Two-phase Flow Situations Research Articles

Numerical investigation of the effects of dry gas model and wet steam model in solar-driven refrigeration ejector system

Abstract Nowadays, the use of equipment with little pollution is essential due to the increase in the planet’s temperature. Ejectors are considered one of the equipment with no pollution, and their failure rate is low due to the lack of moving parts. Also, scholars have recently focused on improving the efficiency of industrial equipment. The use of accurate modeling is required to improve steam ejector performance. In a steam ejector, non-equilibrium condensation creates a two-phase flow situation. The wet steam model, used in this study, characterizes this two-phase flow. The study’s objective was to compare this wet steam model with the dry gas model. In the wet steam model, the liquid mass fraction is 0.25, and its calculated entrainment ratio is lower than the dry gas model, closely matching experimental observations. The dry gas model reaches a maximum Mach number of about 5, while the wet steam model approximates 4. A significant temperature difference exists between the two models, with the dry gas model indicating lower temperatures compared to the wet steam model. Diagonal shocks and expansion waves are evident in the mixing chamber, fixed cross-section, and diffuser. These phenomena occur with greater intensity and a slight delay in the wet steam model compared to the dry gas model.

A novel theoretical model of gas–solid two-phase flow mixed dielectric discharge

A theoretical physical model of gas–solid two-phase flow mixed dielectric discharge in a uniform field based on Townsend's discharge theory is presented. This model extends the classical Townsend's theory to be applicable to the quantitative analysis of dielectric discharge questions related to gas–solid two-phase flow environments, reveals the influence mechanism of flowing gases and solid-phase particles on discharge, and provides a theoretical basis for expanding the application of discharge plasma technology in various fields. In the model, based on the basic physical process of gas discharge and our previous studies, the effects of the attraction and obstructive factors of solid-phase particles on the number density of electrons or ions and the local space electric field in the inception and development of gas discharge were taken into account. On this basis, the analytical expression of the breakdown voltage in a gas–solid two-phase flow mixed dielectric is obtained, Paschen's law of gas breakdown is modified, and Townsend's breakdown criterion for gas–solid two-phase flow situation is proposed. It is shown that the breakdown voltage of the gas–solid two-phase flow mixed dielectric decreases with increasing gas flow velocity. The gas flow velocity is the main factor affecting the variation trend of the breakdown voltage. The concentration and size of solid-phase particles determine the values of breakdown voltage. The breakdown voltage of the smaller size and higher concentration of solid-phase particles is greater, which has a stronger suppression effect on the discharge.

DNS and Highly-Resolved LES of Heat and Mass Transfer in Two-Phase Counter-Current Condensing Flow

A comprehensive study of direct-contact condensation heat transfer for turbulent, counter-current, liquid/vapour flow in a nearly horizontal channel at high pressure (i.e. 5 MPa) has been carried out based on Direct Numerical Simulation (DNS) and highly-resolved Large Eddy Simulation (LES) approaches. To simulate the two-phase flow situation, driven in this case by a constant pressure gradient, a single set of Navier–Stokes equations, coupled with an enthalpy conservation equation, have been employed. The interfacial mass transfer, seen in this case to be dominated by condensation, has been calculated directly from the heat flux at the liquid/vapour interface. To investigate the effect of condensation on the turbulence phenomena, and vice versa, cases have been considered involving two friction Reynolds numbers: namely Re_{*}=u_{*}h/nu =178 and Re_{*}=u_{*}h/nu =590 (u_{*}=(hvarDelta P/rho )^{1/2}). At the lower Reynolds number, three levels of water subcooling—0 K, 10 K and 40 K—have been investigated. The use of water subcooling of 0 K has enabled the validation and verification procedures associated with the numerical approach to be compared against experimental and numerical data reported in the literature. The choice of the maximum degree of water subcooling is dictated by the need to justify the periodic boundary conditions applied in this numerical study. In the simulation for the higher Reynolds number, only the case of 10 K subcooling has been included, as a consequence of the very high computation effort involved. A detailed statistical analysis of the DNS and LES data obtained from the application of the well-known wall laws has also been assessed. In the vicinity of the liquid/vapour interface, the characteristics of the turbulent motions appear somewhat diverse, depending on whether the interface is basically flat or wavy in character. For a flat interface, some damping effect of the presence of the interface on the turbulence intensity has been observed, a feature which becomes enhanced as the level of liquid subcooling is increased. In the case of a wavy interface, the damping effect is predicted as considerably less pronounced.

A methodology to devise consistent probability density function models for particle dynamics in turbulent dispersed two-phase flows

The purpose of this article is to propose a generic methodology to build consistent Lagrangian models for polydisperse turbulent two-phase flows where the main issue is to devise a stochastic model for the velocity of the fluid seen by discrete particles. By consistent, it is meant that such models should meet the requirements set forth by Minier et al. (Phys. Fluids, 26, 113303, 2014) and, in the limit of vanishing particle inertia, retrieve the state-of-the-art stochastic models referred to as generalized Langevin models (GLMs) used for the simulation of turbulent single-phase flows. The methodology is generic in the sense that the resulting stochastic models for polydisperse two-phase flows are not limited to one particular fluid model but allows extending any GLM formulation to the two-phase flow situation. This is obtained by introducing a specific operator, which represents how statistical characteristics of fluid particles are transformed, or mapped, to the ones pertaining to the velocity of the fluid seen. In practice, this operator can be worked out separately from first principles or by resorting to some physical inputs. Once it is expressed, the present methodology shows how to extend a GLM for fluid particles to obtain a two-phase GLM formulation in a consistent manner. This is helpful to decouple physics-based developments used to obtain such an operator from the construction of practical stochastic models while ensuring that they remain consistent with fluid descriptions.

Orientation effect on heat transfer coefficient of a downward surface for flow boiling in a rectangular channel under low flow rate

Natural convection boiling in channel with Arc-Shaped will be encountered in the IVR-ERVC (In-Vessel Retention measure by External Reactor Vessel Cooling) system in nuclear power plant under severe accident. The flow and heat transfer characters in this situation is simulated by flow boiling of deionized water in an inclined rectangular channel under low flow rates. This paper aims to separate various parameters (such as orientation, mass flow rate and inlet quality, etc.) to investigate their individual effects on heat transfer coefficient (HTC) in a rectangular channel with cross section of 17 mm × 10 mm. By using a preheater at the inlet of the rectangular channel, the inlet quality could be controlled and the two-phase flow situation could be observed before the fluids entering into the main heater region on one side of the channel wall in downstream. Thus the characteristics of HTC on the main heater could be investigated at different flow patterns. The channel orientations vary from 15 to 90°, the mass flow rates vary from 110 to 288 kg/(m2s) and the qualities vary from 0.003 to 0.036, respectively. Experimental results show that the mass flow rate and quality effects on the HTC are very weak in this study. However, the orientation angle effect on HTC shows an transition region within 45°~60°, while it slowly changes when the orientation angle is smaller than 45° and bigger than 60°. Such tendency could be well formulated by the error function. Compared with different empirical formulas of saturated boiling HTC, it is found that the Liu & Winterton correlation can well predict the experimental HTC results in 90° orientation channel. Based on such correlation and coupled with the error function, a new model was developed by considering the orientation effect, which has an error of ±15% comparing with the experimental data.

Modelling compressible dense and dilute two-phase flows

Many two-phase flow situations, from engineering science to astrophysics, deal with transition from dense (high concentration of the condensed phase) to dilute concentration (low concentration of the same phase), covering the entire range of volume fractions. Some models are now well accepted at the two limits, but none are able to cover accurately the entire range, in particular regarding waves propagation. In the present work, an alternative to the Baer and Nunziato (BN) model [Baer, M. R. and Nunziato, J. W., “A two-phase mixture theory for the deflagration-to-detonation transition (DDT) in reactive granular materials,” Int. J. Multiphase Flow 12(6), 861 (1986)], initially designed for dense flows, is built. The corresponding model is hyperbolic and thermodynamically consistent. Contrarily to the BN model that involves 6 wave speeds, the new formulation involves 4 waves only, in agreement with the Marble model [Marble, F. E., “Dynamics of a gas containing small solid particles,” Combustion and Propulsion (5th AGARD Colloquium) (Pergamon Press, 1963), Vol. 175] based on pressureless Euler equations for the dispersed phase, a well-accepted model for low particle volume concentrations. In the new model, the presence of pressure in the momentum equation of the particles and consideration of volume fractions in the two phases render the model valid for large particle concentrations. A symmetric version of the new model is derived as well for liquids containing gas bubbles. This model version involves 4 characteristic wave speeds as well, but with different velocities. Last, the two sub-models with 4 waves are combined in a unique formulation, valid for the full range of volume fractions. It involves the same 6 wave speeds as the BN model, but at a given point of space, 4 waves only emerge, depending on the local volume fractions. The non-linear pressure waves propagate only in the phase with dominant volume fraction. The new model is tested numerically on various test problems ranging from separated phases in a shock tube to shock–particle cloud interaction. Its predictions are compared to BN and Marble models as well as against experimental data showing clear improvements.

Validation of system codes RELAP5 and SPECTRA for natural convection boiling in narrow channels

Safety analyses of LOCA scenarios in nuclear power plants are performed with so called thermal–hydraulic system codes, such as RELAP5. Such codes are validated for typical fuel geometries applied in nuclear power plants. The question considered by this article is if the codes can be applied for LOCA analyses in research reactors, in particular exceeding CHF in very narrow channels. In order to answer this question, validation calculations were performed with two thermal–hydraulic system codes: RELAP and SPECTRA. The validation was based on natural convection boiling in narrow channels experiments, performed by Prof. Monde et al. in the years 1990–2000. In total 42 vertical tube and annulus experiments were simulated with both codes. A good agreement of the calculated values with the measured data was observed. The main conclusions are:✓The computer codes RELAP5/Mod 3.3 (US NRC version) and SPECTRA 3.61 have been validated for natural convection boiling in narrow channels using experiments of Monde. The dimensions applied in the experiments were performed for a range that covers the values observed in typical research reactors. Therefore it is concluded that both codes are validated and can be used for LOCA analyses in research reactors, including natural convection boiling. The applicability range of the present validation is: hydraulic diameters of 1.1⩽Dhyd⩽9.0mm, heated lengths of 0.1⩽L⩽1.0m, pressures of 0.10⩽P⩽0.99MPa. In most calculations the burnout was predicted to occur at lower power than that observed in the experiments. In several cases the burnout was observed at higher power. The overprediction was not larger than 16% in RELAP and 15% in case of SPECTRA. Since the experimental uncertainty is 11%, the required safety margin is in this case 27% and 26% for RELAP and SPECTRA respectively.✓The range within which both codes gave conservative results is: hydraulic diameters of 1.1⩽Dhyd⩽2.0mm, heated lengths of 0.5⩽L⩽1.0m, pressures of 0.10⩽P⩽0.99MPa. In this range the required safety margin is in this case 11%, due to experimental uncertainties.✓The validation was performed using the standard version of the RELAP and SPECTRA codes. Calculations showed that burnout is caused in most cases by a complete dryout in the upper part of the cooling channel rather by exceeding CHF in two-phase flow situation. Therefore CHF correlation is in fact not affecting the results.✓A new correlation is proposed that gives a conservative estimation of the heat flux at which burnout occurs. The correlation is similar to the Monde correlation and is based on global conditions (total power, length, diameter). It is not suitable for implementation into a thermal–hydraulic code, however, it is very convenient for a quick engineering check of margin to burnout.

Guidelines for the formulation of Lagrangian stochastic models for particle simulations of single-phase and dispersed two-phase turbulent flows

In this paper, we establish a set of criteria which are applied to discuss various formulations under which Lagrangian stochastic models can be found. These models are used for the simulation of fluid particles in single-phase turbulence as well as for the fluid seen by discrete particles in dispersed turbulent two-phase flows. The purpose of the present work is to provide guidelines, useful for experts and non-experts alike, which are shown to be helpful to clarify issues related to the form of Lagrangian stochastic models. A central issue is to put forward reliable requirements which must be met by Lagrangian stochastic models and a new element brought by the present analysis is to address the single- and two-phase flow situations from a unified point of view. For that purpose, we consider first the single-phase flow case and check whether models are fully consistent with the structure of the Reynolds-stress models. In the two-phase flow situation, coming up with clear-cut criteria is more difficult and the present choice is to require that the single-phase situation be well-retrieved in the fluid-limit case, elementary predictive abilities be respected and that some simple statistical features of homogeneous fluid turbulence be correctly reproduced. This analysis does not address the question of the relative predictive capacities of different models but concentrates on their formulation since advantages and disadvantages of different formulations are not always clear. Indeed, hidden in the changes from one structure to another are some possible pitfalls which can lead to flaws in the construction of practical models and to physically unsound numerical calculations. A first interest of the present approach is illustrated by considering some models proposed in the literature and by showing that these criteria help to assess whether these Lagrangian stochastic models can be regarded as acceptable descriptions. A second interest is to indicate how future developments can be safely built, which is also relevant for stochastic subgrid models for particle-laden flows in the context of Large Eddy Simulations.

Analyses of Flow Field and Heat Exchange about Limestone Grain in the Preheater

According to the structural features of the vertical preheater, the 2-dimention finite element models are set up for the section of the preheater with different arrangement and diameters for the limestone grains. The situation of gas-solid two-phase flow between the limestone grains and the hot air in the preheater is simulated by the discrete particle model. Based on the heat transfer theory, the convection heat transfer coefficient between the limestone grains and the hot air is deduced by means of the experimental correlation of the air cross-flow tube bundles. On the simulation of the 2-dimention finite element model of the vertical preheater based on the FLOTRAN module of ANSYS, the temperature field, pressure field and velocity field of the limestone grains are obtained. The results show that there is a little change of convection heat transfer coefficient by varying the arrangement of limestone grains. The convection heat transfer coefficient is improved by employing the limestone grains with little diameter. The larger the gap between the grains is, the larger the convection heat transfer coefficient is.

Numerical Study of Frictional Resistance in Vertical Helically Coiled Pipe

Based on the heat and mass transfer theory and characteristics of the FLUENT software, a numerical simulation platform had been developed to study the heat exchange and flow situation of gas-liquid two-phase flow in vertical helically coiled pipe. The frictional resistance of gas-liquid two-phase flow in the pipe was investigated by this platform. The results show that frictional pressure drop is relatively small under high pressure condition and increases with the increasing gas quality. This platform could provide a good support for the design and research of vertical helically coiled pipe.

A Sharp Analytical Bound on the Spatiotemporal Locality in General Two-phase Flow and Transport Phenomena

The objective is to understand, for any two-phase flow situation, the instantaneous spatiotemporal nature of the domain-of- dependence. The focal setting is generally nonlinear and heterogeneous, compressible two-phase flow and transport in porous media. The analytical approach develops a sequence of approximations that ultimately recast the general conservation equa- tions into an infinite-dimensional Newton process. Within this process, the spatiotemporal evolution is dictated by linear dif- ferential equations that are easily analyzed. We develop sharp conservative estimates for the support of instantaneous changes to flow and transport variables. Several computational examples are used to illustrate the analytical results.

Exchange flow of two immiscible fluids and the principle of maximum flux

The steady, coaxial flow in which two immiscible, incompressible fluids of differing densities move past each other slowly in a vertical cylindrical tube has a continuum of possibilities due to the arbitrariness of the interface between the fluids. By invoking the presence of surface tension to at least restrict the shape of any interface to that of a circular arc or full circle, we consider the following question: which flow will maximise the exchange when there is only one dividing interface Γ? Surprisingly, the answer differs fundamentally from the better-known co-directional two-phase flow situation where an axisymmetric (concentric) core-annular solution always optimises the flux. Instead, the maximal flux state is invariably asymmetric either being a ‘side-by-side’ configuration where Γ starts and finishes at the tube wall or an eccentric core-annular flow where Γ is an off-centre full circle in which the more viscous fluid is surrounded by the less viscous fluid. The side-by-side solution is the most efficient exchanger for a small viscosity ratio β ≲ 4.60 with an eccentric core-annular solution optimal otherwise. At large β, this eccentric solution provides 51% more flux than the axisymmetric core-annular flow which is always a local minimiser of the flux.

Numerical Simulation of Oil-water Hydrocyclone Using Reynolds-Stress Model for Eulerian Multiphase Flows

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Measurement of Void Fraction in Gas-Liquid Two-Phase Flow by Using Magnetic Fluid

Thispaperdescribesanewmeasuringtechniqueofvoidfractionusingmagnetic fluid.Theproposedmeasurement can be realized with simple devices. The present study consists of two appropriate experiments termed “static experiment” (“calibration test”) and “flow experiment” (“actual flow test”). In the present study, the experiments have been performed with fully diluted magnetic fluid for the purpose of applying to common two-phase flow situations such as air–water. Results of the experiments have concluded that the proposed measuring technique of voidfractioncanmeasurevoidfractioninair–watertwo-phase flowsituationsbyaddingasmallamountofmagnetic fluid to fluids of interest. The present method has promise for other fluids. Nomenclature A = area of cross section of pipe, m 2 B = magnetic flux density, T for tesla H = magnetic field, A=m for ampere=m H = magnitude of magnetic field, A=m j = volumetric flux, m=s k = Boltzmann constant, J=K M = magnetization, A=m

Neutron radiography imaging, porosity and permeability in porous rocks

Research Article| December 01, 2006 Neutron radiography imaging, porosity and permeability in porous rocks F.C. de Beer; F.C. de Beer Building 1500, NECSA, Pelindaba, PO Box 582, Pretoria, 0001, e-mail: fdebeer@necsa.co.za Search for other works by this author on: GSW Google Scholar M.F. Middleton M.F. Middleton Geological Survey of Western Australia (GSWA), Australia; e-mail: mfmiddleton@iinet.net.au Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2006) 109 (4): 541–550. https://doi.org/10.2113/gssajg.109.4.541 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation F.C. de Beer, M.F. Middleton; Neutron radiography imaging, porosity and permeability in porous rocks. South African Journal of Geology 2006;; 109 (4): 541–550. doi: https://doi.org/10.2113/gssajg.109.4.541 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract The visualization of fluid flow and characterization of porous media, by means of Neutron Radiography (NRad), has been tested using the Safari-1 nuclear reactor in South Africa – the only neutron imaging facility available in South Africa for R & D purposes and yet not fully explored by scientists, engineers and researchers. This has been enhanced by the capabilities of neutron imaging equipment and the subsequent improvement of imaging software features that support this type of characterization and analysis.A low light level CCD Camera that was converted into a neutron detection system was used to study the porosity and permeability features of laboratory scale porous sandstone and concrete samples and to compare the results obtained from conventional detection methods.The study develops the theory of mensuration of water saturation fluid of porous media of various types, but specifically concrete and sandstone, using Nrad techniques. Examples of these are fully developed.The study also investigated a number of often poorly understood situations of liquid with full and partial saturations of porous media, including concrete, sandstone aquifers and petroleum reservoir rocks. Never previously studied using NRad is the situation of two-phase fluid flow through oil-bearing rock inside a Hassler high-pressurized apparatus. The Hassler cell is used to reproduce deep reservoir pressure conditions, and is a standard petroleum petrophysical apparatus. For the present experiments, the Hassler cell was constructed of stainless steel (which is not optimum for neutron attenuation), and “heavy water” (D2O) was used instead of water in order to enhance the neutron attenuation contrast between petroleum and the resident liquid (normally water, but in this case deuterium). The heavy water and petroleum combination permitted sufficient contrast in neutron attenuation to enable resolution of D2O versus petroleum concentration within the pore volume to about 10% of the total concentration. Results were obtained of significant quality to suggest that NRad may supply valuable information on multi-phase fluid behaviour in such experimental apparatus.This study shows the capability and ability of neutron radiation imaging to obtain information about porous media and to validate existing data obtained through conventional methods. Although this study concentrate only on 2D neutron imaging and its advantages, exciting new 3D tomography imaging equipment are envisaged at SAFARI-1 that will enhance these R & D capabilities through future studies. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Simulation of a small loss of coolant accident by using RETINA V1.0D code

RETINA has been developed for modeling of two-phase flow situations in full-scope simulators of nuclear power plants. A special feature of RETINA is that both RETINA V1.0D (drift-flux — 5 equations) and RETINA V1.0-2V (two-fluid — 6 equations) approach are available in the code and the same constitutive relations are used in both cases. The governing equations are discretized implicitly, and an automatic derivation algorithm determines the Jacobian matrix, which is partitioned taking into account the special structure of nuclear power plants. Partitioned inverse formula is used to solve the global equation system providing the possibility of multi-level parallelization. Heat structures are modeled in two dimensions and are coupled to the flow equations explicitly. Since the code will be used in real-time simulators, we paid special attention to time-effective solution. In this paper, we demonstrate the ability of our code by simulating a small loss of coolant accident Paks Model Circuit (PMK). The simulation results are compared to real measurements obtained by Paks Model Circuit.

Three-dimensional bubbly flow measurement using PIV

The experimental flow visualization tool, Particle Image Velocimetry (PIV), is being extended to determine the velocity fields in three-dimensional, two-phase fluid flows. In the past few years, the technique has attracted quite a lot of interest. PIV enables fluid velocities across a region of a flow to be measured at a single instant in time in the whole volume (global) of interest. This instantaneous velocity profile of a given flow field is determined by digitally recording particle (microspheres or bubbles) images within the flow over multiple successive video frames and then conducting flow pattern identification and analysis of the data. This paper presents instantaneous velocity measurements in various three-dimensional, bubbly two-phase flow situations. This information is useful for developing or improving existing computer constitutive models that simulate this type of flow field. It is also useful for understanding the detailed structure of two-phase flows.

Influence of two-phase flow characteristics on critical heat flux in low pressure

Abstract In the present study, the critical heat flux (CHF) was measured over wide quality range from subcooled boiling to annular-mist flow at the atmospheric pressure. At a low pressure around the atmospheric pressure, the two-phase flow pattern and void fraction changes easily along the heated length and the flow becomes more complex due to the low density of gas phase. By using Pyrex glass tube as a test channel installed with a heater rod at the centerline, the two-phase flow situation was observed visually. The flow pattern was kept nearly constant over the length of test section due to the low power input into the fluid. Therefore, the characteristics of CHF could be investigated at each flow patterns of bubbly, slug, annular and annular-mist flow. In the subcooled boiling region of bubbly flow, the CHF decreased with increase of quality and was less sensitive to flow rate. In the slug flow region, the CHF showed a minimum value. With more increase of quality in the annular flow, the CHF increased and reached a peak value at a certain quality depending on flow rate. The peak of CHF occurred almost at a constant vapor mass velocity. In the annular-mist flow region, the CHF decreased with increase of quality. In the region, the effect of heated length on the CHF was systematically measured and validity of an analytical model of CHF considering dryout of liquid was investigated.

Numerical Simulation of the Gas-Liquid Flow in a Rotary Gas Separator

The presence of free gas at the pump intake adversely affects the performance of an electrical submersible pump (ESP) system, often resulting in low efficiency and causing operational problems. One method of reducing the amount of free gas that the pump has to process is to install a rotary gas separator. The gas-liquid flow associated with the down hole installation of a rotary separator has been investigated to address its overall phase segregation performance. A mathematical model was developed to investigate factors contributing to gas-liquid separation and to determine the efficiency of the separator. The drift-flux approach was used to formulate this complex two-phase flow problem. The turbulent diffusivity was modeled by a two-layer mixing-length model and the relative velocity between phases was formulated based on published correlations for flows with similar characteristics. The well-known numerical procedure of Patankar-Spalding for single-phase flow computations was extended to this two-phase flow situation. Special discretization techniques were developed to obtain consistent results. Special under relaxation procedures were also developed to keep the gas void fraction in the interval [0, 1]. Predicted mixture velocity vectors and gas void fraction distribution for the two-phase flow inside the centrifuge are presented. The model’s predictions are compared to data gathered on a field scale experimental facility to support its invaluable capabilities as a design tool for ESP installations.

Fin efficiency of extended surfaces in two-phase flow

Abstract Compact plane-fin heat exchangers and other extended surface heat exchanges are used in several two-phase flow applications, such as condensation, boiling, or moist-air cooling. Because of simplicity and lack of better information, designers often use fin efficiency formulas for two-phase flows that are derived based on uniform heat transfer coefficient. However, the heat transfer coefficient h on the fin surface under condensation or boiling may vary significantly compared to the single-phase forced convection attributable to the possible existence of different two-phase flow regimes on the fin surface, including the condition of partial dryout. Thus, the use of constant h fin efficiency formulas (of single-phase flow) for two-phase flow situation may result in serious errors. A critical assessment is made in this paper of the various methods available for calculating fin efficiency in practical fin configurations with condensation, moist air cooling, and boiling on the extended surfaces. Conditions under which the use of constant h can be made are identified. Based on the literature review, some specific design recommendations are made for the determination of the fin efficiency for two-phase flow heat exchanger applications.