نشریه سوخت و احتراق
سال هشتم شماره 1 (پیاپی 15، بهار و تابستان 1394)

Homogeneous Charge Compression Ignition (HCCI) engines due to reduction of NOx and soot emission and also their high flexibility in using different fuel blends, are suitable substitute for conventional diesel engines. Control of combustion initiation is the main challenge in using these engines. However, the use of natural gas as a clean fuel has always been an attractive choice for many numerical and experimental researchers. Due to high octane number of natural gas, this fuel is resistant to auto-ignition; so in order to improve ignition timing, some additives should be supplied. In this study, the effects of added reformer gas on natural gas combustion have been investigated using a multi zone thermodynamic model. Studies have been performed on the CFR engine. Also, probability distribution function for initial temperature stratificationand the effects of residual gas in the engine is considered. Some important parameters such as factors affecting start of combustion, output power, efficiency, and engine emissions have been investigated. The results show that the added synthetic gases can be an appropriate candidate for ignition timing control. Added inlet synthetic gas has a negligible effect on power and IMEP, but causes advanced ignition and increases heat release rate. With increasing heat release rate and maximum temperature, the thermal efficiency will be decreased and NOx production will be increased.

Low swirl combustion is a novel method to stabilize lean premixed flames. In order to utilize this stabilization method in gas turbines, it is required to gain a better understanding of flow and combustion characteristics of low swirl combustion in different working condition. This paper utilizes large eddy simulation (LES) and a thickened flame model to investigate the characteristics of a low swirl flame under two swirl number conditions. Results from simulation of the low swirl flame show that the lifted flame is stabilized above the burner exit by means of a low velocity zone, rather than the presence of a central recirculation zone. With the increase of swirl number from 0.5 to 0.65, the flame and the central recirculation zone move upstream; however, the stabilization mechanism does not change.

Different catalysts have been developed for in-situ hydrogen production via steam methanol reforming reaction. Attempts are focused on producing catalysts with high conversion and selectivities to produce maximum hydrogen content and minimum amount of carbon monoxide. In the meantime, there are several ways to improve the catalytic properties which can be classified into two major categories: a) addition of promoters; b) change in the synthesis parameters. The purpose of this study is to determine the effects of alumina precursor in the combustion synthesis method for synthesizing nanocatalysts promoted with 5%wt cerium oxide. Two samples with boehmite and aluminum nitrate precursors were synthesized and analyzed via XRD, BET, FESEM, EDX and FTIR to determine their physicochemical properties. CuO and ZnO were detected in XRD patterns and alumina presence was proved via SED-EDX and FTIR, because no peaks could be detected in XRD patterns. FESEM images showed that particles of synthesized samples were in nano range. FESEM analysis also revealed that the particle size of the sample with aluminum nitrate precursor was reduced and a more porous structure was obtained. Catalytic performance studies also exhibited that the sample with aluminum nitrate precursor yields better results in terms of conversion and product selectivities.

This paper is aimed to simulate a jet-stabilized combustor three dimensionally and also investigate the influence of the injection direction of the jet on the combustion characteristics and NOX emissions. A Finite Volume method is adopted to discretize the transport equations. The advection terms of all the transport equations are discretized by power law scheme. An Euler/Lagrangian approach is employed to take into account the gas-liquid interactions and model the spray combustion. Due to fluctuating characteristics of the flow, the presumed Probability Density Function (PDF) method is employed to investigate the chemistry-turbulence interactions. In order to estimate the turbulent behavior of the flow, realizable k-epsilon model is chosen and the discrete ordinates model is applied for predicting radiation heat trasfer. The present model of the jet-stabilized combustor is in a good agreement with the measurements. The results have shown that the injection towards upstream enlarges the recirculation zone and the maximum combustion temperature. Also, the higher combustion temperature at the recirculation zone leads to an increase in the NOX formation. Furthermore, injecting the air jet more towards downstream, yields a more uniform temperature at the combustor exhaust and less NOX formation.

Using fossil fuels in industrial furnaces and power plants has a major role in the production of NOx. In the present study, different chemical kinetics are used to predict NOx reduction by selective non-catalytic reduction method (SNCR). In SNCR method, ammonia is injected into the flue gases in stack, in a temprature rang from 1150 to 1350K and converts NOx to nitrogen and H2O. To validate the present simulations, the results are comapred with experimental data of Ostberg et al. from a cylindrical tube of 5ft length and 5in diameter. Four chemical kinetic mechasnisms offered by Miller, Duo, Glarborg and Brouwer are used in this study to predict the NOx reduction. The results indicate that the Glarborg and Brouwer kinetic mechanisms predict the NOx reduction better than the other mechanisms. In this study the effects of inert materials (Hydrogen and Nitrogen) to improve the reduction of NOx by SNCR is also investigated. An important question that should be considered in SNCR method is the ammonia slip phenomenon.Using Glarborg chemical kinetic mechanism, it is found that theammonia slip decreases by increasing the inlet temperature to 1250K.

Homogeneous Charge Compression Ignition (HCCI) engines are known as a new generation of internal combustion engines. They deliver efficiencies and powers in the range of diesel engines and have achieved better fuel economy and lower NOx emission. Considering the importance of this combustion method, first the 3-D CFD model, coupled with detailed chemical kinetics, was validated with experimental results in the close part of the cycle. The results show good agreement with approximately 5% error in estimating the peak pressure. The model was investigated in different initial pressures and equivalence ratios. Results show that SOC is delayed by decreasing the inlet pressure and increasing the equivalence ratio. Dimethyl ether (DME) was used as the second fuel to reduce the knocking in case of pure methane. This work was done in the constant equivalence ratio that resulted good performance. Also, hydroxyl radical (OH*) was introduced as the controlling species and the increase of OH* had a good agreement with the heat release rate in both cases of pure and blended methane.

Since enhancing the engine power needs redesigning, in this work MTI4.244 diesel engine produced by Tabriz Motorsazan Company has been considered to simulate its combustion process. It has been simulated and linked to the turbocharger and gas exchange models, using integrated simulation in GT-Suite, in order to investigate the possibility of power enhancement and pollutant reduction. Then results of this simulation have been validated by the experimental data of engine which have been carried out in Motorsazan Company. In order to improve engine performance and emission characteristics simultaneously, the effects of injection timing and EGR percentage on combustion, performance and emission of this engine at full load conditions have been studied. For example, at 1350rpm, 9.53% improvement in power and 49.87% reduction in soot have been achieved by advancing injection to 2 CA-BTDC. This change of course causes 54.12% increase in NOx. Finally, using 20% EGR and injection advancing, reduction of soot and NOx together with engine power enhancement have been predicted.