M. Gieras

Ignition and combustion of coal particles at zero gravity

Abstract In the paper, the results of research on ignition and flame propagation between particles of various kinds of coal in conditions at zero gravity are presented. Direct and Schlieren photographs of the process were made. They illustrate the influence of the oxygen concentration in the air, the volatile contents in the coal and the distance between particles in the investigated process. The absence of free convection makes it possible to evaluate directly the influence of two heat transport mechanisms, i.e. the thermal conduction and radiation. There were also presented certain remarks resulting from the investigations of the ignition and burning of single coal particles which may be of use in modelling the coal-dust-air mixture combustion process.

Suppression of dust explosions by means of different explosive charges

Abstract The research was aimed at the development and testing of a super fast explosion suppression system using different explosive charges. The experiments were carried out on a prototype device (steel container) of 2 l capacity, closed by means of a membrane. Below the membrane, there was an exhaust connector pipe, at the end of which was a dispersing head. About 1 kg of extinguishing material was located inside the container. The membrane was ruptured by the explosion of a miniature cumulative charge placed on the membrane surface inside the container. In some experiments the explosive charge was located both on the membrane surface and in the upper part of the container—over the extinguishing material. Pentryt, pyrotechnic charge and powder charge were used as the explosive material. In the first case, the container was filled up (above the extinguishing material surface) with nitrogen under pressure in the range of 30–50 bar. In two other cases, there was no overpressure inside the container. The pyrotechnic charge and powder charge were used as gas generators in order to produce the overpressure only after triggering the system by the signal from the protected volume. Standard electric igniters were applied to initiate the explosive material. The main reason for using the explosive materials for membrane perforation was their high combustion velocity, that enabled opening of the membrane within several microseconds. The dust explosion suppression occurred as a result of the activity of extinguishing material blown out of the container by means of compressed gases. The suppressing powders were used as the extinguishing material. The results of the research into optimization of the shape and miniaturization of the explosive charge mass are presented. Also, the process of shaping of the exhaust stream of the extinguishing material was recorded. Finally, the dust explosion suppression process in a 1.25 m 3 chamber, using the developed super fast extinguishing system was studied.

Experimental and theoretical study of ignition of single coal particles at zero gravity

Abstract In the coal combustion process one of the most important problems is the knowledge of chemical reaction principles concerning combustion of single particles and the knowledge of mutual interaction between the burning particles. In this paper theoretical results are presented, which demonstrate the characteristic ways of ignition between coal particles depending both on volatile content in the coal and oxygen content in the atmosphere. The influence of the particles diameter and the distance between them are also discussed. Based on these experiments, a mathematical and physical model of the process have been made. Due to numerical calculation, the radiation and conduction portion in overall balance of heat exchange between the particle and its surrounding in the course of ignition process for various kinds of cool and oxygen contents in the air have been determined. The presented calculation results show an accordance with data results of experiments carried out at zero gravity conditions.

Experimental Studies of Explosions of Methane-Air Mixtures in a Constant Volume Chamber

Results are presented for the variation of flame propagation velocity with rising explosion pressure for methane-air mixtures. The experiments were carried out in an explosion chamber with a volume of 1.25 m3. Photos of the flame propagating during the explosion processes were obtained through a window in the middle of the front cover of the chamber. The influence of the methane concentration on the time delay between the flame front approaching the chamber wall and the moment when the pressure inside the chamber reached its maximum is also discussed.

Experimental and theoretical investigation into the ignition and combustion processes of single coal particles under zero and normal gravity conditions

The results of research and numerical calculation of ignition and burning process of single dust particles arranged in a row are presented. The influence of particle diameter (0.3–1.2 mm), coal volatile content (2–48%), and oxygen concentration in the gas (21–100%) on the ignition process and the rate of flame propagation were tested. Also the temperature was measured around the particles in the course of ignition and burning processes. Experiments under zero gravity conditions were performed to investigate the process mechanism and to observe ignition which is only controlled by heat radiation and conduction. Comparison of the obtained results with those obtained in normal gravity conditions makes it possible to determine the free convection influence on the investigated process. With the investigated particle diameters ignition occurs on the particle surface and further flame development around the particle depends mainly on the volatile content in the coal. Discrete and continuous flame propagation was observed depending on condition in which the process was carried out. There is an optimal distance between the particles where the maximum flame propagation velocity is obtained. A physical model of the process is presented which explains the flame propagation mechanism and determines the ignition requirements. On the basis of this model, a mathematical model of the process was formulated which enables one to predict the influence of different heat transfer mechanisms on particle ignition and flame propagation.

THEORETICAL STUDIES OF VARIABLE GEOMETRY – HOT SECTION OF THE MINIATURE JET ENGINE

The aim of this article is to present the results of theoretical studies regarding the use of variable geometry hot section of a miniature gas turbine. The variable geometry combustor and variable area nozzle concepts for GTM-120 miniature jet engine are presented in particular. Recent trends of propulsion system size reduction, low-emission combustion and improved fuel efficiency have been considered. A system of variable geometry combustor and variable area nozzle has been proposed as solution. The basic zero-dimensional analytical models for variable geometry combustor and variable area nozzle are developed. Chemkin based model shows significant NOX/CO emissions reduction and combustor outlet enthalpy increases with the use of variable geometry combustor chamber. The analytical model of the variable area nozzle has been proposed. It shows turbine effectiveness increase across its operating range by raising the compressor working line. As a result, noticeable turbine stage efficiency increase has been obtained. Finally, physical implications and future work plans regarding variable geometry hot section of miniature gas turbines are discussed.