Experimental study on micro-channel bare tube heat exchanger in high-altitude and low-pressure environment
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摘要:
为探究高空低压环境对空冷换热器对流流动与传热特性的影响，搭建了低气压对流传热测试风洞实验台，实验台的压力范围为5.5～101.3 kPa，流速范围为0～13.6 m/s。对微通道光管换热器在12～20 km海拔高度环境压力下的流动与传热特性进行了实验和数值研究。结果表明：现有关联式在预测雷诺数低于100的摩擦因数时偏低，在预测雷诺数低于40的努塞尔数时偏高，茹氏公式对努塞尔数预测的最大相对偏差达72.4%。同时，由于空气低体积热容的影响，管排数的增加显著削弱了平均表面传热系数，使得茹氏公式的传热预测偏差随着管排数的增加而显著上升。为此，建立了新关联式，显著提高了对低雷诺数范围的预测准确性，并考虑了管排数对换热的影响，对摩擦因数与努塞尔数的预测与实验数据的相对偏差均在10%以内。
Abstract:To explore the influence of high-altitude low-pressure environment on the convective flow and heat transfer characteristics of air-cooled heat exchanger, a low-pressure convective heat transfer experimental wind tunnel was established. The wind tunnel operated within a pressure range of 5.5 to 101.3 kPa and a flow rate range of 0 to 13.6 m/s. The flow and heat transfer characteristics of micro-channel bare tube heat exchanger at 12—20 km altitude were studied experimentally and numerically. The results indicate that when the Reynolds number is less than 100, the existing correlations underpredict the friction factor. And when the Reynolds number is less than 40, the Nusselt number is overpredicted. The maximum relative deviation of the Zukauskas correlation for Nusselt number is 72.4%. Additionally, due to the low volumetric heat capacity of airflow, the increase of tube rows significantly weakens the average convective heat transfer coefficient, thereby increasing the heat transfer prediction deviation of the Zukauskas correlation. Consequently, a new correlation was established, which significantly improves the prediction accuracy in the low Reynolds number range, and considers the influence of the number of tube rows on heat transfer. The relative deviation between the prediction of friction coefficient and Nusselt number and the experimental data is within 10%.
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图 1 低气压对流传热测试风洞实验台示意图
Figure 1. Schematic diagram of low-pressure heat transfer wind tunnel experimental platform
图 2 低气压对流传热测试风洞实验台
Figure 2. Low-pressure heat transfer wind tunnel experimental platform
图 4 微通道光管换热器管束结构示意图
Figure 4. Schematic diagram of micro-channel bare tube heat exchanger
图 5 常压工况努塞尔数实验数据与茹氏公式对比
Figure 5. Comparison between experimental data and Zukauskas formula of Nusselt number under normal pressure
图 8 摩擦因数数值结果与实验结果的偏差
Figure 8. Deviation between numerical and experimental results of friction factor
图 9 努塞尔数数值结果与实验结果的偏差
Figure 9. Deviation between numerical and experimental results of Nusselt number
图 10 低压压力工况摩擦因数实验值
Figure 10. Experimental values of friction factor under low-pressure operating conditions
图 11 低压压力工况努塞尔数实验值
Figure 11. Experimental values of Nusselt number under low-pressure operating conditions
图 12 不同雷诺数下管束绕流速度流线分布
Figure 12. Flow velocity streamline distribution around tube bundle at different Reynolds numbers
图 13 不同管排数下努塞尔数模拟值
Figure 13. Nusselt number of simulation with different tube row numbers
图 14 不同管排数下空气温度云图
Figure 14. Contours of air temperature under different tube row numbers
图 15 管束中每排管的表面平均热流密度
Figure 15. Average surface heat flux of each tube row in the tube bundle
图 16 摩擦因数实验数据与新关联式
Figure 16. Experimental data and new correlation of friction factor
表 1 换热器管束结构参数
Table 1. Structural parameters of heat exchanger tube bundle
参数 数值 管外径 D/m 0.0009 横向管间距比 St/D 2.89 纵向管间距比 Sl/D 2 管排数 N 8 
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表 2 测量参数及测量仪表
Table 2. Measurement parameters and measuring instruments
测量参数 测量仪表 量程 仪器精度 温度 PT100 温度计 173～223 K ±0.1 K 压力 DOWESTON 压力变送器 0～20 kPa 0.25% 满量程 压差 DOWESTON压差变送器 0～ 1000 Pa0.1% 满量程 质量流量 EMERSON 高准科里奥利流量计 0～50 kg/h 0.5% 读数 长度 游标卡尺 0～300 mm ±0.02 mm 压力采集 COM-4017I模拟量输入模块 4～20 mA 0.3% 温度采集 COM-4015热电阻输入模块 73～473 K 0.1%
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表 3 流体横掠管束摩擦因数经典关联式
Table 3. Classic correlation of friction factor for flow across tube bundle
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表 4 流体横掠管束努塞尔数经典关联式
Table 4. Classic correlation of Nusselt number for flow across tube bundle
研究者 Nu关联式 适用范围 Žukauskas等[18] $ Nu=1.04{{Re}}^{0.4}{Pr }^{0.36}{\left(\dfrac{Pr }{{Pr }_{\text{w}}}\right)}^{0.25} $ 1 < Re < 500 0.7 < Pr < 500 ESDU[15] $ Nu=1.309{{Re}}^{0.36}{Pr }^{0.34}{\left(\dfrac{Pr }{{Pr }_{\text{w}}}\right)}^{0.25} $ 10 < Re < 300 Pr < 600 Chen和Wung[24] $ {Nu}=0.78 {Re} ^{0.45} {Pr}^{0.38} $ 40 < Re < 800
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