Micro channel heat exchanger has become the focus of scholars as a kind of highly efficient heat transfer equipment, and the nanofluid flow boiling heat transfer in microchannels is a hot topic at present. The flow boiling as a vapor-liquid two-phase flow, including a series of sub-processes of generation, growth, detachment and interaction of many boiling bubbles, is complex nonlinear system, and their nonlinear characteristics have an important influence on the boiling heat transfer performance of the entire microchannels. The simple experimental analysis method adopted by scholars does not accurately describe the dynamic characteristics of the flow boiling system in microchannel. Therefore, in order to investigate the heat transfer characteristics, non-linear characteristics and their interrelationships of nanofluid flow boiling system in microchannels, uniform and stable 0.05-0.30 wt% Al2O3/R141b nanofluids were prepared as the experimental working fluid and the experimental platform was built, and the flow boiling test was carried out in the microchannels of 2 mm × 2 mm under the heat flux of 14-46 kW/m2, mass flow rate of 310.5 kg/(m2·s), and system pressure of 165 kPa. The boiling heat transfer coefficient is calculated through the heat transfer model and the univariate time series is established by importing and exporting pressure data through the experimental section. The nonlinear characteristics of the time series are studied by Hurst exponential analysis, correlation dimension, maximum Lyapunov number and Kolmogorov entropy. The relationship between the nonlinear characteristics and the heat transfer performance is also compared. The results show that the boiling heat transfer coefficient of nanofluids first increases and then decreases with the increase of heat flux density under the experimental conditions, and the heat transfer coefficient reaches the maximum at 38 kW/m2heat flux density. The flow boiling of the nanofluid Al2O3/R141b and pure refrigerant R141b in the microchannels shows chaotic characteristics. The Hurst exponent is greater than 0.5, and the correlation dimension, the maximum Lyapunov exponent and the K entropy are all finite values greater than 0. Compared to pure refrigerant, the chaotic degree of the flow boiling system is stronger and the heat transfer performance is also better. The concentration has a significant effect on the nonlinear characteristics and heat transfer coefficient of the nanofluid boiling system, the chaos degree of the nanofluid increases first and then decreases with the increase of the concentration of nanoparticles, and its boiling heat transfer coefficient also increases first and then decreases. Under the experimental conditions, the non-linear characteristics of 0.10% nanofluid reach the maximum and the corresponding boiling heat transfer coefficient is also the largest, and the average boiling heat transfer coefficient is about 76% higher than that of pure refrigerants. The analysis believes this result is the comprehensive effect of nanoparticles on the vapor-liquid interface and its deposition on the channels surface. The effect of nanoparticles on the vapor-liquid interface can make the gas-liquid-solid three-phase line move toward the gas phase, the diameter of bubbles smaller and the frequency of disengagement increase. As a result, the turbulence intensity of the fluid in the microchannels is increased and the chaos of the system is stronger and the heat transfer performance is better. The deposition of nanoparticles on the channels yet can increase the wall thermal resistance and wall wettability, resulting in a smaller number of bubbles, thereby reducing the chaos degree of system and heat transfer efficiency. They are 2 diametrically opposed mechanisms that have led to the above experimental results. In this paper, the method of combining nonlinear analysis and experiment is introduced into the study of flow boiling in microchannels. Compared with the traditional analytical methods, the kinetic characteristics of the flow boiling system in microchannels can be more accurately described and the mechanism of nano-fluid enhanced phase heat transfer is further revealed.%为探究微细通道内纳米流体流动沸腾系统的传热性能、非线性特性及其相互关系,分别以质量分数为0.05 %、0.10 %、0.15 %、0.20 %和0.30 %的Al2O3/R141b纳米流体和R141b纯制冷剂为试验工质,在2 mm×2 mm的矩形微细通道内进行流动沸腾试验,计算得到了不同浓度纳米流体的沸腾传热系数,建立了试验段进出口压差时间序列,运用Hurst指数分析、关联维数、最大Lyapunov数和Kolmogorov熵研究了该时间序列的非线性特征,并比较其与传热系数之间的关系,结果表明:相比纯制冷剂,纳米流体流动沸腾系统的混沌程度更强,传热性能也更好;纳米流体的混沌程度随着浓度的升高先增强后减弱,其沸腾传热系数也随着浓度的升高先增加后减小,试验工况下质量分数为0.1%的纳米流体的各项非线性特征量均达到最大值,混沌程度最强,相应的沸腾传热系数也为最大,其平均沸腾传热系数可达4.25 kW/(m2·K),而纯制冷剂仅为2.42 kW/(m2·K).该文采用非线性分析与试验相结合的方法,更能准确描述微细通道沸腾系统的动力学特征,可为进一步研究微细通道纳米流体相变强化传热机理提供参考.
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