Determining the time-resolved mass flow rates of hybrid rocket fuels using laser absorption spectroscopy

Abstract

A non-intrusive method for determining the time-resolved fuel mass flow rate in a hybrid rocket engine is proposed based on tunable diode laser absorption spectroscopy (TDLAS). The present work performed experimental tests using oxygen/paraffin as the propellant in a laboratory-scale hybrid rocket engine, applying oxidizer-to-fuel ratios in the range of 2.5–2.9 corresponding to combustion chamber pressures of 1.68–2.46 MPa. Variations in temperature and H2O partial pressure at the nozzle outlet were determined adopting TDLAS based on H2O absorption near 2.5 μm. Three H2O absorption lines at 4029.5, 4030.6 and 4030.7 cm−1 were simultaneously measured using only one diode laser at 2.0 kHz repetition rate and adopting a scanned-wavelength direct absorption strategy. The airflow velocity at the nozzle outlet was ascertained in two-dimensional numerical simulations. In this manner, the time-resolved fuel mass flow rate participating in combustion can be derived from the ideal state equation. A detailed discussion of the measurement uncertainty was provided herein considering the influence from determination of temperature, H2O partial pressure and velocity. The effectiveness of the novel method was validated by comparing its results with that acquired with the traditional weight-loss technique. Finally, it is initially analyzed that the combustion efficiency of pure paraffin fuel gradually increases during the combustion process according to the measurement results.