超臨界二氧化碳布雷登循環(Brayton cycle)向心式渦輪之設計與數值分析

Meanline Design and Numerical Analysis of Small and Medium Scale Radial Inflow Turbines for Supercritical Carbon Dioxide Brayton Cycles

王婷娜(Unglaube, Tina)

指導教授 ： 王培仁 ； 蔣小偉

國立清華大學/其它/跨院國際博士班學位學程/博士(2019年)

摘要

近年來由於超臨界二氧化碳布雷登循環擁有高效率、體積小、以及低污染等優勢，所以成為目前學術研究的一個主要方向。而在小型渦輪發電系統方面，向心式渦輪已經成為渦輪擴散器的主要選擇。本論文將針對低溫餘熱回收的超臨界布雷登循環100kW以及500kW級渦輪發電系統，運用子午面(meanline) 葉片設計方法產生多種向心式的渦輪設計方案。運用比速率(specific speed)以及速度比(velocity ratio)這兩項重要的參數，設計出不同的向心式的渦輪，再與計算流體力學軟體(CFD)的模擬結果來做比對與驗證。研究結果成功的設計出能夠產生最大渦輪輸出性能的向心式渦輪設計方案。針對100kW渦輪發電機在每秒3公斤的流量時，最佳比速率(specific speed)是在0.3到0.5，而最佳的速度比(velocity ratio)則是落在0.52到0.68。至於500kW渦輪發電機在每秒10公斤的流量時，無法有效提升比速率(specific speed)超過0.6，原因是已經產生了區域性的穿音速擾流，而且是在低轉速的運作情況下就發生了。至於提高速度比(velocity ratio)則會在定流量的情況下，提升擴散比;而在定擴散比的情況下，降低流量。運用上述兩項最佳化的參數來設計最大效率的向心式渦輪，結果顯示速度比會隨著比速率的降低而減少。總之，本研究所開發的向心式渦輪葉片設計系統顯示可以有效地運用在小型渦輪發電系統，而且能夠達成86% 轉子效率以及84 % 整體渦輪效率。
最後，為了要驗證本研究所開發的設計系統，論文中採用了文獻中所提及的向心式空氣渦輪設計方法(Balje diagram)來做比對。結果顯示本研究適用於超臨界二氧化碳向心式渦輪機葉片設計之餘，同樣也可適用於向心式空氣渦輪機。而且超臨界二氧化碳向心式渦輪機的各方面的損耗，也與向心式空氣渦輪非常類似。同時本研究也驗證了超臨界二氧化碳向心式渦輪不同於超臨界二氧化碳壓縮機的部分，就是渦輪機展現了非常低的可壓縮性而且與理想氣體非常類似。

關鍵字

超臨界二氧化碳 ； 布雷登循環 ； 向心式渦輪 ； 子午面葉片設計

並列摘要

In recent years, supercritical CO2 (sCO2) Brayton cycles have drawn the attention of researchers due to their high cycle efficiencies, compact turbomachinery, and environmental friendliness. For small scale cycles, radial inflow turbines are the prevailing choice for the turboexpander and one of the key components. Different meanline design paths for radial inflow turbines are outlined, and aerodynamic design space exploration is conducted for one 100 kW-class and one 500 kW-class turbine test case for a low-temperature waste-heat utilization sCO2 Brayton cycle. By varying the two design parameters, specific speed, and velocity ratio, different turbine configurations are set up and compared numerically by means of CFD simulations. Results are analyzed to conclude on best design parameters with regard to maximum turbine performance. Specific speeds from 0.3 to 0.5 and velocity ratios from 0.52 to 0.68 are recommended for sCO2 radial inflow turbines with small throughflow (3 kg/s). For the radial inflow turbines intended for the medium scale sCO2 power cycle, despite the higher mass flow rate (10 kg/s), no significant increase in specific speed (above 0.6) could be achieved, as local transonic flow occurred already at considerably lower rotational speeds. Higher velocity ratios result in bigger expansion ratios if the mass flow rate is fixed, and in smaller throughflows, if the expansion ratio is fixed. Pairs of optimum design parameters that effectuate maximum efficiency are identified, with smaller velocity ratios prevailing for lower specific speeds. By achieving total-to-static stage and rotor efficiencies of 84% and 86%, respectively, the developed meanline design procedure has proven to be an effective and easily applicable tool for the preliminary design of small scale sCO2 RIT.
Moreover, the turbine simulation results for sCO2 are compared to well-established recommendations for the design of air turbines from literature, such as the Balje diagram. It is concluded that for the design of sCO2 radial inflow turbines, similar design principles and parameters can be used as those for air turbines. Also, losses occurring in sCO2 radial inflow turbines are similarly distributed as those of air turbines. These outcomes support the understanding that supercritical CO2, in the working range of a sCO2 turbine, shows very little compressibility and behaves similar to ideal gas (in contrary to a sCO2 compressor.)

並列關鍵字

Supercritical CO2 ； Brayton cycle ； Radial inflow turbine ； Meanline design

參考文獻

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