Effects of Surface Characteristics in Pool Boiling Critical Heat Flux Enhancement, and its Prediction Model

Abstract

In nuclear power reactors, most of studies related to thermal-hydraulics have focused on increasing power density, efficiency, and safety of currently operating nuclear power reactors. The primary constraint in the aspect of thermal-hydraulics is determining and controlling of departure of nucleate boiling (DNB), called as critical heat flux (CHF). When the CHF is occurred, bubbles and vapors, which heat transfer coefficient is very low compared to liquid, covered whole heating surface; thus the temperature of heating surface will increase rapidly. The rapid increment of the fuel rod and cladding surface temperature due to the CHF will destroy the second barrier of the defense in depth and may release dangerous fission products in containment building or environment. This inferred that an accurate prediction of the CHF phenomenon is important because it is related to the safety of the environment. In addition, CHF enhancement enables increasing power density and safety margins of reactors; the CHF enhancement study and development of the accurate CHF prediction model should be conducted. In the present research, therefore, CHF enhancement studies based on surface modifications on a plain heating surface and modified CHF model that includes the effects of surface characteristics are presented. For the CHF enhancement studies, surface modification techniques have been widely studied: deposition of nano- or micro-particles, mechanical machining, and micro-electro-mechanical techniques. In the present CHF enhancement study, deposition of nanoparticles (SiC and graphene) on a plain heating surface was used for the porous heating surfaces. Nonporous heating surfaces were also examined to compare with the porous heating surfaces. In addition, a hybrid graphene/SWCNTs heating surface was considered to show the effect of thermal properties for the heating surfaces on boiling performance. Surface characteristics for various heating surfaces were considered as the CHF enhancement parameters. Infrared (IR) thermometry was used to visualize the heating surfaces and several methods were applied on the parameter studies to quantify the enhancement mechanism. The discussed surface parameters were surface wettability, roughness, capillarity, permeability, porosity, and thermal effusivity. Because all the surface parameters are coupled each other, it was difficult to determine the major parameters that influence on the CHF performance. Therefore, several CHF models, such as hydrodynamic instability theory, bubble force balance, microlayer dryout, bubble interaction, and hot/dry spot, were evaluated to determine the appropriate the CHF prediction model in comparison of the experimental results. As a result, modified hydrodynamic instability theory was selected as one of the CHF enhancement model because it can predict the CHF enhancement trends when the effect of surface characteristics was considered as a change of RT instability wavelength. Hydrodynamic instability theory has been widely used as the CHF prediction model because it can predict well under plain heating surface conditions. The hydrodynamic instability theory assumed that there is a critical region that the liquid could not penetrate into the heating surface and this point is called as the CHF point. The critical vapor velocity was derived by Kelvin–Helmholtz (KH) instability. However, the measurement of the critical vapor velocity at near the CHF point is impossible because vigorous boiling is occurred on the heating surfaces under nucleate boiling situation. Instead of measuring critical vapor velocity or KH instability wavelength, Rayleigh–Taylor (RT) instability wavelength was considered as the KH instability wavelength. The present approach is started from the change of the critical vapor velocity due to the change of heating surfaces. Critical and the most dangerous RT instability wavelengths were applied on the hydrodynamic instability model and further approximation to derive the CHF prediction model was conducted. However, there are several assumptions in the hydrodynamic instability theory and these assumptions should be evaluated to provide credibility of the model. Therefore, the relation between RT and KH instability wavelengths and the consideration of the surface effects on the change of RT instability wavelengths incorporated into the hydrodynamic instability theory, called as a modified hydrodynamic instability approach, was studied in the present research. The relation between RT instability wavelength and CHF was examined in a pressurized wire pool boiling facility with various kinds of Ni-Cr wire diameters. The results indicated that that it is possible to make the relation between the RT instability wavelengths and the CHF values by using the modified hydrodynamic instability approach. Not only using the bare Ni-Cr wire surface, various kinds of nanoparticle-deposited heating surfaces were considered to evaluate the modified hydrodynamic instability CHF model. A correlation for the change of RT instability wavelength based on the experimental results was proposed to predict the CHF enhancement based on the heating surface characteristics. Because the RT instability wavelength which was determined by the effects of heater geometries can explain all the reasons of CHF enhancement in pool boiling conditions, the surface roughness and factor was also examined in the validation procedure for the proposed CHF model. The validation results indicated that the modified hydrodynamic CHF model is valid in the present research.