擾流器與管道截面形狀引導之核心流與近壁流對熱交換器熱傳效能之影響

Influence of Core Flow and Boundary Layer Flow Induced by Perturbator and Channel Cross-Sectional Geometry on Heat Transfer Performance in Heat Exchangers

詹舒博(Shu-Po Chan)

指導教授 ： 劉通敏 ； 張始偉

國立清華大學/工學院/動力機械工程學系/博士(2017年)

摘要

本研究分別使用質點影像測速儀（Particle Image velocimetry）、紅外線測溫儀（Infrared Thermometry）及壓力傳感器（pressure transducer）來量測流場結構、壁溫分布及壓力差值，雷諾數（Reynolds number）在壁溫分布及壓差量測實驗中從5,000變化至20,000，而流場結構量測實驗時，雷諾數則訂於10,000。本文將會分成以下三個部分以探討核心流干擾與近壁流干擾對於流體動量及能量傳遞之影響：
本研究的第一部份選擇平滑平行四邊形銳轉雙通道做為基礎案例，此雙通道前將加裝一不對稱突縮入口（ASI）以模擬實際渦輪機葉片冷卻通道之入口條件，藉此探討入口與平行四邊形截面形狀對於流場、熱傳及壓損的綜合效應。為探討流力參數與熱傳表現之間的關係，主要量測的流力參數有管道截面二次流、近壁面速度分布及紊流動能，本研究發現目前所使用之平行四邊形上、下壁面兩側存在不對稱的流場及熱傳分布，此一發現與對稱的方形及矩形管道有著顯著的不同。此外，與先前加裝全展入口（FDI）之平滑平行四邊形雙通道相比，本研究所使用之仿真入口能額外導引出二次流並且提高流體紊動強度，在較低的范寧摩擦係數（f）下提供較高的紐賽數（Nu），進而提升定泵功率下之熱效能係數（TPF），提升之幅度分別在雷諾數5,000及20,000的條件下達到43.5及53.7個百分比。
做為本研究第一部份之延續，本研究的第二部分將三種不同角度（45、−45及90度）的肋條加裝在搭載仿真入口的平行四邊形雙通道中以探討肋條角度與入口之綜合效應，實驗結果主要探討此綜合效應對於平均速度、二次流、紊動強度、紊動動能、雷諾剪切應力、局部與平均紐賽數及摩擦係數之影響。本研究發現此一綜合效應不只影響著離入口最近的第一通道，影響的範圍甚至延伸至整個平行四邊形雙通道，而在三個肋條角度當中，45度肋條能有效增強入口及平行四邊形傾斜側壁之綜合效應，並使得上、下壁面的紐賽數差異再次擴大，相較之下，另外兩個角度之肋條則因無法增強上述之綜合效應（尤以90度肋條最為明顯）而縮小上、下壁面的紐賽數差異。基於增強面平均紐賽數之能力，在平行四邊形上、下壁面最佳之肋條角度分別為45度及90度，此一結果與先前方形及矩形管道中之數據差異甚大。總體來說，三個肋化之平行四邊形雙通道皆能在定泵功率下提供較高的熱傳增益（TPF>1），但是僅有在能增強入口效應之45度肋化管道中，定泵功率下之熱效能係數能一直維持高於搭載仿真入口的平滑平行四邊形雙通道中的值，因此在平行四邊形雙通道中加入肋條增益熱傳前，一定得考慮入口效應來決定放置何種角度之肋條。
本研究的第三部分基於肋條（Rib）及折流板（Baffle）的熱傳機制與優點設計出新式百葉窗型擾流器，百葉窗型擾流器藉由干擾核心流體、壁面衝擊冷卻及延伸散熱面積三種方式來增益管道內熱傳表現，為簡化研究題目並釐清新式擾流器之影響，搭載全展流入口之正方形雙通道被採用做為第三部分的測試管道，實驗探討擾流器間距比（Pi/DH=1, 2, 3, 4, and ∞）及導流板數量（1≤Ns≤4）對於局部與平均紐賽數、摩擦係數及定泵功率下之熱效能係數的影響以找出新式擾流器之最佳設計。實驗結果顯示紐賽數比及摩擦係數比皆隨著間距比的變小及導流板數量的增加而上升，在所有測試的情況當中，擁有最小間距（Pi/DH=1）及最多導流板（Ns=4）的管道提供最高的熱傳增益，其增益幅度為正方形平滑雙通道之兩倍，然而，在此管道當中其壓損係數亦是所有測試管道當中的最大；此外，本研究亦發現正方形雙通道加裝百葉窗型擾流器之後其定泵功率下之熱效能係數不太隨擾流器間距而改變，且當導流板數量大於3時，熱效能係數亦不太隨導流板數量而改變，但當導流板數量小於3時，熱效能係數隨導流板數量減少而增加。綜合以上，在本研究之實驗條件範圍內，加裝越多百葉窗型擾流器且增加導流板數量（干擾核心流）可以大幅增強管道內熱傳表現，另一方面，干擾壁面流體則是較為經濟實惠的熱傳增益方式（定泵功率下之熱效能係數較高）。

關鍵字

質點影像測速儀 ； 紅外線測溫儀 ； 平行四邊形管道 ； 入口效應 ； 肋條角度 ； 百葉窗型擾流器 ； 核心流干擾 ； 近壁流干擾 ； 紊流 ； 熱傳

並列摘要

This work attempts to enlighten the influence of core flow disturbance and boundary layer disturbance on momentum and energy transport. Local temperature distributions over entire heated surfaces with the associated pressure drops are examined using Infrared Thermometry (IT) and pressure transducers with Reynolds number (Re) ranging from 5,000 to 20,000. The detailed turbulent flow features are measured with Particle Image velocimetry (PIV) to provide the flow physics attributed to the heat transfer performances at a fixed Re of 10,000.
The first part of the present study explores the detailed flow fields, heat transfer distributions, and pressure drop penalties in a stationary two-pass smooth parallelogram channel with 180-deg sharp bend as the baseline case. The particular asymmetrically and suddenly contracted inlet condition (hereinafter referred to as “ASI”) emulates the abrupt flow entrance of the real gas turbine blade cooling conditions. The secondary-flow patterns as well as the near-wall streamwise mean velocity components and turbulent kinetic energy are analyzed to correlate the relationship between flow characteristics and heat transfer distributions. The most distinct finding of this study is the asymmetric thermal and fluid flow features on the top and bottom wall side, in contrast to the symmetric ones in the corresponding square and rectangular channels. Compared with the previous fully developed inlet condition (hereinafter referred to as “FDI”) case, it is found that the thermal performance factors (TPF) of the ASI case are respectively 43.5% and 53.7% higher than those of the FDI case at Re=5,000 and 20,000 because of the entrance-induced secondary flow and disturbance. The correlations of Nu and fanning friction factor (f) are obtained to compare with those generated from the corresponding square channels.
The second part of the present study explores the detailed flow patterns and turbulence parameters, including streamwise and spanwise mean flow field, secondary-flow mean velocity map, turbulent intensity, Reynolds stress and turbulent kinetic energy as well as the heat transfer performances for the two-pass ribbed parallelogram channels with three different rib orientations (45-deg, -45-deg, and 90-deg). It is found that the combined effect of inlet condition and rib orientation extends to the entire parallelogram channel rather than the first passage only. Among the rib orientations investigated, the 45-deg ribs enhance the effects of parallelogram slant sidewalls and entrance to extend the Nu differences between the top and bottom walls. In contrast, the other two rib configurations, 90-deg ribs especially, weaken these effects and narrow the corresponding differences. Based on the ability to elevate the surface-averaged Nu/Nu∞ ratios, the respective orders of (45-deg, −45-deg, 90-deg) and (90-deg, 45-deg, −45-deg) for the present top and bottom walls are significantly different from those in square and rectangular channels. The variations of channel-averaged Nu/Nu∞ and f/f∞ with the associated TPF values against Re are compared with the previous results obtained from literature. In general, the three tested angled ribs all provide the TPF values above than unity. Only the TPF values in 45-deg case are always higher than those in corresponding smooth parallelogram channel (ASI case). Divided by the TPF values in ASI case, the normalized TPF values are respectively 1.09-1.04, 1.07-0.87, and 0.83-0.99 for 45, 90, −45-deg ribbed cases. It is thus important to take the entrance effect into account when applying the angled ribs in a two-pass parallelogram channel.
The third part of the present study aims to propose innovative louver-type turbulators to enhance the heat transfer rate via three flow mechanics, namely core flow disturbance, jet impingement, and extended heat transfer surface. These louvers are installed in the twin-pass square channel with a fully developed inlet condition. The parameters are examined to disclose optimal design in terms of the pitch ratio (Pi/DH=1, 2, 3, 4, and ∞) and the number of slat per half louver (1≤Ns≤4). The pressure drop measurements are also performed to estimate the Fanning friction factors (f) and the thermal performance factors (TPF). The results show that both (Nu) ̅/Nu∞ and f ̅/f∞ ratios rise with descending Pi/DH and ascending Ns under the present test conditions. Among all the tested cases, the case with Pi/DH=1 and Ns=4 provides the highest Nu/Nu∞, almost twice the value of smooth reference. Nevertheless, it suffers from high f/f∞ penalty. The TPF level is a relatively weak function of Pi/DH. There exists a critical slat number of Ns=3 above which the TPF value is a weak function of Ns. Below the critical Ns, the TPF value increases with decreasing Ns. From the viewpoint of heat transfer enhancement, one could apply the louvered channel as a heat exchanger with small Pi/DH and large Ns. The boundary layer disturbance, on the other hand, is more cost-effective than core flow disturbance as a mechanism to augment heat transfer from the viewpoint of thermal performance.

並列關鍵字

PIV ； Infrared Thermography ； Parallelogram Channel ； Entrance Effect ； Rib Angle ； Louver ； Core Flow Disturbance ； Boundary Layer Disturbance ； Turbulent Flow ； Heat Transfer

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