, the radiation energy losses, and the energy of the hot gas that is left over after the shock has been propagated away. With spark energies ranging from 15 to 50 mJ, we found that the shock energy was from approximately 70% to 51%, the radiation energy losses were from 22% to 34%, and the energy of the remaining hot gas was about 7% to 8% of the spark total energy. As far as ignition is concerned, the shock propagation and the radiation energies are wasted. The energy of the hot gas is the energy source that causes ignition. Thus, within the limits of error of the present measurements and calculations, the ignition energy obtained by the laser spark ignition does not differ greatly from that obtained by the electric spark ignition as has been reported.

Fully developed flow of a modified second grade fluid with temperature dependent viscosity

SummaryIn this paper we will study the fully developed flow of a modified (and sometimes referred to as the generalized) second grade fluid down an inclined plane. The reasons for using such a model for the flow of non-Newtonian fluids are (i) the capability of predicting the normal stress differences and (ii) allowing for the possibility of shear dependent viscosity. The boundary value problem is solved numerically, and the special case of constant viscosity amends itself an exact solution (as previously reported in the literature) which serves as a test case to check the accuracy of our numerical scheme. The velocity and temperature profiles are obtained for various dimensionless numbers, for the case where the viscosity is also a function of temperature.

On the fully developed flow of a dense particulate mixture in a pipe

Abstract In this paper we present the governing equations for the flow of a dense particulate mixture in a pipe. The governing equations for the individual constituent as well as the mixture are provided based on continuum mechanics. Constitutive relations for the stress tensors and the interaction force are presented and discussed. A model is provided for the viscosity of the mixture. Numerical solution for the steady fully developed isothermal flow of such a mixture in a pipe is presented. The importance of the pressure drop and the skin friction in pipe flow are also discussed.

Experimental studies of the absorption and emissions from laser-induced spark in combustible gases

Abstract This paper reports preliminary results of experimental measurements of the emission and absorption of laser-induced spark in air, O2, N2, H2, and CH4 using a Q-switched Nd-Yag laser operating at 532 nm and 1064 nm, with a pulse duration of 5.5 ns. Line radiation spectra observed for spark in air consist of N and O lines that are distinctly observed in the region from 399.49 to 870.2 nm. The spectra of spark created in CH4 show strong H lines centered at 656.46 nm. Some rather weak emission lines are also observed at 460.4, 497.0, 497.3, and 503.7 nm. Laser wavelength has a strong effect on the absorption data obtained as a function of total laser energy Eo. However, in case of variable pressure, its effect is considered to be within experimental fluctuations.

Rheological behavior of clay-nanoparticle hybrid-added bentonite suspensions: specific role of hybrid additives on the gelation of clay-based fluids.

Two different types of clay nanoparticle hybrid, iron oxide nanoparticle clay hybrid (ICH) and Al(2)O(3)-SiO(2) nanoparticle clay hybrid (ASCH), were synthesized and their effects on the rheological properties of aqueous bentonite fluids in steady state and dynamic state were explored. When ICH particles were added, bentonite particles in the fluid cross-link to form relatively well-oriented porous structure. This is attributed to the development of positively charged edge surfaces in ICH that leads to strengthening of the gel structure of the bentonite susensions. The role of ASCH particles on the interparticle association of the bentonite fluids is different from that of ICH and sensitive to pH. As pH of ASCH-added bentonite suspensions increased, the viscosity, yield stress, storage modulus, and flow stress decreased. In contrast, at low pH, the clay suspensions containing ASCH additives were coagulated and their rheological properties become close to those of ICH added bentonite fluids. A correlation between the net surface charge of the hybrid additives and the rheological properties of the fluids indicates that the embedded nanoparticles within the interlayer space control the variable charge of the edge surfaces of the platelets and determine the particles association behavior of the clay fluids.

High-energy Nd-Yag laser ignition of coals: Experimental observations

Abstract Some experiments were conducted to study the interaction of the Nd-Yag laser radiation with four different coals. All coals were ground and screened to 400 mesh and then pressed into cylindrical pellets of 3 mm diameter and 2 mm length. The coal pellets were prepared by cold pressing using a stainless-steel die to a pressure of about 500 kg/cm 2 . Laser intensities ranging from 0.5 × 10 3 to 1.5 × 10 4 W/cm 2 at 1.064 μm wavelength and a pulse duration of 5 ms were used. For laser intensities less than 800 W/cm 2 , no ignition was observed for all coals. For laser intensities above this value, two ignition mechanisms were observed: the surface ignition followed by the gas-phase ignition when Wyoming subbituminous, Indian lignite, and North Dakota lignite coals were used. For the same range of the laser intensities, however, only the gas-phase ignition was observed when Pittsburgh bituminous coal was used. It was also noted that a significant amount of the external laser radiations were absorbed by the pyrolysis products during the early stages of the ignition period. This process leads to a series of nonlinear phenomena and dictates not only the processes occurring at the coal surface but also in the gas phase.

Transient heating of coal particles undergoing pyrolysis

Abstract Most of the studies reported to date have viewed the coal devolatilization process as occurring isothermally throughout the coal particle, and transient processes occurring in the interior of coal particle have been largely ignored. However, in pulverized coal combustion processes involving several heating rates and several particle sizes, large temperature gradients are often produced within the coal particles that are undergoing pyrolysis. Large temperature gradients inside the coal particle result in pyrolysis of the coal particle in the following manner. The pyrolysis front first initiated at the particle surface propagates inward into the particle, leaving behind a char layer that gradually thickens as the front moves inward. The present work uses the method of line to theoretically investigate the competition between the motion of the pyrolysis front and the intraparticle heating for coal particles as it undergoes heating and pyrolysis under the effects of heating rate, particle size, particle thermal conductivity, and heat of devolatilization. Two intraparticle heating mechanisms have been identified in this study. The first is the intraparticle diffusion heating where the motion of the pyrolysis front is faster than the rise in the surface temperature but is slower than the rise in the temperature of the unreacted core. As a result, the coal particle is totally pyrolized before the surface temperature reaches its oxidation value. The second mechanism is due to the thermal wave moving with the pyrolysis front where the motion of the pyrolysis front is faster than the rise in the temperature of the unreacted core but is slower than the rise in the coal surface temperature. This may lead to a situation where the surface temperature reaches its oxidation value and only a narrow outer layer close to the surface is pyrolized.

The effect of slip boundary condition on the flow of granular materials: a continuum approach

In the previous studies we have looked at a fully developed flow under the assumption that the granular materials adhere to the boundary. Whether one uses the continuum approach or the kinetic theory approach in modeling of the granular materials, slip may occur at the wall, especially when the interstitial fluid is a gas, and therefore the classical assumption of adherence boundary condition at the wall no longer applies. The steady, fully developed flow of granular materials down an inclined plane subject to slip at the wall is studied numerically. This is a non-linear boundary value problem. The results for the velocity profiles are presented in terms of appropriate dimensionless numbers.