The goal of the combustion research is to achieve optimum combustion mode. Reaction Front velocity is one of the most important parameter that is studied in this field. Most biomass consumed at the present time is burned in fixed or moving beds. Fixed-bed combustion systems are characterized by the slow combustion of large particles subjected to an oxidizing ambient. In this paper a model for propagation of a reaction front of wood particles in a fixed Bed is presented. Once the bed is ignited, an apparent flame zone is formed at the bed's top surface and the flame front moves downwards into the bed of fuel at a speed depending on fuel type and operating parameters. Effect of different parameters such as air flow rate through the bed, primary air temperature, moisture content, particle size and number density of fuel particles on the reaction front velocity has been studied. In order to compare the results of this model with the associated experimental data three species of wood fuels are studied and the agreement is found to be satisfactory.

An analytical model for one-dimensional premixed filtration combustion of volatile fuel particles-air mixture is presented. It is presumed that fuel particles first vaporize and a gaseous fuel with definite chemical structure is formed, which is subsequently oxidized in the gas phase. Flame structure is considered in the three zones. In the preheating vaporization zone, the mixture is heated until it reaches ignition temperature. In the reaction zone, the combustible mixture burns and the post flame zone is occupied by the combustion products. The temperature and mass fraction profiles are obtained of gaseous fuel in these three zones at a semi-infinite inert porous media. Thereafter, the effects of various parameters such as gas velocity, porosity, fuel particles diameter, number density of fuel particles, and heat of chemical reaction on the temperature and mass fraction profiles are investigated.

It is observed that a diffusion-controlled mechanism applied to the burning of micron-sized particles is not applicable to the combustion of nano-sized particles burning under kinetically controlled conditions. Furthermore, when heat transfer occurs between micron-sized particles and air, Nusselt number can be assumed to be constant and equal to 2, while this number is a function of Knudsen number when heat transfer occurs between nano-sized particles and air. Ignition temperatures of micron- and nano-sized particles are also different. In this article, mass and energy conservation equations for both particle and gas phases are solved. By doing so, flame velocity is obtained. Afterwards, with respect to different combustion characteristics of micron-and nano-sized particles such as ignition temperature, burning time, and Nusselt number, the effect of particle size on the flame velocity of aluminum particles combustion in air is studied and compared with experimental and numerical results. At the equivalence ratio of 0.85, it is shown that flame velocity is proportional to d−0.94 and d−0.56 for micron- and nano-sized aluminum particles, respectively.

High-speed oblique water entry is an interesting subject, many physical aspects of which remain unknown up to now. Among high-speed air-to-water projectiles, the supercavitating cylindrical-cone (SCC) ones have economic and operational advantages over the other types. However, maintaining stability of the SCC projectiles inside the cavity at shallow entry angles is a challenging issue from both practical and design-related points. The first section of the present study proposes a novel and unique scheme of air-to-water supercavitating projectile design which is called the supercavitating stepped cylindrical-cone (SSCC) projectile. The SSCC scheme is analyzed numerically to investigate the projectile stability improvement at shallow entry angles. The 6DOF dynamics of the SSCC projectile are investigated using the Star-CCM+ commercial code in the presence of three phases of air, water and vapor in a three-dimensional and transient model. Accuracy of numerical results and the model's ability to simulate the physical phenomena of water entry is validated using experimental results from the literature, and both are in good agreement. In the present study, the high-speed oblique water entry dynamics of the SSCC projectile are investigated for five certain entry angles varying from 10° to 60°. The results show that the SSCC projectile faces intensive unstabilizing forces in the water entry process which leads to a heavy pitching moment and, hence, intensive angular velocity (̇) on the projectile. This study also proves that the presence of step enhances the projectile stability in the entry process. The present study shows that, based on their geometry and mass characteristics, each SSCC projectile is capable of withstanding instability up to a critical value of the angular velocity (̇C r). Therefore, projectile stability inside the cavity can be achieved when the value of maximum angular velocity (|̇| max) experienced by the projectile is lower that ̇C r (i.e., |̇| max <̇C r). The results of this study also show that |̇| max is inversely correlated with the , and that it follows a simple equation which is proposed in this study. Therefore, projectile stability inside the cavity can also be practically achieved by adjusting the shooting mechanism at an angle higher than the minimum stable entry angle (min). This study also proposes an effective numerical approach to evaluate min of a supercavitating projectile. It should be noted that determining the value of min is an important factor from both a practical and design-related points of view.

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