A Pressure Pulse Model for Two-Phase Critical Flow and Sonic Velocity

1969 ◽  
Vol 91 (3) ◽  
pp. 371-381 ◽  
Author(s):  
F. J. Moody
Keyword(s):  
Heat Transfer ◽  
Phase Change ◽  
Present Model ◽  
Pressure Pulse ◽  
Critical Flow ◽  
Sonic Velocity ◽  
Phase Pattern ◽  
Two Phase ◽  
Theoretical Formulation ◽  
Pulse Transmission

Pressure pulse transmission in a flowing mixture provides the basis for a theoretical formulation of two-phase critical flow and sonic velocity. Homogeneous and separated phases are considered, showing that phase pattern plays an important role in pulse transmission. Graphs are presented for critical flow and sonic velocity of steam-water mixtures, based on no phase change or heat transfer across the pulse. The present model predicts most available data with homogeneous and separated phase patterns and suggests quality ranges for which each pattern applies.

Phase Change Process Inside a Small-radii Cylinder Subjected to Cyclic Convective Boundary Conditions: a Numerical Study

2021 ◽  
Author(s):  
Yousef Kanani ◽  
Avijit Karmakar ◽  
Sumanta Acharya
Keyword(s):  
Heat Transfer ◽  
Surface Temperature ◽  
Phase Change ◽  
Phase Change Materials ◽  
Numerical Study ◽  
Convective Boundary Condition ◽  
Freezing Process ◽  
Fourier Number ◽  
Two Phase ◽  
Convective Boundary

Abstract We numerically investigate the melting and solidi?cation behavior of phase change materials encapsulated in a small-radii cylinder subjected to a cyclic convective boundary condition (square wave). Initially, we explore the effect of the Stefan and Biot numbers on the non-dimensionalized time required (i.e. reference Fourier number Tref ) for a PCM initially held at Tcold to melt and reach the cross?ow temperature Thot. The increase in either Stefan or Biot number decreases Tref and can be predicted accurately using a correlation developed in this work. The variations of the PCM melt fraction, surface temperature, and heat transfer rate as a function of Fourier number are reported and analyzed for the above process. We further study the effect of the cyclic Fourier number on the periodic melting and freezing process. The melting or freezing front initiates at the outer periphery of the PCM and propagates towards the center. At higher frequencies, multiple two-phase interfaces are generated (propagating inward), and higher overall heat transfer is achieved as the surface temperature oscillates in the vicinity of the melting temperature, which increases the effective temperature difference driving the convective heat transfer.

scholarly journals Prediction of Two-Phase Critical Flow Rate

1965 ◽  
Vol 87 (1) ◽  
pp. 53-57 ◽  
Author(s):  
S. Levy
Keyword(s):  
Flow Rate ◽  
Thermal Equilibrium ◽  
Present Model ◽  
Critical Flow ◽  
Test Results ◽  
Two Phase ◽  
Mean Velocity ◽  
Critical Flow Rate ◽  
Pressure Losses ◽  
Gas Void Fraction

An analytical model to predict two-phase critical flow rate is proposed. The model is based upon thermal equilibrium, a “lumped” treatment of the two-phase velocity (each phase is represented by a single mean velocity), and upon the neglect of frictional and hydrostatic pressure losses. A comparison of the proposed predictions with available test results and previous analyses shows that: (a) The present model agrees very well with the published test data; (b) In contrast to all other analyses, the model requires no assumption about the gas void fraction.

Numerical Modeling and Validation of Supersonic Two-Phase Flow of CO2 in Converging-Diverging Nozzles

2013 ◽  
Vol 136 (1) ◽  
Author(s):  
Miad Yazdani ◽  
Abbas A. Alahyari ◽  
Thomas D. Radcliff
Keyword(s):  
Phase Change ◽  
Velocity Model ◽  
Attractive Alternative ◽  
Sonic Velocity ◽  
Shock Train ◽  
Alternative Refrigerants ◽  
Two Phase ◽  
Expansion Waves ◽  
Fluid Properties ◽  
Layer Flow

Carbon dioxide is an attractive alternative to conventional refrigerants due to its low direct global warming effects. Unfortunately, CO2 and many alternative refrigerants have lower thermodynamic performance resulting in larger indirect emissions. The effective use of ejectors to recover part of the lost expansion work, which occurs in throttling devices, can close this performance gap and enable the use of CO2. In an ejector, the pressure of the motive fluid is converted into momentum through a choked converging-diverging nozzle, which then entrains and raises the energy of a lower-momentum suction flow. In a two-phase ejector, the motive nozzle flow is complicated by the nonequilibrium phase change affecting local sonic velocity and leading to various types of shockwaves, pseudo shocks, and expansion waves inside or outside the exit of the nozzle. Since the characteristics of the jet leaving the motive nozzle greatly affect the performance of the ejector, this paper focuses on the details of flow development and shockwave interaction within and just outside the nozzle. The analysis is based on a high-fidelity model that incorporates real-fluid properties of CO2, local mass and energy transfer between phases, and a two-phase sonic velocity model in the presence of finite-rate phase change. The model has been validated against the literature data for two-phase supersonic nozzles and overall ejector performance data. The results show that due to nonequilibrium effects and delayed phase change, the flow can choke well downstream of the minimum-area throat. In addition, Mach number profiles show that, although phase change is at a maximum near the boundaries, the flow first becomes supersonic in the interior of the flow where sound speed is lowest. Shock waves occurring within the nozzle can interact with the boundary layer flow and result in a ‘shock train’ and a sequence of subsonic and supersonic flow previously observed in single-phase nozzles. In cases with lower nozzle back pressure, the flow continues to accelerate through the nozzle and the exit pressure adjusts in a series of supersonic expansion waves.

A Computational Approach to Study Heat Transfer Enhancement in Film Boiling due to the Addition of Surfactants

2016 ◽  
Author(s):  
Kannan N. Premnath ◽  
Farzaneh Hajabdollahi ◽  
Samuel W. J. Welch
Keyword(s):  
Heat Transfer ◽  
Phase Change ◽  
Surfactant Concentration ◽  
Computational Approach ◽  
Film Boiling ◽  
Two Phase ◽  
Diverse Range ◽  
Study Heat Transfer ◽  
Interfacial Surfactant ◽  
Adsorption Desorption

Two-phase flows involving phase change are ubiquitous in a diverse range of scientific and technological applications. There has been great recent interest in the enhancement of boiling heat transfer processes by means of additives such as surfactants. Surfactants can influence boiling through convection currents in the bulk fluids as a result of changes in the surface tension caused by local surfactant concentration due their adsorption/desorption from the bulk regions. This can result in changes in bubble release patterns and higher heat transfer rates if such changes lead to higher rate of vapor formation. We intend to study this effect in the context of film boiling. Our computational approach augments the CLSVOF method with bulk energy and diffusion equations along with a phase change model and an interface surfactant model. The challenge here is to accurately calculate the tangential gradients of the interfacial surfactant concentration in the presence of discontinuous bulk concentration gradients near the interface. We discuss a simplified model in which the interfacial surfactant concentration is always in equilibrium with the changing bulk concentrations and then present validation results to assess the accuracy of this approach. Finally, initial studies of surfactant enhanced film boiling will be presented and interpreted.

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