نشریه سوخت و احتراق
سال سیزدهم شماره 3 (پیاپی 32، پاییز 1399)

The aim of the present study is to investigate the effect of carbon dioxide injection on the flame structure using numerical simulation of flameless furnace. Using different amounts of carbon dioxide injection leads to the formation of three air-fuel, oxygen enrich and oxy-fuel combustion modes. Numerical simulations are performed using OpenFoam software. The standard k-εmodel for turbulence modeling and the discrete ordinate model for radiation modeling are used, respectively. Four combustion models based on the eddy dissipation concept model have also been used to validation of combustion model. Studies are performed on temperature distribution, ignition delay, flame color, and hydroxyl radical in order to the investigation of flame structure. The results show that by switching from air-fuel flameless combustion to oxygen enrich and oxy-fuel flameless combustions coupled with the replacement of part or all of the mass fraction of nitrogen with carbon dioxide, the maximum flame temperature is reduced due to higher carbon dioxide heat capacity and active presence in the chemical reactions. Moreover, the presence of carbon dioxide in oxygen enrich and oxy-fuel combustion conditions results in a delayed in ignition process and significantly reduces the concentration of methylene radical, which is the driving factor of visible light.

In the present study, combustion characteristics of premixed hydrogen-air mixture in converging - diverging microtubes were investigated using numerical simulation of combustion process. The purpose is investigation of the effect of inlet velocity and converging-diverging angle on the combustion characteristics consist of maximum flame temperature, flame position, upper flammability limit and flame thickness. Governing equations were considered as three dimensional and transient with detailed chemistry mechanisms. Results showed that inlet velocity affect flame position in the microtube and maximum flame temperature related on the flame position will change. Minimum flame temperature in a specific equivalence ratio were occurred when the flame placed in the converging-diverging section. Inlet velocity and throat section have a direct effect on the flame thickness. Increasing the flame thickness at high inlet velocities for microtubes with low throat diameter causes to blow out a part of the flame. Compared to a microtube with the same dimensions, it was found that making a throat zone in the microtubes causes to increase the upper flammability limit.

In this paper, the combustion process in a prototype scramjet (DLR) is numerically simulated and studied. To that end, the scramjet is initially simulated and validated in the non-reacting condition. The results show that the method used has the ability to predict the fields of velocity and pressure with appropriate accuracy. Then, the same scramjet is simulated and validated for the reacting case by considering one-reaction model. The results of these two simulations illustrates that the maximum distance of subsonic area from the strut is 141 mm in the reacting case while this distance is only 22 mm in the non-reacting case. the waves pass through the jet region with a slight deviation in the non-reacting case while they are reflected after hitting the jet region in the reacting case.the waves pass through the jet region with a slight deviation in the non-reacting case while they are reflected after hitting the jet region in the reacting case.

In this study, the effect of a low swirl injector blockage ratio on the flame stability limits is investigated by using a low swirl burner. The blockage ratio is one of the main geometrical parameters of the low-swirl injector which has a significant impact on the stability of the flame. In order to investigate the influence of the blockage ratio of the central channel of low swirl injector on stability, nine swirler with distinct geometries were investigated. The results showed that with increasing the blockage ratio, the flame extinction limit was reduced so that increasing the blockage ratio from 43% to 76% reduced the extinction limit to about 25% and improved flame stability. It was also observed in this study that by increasing the equivalence ratio, the flame undergoes a transition process in which the flame proceeds from a stable lifted bowl shape to a vortex shape attached to the burner nozzle. The transition process can be divided into three different combustion regimes and in this study, these limits presented based on the equivalence ratio and the Reynolds number of the burner inlet. Evaluation of NOx pollutants of swirlers showed that Nox increased with increasing equivalence ratio and at equivalence ratios near stoichiometric values, the Nox level was about 20 ppm.

Transient evaporation of a bi-component droplet at high temperature and pressure condition has been modeled numerically. In the gas phase, the equations of species, momentum, and energy, and in the liquid phase, the equations of species and energy by assuming fugacity equilibrium, and Peng-Robinson equation of state using the finite volume method have been solved. The droplet is assumed to be spherical, the solubility of the gas in the liquid and effect of gravity have been neglected. The results of the proposed model for heptane-hexadecane droplet in various temperatures and pressures have been validated against the available experimental data. Results were in good agreement. The effect of pressure on the evaporation showed that at a constant temperature, by increasing pressure up to 2 MPa the droplet lifetime increases but further increase in pressure (up to 2.5 MPa) reduces droplet lifetime. Evaporation behavior does not change with different compositions. The role of different equations of state on the prediction of binary droplet lifetime was studied, and finally the effect of ambient temperature and pressure on the surface temperature of the droplet was investigated. It was observed that at high enough pressure and temperature, the droplet surface could reach the critical condition.

Homogenization of lean air-fuel mixture decrease flame front temperature and simultaneously reduces NOx and particulate matter. Experimental work has been shown high level of dispersion can be obtained by impingement of diesel jet onto a matrix of cylindrical obstacles in accordance structure close to Porous media. The injected fuel impinges with the first cylindrical obstacle and splits to two smaller jets and with impingement with other obstacles causes to multi-jets formation causes to fuel dispersion. Previous experimental works have been shown geometrical structure and obstacle diameter are of key factors in multi‐jet formation and spatial distribution of the charge. In this paper, computational fluid dynamics models that accurately predict the formation behavior of several transient multi-jet are developed, and after validation, new structures of obstacles with increasing distances between them and an improved structure are proposed as novel innovations. New modeling shows that these structures have better homogenization properties and space distribution is more efficient than the structures previously studied. Also, with increasing the distance between the cylindrical obstacles, the diffusion and penetration of some fuel jets has increased and for all geometrical structures, it has increased the fuel distribution angle. The geometrical structure of the obstacles is very important to achieve the proper fuel distribution pattern and is not dependent on the number of obstacles.

In piston engines, as the engine speed increases, the combustion cycle time decreases. Reducing the engine combustion cycle time, the opportunity for complete fuel combustion is reduced. As a result, in order to complete the fuel combustion, at high engine speeds, the spark plug must ignite sooner. Now that the natural gas flame is slower than gasoline, this problem is becoming more noticeable. As a result, the spark ignition timing should be increased further in CNG fuel mode than in the case of gasoline combustion. This leads to an increase in negative work during the compression phase, resulting in reduced cycle efficiency. Researchers have suggested adding hydrogen to improve natural gas combustion (HCNG). By adding hydrogen, the flammability properties of natural gas are improved. In this study, using Hydrogen generator, hydrogen and oxygen (HHO) gas is produced and consumed water by electrolysis method. The gas (HHO) produced together with the gas (CNG) is combined with the air entering the engine. Then the effect of gas (HHO-CNG) on the parameters of efficiency, power and pollution has been investigated. The results show that the thermal efficiency increases with the addition of HHO. Under these conditions, CO and UHC levels have decreased. This reduces contamination and increases efficiency due to reduced need for spark plug advance and complete fuel combustion.