Mechanical Non-Equilibrium Effect on Choking Flow at Low Pressure in Air-Water Experiment

2002 ◽  
Author(s):  
H. J. Yoon ◽  
M. Ishii ◽  
S. T. Revankar ◽  
W. Wang
Keyword(s):  
Mass Flux ◽  
Critical Mass ◽  
Low Pressure ◽  
Low Flow ◽  
Critical Flow ◽  
Two Phase ◽  
Slip Ratio ◽  
Flow Quality ◽  
Equilibrium Effect ◽  
Non Equilibrium

At low pressure and low flow conditions, the prediction of two-phase flow transients is much more difficult than at relatively higher pressure or at high flow due to the large density ratio and thermal and mechanical non-equilibrium between the phases. A mechanical non-equilibrium effect was studied with air-water two-phase critical flow in pipes at low pressure (< 1 MPa). Critical flow test were conducted in a well-scaled test facility with several on-line instruments. The slip ratio, which is the key factor in the mechanical non-equilibrium, is directly measured at the upstream of the critical section. The break geometry effect was investigated using the nozzle and orifice as critical flow sections. The experimental results showed that the slip ratio increased as the quality increased. The slip ratio value at low pressure was relatively higher than the slip ratio at the high pressure for the same flow quality. The measured critical mass flux for the nozzle was higher than the orifice at the low flow quality. Thus there is a geometry effect on critical mass flux at the low quality region, even though there is no difference in the slip ratio at the upstream of choking plane. Thus, it is concluded that there is a strong mechanical non-equilibrium at the choking plane.

Mach Number Scaling of Single-Component, Two-Phase Flow

1993 ◽  
Vol 115 (4) ◽  
pp. 772-777
Author(s):  
D. E. Nikitopoulos
Keyword(s):  
Experimental Data ◽  
Mach Number ◽  
Mass Flux ◽  
Two Phase Flow ◽  
Critical Mass ◽  
Critical Flow ◽  
Phase Flow ◽  
Homogeneous State ◽  
Two Phase ◽  
Good Agreement

A simple two-fluid formulation is used to investigate compressibility effects and Mach number scaling for equilibrium, evaporating two-phase flow. The definition of the local two-phase Mach number emerges from a critical flow analysis. Comparisons of the theoretical critical mass flux with existing experimental data obtained in steam-water flows show very good agreement for moderate and high qualities over a wide critical pressure range. Within this quality range the predicted critical mass flux is quite insensitive to the velocity ratio. The analysis confirms previous observations, based on homogeneous flow models, indicating that in variable area ducts the critical state does not occur at a geometrical throat. Results of existing critical flow experiments in slowly diverging ducts are discussed in the light of this conclusion. A way from the neighborhood of the flash horizon, pressure-drop and kinetic energy changes are shown to scale with similar local Mach functions as those of single-phase compressible flow. Existing experimental data from vertical-upwards and horizontal two-phase flows in pipes indicate that the Mach number calculated on the basis of the local homogeneous state provides the optimum scaling performance. Scaling of the same experimental data using a Mach number based on the local nonhomogeneous state provides results that are in reasonably good agreement with the theoretical scaling guidelines and predictions, but is handicapped by considerable scatter in the scaled experimental variables.

Analysis of two phase critical flow with a non-equilibrium model

2021 ◽  
Vol 372 ◽  
pp. 110998
Author(s):  
Hong Xu ◽  
Aurelian Florin Badea ◽  
Xu Cheng
Keyword(s):  
Equilibrium Model ◽  
Critical Flow ◽  
Two Phase ◽  
Non Equilibrium

A Choking Model With Thermal Non-Equilibrium for Initially Subcooled Water

2018 ◽  
Author(s):  
Lv Yufeng ◽  
Zhao Minfu ◽  
Li Weiqing
Keyword(s):  
Bubbly Flow ◽  
Flow Regimes ◽  
Critical Flow ◽  
Vapor Bubbles ◽  
Two Phase ◽  
Subcooled Water ◽  
Wide Range ◽  
Frictional Forces ◽  
Flow Closure ◽  
Non Equilibrium

Mechanical non-homogeneous and thermal non-equilibrium phenomenon exists in two-phase critical flow compared with single phase flow. A one-dimensional two-fluid critical flow model is developed for initially subcooled water flowing in pipe or orifices. The model accounts for thermal nonequilibrium between the liquid and vapor bubbles and for interphase relative motion. In this model, an improved correlation to calculate flashing inception location and surperheat is proposed. The model consists of six conservation equations as well as a seventh equation representing bubble growth in bubbly flow. Closure of the set of governing equations is performed with constitutive relationships which determine the interfacial momentum terms due to mass exchange, wall to liquid and wall to vapour frictional forces, liquid to gas interfacial force and interfacial heat transfer rate. The model considers the development of three flow regimes, namely, bubbly, churn and annular flow regimes. Model predictions compare favorably with experimental data over a wide range of pressures and pipe diameters and lengths.

Critical Heat Flux in Microchannels With an Adjustable Inlet Orifice

2012 ◽  
Author(s):  
Brent A. Odom ◽  
Carlos A. Ortiz ◽  
Patrick E. Phelan
Keyword(s):  
Heat Flux ◽  
Pressure Drop ◽  
Critical Heat Flux ◽  
Mass Flux ◽  
Cooling Capacity ◽  
Hydraulic Diameter ◽  
Low Flow ◽  
Two Phase ◽  
Mass Fluxes ◽  
Inlet Conditions

The benefits of eliminating instabilities in two-phase microchannel flow with inlet orifices come with costs. This study describes the tradeoffs between microchannels with and without inlet orifices, focusing on results from critical heat flux data obtained for various orifice sizes and mass fluxes. An adjustable inlet orifice controlled with a micrometer was placed in front of an array of 31 parallel microchannels each with a hydraulic diameter of 0.235 mm and a length of 1.33 cm. For mass fluxes ranging from 186 kg m−2 s−1 to 847 kg m−2 s−1, critical heat flux (CHF) data were obtained for 7 different orifice sizes. For low flow rates that provided a low quality saturated inlet condition, the difference in CHF values was found to be minimal between open and almost closed orifice conditions. The smallest orifice achieved a CHF value of 5 W cm−2 less than the largest orifice size for a mass flux of 186 kg m−2 s−1, and 7 W cm−2 less for a mass flux of 433 kg m−2 s−1. For mass fluxes higher than 433 kg m−2 s−1, subcooled conditions were present at the orifice inlet, and the highest CHF values occurred with an orifice hydraulic diameter of 35 percent of fully open. For the higher mass flux cases, orifice sizes in the range of 1.8 percent to 28 percent of fully open caused CHF to occur at lower values than less restrictive orifice sizes. This was due to loss of cooling capacity from rapid pressure drop through the orifice. Slightly higher average channel pressures also decrease the refrigerant’s latent heat of vaporization. For the orifice sizes from 35 to 70 percent of unrestricted flow, a very minimal increase in pressure drop over fully open inlet conditions occurred and the general trend was higher CHF values. Very small inlet orifices are beneficial for steady state conditions that do not approach CHF; however, overly restricting the flow at the inlet to microchannels reduces cooling capacity significantly and will cause early onset of CHF. A slightly restrictive inlet orifice will increase CHF.

Homogeneous non-equilibrium two-phase critical flow model

1987 ◽  
Vol 10 (1) ◽  
pp. 420-426 ◽  
Author(s):  
Jens Jürgen Schröder ◽  
Nha Vuxuan
Keyword(s):  
Flow Model ◽  
Critical Flow ◽  
Two Phase ◽  
Non Equilibrium

Two-Phase Critical Flow under Diabatic Conditions

1969 ◽  
Vol 184 (3) ◽  
pp. 127-133
Author(s):  
A. E. Bergles ◽  
J. T. Kelly
Keyword(s):  
Experimental Investigation ◽  
Equilibrium Model ◽  
Small Diameter ◽  
Critical Flow ◽  
Subcooled Boiling ◽  
Flow Conditions ◽  
Two Phase ◽  
Wide Range ◽  
Good Agreement ◽  
Non Equilibrium

This paper summarizes an experimental investigation of steam-water critical flow in heated tubes. A wide range of data was taken for water at pressures below 100 lbf/in2 (abs.) in tubes of small diameter. It is demonstrated that critical flow conditions can occur in subcooled boiling at low exit subcoolings. At equilibrium qualities below about 0·04, the data differ significantly from adiabatic data for a similar exit geometry. The deviations can be explained in terms of the additional non-equilibrium effects present in heated flows. For higher qualities, the diabatic data are in good agreement with adiabatic data, and can be approximately predicted by a slip equilibrium model.

Thermodynamics of Void Fraction in Saturated Flow Boiling

2005 ◽  
Vol 128 (6) ◽  
pp. 611-615 ◽  
Author(s):  
Francisco J. Collado ◽  
Carlos Monné ◽  
Antonio Pascau ◽  
Daniel Fuster ◽  
Andrés Medrano
Keyword(s):  
Void Fraction ◽  
Heat Balance ◽  
Flow Boiling ◽  
Essential Element ◽  
Saturated Flow ◽  
Two Phase ◽  
Slip Ratio ◽  
Flow Quality ◽  
The 1960S ◽  
Thermodynamic Quality

Recently, Collado (Proc, IMECE 2001, Symposium on Fluid Physics and Heat Transfer for Macro- and Micro-Scale Gas-Liquid and Phase Change Flows) suggested calculating void fraction, an essential element in thermal-hydraulics, working with the “thermodynamic” quality instead of the usual “flow” quality. The “thermodynamic” quality is a state variable, which has a direct relation with the actual vapor volumetric fraction, or void fraction, through phase densities. This approach provides a procedure for predicting void fraction, if values of “thermodynamic” quality are available. However, the standard heat balance is usually stated as a function of the “flow” quality. Therefore, we should search for a new heat balance between the mixture enthalpy, based on “thermodynamic” quality, and the absorbed heat. This paper presents the results of such analysis based on the accurate measurements of the outlet void fraction measured during the Cambridge project by Knights (1960, “A Study of Two-Phase Pressure Drop and Density Determination in a High-Pressure Steam-Water Circuit,” Ph.D. thesis, Cambridge University Engineering Lab, Cambridge, UK) in the 1960s for saturated flow boiling. In the 286 tests analyzed, the pressure and mass fluxes range from 1.72 MPa to 14.48 MPa and from 561.4 to 1833.33 kgm−2s−1, respectively. As the main result, we find that the slip ratio would close this new thermodynamic heat balance. This has allowed the accurate calculation of void fraction from this balance, provided we can predict the slip ratio. Finally, the strong connection of this new thermodynamic heat balance with the standard one through the slip ratio is highlighted.

Two-Phase Critical Flow Simulations by Using a New Non-Equilibrium Model and a Modified Equilibrium Model

2010 ◽  
Author(s):  
Moon-Sun Chung ◽  
Sung-Jae Yi ◽  
Keun-Shik Chang
Keyword(s):  
Phase Change ◽  
Equilibrium Model ◽  
Phase Changes ◽  
Critical Flow ◽  
Vapor Generation ◽  
Two Phase ◽  
Subcooled Liquid ◽  
Equilibrium Parameter ◽  
Phase Mixture ◽  
Non Equilibrium

An accurate prediction of a critical flow discharged from a pressurized pipe system is of most importance in such a safety analysis of nuclear power plants, since it provides the transient boundary conditions during the depressurization transients initiated by a pipe break in primary or secondary systems and during the over-pressurization transients resulting in a relief of coolant through valves. Mass and energy discharge through the opening of pressure boundary affects the system thermal hydraulic responses, that is, phase changes and flow distribution in the system, and the mass inventory remaining in the system necessary to remove core decay heat of a nuclear reactor. Therefore, the safety significance relating to the critical flow led to a development of various empirical and mechanistic critical flow models. However, the accuracies of these models are still in question especially during two-phase critical flow condition. A good example of that is a homogeneous equilibrium model (HEM). The HEM is the basis of several system codes, such as early versions of RELAP, for nuclear loss-of-coolant accident (LOCA). The major non-equilibrium phenomena that are ignored in the HEM are vapor bubble nucleation and interface heat, mass, and momentum transfer. Henry-Fauske empirically handled non-equilibrium vapor generation by introducing a non-equilibrium parameter that allows only a fraction of the equilibrium vapor generation to occur. This approach boils down in essence to a correlation of the deviation between the measured flow rate and the prediction from the HEM: The details of the flow path do not have to be worked out and only needs to know the upstream conditions. However, if we treat non-equilibrium phenomena with this model, it requires an empirical database of the non-equilibrium parameters or their correlations that are so far unknown. Further, because the coefficients are not applied separately to the subcooled liquid and two-phase mixture, we have not been able to treat the non-equilibrium phenomena with the phase change properly. For this reason, we propose the non-equilibrium parameters for subcooled liquid and two-phase mixture, respectively, and then we adopt their combinations according to the flow conditions through the phase change process using the RELAP5/MOD3 code. In addition, we discuss the assessment results of Marviken LBLOCA tests using these non-equilibrium parameter sets with those from the non-equilibrium model by Trapp-Ransom and Chung et al.

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