Separated Flow Model Research Articles

Transient modeling of a flash tank vapor injection heat pump system – Part I: Model development

This two-part article explores the dynamic behavior of a flash tank vapor injection heat pump system from a numerical simulation perspective. Part I provides a first-principles model describing the transient heat transfer and flow phenomena of the system with detailed modeling techniques for each component. The vapor injection scroll compressor is analyzed with the internal heat transfer between the refrigerant and metallic parts taken into account. Lumped-parameter models are developed for the flash tank and expansion devices. Heat exchangers are modeled using a finite volume approach and accounting for the complex tube circuitry. The separated flow model without interfacial exchange is utilized for two-phase flows in order to incorporate an appropriate void fraction model so that a more accurate prediction for refrigerant mass distribution can be achieved. The modular nature of the component models allows flexibility in the system configuration. Transient simulations are carried out for start-up and shut-down operations. A detailed comparison of model predictions against experimental data is presented in the companion paper.

Micro-channel evaporator for space applications – 2. Assessment of predictive tools

This study is the second part of a two-part study addressing the effectiveness of micro-channel evaporators for space applications. The first part provided pressure drop and heat transfer data for FC-72 that were acquired with a test module containing 80 of 231μm wide×1000μm deep micro-channels. The tests were performed in three flow orientations: horizontal, vertical upflow and vertical downflow over broad ranges of mass velocity and heat flux. The present part uses these experimental results to assess the accuracy of published predictive tools. The two-phase heat transfer coefficient data are compared to predictions of 15 popular correlations, and pressure drop data to 7 mixture viscosity relations used in conjunction of the Homogeneous Equilibrium Model (HEM), and 18 correlations based on the Separated Flow Model (SFM). These models and correlations are carefully assessed in pursuit of identifying the most accurate tools. In addition, three important criteria for implementing micro-channel flow boiling in space systems are proposed: avoiding large pressure drop, avoiding critical heat flux (CHF), and negating the influence of body force. It is shown that micro-channels require significantly smaller mass velocities to negate body force effects than macro-channels, and are therefore very effective for space applications.

SIMULATION OF THE PROCESS OF FLOW SEPARATION AT THE ENTRANCE OF SQUARE ASPIRATING PORT

We consider the flow at the inlet to the suction sq uare hole with sharp edges, which is located in an infinite space. The purpose of this study is to con struct a mathematical model of flow separation at t he entrance to square suction canal with sharp edges, located in infinite space, by using square vortex frameworks. As a part of ideology of the method of discrete vortices in the non-stationary quasiaxisymmetric formulation, we constructed the mathematical model of separated flow at the inlet to the square aspirating pipe and its software-algorithmic implementation. We have determined the velocity field at the entrance to suction channel and a line of flow separation. Determine the velocity field i n typical cross-sections of the suction channel, dime nsions of the efficient absorption, compression rat io of the jet. Were received analytical formulas for calc ulating separation surfaces current. Profiling the entrance opening of the suction hole detected on th e outlines separation surface will improve the acou stic and aerodynamic properties of the exhaust systems. The obtained results can be useful for designing of local exhaust ventilation of reduced energy consump tion.

Experimental Research on an Active Sting Damper in a Low Speed Acoustic Wind Tunnel

Wind tunnels usually use long cantilever stings to support aerodynamic models in order to reduce support system flow interference on experimental data. However, such support systems are a potential source of vibration problems which limit the test envelope and affect data quality due to the inherently low structural damping of the systems. When exposed to tunnel flow, turbulence and model flow separation excite resonant Eigenmodes of a sting structure causing large vibrations due to low damping. This paper details the development and experimental evaluation of an active damping system using piezoelectric devices with balance signal feedback both in a lab and a low speed acoustic wind tunnel and presents the control algorithm verification tests with a simple cantilever beam. It is shown that the active damper, controlled separately by both PID and BP neural network, has effectively attenuated the vibration. For sting mode only, 95% reduction of displacement response under exciter stimulation and 98% energy elimination of sting mode frequency have been achieved.

A flow pattern independent drift flux model based void fraction correlation for a wide range of gas–liquid two phase flow

The main objective of this study is to present new equations for a flow pattern independent drift flux model based void fraction correlation applicable to gas–liquid two phase flow covering a wide range of fluid combinations and pipe diameters. Two separate sets of equations are proposed for drift flux model parameters namely, the distribution parameter (Co) and the drift velocity (Ugm). These equations for Co and Ugm are defined as a function of several two phase flow variables and are shown to be in agreement with the two phase flow physics. The underlying data base used for the performance verification of the proposed correlation consists of experimentally measured 8255 data points collected from more than 60 sources that consists of air–water, argon–water, natural gas–water, air–kerosene, air–glycerin, argon–acetone, argon–ethanol, argon–alcohol, refrigerants (R11, R12, R22, R134a, R114, R410A, R290 and R1234yf), steam–water and air–oil fluid combinations. It is shown that the proposed correlation successfully predicts the void fraction with desired accuracy for hydraulic pipe diameters in a range of 0.5–305mm (circular, annular and rectangular pipe geometries), pipe orientations in a range of -90°⩽θ⩽90°, liquid viscosity in a range of 0.0001–0.6Pa-s, system pressure in a range of 0.1–18.1MPa and two phase Reynolds number in a range of 10 to 5×106. Moreover, the accuracy of the proposed correlation is also compared with some of the existing top performing correlations based on drift flux and separated flow models. Based on this comparison, it is found that the proposed correlation consistently gives better performance over the entire range of the void fraction (0<α<1) and is recommended to predict void fraction without any reference to flow regime maps.

Numerical investigation of vapor–liquid heat and mass transfer in porous media

A modified separate flow model (MSFM) is developed to numerically investigate the heat and mass transfer behaviors in porous media in this paper. In the MSFM, the effects of capillarity, liquid phase change, nonisothermal two-phase region and the local thermal non-equilibrium (LTNE) are considered. The vapor and liquid velocities are both converted into intermediate variables in the simulations and conveniently convergent solutions are obtained because a special upwind scheme for the convection or boiling heat transfer source and variable convergence factors are simultaneously employed. Two typical numerical examples with a one-dimension model of porous media are studied that the high heat fluxes are vertical and parallel to the fluid flow direction, respectively. And the results indicated that the influence of heat flux direction on heat and fluid flow behaviors in porous media is great. The nonisothermal phenomenon in the two-phase region is obvious for the former while the LTNE phenomenon is remarkable in the two-phase region for the latter. The results also showed several similar behaviors that the saturation profile is weakly discontinuous on the phase interface and a countercurrent flow exists in two-phase region.

Effect of rolling motion on transient flow resistance of two-phase flow in a narrow rectangular duct

As a typical ocean environment, rolling motion can induce additional forces on a flow system. Experimental study of the characteristics of air–water mixture flow resistance in narrow rectangular channel(40×1.41mm2) in rolling motions was carried out with the rolling periods and amplitudes of 8s, 12s, 16s, and 10°, 15°, 30°, respectively. Several homogeneous and separated flow models were evaluated against the experimental data to clarify their applicability in predicting the two-phase flow resistance in vertical rectangular channels. The comparison showed that McAdams correlation for calculating the viscosity in homogeneous model is suitable for the present situation to calculate the averaged two-phase flow resistance. Transient flow resistance under rolling condition changed periodically, as a result, attempt was conducted to illustrate the effect of rolling motion. Transient frictional pressure drop strongly depends on the mass quality, Reynolds number, rolling period and amplitude. The fluctuation amplitude of the frictional pressure drop increased with increasing mass quality and rolling amplitude. It also increased with reduction of the rolling period. A new correlation for predicting the transient frictional factor was given by including the influence of rolling parameters and was validated against the experimental data.

Mass Flow Rate Measurement of Oil-Water Two-Phase Flow by a Long-Waist Cone Meter

A long-waist cone meter is proposed for horizontal oil-water two-phase flow rate measurement. A constricting differential pressure Δp and an overall differential pressure δp were tapped for flow rate measurement. Δp is related to flow rate through Bernoulli's equation. A correlation for predicting the mass flow rate by using the overall differential pressure δp is presented through physical analysis. Correlations of Δp and δp are then extended to predict the mass flow rate of two-phase flow based on the separated flow model. A dual differential pressure model is then proposed by combining the two correlations of Δp <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">tp</sub> and δp <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">tp</sub> . The flow regimes in the experiments include water continuous and oil continuous. Prediction on overall mass flow rate is improved by using the dual differential pressure model at an associated error of 2.7% and 4.1%, respectively, for water-continuous flow and oil-continuous flow. This paper indicates that the overall mass flow rate of oil-water two-phase flow can be determined at an accuracy within ±5% with a long-waist cone meter and the dual differential pressure models without the aid of a secondary instrument for the phase fraction measurement, and the results are less affected by phase inversion.

Liquid–liquid segmented flows in polycarbonate microchannels with cross-sectional expansions

Liquid–liquid segmented flows in microchannels fabricated on polycarbonate test chips were investigated experimentally. Three different cross-sectional expansion ratios of 16, 4, and 2 from the injection to the test channels were used to observe the effect of different cross-sectional areas of the injection channel on the segmented flow regimes in the test channel. Flow regime maps and the transitions between flow regimes were determined for the different expansion ratios. The effect of the cross-sectional expansion ratio was determined by mutual comparison of these results. The measured length of the dispersed fluid segments was found to scale inversely with the carrier fluid volumetric flow ratio (βC). A simplified geometric model for the volume of the dispersed and the carrier fluid plugs provided good estimates of the carrier plug length, which scaled inversely with (1−βC). Velocity and pressure drop measurements of the liquid–liquid segmented flows were carried out for all flow regimes, and the associated trends were correlated with changes in the flow topology. Most previous homogeneous and separated flow models based on the viscous frictional pressure drop underestimated the measured pressure drops. Including the contribution of the capillary pressure drop in addition to the frictional pressure drop improved the agreement with the measured data.

수평 사각 채널에서의 상 압력 강하

Two-phase pressure drop experiments were performed during flow boiling to deionized water in a microchannel having a hydraulic diameter of . Tests were made in the ranges of heat fluxes from 100 to , vapor qualities from 0 to 0.2 and mass fluxes of 200, 400 and . The frictional pressure drop during flow boiling is predicted by using two models; the homogeneous model that assumes equal phase velocity and the separate flow model that allows a slip velocity between two phases. From the experimental results, it is found that the two phase multiplier decreases with an increase in mass flux. Measured data of pressure drop are compared to a few available correlations proposed for macroscale and mini/microscale. Among the separated flow models, the correlation model suggested by Lee and Garimella predicted the frictional pressure drop within MAE of 47.2%, which is better than other correlations.

Two-phase pressure drop due to friction in micro-channel

This paper deals with an experimental investigation to measure the frictional pressure drops for two-phase flow boiling in a micro-channel with a hydraulic diameter of 500 µm. First, the experimental study is performed under the test conditions: heat fluxes ranging from 100 to 400 kW/m2, vapor qualities from 0 to 0.2, and mass fluxes of 200, 400 and 600 kg/m2s. Then, the frictional pressure drop during flow boiling is estimated using two models: the homogeneous model and the separated flow model. The experimental results show that the two-phase multiplier decreases with the increase of mass flux. In addition, the measured pressure drops are compared with those from a few correlation models available for macro-scales and mini/micro-scales. Finally, the present paper proposes a new correlation for two-phase frictional pressure drops in mini/micro-scales. This correlation model is developed based on the Chisholm constant C as a function of two-phase Reynolds and Weber numbers. It is found that the new correlation satisfactorily predicts the experimental data within mean absolute error (MAE) of 3.9%.

미소 사각 채널에서의 마찰 압력 강하

An experimental study was carried out to measure frictional pressure drop in flow boiling to deionized water in a microchannel having a hydraulic diameter of 500 μm. Tests were performed in the ranges of heat fluxes from 100 to 400 kW/ , vapor qualities from 0 to 0.2 and mass fluxes of 200, 400 and 600 kg/m 2 s. The frictional pressure drop during flow boiling is predicted by using two models; the homogeneous model that assumes equal phase velocity and the separate flow model that allows a slip velocity between two phases. From the experimental results, it is found that the two phase multiplier decreases with an increase in mass flux. Measured data of pressure drop are compared to a few available correlations proposed for macroscale and mini/microscale. The homogeneous model well predicted frictional pressure drop within MAE of 29.4 % for the test conditions considered in this work.

Numerical simulation of coiled adiabatic capillary tubes in CO2 transcritical systems with separated flow model including metastable flow

A separated flow model of coiled adiabatic capillary tube for CO2 considering metastable flow region is presented. In the case of straight capillary, different friction factors are compared with each other, the model using Churchill and Lin correlations gives a deviation of −1.8% compared with the experimental data for CO2. Three different friction factor correlations (Schmidt, M&N and M&N modified) are discussed for coiled capillary tube. The results show that the M&N modified friction factor gives the best reasonable prediction with a deviation of 4% compared with the experimental data for R22. The discrepancy between the capillary tube length calculated with the separated flow model and that calculated with homogeneous flow model is about 2–7% for the chosen conditions. The mass flow rate for CO2 increases about 7% when the coiled diameter changes from 40 mm to 600 mm.

Measuring two-phase flow in geothermal pipelines using sharp edge orifice plates

The measurement of the two-phase flow rate is very important for the geothermal industry. Among the many different measurement techniques available, the orifice plate stands out for its simplicity, low cost, and the ability to provide real time measurements. Several data sets from different geothermal fields in New Zealand, Indonesia, and the Philippines are used to evaluate existing correlations all of which are based on homogenous flow models. Then derive and test a new correlation based on separated flow model with time discrete liquid phase flow for a more accurate measurement of two-phase flow in geothermal pipelines using the orifice plate. The new correlation is now being implemented successfully in New Zealand and other countries.

CFD Analysis of Cable-In-Conduit Conductors (CICC) for Fusion Grade Magnets

In the present work, dual channel Cable-in-Conduit Conductor (CICC) is considered for Computational Fluid Dynamics (CFD) analysis to understand the complex behavior of flow. A two dimensional (2D) axisymmetric computational model mimicking the CICC is generated and meshed in GAMBIT 2.0. The meshed model is exported to FLUENT 6.3, a commercial CFD code, for further analysis. Annular bundle channel of CICC is assumed to be porous medium with porosity of 0.37 and the central channel is assumed as clear region. The effects of variations in mass flow rate (6 g/s to 10 g/s) of Supercritical helium (SHe), which is used as coolant, through CICC on the pressure gradients and velocity gradients are studied. Reynolds Averaged Navier Stokes (RANS) model is considered for the analysis with standard k-epsilon (k - e)turbulence, which is relevant to the separated flow models. Axial and Radial pressure gradients are calculated along the CICC axis and along the centre line of bundle channel. Friction factor is calculated using the shear stresses obtained from CFD analysis.

A comparison of homogenous and separated flow assumptions for adiabatic capillary flow

Homogenous and separated flow models are investigated for use in modeling one-dimensional adiabatic capillary tube flow. While these methods have been utilized extensively within the literature, the current work provides a rigorous, quantitative comparison of their accuracy using recent experimental data. Simulations utilizing the working fluids R134a, R600a, and R744 are performed for both methods and validated against experimental data. The mean error of the homogenous flow method is 8.55%, 5.4%, and 8.13%, respectively for R134a, R600a, and R744. The mean error of the separated flow method is 5.77%, 4.57%, and 8.03%, respectively for R134a, R600a, and R744. The separated flow method was found to have a smaller mean error and to perform better than the homogenous method as determined by non-parametric statistical tests.

Universal approach to predicting two-phase frictional pressure drop for adiabatic and condensing mini/micro-channel flows

Previous models and correlations for the prediction of pressure drop in adiabatic and condensing mini/micro-channel flows have been validated for only a few working fluids and relatively narrow ranges of relevant parameters. A universal predictive approach for these flows must be capable of tackling many fluids with drastically different thermophysical properties and very broad ranges of all geometrical and flow parameters of practical interest. To achieve this goal, a new consolidated database of 7115 frictional pressure gradient data points for both adiabatic and condensing mini/micro-channel flows is amassed from 36 sources. The database consists of 17 working fluids, hydraulic diameters from 0.0695 to 6.22mm, mass velocities from 4.0 to 8528kg/m2s, liquid-only Reynolds numbers from 3.9 to 89,798, flow qualities from 0 to 1, and reduced pressures from 0.0052 to 0.91. It is shown that, while a few prior models and correlations provide fair predictions of the consolidated database, their predictive accuracy is highly compromised for certain subsets of the database. A universal approach to predicting two-phase frictional pressure drop is proposed by incorporating appropriate dimensionless relations in a separated flow model to account for both small channel size and different combinations of liquid and vapor states. This approach is shown to provide excellent predictions of the entire consolidated database and fairly uniform accuracy against all parameters of the database. This approach is also capable of tackling single and multiple channels as well as situations involving significant flow deceleration due to condensation.

Modeling of turbulent separated flows using OpenFOAM

Turbulent separated planar bluff-body flows were numerically analyzed using the state-of-the-art OpenFOAM and ANSYS FLUENT technologies, based on the conventional URANS approach. Several popular in fluid dynamics test problems such as laminar and turbulent flows over a circular cylinder and turbulent fully developed flows over a triangular cylinder in a channel were numerically replicated with the goal of validation of the selected numerical methods. The detailed, face-to-face comparison between OpenFOAM, FLUENT and experimental data was discussed. Parallel performance in the terms of a strong and weak scalability was assessed up to 1024 cores and compared as well. In general, the present results demonstrated minimum deviations between OpenFOAM and FLUENT and agreed fairly well with the experimental data and other numerical solutions.

Theoretical study on the characteristics of critical heat flux in vertical narrow rectangular channels

Abstract A mathematical separated flow model of annular upward flow has been developed to predict the critical heat flux (CHF) in uniformly heated vertical narrow rectangular channels. The theoretical model is based on fundamental conservation principles: the mass, momentum, and energy conservation equation of the liquid film and the momentum conservation equation of the vapor core together with a set of closure relationships. The comparisons between the theoretically predicted results and the experimental data demonstrate that a good agreement between them has been achieved. With the applications of the present model, the influences of several parameters (such as system pressure, inlet mass flux and channel size) on the CHF in rectangular channels are analyzed, and the CHF is compared with that in annular channels.

Adiabatic air–water two-phase flow in circular microchannels

Gas–liquid two-phase flow in microchannels with hydraulic diameters of 100–500 μm exhibits drastically different flow behaviors from its counterpart in conventional macroscopic channels. Of particular interests are the two-phase flow patterns and the two-phase frictional pressure drop for given flow conditions in these microchannels. This paper presents an experimental study of the effects of channel size and superficial phasic velocity on the two-phase flow pattern and pressure drop of air–water mixture in circular microchannels with inner diameters of 100, 180 and 324 μm. Two-phase flow patterns were visualized using high-speed photographic technique. Four basic flow patterns, namely, bubbly flow, slug flow, ring flow and annular flow, were observed. The two-phase flow regime maps were constructed and the transition boundaries between different flow regimes identified. In an effort to unify the flow transition boundary in microchannels of different sizes, a new flow map was developed using the modified Weber numbers as the coordinates. The two-phase frictional pressure gradient in the microchannels was measured and the data were compared with predictions from the separated flow model, the homogeneous flow model and the flow pattern-based phenomenological models. Results show that the flow pattern-based models provide the best prediction of the two-phase pressure drop in the microchannels.