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HEM_web - ENU 4134 – Homogeneous Equilibrium Model D...

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Unformatted text preview: ENU 4134 – Homogeneous Equilibrium Model D. Schubring September 18, 2009 Modeling of Two-Phase Flow I Averaging, averaged parameters (2) I Transport equations (2) I Homogeneous equilibrium model (1+) I Separated flow model(s) (1+) I Choked (critical) flow (1+) Homogeneous Equilibrium Model The HEM consists of making the following modeling assumptions: I The average phase velocities, { v k } k and { v k } k are the same (call these v k and v k ). Further, the velocities are assumed uniform within the channel. I The two phases are in thermodynamic equilibrium – temperature and pressure. The accuracy of a model is often linked to the accuracy of its assumptions. The second assumption is reasonable for many situations, particularly those with no or fairly low heat transfer. HEM Velocity Assumption The assumption in the HEM regarding velocities is much weaker. The second component – uniform velocities in the channel – is highly inaccurate. In the development of the HEM pressure drop model, however, single-phase turbulent friction factors are used that limit this effect. The first component of the assumption – equal average velocities – is only a good approximation in certain systems: I Air-particulate transport (dilute pollutants) I Certain types of sprays I Systems where ρ f and ρ g are not “too” different (near the critical point) HEM for Gas-Liquid Flows With respect to the four flow regimes of vertical flow, the HEM will produce poor results for bubbly flow in most circumstances. In slug, churn, and annular flow, the results are worse (unacceptably bad)....
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HEM_web - ENU 4134 – Homogeneous Equilibrium Model D...

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