高旋轉數下利用閃頻顯影與穩態感溫液晶顯影技術於鰭片通道表面之熱傳研究

Endwall Heat Transfer Measurement Using Steady State Liquid Crystal Thermography with Stroboscopy in a Rotating Pin Fin Channel at High Rotation Number

黃思齊(Szu-Chi Huang)

指導教授 ： 劉耀先

國立交通大學/工學院/機械工程系所/博士(2019年)

摘要

燃氣渦輪機被廣泛得應用在發電機組及推進系統中，為了符合經濟效應與減低排放量，提升燃氣渦輪機的熱效率為勢在必行。藉由熱力學定律可知，提升燃氣渦輪機的入口溫度可大幅提升燃氣渦輪機之熱效率。然而，過高的入口溫度將使渦輪機葉片受到熱損害並減少葉片壽命。為了延長葉片的使用壽命，不同種類的冷卻方法伴隨著葉片的改良與發展也逐漸成形。本研究利用寬高比為4的矩形冷卻通道並分別置入交錯型與對正型圓形鰭片陣列來模擬燃氣渦輪機葉片尾端之結構。除此之外，兩種相對於旋轉平面之冷卻通道方位角(90°和150°)也分別於靜止與旋轉中進行測試。實驗中，三種雷諾數(5,000、10,000和20,000)分別在通道旋轉速度為0、180和300RPM時測試，使其相對應之最高旋轉數達到0.39。為了量測冷卻通道壁面上的詳細熱傳分布，本研究藉由感溫液晶對於不同溫度有不同顏色的特性，配合開發閃頻顯影技術使得旋轉鰭片冷卻通道中的熱傳分布現象得以進行分析。
閃頻顯影技術其原理為視覺暫留，當閃爍頻率與旋轉目標物之速度達到一致時，旋轉物體將呈現靜止狀態，因此，透過閃頻顯影技術相機可拍攝旋轉壁面上之感溫液晶顏色並進行分析。藉由此原理，本實驗利用波形產生器同步控制影像感影器和閃頻對旋轉冷卻通道中的壁面溫度變化進行拍攝。為了驗證此顯影技術用於熱傳量測之正確性，校正過程中分別比較光源曝光時間、閃爍頻率、對正角度和液晶重複性對於色調值(hue)與溫度關係曲線的影響。藉由建立與真實拍攝環境相同的液晶校正曲線可將此感溫液晶之誤差降低至2.5%，其主要來源源自於些微的模糊度和相機與拍攝平面的角度誤差，因此閃頻顯影技術可應用於本熱傳量測實驗。同時，閃頻顯影技術的開發在未來也可量測高旋轉速下的真實渦輪機葉片之熱傳，藉由此種全域式的量測方法也可針對葉片上熱應力集中之地方做改善。
對於旋轉鰭片通道中的壁面熱傳結果而言，本實驗分析位於鰭片陣列中三至六排(x/Dh=1.58~3.44和y/Dh=0.54~1.46)且提供每平方公分9472像素的熱傳分布。結果顯示，鰭片前方的馬蹄型渦流與後方的尾流區域因受到旋轉通道方位角(150°)的影響產生了大幅度的偏移，同時，展向上的局部熱傳分布也呈現非對稱的分布。分析區域內的熱傳結果同時也顯示，對正型鰭片陣列相較於交錯型鰭片陣列對於旋轉所能提升之熱傳有較大幅度的上升，其主因在於柯式力所產生之二次渦流在對正型鰭片陣列中有較寬廣的空間得以流動使得冷卻效果較為明顯。反之，在交錯型鰭片陣列中，由於鰭片所產生之渦流破壞力極強，使得旋轉效應大為減弱。除此之外，旋轉通道中對正型鰭片陣列之熱傳在方位角90°時相比於靜止時分別在翼前緣面與翼後緣面上升約40%與90%。

關鍵字

燃氣渦輪機熱傳 ； 液晶顯影 ； 閃頻顯影

並列摘要

Gas turbine is widely used for power generator and electricity production. Thermodynamically, higher thermodynamic efficiency is achieved by using higher turbine inlet temperatures. To protect turbine blades in long continuous operation, several cooling technologies are designed to increase the lifetime of gas turbine blade. In this study, a rectangular channel with an aspect ratio of 4:1 was selected to model internal cooling channel near the trailing edge of a turbine blade. The pin-fin arrays were designed in either inline or staggered patterns, and the Reynolds number ranged from 5,000 to 20,000. The pressurized air flow acted as the working fluid, and the highest rotation number was 0.39. Heat transfer was experimentally measured using a rotating pin-fin channel involving two channel orientations, namely 90° and 150°, with respect to a rotating axis.
A newly developed method of liquid crystal thermography along with stroboscopic photography was used to obtain detailed heat transfer contours on the endwall surface. A detailed calibration was performed to quantify the uncertainties in the liquid crystal measurement from the stroboscope frequency, flash duration, and viewing angle. Results showed that, with the careful calibration process, the wide band liquid crystal thermography was valid for measuring heat transfer distribution of the rotating objects. The uncertainty in the temperature measurement was within 2.5%, and the primarily source of uncertainty was from the imperfect synchronization and marginal blur during rotation. This technique was useful to validate the rotating heat transfer distribution with high rotational speed in the future. For the heat transfer results, the influence of rotation was greater on the inline array than on the staggered array, engendering higher heat transfer enhancement on the leading and trailing surfaces. Compared with the other orientation, the inclined orientation (150°) engendered a larger spanwise heat transfer variation because of the shifted rotation-induced secondary flows. The inline array exhibited the highest rotation-induced heat transfer enhancement at the 90° orientation, and the highest enhancement levels were 90% and 40% on the trailing and leading surfaces in the analyzed region, respectively.

並列關鍵字

gas turbine ； heat transfer ； liquid crystal thermography ； stoboscopic photography

參考文獻

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