Modeling and Simulation of Liquid Monopropellant Combustion in Microthrusters

Abstract

A comprehensive numerical analysis is developed to study the combustion of liquid monopropellant in a small-volume vortex chamber. The instantaneous burning rate of the liquid propellant is obtained from well-caliberated experiments as a function of local pressure and temperature. The basis of the present work is a two-phase flow analysis based on the level-set approach. The model allows for a detailed investigation of the liquid-film motion and gas-phase flow development. As a specific example, the case with liquid nitromethane in a volume of 108 mm 3 is examined systematically. I. Introduction ith the fast development of Micro Electro Mechanical Systems (MEMS) in the past 10 years, the miniaturization of small-size power supply devices with high energy density has been receiving considerable attention. The use of combustion for power generation provides enormous advantages over currently available batteries in terms of specific power generation, even when the conversion efficiency from thermal to electrical energy is taken into account. The challenges associated with micro-scale combustion modeling are considerably different from those of macro-scale devices. The chamber internal flow is predominantly laminar, and the surface phenomena play an important role in dictating the system characteristics. The effects of miniaturization on the fluid mechanics, heat transfer, and combustion characteristics involved in micro power devices have recently been analyzed by Fernandez-Pello 1 and Ketsdever et al. 2 . As first described by Waitz et al. 3 , a micro-scale combustor is more constrained by an inadequate residence time for complete combustion and a high rate of heat loss from the combustor. Both features can lead to incomplete reaction or even quenching of chemical reaction. The commonly used methods to acquire stable combustion in a micro-scale combustor are (1) balancing the flow residence time and chemical reaction times and (2) optimizing the thermal conditions, such as the “Swiss roll” burner developed by Weinberg et al. 4 . For liquid propellants, one major challenge for all miniature combustion concepts is the increasing surface-tovolume (S/V) ratio with decreasing size. At small scales, a high S/V ratio often leads to flame quenching, particularly for premixed flames. One alternative for high S/V combustors is to deliver the liquid fuel as a film on the combustor surface. Such a propellant delivery technique simultaneously cools the combustor wall and increases the liquid surface for vaporization. In current larger combustion systems, to keep the ratio of liquid surface area to volume large enough to sustain high fuel vaporization rates, the fuel is injected as a spray. The intention is to vaporize the liquid as a spray before the liquid deposits on the combustor wall. If the fuel was filmed in these larger devices, the liquid surface area would not be large enough to sustain the needed vaporization rate. On the other hand, because the specific area of the wall film increases as the combustor volume decreases, the liquid film can offer a surface area for vaporization as large as a vaporizing spray in the subcentimeter-size range. At one atmosphere, a 10 mm diameter combustor has a film surface area greater than would occur for a droplet spray with 10 µm mean radius 5 . The film has more surface area per volume of liquid than the spray if the combustor diameter is sufficiently small or the pressure is sufficiently low. Furthermore, the liquid film offers protection from heat losses and quenching. With the liquid film on the solid surface, the wall temperature will not exceed the boiling point of the liquid. Although the film thickness is on the order of tens of microns, the Reynolds number is larger than unity, indicating that viscous forces do not prevent the movement of liquid along the solid surface. Generally the liquid film is always in the laminar range.