نشریه سوخت و احتراق
سال چهاردهم شماره 4 (پیاپی 37، زمستان 1400)

The non-premixed autoignition characteristic of a hot turbulent jet ejecting into the quiescent air was studied experimentally. The jet was the combustion product of fuel rich natural gas/air mixture burn in a thermally isolated combustion chamber. The combustion products discharge through a nozzle (13 and 20 mm in diameter) into the ambient air. The pre-chamber equivalence ratio, jet Reynolds number, and temperature are the governing parameters which are controlled by mass flow controller and thermocouples. The results of these experiments provide insight into the temporal and spatial non-premixed autoignition of the high-temperature combustible jet. Observations of ignition-flame formation –extinction behaviors of these jets are categorized into 1-no ignition 2- anchored diffusion flame 3- lifted diffusion flame 4- unstable diffusion flame 5- random autoignition kernel. The high-speed imaging helps to measure the location, lifetime and frequency of autoignition kernel using the in-house image processing developed code. Furthermore, the CH chemiluminescence imaging via an optical band-pass filter ensures that the jet composition is chemically quenched at the nozzle tip. The autoignition kernel can occur in radial and axial domain determined by the jet temperature and Reynolds number. The high-speed imaging shows that the most probable zone for autoignition kernel formation is on the jet axis and the jet Reynolds number has the major effect on axial distance. The jet temperature development merely extends this domain in the axial direction. Moreover, the temperature directly affects the lifetime and frequency of autoignition kernel.
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Samples of low-cost activated carbons were synthesized from rose damascena waste by chemical activation using KOH as an activator with different KOH to rose damascena waste ratios (0.5, 1 and 2) as named ACR-0.5, ACR-1, ACR-2.  One sample was loaded with copper nanoparticles (ACR-2-Cu). The synthesized activated carbons have been characterized using FESEM (Field Emission Scanning electron microscopy), FTIR (Fourier transform infrared spectroscopy) and BET surface area analyzer. All activated carbon samples were used in desulfurization of a model diesel fuel composed of n-Heptane   4,6-dimethyldibenzothiophene (4,6-DMDBT) as sulfur containing compound. Results showed that ACRs with BET surface areas up to 1330-2155  were obtained. The BET surface areas and pore volumes increased with KOH to rose damascena waste ratio and Cu functionalization. More than 95% of 4,6-DMDBT adsorption capacity was reached in the first 10 min of adsorption experiment. The efficiency of sulfur removal by ACR was increased by enhancement the KOH to rose damascena waste ratio and by Cu functionalization. The 4,6-DMDBT adsorption capacity follows the order: ACR-2-Cu> ACR-2> ACR-1> ACR-0.5. The maximum adsorption capacity of ACR-2-Cu sample to 4,6-DMDBT was 24 mg S/gr adsorbent and the adsorption capacity was related to the volume of narrow micropores and Cu nanoparticles. The kinetic data was well described by pseudo-second and first-order kinetic models and the equilibrium adsorption data was well fitted to the Langmuir and Freundlich models to estimate the adsorption parameters.

One of the methods of investigating combustion instability is to simulate the flame response to acoustic oscillations. In the present study, in order to model the acoustic oscillations, the artificial excitation method "single frequency harmonic velocity" in the longitudinal direction has been used. The effect of this oscillation on the flame is expressed by numerical solution of LES in the form of flame transfer function (FTF) that chemical kinetics and combustion model have a great effect on determining this function. The combustion simulation results for methane premixed flame showed that although the 17-component EDC combustion model has twice the computational cost compared to the 6-component TF combustion model, it reduces the numerical solution error for calculating the FTF function to less than 5%. After simulation by using Fourier transform of FTF, the amplitude and phase are calculated. The FTF amplitude shows two maximum values at 30-20 Hz and 170 Hz and a minimum point at 80-110 Hz. Combustion instability can be investigated by solving the acoustic equation (Helmholtz) and placing the amplitude and phase of the FTF function as the heat release term in this equation.

The rocket design with low motor weight and more power paves the way for researchers to develop innovations in rocket engines. Combined propulsion design focuses on the simultaneous combustion of solid and liquid propellant components in the combustion chamber. In the present work, simultaneous combustion modeling of propellants (solid and liquid) was done. By writing house code and applying simplifying assumptions, the propulsion system behavior was analyzed. Different compositions were considered for systems with 250, 500, 750, and 1000 kg total mass of propellants. It shows by increasing the solid propellant percentage to 60%, "the maximum thrust" was reached its maximum value while decreasing the motor combustion time. When the solid propellant percentage increases, "total impulse" increases accordingly, and the specific impulse decreases. In another comparison, for a propellant combination with 150 kg of the solid propellant charge and 100 kg of liquid propellant charge, the injection rate related to liquid propellant was adjusted at 4, 8, and 16 liters per second. The results show by increasing injection rate, maximum thrust increased, and total impulse decreased. The "specific impulse" was almost constant.

This efficiency is also associated with the lowest emissions of NOx and CO pollutants, which are 2.2ppm and 4ppm, respectively. Also, the convective heat transfer of the bottom of the dish and its wall, respectively, 58% and 28%, along with radiation from the surface and flame radiation, which account for 2% and 12% of the total heat transfer, respectively. In another experiment, with the constant heat output of the burner, the burner equivalence ratio was investigated and the highest thermal efficiency, corresponding to φ = 0.998 and equal to 23.9%, was obtained.

Increasing the flame propagation speed with aid of centrifugal force proposed by Lewis is a new challenge that can reduce the length of the combustion chamber and thus increase the thrust to weight ratio. In this study, large eddy simulation of premixed combustion of air-propane mixture in a two-dimensional tube with closed ends have been implemented in OpenFoam Software to investigate the effect of centrifugal force on the flame propagation speed. Comparison of numerical solution results with experimental data showed about 8% error in the most critical centrifugal acceleration (3000g). To investigate the effect of the turbulence model, the combustion simulation using the k turbulence model for 3000g, was compared with the LES model and it was observed that the LES model, investigate propagation speed and flame wrinkling with higher accuracy. Considering the flame surface wrinkling parameter, it was observed that at a centrifugal acceleration equal to 4000g, the flame surface wrinkling, suddenly increased and then decreased rapidly that indicates the extinction of the flame. Then, the effect of pipe length on flame propagation speed and flame wrinkling at 2000g was investigated and it was observed that increasing pipe length and distance from the origin of rotation, the induced centrifugal force increased and as a result, propagation speed and wrinkling Increases.

The objective of this paper is two-fold: a comprehensive literature review of the combustion model based on the laminar flamelet assumption; and implementation, application and sensitivity analysis of one of these flamelet models for simulation of turbulent premixed flames. Large-eddy simulation is one of the most reliable approaches in turbulence modelling. Since the computational cost of this approach is substantially more significant than the Reynolds-averaged Navier-Stokes models, the most economical and thus widely-used combustion models in the context of large-eddy simulation are models based on the flamelet assumption. Nevertheless, flamelet models have known shortcomings presented and discussed in detail in this work. The Flamelet-Generated Manifold (FGM) model is one of these models utilized in this work for a large-eddy simulation of a turbulent flame stabilized behind a bluff body. The results show that the accuracy of this model depends on the sub-grid scale variance sub-model its parameters. An algebraic model was used to approximate the variance, but the results were not accurate enough; so that the flame height was under-estimated by approximately 30%, and the error in the mean axial velocity was more than 60% at some points in the domain. However, solving a transport equation for this quantity improves the accuracy of the predictions.