Two-zone model for combustion of a composite solid propellant with a coolant

A mathematical model for combustion of a propellant with a coolant taking into account the two-zone structure of the combustion front due to condensed- and gas-phase reactions was developed and examined. Formulas for the burning rate, reaction-zone temperature, and burning-rate sensitivity to internal and external parameters of the system were derived in analytical form. The burning rate was examined numerically as a function of pressure, the characteristic size of the system, component content, and thermodynamic and kinetic parameters. Recommendations are given on methods of obtaining particular ballistic properties by varying the initial parameters of the system.     

The application of a multistage-bed model for residence-time analysis in chemical-looping combustion of solid fuel

The behavior of a 10 kW chemical-looping combustor for solid fuels, normally operated in continuous mode, has been studied by addition of fuel batches. From analysis of gas leaving the air reactor, it was possible to determine the residence time and residence-time distribution of particles in the fuel reactor. Knowing the solids inventory in the fuel reactor, the circulation mass flow could be directly correlated to measured operational data, i.e. pressure drop, temperature and gas flow in air reactor riser. Using results for carbon-capture efficiency and residence-time distribution, a model was developed which could determine a mass-based reaction-rate constant for char conversion. The reaction-rate constant of a Mexican petroleum coke at both 950 and 970 °C was calculated to 8.2 and 28.8 wt%/min, respectively. The reaction-rate constant of a South African coal at 950 °C was calculated to 26.1 wt%/min. This reaction-rate constant could also be determined independently from the conversion rates of char during the batch tests. The results showed a good agreement between the two approaches, indicating that the model well describes the behavior of the unit. From the determination of the circulation mass flow, and comparison with previous testing with different circulation, it was also possible to estimate the limit for which the ilmenite was unable to supply sufficient oxygen to achieve good conversion. This limit was compared to theoretical limits for ilmenite as well as the limit set by a heat balance. It was concluded that the latter will be the one setting the limit.     

Modeling filtration combustion of a pyrolyzing solid fuel

A model is proposed for the steady-state combustion of a mixture of a pyrolyzed solid fuel and an inert material in a countercurrent gaseous oxidizer. The chemical scheme includes the pyrolysis of the original fuel with the formation of a coke residue and gaseous products (pyrolysis tar), oxidation of pyrolysis tar, and oxidation of the coke residue. The process in an infinite nonadiabatic reactor is considered. The one-dimensional single-temperature model includes the energy conservation equation for the system and the mass conservation equations for each species. The original system of equations is solved for each type of thermal structure of the combustion wave (normal and inverse) using an asymptotic method and assuming a narrow combustion zone. Analytical relations between the main macrokinetic parameters of the process are obtained. It is shown that at la ow content of the inert components (in the parametric domain of inverse waves), pyrolysis completely proceeds in a zone distant from the combustion front. In the region of normal waves, more complete combustion of the fuel is observed, which is provided by oxidation of part of pyrolysis tar.     

Three-dimensional random resistor-network model for solid oxide fuel cell composite electrodes

A three-dimensional reconstruction of solid oxide fuel cell (SOFC) composite electrodes was developed to evaluate the performance and further investigate the effect of microstructure on the performance of SOFC electrodes. Porosity of the electrode is controlled by adding pore former particles (spheres) to the electrode and ignoring them in analysis step. To enhance connectivity between particles and increase the length of triple-phase boundary (TPB), sintering process is mimicked by enlarging particles to certain degree after settling them inside the packing. Geometrical characteristics such as length of TBP and active contact area as well as porosity can easily be calculated using the current model. Electrochemical process is simulated using resistor-network model and complete Butler-Volmer equation is used to deal with charge transfer process on TBP. The model shows that TPBs are not uniformly distributed across the electrode and location of TPBs as well as amount of electrochemical reaction is not uniform. Effects of electrode thickness, particle size ratio, electron and ion conductor conductivities and rate of electrochemical reaction on overall electrochemical performance of electrode are investigated.     

Method of calculating the bed combustion of a solid fuel coke residue

An approximate method of solving the steady-state problem of the bed combustion of a solid fuel coke residue is proposed. The method is based on splitting the initial problem into thermal and chemical problems. The former represents the problem on the hydrodynamics and heat transfer in a blown heat-generating granular bed. The latter is the problem on the chemical conversion of a mixture of reactants in a specified temperature field. These auxiliary problems are combined into an iteration cycle to match their solutions. The model problem on the combustion of a milled peat coke residue was calculated for the cocurrent and countercurrent process schemes. The bed combustion of coke is calculated for the experimental conditions published in the literature. The calculated results are compared with the experimental data obtained under these conditions.     

Modeling the structure of composite solid rocket fuel

Results are reported on the creation of a model for the structure of a composite solid rocket fuel in terms of such concepts as “pockets” and “interpocket bridges.” Appropriate model calculations and experimental data are presented, and the possibility of using the model for predicting the characteristics of the combustion process is pointed out. Translated fromFizika Goreniya i Vzryva Vol. 35, No. 2, pp. 35–40, Mach–April 1999.     

Neutron-radioisotope monitoring of the heat of combustion of solid fuel

A radioisotope method is proposed for determining the heat of combustion of coal. This method is based on recording of the ?? radiation associated with the inelastic scattering of fast neutrons and the radiative capture of thermal neutrons by elements in the fuel.     

Simulation of a composite cathode in solid oxide fuel cells

Incorporating the mechanistic model for oxygen reduction at YSZ/LSM interface, a complete micro-model for YSZ/LSM composite cathode considering all forms of polarization was developed which established the interrelationship among the transport phenomena, electrochemical processes and the microstructure of the composite cathode. The exchange current densities of the rate-limiting steps used in the simulation were obtained by fitting the proposed mechanistic model to the DC polarization curves. Simulation was conducted to predict the optimal design parameters, e.g. cathode thickness, particle size, particle size ratio and YSZ volume fraction, for a LSM/YSZ composite cathode. Except for the YSZ volume fraction beyond the percolation thresholds, the predicted results seem to be in good agreement with the experimental and literature data. Incorporating with reliable experimental data, the model can be used as a tool to guide the design of high performance cathodes.     

Film percolation for composite electrodes of solid oxide fuel cells

A film percolation model is proposed for composite electrodes of solid oxide fuel cells (SOFCs). The model is developed to predict the percolation properties of 2D-infinite structures which represent the structural characteristics of composite electrodes of electrochemical devices such as SOFCs. The model can be used to estimate electrode properties, such as percolation probability, active three-phase boundary length and interfacial polarization resistance. Compared with the classic percolation theory, which is particularly applicable to 3D-infinite bulks, the model can explicitly capture the effects of thinly layered nature of composite electrodes, and describes a cross-over between 2D-infinite films and 3D-infinite bulks. It also permits the prediction within whole electrode composition range, and can be easily applied in SOFC modeling.     

Reduced model for the planar solid oxide fuel cell

3D modeling for fuel cells is generally computationally expensive, especially for stacks. In order to reduce computational cost, spatial smoothing over the parallel plain channels in flow fields is introduced and applied to a 3D steady-state isothermal planar solid oxide fuel cell model, which is validated with experiment from literature. The 3D model is reduced to 2D coupled with effective parameters and correlation factors, and then asymptotically reduced to parabolic PDEs and ODEs associated with space marching. The correlation factors, which are derived based on a full set of governing equations for electrokenitics over a cell cross section, can handle not only variations in diffusion pathways due to ribs but also the coupling effect between governing equations. The reduced models are verified with the 3D counterpart in view of global and local properties. Good agreement with a quantitative loss of information is achieved. The reduction in computational cost is investigated.     

A diffusion-relaxation model of pulverized fuel combustion

A model for pulverized fuel combustion with allowance for the microstate of the fuel is proposed. It was shown that the burning of a fuel particle having a microstructure follows the laws of external (diffusion) and internal (relaxation) kinetics. The influence of the diffusion-relaxation mechanism on the burnout time of a fuel particle with a polymeric microstructure, its stressed state, and the pulverized fuel flame length was analyzed. The ranges of pulverized-fuel combustion parameters in which the flame length is determined by the fuel combustion or the gas jet mixing mechanism were distinguished.     

Effect of preheating on liquid fuel evaporation on combustion in a model combustion chamber

A combustion model for a sprayed liquid fuel is developed, starting from the assumption that combustion of an individual drop is impossible, and the droplets are completely entrained in the moving medium. The model gives the a flame length close to that observed experimentally. It is established that the preheat time of the droplet is roughly the same as its evaporation time, and the temperature change in the air basically affects the droplet preheating and only weakly affects the droplet evaporation.Moscow. Translated from Fizika Goreniya i Vzryva, Vol. 27, No. 6, pp. 45–52, November–December, 1991.     

Electrospun composite nanofibers for intermediate-temperature solid oxide fuel cell electrodes

Considering the dominant loss associated with surface oxygen exchange reactions among other complex electrochemical processes, the design of an electrode structure for the reasonable operation of solid oxide fuel cells is challenging. The surface oxygen exchange reaction can be considerably facilitated using composite nanofibers containing the electrode, electrolyte, and catalyst. The composite nanofiber electrodes containing Pd show the smallest polarization resistance of 0.031 Ωcm² and the maximum power density of 0.7 W/cm² at 650 °C, which are 39.2% and 12.5% improved values compare to the catalyst-free composite nanofiber electrodes, respectively. These results provide an facile fabrication strategy for developing high-performance electrodes for use in solid oxide fuel cells.     

Mechanism and laws of combustion of composite solid propellants with a coolant

The model of combustion of a composition consisting of a quasi-homogeneous composite propellant (matrix) and coolant particles is considered. The model is based on the leading role of exothermal decomposition of the matrix and on the cooling effect of the second component by virtue of transverse heat transfer between the components in the condensed and gas phases. Formulas for combustion characteristics (temperature, burning rate, its sensitivity to pressure, and initial temperature) are derived and analyzed. The calculated dependences of these characteristics on pressure, particle size, concentration, and thermal effects of decomposition of the components show that transitional regimes with a stronger dependence of the burning rate on pressure than that of the initial propellant are reached in a certain range of parameters. An algorithm is proposed, and a parametric identification of the model on the basis of experimental data is performed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 6, pp. 51–60, November–December, 2007.