The effects of nano-scale additives on the linear burning rate of nitromethane at high pressure
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This paper examines the effects of various nanoparticle additives on the combustion behavior of nitromethane, using an alternate method recently developed by the authors to measure the linear burning rates of liquid monopropellants. Nano-scale aluminum (100 nm diameter) was used to increase the overall energy density of the mixture, and fumed silica powder (200 to 300 nm aggregate) was used to increase the mixture thickness and encourage aluminum suspension. Nano-scale titania (20 nm diameter) was also included as a useful additive, based on previous studies that showed marked increases in the burning rates of solid monopropellants with added titania. The effects of these added particles on linear burning rates were tested at chamber pressures ranging from 3 to 13 MPa, using a technique that functioned without direct observation of the burning front progression. The added amount of fumed silica powder was varied from 1% to 3% by mass, aluminum was varied from 5% to 13.5% by mass, and titania was added at 1% by mass for all relevant trials. The mass percentage of suspended aluminum was limited by the viscosities of the resulting mixtures and represented the maximum amounts that still allowed the mixture to flow as a liquid. The use of fumed silica powder yielded increased linear burning rates compared to neat nitromethane, and the pressure exponent of the burning rate equation shifted from lower to higher than neat nitromethane as more silica was added to the mixture. The addition of aluminum to this silica mixture led to further increases in linear burning rates, and the exponential factors consistently showed increased pressure sensitivities for those mixtures containing aluminum. The combination of nano-scale titania and nitromethane led to higher linear burning rates with increased pressure sensitivity and combustion instability for chamber pressures above approximately 8 MPa, but decreased pressure sensitivity for the same mixture below 8 MPa. However, the addition of titania to mixtures already containing silica and aluminum slightly decreased their burning rates, reversing the accelerating effect seen in mixtures that contained only nitromethane and titania.
