نشریه سوخت و احتراق
سال دوم شماره 2 (پیاپی 4، پاییز و زمستان 1388)

This paper presents a dynamic investigation of fuel droplets breakup at the vicinity of critical Weber number. Based on prior research, the critical Weber number depends on density ratio. However, in traditional diesel spray droplet breakup models such as TAB, the critical Weber number is considered to be constant at a value of 12. In this article, some modification has been proposed in order to improve Taylor analogy breakup model with respect to density ratio effects. The results of the present research indicate that at higher combustion chamber pressure levels, critical Weber number is greater than the custom value of 12 in novel diesel engines. In addition, the results from our proposed model are in good agreement with experimental data reported by other researchers. It is, therefore, concluded that the new modified model is effectively more reliable in comparison with TAB model.

In this paper, a metal porous radiant burner is studied experimentally. Two porosities are tested both individually and in combination. Surface temperature and radiation efficiency of the burner are considered as assessment criteria. The experiments show that the surface temperature rises with an increasing firing rate. This increase in temperature is fast in low firing rates, and slow in high firing rates. In a uniform porous media (non-combined), with a rise in the thickness of the media, the surface temperature and radiation efficiency both decrease. However, with an increase in porosity, the surface temperature as well as radiation efficiency decreases. In such cases, radiation efficiency initially increases, and then remains almost constant. In a combined (two porosity) medium, the surface temperature is maximum for a specific thickness, and although radiation efficiency initially increases for a rising firing rate, it then decreases again.

In this study, a two-dimensional, planar, numerical solution has been used to model the chemically reactive viscous flow in hybrid rocket motors for the purpose of determining the e solid fuel regression rate. The solution employs an implicit finite-volume, lower-upper Steger-Warming scheme (LU-SW) based on Van Leer’s flux vector splitting method together with MUSCL technique, which includes minmod flux-limiter function. In agreement with other experimental studies, C4H6 species is considered as the main gaseous product of HTPB pyrolysis. The rate of pyrolysis is described by means of an Arrhenius-type relationship. In the present work, the chemical reactions between Oxygen and C4H6 are presented through an 11–species and 20–Step chemistry model. Also, turbulence is simulated using the Baldwin-Lomax algebraic eddy viscosity model. The characteristics of reactive flow in port and nozzle such as temperature distribution, Mach number, regression rate and surface temperature are calculated. Numerical simulation of a lab scale motor firing is presented, whereby comparison with other computational and experimental data shows good agreement between the predicted and measured regression rate.

In this work, a detailed combined chemical kinetics mechanism (76 chemical species, 464 reactions) for a mixture of natural gas and n-heptane with arbitrary mass fractions of natural gas between 36 and 85 percent was developed from the combination of the detailed reaction schemes for natural gas and n-heptane fuels. Then, essential reactions were determined through performing a sensitivity analysis on the combined mechanism. In addition, genetic algorithm was applied to optimize the Arrhenius rate coefficients of the specified reactions by means of sensitivity analysis. Finally, accuracy of the presented mechanism was investigated through two different zone configurations (6 zones and 11 ones) of a multi-zone combustion model as well as available laboratory results. Also, the results of the two considered zone configurations from the multi-zone combustion model were compared, whereby it was found that the two zone configurations considered could properly predict combustion, performance parameters (i.e. start of combustion), burn duration, indicated mean effective pressure, indicated thermal efficiency as well as important HCCI engine emissions (i.e. unburned HC and CO emissions) in good accord with the experimental data. Also, it was discovered that the results of the 6 zone combustion model were closer to the results of the 11 zone combustion model. But the required computational time for the 11 zone combustion model was approximately twice that of the 6 zone combustion model.

Catalytic propulsion system is used in adjustment of satellite orbits. Hydrazine is conventionally used as propellant of the system, whose decomposition in the presence of Ir/γ-Alumina catalyst causes hot gases, which leads to the propulsion force for the above purpose. This decomposition process, which is a catalytic combustion, comprises production and subsequent decomposition of ammonia. The specific impulse of such a system is 180-240 seconds. If there is any problem in the catalyst which causes hydrazine decomposition to result only in ammonia, the combustion is incomplete, in which case system performance will be below optimum. In this paper, the performance of the system is modeled in the incomplete combustion state, whereby the results show that the specific impulse in the condition is 150 seconds. This model is expected to provide proper insight into the relationships governing the system. Existing experimental data are also in line with those of the present model.

Mild is an acronym for moderate or intense low-oxygen dilution and refers to oxidizer high preheating and dilution. The Mild combustion has features which set it apart from other combustion systems, such as higher reaction zone volume, more uniform temperature distribution, lower reaction rate, lower heat release rate and lower Damkohler number in comparison with the ordinary combustion regime. In this paper, characteristics of the Mild regime were studied numerically. The experimental conditions of Dally et al. [Proc. Combust. Inst. 29 (2002) 1147–1154] were used for modelling in the present study. The EDC model was used to describe the turbulence-chemistry interaction. The DRM-22 reduced mechanism and the GRI 2.11 full mechanism were used to represent the chemical reactions of H2/methane jet flame. The distribution of temperature, some species, reaction rates and also the importance of molecular diffusion for various O2 levels and jet Reynolds numbers were investigated. The results show that the molecular diffusion in Mild combustion cannot be ignored in comparison with the turbulent transport. Also, the method of inclusion of molecular diffusion in combustion modelling has a considerable effect on the accuracy of numerical modelling of Mild combustion. By decreasing the jet Reynolds number and reducing the oxygen concentration in the airflow, the influence of molecular diffusion on Mild combustion increases. Moreover, the effect of the molecular diffusion reduces on reaction zone when moving away from the nozzle. Although the agreement between numerical and experimental results are very satisfactory, by distancing from the nozzle, the rate of reactions increases while deviation from Mild condition and numerical errors grows.

In this paper, the performance of a natural gas HCCI engine is studied through a thermodynamic model along with its detailed chemical kinetics. The influence of using formaldehyde as an additive on the engine characteristics is also investigated. The results show that it is possible to change the engine working limit using this additive. Furthermore, there is an optimum additive content for each operating condition, which leads to higher output work and power. It is also shown that the air/fuel mixture will ignite earlier using this additive, so it conceivable to reduce inlet mixture temperature resulting in better performance due to higher volumetric efficiency.