Limit Of Stable Operation Research Articles

Mild alkaline conditions affect digester performance and community dynamics during long-term exposure

This paper examines the adaptive responses of microbial communities to gradual shifts in pH toward the mild alkaline range in anaerobic digestion (AD) systems. The results indicate that a pH of 8.0 serves as a critical upper limit for stable AD operation, beyond which microbial efficiency declines, underscoring the importance of microbial resilience against elevated pH stress. Specifically, hydrolysis genera, e.g. Eubacterium and Anaerobacterium, and syntrophic bacteria were crucial for reactor stability. Fibrobacter had also been shown to play a key role in the accumulation of propionate, thus leading to its dominance in the volatile fatty acid profile throughout the experimental phases. Overall, this investigation revealed the potential adaptability of microbial communities in AD systems to mild alkaline pH shifts, emphasizing the hydrolysis bacteria and syntrophic bacteria as key factors for maintaining metabolic function in elevated pH conditions.

Stable operating limits and improvement methods for hydropower and photovoltaic integration through MMC-HVDC systems

This paper addresses the critical need to determine the stable operating limit of modular multilevel converter-based high voltage direct current (MMC-HVDC) systems, particularly concerning the integration of extensive renewable energy sources. To achieve this, the steady-state mathematical model and state-space model of bundled hydropower and photovoltaic integration through MMC-HVDC systems are established. A novel methodology considering steady-state and small-signal stability constraints is proposed to compute the stable operating region of the system. The quantitative assessment reveals that diminishing AC system short-circuit capacities amplify restrictions from small-signal stability constraints, thereby reducing the system's stable operating region. Eigenvalue and participation factor analyses shed light on the pivotal factors affecting small-signal stability in weak AC systems. To expand the system's stable operating region, a supplementary frequency damping control strategy is proposed. The theoretical analysis and calculation results are validated by building a simulation model for the bundled hydropower and photovoltaic integration through MMC-HVDC systems in PSCAD/EMTDC.

Analysis of the Exothermic Reaction of Flame Ignition in the Combustion Chamber of a Gas Turbine Unit

This article analyses the exothermic reaction of flame ignition in the combustion chamber of a gas turbine unit, which is characteristic of combustion chambers operating on traditional hydrocarbon fuels. The combustion of gases as an explosive process in confined and semi-enclosed areas remains a poorly understood section of thermal physics. Without a detailed review of the physical and chemical processes taking place in the combustion chamber, it cannot be said whether the gas turbine unit will run sustainably. It is also important to know what combustion modes are in principle possible after a loss of stability in the combustion chamber in order to take action against this in advance. To describe flame ignition and quenching in the flow of the fuel–air mixture through a combustion chamber, a system of differential conservation equations of energy and reactive species supplemented with the equation of state is used. Nonstationary combustion processes in gas-turbine engines were studied, and flame ignition and blow-off were determined by the heat balance and by the continuity of chemical processes. Calculation methodologies for various operating modes of the combustion chamber of a gas turbine unit are developed and realized. The results of the calculations that were carried out are presented with graphical interpretation and with their analysis provided in sufficient detail. Based on this analysis, recommendations are then provided. From the graphs, it can be observed that the combustion chamber of a gas turbine unit reaches its maximum limit of stable operation at the optimum value of the reduced flow velocity in the openings of the air supply to the combustion and the mixing zones of the flame tube (λOC)opt = 0.22 when the fuel–air mixture is at maximum depletion, ensuring that combustion does not stop and flame failure does not occur. The topic of this article relates to the intensification of hydrocarbon fuel combustion and the technological improvement of combustion chambers in gas turbine units. This topic is of exceptional importance and relevance, emphasizing its significance. The purpose of this work is to develop and implement a methodology for calculating various modes of operation of the combustion chamber of a gas turbine unit.

Ultra-low emission power generation utilizing chemically stabilized waste plastics pyrolysis oil in RCCI combustion concept

Emission standards in European Union, designed to reduce the environmental impact of power generation, present a significant challenge for fast-response distributed power generation systems based on internal combustion engines. Regulated emissions, such as NOx and particulate matter present a major concern due to their adverse number of environmental and health effects. Simultaneously, European Union strives towards sustainable management of plastic waste and seeks the ways for its upcycling and production of new fuels and chemicals. As an answer to the presented challenges, the present experimental study addresses the potential for use of chemically stabilized Waste Plastics Oil (WPO), a product of pyrolysis process of waste plastics in a Reactivity Controlled Compression Ignition (RCCI) combustion concept. To establish a reactivity-controlled combustion, the study uses a combination of methane (a model fuel for biomethane) and WPO to a) simultaneously reduce NOx and particulate matter emissions due to low local combustion temperatures and a high degree of charge homogenization and b) address waste and carbon footprint reduction challenges. Through experiments, influence of direct injection timing and energy shares of utilized fuels to in-cylinder thermodynamic parameters and engine emission response were evaluated in engine operating points at constant indicated mean effective pressure. Acquired results were deeply investigated and benchmarked against compression ignition (CI) and RCCI operation with conventional diesel fuel to determine potential for WPO utilization in an advanced low-temperature combustion concept. Results show that chemically stabilized WPO can be efficiently utilized in RCCI combustion concept without adaptation of injection parameters and that with suitable control parameters, ultra-low emissions of NOx and PM can be achieved with utilized fuels. For diesel/methane mix, NOx and PM emissions were reduced compared to conventional CI operation for 82.0% and 93.2%, respectively, whereas for WPO/methane mix, NOx and PM emissions were reduced for 88.7% and 97.6%, respectively, which can be ascribed to favourable chemical characteristics of WPO for the utilized combustion concept. In the least favourable operating point among those studied, indicated mean effective pressure covariance was kept below 2.5%, which is well below 5% being considered the limit for stable engine operation.

Formation of Hot Spots in Coated Conductors During Static and Dynamic DC Loading

High voltage DC transport at long distances represents an opportunity for wider use of superconductors. In particular, the second generation of high temperature superconductor tapes produced in form of coated conductors (CC) offers a unique solution for protecting a high-capacity connection against fault currents. In the resistive superconducting fault current limiter (R-SFCL), the CC tape would emerge as a substantial resistance in case of overcoming its critical current, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c</i> . Non-uniformity of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c</i> along the CC length is a common issue requiring attention. Because of highly nonlinear current/voltage dependence, the spot with critical current <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_cmin</i> < <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c</i> could transform into the hot spot with rapid rise of local temperature. Due to its small dimension, it would not create an observable voltage at the device terminals, thus may escape attention of a quench detection system. We have investigated the mechanism of hot spot creation at the DC currents between <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_cmin</i> and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c</i> , and the analytical formulas have been derived predicting the limits of stable operation. Now we extend the analysis to the case of the limitation event, when for a fraction of second the current exceeds also the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c</i> . It is desirable that the temperature rise happens along all the conductor length, leading to a quick reduction of transported DC. Numerical modelling has been utilized to analyze such an event, and a simplified analytical model has been developed to predict the range of currents causing the formation of hot spots.

Experimental verification between the functioning of tribosystems in the conditions of boundary lubrication

The paper presents an experimental test of modeling the limits of stable operation of different structures of tribosystems (robustness criteria) in the conditions of extreme lubrication. The results of the experimental test confirmed the previously concluded conclusion that not all structures of tribosystems lose stability in terms of the coefficient of friction, i.e. the appearance of burrs on the surfaces of the friction. At low values of the coefficient of shape and low values of the quality factor of the tribosystem, the loss of stability occurs due to accelerated wear of materials.Expressions for calculation of criteria of robustness of tribosystems taking into account speed of change of loading on tribosystem are received. The rate of change of load is taken into account by the coefficients of dynamism, which are obtained taking into account the right-hand side of the differential equation of the dynamics of the functioning of tribosystems. Analysis of the obtained theoretical results on the assessment of the robustness of tribosystems and their comparison with the results of the experiment, suggest that the obtained conditions for stable operation of tribosystems (criteria of robustness) allow theoretically, with error 10,3 - 13,3 %, determine the boundaries of sustainable work. Criteria for the robustness of the tribosystem by wear rate and friction coefficient should be used in the design of tribosystems.

Effect of combustion zone material on the thermal performance of a biogas-fuelled porous media burner: Experimental studies

The performance of a porous media burner (PMB) is immensely dependent on the selection of porous material. Present experimental work investigates the impact of burner’s material on the thermal performance of a crude biogas-fuelled, in house developed PMB. This includes the study of burner stable operating limits, location of reaction zone, estimation of radiation efficiency and exhaust gas emission analysis. The PMB consists of two sections of porous material, one is the combustion zone and the other is the preheat zone. In the present research, the impact of combustion zone material on performance of the PMB is tested, while keeping the material of preheat zone unchanged. Silicon carbide (SiC) and zirconia (ZrO2) foams, both having porosity of 10 ppi, have been chosen as combustion zone. Experimental investigation shows that for the whole input load range of 5–10 kW, the SiC foam offered lean working condition, while in case of ZrO2, similar condition was limited to 5–8 kW. Within these stable operating limits, the axial temperature mapping of the burner highlights the location of reaction zone. Combustion in SiC foam is found to offer higher radiation efficiency of maximum 23.7%, as compared to that of ZrO2, where it is only 8%. Exhaust gas analysis shows that CO and NOx concentrations of both burners were low and the maximum values were limited to 156 ppm and 20 ppm, respectively. Overall performance assessment confirms that SiC is a better choice over ZrO2 for lean combustion of crude biogas in the developed PMB.

Modelling and voltage control of the solar-wind hybrid micro-grid with optimized STATCOM using GA and BFA

Electricity generation from the wind and solar photovoltaic (PV) systems are highly dependent upon weather conditions. Their intermittent nature leads to fluctuations in their output. Therefore, the need for rapid compensation for energy transmission and distribution systems is increasingly important. Static Synchronous Compensator (STATCOM) can be adopted for reactive power compensation and for decreasing the voltage fluctuation caused by the system and renewable energy sources. This study presents modelling of a Solar PV-Wind Hybrid Micro-grid and the increase of the stable operating limit of the system in case of the incorporation of STATCOM is examined. The major contribution of this paper is the optimization of gain parameters of four PI controllers in STATCOM control circuit based on genetic algorithms (GA) and Bacteria Foraging Algorithm (BFA) and therefore obtaining better responses and voltage stability in terms of nonlinear nature of solar-wind hybrid micro-grid. The Simulink models of the system architecture include a wind turbine model, a solar PV power system model and a STATCOM. It is certified that the voltage fluctuation at the end of the bus bar is reduced by 8% using conventional PI controller, by 10% for GA-based PI controller, and by 15% for BFA based PI controller under variable load. The results obtained by GA and BFA-based optimization of PI controllers are compared with that of the conventional controller and better results attained.

An Active Magnetic Bearing for Thin-Walled Rotors: Vibrational Dynamics and Stabilizing Control

This paper describes a novel active magnetic bearing with distributed actuation, designed for contact-free support of a thin-walled rotor. A full-dynamic analysis, control formulation, and experimental evaluation are presented. Important aspects of the bearing operation include the vibration excitation of the rotor wall due to asymmetries in manufacture and assembly, and the limits of stable operation with respect to static loading of the bearing. For the proposed design, the vibratory dynamics for flexure of the rotor wall are accounted for explicitly in the actuator coil-winding and driving scheme so that the coupling of the levitation control with the wall dynamics is minimized. This enables the selection of control feedback laws based on rigid-body dynamic considerations alone. Dynamic performance is investigated by theoretical and experimental study involving a prototype rotor-bearing system. The operating principles are validated and the dynamic behavior is shown to align well with the theoretical predictions.

A simplified CFD model for spectral radiative heat transfer in high-pressure hydrocarbon–air combustion systems

Detailed radiation modeling in piston engines has received relatively little attention to date. Recently, it is being revisited in light of current trends towards higher operating pressures and higher levels of exhaust-gas recirculation (EGR), both of which enhance molecular gas radiation. Advanced high-efficiency engines also are expected to function closer to the limits of stable operation, where even small perturbations to the energy balance can have a large influence on system behavior. Detailed radiation modeling using sophisticated tools like photon Monte Carlo/line-by-line (PMC/LBL) is computationally expensive. Here, guided by results from PMC/LBL, a simplified stepwise-gray spectral model in combination with a first-order spherical harmonics (P1 method) radiative transfer equation (RTE) solver is proposed and tested for engine-relevant conditions. Radiative emission, reabsorption and radiation reaching the walls are computed for a heavy-duty compression-ignition engine at part-load and full-load operating conditions with different levels of EGR and soot. The results are compared with those from PMC/LBL, P1/FSK (P1 with a full-spectrum k-distribution spectral model) and P1/Gray radiation models to assess the proposed model’s accuracy and computational cost. The results show that the proposed P1/StepwiseGray model can calculate reabsorption locally and globally with less than 10% error (with respect to PMC/LBL) at a small fraction of the computational cost of PMC/LBL (a factor of 30) and P1/FSK (a factor of 15). In contrast, error in computed reabsorption by the P1/Gray model is as high as 60%. It is expected that the simplified model should be broadly applicable to high-pressure hydrocarbon–air combustion systems, with or without soot.

Operation of a liquid-fueled and valveless pulse detonation rocket engine at high frequency

To increase the operating frequency and enhance the propulsive performance of a pulse detonation rocket engine (PDRE), a valveless mode with the purge gas based on liquid fuel was developed. Liquid gasoline, oxygen-enriched air, and nitrogen were utilized as fuel, oxidizer, and purge gas respectively. Supply of these propellants was controlled by the periodic pressure oscillations in the detonation tube. The purge gas with the highest supply pressure was filled into the detonation tube first. The fuel and oxidizer were injected after the inner pressure decaying to a relatively low level. This delay and the vaporization of the liquid fuel prevented pre-ignition of the fresh detonable mixture. The PDRE can work stably from 10Hz to 160Hz in this scheme. Fully-developed detonation waves were proved to be obtained successfully at a maximum frequency of 130Hz. Both oxygen volume fraction and supply pressure of the purge gas were tested to determine the limits for stable operation. The propulsive performance of a practical PDRE at a wide operating frequency range was also tested and the maximum thrust was obtained at 140Hz. Compared to the results without purge gas, introduction of a purge gas extends the ranges of supply pressure and operating frequency.

Dynamic Droop-based Inertial Control of a Wind Power Plant

The frequency of a power system should be maintained within the allowed limits for stable operation. When a disturbance such as generator tripping occurs in a power system, the frequency is recovered to the nominal value through the inertial, primary, and secondary responses of the operating synchronous generators (SGs). However, for a power system with high wind penetration, the system inertia will decrease significantly because wind generators (WGs) are operating decoupled from the power system. This paper proposes a dynamic droop-based inertial control for a WG. The proposed inertial control determines the dynamic droop depending on the rate of change of frequency (ROCOF). At the initial period of a disturbance, where the ROCOF is large, the droop is set to be small to release a large amount of the kinetic energy (KE) and thus the frequency nadir can be increased significantly. However, as times goes on, the ROCOF will decrease and thus the droop is set to be large to prevent over-deceleration of the rotor speed of a WG. The performance of the proposed inertial control was investigated in a model system, which includes a 200 MW wind power plant (WPP) and five SGs using an EMTP-RV simulator. The test results indicate that the proposed scheme improves the frequency nadir significantly by releasing a large amount of the KE during the initial period of a disturbance.

가정용 열병합 발전을 위한 스털링 엔진의 열원 온도 및 냉각수 유량에 따른 성능 실험

A Beta-type Stirling engine is developed and tested on the operation stability and cycle performance. The flow rate for cooling water ranges from 300 to 1500 ml/min, while the temperature of heat source changes from 300 to 500 ℃. The internal pressure, working temperatures, and operation speed are measured and the engine performance is estimated from them. In the experiment, the rise in the temperature of heat source reduces internal pressure but increases operation speed, and overall, enhances the power output. The faster coolant flow rate contributes to the high temperature limit for stable operation, the cycle efficiency due to the alleviated thermal expansion of power piston, and the heat input to the engine, respectively. The experimental Stirling engine showed the maximum power output of 12.1 W and the cycle efficiency of 3.0 % when the cooling flow is 900 ml/min and the heat source temperature is 500 ℃.

Regenerative Instability of Impact-cutting Material Removal in the Grinding Process Performed by a Flexible Robot Arm

Robotic machining is emerging as a viable alternative for some conventional machining tasks. Nevertheless, a challenge to cutting performance remains: vibrational instability is reported to be more severe with robotic cutting due to robot compliance. In this paper, the regenerative vibrational instability of a robotic grinding operation is studied. The objective is to find the limit of stable operation when material removal is performed by a compliant robot. The traditional approach to regenerative chatter analysis is revisited for a robotic machining process developed at Hydro-Québec's research institute for tasks on hydropower equipment. Stability lobes are established with respect to system gain (ratio of cutting rigidity over robot stiffness) versus repeat frequency (ratio of spindle frequency over the robot's first natural frequency). Robotic machining is found to be far upper-right of the first stability lobe minimum, where system gain and repeat frequency are more than one order of magnitude larger than for conventional machining. The cyclic impacting dynamics of material removal in the operation under study is invoked to investigate instability in this region. A SDOF dynamic model for the robot is excited by an impact-cutting force and the stability of the simulated response is verified while increasing the cutting depth. The limit of stable impact cutting is thus determined for the process at typical rotational speeds, from which a stability boundary is plotted versus the repeat frequency. The boundary is found to be very close to the limit predicted using the traditional approach. It is concluded that the large gain is typical for robotic machining. The impacting dynamics of material removal due to robot compliance must be considered to understand such large gain values, never occurring in conventional machining.

Octanol ethoxylation in microchannels

The propagation kinetics of the ring opening polymerization of ethylene oxide with octanol (“octanol ethoxylation”) was studied in a continuous microstructured reactor. Temperature was varied from 130°C to 240°C, and application of elevated pressure ensured a homogeneous liquid phase process. The activation energy was found to be EA=72.2±0.9kJmol−1. A numerical model was developed which was able to predict ethylene oxide consumption as function of residence time.The new kinetic parameters were used to determine the process intensification potential of this reaction and to study the limits of stable reactor operation.

Constriction factor based particle swarm optimization for analyzing tuned reactive power dispatch

The reactive power dispatch (RPD) problem is a very critical optimization problem of power system which minimizes the real power loss of the transmission system. While solving the said problem, generator bus voltages and transformer tap settings are kept within a stable operating limit. In connection with the RPD problem, solving reactive power is compensated by incorporating shunt capacitors. The particle swarm optimization (PSO) technique is a swarm intelligence based fast working optimization method which is chosen in this paper as an optimization tool. Additionally, the constriction factor is included with the conventional PSO technique to accelerate the convergence property of the applied optimization tool. In this paper, the RPD problem is solved in the case of the two higher bus systems, i.e., the IEEE 57-bus system and the IEEE 118-bus system. Furthermore, the result of the present paper is compared with a few optimization technique based results to substantiate the effectiveness of the proposed study.

Evaluation of the use of ceramic foams as catalyst supports for reverse-flow combustors

The performance of a silicon carbide foam supported palladium catalyst (Pd/β-SiC) in a reverse flow reactor for the abatement of methane in diluted streams (2700–5400ppm) has been studied in this article. The limits of stable operation for this configuration have been experimentally established, being narrower than the corresponding to particulated and honeycomb monolith beds because of the lower thermal inertial of foamed beds. A heterogeneous one-dimensional model has been developed (including the use of specific correlations for mass transfer and dispersive terms) and experimentally validated for predicting the behaviour of these reactors.Finally, the numerical model has been used for simulating reverse-flow reactors equipped with different catalyst shapes (e.g. Raschig rings, honeycomb monoliths and foams) of equivalent geometry. Results for different superficial velocities (0.25–1.5m/s) are compared in terms of transport properties and performance, concluding that honeycomb monolithic beds are the most appropriate configuration for these devices.

Syngas production from burner-stabilized methane/air flames: The effect of preheated reactants

The effect of preheated reactants on syngas production from a methane/air flame was investigated over a range of inlet temperatures up to 630K. In addition to experimental measurements, the results from a burner-stabilized flame and freely-propagating flame models are presented. A comparison of the modeling and experimental results in terms of flame standoff distance, stability limit conditions and species yields show excellent agreement across a broad range of equivalence ratios and preheat temperatures. Preheating of reactants increased the rich limit for stable operation from 1.26 to 1.75 for a given inlet velocity, and syngas yields were shown to increase with equivalence ratio. The preheat temperature of the reactants was shown to have little impact on syngas yields beyond extending the limits of stable operation. The results of this study are useful for the design and analysis of heat recirculating reactors and other reactors that are designed for producing syngas through the combustion of rich mixtures.

Centrifugal inertia effects in high-speed hydrostatic air thrust bearings

A modified Reynolds equation for compressible flow is used to model the dynamics of pressurised air bearings in a simplified axisymmetric geometry. The formulation incorporates the effect of centrifugal inertia for high-speed operation. A steady-state analysis is presented with a fixed rotor–stator clearance. The load-carrying capacity of the bearing is assessed for both inward and outward pressurisation and the air-flow characteristics are seen to depend on the level and direction of pressurisation. A critical shaft speed is identified that maintains no-net flow by balancing inertia and pressurisation effects. The nonlinear air–rotor–stator dynamics are investigated by modelling the axial stator position using a spring–mass–damper system coupled to the air-film dynamics. Solutions are presented to illustrate the effect of various frequencies and amplitudes of forcing for a range of rotation speeds. The time-averaged axial force and mass flow of air are used as characteristic measures of bearing performance, and further numerical results are presented as part of a parameter space analysis. Using the method of arc-length continuation key measures of the numerical solutions are tracked for changing values of the physical parameters. The minimum rotor–stator clearance is used to evaluate the limit of stable periodic operation without resonant stator dynamics and incorporating high operating speeds.

Benefits of Active Compressor Stability Management on Turbofan Engine Operability

Active compressor stability management can play a significant role in the intelligent control of gas turbine engines. The present work utilizes a computer simulation to illustrate the potential operability benefits of compressor stability management when actively controlling a turbofan engine. The simulation, called the modular aeropropulsion system simulation (MAPSS) and developed at NASA Glenn, models the actuation, sensor, controller, and engine dynamics of a twin-spool, low-bypass turbofan engine. The stability management system is built around a previously developed stability measure called the correlation measure. The correlation measure quantifies the repeatability of the pressure signature of a compressor rotor. Earlier work has used laboratory compressor and engine rig data to develop a relationship between a compressor’s stability boundary and its correlation measure. Specifically, correlation measure threshold crossing events increase in magnitude and number as the compressor approaches the limit of stable operation. To simulate the experimentally observed behavior of these events, a stochastic model based on level-crossings of an exponentially distributed pseudorandom process has been implemented in the MAPSS environment. Three different methods of integrating active stability management within the existing engine control architecture have been explored. The results show that significant improvements in the engine emergency response can be obtained while maintaining instability-free compressor operation via any of the methods studied. Two of the active control schemes investigated utilize existing scheduler and controller parameters and require minimal additional control logic for implementation. The third method, while introducing additional logic, emphasizes the need for as well as the benefits of a more integrated stability management system.