Features of paraffin-based fuel conversion for hybrid engines

Abstract

The subject of this study is the evaporation and combustion processes of paraffin droplets in a gaseous medium containing an oxidant (air). Paraffin is a low-melting material that can be used as an environmentally friendly and high-energy fuel for hybrid rocket engines. The alkane – docosane (pure), a saturated hydrocarbon with the chemical formula C₂₂H₄₆, was used as the object of the study. To develop effective fuel compositions based on paraffins (alkane mixtures) for hybrid engines, data on the characteristics of evaporation, ignition, and combustion of individual alkanes and their use conditions are necessary. The purpose of this work is to study the kinetics of docosane droplet evaporation and combustion, determine the evaporation and combustion rate constants, and determine the diffusion coefficient of docosane vapor in air. The main tasks: a) development of methods for determining the rate of evaporation and combustion of docosane droplets with real-time registration of processes and computer processing of digital images; b) establishment of physical mechanisms of evaporation and burning of docosane droplets, measurement of constant evaporation and burning rates. The following results were obtained. The burning of docosane droplets with a diameter (d) of up to 2 mm from a powerful spark discharge was studied. The change in drop diameter and flame height during the burning process was studied. The diffusion mechanism of burning was confirmed for all the drops studied, and the constant burning rate and the ratio of the flame height to the initial drop diameter were determined, which is an indicator of burning efficiency. The evaporation process of docosane droplets in air at different temperatures also confirmed the diffusion mechanism of evaporation (d² - law). The diffusion coefficient of docosane vapor in air was estimated based on the experimentally found constants of drop evaporation rates. The experimental studies of the evaporation and burning processes of docosane droplets in air allowed us to draw the following conclusions. The kinetics of evaporation and combustion of docosane droplets in the investigated diameter range (1-2 mm) and temperatures 473-523 K is described by the d² - law. The burning constant of docosane droplets under normal conditions was determined, which on average is equal to K_bur = 1.5 mm²/s. The evaporation constant of docosane droplets in the air temperature range 473-523 K was determined. The diffusion coefficient of docosane vapor in air was calculated using the obtained experimental data on the constant evaporation, the value of which is equal to D=1.6 10^-7 m²/s in the temperature range of 473-503K.