應用混合增強器的N2O及單孔HTPB混合式燃料火箭推進系統之數值模擬與實驗驗證

Abstract

近年來混合式火箭因其具高度安全性、低成本與良好的理論推進效率（比一般固態燃料火箭優異），使之相當適合學術機構進行研究發展。然而因其擴散焰燃燒特性，導致燃燒效率不佳，因此如何增加其燃燒效率成為重要的課之一。HTPB因為擁有合適之機械與化學性能，所以選為本研究使用的燃料；另因N2O在商用市場取得容易及其自壓(self-pressurized)的特性，因此選為氧化劑。在本研究中提出使用混合增強器在單一圓孔流道結構的藥柱的腔體中，使用數值模擬及實驗探討其對氧化劑和燃料之混合效率之影響。 在數值模擬方面，我們使用一平行計算流體動力學模擬程式分別進行冷流場與化學反應熱流場的數值模擬。在冷流場模擬中我們使用簡化燃燒艙環境分別針對混合增強器葉片數量、葉片攻角及翼展參數作變化以其找出最佳化設計。結果發現在30 mm內徑流道中，含8片葉片、攻角23.5度、翼展6 mm的葉片構型下混合程度為最佳狀態；而在化學反應流場模擬中，使用300公斤推力等級單孔燃燒艙構型進行混合增強器效果之驗證，模擬結果顯示裝設混合增強器之燃燒艙與無安裝者相較擁有較佳之真空Isp (235 s, 提升14%)、C*以及較低的O/F 比值。如裝設第二段混合增強器，其性能能更進一步上升(真空Isp 255 s, 提升達24%)。O/F比值範圍則在10.87和7.21之間，屬於貧油範圍。接下來在放大尺寸1.87倍之1000公斤推力等級的模擬中進行藥柱縮短之模擬，發現混合增強器雖然可增加混合效率，但依然需要充分時間進行混合進行化學燃燒反應。之後在流道內徑擴大模擬中出現性能變差之結果，部分顯示出真實燃燒中藥柱擴大之相關性能變化情形。 在地面靜試推力實驗中，進行了50公斤級、100公斤級、300公斤級及1000公斤級單一流道燃料柱地面推力測試。100公斤級推力測試顯示相關性能有了30%以上之提升，已初步顯示混合增強器之功用。300公斤級也進行一連串之地面測試實驗，並藉此技術開發出實用化之混合式火箭推進器，並已用在HTTP-1及HTTP-2火箭計畫中且成功完成飛試。1000公斤級推力實驗中首次進行不同氧化劑流率對燃燒艙產生之影響，可以觀察到可能是因為O/F比值不同所產生之性能改變；不同氧化劑流率實驗結果對將來推力控制引擎之研發提供非常重要之實驗資料。為取得更可信的實驗結果，另行進行50公斤級推力實驗，從實驗結果已證明混合增強器有明顯增益效果。另外，藥柱不均勻燒蝕現象及數值模擬中藥柱表面溫度分布相比較下，發現藥柱退縮率和燃燒溫度和火焰分布有非常大之關聯性。實驗中發現混合增強器產生之渦流流場雖然可增加混合效率，但同時也對藥柱和噴嘴造成不均勻燒蝕，這也是將來極待解決之課題。 最後將總結本論文主要研究發現，並詳列對未來相關研究建議方向。

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.