The onset of stochastic oscillations in gyrotrons is studied by means of the self-consistent theory describing nonstationary processes. Complicated alternating sequences of regions of stationary, automodulation, and chaotic oscillations are found in the plane of the generalized gyrotron variables: cyclotron resonance mismatch and dimensionless current. The results of the investigations are important in connection with attempts to increase the output power of gyrotrons by raising the current.

We study the flow-induced orientation dynamics of semiflexible fibers in dilute fiber suspensions. Starting from the equations of motion for a two-rod model of flexible fibers in Stokes flow, the Smoluchowski equation for a connected monomer orientation distribution function is derived. We then obtain a set of equations for the time dependence of the first and second moments of the orientation distribution function, thus extending the Folgar Tucker equations for short rigid fiber suspensions to flexible fiber suspensions. The resulting generalized equations for the orientation dynamics of a suspension of flexible fibers are solved for simple channel flow. It is shown that all qualitative effects of bending and straightening of fibers and their influence on the orientation of flexible fibers are captured within our model. A scalar measure for the distribution of bending in a flow is introduced, which allows to detect the degree of bending of fibers

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Abstract. To promote more effective utilization of wheat straw for cleaner heat energy production with reduced greenhouse carbon and polluting emissions, experimental study and mathematical modelling of the processes of wheat straw co-combustion with wood and peat pellets are carried out with the aim to assess the impact of the mixture composition on the development of the main gasification/combustion characteristics and heat energy production. The processes of thermal decomposition of wheat straw mixtures and combustion of volatiles were studied experimentally by varying the mass load of wheat straw pellets in the mixtures. The development of the main gasification/combustion characteristics was studied experimentally using a batch-size pilot setup with a heat output up to 2 kW, which combines a biomass gasifier and a combustor. The results of the experimental study and mathematical modelling suggest that creating a mixture of fuels, which have different elemental compositions and heating values of the components, makes it possible to control the thermal decomposition of wheat straw by varying the mass flow of volatiles entering the combustor and so improving the combustion conditions in the flame reaction zone, which results in faster ignition with more complete combustion of the volatile compounds and in cleaner heat energy production.Keywords: gasification, combustion, wheat straw mixtures. IntroductionBiomass pellets (wood, wheat straw, rape straw, and peat) as fuels are of great interest for district heating because they can contribute to greenhouse gas emission reductions and provide a more efficient use of local energy resources. The test experiments of their applicability for the heat energy production have shown that wood pellets are most suitable as a fuel in boilers without any operational problems [1]. Problems can arise if wheat straw or peat pellets are used as a fuel for the heat production, because they have a higher ash and nitrogen content and a higher release of hazardous emissions (PAHs, CO, HCl) during their thermal decomposition, with serious impact on the boiler operation, human health and on the environment [2; 3]. Actually, the operational and ash-related problems of boilers can be partially reduced by the co-combustion of such mixtures as straw with wood, coal or peat [4; 5]. Co-combustion of different solid fuels allows to improve the boiler operation, to control the slagging and fouling tendency, ash deposition and bed agglomeration as well as the formation of polluting emissions. The results of advanced studies of these processes [4][5][6] show that the development of these processes is highly influenced by the variation of the chemical and elemental composition of biomass pellets to be mixed and depends on the mass load of fuels in the mixture. In fact, it is very difficult to predict and extrapolate the results of experimental studies which might be obtained for a specific fuel mixture to biomass mixtures with a different elemental and chemical composition of the components. Theref...

The aim of this study was to provide more effective use of straw for energy production by co-firing wheat straw pellets with solid fuels (wood, peat pellets) under additional electric control of the combustion characteristics at thermo-chemical conversion of fuel mixtures. Effects of the DC electric field on the main combustion characteristics were studied experimentally using a fixed-bed experimental setup with a heat output up to 4 kW. An axisymmetric electric field was applied to the flame base between the positively charged electrode and the grounded wall of the combustion chamber. The experimental study includes local measurements of the composition of the gasification gas, flame temperature, heat output, combustion efficiency and of the composition of the flue gas considering the variation of the bias voltage of the electrode. A mathematical model of the field-induced thermo-chemical conversion of combustible volatiles has been built using MATLAB. The results confirm that the electric field-induced processes of heat and mass transfer allow to control and improve the main combustion characteristics thus enhancing the fuel burnout and increasing the heat output from the device up to 14% and the produced heat per mass of burned solid fuel up to 7%.

Abstract. This paper deals with a simplified model taking into account the interplay of compressible, laminar, axisymmetric flow and the electrodynamical effects due to Lorentz force's action on the combustion process in a cylindrical pipe. The combustion process with Arrhenius kinetics is modelled by a single step exothermic chemical reaction of fuel and oxidant. We analyze non-stationary PDEs with 6 unknown functions: the 3 components of velocity, density, concentration of fuel and temperature. For pressure the ideal gas law is used. For the inviscid flow approximation ADI method is used. Some numerical results are presented.

Jeffery's equation describes the dynamics of a non-inertial ellipsoidal particle immersed in a Stokes liquid and is used in various models of fiber suspension flow. However, it is not valid in close neighbourhood of a rigid wall. Geometrically impossible orientation states with the fiber penetrating the wall can result from this model. This paper proposes a modification of Jeffery's equation in close proximity to a wall so that the geometrical constraints are obeyed by the solution. A class of models differing in the distribution between the translational and rotational part of the response to the contact is derived. The model is upscaled to a Fokker–Planck equation. Another microscale model is proposed where recoiling from the wall upon the collision is permitted. Numerical examples illustrate the dynamics captured by the models.

Abstract. The recent research is focused on experimental study and mathematical modelling of the development of combustion dynamics at thermo-chemical conversion of biomass mixtures (straw pellets with crashed coal) with the aim to better understand the effect of electric field on the formation of the main gasification/ combustion characteristics when co-combusting straw with crashed coal. The experimental study and numerical modelling of the electric field effects on the combustion dynamics, when co-combusting straw pellets with coal, were carried out to ensure wider use of straw as a fuel for energy production providing the electrodynamic process control. The mathematical model considers the electric field influence on the combustion characteristics using the approximation of 2D axially symmetric compressible swirling flow and chemical reactions with account of the development of A→B↔C kinetics (A -reactant, B -intermediate product, C -final product) downstream the cylindrical combustor.Keywords: biomass pellets, electric field, chemical reactions, mathematical model. IntroductionThe electric field effect on diffusion and premixed flames attract attention as a tool, which allows to control the flame shape, structure and the main flame characteristics, such as the flow velocity, flame temperature, composition, equivalence ratio and products composition [1][2][3][4][5][6]. There are different mechanisms of the electric field effect on the main flame characteristics. First, at high current density, the electric field effect on the flame can be related to the flame heating, with an additional heat input into the flame (plasma support of combustion). Next, the electric body forceinduces an ion drift motion in the field direction.Inelastic collisions between the flame ions and the neutral flame species can then cause the ion wind effect promoting the interrelated processes of field-enhanced heat and mass transfer in the field direction. This can cause variations of the thermal decomposition of solid fuels and the combustion of volatiles. In addition, the electric field-induced inelastic collisions between the electrons and the flame species can cause variations of the rate of reactions. Finally, the flame dynamics can be controlled using the Lorentz force, when the electric field-induced current in the flame reaction zone creates a field-induced magnetic field with axial and radial components of the electromagnetic force, which influence the evolution of flow dynamics and flow vorticity [7]. Thus, multiple factors can influence the development of combustion dynamics, when the electric field is applied to the flame. In order to obtain predictable and controllable field effect on the main flame characteristics, one can vary such parameters as the electric field polarity, bias voltage and current density between the electrodes.More precise and systematic studies of the electric field effect on the combustion dynamics for different types of flames (diffusion, swirling, etc.) and fuels (gaseous, solid and their mixtures)...