Conference Agenda

The pool boiling experiments were conducted on a surface with micro-cavities, where the pitch and depth of the micro-cavities were kept constant at 120 μm and 20 μm, respectively. The cavity diameter (CD) ranged from 5 to 70 μm, and visualization was performed using visible ray and X-ray. The results of the experiments showed that at low heat flux, the heat transfer coefficient (HTC) was highest in the CD10-20 range, and there was a proportional relationship between nucleation site density and HTC. At high heat flux, excluding CD5, there was a trend of decreasing HTC with increasing cavity diameter. In this case, there was no significant difference in bubble behavior across the entire surface, and it is speculated that HTC decreases as the area of cavities filled with bubbles increases. Additionally, CD5 exhibited different bubble behavior compared to surfaces with CD10 and above, requiring different interpretation.

Capillary wicking can transport water effectively and can be applied in thermal management. Nanostructured materials have strong capillary wickability, which can promptly furnish water to the heating surface and enhance its boiling heat transfer characteristics. However, when the nanostructured surface is placed in an atmosphere environment, it will continuously adsorb volatile organic compound (VOC) from air, which can lead to a wicking deterioration. Therefore, this article mainly explores the effect of VOC adsorption or removal on the capillary wicking and boiling heat transfer characteristics of nanostructured materials. The contact angle and XPS results indicated that VOC adsorption could increase the pollutant content and hydrophobicity of nanostructured surface, while argon plasma treatment could remove VOC and enhance hydrophilicity. The pool boiling experiment showed that capillary wicking can greatly improve the threshold of boiling heat transfer and critical heat flux (CHF), while VOC adsorption can lead to a decrease in the capillary wicking performance of nanostructure, which can cause a deterioration of boiling heat transfer characteristics. After removing some VOCs through argon plasma treatment, the wicking and boiling heat transfer characteristics of nanostructure were partially restored. This work is beneficial for promoting the understanding of the impact of VOC on the heat transfer characteristics of wicking structures.

As a type of laser modification method, femtosecond laser surface modified technology can fabricate microstructures on surface of stainless steel, zironium alloys and nickel alloys, with special surface morphology such as honeycombs, humps and grooves. Modified surface with microstructures has significant impact on the heat and mass transfer. In order to explore the mechanism of enhanced boiling heat transfer on modified surfaces, visual research was conducted on nucleated bubbles on modified surfaces. It is possible to obtain the unique bubble behavior of modified surfaces, reveal the mechanism of enhanced heat transfer, and construct a model for bubble nucleation.Research can provide technical guidance and data support for the application of femtosecond laser modification technology in typical channels and various heat exchange devices.

Surface engineering has demonstrated significant potential for enhancing nucleate boiling heat transfer performance. However, the underlying mechanism remains unclear, especially the role of microlayer evaporation underneath bubbles. In this work, we systematically investigate the effect of surface micro-pillars on the microlayer morphology and the corresponding microlayer heat transfer performance. Using Direct Numerical Simulations, the microlayer formation and evaporation in the early diffusion-controlled bubble growth stage on various micro-pillar arrayed surfaces are reproduced. We reveal three distinctive microlayer morphologies on the micro-pillar arrayed surface: the disturbed microlayer, disrupted microlayer, and undisturbed microlayer. In general, a disrupted microlayer results in a reduced average thickness, increasing the transient heat transfer coefficient. Conversely, a disturbed microlayer retains more liquid, enhancing the microlayer heat transfer potential throughout its life cycle. Isolated bubble nucleate boiling experiments are performed to examine and further extend these findings throughout the entire bubble life cycle in nucleate boiling. The bubble dynamics are statistically analyzed. In addition, a preliminary experiment using synchrotron X-ray imaging is performed to directly capture the microlayer morphology. The experimental results align closely with the simulation results. Moreover, the experimental results indicate that a critical average microlayer thickness can be achieved by optimizing surface modifications, ensuring efficient evaporation throughout the bubble life cycle without significant depletion. This work provides a novel and practical way to optimize surface engineering for enhanced nucleate boiling heat transfer.

Heterogenous nucleate boiling observed in industrial scenarios, such as pressurised water reactors, originates at the smallest temporal and spatial scales. On the microscale, the evolution of the liquid-vapour interactions are directly influenced by the morphology of a heater’s surface. At this scale, nucleation mechanisms such as gas/liquid trapping, interface retention, and subsequent nucleation site activation are driven by hydrodynamic and thermal effects. In particular, the hydrodynamic phenomena become more pronounced when examining surfaces with increased surface intricacies. For example, the porosity or roughness can result in dominating capillary forces with complex surface tension and vapour diffusion effects. This process is further complicated by related thermal effects such as Conjugate Heat Transfer (CHT) from the solid to the fluid phases. This work aims to quantify the effect of nucleation mechanisms at the microscale and their impact on thermal efficiency and subsequent nucleate boiling. The progression of nucleate boiling is examined, using detailed Computational Fluid Dynamics (CFD) and the Volume of Fluid (VOF) method. Low capillary number simulations are performed using surfaces with porosity and roughness representative of the zirconium oxide layer on the cladding found in water-cooled nuclear reactors to discern more about the hydrodynamic and thermal physical processes that occur on such a surface. This is of particular interest, due to the challenges associated performing prototypic measurements of a water-cooled nuclear reactor across the range of length- and time-scales.

Point of Net Vaper Generation (PNVG) marks the transition in the bulk flow from single-phase to two-phase. Its prediction is paramount to modeling subcooled boiling and has therefore drawn consistent research attention. By searching for the incipience of bubble detachment via high-speed imaging, a recent flow-boiling dataset has identified hydrodynamically controlled PNVG in an upward annular channel. These data - which feature near-inlet PNVGs and a partially heated wetted perimeter - are compared with predictions by several well-known correlations. Some tested correlations were developed based on the dedicated mechanism of bubble detachment, and most of them claimed acceptable performance in fitting their own benchmarks. However, noticeable discrepancies are often found between the current data and these correlations, as well as among different correlations. In addition, certain correlations were originally reported with acceptable uncertainties in terms of dimensionless groups. These uncertainties are found propagating to unsatisfactorily low confidence in more sensible dimensional parameters such as the PNVG location, due to the nature of current conditions. These discrepancies and challenges faced by existing correlations are discussed with quantification and reasoning. Improvement is also attempted, and an analytically modified Levy’s model achieves acceptable consistency with the current experiment. The limited capability in predicting PNVG in the current configuration needs awareness and calls for further modeling improvement and validation.

During two-phase annular flow boiling at prototypical conditions in nuclear reactor fuels, liquid transport to the regions prone to CHF is an important consideration in the mechanistic prediction of dryout. Water flows to the CHF-prone region through a base liquid film flowing along the fuel sheath and through droplets that are entrained in the vapour flow, while water can be removed from the base film through entrainment processes. The base-film flow rate is related to the liquid superficial velocity while the droplet velocities move near the vapour core velocity. Recent work has shown that the presence of large liquid interfacial waves may be responsible for a large fraction of the liquid transport to the CHF region and with a speed in between that of the liquid and vapour. This paper presents the results of a development program to measure these interfacial phenomena in steam water boiling flows in a heated tube. A high output X-Ray source and single-photon detector counting array are used to record the characteristics of the interior flow patter at a speed of 250 frames per second. X-Ray energy was optimized to ensure transmission through the Inconel test section wall as well as to ensure differentiation in the cross sections of liquid and steam. Flow rates were limited to those where interfacial wave speed was less than 4m/s due to frame speed limitations. Measurements include wave speed, heigh, mass flow, and frequency. Important observations on the relationship between wave height, speed, and frequency will be presented.