The synergistic effect of equivalence ratio and initial temperature on laminar flame speed of syngas

The laminar flame speed of syngas (CO:H₂ = 1:1)/air premixed gas in a wide equivalence ratio range (0.6–5) and initial temperature (298–423 K) was studied by Bunsen burner. The results show that the laminar flame speed first increases and then decreases as the equivalence ratio increasing, which is a maximum laminar flame speed at n = 2. The laminar flame speed increases exponentially with the increase of initial temperature. For different equivalent ratios, the initial temperature effects on the laminar flame speed is different. The initial temperature effects for n = 2 (the most violent point of the reaction) is lower than others. It is found that H, O and OH are affected more and more when the equivalence ratio increase. When the equivalence ratio is far from 2, the reaction path changes, and the influence of initial temperature on syngas combustion also changes. The laminar flame speed of syngas is more severely affected by H + O₂ = O + OH and CO + OH = CO₂ + H than others, which sensitivity coefficient is larger and change more greatly than others when the initial temperature and equivalence ratio change. Therefore, the laminar flame speed of syngas/air premixed gas is affected by the initial temperature and equivalence ratio. A new correlation is proposed to predict the laminar flame speed of syngas (CO:H₂ = 1:1)/air premixed gas under the synergistic effect of equivalence ratio and initial temperature (for equivalence ratios of 0.6–5, the initial temperature is 298–423 K).     

Dynamic effects of turbocharging on adaptive speed control ofdiesel driven power-plants

The adaptive control scheme introduced by S. Roy et al. (1991) for naturally aspirated diesel prime-movers has been applied to turbocharged generation plants. The performance has been studied in comparison to that of a fixed PI (proportional plus integral) controller. It is seen that since the nonlinear effect of air dynamics now affects the dynamic fuel consumption, the turbocharger is a major determinant of the adaptive controller performance. Furthermore, the performance is found to have a very significant effect due to the droop. It can in general be concluded that the turbocharger inertia should be low for high-droop operations. However, despite this predominant factor, the adaptive scheme can achieve good improvement over the conventional controller     

The impact of equivalence ratio oscillations on combustion dynamics in a backward-facing step combustor

The combustion dynamics of propane-air flames are investigated in an atmospheric pressure, atmospheric inlet temperature, lean, premixed backward-facing step combustor. We modify the location of the fuel injector to examine the impact of equivalence ratio oscillations arriving at the flame on the combustion dynamics. Simultaneous pressure, velocity, heat-release rate and equivalence ratio measurements and high-speed video from the experiments are used to identify and characterize several distinct operating modes. When the fuel is injected far upstream from the step, the equivalence ratio arriving at the flame is steady and the combustion dynamics are controlled only by flame-vortex interactions. In this case, different dynamic regimes are observed depending on the operating parameters. When the fuel is injected close to the step, the equivalence ratio arriving at the flame exhibits oscillations. In the presence of equivalence ratio oscillations, the measured sound pressure level is significant across the entire range of lean mean equivalence ratios even if the equivalence ratio oscillations arriving at the flame are out-of-phase with the pressure oscillations. The combustion dynamics are governed primarily by the flame-vortex interactions, while the equivalence ratio oscillations have secondary effects. The equivalence ratio oscillations could generate variations in the combustion dynamics in each cycle under some operating conditions, destabilize the flame at the entire range of the lean equivalence ratios, and increase the value of the mean equivalence ratio at the lean blowout limit.     

Adaptive speed control of hydrogenerators by recursive leastsquares identification algorithm

An inherent challenge to hydrogenerator speed regulation is the nonlinear and time varying nature of the process. When a fixed gain controller is used to regulate such a process, globally stable system response can be achieved only at the expense of degraded off design-point performance. This paper considers the application of an adaptive control methodology that allows the controller gains to automatically adjust to changing process variables and thereby provide uniform closed loop response over a wide range of operating conditions. Preliminary findings derived from a calculated plant model, and a standard recursive identification technique, indicate that the adaptive system provides desirable dynamic response despite changes in system operating dynamics, maintaining stable operation in situations where constant gain schedules fail     

Model of flame dynamics of laminar premixed flame subject to the low frequency equivalence ratio oscillations

The effect of the non-uniform profile of scalar variables, such a fuel at the upstream and temperature at the downstream of the flame zone was discussed theoretically to elucidate; (1) the deviation of motion from the steady state case and (2) the hysteresis of premixed flames response to the equivalence ratio oscillations seen in an experimental and numerical works. One-dimensional integral model for the non-uniform scalar variable profile with low frequency equivalence ratio oscillation has been developed. Here, the wavelength of the oscillation is assumed to be larger than the nominal flame thickness. Through the integral analysis, we obtained the relation of the flame propagation speed for steady and unsteady cases depending on the non-uniform scalar profile at the upstream and downstream of the flame zone. Hysteresis of the flame propagation speed is found due to the transport of fuel and heat by the non-uniform scalar profile at the upstream and downstream of the flame zone. This result qualitatively agreed with the numerical results of a response of the stagnation laminar CH₄/air premixed flames for a low equivalence ratio oscillation frequency.     

Effect of compression ratio, equivalence ratio and engine speed on the performance and emission characteristics of a spark ignition engine using hydrogen as a fuel

The present energy situation has stimulated active research interest in non-petroleum and non-polluting fuels, particularly for transportation, power generation, and agricultural sectors. Researchers have found that hydrogen presents the best and an unprecedented solution to the energy crises and pollution problems, due to its superior combustion qualities and availability. This paper discusses analytically and provides data on the effect of compression ratio, equivalence ratio and engine speed on the engine performance, emissions and pre-ignition limits of a spark ignition engine operating on hydrogen fuel.These data are important in order to understand the interaction between engine performance and emission parameters, which will help engine designers when designing for hydrogen.     

空气当量比对生物质和煤共气化影响的研究

采用新型床料对松木屑与烟煤的流化床共气化进行研究.在生物质掺混比例为50%的工况下,当空气当量比(ER)从0.2增加到0.28时,产气中H2的体积分数从14.1%上升到26.9%,CO的体积分数从28.9%减小到21.8%,CO2的体积分数呈上升趋势,CH4和CnHm的体积分数逐渐下降;燃气热值在ER为0.25时最大,大约为7 180 kJ/m3;气化效率为44%～53%;混合燃料的碳转化率为74%～76%.随着ER的增加,共气化的主要反应、燃料有机特性、松木屑的灰特性呈现不同的变化规律,从而对共气化参数产生影响.探求气化参数的变化规律将为共气化反应器的结构设计和运行参数的选择提供依据.     

Adaptive finite element analysis of microwave driven convection

This study describes an adaptive finite element methodology for heat transfer by convection applied to microwave heating of liquids. This is the first attempt to model such type of problems employing the concepts of error estimation and mesh adaptivity. The proposed methodology is generic and can be applied to steady-state, transient, linear and nonlinear problems involving heat transfer by conduction and convection. There was very good agreement between simulation and experimental results.     

Postflame reaction chemistry of dichloromethane: variations in equivalence ratio and temperature

We report on the destruction pathways and byproduct formation of dichloromethane (CH₂Cl₂) in conditions typical of incinerator postflame regions (injection temperature = 900–1200 K; equivalence ratio = 0.6, 0.9, 1.0, 1.1; residence time = 0.28–0.35 s). This is the first study to independently vary equivalence ratio and temperature, and evaluate their impacts on byproduct yield and destruction efficiency. We inject 750 ppm CH₂Cl₂ into postflame combustion products and measure byproducts with extractive FTIR spectroscopy. We use a detailed chemical kinetic mechanism and reaction rate analysis to predict the changes in reaction pathways as a function of equivalence ratio. The predictions for major products and several intermediate species compare well with experiments; the largest disparities are an underprediction of phosgene (CCl₂O) and an overprediction of methyl chloride (CH₃Cl). Both the experiment and the numerical predictions show increased destruction at lower equivalence ratios. However, the experiments reveal increased levels of stable chlorinated organics at lower equivalence ratios, opposite to the numerical prediction. We discuss reasons for this discrepancy and implications of these results for designing control strategies to promote full conversion to HCl and to reduce chlorinated byproduct emissions.     

Response of partially premixed flames to acoustic velocity and equivalence ratio perturbations

This article describes an experimental investigation of the forced response of a swirl-stabilized partially premixed flame when it is subjected to acoustic velocity and equivalence ratio fluctuations. The flame’s response is analyzed using phase-resolved CH^* chemiluminescence images and flame transfer function (FTF) measurements, and compared with the response of a perfectly premixed flame under acoustic perturbations. The nonlinear response of the partially premixed flame is manifested by a partial extinction of the reaction zone, leading to rapid reduction of flame surface area. This nonlinearity, however, is observed only when the phase difference between the acoustic velocity and the equivalence ratio at the combustor inlet is close to zero. The condition, Δφ_Φ^′-V^′≈0°, indicates that reactant mixtures with high equivalence ratio impinge on the flame front with high velocity, inducing large fluctuations of the rate of heat release. It is found that the phase difference between the acoustic velocity and equivalence ratio nonuniformities is a key parameter governing the linear/nonlinear response of a partially premixed flame, and it is a function of modulation frequency, inlet velocity, fuel injection location, and fuel injector impedance. The results presented in this article will provide insight into the response of a partially premixed flame, which has not been well explored to date.     

A new control strategy for a stand-alone self-excited induction generator driven by a variable speed wind turbine

This paper presents a new control strategy of a stand-alone self-excited induction generator (SEIG) driven by a variable speed wind turbine. The proposed system consists of a three phase squirrel-cage induction machine connected to a wind turbine through a step-up gear box. A current controlled voltage source inverter (CC–VSI) with an electronic load controller (ELC) is connected in parallel with the main consumer load to the AC terminals of the induction machine. The proposed control strategy is based on fuzzy logic control principles which enhance the dynamic performance of the proposed system. Three fuzzy logic PI controllers and one hysteresis current controller (HCC) are used to extract the maximum available energy from the wind turbine as well as to regulate the generator terminal voltage simultaneously against wind speed and main load variations. However, in order to extract the maximum available energy from the turbine over a wide range of wind speeds, the captured energy is limited due to electrical constraints. Therefore the control strategy proposed three modes of control operation. The steady state characteristics of the proposed system are obtained and examined in order to design the required control parameters. The proposed system is modeled and simulated using Matlab/Simulink software program to examine the dynamic characteristics of the system with proposed control strategy. Dynamic simulation results demonstrate the effectiveness of the proposed control strategy.     

An optimized control method for a high utilization ratio and fast filling speed in hydrogen refueling stations

A well-designed control system with a high utilization ratio of hydrogen and a fast filling speed are two critical objectives to ensure the reduction of cost and time required in the refueling process. In this paper, the popular three-stage refueling process is modeled with the aim to address both objectives. Using the real gas law of hydrogen, the utilization ratio of hydrogen filling is analyzed and the filling flowrate and time of each stage are evaluated. A multi-objective iterative optimization model is established and an optimization algorithm for the filling process is proposed to achieve both fast refueling and high utilization. Numerical results can be applied to the optimization of an actual hydrogen filling process. Besides, the tests show that an optimized control method can significantly improve the utilization ratio and allow refueling in a widely acceptable time.     

Drive train dynamics assessment and speed controller design in variable speed wind turbines

This paper deals with the speed controller design in DFIG based wind turbines, and investigates stability and performance of the drive train dynamics against different control strategies. It is shown that speed controller design based on the single mass drive train model may result in unstable mechanical modes, because it ignores the dynamics of the flexible shaft. Then, another control approach, known as feedforward compensation of the shaft torsional torque, is examined. It is shown that this control method results in poorly damped oscillations of torsional torque and turbine speed during the transient conditions. The open loop transfer function from the electromagnetic torque to the generator speed contains a dual quadratic function representing the dynamics of flexible shaft. The dual quadratic function comprises resonant and anti-resonant frequencies that greatly affect the stability of the drive train dynamics. Next, a step-by-step procedure for designing the speed controller based on the two-mass drive train model is proposed. The proposed speed controller provides stable closed loop system, zero tracking error, low-frequency disturbance rejection, and open-loop gain attenuation at the resonant frequency. At the end, performance of the proposed controller is investigated by the time domain simulations.     

Effect of pressure and equivalence ratio on the ignition characteristics of dimethyl ether-hydrogen mixtures

Experimental and numerical study on the effect of pressure and equivalence ratio on the ignition delay times of the DME/H₂/O₂ mixtures diluted in argon were conducted using a shock tube and CHEMKIN II package at equivalence ratios of 0.5–2.0, pressures of 1.2–10 atm and hydrogen fractions of 0–100%. It was found that the measured ignition delay times of the DME/H₂ mixtures demonstrate three ignition regimes. For the DME/H₂ mixture at XH₂$X_{{\text{H}}_2}$ ≤80%, the ignition is controlled by the DME chemistry and ignition delay times present a typical Arrhenius pressure dependence and weak equivalence ratio dependence. For the DME/H₂ mixture at 80% < XH₂$X_{{\text{H}}_2}$ < 98%, the ignition is controlled by the combined chemistries of DME and hydrogen, and the ignition delay times give higher ignition activation energy at higher pressures and a typical Arrhenius equivalence ratio dependence. However, for the DME/H₂ mixture at XH₂$X_{{\text{H}}_2}$≥98%, the ignition is controlled by the hydrogen chemistry and ignition delay time shows complex pressure dependence and weak equivalence ratio dependence. Comparison of the measurements of neat DME and neat hydrogen with the calculations using three generally accepted mechanisms, NUIG Aramco Mech 1.3 [1], LLNL DME Mech 2, 3 and 4 and Princeton-Zhao Mech [5], shows that NUIG Aramco Mech 1.3 gives the best predictions and can well capture the pressure and equivalence ratio dependence at various hydrogen fractions. The sensitivity and normalized H-radicals consumption analysis were performed using NUIG Aramco Mech 1.3 and the key reactions that control the ignition characteristics of DME/H₂ mixtures were revealed. Further chemical kinetic analysis was made to interpret the ignition delay time dependence on pressure and equivalence ratio at varied hydrogen fractions.     

Concept risk assessment of a hydrogen driven high speed passenger ferry

A concept risk assessment of a hydrogen and fuel cell driven high speed passenger ferry has been performed. The study focused on fatality risk related to the hydrogen systems on the vessel, both during operation and while moored in harbour overnight. The main objective with the study was to evaluate whether the risk related to the hydrogen systems is equivalent to that of conventionally fuelled vessels and can be considered acceptable according to the requirements of the IGF-code (International Code of Safety for Ships Using Gases or Other Low-flashpoint Fuels). Since hydrogen behaves differently than other flammable gases, some adjustments to existing models and vulnerability criteria have been proposed. The conclusion of the study is that the estimated risk related to hydrogen systems is relatively low, and much lower than the expected acceptable risk tolerance level of 0.5–1.0 fatalities per 10⁹ passenger km. Furthermore, for the overnight mooring in harbour the estimated risks are well within acceptable limits. The work presented is part of a maritime case study performed within MoZEES, a Norwegian research centre for environmentally friendly technology and zero emission transport.     

风力机最佳攻角与最大升阻比攻角非等同性研究

为提高风力机风能利用率,追求最大风能利用系数,对水平轴风力机最佳攻角与最大升阻比攻角的非等同性进行了理论分析,并采用叶素动量理论对其进行实例论证,以及通过计算流体力学(CFD)对结论进行仿真验证。研究结果表明,二者具有非等同性,最佳攻角略大于最大升阻比攻角,处于最佳工作点的风力机具有更高的风能利用系数,同时具有最大的速度比阈值区间。研究进一步发现,对于目前工作于最大升阻比攻角状态的风力机,通过减小叶尖速比的方式可以使其过渡到最佳攻角状态。     

Model for optimizing energy efficiency through controlling speed and gear ratio

Specific fuel consumption of vehicles has been improved through technical changes in the last three decades. There has been little study of the impact of behavioral aspects on the specific fuel consumption of vehicles. Analytical tools developed recently have included options for quantifying the specific fuel consumption in certain predefined driving cycles. Empirical studies have shown that real driving cycles are different from predefined ones. It has also been observed that optimal coordination of both speed of vehicle and gear ratio are detrimental to the improved performance of a vehicle. Previous studies have only considered the vehicle speed as the explanatory parameter, and gear ratio has rarely been discussed. Therefore, an optimal model of vehicle fuel consumption has been developed on the basis of microeconomic theories. The application of this model helps to identify the optimal driving strategy of a vehicle in a certain real driving cycle. The model has then been applied to estimate the optimal fuel consumption of a vehicle in a given real case, and the results of model estimate have been compared with measured data. It has been found that the implementation of optimal driving strategy based on the model estimates may help to reduce specific fuel consumption by 3.2 l/100 km on average (37% of actual specific consumption). It is also observed that the potential for energy saving prevails when the traffic flow is dense and slow. Hence, the implementation of optimal driving strategy based on the coordination of speed and gear ratio would lead to the realization of energy saving potential that is considerable.

The nonlinear response of inverted “V” flames to equivalence ratio nonuniformities

Recent studies indicate that heat release fluctuations generated by equivalence ratio perturbations may constitute sources of instability with effects similar to those induced by acoustic perturbations. The present article addresses this issue by considering the dynamics of an inverted laminar dihedral (“V”) flame spreading in an open geometry when this flame is submitted to equivalence ratio modulations. The problem is investigated with numerical simulations by first establishing a steady state flame which then evolves in a uniform flow transporting a fixed level of equivalence ratio perturbations. The flame features wrinkles of increasing amplitude locking on the convected composition perturbations. The wrinkling amplitude grows with distance from the injector. For sufficiently large wrinkle amplitudes, the flame interacts with the fresh mixture outer boundary, giving rise to sudden disruptions of the flame sheet. The rapid burning of fresh mixture pockets generates a nonlinear heat release signal with abrupt changes in the waveform. It is found that high levels of modulation induce axial velocity perturbations, which in turn interact with the flame and modify the response. Calculations described in this article may serve to guide analytical modeling of the response of combustion to equivalence ratio inhomogeneities. A simple model is devised on this basis to distinguish regimes corresponding to weak and strong interactions.     

Environmental performance and economic analysis of all-variable speed chiller systems with load-based speed control

There are increasing views on implementing all-variable speed chiller plants in place of conventional constant speed plants. Supporters of these views claim that all-variable speed chiller systems can operate much more efficiently at part load in response to changes in building cooling load. This paper introduces load-based speed control for all-variable speed plants to optimize their environmental performance. Thermodynamic-behaviour chiller system models were developed to perform environmental assessment (in terms of annual electricity and water consumption) for typical constant speed and all-variable speed chiller systems operating for the cooling load profile of a local office building. Operating cost differences between the two systems were calculated and compared in an economic analysis. Applying load-based speed control to the variable speed chiller plant can decrease the annual total electricity use by 19.7% and annual water use by 15.9% relative to the corresponding constant speed plant. The significance of this study is to provide more insights into how to make chiller systems more sustainable.