3. HEM_web

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

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Unformatted text preview: ENU 4134 – Homogeneous Equilibrium Model D. Schubring July 26, 2010 Learning Objectives I 1-c-i Identify the assumptions used in the homogeneous equilibrium model (HEM) and analyze the appropriateness of these assumptions for a two-phase flow of interest (feeds into 5-d ) I 1-c-ii Use the HEM for an estimate of pressure drop (feeds into 5-b ) 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 a good approximation in certain systems: I Air-particulate transport (dilute pollutants) I Certain types of sprays It is an assumption that can lead to reasonable global results (such as pressure drop) for high pressure, high mass flux flow. For example, at BWR/PWR conditions and a mass flux of 2000 kg m- 2 s- 1 , the HEM can provide as good an estimate of pressure drop as many more complex models. Dynamic Density Under the restrictions of the HEM and constant fluid properties, the brackets aren’t necessary in the definition of...
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## This note was uploaded on 02/07/2011 for the course ENU 4134 taught by Professor Schubring during the Spring '11 term at University of Florida.

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

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