محمود گرجی

Recognizing the effective parameters on liquid fuel degradation and its shelf life prediction is an important issue. Most of the researches done in this area are limited to synthetic fuels. In this work, the shelf life of hydrocarbon fuel and the effect of vessel material, blanketing gas and vessel temperature on the amount of generated gum in the fuel have been investigated. The results showed that both aluminum and stainless steel materials are suitable for storage vessel. Moreover, the amount of gum reduces to one-ninth by nitrogen rather than oxygen. Therefore, the stainless steel vessels with nitrogen blanketing gas have been utilized for accelerated ageing tests. According to kinetic studies, the reaction of gum generation was of zero order and the reaction rate constants and the shelf life of fuel at 20, 30 and 40 °C were predicted to be 0.00254, 0.00616 and 0.0141 mg/ 100 ml. day and 43.4, 17.9 and 7.8 months for Arrhenius approach and 0.00299, 0.00639 and 0.0136 mg/ 100 ml. day and 36.6, 17.2 and 8.1 months for Berthelot approach, respectively.

Shelf life prediction for liquid fuels depends on a reliable and efficient accelerated ageing method. Most of the available researches in this area are limited to isothermal ageing. Therefore, these isothermal ageing methods are not capable to predict shelf life for fuels at non-isothermal ageing conditions. In the previous work, a new approach called reaction severity index was proposed for the prediction of liquid fuels shelf life at isothermal aging behavior and the results of this approach compared with classical Arrhenius one. In this work, at first this approach has been used to predict the Samine fuel shelf life at alternate temperature cycles of 50 and 60 °C. The predicted shelf life based on Arrhenius and Berthelot reaction rate are 104.3 and 106.2 days, respectively that shows a good agreement with the empirical value of 110 days. Then the fuel shelf life was predicted using different temperature patterns. The results showed that the Berthelot approach predicts lower values of shelf life rather than the Arrhenius approach at ambient temperatures, while at higher temperatures the predicted values are approximately the same. Ultimately, the predicted shelf life at temperature patterns of 20,60, 30,60, 4,60, and 50, 60 °C showed that the predicted shelf life of all patterns are in the vicinity of the fuel shelf life at 60 °C. Therefore, the reliability of this approach makes it possible to predict the fuel shelf life for storage conditions at ambient of non-isothermal temperatures.

Due to prolonged storage time of fuel, recognizing the effective parameters on fuel degradation and its shelf life prediction is important. Regarding the need of using a reliable and efficient accelerated ageing method for prediction of fuel shelf life, introducing an efficient approach is necessary for this purpose. Most of the previous researches are limited to Arrhenius classical approach, and are rarely based on Berthelot one. The main objections of these two approaches are their exclusive application for isothermal ageing. In this work, a new approach called reaction severity index is proposed for predicting the shelf life of liquid fuels. Then, the shelf life of Samine fuel is predicted using this approach and compared with Arrhenius results at isothermal conditions. The predicted shelf life using this approach at 20, 30, and 40 °C is 5.64, 2.37, and 1.05 years, respectively. These results are in good agreement with the shelf life predicted using Arrhenius classical approach that predicts 5.69, 2.39, and 1.06 year, respectively. Therefore, the reliability of this approach makes possible to predict the fuel shelf life for storage conditions at ambient temperature.

In simulation and modeling of mass transfer phenomena, design of separation equipment; and predicting the mass transfer of miscible fuels, the mass transfer coefficients is required. In this study, a diaphragm-cell was manufactured to measure the liquid-liquid diffusion coefficient. The cell constant was obtained to be 0.188 cm-2 with 0.5 molar solution of potassium chloride solution. Based on this value, the binary diffusion coefficients of 0.5 molar sodium chloride and heptane-benzene solutions were obtained as 1.6×10-5 and 2.38×10-5 cm-2/s at 25 °C, respectively. These coefficients have been previously reported respectively as 1.5×10-5 and 2.29×10-5 cm2/s at 25 °C. Afterward, the infinite dilution diffusion coefficients of heptane in benzene and benzene in heptane at 25 °C were obtained as 3.14×10-5 and 1.88×10-5 cm2/s, respectively. These coefficients have also been previously reported as 3.4×10-5 and 2.10×10-5 cm-2/s respectively. The measured diffusion coefficients showed a good agreement with literature values. Finally, investigating the effect of liquid density of upper and lower part of the diaphragm cell on the measured value of diffusion coefficient showed that the solution with higher density should be charged in the lower part of the cell.

In a motor with a liquid hydrocarbon fuel، the hydrocarbon fuel diffuses into the amine fuel and the fuel concentration changes with time. Therefore، the variation of amine fuel at each time should be predicted to ensure the safe start of the motor and the fuel storage should be recharged if the amine fuel concentration is out of specific range. In this work، the amount of hydrocarbon fuel diffusion in the amine fuel and the variation of the amine fuel concentration at different times was obtained numerically and compared with experimental data. For this reason، the corresponding diffusion coefficient was obtained firstly. The results showed that the diffusion coefficient of 9. 4*10-6cm2/s with an absolute average deviation of 0. 037 has a good conformity with experimental data at 20°C. To investigate the effect of ambient temperature on the diffusion phenomenon، a correlation was proposed to predict the variation of diffusion coefficient with temperature. The predicted concentration with this predicted diffusion coefficient showed a good agreement with experimental data. Ultimately، a correlation was proposed to predict the amine fuel concentration versus a dimensionless parameter ofz/√Dt. The prediction showed a good agreement with empirical data.

The JPX rocket fuel is a mixture of unsymmetrical dimethyl hydrazine and a nearly light hydrocarbon fuel. Regarding the presence of unsymmetrical dimethyl hydrazine and olefin compounds in the JPX fuel, emission of its vapors during fuel charging can be hazardous. On the other hand, after fuel charging, the internal surface of pipeline and related equipment is wetted with the liquid fuel. This fuel is hazardous from safety and environmental viewpoint and it is necessary to neutralize the vapors and wash the related equipment with an appropriate solvent. Aqueous solution of ethanol (5%) was selected for the equipment washing. Various oxidants were also compared for vapor neutralization and potassium permanganate was selected. Results showed that the oxidation reaction consists of two steps: in the first step, the potassium permanganate is reduced to manganese dioxide and in the second step, manganese dioxide is reduced to Mn(II) ion. The molar ratio of potassium permanganate to dimethyl hydrazine was found to be 0.506. Assuming the second order reaction with respect to dimethyl hydrazine, the rate constant was determined to be 6.1lit.mol-1min-1 at 25°C. The main products of the oxidation reaction included formaldehyde dimethyl hydrazone, dimethyl amine, ammonia, and water.

In this work, a two dimensional CFD model has been used to investigate both hydrodynamics and mass transfer and their effects on hydrogen solubility in a hydrocarbon fuel. This work has been done based on two fluid (Eulerian-Eulerian) model and finite volume method, along with standard k-e model to address turbulent behavior of both phases. The bubble rise velocity obtained using the present model has been compared with the experimental results and showed a good agreement with experimental data. The CFD model correctly exhibited the general behavior expected from hydrodynamics of the system. For gas superficial velocities up to 3.0 cm/s, the gas volume fraction and volumetric mass transfer coefficient were obtained up to 0.091 and 0.054 s-1, respectively. Results of mass transfer run showed that hydrogen concentration in liquid fuel is increased progressively until reaching its equilibrium value. For hydrogen mass transfer coefficient of 0.0004, 0.0008 and 0.002, the time required to reaching the equilibrium liquid concentration is about 120, 60 and 25 sec. respectively. Therefore, through selection of an appropriate system geometry and gas velocity, an appropriate mass transfer and gas distribution in liquid for hydrogen solubility in liquid fuel can be achieved.

One of the difficulties in the liquid fuels is theirs shelf life. These fuels, like other chemicals, dont meet the military specifications after a period of time due to their destructive reactions. This variation from the specifications, changes the chemical and physical properties of the fuel and its combustion performance. This subject is significant because of the fuel long time storage requirements. Thus, it is necessary to specify, control, and decrease the effective parameters and increase the fuel shelf life. One of the important fuels used in aerospace industries, is unsymmetrical dimethyl hydrazine. Studies show that oxygen, temperature, and storage material of construction are parameters affecting the fuel shelf life. Regarding shelf life conditions, presence of oxygen is the most important parameter affecting the storage shelf life of the fuel. Thus, the effect of oxygen on shelf life of unsymmetrical dimethyl hydrazine was investigated empirically. Results show that the shelf life of this fuel strongly depends on the molecular oxygen. The major products of this oxidation are formaldehyde dimethyl hydrazine, dimethyl amine, ammonia, and water. Assuming the zero and first order reactions, the oxidation rate constants were determined at storage temperature (25 °C) to be 0.021mol.lit-1.min-1 and 0.00023 min-1, respectively.

The hydrogenation kinetic of NDMA to UDMH on a 5% Pd/C in aqueous solution of NDMA was studied and optimum conditions of the reaction for maximum yield of UDMH in the presence of undesired dimethyl amine (DMA) product was investigated. Experiments were carried out in a semi-batch three phase stirred tank reactor under constant pressure of 15 bar, temperature of 40 to 70°C and NDMA concentration of 40 to 70 wt%. Effects of temperature, pressure and NDMA concentration on yield of UDMH product in presence of undesired dimethylamine (DMA) were studied and the optimum conditions for selective production of UDMH were obtained. For these optimum conditions, a kinetic scheme based upon Eley-Rideal (ER) model was proposed to predict UDMH and DMA production rates. Based on this model, the reaction parameters for these two products were estimated and the activation energies of these two reactions were determined.