Mechanistic model to predict frequency and amplitude of Density Wave Oscillations in vertical upflow boiling

• Published in: International journal of heat and mass transfer 2018-08, Vol.123, p.143-171
• Main Authors: O'Neill, Lucas E., Mudawar, Issam
• Format: Article
• Language: English
• Subjects: Amplitudes; Analytic model; Boundary layer; Conservation equations; Density; Density Wave Oscillations; Enthalpy; Flow boiling; Flow separation; Flow velocity; Flow visualization; Frequency and amplitude; Frequency distribution; Heat conductivity; Heat transfer; Mathematical models; Model accuracy; Modelling; Oscillations; Prediction; Predictions; Pressure drop; Pressure effects; Pressure oscillations; Stability; Two phase flow; Vapor phases; Vertical oscillations
• ISSN: 0017-9310; 1879-2189
• DOI: 10.1016/j.ijheatmasstransfer.2018.02.078

•This study presents a new mechanistic model for Density Wave Oscillations (DWOs) in vertical upflow boiling.•The model draws on flow visualization records and transient experimental data.•The model captures the cyclical propagation and growth of a high density liquid front along the flow channel.•The model is successful at predicting both frequency and amplitude of pressure oscillations. Modeling of two-phase flow transient behavior and instabilities has traditionally been one of the more challenging endeavors in heat transfer research due to the need to distinguish between a wide range of instability modes systems can manifest depending on differences in operating conditions, as well as the difficulty in experimentally determining key characteristics of these phenomena. This study presents a new mechanistic model for Density Wave Oscillations (DWOs) in vertical upflow boiling using conclusions drawn from analysis of flow visualization images and transient experimental results as a basis from which to begin modeling. Counter to many prior studies attributing DWOs to feedback effects between flow rate, pressure drop, and flow enthalpy causing oscillations in position of the bulk boiling boundary, the present instability mode stems primarily from body force acting on liquid and vapor phases in a separated flow regime leading to liquid accumulation in the near-inlet region of the test section, which eventually departs and moves along the channel, acting to re-wet liquid film along the channel walls and re-establish annular, co-current flow. This process was modeled by dividing the test section into three distinct control volumes and solving transient conservation equations for each, yielding predictions of frequencies at which this process occurs as well as amplitude of associated pressure oscillations. Values for these parameters were validated against an experimental database of 236 FC-72 points and show the model provides good predictive accuracy and capably captures the influence of parametric changes to operating conditions.