Abstract
A simple method of pressure drop calculation for two-phase flows of different fluids during convective boiling in channels is presented. It is based on experimental data of pressure drop multiplier R and void fraction φ obtained by Martinelli and Nelson for boiling of water in vertical tubes. The data cover the whole two-phase domain from ambient to critical pressure. Unfortunately, they have been presented in graphical forms. The first step in the procedure proposed in the paper was a transformation of the graphical data into analytical formulas which contain such dimensionless quantities as steam quality x , Martinelli parameter X , multiplier Φ l 2 and dimensionless coefficients D , m , E and k . In the second step, simple analytical formulas were determined to express the dimensionless coefficients as a function of physical property parameter K . In this way two simple analytical expressions for the multiplier R and void fraction φ were obtained. They are in analytical dimensionless form so they may be used directly for different fluids, not only for water. This is the main advantage of the proposed method.
Highlights
In a number of engineering systems it is important to be able to predict the pressure drop during boiling of twophase medium, such as water or other fluids
The first one is the well-known Lockhart– Martinelli (LM) correlation for calculation of both quantities, the second one is developed by the present authors
The second parameter is known as Martinelli one and is defined as dz)l dz )g what means that it is a ratio of pressure gradient due to friction of liquid phase flowing at mass rate equal to ṁ = ṁ l to the pressure gradient for a gas stream flowing alone in the same channel at the mass flow rate ṁ g = xṁ o
Summary
In a number of engineering systems it is important to be able to predict the pressure drop during boiling of twophase medium, such as water or other fluids. Total two-phase pressure drop for flows in vertical channels consists of three components – frictional loss ΔpTPf, momentum change Δpa and elevation pressure drop Δph arising from the effect of the gravitational force field. The current methods for modelling of twophase pressure drop fall into two categories: homogenous model and separated model. The latter is chosen in the paper for particular consideration. According to Collier and Thome [1], the total two-phase pressure drop may be written as. It order to apply the above equation it is necessary to develop two expressions for multiplier R and void fraction φ. The first one is the well-known Lockhart– Martinelli (LM) correlation for calculation of both quantities, the second one is developed by the present authors
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