Combustion Characteristics and Flame Structure of Nitromethane Liquid Monopropellant

Open Access
Boyer, J. Eric
Graduate Program:
Mechanical Engineering
Doctor of Philosophy
Document Type:
Date of Defense:
November 11, 2005
Committee Members:
  • Kenneth K Kuo, Committee Chair
  • Paul Wencil Brown, Committee Chair
  • Thomas Litzinger, Committee Member
  • Stefan Thynell, Committee Member
  • Vigor Yang, Committee Member
  • monopropellant
  • liquid propellant
  • nitromethane
  • combustion
The push for higher performance and reduced toxicity monopropellant has led to the search for a hydrazine replacement. One promising candidate, nitromethane, was investigated in this study. To assist the design and evaluation of future systems, measurement of basic propulsion parameters such as burning rate were made over a wide range of conditions for both static (tube) and feeding tests. Three pressure regimes were found, distinguished by pressure exponent slope breaks: rb(mm/s) = 0.173[P(MPa)]^1.17 (for 3<P&#8804;15 MPa) rb(mm/s) = 0.009[P(MPa)]^2.33 (for 15<P&#8804;70 MPa) rb(mm/s) = 4.153[P(MPa)]^0.86 (for 70<P&#8804;170 MPa) Temperature sensitivity of burning rate was determined to be a low value of about 2.6x10-3 at 3 MPa, decreasing with pressure. Observations and temperature profiles measured using microthermocouples showed a very thin reaction zone and low luminosity flame over a stable, smooth liquid surface. A comprehensive detailed model for linear regression was developed and exercised. The model considered one-dimensional behavior with surface vaporization and detailed gas-phase kinetics based on the RDX mechanism of Yetter, et al. combined with the nitromethane decomposition of Glarborg, Bendtsen, and Miller. It was implemented using a custom FORTRAN code wrapping the CHEMKIN PREMIX gas-phase code coupled with the condensed-phase model. Predicted burning rates using the model showed good agreement with measured rates over the subcritical range of 3 to 6 MPa, based on review of recent literature, two rates were adjusted slightly to improve the match. Calculated species and temperature profiles showed three regions that could be distinguished based on species. CH4 and NO were the most important intermediate species, with smaller percentages of CH2O, N2O, HNO, and HONO present. The first stage was marked by decomposition of nitromethane, the second by consumption of all intermediate species except CH4 and NO, and final by rise to final temperature and species concentrations. Sensitivity analysis identified the importance of HNO reactions to the temperature profile, and therefore burning rate. Although the absolute levels of NH and HCO were low, they served as an important intermediate species transporting nitrogen and carbon, respectively, between other higher-concentration species. Although chemically nitromethane should be simpler than energetic molecules such as nitramines, the combustion characteristics (no stable combustion at low pressure and very thin reaction zone) made it more difficult to study. Many of the techniques that have been applied to nitramines cannot be used with nitromethane because of its nature as a clear liquid. Future refinement of the current model can be aided by the development of higher-resolution measurement techniques or methods to stretch the nitromethane reaction zone so that current techniques can more easily resolve species and temperature profiles.