Onset Of Flow Instability Research Articles

Two-phase flow instabilities during microgravity flow boiling onboard the International Space Station

This study is an elaboration on flow instabilities observed during flow boiling experiments conducted onboard the International Space Station (ISS) as part of the Flow Boiling and Condensation Experiment (FBCE). During highly subcooled flow boiling, liquid backflow into the channel that rapidly condenses vapor within the channel was observed. Experiments were conducted with an enhanced sampling frequency of 30 Hz and an extended image sequence recording duration of at least 4 seconds at 500 frames per second to further investigate the instability. Identical experiments were performed both in microgravity onboard the International Space Station (ISS) and in Earth gravity during vertical upflow. Instabilities in microgravity are more severe than those in Earth gravity and, at their most severe, propagate to the channel's upstream region. Instabilities are observed within the channel when the intensity, I, which is dependent on inlet pressure fluctuations and mass velocity, exceeds 1.8 × 105 W/m2. Parametric trends of the frequency and amplitude of inlet pressure fluctuations during instability are examined, which reveal instabilities are most severe at low flow rates, high inlet subcoolings, and high heat fluxes. Various stability maps proposed in the literature are evaluated against the present database, and instabilities only manifest for subcooling numbers greater than 14. A subset of the database containing the Onset of Flow Instability (OFI) point is extracted to first evaluate correlations available in the literature and then develop a new correlation. The new correlation is applicable in both microgravity and Earth gravity conditions and predicts the database with a Mean Absolute Error (MAE) of 1.3%.

Pressure fluctuation analysis for identifying the CHF approaching condition during subcooled flow boiling through a mini-channel

This study presents a new method for predicting the Critical Heat Flux (CHF) approaching condition in subcooled flow boiling based on pressure fluctuation analysis. Through experimental investigations, water flow in an upward direction was conducted within a mini-channel with one side heated to analyze the sequential boiling phenomena, including ONB (Onset of Nucleate Boiling), OFI (Onset of Flow Instability), and CHF. While ONB was determined using the slope of the wall temperature–heat flux, OFI was determined by considering the pressure drop through the channel and outlet pressure fluctuation. By analyzing the inlet pressure fluctuations, a new condition called “the CHF approaching condition” was introduced, which serves as a CHF precursor. The CHF approaching condition was found to be approximately 90% of the CHF value across all experimental scenarios. The proposed method provides an effective tool for predicting and preventing CHF, thereby reducing the potential risk of damage to the heating surface.

Terrestrial solute fingering flow behind subsurface physical barriers during seawater intrusion

Blocking seawater intrusion in coastal aquifers through impervious subsurface physical barriers is a commonly used method, which creates a relatively closed hydrodynamic environment behind the barrier for the transport of land-sourced contaminants. This study investigated the phenomenon of terrestrial solute fingering flow behind subsurface physical barriers for the first time under field-scale hydrological and hydrogeological conditions. A prediction model for the onset of flow instability and the size of subsequent solute fingers was proposed based on the identified effects of these conditions. Once a modified Rayleigh number exceeded 4000, solute fingering flow behind subsurface physical barriers would occur, and the size of individual fingers followed a sigmoidal function growth with increasing Rayleigh number. Solute fingering flow resulted in a series of morphological changes in individual fingers from finger-like to foot-like with decreased salinity. The movement of solute fingers altered the trajectory and residence time of tracer particles, which led to an early particle arrival for those initially located behind. Compared to a no-finger situation, solute fingering flow accelerated the movement of tracer particles out of the suspended barriers with a reduced residence time by 45%. Shortened residence times and changed transport patterns as a result of solute fingering flow may have significant implications for the fate of terrestrial contaminants. Thus, caution should be exercised in the interpretation of field measurements on groundwater contamination in coastal aquifers if solute fingering flow behind subsurface physical barriers is expected, e.g., from the present prediction model.

Inception of macroscopic shear bands during hot working of aluminum alloys

Macroscopic shear bands (MSB) may develop during hot working of metallic materials. They are well-understood as a physical manifestation of flow instability, and the processing regimes wherein they form are well-charted. However, the microstructural transitions that occur between the onset of flow instability and MSB inception are not fully understood. In order to elucidate them, several aluminum alloy specimens were subjected to strip testing in a thermomechanical simulator (Gleeble™) at 298 K and 573 K. Prominent MSB were observed along the diagonals of the strip volume of only Aluminum-6 wt% Magnesium alloy specimen deformed at 573 K. Comparing the experimental grain morphology and crystallographic textures with those from plastic flow models revealed that MSB inception occurred only after ∼0.20 homogeneous plane strain deformation. However, classical flow instability was predicted at much smaller strain. This ‘delay’ was explained experimentally by showing that clusters of neighbouring severely deforming, fragmenting, mostly soft-oriented grains gradually developed due to lattice rotations along the specimen diagonals, and that MSB inception corresponded to the formation of a percolating network of such grains spanning the specimen. Further, clear experimental evidence revealed that differential dynamic recovery between hard- and soft-oriented grains was essential for MSB formation.

Effect of rotating magnetic field on the stability of thermocapillary flow in a gallium arsenide liquid bridge between unequal ends

In this study, we investigated the impact of a rotating magnetic field on the stability of a thermocapillary flow in a gallium arsenide liquid bridge (Prandtl number Pr = 0.068) situated between two unequal disks, considering two different scenarios with radius ratios of Γr = 0.98 and Γr = 0.60 for the upper heated disk. By utilizing linear stability analysis based on the Legendre spectral element method, we first identified the critical parameters of the onset of flow instability, including critical Marangoni number (Mac), dimensionless oscillation frequency (fc), and azimuthal wavenumber (m). Then, we employed kinetic energy budget analysis to uncover the underlying instability mechanism. For radius ratio Γr = 0.98, three transitions between axisymmetric steady flow and three-dimensional oscillatory flow in the narrow range of Taylor number Ta (8700≤Ta ≤ 9500) are observed; these transitions arise due to the interplay between the flow induced by rotating magnetic field and thermocapillary flow. For the Γr = 0.60 scenario, the rotating magnetic field is observed to significantly enhance the flow stability. Additionally, our analysis identifies four instability types dominated by the hydrodynamic mechanism. In the meantime, the thermocapillary mechanism also contributes to flow instability in the specific region of Taylor number Ta (1250≤Ta ≤ 8000) for radius ratio Γr = 0.98.

Deformation Mechanisms and Processing Maps for High Entropy Alloys (Presentation of Processing Maps in Terms of Zener-Hollomon Parameter): Review.

In this review paper, the hot compressive deformation mechanisms and processing maps of high-entropy alloys (HEAs) with different chemical compositions and crystal structures are analyzed. The stress exponent (n1) values measured from the series of compression tests for the HEAs performed at different temperatures and strain rates are distributed between 3 and 35, and they are most populated between 3 and 7. Power law breakdown (PLB) is found to typically occur at T/Tm ≤ 0.6 (where T is the testing temperature and Tm is the melting temperature). In AlxCrMnFeCoNi (x = 0-1) and AlxCrFeCoNi (x = 0-1) HEAs, n1 tends to decrease as the concentration of Al increases, suggesting that Al acts as a solute atom that exerts a drag force on dislocation slip motion at high temperatures. The values of activation energy for plastic flow (Qc) for the HEAs are most populated in the range between 300 and 400 kJ/mol. These values are close to the activation energy of the tracer diffusivity of elements in the HEAs ranging between 240 and 408 kJ/mol. The power dissipation efficiency η of the HEAs is shown to follow a single equation, which is uniquely related to n1. Flow instability for the HEAs is shown to occur near n1 = 7, implying that the onset of flow instability occurs at the transition from power law creep to PLB. Processing maps for the HEAs are demonstrated to be represented by plotting η as a function of the Zener-Hollomon parameter (Z = expQcRT, where R is the gas constant). Flow stability prevails at Z ≤ 1012 s-1, while flow instability does at Z ≥ 3 × 1014 s-1.

Experimental study on flow excursion instability of supercritical hydrocarbon fuel in scramjet regenerative cooling parallel channels

Flow instability of supercritical hydrocarbon fuel is a crucial issue in scramjet regenerative cooling structure. In this study, flow excursion instability and flow distribution in parallel tubes were experimentally studied for supercritical fluids. Two types of flow excursion occur in a single tube. Type I and Type II excursions, and they are corresponding to decreasing and increasing flow rate respectively. They can trigger flow maldistribution between parallel tubes and the hysteresis phenomenon of flow distribution. The effects of system parameters, including inlet temperature, system pressure, and heat flux, on flow distribution were analyzed. In addition, the relationship between flow excursion and the pseudo-critical interval proposed in the literature was established according to the heated tube outlet temperature at the onset of flow instability. Finally, the flow excursion instability boundary was obtained using two dimensionless parameters. These experimental results can provide helpful insight on the mechanism of Scramjet regenerative cooling.

Determination of heat flux leading to the onset of flow instability in MTR reactors

Abstract The prediction of heat flux leading to the Onset of Flow Instability (OFI) phenomena is an important consideration in the design of Material Testing Reactors (MTR) due to the possibility of flow excursion during postulated accident. From the thermal-hydraulic point of view, OFI is the critical phenomenon limiting MTR reactor power. In a previous work, an empirical correlation is developed to predict the subcooling at OFI in narrow vertical rectangular channels simulating a coolant channel of MTR. In the present work, an innovative model to determine the heat flux leading to OFI in MTR reactors is introduced based on the previous correlation. The developed model gives a very low deviation of only 1.65% from the experimental data of Whittle & Forgan that covers a wide range of MTR operating conditions. The heat flux leading to OFI is also predicted by both Whittle & Forgan and Fabrega correlations for comparison. The present model is then applied on the IAEA 10 MW MTR generic reactor to predict the Best-Estimate (BE) and Best-Estimate-Plus-Uncertainty (BEPU) Onset of Flow Instability Ratio (OFIR) and the power leading to OFI as well as the bubble detachment parameter under different coolant velocities and inlet temperatures. The model is also used to predict both the OFIR and bubble detachment parameter in the reactor under unprotected Loss-of-Flow transient for exponential flow decay with a time constant of 1.0 s (fast LOFA), 10, 15 and 25 s (slow LOFA) from a power level of 10 MW. For BEPU calculation, a combined statistical method with direct propagation of errors is adapted to treat the uncertainty factors for fuel fabrication and measured parameters in the BEPU calculation. The model results is analyzed and discussed.

Numerical Analysis of Water Jet Injection in the Draft Tube of a Francis Turbine at Part Load Operations

Abstract The off-design operation of Francis turbines results in the onset of flow instabilities. These instabilities lead to severe pressure pulsations, power swings, fatigue damage, and torque fluctuations in the turbine unit. Axial water jet injection in the draft tube is a relatively recent method proposed to reduce the detrimental effects of flow instabilities on turbine performance. However, its efficacy at different operating points needs to be ascertained before implementing in actual prototype turbines. This work reports the findings of numerical investigations performed with water injection at three different part-load conditions. These operating points represent distinct flow regimes in the draft tube. The effect of water injection on the velocity and pressure fields in the draft tube is investigated. The results indicate that the water jet strongly influences the turbine performance at part-loads involving a precessing vortex rope. However, little influence of water jet is observed at deep part-load operation. The interaction of the jet with the draft tube bend is also investigated. The results show that the amount of water jet needs to be cautiously controlled as higher water jet injection impacting the bend may deteriorate the performance. The influence of water jet injection on the pressure recovery, power output, and efficiency of the turbine unit is also reported.

Steady state thermal hydraulic modelling of WWR-S tank-in-pool research reactor for the purpose of its power upgrading

Abstract This paper presents a newly developed steady state core thermal hydraulic model (named SSTH-RR10 model) for upgrading the Egyptian first Research Reactor (ETRR-1), from its original power of 2 MWth to a higher level of 10 MWth, by considering different types of nuclear fuels. The SSTH-RR10 model is capable to predict and calculate, by means of a developed computer program, all the steady state thermal hydraulic parameters for the defined core configuration for each fuel type at 10 MWth. Three different fuel types were investigated: the reference fuel EK-10 rod type, the MTR plate type, and the IRT-4M ducted type. For each fuel type, the distribution of central fuel, clad, and coolant temperatures for average and hot channels of the core were predicted in the axial direction. Power distributions and pressure gradients were predicted as well. Moreover, the program calculates the safety limits and margins against the critical phenomena encountered such as the Onset of Nucleate Boiling (ONB), Departure from Nucleate Boiling (DNB), and the Onset of Flow Instability (OFI). Results of the SSTH-RR10 program for benchmarks of powers of 2 and 10 MWth are verified by comparing it with the published results of the International Atomic Energy Agency (IAEA), and those published for other programs such as PARET code, and very good agreement is found. The safety margins against ONB and DNB were evaluated in which the minimum DNB ratio was found to be about 3.1, which gives a sufficient margin against the DNB. The present work gives confidence in the model results and applications.

Experimental study of heat distribution in a Non-uniform heat source of a natural circulation loop

An experimental investigation has been carried out on heat distribution in a non-uniform heating channel of a natural circulation (NC) loop under single-phase condition. A heated channel with a truncated axial cosine heat flux distribution was used in the NC circuit and the non-uniformity of the channel is defined in its outer dimension which is convex in shape. The NC experimental loop similar to the primary loop of a Pressurized Water Reactor (PWR) was first reviewed and adjusted to determine the various critical positions for the thermocouples along the heated and flow channels. The experimental procedures involve setting the heating channel inlet temperature at different heating power, followed by observation of the thermal hydraulic responses. When a stable flow is achieved at each inlet parameters, data are recorded till the onset of flow instability. The findings through the results obtained, extend the knowledge on adoption of the NC systems and contribute to a better understanding of the impact of irregular heat distribution on heat transfer capability of the NC loop.

Dimensional analysis on incipient flow instability in one-side high heat loaded rectangular flat heat sink under sub-cooled flow boiling conditions

The onset of flow instability (OFI) is a major threat to the operation of a fusion power plant. In particular, because the divertor is loaded with a high heat flux of up to 20 MW/m2, it may be vulnerable to OFI. Therefore, in this study, the OFI of a one-side heated flat heat sink was experimentally analyzed. When the effect of the system parameters on the OFI was analyzed, the faster the vapor inside the channel could be removed, the higher was the OFI that was experimentally recorded. As the flow rate and degree of subcooling increase, the OFI increases because it induces an enhancement of the forced convective heat transfer performance and a rapid coding rate, respectively. However, when the pressure is increased, the latent heat and liquid surface tension are reduced; thus, boiling occurs at a lower heat rate, which leads to a decrease in OFI. When the prediction performance of the existing OFI correlations developed under the subcooled flow boiling condition was evaluated, it showed a tendency to overpredict this experimental value significantly. This is because the evaluated correlations were developed based on relatively low heat flux conditions and narrow rectangular channels. Therefore, in this study, we developed a new OFI correlation optimized for experimental values using an artificial intelligence (AI) regression method. Because this correlation can be utilized under one-side high heat load conditions, it is expected that it will be especially useful when establishing the operation strategy of the cooling system of a fusion power plant.

Surfactant effects on microfluidic extensional flow of water and polymer solutions

Surfactants are often added to particle suspensions in the flow of Newtonian or non-Newtonian fluids for the purpose of reducing particle-particle aggregation and particle-wall adhesion. However, the impact on the flow behavior of such surfactant additions is often overlooked. We experimentally investigate the effect of the addition of a frequently used neutral surfactant, Tween 20, at the concentration pertaining to microfluidic applications on the entry flow of water and three common polymer solutions through a planar cavity microchannel. We find that the addition of Tween 20 has no significant influence on the shear viscosity or extensional flow of Newtonian water and Boger polyethylene oxide solution. However, such a surfactant addition reduces both the shear viscosity and shear-thinning behavior of xanthan gum and polyacrylamide solutions that each exhibit a strong shear-thinning effect. It also stabilizes the cavity flow and delays the onset of flow instability in both cases. The findings of this work can directly benefit microfluidic applications of particle and cell manipulation in Newtonian and non-Newtonian fluids.

Onset of flow instability during subcooled flow boiling of seawater in a vertical annulus

The present study investigates the onset of flow instabilities during subcooled flow boiling of 3.5 wt% artificial seawater in a vertical annulus. All flow instability experiments are conducted by decreasing mass flux gradually under a specific condition of inlet temperature and heat flux. The results reveal that, comparing with de-ionized water, the artificial seawater is more susceptible to the flow instability and demonstrates 12–79% larger mass fluxes for onset of flow instability (OFI) due to the unique two-phase flow pattern of densely bubbly flow or bubble foam flow during the subcooled flow boiling of artificial seawater. After OFI, unstable bubble foam with slug bubbles resulting from foam ruptures is revealed in artificial seawater resulting in significant pressure drop oscillations at lower inlet temperature of 55 °C, 65 °C and 75 °C. The existing criteria or correlations, developed based on the data of de-ionized water, may not predict the OFI for artificial seawater. A new empirical correlation to predict the mass flux at OFI is proposed. A two-phase flow instability map is established on the plane of phase change number versus subcooling number.

Onset of flow instability with one-side heated swirl tube for fusion reactor safety

In this study, the onset of flow instability (OFI) heat flux of a one-side heated swirl tube is experimentally investigated. The OFI heat flux means the minimum heat flux that can cause flow instability by the vapor generated in the flow path. An analysis of the effect of system parameters on the OFI heat flux indicates that as the pressure increases, the bubble size decreases. Therefore, the void fraction decreases and, consequently, the OFI heat flux tends to increase. Similarly, the higher the flow rate and degree of subcooling, the faster the vapor can be removed; thus, the OFI heat flux increases. In addition, the prediction performances of the existing OFI correlations developed under the subcooled flow-boiling condition are evaluated. Therefore, although the Wang correlation indicates the lowest error rate, it yields a high mean absolute error rate of 87.75%. Thus, it is difficult to predict the OFI heat flux of a one-side heated swirl tube using the existing OFI correlations. Therefore, in this study, a new correlation is developed using a Python code created by employing an artificial intelligence regression method. The developed correlation incorporates the impact of one-side heating, swirl tape, mass flow rate, subcooling, and pressure (mean absolute error = 12.17%, root mean square error = 14.99%).

Experimental study of thermal limits with one-side heated smooth channel for plasma facing component safety

In order to stably operate the equipment inside the tokamak, which is loaded with a heat flux of several MW m−2 under the one-side heating condition, it is necessary to thoroughly prepare for various thermal engineering limits that may occur under the high heat flux load condition. In this study, we have experimentally explored critical heat flux (CHF) and onset of flow instability (OFI), which are considered potential threats in a DEMO fusion power plant. Specifically, the effect of system parameters on CHF was investigated. The results indicate that with an increase in subcooling and mass flux, the CHF increased, as it induced a faster bubble condensation near the CHF. As the system pressure increased, the CHF also increased. This is because the bubble size reduction effect was dominant in the pressure range of 1–10 bar. Most of the existing CHF correlations could evaluate the CHF with reasonable accuracy of within 25%; especially, the Boscary CHF correlation yielded the highest accuracy with an average error of 12%. Similar to CHF, OFI, which is a measure of the sudden fluctuations in the system pressure caused by a large amount of vapor generated due to the high heat flux, tended to increase as the subcooling, mass flow rate, and system pressure increased. Most of the existing OFI correlations yielded large error rates (more than 135%) as these correlations were primarily developed for micro-channels. Therefore, in this study, a new OFI correlation was developed using a Python code, in combination with an artificial intelligence (AI) regression method. The developed correlation can be used in the cooling system design of tokamaks, which involve a high-heat load condition on one-side of the reactor.

New correlation to predict the onset of flow instability (OFI) heat flux of one-side heated hypervapotron channel for fusion reactor application

In this study, the onset of flow instability (OFI) of the hypervapotron channel was experimentally explored under one-side high heat load conditions. The OFI was detected based on the heat flux () where the standard deviation of the inlet pressure rapidly increased. Moreover, the effect of the system parameters was analyzed, showing that the OFI increased as the inset subcooling and mass flow rate increased. The reason is the high subcooling and flow rate that rapidly condense the vapor, which is the leading cause of OFI. Unfortunately, because most of the OFI correlations in previous studies were developed based on micro-channels, these correlations could not predict the hypervapotron OFI properly under one-side high heat load conditions. Finally, the authors found that the Reynolds number and Jakob number were inversely proportional to the ratio of the OFI heat flux () and saturation heat flux (). Furthermore, a new correlation was developed using a PYTHON code and an artificial intelligence technique (mean absolute error rate = 12.89%, root mean square error rate = 14.81%).

Stagnation points control chaotic fluctuations in viscoelastic porous media flow

Viscoelastic flows through porous media become unstable and chaotic beyond critical flow conditions, impacting widespread industrial and biological processes such as enhanced oil recovery and drug delivery. Understanding the influence of the pore structure or geometry on the onset of flow instability can lead to fundamental insights into these processes and, potentially, to their optimization. Recently, for viscoelastic flows through porous media modeled by arrays of microscopic posts, Walkama et al. [D. M. Walkama, N. Waisbord, J. S. Guasto, Phys. Rev. Lett 124, 164501 (2020)] demonstrated that geometric disorder greatly suppressed the strength of the chaotic fluctuations that arose as the flow rate was increased. However, in that work, disorder was only applied to one originally ordered configuration of posts. Here, we demonstrate experimentally that, given a slightly modified ordered array of posts, introducing disorder can also promote chaotic fluctuations. We provide a unifying explanation for these contrasting results by considering the effect of disorder on the occurrence of stagnation points exposed to the flow field, which depends on the nature of the originally ordered post array. This work provides a general understanding of how pore geometry affects the stability of viscoelastic porous media flows.

Improvement of the subcooled boiling model for the prediction of the onset of flow instability in an upward rectangular channel

The MARS code has been assessed for the prediction of onset of flow instability (OFI) in a vertical channel. For assessment, we built an experiment database that consists of experiments under various geometry and thermal-hydraulic condition. It covers pressure from 0.12 to 1.73 MPa; heat flux from 0.67 to 3.48 MW/m2; inlet sub-cooling from 39 to 166 °C; hydraulic diameters between 2.37 and 6.45 mm of rectangular channels and pipes. It was shown that the MARS code can predict the OFI mass flux for pipes reasonably well. However, it could not predict the OFI in a rectangular channel well with a mean absolute percentage error of 8.77%. In the cases of rectangular channels, the error tends to depend on the hydraulic diameter. Because the OFI is directly related to the subcooled boiling in a flow channel, we suggest a modified subcooled boiling model for better prediction of OFI in a rectangular channel; the net vapor generation (NVG) model and the modified wall evaporation model were modified so that the effect of hydraulic diameter and heat flux can be accurately considered. The assessment of the modified model shows the prediction of OFI mass flux for rectangular channels is greatly improved.

A Numerical Study on the Exact Onset of Flow Instabilities in Thermo-Solutal Marangoni Convection Driven by Opposing Forces in a Half-Zone Liquid Bridge under Zero Gravity

The theoretical onset of the Marangoni convection instability occurring in a half-zone liquid bridge of the floating zone (FZ) growth of SixGe1–x has been investigated by numerical simulations. A half-zone model of the liquid bridge between a cold (bottom plane) and a hot (top plane) disk is considered. The highest Si concentration is on the top of the liquid bridge. Therefore, the driving forces for thermal and solutal Marangoni flows are in opposite directions in this configuration. The nonlinear equilibria and periodic orbits are obtained using the Newton–Krylov method, and their stability is analyzed with global linear stability analysis. A flow bifurcation analysis is conducted. The results show that when the solutal Marangoni number, MaC, ≤360, the primary flow bifurcation is from 2D axisymmetric to 3D steady flow. When MaC>360, the 2D axisymmetric flow becomes weakly periodic 2D axisymmetric, and eventually becomes chaotic. The critical thermal Marangoni number is monotonically increasing as MaC increases, and the flow is 2D axisymmetric when the solutal Marangoni forceis dominant. The stability evaluation for various thermal Marangoni number values at MaC=893 reveals that the onset of transition is from steady to periodic. However, the reverse transition does not occur at the same Marangoni number value. The simulation results show that the hysteresis behavior of the flow field exhibits an approximately 1% difference between two critical thermal Marangoni numbers. The present study suggests that the control parameters of FZ crystal growth should be smaller than the calculated critical values to avoid any flow-induced disturbances.