Numerical Study of Geometric Effects on Burning Velocity Measurement by Constant Volume Method

Three axisymmetric 2D numerical model including a spherical and two cylindrical constant volume combustion vessel with different diameter to length ratio are constructed to explore the geometric effects and possibility of adopting cylindrical vessel in measurement of laminar burning velocity (LBV) by constant volume method (CVM). Two methods are adopted. First, the exact combustion properties from simulation are used to extract the LBV. Second, the estimated combustion properties based on the experimental techniques are applied. The results show that the combustion pressure and pressure rise rate are almost same in three different chambers before flame touching the wall, but in cylindrical vessel, flame surface area is changed by cylindrical confinement. The violation of spherical flame assumption applied in CVM results in the obvious overestimation of LBV. However, the discrepancy of LBV is not shown when the estimated combustion properties commonly adopted in practice are used, because the simplified properties do not capture the true burned mass fraction. Therefore, the pressure data before flame hitting the chamber wall in cylindrical vessel could perform same accuracy with that of spherical vessel in experiment. The flame impacting timing could be easily identified by the sudden drop of pressure rise rate. The only concern is that the final pressure used in calculating the burned mass fraction may be reduced by heat loss in cylinder vessel. It is suggested that before the pressure date in cylindrical vessel is utilized in determination of LBV, the final combustion pressure obtained in cylindrical vessel should be compared with the data in spherical vessel, at least compared with ideal constant volume equilibrium pressure to evaluate the deviation.