Estimate of Internal Losses in a Liquid Fuel Jet Engine Chamber

Abstract

I THIS ARTICLE we propose a system for estimating the thrust losses in a liquid fuel jet engine, based on the use of thermodynamic calculations and simple experimental facts. Such systems originated with M. V. MePnikov, who reconciled the experimental facts with computations by using maximum simplifying assumptions. The definitions of the coefficients used in this article have the same meaning as the definitions adopted in heat power engineering, and the relations between them make it possible to strike a more correct loss balance. It is assumed in thermodynamic calculations for liquid fuel jet engines that the cycle is performed by a gas with certain known real properties (the presence of dissociation and a change in composition during the time of equilibrium expansion). However, the real cycle is accompanied by collateral processes which occur because of the superposition of mixtureformation processes on the main cycle, and because of an interaction between the working medium and the walls of the chamber and the nozzle. This causes the gas to have different states at different points in the chamber cross section. The collateral processes are irreversible and lead to losses of energy and thrust, usually called internal losses. The power losses are characterized by the efficiency (77), and the thrust losses by the thrust coefficients (<p and \j/). At the present time it is difficult to obtain reliable results from calculations of the internal losses. This is due above all to the complexity of the collateral processes and to the great difficulties in the performance of experiments that would yield direct indications concerning the magnitude of the more important losses. Thus, for a direct estimate of the chemical losses, it is necessary to have data on the composition of the gas and on its temperature at characteristic cross sections of the chamber. However, if the gas has a complicated composition, if its thermal and chemical structure are not uniform, and if high chemical reaction velocities and high temperatures prevail, it is impossible to obtain satisfactory accuracy in the measurements of the temperature and composition of the gas. I t would appear that an experimental determination of the hydraulic losses would be less complicated, but the gasdynamic processes in the nozzle channel combine with chemical and thermal processes, from which it is quite difficult to separate them. I t must be assumed that it is possible to determine in the experiment, with sufficient accuracy, the flow of working medium, the pressure in the chamber and the thrust. Starting with these reliable data and with data obtained by thermodynamic calculations, it is possible to obtain an estimate of the internal losses in the liquid fuel jet engine; this estimate is useful for work on the final detailed design of the engine. It must be borne in mind that its accuracy will affect not only the quality of the experiment, but also the accuracy of the thermodynamic computation, which contains simplifying assumptions. By thermodynamic computations we obtain