Institute of Combustion & Advanced Technologies
of the I.I. Mechnikov Odessa National University
Institute of Combustion and Dept. of General Physics Odessa I.I. Mechnikov National University as research centers specialize on a dispersion systems’ study. They cover ignition and combustion of disperse systems as single particles, agglomerates and dust.
Object of research – powdered metals as aluminum, magnesium, ferum, zirconium, titanium, boron, compounds thereof; organic solid minerals as coal, plastics (polystyrene, polyethylene etc.)., flour, starch, wood dust, manure, humus and other biological hybrid mixtures as methane-coal-air, mixtures of different metals (Al-Mg, Al-Zr, Al-Fe etc.), metal hydrides, polydisperse powders.
Subject of research:
1) Critical temperature and the self-ignition delay times of individual particles, conglomerates, dusts and hybrid systems, depending on the composition, dispersity and concentration of the solid phase. The centers developed the analytical models of ignition of these systems. Experimental study is conducted by using electric ovens adapted for every object: single particles are studied in track installations; conglomerates – in heated air stream on the cut of the furnace; dusts – by a pneumatic pulse injection into a control heated volume.
2) Hydrodynamic modes and mechanisms of the flame expansion in dusts.
There are three possible regimes of combustion depending on the physical and chemical characteristics and hydrodynamic conditions of the process – laminar, vibration and turbulent combustions. Special attention is paid to the laminar flame expansion. The normal (fundamental) flame expansion velocity is the objective characteristic of the combustible mixture.
Experimental study can be performed in several different ways depending on the goals and mixtures’ dispersion characteristics:
- a) vertical pipes of different lengths (L = 1 ÷ 3.5 m) and diameters (4-12 sm) with two flame expansion vector’s orientations – along and against the gravity vector.
Finely dispersed powders (d≤20mm) are studied in a piston gear. Disaggregation and removal of the powder are realized in high-speed jets. Ignition is performed at the open upper end of a pipe by an electric spark of a gas burner or by a pyrotechnic composition.
The implemented combustion’s regime – the transitions’ cascade: laminar flame – vibrating flame I type with longitudinal front vibrations → vibrating flame type II with transverse waves in the front → turbulent flame.
The main characteristics of the laminar flame are the flame expansion limit, the apparent velocity and the flame surface (high-speed video registration), which determine the normal velocity. The dependences of the normal velocity on the particle size and the fuel concentration are studied. Based on those we make conclusions about the mechanism of the particle combustion in the combustion wave and the mechanism of the heat transfer in the combustion wave. Analytical models of laminar flame considering conductive and radiative heat transfers in the combustion wave, the differences in temperature and velocity of the solid and gas phases are developed.
The basic characteristic of the vibrating flame is the concentration limit of the vibration regime’s realization depending on the particle size and the composition of the dust, the phase relations in the combustion wave, the critical amplitude of oscillation velocity and the pressure of the flame type transition to the II type.
Coarse dispersed powders are studied in the special pipes in the following way: the dust is created at the upper closed end and ignited at the lower open end of the pipe.
Various schemes of the dust creation are applied – a disc dispenser and a vibro-sieve dispenser. The procedure is similar to the measurements described above. Furthermore, in these conditions is possible to study the concentration flame instability caused by the hydrodynamic phase interaction and dependences of the lower concentration limits on the dust composition.
The fine dust laminar flames study is performed in a small-volume clouds with volumes ~ 1,32 gal. The cloud is created by the pneumatic pulse technique and poured into the center of the volume. The apparent velocity and the flame front surface are registered and the normal flame expansion velocity is calculated accounting the thermal expansion of the combustion products. This scheme allows us to determine the effective flame temperatures using electron-optical techniques.
Another methodic for the normal velocity of the flame expanding in the fine dispersion dusts determination is realized in the dust plumes for the premixed system of fuel and oxidizer.
Cone-shaped flame is formed on a cut of the metal pipe, where from a special dispenser the premixed systems of fuel and oxidizer in needed proportions is supplied. Consumption of the components is chosen equal to the mass burning velocity in the plume, ie the plume is self-sustaining. This technique is good applied to a convenient organization of the normal flame velocity study (with measured consumption and the front surface) and to realize optical-spectral and electrical studies of the burning plume as well.
The combination of these methods allows us to objectify the concept of the normal flame velocity as a physics-chemical characteristic of the dust by converting the apparent flame velocities for significantly different hydrodynamic conditions into their normal values.
We also note that the method of plume combustion allows us to study the flame velocity dependence on the initial temperature of dusts. This study is realized through the pre-heating the mixture in the delivery pipe. In addition, this method is only possible for the low-calorie dusts mixing (as coal, wood dust, manure dust, etc) combustion.
Conditions of the combustion of the low-calorie combustible substances and combustible garbage, the length and gas parameters dependences on the fuel and oxidizer characteristics can be also determined by the technique described.
Diffusion dispersed plume is performed by a similar tecnique, but with a separate fuel and oxidizer supply. A solid fuel component is delivered with an inert gas via the inner pipe into the ambient air or via the coaxial outer pipe (in the case of controlled oxidizing medium).
Investigation of the explosive characteristics of dusts is performed in a constant-volume reactor (V = 1,057 gal) – a Hartman bomb analogue. Preliminary studies by the standard method showed unsatisfactory disaggregation of powder and pollution of the reaction volume by the dust. We modified the technique and developed the special pneumatic pulse method of the dust creation by high-speed jets of air (delivery is performed from the bottom of the container) and the central ignition by an electric spark, a rapidly combusted wires or a pyrotechnic igniter. The ignition delay time is regular to account the damping of the turbulent fluctuations caused by pneumatic pulse way of the duct spraying. The time dependence of the pressure, which determines the maximum pressure, average and maximum velocity of the pressure rise is automatically registered. Experiments performed with aluminum aero-suspension showed a very good agreement between the data obtained by the technique described and the data obtained on European technique (V = 5,283 gal).