| Summary: | 博士 === 國立交通大學 === 機械工程系所 === 103 === Recently, hybrid rocket propulsion has attracted much attention because of many advantages, including high safety, high performance and low cost. Hybrid rocket system has better propulsion performance as compared to solid rocket system, but combustion efficiency of diffusion flame is generally low because of poor mixing between fuel and oxidizer. Thus, how to improve the mixing efficiency between fuel and oxidizer while keeping the simplicity of the system is one of the major research topics in hybrid propulsion. In this thesis, mixing enhancers are proposed to improve mixing efficiency between fuel and oxidizer in a single-port combustor and are investigated in detail both numerically and experimentally.
In the numerical part, a parallel computational fluid dynamic solver using unstructured grids was used to simulate both the cold flow and reacting flow in a hybrid combustion chamber. In the cold flow study, a series of parametric study, including variations of blade number, angle of attack, span, and chord length, was performed to investigate their influence on the axial vorticity generated in the port. The results show that the highest axial vorticity can be generated using a configuration of mixing enhancers consisting of 8 blades, 23.5 degrees of angle of attack, 6 mm of span length, and 15 mm of chord length. In the reacting flow study, by using one and two stages of mixing enhancers, an appreciable improvement of combustion efficiency, up to 14% (vacuum Isp: 235 s) and 24% (vacuum Isp: 255 s), respectively, as compared to the case without mixing enhancer in a single-port hybrid combustor was demonstrated with a thrust level of 300 kgf. Corresponding O/F ratio ranges from 10.87 to 7.21, which is a typical fuel-lean combustion. Through the simulated temperature distributions in the chamber, several important features of mixing caused by the use of the mixing enhancers are clarified and explained. In the scale-up study based on geometric amplification concept (e.g., 1.87 from 300 kgf to 1,000 kgf level), we have found that the reduction of the fuel grain axial length, as compared to the standard amplification, downgrades the combustion efficiency. This leads us to conclude that even with two stages of mixing enhancers we still need enough port length for the fuel and oxidizer to mix more thoroughly for a better combustion. Similarly, increase of port diameter also leads to deteriorated performance caused by the relatively poor mixing.
In the experimental part, we have performed several key static-burn tests for a single-port combustor design with various stages of mixing enhancers, which include different levels of thrust considering the scale-up effect (50 kgf, 100 kgf, 300 kgf, and 1,000 kgf). In the test of 100 kgf level of combustor, the results showed 30% increase of vacuum Isp (up to 219 sec) with one stage of mixing enhancer, which coincides with the findings of the numerical simulations. Based on this finding, we have performed several tests for the 300 kgf level. The results with two stages of mixing enhancers showed an impressive vacuum Isp of 236 s, which is better than most of the solid propulsion systems. The motors with one and two stages of mixing enhancers have been employed successfully in the flight tests of HTTP-1 and HTTP-2beta in 2010 and 2013 respectively. In the tests of 1000 kgf level, we have changed the N2O flow rate which were used to test the performance of thrust throttling caused by changes of the flow rate and possibly the O/F shift issue. In addition, we have also observed uneven burned port surfaces of HTPB which correlates well with the simulated temperature distribution. In order to obtain more reliable experimental data, we redesign the 50 kgf chamber for pure experiment purpose, which leads to further confirmation of the benefits of using mixing enhancer.
Major findings and recommendations of future work are summarized at the end of thesis.
|
|---|
