Unsteady characteristics of turbulent heat transfer in a circular pipe upon sudden acceleration and deceleration of flow
Unsteady characteristics of turbulent heat transfer in a circular pipe upon sudden acceleration and deceleration of flow
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DOI：
10.1016/j.ijheatmasstransfer.2017.05.077
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发表时间：
2017-10
影响因子：
5.2
通讯作者：
N. Shiibara;Hajime Nakamura;S. Yamada
中科院分区：
文献类型：
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作者：
N. Shiibara;Hajime Nakamura;S. Yamada
本研究的目的是阐明突然加速和减速时管道流中强化传热的机制。因此，我们应用红外热成像技术来测量水管流的时空传热。通过打开和关闭安装在测试段下游的电磁阀产生矩形脉动流，传热波动与管道内流量和速度波动等流动特性同时测量。首先，将脉冲周期的周期设置得较长（T=8s，ω′=10），以研究流动突然加速和减速引起的传热的基本特性。阀门打开和阀门关闭条件的上限和下限雷诺数分别设置为 Re D= 12,000 和 3000。占空比，即阀门打开时间与总时间的比率，设置为50%。当流动突然减速时，会出现具有高传热点的复杂结构，并且该结构在扩散过程中会维持一段时间。尽管流速迅速下降，但这导致传热系数逐渐下降。相反，当流动突然加速时，由于重新层化现象，传热系数在一段时间内保持较低水平，直到引发流动湍流。基于这些结果，我们修改了脉动条件来探索强化传热的可能性。上限和下限雷诺数分别设置为 Re D= 8000 和 0。占空比为25%时，循环周期变化为T=3s、6s、12s、24s和48s（ω'=16∼4）。结果，与相同流量下的非脉动流相比，在 T= 6 s (ω'= 11) 时，传热增强了 50% 以上。
The goal of the present study is to clarify the mechanism of heat transfer enhancement in a pipe flow upon sudden acceleration and deceleration. As such, we applied a technique using infrared thermography to measure the spatio-temporal heat transfer to a water pipe flow. A rectangle-shaped pulsating flow was generated by opening and closing a solenoid valve installed downstream of the test section, and the fluctuation of heat transfer was measured simultaneously with the flow properties, such as the fluctuations of the flow rate and velocity in the pipe. First, the cycle of the pulse period was set to be relatively long (T= 8 s, ω′= 10) in order to investigate the basic characteristics of the heat transfer caused by a sudden acceleration and a deceleration of the flow. The upper and lower Reynolds numbers for the valve-open and valve-closed conditions were set to Re D= 12,000 and 3000, respectively. The duty ratio, which is the ratio of the period of valve opening to the total time, was set to 50%. Upon sudden deceleration of the flow, a complicated structure with high-heat-transfer spots appeared, and the structure was maintained for some time while diffusing. This resulted in a gradual decrease in the heat transfer coefficient, although flow rate decreased rapidly. In contrast, upon sudden acceleration of the flow, the heat transfer coefficient remained low for some time due to the re-laminarization phenomenon, until flow turbulence was initiated. Based on these results, we modified the pulsation condition to explore the possibility of heat transfer enhancement. The upper and lower Reynolds numbers were set to Re D= 8000 and 0, respectively. The cycle period was varied as T= 3 s, 6 s, 12 s, 24 s, and 48 s (ω′= 16∼ 4) at a duty ratio of 25%. As a result, the heat transfer was enhanced more than 50% at T= 6 s (ω′= 11) compared to that for the non-pulsating flow at the same flow rate.