نشریه سوخت و احتراق
پیاپی 1 (شهریور 1387)

In the present study, propane/ oxygen and city gas/ oxygen diffusion flames within a laminar regime have been investigated in two parts. First, the effect of oxygen dilution with nitrogen and carbon dioxide gases has been investigated. In this section, stability and flame structure variations against the dilution process are studied. In the second part, oxidant stream preheating up to 480 K, and contemporaneous diluting with nitrogen or carbon dioxide are investigated and the results are compared against non-preheating tests. Preheating causes more flame stability in comparison to the dilution process. Because of the temperature rise in the combustion of such products during preheating, these flames are more luminous in comparison with normal temperature flames.

In the present work, numerical simulations of the pure diesel and diesel-isopropyl alcohol cold sprays are performed using OpenFoam software. These sprays are formed through injecting liquid fuel into an initially quiescent air in a constant volume high pressure chamber. The hybrid, Eulerian-Lagrangian formulation, along with a compressible version of k-ε model, is used. The numerical simulation consists of various sub- models for atomization, droplets breakup, drag and gravitational forces. The ability of the OpenFOAM software to predict spray tip penetration is validated against four cases by comparing the predictions against the available experimental data. In order to complete the input data for the simulations, physical properties of DIPA and Diesel (needed for cold spray simulation) are measured experimentally. Simulations are done in two chamber pressures (10 bar and 50 bar). Spray tip penetration, SMD, droplet size distribution and spray structure for DIPA and Diesel are presented and compared. The results show that the SMD of DIPA20 is smaller than that of pure Diesel. They also indicate that the spray tip penetrations for the two fuels are nearly the same regardless of the properties of the two fuels. It is concluded that DIPA20 could be used in traditional Diesel injection systems without any modifications to the engine geometry.

Increase in environmental pollutions due to application of fossil fuels has encouraged researchers to investigate the renewable and clean fuels. One of the most important renewable fuels is bioethanol which makes a suitable fuel when it has with gasoline. The lower percentage volume of bioetanol blended with gasoline is called gasohol. In the present paper, the effect of gasohol which is a clean fuel on a spark ignition exhaust emissions of: CO, CO2, HC and NOX at 25, 50, 75 and 100% loads and speeds of 2000 and 4000 rpm was investigated. Bioethanol fuel was added to gasoline with different volume percentages (E0, E5, E10, E15 and E20). A gas analyzer was used for measuring exhaust emissions. Results of this study showed that increasing the percentage of bioethanol in gasoline fuel blends decreased CO and HC, while CO2 was increased. Also, for 25, 50 and 75% loads, NOX was decreased, but it was increased in full load conditions.

Homogenous charge compression ignition (HCCI) engines are being actively developed worldwide as they can have efficiencies close to that of diesel engines, with low levels of oxides of nitrogen as we particulate matter emissions. There are challenges associated with the successful operation of HCCI engines particularly with combustion phasing controls. Thus, auto-ignition models are proposed to be used in engine control systems. Hu and Keck model, Shell model, and Knock integral method are selected. Two equations in Knock integral method are proposed for the prediction of start of ignition of the first- and second-stage combustion. The comparison of average differences in auto-ignition delay between the models, experimental data and time calculation indicates that Knock integral model has the highest accuracy as well as shorter time.

Today biodiesel is has known as an alternative to diesel fuel. It is mainly produced from different kinds of vegetable oils. Regarding the high cost of biodiesel made from edible vegetable oils, the waste cooking oils and inedible oils are preferred as low cost potential for biodiesel production. Therefore in this research work, first the biodiesel was produced from waste restaurant oil using transesterification reaction. Then the main characteristics of produced fuel were compared with the ASTM D-6751 standard. With confidence from standardization of the produced biodiesel, the changes in the performance of 6 cylinder Perkins engine was investigated using the blends of biodiesel and diesel fuel at full load. Test results showed that by adding the biodiesel to diesel fuel, due to complete combustion, the power and torque of the engine increases. With regard to the low caloric value of the biodiesel, the specific fuel consumption also slightly increases. The qualitative observations showed that the rate of odor and smoke decreases and the change in odor at high ratios of biodiesel is completely evident. Generally it was found that by adding 25 percent biodiesel made from the waste oil to the diesel fuel, without any change and modification, the engine performance is improved.

The low frequency instability in LRE is modeled and analyzed using the double time lag model in the present work. To this end, the interaction between the fluctuations of injectors representing feed system oscillations on the one hand, and fluctuations of chamber pressure representing combustion oscillations on the other hand have been considered. The dynamics of chamber pressure is modeled through the double time lag or evaporization and mixing time lag model. This way, the low frequency instability boundary in liquid-gas and liquid-liquid engines is analyzed and their corresponding frequencies are determined. It is shown that increasing the pressure drop of injectors and gas residence time can stabilize the engine. Decreasing the mixing time in liquid-gas engines has a stabilizing effect, but this is not true for liquid-liquid engines. In these engines, the mixing time has different effects on the stability boundaries depending on the evaporization time of fuel and oxidizer. It is also concluded that low frequency instability can be initiated at special domains of frequency. The results are in agreement with the available theoretical and experimental results.

In this paper, a thermochemical equilibrium model is used to predict the performance of a downdraft biomass gasifier. Numerical results are shown to be in good agreement with those of the experiments. Different biomass materials are tested using the model, and forest residual is shown to be the most energetic one. For this material, the gasification temperature, syngas composition and calorific value are calculated. The effects of moisture content, air/fuel ratio, air inlet temperature and steam/fuel ratio are also investigated. The air inlet temperature is found to be the only way to increase syngas calorific value and cold gas efficiency. The steam/fuel ratio, on the other hand, plays a key role in controlling the gasification temperature and H2/CO ratio.

A computational study aiming to investigate the effect of combustion chamber geometry on combustion process and emission has been carried out in a direct injection diesel engine. The combustion process and emission of three different combustion chamber geometries were considered, and combustion process behaviors such as variation of mean pressure, velocity, heat release rate, emission production and flame movement were revealed. The results also proved that the chamber shape has significant effects on the combustion and emission behaviors. They also showed that reentrant combustion chamber released less Soot and NOx emissions because of intense swirl, tumble and low temperature combustion. Also it was known that depth of chamber is an effective parameter on NOx and soot formation. The results of this model for cylindrical combustion chamber geometry were compared with the corresponding experimental data and proved to be good agreement. Generally it was found that the shape of Omega is the best selection for piston head, but it strongly needs strongly to match chamber geometry with spray characteristics.