Numerical study of pipeline distribution effect on the performance of pasty propellant rocket motor

First, authors introduce the simulation module and the verification of the simulation. The simulation model of a laboratory-designed PPRM is shown in Fig. 1. The physical and chemical processes of the propellant from the propellant storage tank to the nozzle such as viscoelastic flow, chemical reactions, turbulent flow, and gas–solid heat transfer are taken into account, to investigate the working characteristics of PPRM under different pipeline distributions. The whole work is divided into 2 steady-state CFD simulation parts and a transient-state CFD simulation: propellant feed pipeline steady-state simulation module, combustion chamber steady-state simulation module and combustion chamber LES simulation module. The propellant feed pipeline simulation module equates the pipeline distribution to different orifice positions on the inlet plane of the combustion chamber, and designs three pipeline distribution schemes, namely, single loop/double ring/triple ring, in order to study the effect of pipeline distribution on the flow characteristics of the feed system. In addition, the paper chooses the ratio (α) of the outer pipeline diameter to the inner pipeline diameter as 1/1.1/1.2, respectively, to study the effect of nonuniformity of pipeline diameter through simulation, and the schematic diagram of the pipeline distribution is shown in Fig. 3. The theoretical simulation of the combustion chamber simulation module based on the conical combustion surface of the pasty propellant obtains the results consistent with the literature experiments: the maximum relative error of the combustion chamber pressure and the characteristic velocity efficiency is less than 3%, which proves that the numerical simulation of the above feed system and combustion flow field is reliable.

Fig. 1. Axonometric view of the structure from the storage tank to the nozzle.

Fig. 3. Schematic drawings of the single-ring (left), double-ring (middle), and triple-ring (right) distribution.

On this basis, authors present the simulation results and numerical analysis conclusions. In terms of the combustion flow characteristics in the combustion chamber, the combustion-flow behavior of gas is similar to that of the traditional SRM, which can be divided into 2 stages: thin-layer combustion upstream and turbulent flow downstream. As for effects of pipeline location, under the same mass flow rate, the pipeline location distribution has almost no effect on the pipeline pressure drop and the average velocity at the exit of each layer of pipelines, while the combustion chamber pressure oscillation characteristics show a large variation with the pipeline distribution, as shown in Fig. 13.. As for effects of the nonuniformity of pipeline diameter, the average velocity with large-diameter pipelines is significantly larger than that with small-diameter pipelines at the same mass flow rate, and the velocity difference gradually increases with α. Meanwhile, as shown in Table 2, although α has little influence on the oscillation frequency, α has a great influence on the amplitude. Moreover, the paper calculates the PPRM thrust, and the results show that it is not much affected by the pipeline distribution.

Fig. 13. Pressure curve and FFT analysis of (A) case 5, (B) case 10, and (C) case 25.

Finally, authors draw the conclusion as follows. (1) The combustion flow presents typical 3-dimensional feature, and different from the common mechanism of vortex shedding in traditional SRM, a new type of vortex shedding mechanism mainly caused by the special conical shape of the pasty propellant grain (VSG) has been presented. (2) The pipeline distribution has little effect on the flow velocity distribution at the outlet of the pipeline. However, when the pipeline diameters are not evenly distributed, at the same mass flow rate, the flow velocity with the pipe- line of a larger diameter is significantly higher due to the non-Newtonian flow property of the pasty propellant. At the same time, the nonuniformity of the pipeline diameter helps reduce the pressure drop. (3) The pipeline distribution greatly affects the structure of the internal combustion flow field, mainly by forming a large vortex at the gap among the pipelines, which leads to low-velocity zones. When the pipeline diameter is unevenly distributed, the position of the vortex changes accordingly. Pipeline distribution has little effect on pressure oscillation frequency, which is near the first-order natural sound frequency of the combustion chamber (about 3,180 Hz); in general, the more layers the pipeline is distributed, the greater the amplitude of oscillation is, which is caused by the more complex flow field structure. (4) The pipeline distribution has little influence on the total thrust. The difference among all the distribution schemes is almost negligible (the relative deviation is less than 0.2%). This is because the thrust of the motor is mainly provided by the compression–expansion process in the Laval nozzle, so the flow characteristics in the combustion chamber have little influence on the acceleration of the gas.