Flow Rate Oscillations Research Articles

Experimental studies on mechanism whereby premixed chamber length and equivalence ratio collaboratively influence self‐excited thermoacoustic instability

AbstractSelf‐excited thermoacoustic instability (SETAI) is a dangerous phenomenon in combustion equipment. While it is widely acknowledged that SETAI behavior is determined by the couple between pressure and heat release oscillation, their phase difference is difficult to predict, which impedes the development of SETAI control technology. With the aim of passive control technology development, this paper conducted experiment on a premixed hedge combustor to explore the mechanism whereby premixed chamber length (LP) and equivalence ratio (φ) collaboratively influence SETAI behavior. Results showed LP mainly affects the pressure mode shape within premixed chamber and consequently alters the phase difference between pressure and flowrate oscillation at combustion chamber inlet. Changing φ gives rise to different reaction time‐lag (τ), thus altering the phase difference between flowrate and reaction heat release oscillation. By introducing this flowrate oscillation, how LP and φ collaboratively determine phase difference between pressure oscillation and heat release oscillation was clarified. The mechanisms identified in this study are consistent with the emerging rationalization of the factors contributing to SETAI, and also provides better understanding on Rayleigh criterion and guidance for SETAI control. With further work on heat release and flow rate measurement, as well as the development on τ description, SETAI can be better predicted and controlled.

Applicability research and experimental verification based on the coupling of turbulence model and mesh types to capture jet characteristics

Through the experimental verification, the accuracy of the cavitation jet feature captured by the coupling numerical calculation with different mesh types and different turbulence models is evaluated. Firstly, we built a high-speed jet experimental platform, and a high-speed camera and a paperless recorder are used to record the pressure, flow rate, and jet cloud oscillation of the jet pump in different states. The oscillation period and length of the jet cloud under different cavitation numbers are analyzed by the gray value method. The numerical results are compared with the experimental results. It is found that after the numerical calculation converges, the states of the jet cloud calculated by K–O and LES coupled with different meshes are periodically oscillating, while K-E does not. However, the maximum jet cloud lengths calculated by K-E and K–O turbulence models under different mesh types are nearly the same. Overall, the oscillation period calculated by the LES model is more in line with the experimental results, but its amplitude and maximum length are more different from the experimental results. In terms of capturing the jet cloud, critical pressure ratio, turbulent state and vapor volume fraction, the accuracy order of numerical calculation results are P–H (polyhedron-hexahedron mesh) ≈ P (polyhedron mesh) > H-T (hexahedral-tetrahedral mesh) > T (tetrahedral mesh), P–H ≈ P > T > H-T, P–H ≈ P > H-T > T and P–H > P > T > H-T, respectively. This research can provide more appropriate solutions for different concerns, as well as guidance for improving the accuracy and efficiency of simulations, and provides a basis for optimizing and developing specialized mesh types and turbulence models.

Delay Compensation in a Feeder-Conveyor System Using the Smith Predictor: A Case Study in an Iron Ore Processing Plant.

Conveyor belts serve as the primary mode of ore transportation in mineral processing plants. Feeders, comprised of shorter conveyors, regulate the material flow from silos to longer conveyor belts by adjusting their velocity. This velocity manipulation is facilitated by automatic controllers that gauge the material weight on the conveyor using scales. However, due to positioning constraints of these scales, a notable delay ensues between measurement and the adjustment of the feeder speed. This dead time poses a significant challenge in control design, aiming to prevent oscillations in material levels on the conveyor belt. This paper contributes in two key areas: firstly, through a simulation-based comparison of various control techniques addressing this issue across diverse scenarios; secondly, by implementing the Smith predictor solution in an operational plant and contrasting its performance with that of a single PID controller. Evaluation spans both the transient flow rate during step change setpoints and a month-long assessment. The experimental results reveal a notable increase in production by 355 t/h and a substantial reduction in flow rate oscillations on the conveyor belt, evidenced by a 55% decrease in the standard deviation.

A Novel 1D Approach for Modelling Gas Bladder Suppressors on the Delivery Line of Positive Displacement Pumps

This paper concerns the utilisation of a gas bladder hydraulic suppressor to mitigate oscillations in the delivery flow rate of positive displacement machines. The research focuses on two primary objectives: first, the experimental validation of the potential of this solution and second, the formulation of a one-dimensional fluid dynamic model for the suppressor. The foundational framework of the fluid dynamic model is based on the equations governing fluid motion with a one-dimensional approach. To accurately depict the fluid dynamics within the suppressor, a unique approach for determining the speed of sound was incorporated, and it implemented the instantaneous cross-sectional area and the inertial effect of the bladder. This paper is a development of a previous work to also investigate the positioning along the delivery pipe of the suppressor with respect to the pump. The study presents the performance of the suppressor and points out the effects of its relative position with respect to the pump that becomes particularly relevant at high speeds.

Analysis of the Flow Structure in a Supersonic Channel with Cavity

The results of numerical study of supersonic flow in a channel with cavity are given. The calculated oscillation spectra are analyzed using the fast Fourier transform. Two types of oscillatory modes can be distinguished in the resulting periodic self-oscillatory regime. The first type of the modes corresponds to acoustic vibrations caused by the passage of sound waves along the cavity and calculated using the modified Rossiter formula. The second type of the modes corresponds to the frequencies of flow-rate oscillations caused by mass transfer between the cavity and the external flow. It is shown that the flow structure is modified when fuel is supplied in front of the cavity. Active combustion occurs in the layer of mixing fuel and oxygen from air. The flow pattern demonstrates the onset of Kelvin–Helmholtz instability on the interface between the main flow and the reacted gas. It is shown that an increase in the supplied fuel pressure leads to a decrease in the oscillation frequency and an increase in the characteristic size of oscillations.

Experimental and numerical investigation of transverse combustion instability in a rectangle multi-injector rocket combustor

Rocket engines are prone to suffer from transverse combustion instabilities, manifesting as periodic oscillations in pressure and heat release rate. Self-excited transverse instabilities in a rectangle multi-injector rocket combustor powered by n-C12H26 and O2, are investigated experimentally and numerically in this paper. Transverse combustion instabilities are captured during repeatable experiments, while dynamic characteristics in the combustion chamber and driving mechanisms of instabilities are analyzed via numerical simulation based on stress-blended eddy simulation. Dynamic mode decomposition analysis reveals coupling behaviors between the transverse mode in the combustion chamber and longitudinal mode in the oxidizer post, resulting in propellant mass flow rate oscillations. Substantial evidence demonstrates that the intensities of heat release rate near the end walls are higher than that detected around the chamber center due to its intense coupling to the local first transverse (1W) acoustic mode. Flow field animations indicate strong deflection of oxygen posts and liquid particles by a transverse moving gas mixture leading to flame interactions and collision between adjacent share injectors. The propellant injected into the chamber does not burn immediately and is transported to the combustor end wall. Propellant mixing performance is significantly enhanced during interactions between acoustic pressure and end wall, causing a sudden heat release near 1W pressure anti-node, thereby strengthening pressure oscillations. Finally, a combustion instability driving mechanism associated with propellant mass flow rate oscillations, and interactions between unsteady acoustic pressure and heat release rate is proposed. The insights obtained from this work can aid comprehension of transverse combustion instabilities encountered by rocket engines.

Effect of inlet mass flow rate oscillation on simplex swirl injector flow based on VOF-to-DPM model

The flow characteristics of injectors with oscillations play a significant role in the mechanism of combustion instability. And combustion instability reduces engine reliability. Therefore, an efficient VOF-to-DPM model-based method is proposed to evaluate the effect of inlet mass flow rate oscillation on the injectors in liquid rocket engines. In this method, the VOF-to-DPM model, which can portray the complete spray process with less computational cost than the VOF method, is adopted to simulate the whole spray process of the injector under oscillations. The proposed method is first verified by the reported simulation work. Then, it is applied to study the flow characteristics (response and spray characteristics) of the simplex swirl injector under different inlet mass flow rate oscillations. The result of the proposed method indicates that the simplex swirl injector is sensitive to the oscillation of 200–300 Hz, which is consistent with the reported experimental work. Additionally, inlet oscillation frequency dominantly increases the phase difference while frequency and amplitude jointly determine the amplification or attenuation of the outlet mass flow. The injector with 250 Hz has a higher velocity of downstream recirculation and smaller droplets than other oscillation frequencies, which may provoke combustion instability. The numerical results are beneficial to injector design and reduce the possibility of combustion instability by active regulation. The proposed method contributes to revealing the mechanism of combustion instability and improving the reliability of liquid rocket engines.

Studies of liquid metal boiling in fuel assemblies of fast reactors in accident conditions

The paper presents the results of experiments carried out at IPPE JSC on heat transfer and circulation stability at sodium–potassium alloy boiling on models of single fuel assemblies and in a system of parallel fuel assemblies, taking into account the influence of various factors on the boiling process at coolant natural convection.The obtained experimental data have shown that the stable nucleate boiling in the simulated fuel assemblies is observed only in a restricted region of heat fluxes, its transition to unstable pulsating slug boiling is determined by various factors, the transition boundary from nucleate boiling to slug, annular-dispersed and dispersed regimes of two-phase liquid metal flow in fuel rod assemblies are approximated by simple dependences.The onset of an oscillation process at coolant boiling in one of the parallel fuel assemblies leads to an antiphase oscillation process in another fuel assembly, the hydrodynamic interaction of the loops in various fuel assemblies may cause a significant increase in the amplitude of coolant flow rate oscillations (“resonance” of flow rate oscillations) and “choking” or inversion of the coolant flow rate in the loops, the temperature rise in the coolant and fuel rod cladding (the effect of inter-channel instability) and, eventually, a critical heat flux (CHF).In the assembly with low surface roughness of the rod simulators, evolution of an unstable (slug) regime with sharp coolant flow rate oscillations and overheating of the simulator wall results in a CHF, . For the fuel rod simulators with industrially-manufactured surface roughness, due to the appearance of a liquid film on the surface of the simulators, a transition from the unstable slug regime to the stable annular-dispersed one is observed.For sodium boiling in the fuel assembly model with a sodium “cavity“ located above the reactor core, which is designed for compensation of the positive sodium void reactivity effect in case of boiling onset, the feasibility of long-term sodium cooling of fuel rod simulators in the fuel assemblies has been demonstrated for these conditions.The data on liquid metal boiling heat transfer in the assemblies were generalized a cartogram of the flow regimes for a liquid metal two-phase flow in the assemblies was constructed, which significantly differs from a cartogram of the flow regimes for water. The paper presents the results of the calculated and experimental data comparison.

Tangential Shear Stress in Oscillatory Flow of a Viscoelastic Incompressible Fluid in a Plane Channel

The problems of oscillatory flow of a viscoelastic incompressible fluid in a plane channel are solved for a given harmonic oscillation of the fluid flow rate. The transfer function (amplitude–phase frequency response) is determined. Using this function, the effect of the acceleration oscillation frequency and the relaxation properties of fluid on the ratio of the tangential shear stress on channel wall to the velocity averaged over the channel cross-section (cross-sectional velocity) is determined. It is shown that the viscoelastic properties of fluid, as well as its acceleration, are the limiting factors for using the quasi-stationary approach. The found formulas for determining the transfer function for viscoelastic fluid flow in the case of non-stationary stream make it possible to determine the dissipations of mechanical energy in a non-stationary flow of the medium which are of importance for calculation of the control of hydraulic and pneumatic systems.

One-Dimensional Fluid Dynamic Modeling of a Gas Bladder Hydraulic Damper for Pump Flow Pulsation

Positive displacement pumps produce pressure ripple that can be reduced with the attenuation of the generated flow ripple. This paper presents the application of a gas bladder hydraulic damper with the aim of reducing the oscillations of the delivery flow rate of positive displacement machines. This work is focused on the development of a 1D fluid dynamic model of the damper, which is based on the fundamental fluid motion equations applied for a mono-dimensional flow. In order to represent the fluid flow inside the damper, a particular evaluation of the sound speed has been implemented. Experimental tests have been performed involving an axial piston pump with the damper installed in the delivery pipe to validate the model; tests were carried out at different pump working conditions and with different gas precharge pressure of the damper. The test results confirmed the effectiveness of the device, and the comparison with numerical results demonstrated a good agreement. Simulations have been carried out to investigate the influence of various parameters on damper effectiveness.

Visualized experimental study on pressure drop oscillations of a natural circulation system under low pressure condition

By using deionized water, a visualized experimental study on flow instability phenomenon has been performed in the natural circulation system under condition of low pressure 0.14–0.25 MPa and inlet temperature 45–65 °C.The two-phase flow pattern is recorded by high speed camera and the types of flow instability under different heating power are identified and studied. Besides, the influence of heating power, inlet pressure and temperature on the pressure drop oscillations (PDO) is researched. The experimental results in this paper show that the flow has experienced three stages which are single-phase, sub-cooled boiling two-phase and unstable saturated boiling two-phase, with the increase of heating power. For unstable two-phase flow, both density wave oscillations (DWO) and pressure drop oscillations (PDO) with superimposed DWO are observed. Meanwhile, it is found that the mass flow rate oscillation amplitude of PDO increases while the period of that decreases when increasing the heating power. The parameters analysis results show that the critical heating power of PDO increases with system pressure and decreases with inlet temperature of test section. The experimental results can help to understand the flow instability characteristics and improve the flow stability in the natural circulation loop under low pressure condition.

Oscillatory valve effect on pressure drop oscillation in microchannel cooling system

Pressure drop oscillation (PDO) is one of the most common flow instabilities occurred in microchannel cooling systems, which could result in partial dry-out, heat transfer deterioration, and temperature fluctuation. In this study, the effects of three oscillatory electronic valve settings (square, sinusoidal, and sawtooth) on mass flow rate and PDO characteristics are examined numerically and experimentally. A one-dimensional lumped model is developed for the microchannel cooling system, which indicates that a specific range of valve oscillation amplitude and frequency can reduce PDO amplitude significantly. The PDO amplitude can be reduced by 85 % when the valve amplitude is 30 % and the frequency is 0.1 Hz. A smaller valve oscillation amplitude shows a weaker effect on the suppression of PDO. The square function has a better PDO suppression than the sinusoidal and sawtooth functions under the same pulsating valve amplitude and frequency. Experimental results demonstrate a similar effect of valve setting on PDO, for instance, a valve setting oscillates from 20 % to 80 % and a fixed frequency of 0.05 Hz can reduce pressured drop amplitude from 3 kPa to 0.5 kPa, and reduce the mass flow rate oscillation amplitude from 60 ml/min to 10 ml/min. The overly large oscillation amplitude of the valve setting could increase the oscillation amplitude of the mass flow rate and the overall pressure drop significantly. Thus, both the pressure drop through the microchannel evaporator and the valve should be considered in the selection of the oscillatory valve parameters. Based on the analysis of this study, an active control strategy could be developed to suppress flow oscillation and optimize the overall cooling performance using the oscillatory valve setting.

Mathematical modeling of hydrodynamic resistance in an oscillatory flow of a viscoelastic fluid

The problems of the oscillatory flow of a viscoelastic fluid in a flat channel for a given harmonic oscillation of the fluid flow rate are solved based on the generalized Maxwell model. The transfer function of the amplitude-phase frequency characteristics is determined. Using this function, the dependence of the hydrodynamic resistance on the dimensionless oscillation frequency is studied for various values of the elastic Deborah number and the concentration of the Newtonian fluid. It is shown that in an oscillatory flow of a viscoelastic fluid, the hydrodynamic resistance decreases depending on the Deborah number. With an increase in this number, the decrease becomes more pronounced than before. This effect allows us to evaluate the hydrodynamic resistance for a given law; the change in the longitudinal velocity averaged over the channel section and for the motion of a viscoelastic fluid in an unsteady flow allows us to determine the dissipation of the mechanical energy of the medium, which is important in the regulation of hydraulic and pneumatic systems.

Large eddy simulation of combustion instabilities in multiple combustors densely interacting with each other

Large eddy simulation (LES) of combustion instabilities in the can-annular gas turbine combustors are performed. Naturally occurring thermoacoustic instability is simulated in the can-annular combustors for the first time, and the resulting dynamic flames are investigated. Premixed swirl flames are generated in the two combustors, and the combustor-combustor interaction of pressure wave occurs in the cross-talk connecting each combustor. Both symmetric and asymmetric conditions are considered for explaining their effects on flame perturbation and instability mode during the operation. In the symmetric inlet condition, longitudinal pressure wave induces the flame fluctuation resulting in a push-push mode in the combustion system, which indicates that the instability phases in two combustors are identical. In the asymmetric case, an inlet velocity pulsation with an opposite phase of combustion instability is applied, which results in the suppression of instability and the attenuation of acoustic energy transferred to the whole system through the cross-talk. This work expands the scope of LES together with high-fidelity turbulent combustion models for addressing the combustion instability in the can-annular combustors. The results indicate that asymmetry in flow rate oscillations provides an effective actuation for the attenuation of combustion instabilities.

Experimental study on heat transfer characteristics of LBE flow in a vertical tube under static and rolling conditions

The heat transfer characteristics of liquid metals get much attention because of their low Prandtl number. In this paper, the heat transfer characteristics of a heavy liquid metal (LBE) in a vertical circular test section under static and rolling motions are experimentally investigated. The results show that the additional forces induced by rolling motion cause a periodic change in the flow rate of the forced circulation. The rolling motion reduces the mean circulation flow rate, especially in the low flow rate region. The cycle-averaged heat transfer characteristics are significantly enhanced in the low Peclet number region (<2500). In the high Peclet number region (>2500), the cycle-averaged heat transfer characteristics are not significantly different compared to those under static condition. At low flow rates, flow rate oscillations may lead to counterflow. The amplitude of the Reynolds number oscillation, the cycle-averaged Reynolds number, the roll period and the roll amplitude jointly determine the cycle-averaged Nusselt number. The correlations under static conditions cannot predict the Nussle number under rolling conditions well. Therefore, a correlation is developed to analyze the heat transfer characteristics of LBE under rolling conditions. This study may contribute to the design and safety analysis of liquid metal heat transfer when applied to moving conditions.

Flame stabilization of liquid oxygen/kerosene bi-swirl injector at elevated pressure

Throttleable liquid-propellant rocket engines have wide range and economical applications. However, low frequency combustion instabilities were often excited in the oxygen-rich preburners of liquid oxygen (LOx) and kerosene rocket engines as the engines throttling. To understand the mechanisms and eliminate these instabilities, the detail features of the instabilities need to be described. Bi-swirl injectors with deep recess are used in the oxygen-rich preburners to spray propellants. This paper presents the non-premixed flame dynamic of a LOx/kerosene bi-swirl injector under high-pressure conditions. High-speed cameras captured OH* and CH* chemiluminescence images of the oxygen-rich flame for the first time. Results indicate that a stable flame was anchored inside the recess region and featured by a V-shaped, swirl flame. The flame-spreading angle under supercritical conditions was slightly larger than the liquid spray angle. Intense low-frequency combustion instabilities at 16–37 Hz were developed when the LOx/kerosene mixing ratio (ROF) was greater than threshold value. The flame oscillation process was coupled to the oscillations of the mass flow rate and the ROF. As the oxygen-rich level decreased, the flame oscillation pattern gradually shifted from flame blow-out/auto-ignition mode to flame length oscillation mode. Both modes were characterized by violent flashback and blow-off movement of the flame and large pressure variations. The instability was one type of “lean-limit” instability. A discussion of the oscillation periods qualitatively explains why the oscillation periods were found to decrease as the ROF decreases. The axial velocity of the kerosene is a controlling parameter of the oscillation period.

Effect of orientation on the steady-state performance of vapor compression cycles

• Sinusoidal response of vapor compression cycle to inclination changes covering 360°. • Observed varying degress of suction line mass flow rate oscillations at specific range of angles for three different mass fluxes. • Hypothesis of charge migration due to inclination changes presented and supported through comparison of experimental and modeling results. The number of vapor compression cycles in microgravity is still small, especially relative to the increasing rate of developments supporting space exploration. Ground-based inclination testing helps to understand the effect of gravity on two-phase cycles and can increase confidence into their operability in space. An investigation was conducted by operating a vapor compression cycle at different orientations, always for a long enough time to achieve steady-state operation. Experiments were conducted with R134a and R1234ze(E) across three different mass fluxes. In general, both refrigerants reacted similarly to inclination changes. Significant mass flow rate oscillations were observed in the suction line due to inclination changes in a transient study. These had larger amplitudes at lower flow rates. The steady-state conditions plotted as a function of the inclination angle for one set of control parameters resulted in sinusoidal behaviors with varing “amplitudes”. A semi-mechanistic heat exchanger model was leveraged to track the hydrostatic pressure drop of all coil-segments as the test rig was rotated. Based on comparing experimental and model results, it is hypothesized that changes in orientation led to an accumulation of refrigerant in the evaporator causing a higher pressure drop not captured by the model.

Behaviors of type-I instabilities in natural circulation with a heating channel under ocean conditions

Characteristics of two-phase natural circulation and density wave instability with a single-heating channel under ocean conditions were studied by using PNCMC PROGRAM (Program for Natural Circulation under Motion Condition). The superposition laws of oscillations of both mass flow rate and density wave caused by ocean conditions are found, and the corresponding stability boundaries of the natural circulation system are also proposed here. The use of a simplified mathematical description of fluid flow to describe the additional motions with waves reduces the difficulty of analysis. Specifically speaking, based on the study of stability boundary in vertical condition, first of all, it was found that inclined ocean condition can weaken the ability of natural circulation and can move down stability boundary of natural circulation, and the greater the inclination angle, the greater the amount of going-down. Secondly, the stability boundary of the system moves up in heaving circumstances, and the smaller the period, the greater the amount of going-up. Thirdly, the amplitude of rolling plays an important role in the rolling situation. In the case of rolling with large amplitude, the period of flow complex oscillation is clear, which is half the rolling period. However, in the case of rolling with a short period, the period of flow complex oscillation is ambiguous, the oscillation frequency of density wave under vertical condition is preserved in the flow complex oscillation. The stability boundary of natural circulation is moved down by rolling with a short period, where the frequency of flow complex oscillation was equal to that of density wave oscillation under vertical conditions. In contrast, the stability boundary of natural circulation is moved up by rolling with a long period, where the short-period local oscillation appears in the process of flow complex oscillation. Finally, the oscillation superpositions of rolling and heaving motion will result in a complex long-period oscillation with motion amplitudes being important.

Multi-faceted monitoring of powder flow rate variability in directed energy deposition

• Powder delivery systems for directed energy deposition processes can exhibit oscillatory variation in mass flow rates, matching the characteristic speed of individual hopper components. • Flow rate variation can be observed at the nozzle exit, both by offline weight measurement and by imaging the powder flow. • Flow rate variation is observed to have a measurable effect on melt track height, with the potential to cause build quality issues. • Melt pool imaging shows a correlating pattern, indicating that flow rate variation can affect melt pool image brightness, and may influence the behaviour of image based control approaches. Powder flow rate is a key parameter in Directed Energy Deposition (DED) processes. During a typical build, if powder flow rate is reduced for just 1 second, 30 mm of melt track is affected. Consequently, even a small variation in powder flow rate can have significant implications on build quality. In this work, the powder flow stability for different types of 316 L steel powders was quantified using a combination of methodologies including offline weight measurements, flow imaging, in-situ build data and coaxial melt pool imaging. Flow rate oscillation was observed, correlated with the periodicity of powder hopper turntable rotation, at sufficient magnitude to cause build quality effects and be identifiable in coaxial melt pool imaging. The implications of flow rate variation on the use of melt pool imaging for closed-loop control are discussed.

Analysis of self-excited transverse combustion instability in a rectangular model rocket combustor

A methane/oxygen mixture is considered to be an appropriate propellant for many future rocket engines due to its practicality and low cost. To better understand the combustion instability in methane/oxygen-fed rocket engines, the spontaneous transverse combustion instability in a rectangular multi-element combustor (RMC) was analyzed both experimentally and numerically. Severe combustion instabilities occurred in the RMC during repeatable hot-fire tests. The physical mechanisms were systematically investigated through numerical simulations based on the stress-blended eddy simulation and flamelet-generated manifolds method with detailed chemical mechanisms (GRI Mech 3.0). The numerical results for the frequency spectrum and spatial modes agree well with the theoretical analysis and experimental data. The driven regions of the combustion instability were identified on both sides of the combustion chamber through a Rayleigh index analysis. The longitudinal pressure oscillations in the oxidizer post were found to be coupled with the transverse pressure waves in the combustion chamber and led to periodic oscillations of the mass flow rate of propellant. Moreover, the mixing was highly enhanced when the pressure wave interacted with walls of the combustion chamber. Therefore, a sudden release of heat occurred. The pressure oscillations were enhanced by pulsated heat release. A closed-loop system with positive feedback associated with periodic oscillations mass flow rate of the propellant, and sudden heat release, was believed to account for the present combustion instability.