Single-Phase Pressure Drop Analysis

The requirement for both dynamic and static instabilities is that the increase in the two-phase pressure drop should be either equal to or greater than the decrease in the single-phase pressure drop as the inlet flow decreases. The relevant limit is actually the static (non-linear) instability boundary, which may lead to CHF, has been called the "zeroth mode" of dynamic instability. Thus, in dynamic dispersion-type analysis, it corresponds to the time-independent, zero-frequency (or infinite wave number), real wave number case which, corresponds precisely to the homogeneous equilibrium limit for the flow. In non-linear (called excursive instability ), the channels could switch from one flow rate to another while maintaining the same total pressure drop. When non-linearly unstable, the channel flow fluctuates, or reverses, and dryout can ensue. ... 

EXCHANGER SINGLE-PHASE HEAT TRANSFER AND PRESSURE DROP ANALYSIS... 

Pressure-Driven Single-Phase Liquid Flows, Fig. 9 Model for pressure drop analysis of multiple-microchannel systems... 

Methods for determining the drop in pressure start with a physical model of the two-phase system, and the analysis is developed as an extension of that used for single-phase flow. In the separated flow model the phases are first considered to flow separately and their combined effect is then examined. 

For gas-liquid flows in Regime I, the Lockhart and Martinelli analysis described in Section I,B can be used to calculate the pressure drop, phase holdups, hydraulic diameters, and phase Reynolds numbers. Once these quantities are known, the liquid phase may be treated as a single-phase fluid flowing in an open channel, and the liquid-phase wall heat-transfer coefficient and Peclet number may be calculated in the same manner as in Section lI,B,l,a. The gas-phase Reynolds number is always larger than the liquid-phase Reynolds number, and it is probable that the gas phase is well mixed at any axial position therefore, Pei is assumed to be infinite. The dimensionless group M is easily evaluated from the operating conditions and physical properties. 

Counter-current gas/vapor-liquid film flows in SP above the load conditions are extremely complicated. For this reason, it appears improbable that the CFD-based virtual experiments replace real experiments entirely in the near future. However, even single-phase CFD simulations can improve predictivity of pressure drop models, since all correlations pressure drop - gas load used in practice contain some dry pressure drop correlation as a basic element. Replacing this correlation by the rigorous CFD analysis helps to avoid heuristic assumptions on possible correlation structure, which are inevitable both in conventional mechanistic models (Rocha et ah, 1993) and in more sophisticated considerations (Olujic, 1997). 

Estimate thermofluid characteristics of liquid-vapor phase change and related heat transfer processes such as circulation rate in natural or forced internal or external fluid circulation, pressure drops, and single- and two-phase vapor-liquid flow conditions. The initial analysis should be based on a rough estimation of the surface area from the energy balance... 

One of the objectives of this program was to prove or modify the basic equation of single-phase flow. A brief analysis of the theoretical friction factors, Reynolds number, pressure drop, and insulation may help in understanding the test data. Formulas and their derivation will not be included in this report but will be found in the final program report. 

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