Two-phase Flow Instability Research Articles

The analysis of density wave oscillation in ocean motions with a density variant drift-flux model

The two-phase flow instability in heated channels is an important phenomenon in industrial facilities. The effect of ocean motions upon the flow instability should be predicted accurately for the heat exchangers in floating platform. The reduced order model with drift-flux approach is developed to analyze the density wave oscillation in a uniformly heated channel in ocean motions. The density variation of coolant flow is considered. The stability maps can be predicted precisely on experimental data. The effect of ocean motions on the stability boundary is investigated quantitatively by the defining of the degree of instability. The subcritical Hopf bifurcation in ocean motions is also studied.

Numerical Investigation on Thermal Hydraulic and Transit time Characteristics of Density Wave Oscillations

The study of two-phase flow instability attracts lots of attention due to its significance and complexity. Density wave oscillations are the most common dynamic instability, one type of flow instability, in two-phase systems. In this paper, various channe l models are constructed based on RELAP5. The 3×3 channel is chosen to analyze the stability and operation security of the system based on the comparison of stability boundaries. For the understanding of basic phenomenon, thermal hydraulic and transit time characteristics in the 3×3 channel are analyzed. The period of flow rate oscillations is an almost constant value along the axial direction. There is a phase lag between inlet and outlet which is nearly equal to 1/2 oscillating period. Flow oscillations may cause dryout when oscillations are divergent. This phenomenon is dangerous for reactors, and should be avoided. For the analysis of the decrease of CHF induced by oscillatory flow, the transit time is taken into consideration as an crucial parameter. The correlation developed by Masini et al. is the most accurate for transit time evaluation by comparison of different methods.Based on the analysis of parameters effect, transit time is positive with subcooled number Nsub. Transit time decreases linearly, and then transit time becomes flat with the increase of pressure when Nsub is fixed.

Effect of groove configuration on two-phase flow instability for Ultra-Thin Looped Heat Pipes in thermal management system

Aims to solve the increasingly serious safety issue in thermal management system applied for power sources, the typical thermo-hydraulic devices—heat pipe, has been widely adopted. In this paper, the start-up characteristics at low heat load as well as the dry-out limits at high heat load of the Ultra-Thin (1.5 mm in thickness) Looped Heat Pipes (ULHPs) with three various groove configurations are investigated via experiments. Gravity instability and density instability are considered as the two main types of flow instability occurred in the ULHP system and have significant influences on the stable operation. The effects of the groove configurations on suppressing the flow instability and stabilizing the operation of two-phase flow in the ULHPs are both analyzed. The boundaries and the corresponding areas of the two types of instability for different configurations are plotted with well-known Nsub-Npch figure. The shape factor ξ and the contribution effect φ are proposed to effectively quantify the function of the groove configuration on stabilizing the system operation. The effectiveness of the method is verified with the experimental results and the theoretical analysis, providing possible tools for the further design and optimization in heat pipe systems.

Non-Darcy interfacial dynamics of air-water two-phase flow in rough fractures under drainage conditions

Two-phase flow interfacial dynamics in rough fractures is fundamental to understanding fluid transport in fractured media. The Haines jump of non-Darcy flow in porous media has been investigated at pore scales, but its fundamental processes in rough fractures remain unclear. In this study, the micron-scale Haines jump of the air-water interface in rough fractures was investigated under drainage conditions, with the air-water interface tracked using dyed water and an imaging system. The results indicate that the interfacial velocities represent significant Haines jumps when the meniscus passes from a narrow “throat” to a wide “body”, with jump velocities as high as five times the bulk drainage velocity. Locally, each velocity jump corresponds to a fracture aperture variation; statistically, the velocity variations follow an exponential function of the aperture variations at a length scale of ~100 µm to ~100 mm. This spatial-scale-invariant correlation may indicate that the high-speed local velocities during the Haines jump would not average out spatially for a bulk system. The results may help in understanding the origin of interface instabilities and the resulting non-uniform phase distribution, as well as the micron-scale essence of the spatial and temporal instability of two-phase flow in fractured media at the macroscopic scale.

Numerical Investigation on the Natural Circulation Thermal Hydraulic Characteristics of a Pressure Tube Type Boiling Water Reactor

Natural Circulation Boiling Water Reactors (BWRs) are susceptible to boiling two-phase flow instabilities under certain conditions, which can lead to flow oscillations in the reactors. These oscillations could be in-phase or out-of-phase in nature depending on the geometry and operating conditions of the system.This paper reports on a study on the effect of both thermal hydraulics as well as neutron kinetics on the characteristics of boiling two-phase natural circulation flow instabilities in a pressure tube type natural circulation BWR. RELAP5/MOD3.2 code has been used to simulate the natural circulation behavior in multiple channels of the reactor. Before applying the RELAP5 model for simulation of natural circulation in this reactor, the code was benchmarked with the experiment conducted in a multichannel boiling natural circulation loop, having geometry similar to this reactor. The results showed that the RELAP5 model is able to capture the boiling induced-flow instabilities. Then the model was applied to simulate the reactor behavior. The prediction showed that the reactor could experience both Type-I and Type-II density wave oscillations depending on the channel inlet subcooling and channel power. Unlike Type-II instability wherein clear cut outs of phase oscillations among multiple channels were observed, in Type-I instability it was observed that mixed mode oscillations could be present, especially at low subcooling. The phase difference among the channels were found to change with time in Type-I instability. These are completely new findings with regard to characteristics of boiling two-phase natural circulation.The fuel in this reactor is a combination of (Th-233U)O2 and (Th-Pu)O2, which is different from conventional BWRs. Also, the coolant (light water) is present in different pressure tubes which are physically separated from moderator (heavy water). The effect of neutronic feedback due to this fuel and geometrical configuration on characteristics of Type-I and Type-II instabilities has not been investigated before. In view of this, a systematic investigation was done to study the effect of neutronic feedback on Type-I and Type-II oscillations observed in this reactor. The simulation showed that the threshold power for both Type-I and Type-II instability slightly stabilizes with introduction of neutronic feedback. Since the magnitude of void reactivity feedback is very small in the present fuel composition, the stability boundary was only slightly altered with the introduction of neutronic feedback. Regarding the oscillation characteristics, it was found that change in magnitude of void reactivity has almost no effect on Type-I oscillations whereas the Type-II oscillations get stabilized when void reactivity magnitude was increased. This kind of effect due to void reactivity feedback is in contrast to findings based on conventional BWR and is an important finding of the present study.

Investigation of density wave instability in once-through superheated steam generators using SIGHT

Density-wave oscillation is the most common type of two-phase flow instability in the steam generators. A computer code SIGHT was derived to study on flow instability for the superheated steam generator under high pressure. The drift flux model was used for two-phase flow analysis. Linear, frequency-domain method was used for modeling and the flow stability was evaluated by Nyquist stability criterion. The unified and representative variables, namely water/steam mass flow density, water/steam enthalpy and liquid sodium temperature were selected to describe steady state and perturbations. The density perturbation of subcooled water was taken into account since the large subcooling degree of feedwater. The code has been tested against the existing experimental data and compared with results of past analysis code. Also the effect on flow stability caused by system parameters and density changes in subcooled water was analyzed. Lastly, preliminary stability thresholds were given for Steam Generator of China Fast Reactor 600 (CFR600) using SIGHT.

Effect of operational parameters on flow boiling heat transfer performance for porous interconnected microchannel nets

A porous interconnected microchannel net (PIMN) is proposed as an effective structure in this study. As a combination of IMN and sintered porous material, the PIMN was fabricated via solid-phase sintering and wire electric discharge machining with orthogonally interconnected microchannels on the both side of the porous matrix. Two-phase heat transfer performance of the PIMN was investigated and evaluated through flow boiling experiments utilizing deionized water as the coolant at different operation parameters (mass flux and inlet subcooling). The results showed that the heat transfer enhancement by the PIMN was outstanding comparing to the reentrant porous microchannels (RPM) thanks to the interconnected microchannels and porous structure. The PIMN under the condition of higher mass flux or inlet subcooling yielded better two-phase heat transfer performance, including the higher heat transfer coefficients and lower pressure drop. Visualization images captured by high speed camera revealed that the flow pattern inside the PIMN transformed from bubbly flow to annular flow as the boiling mechanism developing from nucleate boiling into convective boiling. Further study in two-phase flow instabilities showed that more stable two-phase flow for the PIMN leads to better pressure drop and boiling heat transfer performance.

Experimental study for pressure drop and flow instability of two-phase flow in the PCHE-type steam generator for SMRs

Experiments have been carried out to apply PCHE as a steam generator in Small Modular Reactors. KAIST designed the straight-type PCHE for the application of the steam generator, and constructed a high-pressure test facility. We investigated the effects of system pressure, inlet water temperature and heat input on the thermal-hydraulic characteristics of PCHE. For each case, we varied the water flow rate over a wide range in order to identify the pressure drop characteristic curve of PCHE. In order to predict the pressure drop caused by PCHE, we compared and analyzed various existing correlations. In addition on the steady-state characteristic curve, the occurrence of instability at each point and the criteria of DWO and Ledinegg instability were experimentally determined. The super-heated steam test at low flow rate resulted in severe DWO with high fluctuation. Since the stable operation in the super-heated region is crucial for the PCHE-type steam generator, we conducted a stabilization experiment using an inlet throttling valve to verify such possibility.

Simulation of reactive polydisperse sprays strongly coupled to unsteady flows in solid rocket motors: Efficient strategy using Eulerian Multi-Fluid methods

In this paper, we tackle the issue of the accurate simulation of evaporating and reactive polydisperse sprays strongly coupled to unsteady gaseous flows. In solid propulsion, aluminum particles are included in the propellant to improve the global performances but the distributed combustion of these droplets in the chamber is suspected to be a driving mechanism of hydrodynamic and acoustic instabilities. The faithful prediction of two-phase interactions is a determining step for future solid rocket motor optimization. When looking at saving computational ressources as required for industrial applications, performing reliable simulations of two-phase flow instabilities appears as a challenge for both modeling and scientific computing. The size polydispersity, which conditions the droplet dynamics, is a key parameter that has to be accounted for. For moderately dense sprays, a kinetic approach based on a statistical point of view is particularly appropriate. The spray is described by a number density function and its evolution follows a Williams–Boltzmann transport equation. To solve it, we use Eulerian Multi-Fluid methods, based on a continuous discretization of the size phase space into sections, which offer an accurate treatment of the polydispersion. The objective of this paper is threefold: first to derive a new Two Size Moment Multi-Fluid model that is able to tackle evaporating polydisperse sprays at low cost while accurately describing the main driving mechanisms, second to develop a dedicated evaporation scheme to treat simultaneously mass, moment and energy exchanges with the gas and between the sections. Finally, to design a time splitting operator strategy respecting both reactive two-phase flow physics and cost/accuracy ratio required for industrial computations. Using a research code, we provide 0D validations of the new scheme before assessing the splitting technique's ability on a reference two-phase flow acoustic case. Implemented in the industrial-oriented CEDRE code, all developments allow to simulate realistic solid rocket motor configurations featuring the first polydisperse reactive computations with a fully Eulerian method.

Study on the Instability of Two-Phase Flow in the Heat-Absorbing Tube of Trough Solar Collector

The Marangoni effect and Rayleigh-Benard effect in the two-phase region of solar trough heat-absorbing tube are simulated by FTM (front tracking method). Considering the Marangoni effect alone, although surface tension gradient and surface tension affect the interface wave, the two effects have different characteristics. The surface tension gradient caused by the temperature gradient is one of the factors that swing the interface. The amplitude attenuation of the interface wave decreases with the increase of the Marangoni number (Ma). In general, the surface tension gradient enhances the convection opposite to the temperature gradient. Under the gravity field, the Rayleigh-Benard effect influences the development of the vortex structure in the flow field, which in turn affects the velocity gradient near the interface to influence the evolution of the interface fluctuation. In a small Rayleigh number (Ra), the buoyancy convection reduces the velocity gradient, thus suppressing the evolution of the interfacial wave. In the range of Ra < 4.0E4, the larger the Ra, the stronger the inhibitory effect. However, when the Ra number is large (Ra > 4.0E4), the situation is just the opposite. The larger the Ra is, the stronger the promoting effect is.

On the flow boiling enhancement in interconnected reentrant microchannels

Interconnected reentrant microchannels (IRM) were developed to explore the feasibility of flow boiling enhancement and applications in high heat flux dissipations. They consisted of 14 parallel semi-closed Ω-shaped microchannels and 8 transverse ones each with a hydraulic diameter of 781μm. These microchannels were fabricated in copper by wire electrical discharge machining process. One-to-one comparative studies between the IRM and parallel reentrant microchannels (PRM) with the same reentrant configurations were conduced in flow boiling experiments. Two coolants tests of deionized water and ethanol were performed at inlet subcoolings of 10 and 40°C and mass fluxes of 125–300kg/m2s. Significant flow boiling heat transfer augmentation, i.e., an enhancement ratio of 40–280% in water tests, and 28–61% in ethanol tests, were achieved for the interconnected reentrant microchannels. This can be attributed to that the addition of transverse microchannels provided more flow passages, diminished the confinement effects for vapor flow and introduced intense disruption and re-initializations of the liquid thin film. Moreover, the two-phase flow instabilities were mitigated considerably, and the stream-wise temperature uniformity was also improved for the IRM. All these features of interconnected reentrant microchannels indicate that they could be a promising option for the application in high heat flux dissipations.

Experimental study and optimization of pin fin shapes in flow boiling of micro pin fin heat sinks

Micro pin fin heat sinks show their great merits of thermal management in high heat flux devices in microelectronic, energy, aerospace, and military areas. The design optimization of micro pin fin shapes is a critical issue for their applications, especially in two-phase flow boiling conditions. In this study, four types of staggered micro pin fins with different cross-section shapes, i.e., square, circular, diamond and streamline, were fabricated by a laser micromilling method, and constructed for heat sinks cooling systems. Flow boiling performance of the micro pin fin heat sinks (MPFHSs) was characterized using deionized water as coolant. The effect of cross-section shape on flow boiling characteristics of MPFHSs was examined. Test results showed that the square micro pin fins presented the best boiling heat transfer, followed by circular and streamline ones. The diamond micro pin fins performed worst in boiling heat transfer and suffered severe two-phase flow instabilities at moderate to high heat fluxes, whereas they introduced the smallest pressure drop. The streamline micro pin fins presented the largest two-phase pressure drop. The square and circular micro pin fins showed their superiority in the mitigation of two-phase flow instabilities. The square micro pin fins seem to be the optimum choice and should be selected for heat sink cooling systems. The results in this study provide critical information for the design optimization of micro pin fins heat sinks for the high heat-flux applications, and are of considerable practical importance.

Flow boiling of ammonia and flow instabilities in mini-channels

A simultaneous visualization and measurement study has been carried out to investigate the flow boiling and two-phase flow instabilities of ammonia in a mini-channel evaporator at various heat and mass fluxes. The evaporator consisted of 4 parallel 1×1.1mm channels with a uniformly heated length of 250mm. The visualization study showed that periodic reverse flow occurred in the mini-channel evaporator with liquid rewetting stage and annular film evaporating stage repeated alternately. Both damped and stable oscillations of pressure drop, wall temperature and system pressure were observed. The liquid rewetting stage approximately coincided with the decreasing stage of system pressure. Three typical two-phase flow patterns: bubbly flow, elongated bubble/slug flow and annular flow were identified. Low-frequency high-amplitude oscillation and high-frequency low-amplitude oscillation were identified through wavelet analysis. The high-frequency oscillation mainly occurred at the wave crest of the low-frequency oscillation. In addition, high-order harmonics were found in almost all the low-frequency oscillation waves. The effects of mass flux and saturation temperature on flow boiling instabilities were also investigated.

Experimental investigation on two-phase flow instabilities of ultra supercritical pressure boiler with spirally water wall tubes

For a better design of 1000MW ultra supercritical pressure boiler with spirally water wall tubes, an experimental study has been performed in the State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University to investigate the instability of the steam-water two-phase flow inside a smooth tube with an inclined angle of 20degree from the horizon. The static hydrodynamic characteristic curves are obtained and the dynamic instabilities are also analyzed. The boundary conditions of the pressure drop oscillation and the density wave oscillation are proposed. The effects of pressure, mass flux, inlet and outlet throttle, inlet subcooling and the compressible volumes are analyzed primarily. The results demonstrate that the above parameters significantly influence the critical heat flux for the mentioned two types of oscillations and the density wave oscillations are the major type of oscillations in boilers and steam generators. The density wave oscillations occur at a higher heat flux than the pressure drop oscillations with the other operating parameters remaining constant. The inducing mechanisms of the above two types of oscillations are clarified. The variation ranges of critical heat flux and critical vapor quality are illustrated. Two empirical correlations predicting the onset of pressure drop oscillation and density wave oscillation are derived from the experimental data.

Flow boiling enhancement of structured microchannels with micro pin fins

Structured microchannels with micro pin fins (SM-MPF) were developed for the utilizations in advanced microchannel heat sinks to cool high heat-flux devices. These micro cone pin fins were fabricated on the bottom surface of rectangular microchannels by a laser micromilling method. Flow boiling performance of these structured microchannels with micro pin fins was experimentally explored together with comprehensive comparisons with conventional rectangular microchannels with smooth bottom walls surfaces. Flow boiling tests with two coolants, deionized water and ethanol, were performed at inlet subcoolings of 40°C and 10°C, mass fluxes of 200–300kg/m2·s and effective heat fluxes up to 1067kW/m2. Experimental results demonstrated that the structured microchannels presented significant flow boiling heat transfer enhancement, i.e., an enhancement of 10–104% in water tests, and 90–175% in ethanol cases in general, compared to the conventional rectangular microchannels. Moreover, the critical heat fluxes were promoted notably, and the severe two-phase flow instabilities were also mitigated for the structured microchannels in the low inlet temperature cases. The structured microchannels provided lots of stable nucleation sites by producing lots of tiny reentrant cavities, and introduced significant wicking effects to maintain the liquid rewetting and hinder the local dry-out, which contributed to the above notable flow boiling enhancement.

Effects of structural parameter on flow boiling performance of interconnected microchannel net

Developed thermal management solutions for microelectronic fields have got extensive attentions these years. In this study, an interconnected microchannel net (IMN) was developed via micro wire electrical discharge machining process to explore the feasibility of flow boiling heat transfer enhancement. The experiments for the IMNs with different structural parameters were conducted using deionized water as the coolant with variation in the heat flux and under an inlet subcooling of 40°C and constant mass flow rate of 180kgm−2s−1. Four IMN samples with different channel width, i.e., 0.25, 0.4, 0.55 and 0.7mm, were investigated to study the effects of channel size and heat flux on flow boiling characteristics, i.e., two-phase heat transfer, pressure drops and two-phase flow instabilities, were proceeded in order to optimize the structural design of the IMN. Test results indicate that the sample IMN-2 with the channel width of 0.4mm manifests the overall best flow boiling performance, which exhibits more favorable two-phase heat transfer and pressure drop characteristics, as well as the prominent superiority for the mitigation of two-phase instability. This reveals a highlight for the potential application in the flow boiling enhancement of microchannel heat transfer.

Application of cross recurrence plot for identification of temperature fluctuations synchronization in parallel minichannels

The temperature fluctuations occurring in flow boiling in parallel minichannels with diameter of 1 mm have been experimentally investigated and analysed. The wall temperature was recorded at each minichannel outlet by thermocouple with 0.08 mm diameter probe. The time series where recorded during dynamic two-phase flow instabilities which are accompanied by chaotic temperature fluctuations. Time series were denoised using wavelet decomposition and were analysed using cross recurrence plots (CRP) which enables the study of two time series synchronization.

Investigation on the effect of polymer in vertical oil-water two-phase flow using nonlinear analysis

This paper is devoted to investigating the effect of polymer on vertical oil-water two-phase flow using nonlinear analysis. For the comparison of flow characteristics in oil-water flows with different polymer dosages, an experiment in a vertical upward pipe with 20mm inner diameter (ID) is conducted to obtain fluctuating signals of dispersed oil-in-water slug flow (D OS/W), dispersed oil-in-water flow (D O/W) and very fine dispersed oil-in-water flow (VFD O/W) using vertical multi-electrode conductance array sensor (VMEA). With the aid of recurrence plot, the effect of polymer on flow structure variation is qualitatively characterized. Then, recurrence quantification analysis and multi-scale weight complexity entropy causality plane (MS-WCECP) are respectively utilized to uncover the determinism and instability of the complex oil-water system with polymer addition. The results indicate that the recurrence plot can be effectively used to identify flow pattern and reveal the differences of flow structures in oil-water flows with and without polymer. The multi-scale weight complexity entropy causality plane can satisfactorily reflect the instability of polymer-added oil-water two-phase flow.

High-Flux Thermal Management With Supercritical Fluids

A novel thermal management approach is explored, which uses supercritical carbon dioxide (sCO2) as a working fluid to manage extreme heat fluxes in electronics cooling applications. In the pseudocritical region, sCO2 has extremely high volumetric thermal capacity, which can enable operation with low pumping requirements, and without the potential for two-phase critical heat flux (CHF) and flow instabilities. A model of a representative microchannel heat sink is evaluated with single-phase liquid water and FC-72, two-phase boiling R-134a, and sCO2. For a fixed pumping power, sCO2 is found to yield lower heat-sink wall temperatures than liquid coolants. Practical engineering challenges for supercritical thermal management systems are discussed, including the limits of predictive heat transfer models, narrow operating temperature ranges, high working pressures, and pump design criteria. Based on these findings, sCO2 is a promising candidate working fluid for cooling high heat flux electronics, but additional thermal transport research and engineering are needed before practical systems can be realized.

Modeling analyses of two-phase flow instabilities for straight and helical tubes in nuclear power plants

The effects of system pressure, mass flux and inlet subcooling on two-phase flow instability for the test section consisted of two heated straight channels or two helical channels are studied by means of RELAP5/MOD3.3 and multi-variable frequency domain control theory. The experimental data in two straight channels are used to verify the RELAP5 and multi-variable frequency domain control theory results. The thermal hydraulic behaviors and parametric effects are simulated and compared with the experimental data. The RELAP5 results show that the flow stability increases with the system pressure, mass velocity, and inlet subcooling at high subcoolings. The frequency domain theory presents the same results as those given by the time domain theory (RELAP5). The effects of system pressure, mass velocity and inlet subcooling are simulated to find the difference between the straight and the helical tube flows. The RELAP5 and the multi-variable frequency domain control theory are used in modeling and simulating density wave oscillation to study their advantages and disadvantages in straight and helical tubes.