نشریه سوخت و احتراق
سال پانزدهم شماره 2 (پیاپی 39، تابستان 1401)

In this paper, numerical simulation of ignition of fuel-air mixture in a cylinder of a v-type engine has been done by the method of computational fluid dynamics and premixed mixture. The premixed mixture consists of iso-octane and air with an equivalence ratio of 0.9. The existing simulation is performed using the finite volume method and combustion of the premixed mixture. Besides, the dynamic mesh in the piston motion is also used. In this regard, the trend of temperature and pressure changes with respect to the crank angle has been investigated. In order to validate, the results obtained by mixed mixture method were compared to that of a research engine. The simulation results show maximum error of only 2.2% when compared to the experimental results. Therefore, the proposed numerical method can be used in the design of spark ignition engines. In order to optimize the combustion and better spreading the flame front in the pent-roof type combustion chamber, the location of the spark plug has been investigated with several simulations at different spark locations. According to the data of pressure and temperature and either flame front contours and their comparison in different analyzes, the optimal range was achieved.

The increase in waste and used tires has become a serious problem in environmental pollution. As vehicles increase and the average mileage of each vehicle increases, this pollution increases every day. Having a significant amount of thermal energy in tires has made their recycling very attractive to solve environmental problems, so that the recycling of waste tires not only has no harmful effects on the environment, but also the source of industrial development and economic profitability and processing and production of other products. In this research, the aim of this study is to investigate the two-stage heat-catalytic pyrolysis process of waste tires on ZSM-5, Y and β zeolite catalysts in order to produce quality of liquid fuel. The properties of the catalysts were evaluated using X-ray diffraction (XRD) spectroscopy, scanning electron microscopy (SEM) and nitrogen adsorption-desorption analysis. The performance of commercial catalysts in the catalytic cracking of liquid product resulting from thermal pyrolysis of waste tires in the presence of nitrogen gas was investigated. The results showed that zeolite Y with liquid yield of 78.91% by weight had better performance in the above process than the other two catalysts.
.

This numerical study the simultaneous impacts of diesel direct injection timing (pilot injection at 25-40-55 Crank Angle (CA) Before Top Dead Center (BTDC) and main injection at 10 BTDC), combustion chamber geometry (re-entrant (baseline), and wide-shallow chamber), and applying syngas, 20% and 40% of total energy per cycle, in a heavy-duty off-road RCCI engine. This numerical research is conducted using CONVERGE computational fluid dynamic code. The SAGE combustion model was used coupled with a detailed chemical kinetic mechanism consist of 72 species and 360 reactions. The results showed that increasing the syngas to diesel ratio up to 40% caused the combustion speed increased compared to the baseline pure diesel combustion and the start of combustion occurred near the top dead center. Also use of the wide-shallow combustion chamber along with diesel injection at 55 CA BTDC at diesel- 40% syngas combustion operating condition significantly reduced the maximum pressure rise rate compared to other combustion conditions. Additionally, at this combustion condition emissions of Nitrogen Oxides (NOx), soot and Hydro-Carbons are 17.18, 0.015, 0.1 g/kg of fuel which are decreased by 63.5%, 96.5% and 80.2%, respectively, compared to the baseline pure diesel combustion. However, the use of syngas increased the emission of carbon monoxide.
 </strong>
.

In this article, Cambridge-</strong>Sandia stratified burner has been simulated by adding two different amounts of hydrogen to the fuel in order to investigate the effects of molecular diffusion in stratified and premixed combustion. For this purpose, two different models have been considered. In the first model, the Schmidt number is assumed to unity for all species, and in the second model, different Schmidt numbers have been used for each chemical species. The results show that by adding hydrogen in the stratified case, preferential diffusion of lighter species, affects the mass fraction of hydrogen, especially near the surface of the bluff body, and causes a 4-fold increase in the calculated value for its mass fraction. Therefore, molecular diffusion effects cannot be ignored in the governing equations. In the premixed case, although the molecular diffusion has little effect on the species concentration, it still is effective on the temperature distribution. By adding 40% of hydrogen, the maximum temperature increases by 2% and 5% and the average temperature increases by 12% and 15% in premixed and stratified cases, respectively. In general, increasing the amount of hydrogen changes the flame structure more significantly, especially regarding the recirculation zone above the bluff body, which becomes shorter with hydrogen addition.
.

In this work, a non-intrusive method is developed to measure the CH* chemiluminescence in a laminar non-premixed flame. In this method, a digital camera and optical band-pass filters are used. CH* chemiluminescence happens in a sharp band around 430 nm, but in the case of a soot formation in the flame, the resulting thermal radiation partly happens in the CH* chemiluminescence band. In this work, two other band-pass filters with central frequencies of 420 and 440 nm are used to estimate the thermal radiation of soot particles near 430 nm. Using this approximation, the effect of the thermal radiation of soot particles on CH* chemiluminescence is removed. Comparing the experimental results in this work with the numerical simulations show that the proposed method can effectively remove the soot thermal radiation when the soot concentration is low. However, as the soot concentration increases, resulting in enhanced thermal radiation, the accuracy of the method decreases.

Computing the evaporation rate of fuel is one of the challenges and complexities of pool fire simulations. In this study, the evaporation rate of methanol in a shallow pool fire problem is computed using an algorithm that includes analytical and empirical relations. Modeled radiative heat flux and surrounding air temperature are considered transient. Heat and mass transfer phenomena are included in this algorithm, and their effects are discussed. The relative error of results to other numerical studies was below 3 percent. Also, by comparing steady results to experimental results, the relative error was 1 percent. Results show that the evaporation rate in transient radiative heat flux case is delayed 50 seconds to constant radiative flux. In addition, Nusselt number and Grashof number are independent of radiative flux, and depend on surrounding air temperature.

The variety of multiple physical phenomena in the combustion chamber like chemical kinetics, heat transfer (convection and radiation), multiple phases, and thermal decomposition in the absence of oxygen (pyrolysis) related to solid fuel with the accumulation of particles and the liquid film are combustion challenges in the air-breathing combustion systems (ramjet). Understanding the physical and chemical processes involved in combustion is required to predict the physical procedures governing fluid flow, combustion initiation, regression rate, heat release rate, flame, and concentration of species generated during combustion. According to simultaneously solving the combustion and turbulence models, applying the numerical methods in analyzing the governing equations leads to some results, including the effect of variables, performance characteristics, combustion properties, and finally, access to fuel-rich propulsion combustion efficiency in solid fuel ramjet systems. The chemical reactions in the combustor, pyrolysis of Hydroxyl-terminated polybutadiene (HTPB), and the combustion of the pyrolysis products are investigated based on the reduced kinetic mechanisms. Then, by eddy dissipation and finite-rate chemistry models accompanied by K-ε standard and K-ω-SST models in the modeling of solid fuel ramjet combustor, the simulation results are compared with CEA data to identify the best models for accurate combustion prediction. The results showed that the finite-rate model predicts the combustion characteristics of the solid fuel ramjet with much error compared to the eddy dissipation model due to ignoring the effects of flow turbulence and the amount of fuel and air mixing in the combustion chamber. According to the thermodynamic and combustion results, the K-ω/eddy dissipation case was in good agreement with CEA results.
.