Minimum Film Boiling Temperature Research Articles

Experimental study of minimum film boiling temperature and quench front propagation velocity characteristics during the reflooding process

The plate-type fuel assembly stands as a prevalent choice in research reactors and naval power reactors, owing to its expansive heat transfer area and high heat transfer efficiency. Examining the flow heat transfer during the reflooding process and conducting safety analyses pertaining to this configuration emerge as a matter of great significance. Within the confines of this study, the ramifications brought by distinct reflooding velocity, coolant subcooling, heat flux, and initial wall temperature on the minimum film boiling temperature (MFBT) and quench front propagation velocity (QFPV) is delved. By employing a meticulous analysis of the standard regression coefficients, we discern the sensitivity of MFBT and QFPV under each individual condition. The results indicate that reflooding velocity and subcooling meet a significantly positive effect on MFBT and QFPV. The reflooding velocity has the most significant impact on the two key parameters, with an SRC of 0.603 for MFBT and 0.577 for QFPV, respectively. However, the initial wall temperature has a negative effect, while the heat flux does not play a critical role, as indicated by an SRC of only 0.135 for MFBT. The findings presented in this paper offer valuable data support for the safety design of nuclear reactors, and the conclusions drawn from the sensitivity analysis provide refined guidance for future research direction.

Prediction of the Minimum Film Boiling Temperature of Quenching Vertical Rods in Water Using Random Forest Machine Learning Algorithm

A great amount of research is focused, nowadays, on experimental, theoretical, and numerical analysis of transient pool boiling. Knowing the minimum film boiling temperature (Tmin) for rods with different substrate materials that are quenched in distilled water pools at various system pressures is known to be a complex and highly non-linear process. This work aims to develop a new correlation to predict the Tmin in the above process: Random forest machine learning technique is applied to predict the Tmin. The approach trains a machine learning algorithm using a set of experimental data collected from the literature. Several parameters such as liquid subcooling temperature (Tsub), fluid to the substrate material thermophysical properties (βf/βw), and system saturated pressure (Psat) are collected and used as inputs, whereas Tmin is measured and used as the output. Computational results show that the algorithm achieves superior results compared to other correlations reported in the literature.

Quenching experiment on Cr-alloy-coated cladding for accident-tolerant fuel in water pool under low and high subcooling conditions

A transient pool boiling experiment was conducted to investigate the quenching performance of Cr-alloy-coated cladding in the simulated conditions of a loss-of-coolant accident (LOCA). Cr-alloy-coated cladding is a potential candidate for accident-tolerant fuel (ATF), which is developed to suppress hydrogen generation at high temperature in the event of an LOCA or beyond design basis accident. Cr-alloy-coated cladding and Zircaloy-4 cladding were used in the quenching experiment under low and high subcooling conditions, i.e., 0 °C and 50 °C. The thicknesses of the Cr-alloy-coated layer were 20 and 50 μm. Each cladding specimen was heated up to 600 °C in a furnace and quenched in a water pool at constant temperature. The quenching experiment showed the formation of film boiling and transition to nuclear boiling at the low subcooling temperature of 0 °C. No film boiling was observed at high subcooling at 50 °C. The effect of surface roughness was found to be insignificant for quenching performance. Rather, cladding oxidation, particularly for the Zircaloy-4 specimens, had a remarkable influence on the minimum film boiling temperature (MFBT) and quenching time. The Cr-alloy-coated cladding did not show the effect of surface oxidation as expected because of very low cladding oxidation. There is no significant difference in the quenching performance of the Zircaloy-4 cladding and the Cr-alloy-coated cladding at the high subcooling temperature of 50 °C. Furthermore, the Cr-alloy-coated cladding tends to decrease the quenching time with higher MFBT as the coating thickness decreases.

Impact of system parameters on quenching heat transfer in the candidate materials for accident tolerant fuel-cladding in LWRs

We investigated a potential impact of the candidate materials (FeCrAl, SiC-CVD, and Zry-4) for the Accident Tolerant Fuel-cladding (ATF) in the quenching heat transfer according to the system parameters such as liquid subcooling, pressure, and flooding rates. Results obtained by water quenching experiment and system code analysis using MARS 3D-VESSEL are presented that the quenching heat transfer including the post-CHF parameters (i.e., minimum film-boiling temperature) is more influenced by the system parameters compared to change of the cladding materials. Current models of minimum film-boiling temperature in system code are also discussed for the application of ATF and its oxidation. Select option in the model library is proposed to consider the surface parameters using weighting term in thermal-hydraulic system code.

Minimum film boiling temperature and minimum heat flux in pool boiling of high-temperature cylinder quenched by aqueous surfactant solution

The use of surfactant has been known as one of effective ways to improve the boiling heat transfer. However, the most of boiling research using the surfactant has focused on the nucleate boiling regime, and the surfactant effect on the MFB (minimum film boiling) point, identified with MFBT (minimum film boiling temperature) and MHF (minimum heat flux) has rarely been studied. In the present study, the effects of surfactant and liquid subcooling on MFB point of vertical stainless steel cylinder were investigated experimentally using the quenching method. As the test fluids, the aqueous SDS (sodium dodecyl sulfate) solutions (1000 wppm and 2000 wppm), known to enhance the nucleate boiling heat transfer, were tested with pure water, where their liquid subcoolings covered from 0 °C to 39 °C. Based on the present experimental data, it was revealed that the SDS surfactant can impede the MFB point, while the increase of liquid subcooling leads to accelerating the MFB point. In addition, the influence of contact angle on MFB point was examined and discussed in detail. It was found that it could be insufficient and limited to explain the general trends of MFB point using contact angle alone and the fundamental factors responsible for changing the contact angle and thermal-hydraulic conditions should be considered.

Effects of Substrate Materials and Surface Conditions on the Minimum Film-Boiling Temperature

Film boiling is an important phenomenon in the evaluation of an emergency core cooling system following a hypothetical loss of coolant accident in a nuclear reactor. This study investigates the effects of liquid subcooling, surface oxidation, and surface materials on the minimum film-boiling temperature . Quenching experiments were performed using stainless steel and zirconium (Zr) test samples. The samples were heated to a temperature well above then plunged vertically in various degrees of liquid subcooling pools. A visualization study using a high-speed camera was conducted to capture the quenching behavior. Additionally, surface characterization analyses including X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy were performed to quantify the surface conditions. Results indicate that liquid subcooling has a strong influence on . The visualization study shows a very thin vapor formation around the test sample for higher subcooling pools which explains the enhancement in the heat transfer. It is observed from the surface characterization analyses that the variations in the surface condition of the stainless steel and Zr causes the vapor bubbles to depart differently in the nucleate boiling regime. Furthermore, the effect of surface oxidation is clearly noticeable in the Zr test sample compared to the stainless steel test sample due to the oxidation kinematic of each substrate material. It is found that the substrate thermophysical properties have a significant impact on . Comparing the bare substrates shows that for the same degrees of liquid subcooling pool, the value of for the Zr sample is ∼30°C to 60°C higher compared to the stainless steel sample. Moreover, increasing the degrees of liquid subcooling contributes to a significant increase in that varies between ∼50°C and 70°C for both samples.

Minimum heat flux and minimum film-boiling temperature on a completely wettable surface: Effect of the Bond number

We investigated the effect of Bond number of sphere Bos and surface super-hydrophilicity at minimum film-boiling temperature TMFB and minimum heat flux q″min using quenching experiment at atmospheric pressure and the saturation temperature of water. In particular, we focused on the vapor-releasing dynamics in film boiling and evaluated the main parameters such as vapor-bubble releasing frequency fb and vapor-bubble departure diameter Db. We selected two sizes of quench sphere (sphere diameter Ds = 15 mm and 25 mm) based on critical Bond number BoC to evaluate the vapor-releasing dynamics depending on the Bos. The super-hydrophilic surface was prepared by the anodic oxidation on zirconium (Zr-702) sphere. High speed visualization and inverse heat transfer calculation facilitate a qualitative and quantitative analysis of film boiling heat transfer. The surface super-hydrophilicity of the quench sphere increases TMFB and q″min: 12% and 366% increase for Ds = 15 mm and 20% and 305% increase for Ds = 25 mm, respectively. Db strongly depends on Ds and exhibits a relatively weak dependency to the surface super-hydrophilicity. fb is affected by the Ds and the surface super-hydrophilicity. The increase in TMFB is explained by the liquid-solid contact in film boiling. The D25-CWS exhibits the large area fraction of liquid-solid contact versus total heat transfer surface compared to the D15-CWS. The increase in q″min is related to minimum frequency of vapor-bubble releasing to sustain the stable liquid-vapor interface fb,min because the large fb,min indicates the fast destabilization of the liquid-vapor interface in film boiling during quenching.

Quenching of candidate materials for accident tolerant fuel-cladding in LWRs

A quenching experiment with a vertical rod (diameter D = 10 mm and length L = 60 mm) was conducted under atmospheric pressure and saturation temperature of water to investigate the boiling parameters in the post-critical heat flux (CHF) regime for the candidate materials of accident tolerant fuel-cladding (ATF): FeCrAl(APMT) and SiC-CVD, compared to commercial cladding material Zircaloy-4(Zry-4) in light water reactors (LWRs). We prepared two test groups (plain surface PS and oxidized surface OS) and evaluated their surface parameters (roughness ε, contact angle θc, oxygen content of the surface, and oxide layer thickness δoxide) and thermal properties (heat capacity ρCp and thermal conductivity k) that influence the quenching performance. We analyzed the minimum film-boiling temperature TMFB, minimum heat flux q″min, and the heat transfer coefficient of film boiling hfilm under all test conditions. In particular, the surface damage of the SiC-CVD after quenching was analyzed by considering the post-CHF heat transfer condition, and its impact was evaluated at TMFB during quenching.

Subcooled water quenching on a super-hydrophilic surface under atmospheric pressure

The goal of this work is (i) to evaluate the cooling rate on a super-hydrophilic surface asa function of the subcooled degree ΔTsub of the liquid coolant, (ii) to analyze the contact heat transfer q″c of the liquid-solid contact, and (iii) to investigate the mechanism of microbubble emission boiling (MEB). We fabricated a super-hydrophilic surface by anodic oxidation of a zirconium vertical rod, so called completely wettable surface (CWS), which had surface microstructures with super-hydrophilicity. The CWS results in a decrease of the cooling time tcool as compared with the Bare Zirconium surface (BZS) results under small ΔTsub (tcool∼50% decrease for ΔTsub=0, 15, and 40K, respectively). However, its surface effect is limited in the case of large ΔTsub (tcool∼within 5% for ΔTsub=60 and 75K). The fast quench on the CWS under ΔTsub, explained by the increase in minimum film-boiling temperature TMFB and rewetting velocity U, is due to the liquid-solid contact. We evaluate the contact area Ac and volumetric absorption rate of the liquid dV/dt by conducting liquid absorption experiments. The increase in Ac and dV/dt contribute to an increase in q″c, by forming the liquid film at the liquid-solid contact spot. The orders of the time scale between capillary-wicking and liquid-solid contact are comparable. Destabilization of the large vapor bubble is caused by an increase in q″c, which is a major reason for MEB generation, and this mechanism enables the q″ to be significantly high on the CWS under subcooled quenching.

A Review on Sodium Boiling Phenomena in Reactor Systems

Sodium boiling phenomena in nuclear reactors have been reviewed in the context of the renewed interest in sodium-cooled fast reactors. This paper presents all properties that influence sodium boiling behavior, including thermodynamic and transport properties, as well as the typical composition of reactor-grade sodium, the surface wetting, radiative heat transfer properties, and noncondensable behavior. Starting from these properties, the tendency for high superheat is explained, together with the reasons that the problem of superheat can be neglected for reactor systems. The peculiar boiling behavior of sodium in assemblies is explained on the basis of the temperature profile. This leads us to conclude that a typical slug flow pattern prevails for sodium boiling. The boiling heat transfer for pool film boiling is also given, deducing that the critical heat flux phenomena for sodium boiling in reactor systems is mainly related to dryout and not to the departure from nucleate boiling. The correlations that exist for the minimum film-boiling temperature are discussed in light of their applicability to liquid sodium. Although there are already a large amount of data, gaps in the current understanding of sodium are highlighted.

LOFT Experiment LP-02-6 Analysis by RELAP5/MOD2 Code with Improved Minimum Film Boiling Temperature

LOFT Experiment LP-02-6 Analysis by RELAP5/MOD2 Code with Improved Minimum Film Boiling Temperature

On the Use of Minimum Film Boiling Temperature as a Criterion for Coherent Molten Metal-Coolant Interactions

Minimum film boiling temperature data obtained on 19- and 25-mm-diam spheres quenched in subcooled water are used to establish a relationship between the temperature of the hot molten material and ...