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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Development of a mechanistic model for sliding bubbles growth prediction in subcooled boiling flow

In this paper, a new mechanistic model for predicting the growth of sliding bubbles in subcooled boiling flow has been developed. The basis of this model is the assumption that the fundamental heat transfer mechanisms acting on a stagnant bubble growing at a nucleation site can also be applied to the growth of sliding bubbles. The physical assumptions of the existing bubble growth models were modified to reflect the intrinsic physics associated with the sliding bubble growth. The heat transfer mechanisms considered are microlayer evaporation, evaporation of superheated liquid, and condensation. The present model has been successfully validated against a broad set of subcooled flow boiling data, generated by three separate research groups using different working fluids and different heater surfaces. The model successfully reproduced the sliding bubble growth dependencies on mass flux, wall superheat, and liquid subcooling level as observed in the experiments. Finally, on the other hand, in a few cases where the bubble growth inertia was substantially involved in the sliding bubble growth, the predictive ability was limited.

42 ENGINEERING↗
DOE OSTI2020

Separate Effects Validation for Subcooled Boiling in CTF

In 2010, the U.S. Department of Energy created its first Energy Innovation Hub, which is focused on developing high-fidelity and high-resolution modeling and simulation (M&S) tools for modeling of light water reactors (LWRs). This hub, the Consortium for Advanced Simulation of LWRs (CASL), has developed an LWR simulation tool called the Virtual Environment for Reactor Applications (VERA). The multi-physics capability of VERA is achieved through the coupling of single-physics codes, including CTF (the CASL version of Coolant Boiling in Rod Arrays— Three Field (COBRA-TF)), Michigan Parallel Characteristics Transport (MPACT), BISON, and Materials Performance and Optimization (MPO) Advanced Model for Boron Analysis (MAMBA). As part of its M&S efforts, CASL has identified various challenge problems, including Crud Induced Power Shift (CIPS), Crud-Induced Localized Corrosion (CILC), Pellet-Cladding Interaction (PCI), and Departure from Nucleate Boiling (DNB). This work addresses CASL milestone L2:VVI.P19.03, which focuses on uncertainty quantification of crud, which is relevant to both CIPS and CILC. This is achieved through an analysis and separate effects validation of the thermal hydraulic phenomenon known as subcooled boiling. As part of this work, various sources of experimental data are examined and compared to different options for empirical modeling of subcooled boiling. Through this analysis, a complete understanding of the underlying models and their implementation details are understood. A subset of these data are incorporated into a separate effects validation study of CTF. The Westinghouse Advanced Loop Tester (WALT) and Rohsenow experiments are modeled, and it is shown that the newly-implemented Gorenflo correlation is more accurate than the existing Chen and Thom correlations.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2020

Force Balance Model Assessment for Mechanistic Prediction of Sliding Bubble Velocity in Vertical Subcooled Boiling Flow

The bubble sliding after departing from nucleation site is frequently observed in flow boiling systems and, the crucial impact on wall heat transfer has been evidenced through many experiments. As a result, the heat transfer modeling associated with sliding bubble has become one of the subjects of great attention in CFD boiling heat transfer community. The modeling efforts are primarily aimed at improving the existing Heat Flux Partitioning (HFP) model via the implementation of sliding bubble-induced heat transfer. The performance of HFP model depends inherently on the fidelity of sub-models used to predict the fundamental bubble parameters (e.g., bubble departure/lift-off diameter). In the same context, the accurate prediction of sliding bubble parameters (e.g., sliding bubble growth, sliding bubble velocity) is essential to achieving the successful heat transfer modeling associated with sliding bubble. Of many sliding bubble parameters, this paper deals with the sliding bubble velocity. Specifically, the force balance model was assessed in view of the sliding bubble velocity prediction. The parametric effect of key sub-models (e.g., drag force, bubble growth models) used in the force balance equation was investigated. The experimental data from Maity (2000) and Yoo et al. (2016) were used for demonstrating the model predictive performance. It was found that the force balance model proposed in this study was able to predict well the bubble sliding velocity based on the accurate prediction of bubble growth during sliding. The predictive performance was proven with the experimental data measured under various subcooled boiling conditions of both water and refrigerant (NOVEC-7000) flowing upward in vertical channels.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2019

Assessment on DNB performance for high pressure subcooled boiling with modified CASL boiling model

Understanding and predicting the Departure from Nucleation Boiling phenomena in the nuclear power industry has been a grand challenge due to its large impact on the reactor safety and operational economics. Over the past several years, CASL THM team has extensively investigated and demonstrated a state of art Multiphase CFD framework to provide a high-fidelity DNB simulation capability based on the two-fluid Eulerian model in conjunction with a wall heat flux partitioning approach. The overarching goal of this task is to develop and demonstrate a robust CFD-based DNB model which is capable of predicting an entire boiling history up to the DNB condition in which the wall boiling mode is transitioning from the nucleate boiling mode to the film boiling mode. Three different flow channel configurations (pipe flow, 5x5 fuel bundle with and without mixing vane tests) were studied and thorough closure sensitivity studies were performed to evaluate the wall boiling and momentum closure effect on the DNB performance. Predicated DNB values were compared to the measurements from high pressure subcooled flow boiling tests resulting in a consistent under-predictive trend for all three validation campaigns in FY16-18. With extensive closure sensitivity study, a modified CASL boiling closure model (i.e. Gen1-FY19) is introduced with the following key changes: 1) improved wall boiling closure for high pressure application, 2) newly calibrated lift and wall lubrication force models with modified interfacial length scale model, and 3) updated liquid phase condensation model calibrated for the subcooled boiling application.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2016

Improved Understanding toward Sliding Bubble Effect on Wall Heat Transfer in Subcooled Boiling Flow

The significance of bubble sliding on wall heat transfer under boiling and condensing conditions has consistently been reported by previous researchers, causing increasing attention within boiling heat transfer community. The authors' recent work for subcooled flow boiling experiment, also clearly shows the strong influence of sliding bubbles in wall heat transfer. However, despite the importance as well as the interest for the sliding bubbles both in experiment and CFD modelling, the mechanism of wall heat transfer induced by sliding bubbles has still barely been known. This mainly results from the fact that proper experimental data and physical insight based on them are largely lacking. This paper presents the authors' work to improve the insight toward the wall heat transfer mechanism associated with sliding bubbles. In particular, effort has been dedicated to identifying the knowledge gap between the actual physics and the CFD boiling models. Specifically, of major interest is the 'wall area of bubble influence' which is one of the important factors considered in most wall heat flux partitioning models to evaluate the heat transfer process like quenching. The experimental observation presented in this paper reveals the limitation of existing models and suggests the new physical insight needed to improve them. (authors)

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2018

Area of bubble influence due to sliding bubbles in subcooled boiling flow

The heat transfer effect of sliding bubbles is discussed in two aspects based on new experimental findings: (i) the area of bubble influence and (ii) bubble sliding characteristics influencing the degree of wall heat transfer. One notable finding was that the bubble sliding behavior, characterized by sliding trajectories, played a crucial role in determining the area of bubble influence and the bubble influence factor (K). Also, such sliding characteristic was observed strongly dependent on the sliding bubble size. An empirical model has been proposed based on this finding to evaluate the bubble influence factor (K) for the sliding bubbles. Another experimental finding was that the bubble sliding characteristic, represented by bubble spreading factor (S), had a significant impact on the degree of wall heat transfer induced by sliding bubbles. This is due to the fact that the bubbles spread across the heater width as they slid downstream, which reduced the effective frequency of sliding bubbles per unit wall area. These new findings not only improve our understanding of the heat transfer associated with sliding bubbles, but also are expected to contribute to improving the insight in the existing CFD boiling heat transfer models.

42 ENGINEERING↗
DOE OSTI2016

Effect of Local Void Fractions on Subcooled Boiling Oscillations in Natural Circulation Boiling Loop

Passive heat removal using natural circulation boiling loops (NCBLs) is well established from the view point of the safety of boiling water reactors (BWRs). Nevertheless, NCBLs suffer from a large number of nonlinear flow oscillations due to low driving force for circulations. The local measurements of void fraction a are essential to clearly understand the mechanism of flow oscillations. In this work, we have measured the impedance from the needle probe using LCR meter and quantified time-averaged local a from the measured impedance under oscillating flow conditions in the NCBL. Impedance probes have been successfully used in estimating void fractions and bubble frequencies. (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗