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This work numerically and experimentally investigated the effect of the characteristic structure of nested composite fuel grains on their combustion properties. The characteristic structure comprised multiple helical grooves inside the fuel grain port, which formed during the combustion process. Three-dimensional steady-state simulations were employed to analyze the burning behavior of composite fuel grains with different scales of the characteristic structure (i.e., groove depth) of 0, 1, and 2 mm. An obvious swirl-gas flow field and large tangential velocity appeared for fuel grains with the characteristic structure, which was conductive to enhancing the propellant mixing. A larger flame region and more intense burning were correspondingly achieved, during which greater amounts of H 2 O and CO 2 were generated than by fuel grain without the characteristic structure. Numerical results verified that the characteristic structure was theoretically beneficial to improving the combustion properties of the grains. Firing experiments were conducted using a laboratory-scale hybrid rocket engine with oxygen as the oxidizer. The flame structure and pulsation features were synchronously analyzed using a radiation imaging technique. Pure paraffin-based fuel grains with a circular port were tested for comparison. Both regression rates and combustion efficiencies of the composite fuel grains with characteristic structure scales of 1 mm and 2 mm showed significant improvement. The flame images exhibited an obvious helical shape and large area, which was consistent with the numerical analysis. Both the numerical and experimental results confirmed that the helical characteristic structure in the composite fuel grain played a key role in enhancing the combustion properties and demonstrate potential for further development. • The characteristic structure improved the combustion properties of composite grain. • Combustion properties improving mechanisms were studied numerically and experimentally. • Radiation imaging was employed to assess the flame pulsation features. • Experimental results were consistent well with the numerical results. [ABSTRACT FROM AUTHOR]

To satisfy the demand for thrust control on solid rocket motors and solid ramjet engines, numerical simulation of the effect of high-voltage electric fields on the combustion characteristics of polymethyl methacrylate (PMMA) under lateral inflow was undertaken. The mechanism of the effect of electric fields on the regression rate and mass flow rate of PMMA was analyzed. The results show that PMMA mass flow rate can increase by about 54% when subjected to positive voltages. The PMMA mass flow rate can not only increase by about 5.5% but also decrease by about 14% under the action of negative voltages. When the oxygen mass fraction is 54.5% at 0.101 MPa, the effect of electric fields on PMMA mass flow is weakened as the lateral velocity increases. When the inflow velocity is 1.95 m s−1 and oxygen mass fraction is 23.2%, the effect of electric fields on PMMA mass flow is decreased as the pressure rises. The effect of electric fields on PMMA regression rate is mainly due to the electric-field-induced volumetric body force. It will result in large-scale bulk flow movement, known as ionic wind, which changes the distribution of species and affects the heat of reaction, thus causing the changes in flame temperature, finally altering the thermal feedback to the solid phase. • Electric fields can change PMMA regression rate and mass flow rate. • The change of PMMA mass flow rate was closely related to applied voltages. • The change of the regression rate was due to electric body force. [ABSTRACT FROM AUTHOR]