Barclay G. Jones

Studies of the Behavior of heavy Particles in a Turbulent Fluid Flow

Abstract A statistical analysis has been made of individual particle transport in a homogeneous, turbulent fluid flow. Expressions for dispersion, correlation coefficients, and turbulent energy content have been obtained. In the course of the development two parameters were found to characterize particulate transport, one of which relates to inertial effects acting on the particle, while the other describes the effects of crossing trajectories. As in previous studies by others, crossing-trajectories effects are found to he of particular importance; inertial effects, however, even for heavy particles, are not insignificant. Comparison of theoretical predictions with experimental data shows good agreement.

Aerodynamic disturbances of hot-wire probes and directional sensitivity

Directional responses of hot-wire probes are studied experimentally, and a theory is developed which attributes pitch response to aerodynamic disturbances caused by the probe and yaw response to a combination of aerodynamic disturbances and heat transfer effects. The predicted cooling velocity has the same form as Jorgensens equation, and the pitch coefficient, predicted in terms of the probe geometry, compares favorably with experiments using ideal probes. Comparisons with real probes indicate the limits of validity imposed by hot-wire probe tolerances. The accuracy of Jorgensens equation for combined yaw and pitch has been measured experimentally.

Droplet contact angle behavior on a hybrid surface with hydrophobic and hydrophilic properties

A hybrid surface consisting of an array of hydrophobic and hydrophilic sites was designed and fabricated in an effort to better understand the effects of microscale surface features and chemistry on wettability. A model based on energy minimization was developed to design and predict the wettability of hybrid surfaces. Measured advancing, receding, and equilibrium contact angles fit the proposed model well. Experiments show that a higher degree of hydrophobicity results in higher contact angles and that contact angle hysteresis increases with decreasing micropillar spacing (b/a). Moreover, measured roll-off angle as an indicator of droplet shedding, decreases with b/a.

A Statistical Model of Bubble Coalescence and Its Application to Boiling Heat Flux Prediction—Part I: Model Development

In this work a statistical model is developed by deriving the probability density function (pdf) of bubble coalescence on boiling surface to describe the distribution of vapor bubble radius. Combining this bubble coalescence model with other existing models in the literature that describe the dynamics of bubble motion and the mechanisms of heat transfer, the surface heat flux in subcooled nucleate boiling can be calculated. By decomposing the surface heat flux into various components due to different heat transfer mechanisms, including forced convection, transient conduction, and evaporation, the effect of the bubble motion is identified and quantified. Predictions of the surface heat flux are validated with R134a data measured in boiling experiments and water data available in the literature, with an overall good agreement observed. Results indicate that there exists a limit of surface heat flux due to the increased bubble coalescence and the reduced vapor bubble lift-off radius as the wall temperature increased. Further investigation confirms the consistency between this limit value and the experimentally measured critical heat flux (CHF), suggesting that a unified mechanistic modeling to predict both the surface heat flux and CHF is possible. In view of the success of this statistical modeling, the authors tend to propose the utilization of probabilistic formulation and stochastic analysis in future modeling attempts on subcooled nucleate boiling.

Heat Transfer For Subcooled Flow Boiling In Hypervapotron Configuration

Abstract This work reports on experimental studies that examine subcooled boiling on the enhanced heat transfer surface of hypervapotron structures. The use of simulant fluid (refrigerant R134a) instead of prototypic water allows examination of a full range of subcooled boiling, including up to critical heat flux (CHF). The experimental results are compared to Bjorge’s model and Kandlikar’s heat transfer correlation in the subcooled boiling region. It is found that the fully developed boiling curve has a slope relation of ˜2.96(q′ [similar] Δ, which shows good agreement with Bjorge’s correlation for flat surface channels. In addition, Kandlikar’s correlation is also able to predict the heat transfer coefficient for the range from net vapor generation to the fully developed boiling region with acceptable accuracy. However, the heat transfer curve shows a significant deviation when subcooled boiling approaches CHF.

Boron Concentration Model and Effects of Boron Holdup on Axial Offset Anomaly (AOA) in PWRs

A significant number of current PWRs are experiencing axial offset anomaly (AOA), a condition that continues to elude prediction and thus creates an operational difficulty of being unable to accurately estimate safety margin. The cause of AOA is complex, involving both thermal-hydraulics and chemistry of core operation and design. This paper considers the structure of crud layers, which lead to AOA, and presents a thermal performance and neutron absorber concentration model for the crud layer that satisfactorily predicts the amount of boron accumulated to match that required to cause the observed power shifts.Copyright

Measurement and Analysis of Bubble Behavior in Subcooled Nucleate Boiling Flow Field With High Fidelity Imaging System

To date, more than twenty PWRs have been affected by axial offset anomaly (AOA) or crud-induced power shift (CIPS), an unexpected deviation in the core axial power distribution from the predicted curve during operation. AOA is a current major consideration for reactors operating at increased power levels and is becoming immediate threat to nuclear power’s competitiveness in the market [1]. Despite much effort focusing on this topic, a comprehensive understanding is far from being developed. However, previous research indicates that a close connection exists between subcooled nucleate boiling occurring in core region and the formation of crud, which directly results in AOA phenomena. It is well established that deposition is greater, and sometimes much greater, on heated than on unheated surfaces. [2] A number of researchers have suggested that boiling promotes deposition, and several observed increased deposition in the subcooled boiling region [2]. Limited detailed information is available on the interaction between heat and mass transfer in subcooled nucleate boiling (SNB) flow. It is speculated that direct prediction of the AOA from SNB is difficult. Moreover, bubbles formed in SNB region play an important role in helping the formation of crud. Therefore, we are encouraged to get a better understanding of SNB phenomena and the behavior of the bubbles in SNB. This research examines bubble behavior under SNB condition from the dynamic point of view, using a high fidelity digital imaging apparatus. Freon R-134a is chosen as a simulant fluid due to its merit of having smaller surface tension and lower boiling temperature. The apparatus is operated at “reduced” pressure. Series of images at frame rates up to 4000 frames/s were obtained, showing different characteristics of bubble behavior with varying experimental parameters e.g. flow velocity, fluid subcooled level, etc. Analyses that combine the experimental results with analytical result on flow field in velocity boundary layer are considered. A tentative suggestion is that a rolling movement of a bubble accompanies its sliding along the heating surface in the flow channel. Numerical computations using FLUENT v5.5 are performed to support this conclusion.Copyright

Study of Mechanism of Initial Crud Formation on Fuel Cladding in Subcooled Boiling Region in PWR

A hemispherical vapor bubble model has been constructed, and thermocapillary flow driven by surface tension gradient, due to temperature difference, around bubble attached to a plane heated surface, and it has been examined using numerical methods. LiOH solute concentration distribution around the bubble has been obtained. Some parameters under PWR operating conditions have been discussed: bubble size, mass diffusion coefficient and vapor heat transfer coefficient. Effects of some important parameters, such as heat flux and bubble size, have been examined. The study shows that the thermocapillary flow around a static hemispherical vapor bubble can not generate sufficient solute concentrations to initialize crud formation under PWR operating condition.Copyright

On the Influence of Heating Surface Structure on Bubble Detachment in Sub-Cooled Nucleate Boiling Flows

This research examines the influence of heating surface structure on bubble detachment, which includes bubble departure and bubble lift-off, under sub-cooled nucleate boiling condition, in order to obtain better understanding to the bubble dynamics on horizontal flat heat exchangers. Refrigerant R-134a is chosen as a simulant fluid due to its merits of having smaller surface tension, reduced latent heat, and lower boiling temperature than water. Experiments were run with varying experimental parameters e.g. pressure, inlet sub-cooled level, and flow rate, etc. High speed digital images at frame rates up to 4000 frames/s were obtained, showing characteristics of bubble movement. Bubble radius and center coordinates were calculated via Canny’s algorithm for edge detection and Fitzgibbon’s algorithm for ellipse fitting. Results were compared against the model proposed by Klausner et al. for prediction of bubble detachment sizes. Good overall agreement was shown, with several minor modifications and suggestions made to the assumptions of the model.Copyright

Visualization of Sub-Cooled Flow Film Boiling in Horizontal Channel on Flat Heat Exchangers

The observation of sub-cooled flow film boiling was performed on a single-side-heated flat heat exchanger by using refrigerant R134a as the testing fluid. A stable vapor film was observed with the co-existence of sub-cooled bulk liquid. Vapor bubbles might release from the film, with the number and frequency changing under different conditions. The purpose of this paper is to describe these observations, and to provide a quantitative analysis of the phenomena. Results are compared with Berenson’s model on horizontal heating surface, with discussion and suggestion made.© 2006 ASME