Two-phase Flow Instability Research Articles

Effects of T-junction size on bubble generation and flow instability for two-phase flows in circular microchannels

A visualization experiment was conducted to detect the effects of the T-junction size on bubble generation and air–water two-phase flow instability in circular microchannels. Four test channels were prepared by connecting two glass capillaries (I.D.: 0.486mm) in a connecting chamber, and the gaps between the capillaries (0.14–0.25mm) were used as air injectors. The bubble generation and two-phase flow downstream of the T-junctions were visualized using a high-speed camera. The bubble and slug lengths and bubble generation frequencies were measured from the obtained images. We confirmed that the bubble length can be predicted by a correlation derived from a scaling analysis. However, the correlation was invalid when the flow rate ratio of the two phases was either extremely high or extremely low, or when the bubbles were generated from the smallest T-junction. These flows were defined as unstable or quasi-stable flow, and the transition mechanism to these flows were investigated by analyzing the fluctuations in the Laplace pressure and water phase during the bubble generation process. The time lag in the pressure changes between the Laplace pressure and the water phase destabilizes the flow, and the destabilization increases for small T-junctions and high water velocities.

Classification of natural circulation two-phase flow patterns using fuzzy inference on image analysis

Two-phase flow on natural circulation phenomenon has been an important theme on recent studies related to nuclear reactor designs. The accuracy of heat transfer estimation has been improved with new models that require precise prediction of pattern transitions of flow. In this work, visualization of natural circulation cycles is used to study two-phase flow patterns associated with phase transients and static instabilities of flow. A Fuzzy Flow-type Classification System (FFCS) was developed to classify these patterns based only on image extracted features. Image acquisition and temperature measurements were simultaneously done. Experiments in natural circulation facility were adjusted to generate a series of characteristic two-phase flow instability periodic cycles. The facility is composed of a loop of glass tubes, a heat source using electrical heaters, a cold source using a helicoidal heat exchanger, a visualization section and thermocouples positioned over different loop sections. The instability cyclic period is estimated based on temperature measurements associated with the detection of a flow transition image pattern. FFCS shows good results provided that adequate image acquisition parameters and pre-processing adjustments are used.

Boiling two-phase flow and efficiency of co- and counter-current microchannel heat exchangers with gas heating

To learn how to utilize the exhaust heat from a high-temperature gas product of a methanol reformer, the present study experimentally investigates the boiling two-phase flow in co- and counter-current microchannel heat exchangers (MCHE) with gas heating. Boiling two-phase flow patterns, two-phase flow instability, and efficiency are explored. The working fluid on the hot and cold sides are helium and liquid methanol, respectively. The silicon-based MCHE, which has dimensions of 20mm×20mm, is designed with 18 parallel microchannels on both sides and is prepared using microfabrication processes. Four types of two-phase flow patterns – bubbly-elongated slug flow, annular flow, annular flow with liquid film breakup, and dryout are identified in both types of MCHE that are studied. A flow pattern map is then constructed on the plane of the methanol mass flux versus heat flux for both types of MCHE. In the counter-current MCHE, the efficiency increases significantly with an increase in the mass flux in both the single- and two-phase flow regions, while the effect of mass flux is insignificant in the co-current MCHE. In the two-phase flow region, the efficiency of both types of MCHEs gradually increases with an increase in the hot-side thermal power until the CHF is approached. The highest efficiency obtained in the present study is about 0.85 and 0.90 for the co- and counter-current MCHEs, respectively.

Research of two-phase flow instability in parallel narrow multi-channel system

Two-phase flow instabilities have been highly researched in the past decades. Many researches gave different stability criterions based on experiments or calculations. However, most of the experiments were designed for studying the specific type of instability, but sometimes what we want to know is the stabilities of one specific system. In this paper, a comprehensive study of two-phase flow instabilities in parallel narrow multi-channel system was done by RELAP5/Mod3.4 code. Firstly, a direct comparison was made of the stability boundaries obtained by experimental data and RELAP5/Mod3.4 code to verify the accuracy of RELAP5 code. Good agreement between the experimental and code prediction results was achieved. Then, the two-phase flow instabilities of parallel narrow multi-channel system were investigated in sequence. The in-phase and out-of-phase parallel channel instabilities (PCI), the Ledinegg instability and the out-of-phase density wave instability (DWI) induced by Ledinegg instability in parallel narrow multi-channel system had been investigated respectively. The effects of system parameters on the stability characteristics were also explored. It is found that the instability analysis of parallel narrow multi-channel system should include the above mentioned four types, which can constitute a complete picture of a system’s instability.

Inertia and compressibility effects on density waves and Ledinegg phenomena in two-phase flow systems

The most common kind of static and dynamic two-phase flow instabilities namely Ledinegg and density wave oscillations are studied. A new model to study two-phase flow instabilities taking into account general parameters from real systems is proposed. The stability influence of external parameters such as the fluid inertia and the presence of compressible gases in the system is analyzed. High-order oscillation modes are found to be related with the fluid inertia of external piping. The occurrence of high-order modes in experimental works is analyzed with focus on the results presented in this work. Moreover, both inertia and compressibility are proven to have a high impact on the stability limits of the systems. The performed study is done by modeling the boiling channel using a one dimensional equilibrium model. An incompressible transient model describes the evolution of the flow and pressure in the non-heated regions and an ideal gas model is used to simulate the compressible volumes in the system. The use of wavelet decomposition analysis is proven to be an efficient tool in stability analysis of several frequencies oscillations.

Two-Phase Flow Instabilities in Boiling and Condensing Systems

Two-Phase Flow Instabilities in Boiling and Condensing Systems

RELAP5/MOD3.3 study on density wave instabilities in single channel and two parallel channels

The subject of instability is of great interest in two-phase flow studies and a constantly up to date research field concerns the determination of stable and unstable operating regions of different industrial systems. Density Wave Oscillations (DWOs) are probably the most common type of instability affecting vapor generation in boiling systems. They result from multiple dynamic feedback effects between the flow rate, the vapor generation rate and the pressure drops in the boiling channel. DWOs may constitute a critical issue in all the industrial applications operating with water–steam mixture, as steam generators and boiling water nuclear reactor cores.In this paper, DWOs are investigated by means of the RELAP5/MOD3.3 code. Available literature works address single channel configuration by working with an imposed Δp, kept constant throughout the simulation. In this work the attempt is to reproduce more realistic experimental apparatus for DWO investigation. In experiments, a bypass tube is usually connected to the heated channel to maintain the constant pressure drop boundary condition. The influence on stability of the bypass area is hereby examined. The analysis is completed by studying twin parallel channels. In particular, different configurations are simulated to underline the influence of some geometric parameters on RELAP5 predictions. Channel geometry has been modified to reproduce an experimental facility including two helically coiled tubes. Simulation data are clustered in dimensionless stability maps, generally adopted in such stability investigations.The results aim to be a contribution to the assessment of the code capability to detect the onset of two-phase flow instabilities in boiling channels. Within this respect, RELAP5 can provide useful results for comparison with analytical models and in preparation of experimental campaigns on the matter. Therefore, purpose of the paper is to highlight strengths and weaknesses of the code to simulate DWOs as well as to evaluate carefully the suitable numerical settings necessary to assure a correct prediction of the phenomenon.

Numerical modeling of turbulence-induced interfacial instability in two-phase flow with moving interface

The present paper introduces a new interfacial marker-level set method (IMLS) which is coupled with the Reynolds averaged Navier–Stokes (RANS) equations to predict the turbulence-induced interfacial instability of two-phase flow with moving interface. The governing RANS equations for time-dependent, axisymmetric and incompressible two-phase flow are described in both phases and solved separately using the control volume approach on structured cell-centered collocated grids. The transition from one phase to another is performed through a consistent balance of kinematic and dynamic conditions on the interface separating the two phases. The topological changes of the interface are predicted by applying the level set approach. By fitting a number of interfacial markers on the intersection points of the computational grids with the interface, the interfacial stresses and consequently, the interfacial driving forces are easily estimated. Moreover, the normal interface velocity, calculated at the interfacial markers positions, can be extended to the higher dimensional level set function and used for the interface advection process. The performance of linear and non-linear two-equation k–ɛ turbulence models is investigated in the context of the considered two-phase flow impinging problem, where a turbulent gas jet impinging on a free liquid surface. The numerical results obtained are evaluated through the comparison with the available experimental and analytical data. The nonlinear turbulence model showed superiority in predicting the interface deformation resulting from turbulent normal stresses. However, both linear and nonlinear turbulence models showed a similar behavior in predicting the interface deformation due to turbulent tangential stresses. In general, the developed IMLS numerical method showed a remarkable capability in predicting the dynamics of the considered two-phase immiscible flow problems and therefore it can be applied to quite a number of interface stability problems.

Numerical study of nuclear coupled two-phase flow instability in natural circulation system under low pressure and low quality

Based on the one-dimension two-phase drift flow model, the numerical simulation of two-phase flow stability characteristic on the test loop (HRTL-5) for 5 MW heating reactor (developed by the Institute of Nuclear and New Energy Technology of Tsinghua University, Beijing) is performed with and without coupled point neutron kinetics. The density wave oscillation instability is analyzed in the system under low pressure at 1.5 MPa and low steam quality less than 10%. The effect of inlet subcooling and heating flux on the system instability is simulated under the system pressure P sys = 1.5 MPa. The numerical results show that there exist two instability inlet subcooling boundaries at different heat flux. The numerical results show good agreement with the experimental results on HRTL-5 without consideration of point neutron kinetics. If coupled with point neutron kinetics, the system will exhibit little difference on instability boundaries from that without considering the nuclear characteristics. But the amplitude and the phase of the oscillation of the thermal hydraulic parameters of the system will be somehow affected in unstable zone if the system is coupled with point neutron kinetics.

Experimental Investigation of Two-Phase Flow Instabilities in Low-Mass-Flux Water Wall Tubes of Supercritical Circular Fluidized Bed Boilers

For the purpose of disclosing the hydrodynamic flow characteristics, under the low mass velocity conditions of the 600-MW supercritical circular fluidized beds boilers, experimental studies on instability of two-phase flow in parallel vertical internally ribbed tubes were conducted. Two kinds of oscillations, pressure-drop oscillation and density-wave oscillation, have been observed. In the range of test parameters the effects of pressure, mass flux, inlet subcooling, compressible volume, exit throttle, and asymmetric heat flux to the two-phase instability were explored and analyzed. Indications from experiment data are: To increase system pressure, mass flux and inlet subcooling will intensify the stability of water wall tubes. To increase exit throttle will intensify the instability of water wall tubes. The bounding pressure and bounding mass flux of density-wave oscillations and the bounding pressure of pressure-drop oscillation have been obtained. Based on the results of testing and using a homogeneous model, the threshold relational expressions of instability were obtained. The results may be used for the design and safe operation of parallel vertical rifled water wall tubes of supercritical circular fluidized beds boilers.

Numerical research on oscillation of two-phase flow in multichannels under rolling motion

Two-phase flow instability and dynamics of a parallel multichannels system has been theoretically studied under periodic excitation induced by rolling motion in the present research. Based on the homogeneous flow model considering the rolling motion, the parallel multichannels model and system control equations are established by using the control volume integrating method. Gear method is used to solve the system control equations. The influences of the inlet, upward sections, heating power and rolling amplitudes on the flow instability under rolling motion have been analyzed. The marginal stability boundary (MSB) under the rolling motion condition is obtained. The unstable regions occur in both low and high equilibrium quality and inlet subcooling regions. The multiplied period phenomenon occurs in the high equilibrium quality region and the chaos phenomenon appears on the right of MSB. The concept of stability space is presented.

Instability of two-phase flows: A lower bound on the dimension of the global attractor of the Cahn–Hilliard–Navier–Stokes system

We consider a model for the flow of a mixture of two viscous and incompressible fluids in a two or three dimensional channel-like domain. The model consists of the Navier–Stokes equations governing the fluid velocity coupled with a convective Cahn–Hilliard equation for the relative density of atoms of one of the fluids. We prove the instability of certain stationary solutions for such a system endowed with periodic boundary conditions on elongated domains (0,2π/α0)×(0,2π) or (0,2π/α0)×(0,2π)×(0,2π/β0) for a special class of periodic body forces, provided that α0 and β0 are small enough. As a consequence, we deduce a lower bound for the Hausdorff dimension of the global attractor.

Voltage loss and fluctuation in proton exchange membrane fuel cells: The role of cathode channel plurality and air stoichiometric ratio

Water management is a critical issue in the development of proton exchange membrane (PEM) fuel cells with robust operation. Liquid water can accumulate and flood the gas delivery microchannels and the porous electrodes within PEM fuel cells and deteriorate performance. Since the liquid distribution fluctuates in time for two-phase flow, the rate of oxygen transport to the cathode catalyst layer also fluctuates, resulting in unstable power density and efficiency. This paper reports experimental data on the mean voltage loss and the voltage fluctuations during constant current operation as a function of both the number of parallel microchannels and the air flow rate stoichiometric ratio. We define channel plurality as a flow field design parameter to describe the number of channels per unit of active area. The voltage loss was found to scale proportionally to channel plurality divided by the air stoichiometric ratio. The amplitude of the voltage fluctuations was found to be linearly proportional to channel plurality and inversely proportional to the air stoichiometric ratio squared. By analyzing pressure drop data and power spectra, we conclude that the voltage fluctuations are well-correlated with two-phase flow instabilities in the cathode's parallel microchannels. Finally, a scaling analysis is presented for generalizing the results for fuel cells having different active area and channel cross-section.

Experimental investigation of non-uniform heating effect on flow boiling instabilities in a microchannel-based heat sink

Two-phase flow boiling in microchannels is one of the most promising cooling technologies for coping with high heat fluxes produced by the next generation of central processor units (CPUs). If flow boiling is to be used as a thermal management method for high heat flux electronics it is necessary to understand the behaviour of a non-uniform heat distribution, which is typically the case observed in a real operating CPU. The work presented is an experimental study of two-phase boiling in a multi-channel silicon heat sink with non-uniform heating, using water as the cooling liquid. Thin nickel film sensors, integrated on the back side of the heat sinks were used in order to gain insight related to temperature fluctuations caused by two-phase flow instabilities under non-uniform heating. The effect of various hotspot locations on the temperature profile and pressure drop has been investigated. It was observed that boiling inside microchannels with axially non-uniform heating leads to high temperature non-uniformity in the transverse direction.

Two-phase flow instabilities in horizontal straight tube evaporator

Abstract It is essential to ensure the stability of a refrigeration system if the oscillation in evaporation process is the primary cause for the whole system instability. This paper is concerned with an experimental investigation of two-phase flow instabilities in a horizontal straight tube evaporator of a refrigeration system. The relationship between pressure drop and mass flow with constant heat flux and evaporation pressure is measured and determined. It is found that there is a negative slope section in the middle of positive slope pressure drop versus mass flow velocity thus making the velocity a multi-valued function of pressure drop. Three types of dynamic instabilities including the density-wave instability, pressure drop instability and thermal instability are found under the conditions of heat flux from 5 to 17 kW/m 2 , mass velocity from 150 to 1500 kg/(m 2 s), and evaporating pressure from 0.5 to 0.8 MPa. The density-wave oscillation occurs at almost all mass velocities; its period is about 1–3 s and its amplitude is the lowest. The pressure drop oscillation takes place in a negative slope section; its period is about 10 s. The thermal oscillation can be induced at a high mass flow velocity, and its period is of 60 s and the amplitude is the highest among the three oscillations. The boundary as the onset of pressure drop oscillation and that of thermal oscillation are obtained experimentally. Finally, the empirical equations for the boundaries between three types of oscillations are obtained according to the mathematical model and test data.

Two-phase refrigerant flow instability analysis and active control in transient electronics cooling systems

Two-loop refrigeration systems are being explored for two-phase cooling of ultra high power electronic components. For effective and efficient thermal management of electronic systems, active control methods are desired to suppress inherent flow instabilities especially in transient applications. This paper presents a framework for the transient analysis and active control of pressure-drop flow instabilities under varying imposed heat loads. The external effects on boiling flow characteristics and the boiling oscillatory flow responses to transient heat load changes are studied. Flow instability margins can be quantitatively predicted from an analytical two-phase flow model. In addition, the effects of wall thermal inertia on flow oscillations is systematically investigated. Based on the theoretical analysis of oscillatory flow boiling of refrigerants, a set of active control schemes are developed and studied to suppress flow oscillations and to increase the critical heat flux. With the available control devices – inlet valve and supply pump – different active control schemes are studied to improve the transient two-phase cooling performance.

Experiment investigation on two-phase flow instability in a parallel twin-channel system

In this paper, the behavior of two-phase flow instability of a twin-channel system is studied experimentally. The two-phase flow instability between twin channels (FIBT) was observed in the experiments at three different system pressures, different inlet resistance coefficients and nonuniform heating condition. The instability boundaries of the twin-channel system are obtained in the phase plane of the subcooling and phase change number. The influences of inlet resistance, system pressure and nonuniform heating are obtained. In the end the numerical simulation results based on RELAP5/MOD3.4 code are also discussed.

Investigation of boiling and bubble confinement in a high aspect ratio micro-channel

Abstract In this paper we experimentally investigated the pressure drop fluctuation and liquid/vapour interface behaviours during flow boiling in a single rectangular micro-channel with high aspect ratio. An integrated testing system including syringe pump, pressure sensors, thermocouples, micro-channel and high speed camera was mounted in a 1 m × 1 m × 1 m plexiglass box, the ambient temperature inside of which was controllable. A micro-channel with the hydraulic diameter of 727 μm was chosen, the aspect ratio of which was 10. FC-72 was used as the working liquid, with a saturation temperature of 56 °C at atmospheric pressure. The present study concentrated on the transition from single-phase flow to two-phase flow (including single-phase friction losses and onset of nucleation) and flow instabilities and liquid–vapour interface behaviors during two-phase flow. Pressures at inlet (Pin) and outlet (Pout) of the channel were measured and pressure drop (Pin − Pout) was then obtained. The pressure drop fluctuation was discussed. Besides, various liquid–vapour interface behaviors including interface deformation, recoiling and re-wetting as well as bubble nucleation, bubble growth and bubble geometry were captured and analyzed with the aid of a high speed camera. Additionally, evaporation rates of bubble nose and bubble tail at different mass fluxes and heat fluxes were compared.

Instability of refrigeration system – A review

It is essential to ensure the stability for the normal operation of refrigeration systems. This paper reviews the researches on the theory and solutions of the instability of refrigeration systems. The instability of refrigeration systems includes two aspects: the two-phase flow instability in refrigeration system, the instability on refrigeration system control characteristics. As an inherent characteristic of two-phase evaporating flow, several separate explanations for the formation of oscillation of mixture–vapor transition point in the evaporation process by different scholars had been given but there is no general explanation till now. The investigation of instability on refrigeration system control characteristics focused on both static and dynamic researches. The minimum stable signal line theory, as a very important finding for the static instability of the evaporator and thermal expansion valve control loop, presented the different result to other researches. Dynamic researches on simulation and frequency-domain analysis provided various means for forecast and validation with considerable precision while their application range was still confined. With the development of variable capacity compressor and electronic expansion valve, further researches should be carried out to analyze the instability of the variable capacity refrigeration system with considering the influence of parameter coupling and control algorithm.

Active control of flow and heat transfer in silicon microchannels

Boiling heat transfer in silicon microchannels needs high walls and liquid superheats for bubble nucleation, leading to a strong thermal non-equilibrium between vapor and liquid phases, which not only damages the heat transfer device at the start-up stage, but also causes two-phase flow instabilities. In this paper, the seed bubble technique is used as an active control strategy to improve the flow and heat transfer in silicon microchannels. Seed bubbles are miniature bubbles of micron size, which are produced on a set of microheaters upstream of microchannels driven by pulse voltage signal. They flow downstream of microchannels after they depart from microheaters to decrease and control the thermal non-equilibrium between vapor and liquid phases in microchannels. The working fluid was methanol and the hydraulic diameter of the microchannels was 100 µm. The demand curves of pressure drops versus mass fluxes were examined with and without active control. Four regions (I, II, III and IV) of demand curves were identified. For the flow without active control, the four regions were the subcooled liquid flow, the superheated liquid flow, the unstable boiling flow and the vapor flow at high-vapor-mass qualities. Alternatively, for the flow with active control, the four regions were the subcooled liquid flow, the seed-bubble-triggered boiling flow, the seed-bubble-stabilized boiling flow and the vapor flow at high-vapor-mass qualities. The linear part of the demand curves is shortened when the seed bubble technique is used. The points at which the demand curves deviate from the linear part coincide into one point at different seed bubble frequencies. The seed bubbles have no influence on the subcooled liquid flow (region I) and the vapor flow at high-vapor-mass qualities (region IV). However, seed bubbles not only convert a superheated liquid flow into a quasi-stable boiling flow in region II, but also convert an unstable boiling flow into a quasi-stable boiling flow in region III. Besides, heat transfer coefficients with active control are several times those without active control in regions II and III. The higher the seed bubble frequencies, the more the heater surface temperatures decrease.