Vortex gas accelerator

Abstract

The exhaust velocity of a propellant gas from an isothermal cavity can be significantly increased by a factor of approximately 1.7 above that corresponding to the usual isentropic expansion by utilizing multiple reheat by blackbody radiation from the walls during expansion. In order to reduce the necessary volume and the friclional drag with the walls during this reheat and expansion process, the gas is wrapped up in a rotational flow pattern in which angular momentum per unit mass is constant along all streamlines. As a consequence, the expansion in the nozzle corresponds to the conversion of rotational to linear momentum, rather than the usual thermal to linear. In order to absorb the radiant energy from the walls, the gas must have a sufficient optical opacity (achieved by a small contaminant addition to hydrogen) such that the gas in the cavity is approximately one radiation mean free path thick. R OTATIONAL gas flow can be used to enhance the exhaust velocity from a constant-temperature source provided radiation-transport heating can be made greater than the vortex flow frictional decay time. In simplest terms, a partially isothermal expansion of the working fluid can be achieved as opposed to the usual adiabatic expansion. The additional enthalpy supplied to the fluid during isothermal expansion results in a higher specific impulse for rocket applications. Since this enthalpy must be added to the fluid by radiation flow, the opacity, density, frictional decay rate, and radiation intensity become the determining quantities. There are two general forms of rotational gas flow: one is at constant angular velocity (frequently referred to as rotation) and describes the lowest order state of a gas in equilibrium inside a rotating cylinder; the second form occurs at constant angular momentum per unit mass and, to exist, requires that the rate of angular momentum supplied to the flow pattern must be large compared to the frictional drag, both with the walls and internally. The state of constant angular momentum, therefore, exists either transiently or as the result of a continuous flow of injected angular momentum and partially degraded exhaust. In this paper we are primarily concerned with the second form of rotational gas flow in which the gas is injected tangentially at the periphery of a cylindrical cavity and expands radially toward the axis. The gas leaves the cavity by axial flow through a hole of smaller radius at one end of the cavity (Fig. 1). Both axial and radial velocities are considered as small perturbations to the primary flow, which is circular. The energy transferred in the isothermal expansion from the state of the gas at the outer wall to that at the smaller radius of exhaust is stored in kinetic energy of rotational velocity. This rotational velocity can then be converted to axial velocity in a standard nozzle (Appendix A). It is useful to describe the density and/or pressure distribution of the wheel rotation flow in order to gain a qualitative understanding of the flow pattern near injection. In particular, one would wish to substantiate the possibility of injecting the gas tangentially at the periphery, in a thin layer, at near-constant angular momentum and pressure equilibrium with the surrounding gas. The subsequent radial and rotational flow is assumed to occur at constant