Liquid Monopropellant Combustion in Mesoscale Vortex Chamber

Abstract

A WIDE variety of mesoand microscale combustion devices have been proposed and are under development to meet the power and energy density requirements of propulsion and powergeneration applications [1–4].Most of these devices employ gaseous fuels and, in particular, hydrogen, which has a fast reaction rate and high diffusivity, to demonstrate the feasibility of self-sustained combustion at millimeter scales or below. Liquid-phase storage and operations, however, are essential for taking full advantage of the high specific energy of a combustion-based power generator using hydrocarbon fuels. At macroscales, liquid fuels are typically sprayed into a combustor in droplet form to enhance the vaporization and ensuing burning rates. If the fuel was injected as a film on the chamber wall, the liquid surface area would not be large enough to sustain the needed vaporization rate. In combustors with dimensions in the subcentimeter-size range, however, because the specific area of the wall film increases as the combustor volume decreases, a liquid film can offer a surface area for vaporization that is as large as that of a vaporizing spray. The film on the combustor surface also cools the combustor wall to create a favorably distributed temperature profile that prevents heat losses for both endothermic liquid decomposition reactions and gasification. Heat transferred to the wall, which has been considered as heat loss in most miniaturized combustor applications, can be used for fuelfilm vaporization. Atomizers that are capable of producing submicron-scale droplets may, therefore, not be needed for liquid-fueled combustion in mesoand microscale combustors. Sirignano et al. [5,6] concluded that a 10-mm-diam combustor has a film surface area comparable to a droplet spray with a 10 m Sauter mean radius. The concept using wall film evaporation in small combustion chambers was demonstrated in a 3:14-cm-volume combustor [6] with gaseous air used as the oxidizer. Secondary air injection was recently introduced into the concept to enhance mixing and contain the reaction zone completely within the combustion chamber [7]. Furthermore, a portion of the flow residence time in the chamber was consumed by the mixing between the fuel and oxidizer. To circumvent the problem of short flow residence time in a smallscale combustion chamber and thereby avoid the difficulties associated with the mixing of fuel and oxidizer, we studied the usage of nitromethane (CH3NO2), a liquid monopropellant. A mesoscale vortex combustor developed in our previous work on gaseous combustion [8] was used because of its potential to implement the concept of wall film vaporization.