TED-AJ03-350 An Investigation into the Effect of Subcooled Liquid Inertia on Flowrate Induced Frequency-Response Characteristics of Horizontal Condensing Flow Systems :

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Abstract

The objective of this research is to investigate large-scale transient flow oscillations of the condensate leaving in-tube condensing flow systems, due to perturbations in the inlet vapor flowrate, and the influence of the subcooled liquid inertia of the condensate on these oscillations. In a tube-type condenser involving complete condensation, it has been seen that small changes in the inlet vapor flowrate momentarily cause large transient flow surges in the outlet liquid flowrate. Physically, as the vapor flowrate momentarily cause large transient flow surges in the outlet liquid flowrate. Physically, as the vapor flowrate is increased, lower density vapor tends to displace the higher density liquid in the two-phase region. This displacement, in turn, simultaneously causes a temporary excess of vapor in the two-phase region, and the higher density liquid to leave the condenser, resulting in a flow surge. Once this excess vapor is condensed, the system reaches a new steady-state condition. The liquid inertia will temporarily delay this type of flow surge, but, once it occurs, the inertia will then try to keep the flow surge from decaying back to the new steady-state position, potentially resulting in a surge of larger magnitude than would otherwise occur when inertia effects are small. On the contrary, when the inlet vapor flowrate is reduced, the opposite effect occurs. In this case, the decreased vapor flowrate into the condenser causes a momentary shortage of vapor in the two-phase region. The result is that liquid that would have otherwise left the two-phase region is drawn back to the two-phase region because of this shortage, and accompanying pressure decrease in the two-phase region, thereby momentarily decreasing the outlet liquid flowrate. In some cases, this decrease can be so large that the condensate flowrate actually becomes negative and momentarily heads backwards into the condenser. Once the shortage is made up, a new steady state is then reached. In the case of a sinusoidal inlet vapor flowrate variation, where the vapor flowrate increases, causing a transient flow surge, and then decreases, causing a flow reversal, it is shown that condensers amplify the perturbation. A System Mean Void Fraction (SMVF) Model, a one-dimensional, two-fluid, distributed parameter model of the time-dependent distribution of liquid and vapor within the two-fluid, distributed parameter model of the time-dependent distribution of liquid and vapor within the two-phase region, is developed for predicting these frequency-response characteristics. The salient feature of the SMVF Model is its simplicity. Such simplicity , with an experimentally verified predictive capability, enhances the models' utility not only as an analytical tool, but as an educational tool as well.

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