微型氣渦輪機燃燒室之性能模擬研究-採用四步化學反應

Abstract

本研究以商業套裝軟體CFD-ACE+ 模擬微型氣渦輪機引擎(WREN-MW54)在保持外型結構，但更換原廠預設的液態Jet A1燃料為較低熱值的氣態甲烷混合氣燃料時，其燃燒室的燃燒流場。所得之數值模擬計算結果著重於經燃燒反應後，熱傳效應所造成之襯筒壁高溫與燃燒室出口溫度與實驗數據比較之探討。 本研究模擬不同濃度的氣態甲烷混合氣燃料，並分別在各組的甲烷燃料中混入四種不同二氧化碳質量濃度(10%，20%，30%，40%)來做為稀釋效應的研究。因使用甲烷燃料純度的不同，渦輪機轉速的上限也各有不同。渦輪機轉速的設定範圍在使用90%，80%，70%，與60%甲烷燃料時，分別是40000~50000，40000~65000，40000~80000，40000~90000與轉/分。此外，本研究也比較一步和四步兩種化學反應機制之差異。 由模擬結果可以得知：(1) 燃燒室出口溫度在使用固定燃料濃度時會隨著轉速上升而降低；而在比較不同濃度燃料的相同轉速情況下，溫度則會隨著燃料濃度的上升而降低 (2) 在定轉速的條件下，燃燒室出口均速隨著燃料濃度上升而降低 (3) 使用一步與四步化學反應預測之燃燒室出口溫度與實驗數據相比，四步與實驗結果較相近 (4) 觀察暫態之結果可得知燃燒反應在13秒達到穩定，與穩態之誤差為1.3% (5) 主燃燒區的渦流在3.5秒發展完全，內部流場在13秒達到穩定，燃燒室出口最大流速與穩態之誤差為4.2%。最後也提出幾點建議，可供未來研究工作的改進與延伸。

This study uses a commercial package CFD-ACE+ to simulate the flow and thermal fields in a micro gas turbine (MGT) while applying gaseous fuel (CH4/CO2 mixture). The MGT used in this study is WREN-MW54, whose original fuel is liquid (Jet A1). The numerical results are mainly emphasized on temperature distributions over solid liner walls and the combustor outlet that is compared to the experimental data. In this study, various mass fraction of methane in the fuel mixture are considered. The concentrations of CH4 in fuel are changed from 60% to 90%, and the ones of CO2 decrease from 40% to 10%, correspondently. Because the purity of fuel are different makes the range of related compressor speed are also different. The compressor speed was varied from 40000 to 50000 rpm, 40000 to 65000 rpm, 40000 to 80000 rpm and 40000 to 90000 rpm corresponded to 60%, 70%, 80%, and 90% of methane concentration, respectively. Additionally, two type of chemical mechanisms, including global one-step and four-step ones, are considered in this study. The simulation results are as follows: (1) The outlet maximum temperature declines with an increase of compressor speed under each fixed methane concentration; and also declines with an increase of methane concentration at same rotating speed. (2) The average combustor outlet velocity magnitude declines with methane mass fraction rising at same rotating speed. (3) The four-step reaction mechanism has better temperature prediction than one-step one. (4) The combustion process reaches steady state at 13 seconds. Comparing the maximum temperature with steady state one, the difference is 1.3%. (5) The recirculation in the primary zone is completely developed at 3.5 seconds. The whole flow field inside combustor has reached steady state at 13 seconds. The difference of outlet maximum velocities for transient and steady state cases is 4.2%. Finally, several recommendations are suggested for the possible future improvements and extensions.