Abstract

Several challenges coexist in the field of flow boiling in microchannels, ranging from high superheat required for boiling incipience to boiling instabilities and early dryouts. The aim of this study is to mitigate or solve some of the challenges and develop an image-processing algorithm for analysis of boiling oscillations in multiple parallel channels. The experimental results were acquired on an array of 64 parallel \(25\times 25\) \(\mu\)m microchannels using a synchronized high-speed visualization and measuring system. The small cross section of the microchannels allowed only the formation of annular two-phase flow, and a computer algorithm was developed for tracking the meniscus oscillations during boiling. The applied image analysis focuses on reliability with the simultaneous use of brightness variation and brightness derivative along with image subtraction. Moreover, the images were preprocessed to determine the number of microchannels and their orientation with applying different filtering and Radon transformations. The data extracted from the visualization helped determine the peak-to-peak amplitudes and fundamental frequencies of the oscillating meniscus. The results exhibit lower amplitudes and higher fundamental frequencies with increasing heat flux. The mass flux was kept constant at 83 kg/m\(^{2}\)s, whereas the heat flux varied from 150 kW/m\(^{2}\) to 250 kW/m\(^{2}\). The amplitudes and the fundamental frequencies of the meniscus oscillations determine the length and duration of microchannel with periodically alternating liquid and vapor phases.

中文翻译：

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平行微通道中水流沸腾过程中的振荡特征

摘要

在微通道中的流动沸腾领域中并存着一些挑战，从沸腾初期所需的高过热到沸腾的不稳定性和早期变干。这项研究的目的是减轻或解决某些挑战，并开发一种图像处理算法来分析多个并行通道中的沸腾振荡。实验结果是在64个并行\（25 \ times 25 \） \（\ mu \）的数组上获取的使用同步高速可视化和测量系统的m个微通道。微通道的小横截面仅允许形成环形两相流，并且开发了一种计算机算法来跟踪沸腾过程中的弯液面振荡。应用的图像分析关注可靠性，同时使用亮度变化和亮度导数以及图像减法。此外，通过应用不同的滤波和Radon变换对图像进行预处理，以确定微通道的数量及其方向。从可视化中提取的数据有助于确定振荡弯液面的峰峰值幅度和基频。结果表明，随着热通量的增加，振幅降低，基频升高。\（^ {2} \） s，而热通量从150 kW / m \（^ {2} \） 到250 kW / m \（^ {2} \）。弯月面振荡的幅度和基频确定具有周期性交替的液相和气相的微通道的长度和持续时间。