以固体燃料和液体氧化剂为推进剂组合的固液火箭发动机兼具固体和液体两种发动机的优点，且相对于液体发动机结构更简单，相对于固体发动机更安全，在探空火箭、民用航天、运载火箭等领域有着广阔的应用前景。虽然固液火箭发动机有着诸多优点，但其仍存在一定局限性，其一由于低退移速率导致发动机的推力难以满足要求，第二则是由于边界层的发展使得火焰逐渐远离燃面从而使得药柱沿轴向退移不均匀，增加了燃烧精准控制的难度。本研究的意义即为在变通量下实现发动机高效稳定燃烧。

首先提出了一种均匀涡流复合式药柱，该药柱由丙烯氰-丁二烯-苯乙烯共聚物（acrylonitrile–butadiene–styrene, ABS）基体与石蜡基燃料组成，ABS基体包含外壁和若干组沿轴向螺旋分布的中空叶片，外壁内包含氧化剂通道，且与中空叶片为一体式结构，能够实现氧化剂径向喷注，并引导氧化剂产生旋流，从而降低边界层厚度并增强火焰到燃面的传热，提高退移速率。药柱的制备过程涵盖增材制造、离心铸造以及车削加工。在固液火箭发动机实验台上以氧气为氧化剂对其进行了初步燃烧实验，根据实验结果对药柱结构进行了改进，确认了最终结构方案。

而后，对均匀涡流药柱的燃烧特性进行了实验研究，使用轴向喷注的石蜡基药柱作为对比，氧化剂流量为7.45 – 30.68 g/s，实验台搭载了火焰辐射成像系统。结果表明不同于轴向喷注药柱燃烧时火焰遍布整个前燃烧室，均匀涡流药柱火焰仅存在于药柱孔道内部，且在稳定燃烧阶段尾焰是极度富燃的状态，燃烧后的药柱前端面无严重烧蚀现象，且沿轴向退移更均匀。由于氧化剂通道存在烧蚀现象，考虑所有极限情况确定了退移速率的真实值分布范围，结果表明在氧化剂通量为5.28g/(s·cm²)的条件下，退移速率比石蜡基药柱提升了33.27%-123.61%。同时，对药柱的稳态燃烧过程进行了数值模拟，结果表明均匀涡流药柱前燃烧室无明显高温区，且药柱燃烧通道中燃气旋流强度更高；有利于促进氧化剂与燃料的掺混从而有提升燃烧效率的潜力。实验与数值结果均展示了均匀涡流药柱巨大的发展潜力。

最后，研究了复合式药柱燃烧过程中由于燃料燃速的不同形成的槽状结构对燃烧特性的影响，首先使用三维数值模拟方式研究了无槽状结构以及槽状结构尺度（即槽深）为1, 2 mm的药柱燃烧特性，结果表明槽深1 mm和2 mm的药柱燃烧时会在前燃烧室产生明显的高温区，药柱燃面处存在较大的径向温度梯度，且药柱燃烧通道内的燃气旋流强度更高，这有助于增强传热，改善燃烧特性。对于槽深1 mm与2 mm的药柱，喷管出口处的H₂O和CO₂质量分数比无槽状结构的分别增加了32.2%和48.8%，证明槽状结构有助于使燃烧更充分。在发动机实验台上对三种复合式药柱以及石蜡基药柱进行燃烧实验，结果表明稳定燃烧阶段的燃烧室压强随着槽深的增加而提升，退移速率与燃烧效率亦展示出了相同的规律。其中，当氧化剂通量为3.5 g/(s·cm²)时，槽深1 mm与2 mm的药柱比无槽状结构的药柱退移速率分别提升了9.97%和27.89%。以上结果均证明了槽状结构有助于改善复合式药柱的燃烧特性。

The hybrid rocket engine with solid fuel and liquid oxidizer as propellants has the advantages of solid rocket engines and liquid rocket engines. It has a simpler structure comparing with liquid engine and it is safer than solid engine. Hybrid rocket engine has a broad application prospect in the fields of sounding rockets, civil spaceflight, and launch vehicles. Although the hybrid rocket engine has many advantages, it still has certain limitations. One is that the thrust of the engine is difficult to meet the requirements due to the low regression rate. The second is that the development of the boundary layer makes the flame zone gradually move away from the combustion surface, which makes the fuel grain regresses unevenly along the axial direction. Uneven regression increases the difficulty of the precise combustion control. The significance of this study is to achieve efficient and stable combustion of the engine under variable oxidizer mass flux.

Firstly, a swirl-radial-injection composite fuel grain had been proposed, which was consisted of an acrylonitrile–butadiene–styrene (ABS) substrate and paraffin-based fuel. ABS substrate contains an outer wall and several hollow blades. The blades distributed helically along the axial spiral. The outer wall contains an oxidant channel, and the hollow blades are an integrated structure. The structure of the substrate can realize radial oxidizer injection and induce the oxidizer having a swirl flowing path. Thereby reducing the thickness of the boundary layer and enhancing the heat transfer from the flame to the combustion surface, then increasing the regression rate. The preparation process of the fuel grain includes additive manufacturing, centrifugal casting and turning process. The preliminary combustion experiment was carried out on lab-scale hybrid rocket engine with oxygen as the oxidizer. The structure of the fuel grain was improved according to the experimental results, and the final structural scheme was confirmed.

Then, the combustion characteristics of the swirl-radial-injection fuel grain were experimentally studied Paraffin-based fuel grains with fore-end oxidizer injection were used as a comparison. The oxidizer mass flow rate was 7.45 – 30.68 g/s. A flame radiation imaging system was equipped in the experimental system. The results show that unlike the flame that spreads throughout the pre-combustion chamber for the fuel grain using fore-end injection, the flame only exists inside the port of the column for the swirl-radial-injection fuel grain. The plume is extremely fuel-rich in the stable combustion stage, and there is no serious ablation on the front end of the fuel grain after combustion. The inner diameters only have slight variation along the axial direction. The distribution range of the true value of the regression rate was determined considering all the limit cases due to the ablation of the oxidizer channel. The results showed that the regression rate was increased by 33.27%-127.61% when the oxidizer mass flux was 5.28 g/(s·cm²). At the same time, a steady-state combustion process of the fuel grain is numerically studied. The results show that there is no obvious high-temperature zone in the combustion chamber in front for the swirl-radial-injection fuel grain, and the gas swirl intensity in the combustion channel is higher than that of the fuel grain using fore-end injection. It is conducive to promoting the mixing of oxidizer and fuel and have the potential to improve the combustion efficiency. Both experimental and numerical results show the great potential of this swirl-radial-injection fuel grain.

Finally, the influence of the groove-like structure formed due to the different burning rates of the consisted fuels on the combustion characteristics of the composite fuel grain was studied. Three-dimensional steady-state simulations were employed to analyze the burning behavior of composite fuel grains with different scales of the groove-like structure (i.e., the groove depth) of 1, 2 mm and without groove-like structure. The results showed that a large high-temperature zone in the pre-chamber and higher radial temperature gradient in the burning surface for fuel grains with groove depth of 1 mm and 2 mm. The gas had a high swirl intensity in the combustion channel of the fuel grain, which is beneficial to enhance the heat transfer and improve the combustion characteristics. For fuel grains with structure scale of 1 and 2 mm, the mass fraction of H₂O and CO₂ at the outlet of the nozzle were increased by 32.2% and 48.8% than that without groove-like structure, proving that the groove-like structure helps to make the combustion more sufficient. Firing experiments were conducted on the lab-scale engine. The experimental results show that the combustion chamber pressure in the steady combustion stage increases with the groove depth increases. The regression rate and combustion efficiency also show the same trend. When the oxidizer mass flux is 3.5 g/(s·cm²), the regression rate of the fuel grain with a groove depth of 1 mm and 2 mm increases by 9.97% and 27.89%, respectively, compared with that of without groove-like structure. The above results proved that the groove-like structure can help to improve the combustion characteristics of the composite fuel grain.