Experimental and Numerical Investigations on the CHF Enhancement by Advanced Surface Modifications in Pool Boiling

先進表面結構在池態沸騰中增強臨界熱流密度的實驗和模擬研究

Student thesis: Doctoral Thesis

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Award date14 Apr 2021

Abstract

With the increasing power consumption in the worldwide energy-intensive sectors and the climate change, low-carbon energies such as the nuclear power and solar energy with higher heat utilization become necessary. Besides, the safety concern and the lack of public acceptance after the historic events in Chernobyl and Fukushima, a guaranteed safe operation and the post-accident safety management are essential tasks in the nuclear power plants. Therefore, efficient heat transfer performances are always required. As represented by the critical heat flux (CHF) and the heat transfer coefficient (HTC), the boiling performances of a substrate are continually studied in the thermal engineering field with the aim to obtain further improvement. Numerous advanced approaches have been employed to enhance the CHF and HTC, while the underlying mechanism is still obscure. More investigations regarding the complex mechanism understanding and more explorations with the aim to further enhance the heat transfer performance through advanced approaches are sought after.

First, our study decouples the contributions of the intrinsic surface wettability from the hierarchical (dual-layer) structure on the boiling enhancement by growing nanograss as the substructure and the micro flowers with different cover density as the superstructure. The surface with nano structures shows a maximum critical heat flux (CHF) enhancement of 68% compared with the plain surface. In addition, the effect of the surface orientation on boiling heat transfer has been investigated. For the multi-orientated (from 0° to 180°) substrates, the surface orientation influences the boiling performance through different physical mechanisms. It discloses that the departure time and the thickness of the fully developed vapor film on the inclined surfaces increase with increasing surface orientation, resulting in impeded boiling performance of the downward-facing surfaces. Moreover, enhanced critical heat flux and heat transfer coefficient are obtained for the nanograss structured surface at the downward-facing orientations. Furthermore, new correlations regarding the downward-facing CHF prediction based on the horizontal CHF are proposed.

Second, engineering nano-structured surfaces with mixed/thermo-responsive wettability offer a new approach to improve the boiling performances of advanced thermal systems. Four groups of surfaces: a) nanofilm coated surfaces, b) patterned surfaces with superhydrophilic nanograss, c) patterned surfaces with superhydrophobic nanograss, d) patterned surfaces with thermo-responsive wettable nanograss are investigated for their boiling performances. It is found that the nanofilm coated surfaces show improved maximum heat transfer coefficient (HTCmax) as well as critical heat flux (CHF) compared with the plain surface. The patterned surfaces shift the boiling curves to left, and the CHF increases with increasing nanograss cover density. The surfaces with thermo-responsive wettability, which responses to the external heating/cooling stimuli by gradually increasing or decreasing the wettability, show the most optimal CHF enhancement. This study serves as a proof-of-concept for efficient heat transfer though carefully fabricated nano-structured wettability-enhanced surfaces.

Third, except for the experimental investigation, computational modeling as an efficient approach is also employed in our study. As the validation and extension of the experiment, the lattice Boltzmann method (LBM) is employed to simulate the effect of surface orientation, wettability, geometric size, and subcooling of the downward-facing lower head on the CHF enhancement. Since the enhanced CHF in pool boiling significantly extends the safety limit in the nuclear system, especially under the In-Vessel Retention–Reactor Vessel Cooling (IVR-ERVC) severe accident management strategy to avoid the failure of the lower head reactor pressure vessel, the underlying mechanism needs more study to uncover it. In our research, it was found that for the downward-facing surface, the CHF decreases with increasing surface orientation, which is in agreement with the experimental results. Besides, the surface CHFs for the downward facing angles increase with increasing wettability. The CHF performance of the near-vertical and vertical surfaces exhibits the largest improvement with decreasing the surface contact angle. Moreover, it reveals that with an increased radius of the lower head, the CHF decreases for all curved surfaces, especially obvious for the inclination angle smaller than 90°. Our study also provides a discussion regarding the effect of subcooling on the CHF enhancement. It was found that the CHF increases with the increase of subcooling and the simulation results agree with the Elkassabgi-Lienhard and Zuber correlations. Furthermore, the enhancement of CHF by the Non-gaussian random surface and the ordered sine textured surface was investigated using the fast Fourier transform (FFT) method coupled with LBM. The effects of the rough and regular sine textures are uncovered on the CHF enhancement. An optimal design of the surface with sine structure in mesoscopic scale is proposed.

In summary, we systematically studied the effect of the advanced surface structures on the CHF enhancement in pool boiling by using both experiment and simulation. Monolayer/dual layer of nano/micro-structures, patterned structures, surfaces with homogenous/mixed and thermo-responsive wettability are comprehensively investigated for their contributions in improving CHF. Besides, the orientation as an essential factor coupled with surface modifications are also studied to reveal the unique downward-facing boiling phenomenon. Except for the experimental setup was built in our lab, a set of numerical code was developed in our group for the pool boiling simulation. We believe that our work provides comprehensive and fundamental understanding of the pool boiling process, and new insights regarding the surface structure designs are inspired for the CHF enhancement.