Stable Operating Range Research Articles

Anaerobic digestion of mercury phytoextraction crops with intermediary stage bio-waste polymer treatment

In laboratory experiments, Lepidium sativum L. and Mentha spicata L. were grown in compost spiked with mercury. After cultivation for 20 and 68 days, respectively, translocation factors of 0.05 ≤ TF ≤ 0.2 (Lepidium sativum) and accumulation factors of 2.2 ≤ AF ≤ 12 (Mentha spicata) were recorded. Plants were then harvested and used as feedstock for bench-scale anaerobic digesters. The reactors operated in continuously-stirred batch mode for a period of ten days. Inhibition of anaerobic biogas production was apparent with one sample set evidencing mercury-induced bacteriostatic toxicity. Otherwise, ex-situ characterization of digestate showed that the reactors were within stable operating range. A canola oil-sulphide polymer derived from bio-waste was also used as an intermediary treatment stage to test its capacity for extracting mercury from half the samples prior to anaerobic digestion, and also from the post-experimentation reactor digestate. The polymer removed mercury from digestate with a 40–50% efficacy across all samples, suggesting its potential as a sludge clean-up option. Anaerobic digestion combined with staged polymer extraction offers a potential route for the disposal of phytoremediation crops and ultimately the recovery of mercury, coincident with the production of a bioenergy vector.

Experimental investigation of transient characteristics of mild surge and diffuser rotating stall in a centrifugal compressor with vaned diffuser

The centrifugal compressor is widely used for gas compression in various industrial fields such as aero-engine, gas turbine and turbocharger. However, the stable operating range is usually restricted by the occurrence of flow instability such as stall and surge. The paper experimentally examines the developing process of surge and stall occurring in a centrifugal compressor to advance the understanding on flow mechanism of flow instability. It is found that three types of pressure fluctuation can be observed at low flowrate region. At the critical point, the local stall firstly occurs in some specific diffuser passages and the enlarged local diffuser stall eventually induces the mild surge of compression system indicated by the sinusoidal pressure fluctuation. At lower mass flow rate, the diffuser stall cell begins to circumferentially propagate along the impeller rotating direction at 11% of rotor speed with the existence of mild surge. In comparative analysis of IGV pre-swirl angle on the occurrence of mild surge, the mild surge still occurs at the operating condition when the slope of the characteristic map of test stage is still obviously negative, in which the compression system is supposed to be stable in previous study. And a new suggested criterion for the prediction of mild surge is demonstrated that the occurrence of mild surge depends on the destabi-lization effect of downstream components about diffuser and volute. Combined with the experimental data, the streamwise distribution characteristic of diffuser stall can be used to develop the simplified model of lumped parameters for the analysis on the generation mechanism of the diffuser rotating stall. The quantitative investigation on the relation between the pressure-rise characteristic of subcomponents and the occurrence of mild surge and diffuser rotating stall not only advances the prediction of stability limit but also lays the theoretic foundation for controlling these unsteady behaviors to improve the operating range.

Performance Analysis of Speed-Sensorless Induction Motor Drive Using Discrete Current-Error Based MRAS Estimators

In the literature on sensorless control of induction motors, many algorithms have been presented for rotor flux and speed estimation. However, all these algorithms have been developed in the continuous–time domain. The digital realization of the control systems, requires the implementation of those estimation methods in a discrete–time domain. The main goal of this article is comparison of the impact of different numerical integration methods, used in analogue emulation under the digital implementation of the control systems, to the operation of classical Model Reference Adaptive System; CC-based on two current models (MRASCC) speed estimator and its three modified versions developed for the extension of the estimator stability region. In this paper the generalized mathematical model of MRASCC estimator is proposed, which takes into account all known methods for the extension of the stability region of classical speed estimator of this type. After the short discussion of the discretization methods used for the microprocessor implementation of control algorithms the impact of different numerical integration methods on the stable operation range of the classical and modified MRASCC estimators is analyzed and validated in simulation and experimental tests. It is proved that Modified Euler discretization method is much more accurate than forward and backward Euler methods and gives almost as accurate results as Tustin method, however is much less complicated in practical realization.

Influence of vaned diffuser on the potential effect of volute and stall inception of centrifugal compressor

In the automotive turbochargers centrifugal compressor, the volute has a strong potential effect, leading to circumferential nonuniformity of the impeller flow field and compressor stall. In this study, full-annulus unsteady simulations for centrifugal compressors with vaned/vaneless diffusers are carried out. The influence of the diffuser vane on the potential effect of the volute and stall behavior of a centrifugal compressor is studied in detail. Based on the distribution of the casing static pressure, the formation mechanism of the circumferential distribution of static pressure and the reverse propagation process of pressure waves caused by pressure distortion are revealed. The results of this study show that the diffuser vanes can weaken the potential effect of the volute on the impeller flow field by reducing the degree of static pressure distortion. The number of static pressure peaks in the circumferential direction is related to the number of vanes/blades. The diffuser vanes can change the circumferential position of the stall inception, but cannot eliminate the “locking effect” of the volute tongue on the circumferential position of the stall. In other words, the circumferential position of the stall inception is still determined by the volute tongue for a centrifugal compressor with a vaned diffuser. Although the degree of circumferential static pressure distortion inside the impeller is reduced, the stable operating range of the compressor cannot be significantly widened by adjusting the stagger angle of the diffuser vane when the stall first occurs at the impeller inlet.

Experimental investigation of burning of pulverized peat in a bidirectional vortex combustor

The paper reports on the results of experimental research of bidirectional vortex combustor operation at different modes. This combustor implements a new idea of burning solid pulverized fuel in a bidirectional swirling flow. The experimental results confirmed the thesis about a wide range of stable operation of the combustor, as well as its environmental friendliness: the values of exhaust emission of CO and NOx are significantly lower than those for the direct-flow combustor. Besides, such combustor can operate in two modes: as an igniter or stabilizer of boiler furnace, where it generates a stable high-enthalpy flame as well as an independent device with low emission values. A summary of research results defines that the most environmentally friendly operating mode is achieved at the net air-fuel equivalence ratio λΣ = 1.3 and the ratio of primary and secondary air mass flow rates n = 2.7.

Investigation of methods for equalizing the flow parameters in the axial compressor intake flare

The problem of ensuring a uniform velocity field at the air inlet to the first stage of the gas turbine compressor SGT5-2000E with an angular inlet path, that contains an inlet filter, a silencer and an angular intake flare, is considered. Based on the mathematical modeling of the flow in this flare, it was found that the maximum speed at the compressor inlet is 36% higher than the average flow rate. It leads to the fact that the range of stable operation of the compressor seriously narrows. To solve this problem, it is proposed to change the intake flare design so that a newly developed effective damper of uneven velocity fields is installed in its flow part. Studies have shown that in this way the nonuniformity coefficient was reduced from the above 36% to 6%.

An Optical Method to Characterize Streamer Variability and Streamer-to-Flame Transition for Radio-Frequency Corona Discharges

In recent years, radio-frequency corona ignition gained increasing interest from the engine research community because of its capability to extend the engine stable operating range in terms of lean and EGR dilution. The corona discharge generates streamers coming from a star-shaped electrode, generally consisting of four or five tips. The temporal and spatial variability of such streamers in length, orientation, and branching can be factors that affect the combustion onset and, therefore, engine cycle-to-cycle variability. Generally, the latter is reduced with respect to a conventional spark igniter at the same air–fuel ratio, but still present. In this work, analysis on the corona discharge and on the subsequent combustion onset was carried out in an optically accessible engine by means of the detection, via high-speed camera, of the natural luminosity of streamers and flames. A method to characterize spatial and temporal variability in motored conditions is firstly presented. A statistical analysis of the streamer behavior was performed, by separately analyzing the streamers generated by each tip of the star-shaped electrode. Finally, an original method aimed at determining the moment of the first flame appearance, caused by the combustion onset, is presented. The outcome of this work can be used to improve the knowledge on corona discharge, in particular on the stochastic behavior that characterizes the streamers. The presented optical analysis can also be adapted to other volumetric, single- or multi-point ignition systems.

Diesel/syngas co-combustion in a swirl-stabilised gas turbine combustor

Multiphase fuel combustion was carried out in a swirl-stabilised combustor with the aim of expanding the fuel flexibility of the gas turbine for, at least, land-based applications. Improved capability of the gas turbine in this regard will not only augur well for energy security but also could be useful in tackling harmful emissions. In the study, varying amounts of syngas was premixed with air and swirled into a burning diesel spray, the flowrate of which was altered to maintain the same heat output at all times. Across the several heat outputs tested, the range of stable flame operation was found to reduce as gas content of fuel mix increased. Moreover, for a combined heat output of 15 kW and a global equivalence ratio of 0.7, a steady increase in flame stability was noted and NOx emissions were found to decrease while CO emissions increased as syngas content in fuel mix increased from 10% to 30%. The increase in flame stability, achieved at the cost of lower heat release rate, was attributed to the changes in reacting flow dynamics evinced by the C2* and CH* species chemiluminescence intensity variation as well as chemical kinetics analysis. The NOX and CO emissions trend was ascribed to increasingly inefficient combustion due to the poorer spray quality obtained from pressure atomiser as liquid flow rate reduces and further worsened by the lower heat release rate and decreasing adiabatic flame temperature as gas ratio of combusted fuel increases.

Inducer/Exducer Matching Characteristics inside Tandem Impellers of a Highly Loaded Centrifugal Compressor

A centrifugal compressor usually operates with low isentropic efficiency and a terrible stable operating range, resulting from the complex impeller flow structure companied with the intense interaction among the impeller and the diffuser downstream. In many studies, the potential of centrifugal compressor tandem-impeller configurations for improving the compressor has been demonstrated. Whereas, compared with the convincing results on the tandem-designed axial compressors, the results on tandem impellers are limited and contradictory. Very little insight has been provided into the flow mechanisms inside tandem impellers, which is considered to be the primary reason for the confusion in tandem impeller design and application. Tandem impellers are expected to exhibit a totally different behavior due to the intense aerodynamic interaction between the inducer and the exducer, which substantially contributes to the flow structure and the compressor performance change. In the present study, a numerical study of a highly-loaded centrifugal compressor with various tandem designs was conducted to explore the inducer/exducer matching characteristics and the underlying flow mechanism inside tandem impellers. Two tandem impeller design parameters, namely, the inducer/exducer clocking fraction and the axial gap (overlap), were considered in the tandem impeller design process. The tandem impeller was also compared to the existing conventional impeller which the tandem impeller was redesigned for. The results demonstrated that the tandem-designed impeller can improve the centrifugal compressor stage performance and intense inducer/exducer interaction can be observed with changes in the clocking fraction and the axial gap (overlap). The tandem impeller performance is sensitive to changes in axial gap (overlap) when the suction side of the exducer blade is circumferentially close to the inducer blade. The fundamental reason for the performance variation in the inducer and the exducer lies in the inducer pressure change in the blade trailing edge that is determined by the Kutta condition. Additionally, the correlation between the tandem impeller slip effect and the discharge flow quality should be emphasized in the inducer/exducer gap jet analysis, in which the jet injection angle and the Coanda effect of the exducer suction surface critically affect the discharge flow characteristics.

Operating Range Extension of an Open Impeller Centrifugal Compressor Stage Utilising 3D Diffuser End Wall Contouring

State-of-the-art centrifugal compressor units utilised in pipeline and in energy storage applications face the challenge of flexible and highly efficient operation. Geometric contouring on the hub side near a vaned diffuser affects the flow in a way which increases operational flexibility by delaying the incipience of instability and thereby increases compressor operating range. In the present paper, a hub-side wall contouring is applied within the vaneless space and the vaned diffuser of an open impeller centrifugal compressor stage. The performance characteristic of the novel hub contouring is evaluated in a scale-model test rig and compared against a baseline design. A stable operating range increase of 8% is achieved for the contoured design at Mu 2 = 1.16. 5-hole probe measurements covering a complete diffuser blade-to-blade passage are performed upstream the diffuser and compared both against CFD simulations and against the measurements of the baseline design for an operating point near the stability limit.

The influence of the active control of internal damping on the stability of a cantilever rotor with a disc

The article presents the results of the analysis of the active control of Stodola-Green rotor, made of a cantilevered beam with a rigid disc attached to its free end. The rotor is of a smart type, with piezoelectric patch sensors and actuators glued to its surface. The aim of active control is to decrease the effective internal damping, thus increasing the range of stable operation. The simulation studies show that it is feasible to increase the range of stability of rotors by means of velocity feedback control. Unlike in most studies that utilize this control scheme, negative gains are used, which allows for the effective internal damping to be decreased.

An Adaptive Droop Control Strategy for Islanded Microgrid Based on Improved Particle Swarm Optimization

In an islanded microgrid with multiple distributed generations (DGs), the difference in line impedance may cause local voltage deviation, which leads to a series of problems such as lower power allocation accuracy and bus voltage drop under traditional droop control. In this respect, a method for optimizing the droop control using an improved particle swarm optimization (IPSO) is proposed. Firstly, the microgrid structure and influence of line parameters on traditional droop control strategy is analyzed. Then, an improved particle swarm optimization is proposed. Based on the basic particle swarm optimization (PSO) algorithm, a fuzzy inference system (FIS) is introduced to dynamically adjust the particle swarm optimization, which can effectively improve the global search ability and local search ability of the algorithm. After that, the improved algorithm is applied to the droop controller, simultaneously, the range of stable operation of the system is determined by small signal analysis. Finally, the simulation and experiment results show that the proposed improved droop control strategy can achieve accurate allocation of active and reactive power effectively while maintaining bus voltage and system frequency stability, enhance the dynamic performance and transient stability of the microgrid system.

ЗБІЛЬШЕННЯ ДІАПАЗОНУ СТІЙКОЇ РОБОТИ СТУПЕНЯ ВІДЦЕНТРОВОГО КОМПРЕСОРА З БЕЗЛОПАТКОВИМ ДИФУЗОРОМ

Centrifugal compressors often operate at different capacities, so it is important to ensure their stable operation over a wide flow range. Stages with vaneless diffusers have several advantages compared to stages with other types of diffusers: they are more technologically advanced to manufacture, and more uniform pressure distribution behind the impeller improves the dynamics of the rotor. At low flows, due to the occurrence of a rotating stall and surge, the efficiency of stages with vaneless diffusers rapidly decreases. The occurrence of unstable operating modes of centrifugal compressor stages at low flow rates is associated with the appearance of developed backflows in the flow part. To expand the range of stable operation of the stages, it is necessary to use methods of flow separation control. Separation of the flow can be controlled either by special profiling the flow part channels or by actively influencing the flow, for example, by injecting gas. To solve this problem, a mathematical model of the gas flow in a vaneless diffuser with gas injection is developed. The characteristics and parameters of the flow in the vaneless diffusers with various meridional profiles with and without injecting gas were calculated. A comparison of the calculated and experimental characteristics of the vaneless diffusers and flow parameters in diffusers with different geometries and with different injection modes confirms the adequacy of the mathematical model. Investigations have confirmed the possibility of improving the characteristics of the stages of centrifugal compressors through the use of vaneless diffusers and diffusers with gas injection. Gas injection diffusers extend the stable operation range of the stages. The use of gas injection in a vaneless diffuser allows reducing the power consumption during antisurge control in comparison with the widespread bypass suction system at the entrance to the impeller

Improvement of Stability in a PCM-Controlled Boost Converter with the Target Period Orbit-Tracking Method

Thee peak-current-mode (PCM) control strategy is widely adopted in pulse width-modulated (PWM) DC-DC converters. However, the converters always involve a sub-harmonic oscillating state or chaotic state if the active duty ratio is beyond a certain range. Hence, an extra slope signal in the inductor-current loop is used to stabilize the operation of the converter. This paper presents a new technique for enlarging the stable range of PCM-controlled DC-DC converters, in which the concept of utilizing unstable period-1 orbit (UPO-1) of DC-DC converters is proposed and an implementation scenario based on the parameter-perturbation method is presented. With the proposed technique, perturbations are introduced to the reference current of the control loop, and the converters operating in a chaotic state can be tracked, and thus be stabilized to the target UPO-1. Therefore, the stable operating range of the converters is extended. Based on an example of a PCM-controlled boost converter, simulations are presented as a guide to a detailed implementation process of the proposed technique, and comparisons between the proposed technique and techniques in terms of ramp compensation are provided to show the differentiation in the performance of the converter. Experimental results in the work confirm the effectiveness of the proposed technique.

Kinetic model of hyperbranched polymers formed by self-condensing vinyl polymerization of inimers and trifunctional cores with different reactivities in a continuous-stirred tank reactor

A kinetic model is proposed to describe the formation of hyperbranched polymers, HBPs, by the self-condensing vinyl polymerization, SCVP, of inimers in a continuous stirred tank reactor, CSTR, with the addition of trifunctional cores. The dependence of the average degree of polymerization, DP, and the degree of branching, DB, on the Damköhler number, Da, is calculated by a generating function method. If the SCVP system has no core initiators, the flow in the CSTR may be stopped at a Da of approximately 0.5, which is caused by the formation of HBPs with ultrahigh molecular weights; then, the stable operating range becomes very narrow. The growth of HBPs in the SCVP can be retarded by adding the core with high reactivity into the CSTR. Therefore, formation of HBPs with ultrahigh molecular weights can be avoided even though the A groups are almost completely consumed at a high Da number. For example, the reduced weight-average DP is less than 1000 when the degree of conversion of the A group is 0.99, and only a few unreacted monomers remain in the product. The proposed model is useful for designing and optimizing a flow process for producing HBPs by the SCVP in a CSTR.

Improving the operating range using a centrifugal compressor with a tandem impeller

The centrifugal compressor is being utilized in both commercial and military applications; however, the compressor performance is limited by flow instability affecting the stable operating range. In this paper, a tandem-bladed impeller method is employed to achieve passive flow control to improve the stable operating range of a compressor without control costs or performance penalties. The results inspiringly indicate that the tandem impeller method can achieve a maximum operating range gain of 16.7%. The baseline compressor instability is induced by tip region flow deterioration, which results from tip leakage flow and low-momentum fluid radial transport. For a tandem impeller, the inducer suction side always has a higher trailing edge static pressure compared to the baseline impeller due to the exducer potential influence and the restriction of the Kutta-Joukowski condition, which weakens the radial transport of low-momentum fluid and constrains the leading-edge separation. As a result, the tandem-design compressor operates far from the impeller instability point. Further investigations show that the inducer pressure field varies with the clocking fraction λs, which results in the change of the radial transport at the inducer suction side, finally affecting the flow stability.

Experimental Investigation of PPCI Engine fuelled with Ethanol

Emissions are among those crucial factors that create a huge stress on the people related to engine industry. LTC concepts like HCCI and PPCI are proved as potential solutions for reducing NOX emissions. The impact of ethanol premixed fraction on PPCI mode diesel engine has been investigated in this study. Various ethanol flow rates like 0.27, 0.45, 0.54, 0.72 and 0.81 kg/hr are fumigated in conventional single cylinder diesel engine. Ethanol is used due to the fact that it is available naturally as well as manufactured industrially. The combustion, emission and performance characteristics are plotted for each and every load with various premixed fractions and then the results are compared to pure diesel operation. It is clear from the results that the range of stable operation is being confined by ethanol for different premixed fractions. It can be safely said that for all stable operation, the NOX emission is exceptionally less compared to pure diesel, though, the Un burnt Hydrocarbon & Carbon monoxide emissions are comparatively more. NOX and smoke are decreased for all the premixed fractions at full load. At medium load operations, less NOX, smoke opacity, CO and HC is obtained. Also, at low load operations, there is less NOX and smoke opacity up to premixed fraction 0.3, but HC and CO emissions increases on further increase in the premixed fraction.

Control of MMC-Based STATCOM as an Effective Interface between Energy Sources and the Power Grid

This paper presents a dynamic model of modular multilevel converters (MMCs), which are considered as an effective interface between energy sources and the power grid. By improving the converter performance, appropriate reactive power compensation is guaranteed. Modulation indices are calculated based on detailed harmonic evaluations of both dynamic and steady-state operation modes, which is considered as the main contribution of this paper in comparison with other methods. As another novelty of this paper, circulating current control is accomplished by embedding an additional second harmonic component in the modulation process. The proposed control method leads to an effective reduction in capacitor voltage fluctuation and losses. Finally, converter’s maximum stable operation range is modified, which provides efficiency enhancements and also stability assurance. The proficiency and functionality of the proposed controller are demonstrated through detailed theoretical analysis and simulations with MATLAB/Simulink.

Influence of Fluidic Control in a Staged Lean Jet Engine Burner on Combustor Performance

To improve the turn-down ratio of a lean combustor, which has the greatest potential for reducing NOx emissions from jet engines, fuel staging is commonly employed. To further extend the stable operation range, air staging with a fluidic element is also considered. The influence of fluidic control on combustion was analyzed to better understand fluidic element-burner interactions. The pressure loss of each fluidic element was determined by measuring the pressure at the element exits. The effect of fluidic control on the atomization, fuel distribution, and flow field was investigated using optical, noninvasive techniques. The combustion performance of the burner with the fluidic element was evaluated using exhaust gas analyses. The pressure losses of the swirlers and fuel mixers were varied depending on the bleed air from the fluidic element. Under the idle condition, the reduction of pressure loss in the pilot fuel mixer resulted in inferior atomization due to the reduced gas velocity around the fuel film, which had a positive effect on lean blowout. Under the cruise condition and the staged mode, the reduction of the pilot air flow increased the equivalence ratio of the lean pilot stage and resulted in higher combustion efficiency.

Design of High-Efficiency Refrigerator Test System for Industrial Internet of Things

Smart homes are developing rapidly, and refrigerators are an indispensable part of “smart homes”. Therefore, the research efficiency and quality assurance of refrigerators are highly valued by home appliance companies. Refrigerator test is required in both research and production processes. At present, the quality test of refrigerator has a high degree of manual participation, such as the selection of the stable operating range of refrigerator and the conversion of test stage require manual determination, which cause low test efficiency and high electric energy consumption. In order to solve these problems, this paper design a High-Efficiency Refrigerator Test System (HERTS) for Industrial Internet of Things (IIoT). The core test data intelligent analysis module realizes timely determination of test data and automatic transition of test stage. The proposed HERTS increases the efficiency of automate refrigerator test and greatly reduces the power consumption.