Bypass Valve Opening Research Articles

Study on gasdynamic interaction in a regulated two-stage turbocharging turbines at pulsating conditions

Understanding the response of performance of two-stage turbine to pulsating flow in exhaust manifold system is essential for performance improvement of engine. This paper studies unsteady interaction of two turbines in a two-stage turbocharging system via experimental method and a reduced order model. Firstly, experimental results of the unsteady pressure ratio of two turbines are discussed in cases with different engine speeds, engine loads, and bypass valve openings. Results show that the pressure ratio distribution between two turbines is profoundly influenced by the engine condition. The pressure ratio of low-pressure turbine (LPT) increases consistently with the engine speed and load, while it increases slightly or even remains unchanged for high-pressure turbine (HPT). HPT is de-loaded when the bypass valve is open, and there are large numbers of fluctuations with high frequencies in the trace of pressure ratio when the bypass valve is open. Next, a reduced order model of the two-stage turbine is established for turbine responses at pulsating conditions, and model validations of performance and unsteady pressure in two-stage turbine all show high precision. Finally, the model is applied for performance analysis of two-stage turbine system under pulsating conditions. The results show that with the increase in engine speed, load or bypass valve opening, the hysteresis loops of both turbines are expanded around the steady performance curve. However, the deviation of cycle-average performance from steady case for two turbines is notably different. LPT shows a stronger throttling effect at pulsating inflows. Moreover, compared with steady performance for whole two-stage turbine, cycle-average performance is reduced for all pulsating cases, especially for cases of open valve. The differences in turbine performance of two-stage turbine system caused by gasdynamic coupling effect of two turbines are suggested to be considered in the turbo-engine matching.

An effective PEMFC system shutdown purge strategy for improving the purging effect of liquid water and the dehydration of stack

The shutdown process is one of the critical stages when the proton exchange membrane fuel cell system runs as an actual application. Finding the effect of system operating conditions on shutdown purging could improve the purging effect of liquid water and the dehydration of the stack. In this study, an automotive fuel cell system of 350 single cells was developed on the vehicle scale and tested for factors that influence shutdown process. Seven shutdown purge cases for about 150 s were studied to investigate the shutdown purge strategy. It was found that 100% bypass valve opening reduced the purge time and improved the purge efficiency. The HCP speed should not be too slow or too fast in the shutdown process. The higher temperature of coolant water favored rapid shutdown and purge, and a large stoichiometric ratio of the cathode inlet improved the purge effect but enhanced the energy consumption of the air compressor. Neither too high nor too low purge pressure could blow out the liquid water more efficiently and improve the ohmic resistance. During purging, current density mainly influenced the water production rate and the maintenance state of stack temperature. With a higher stack temperature, purge process with a small current density could quickly reduce the stack humidity and enhance the ohmic resistance. Accelerating the opening rate of the anode purge valve could not effectively accelerate purge effect of the stack, while extending the valve closing time could improve hydrogen utilization. Future studies to investigate the combined impact of these factors on the fuel cell system shutdown purge will be carried out to develop the most effective strategy.

Thermal management of bypass valves for temperature difference elimination in a 5 kW multi-stack solid oxide fuel cell system

Large-scale solid oxide fuel cell (SOFC) stationary power generation systems require multi-stack SOFC integration. System consistency has a significant impact on stable, long-term operations. Investigations on the flow and temperature distributions of the system and effective thermal management are necessary to improve system consistency. In this study, a multi-stack SOFC system homogeneity thermal management method and its decision process are proposed based on the flow network method. The flow and thermal distributions and performance are investigated by considering air stoichiometries of 1.5, 2.0, and 2.5. A thermal management strategy using bypass valves is proposed to solve the maldistribution issue of the system. Both the flow characteristics calculated by the flow network and the electrothermal characteristics of the stack determine the bypass valve opening (or air mixing ratio). The results indicate that a maximum temperature difference of 60 K can be reduced to 0 K through the proposed thermal management strategy in a 5 kW multi-stack system. The application of the bypass valve enables the system to attain temperature consistency, and the thermal efficiency of the entire SOFC combined heat and power system can be increased by up to 50% at the set temperature.

Development of a Zero-Dimensional Model for a Low-Speed Two-Stroke Marine Diesel Engine with Exhaust Gas Bypass and Performance Evaluation

Most large commercial vessels are propelled by low-speed two-stroke diesel engines due to their fuel economy and reliability. With increasing international concern about emissions and the rise in oil prices, improvements in engine efficiency are urgently needed. In the present work, a zero-dimensional model for a low-speed two-stroke diesel engine is developed that considers the exhaust gas bypass and geometry structures for the gas exchange model. The model was applied to a low-speed two-stroke 7G80 ME-C9 marine diesel engine and validated with engine shop test data, which consisted of the main engine performance parameters and cylinder pressure diagrams at different loads. The simulation results were in good agreement with the experimental data. Thus, the model has the ability to predict engine performance with good accuracy. After model validation, the variations in compression ratio, fuel injection timing, exhaust gas bypass valve opening portion, exhaust valve opening timing, and exhaust valve closing timing effects on engine performance were tested. Finally, the influence level of different parameters on engine performance was summarized, which can be used as a reference to determine the reasons for high fuel consumption in some cases. The developed engine performance model is considerable in digital twins for performance simulation, health management, and optimization.

A modeling study on water and thermal management and cold startup of unmanned aerial vehicle fuel cell system

The water and thermal management of hydrogen/air proton exchange membrane fuel cell (PEMFC) system in a variable environment and its fast and efficient cold start at low temperature are the key issues in the practical design and integration of unmanned aerial vehicle (UAV) fuel cell power system. Herein, a complete mathematical model of fuel cell stack and balance of plant (BOP) is established, then the nonlinear relationship between cooling water flow rate, coolant bypass valve opening, and stack temperature at different flight altitudes is studied for a typical UAV configuration, and the safe working range of stack is pointed out. In addition, a cold-start model supporting operation at the high current density is proposed. After analyzing the cold start-up process of the fuel cell, it is found that a reduction of active reaction area caused by icing is the main reason for voltage loss. The optimal self-start current densities of the fuel cell at different low temperatures are detected by the galvanostatic method, as a result, a successful self-startup of the fuel cell is achieved at a temperature of 263.15 K.

Dynamics and control of a turbocharged solid oxide fuel cell system

• A cold bypass valve control can comply with the fuel cell system constraints. • Four controllers were tested on the system during load variation scenarios. • The cascade controllers with feed-forward approaches are the most promising. • A proportional-integrative cannot control the system properly for quick transients. The purpose of this paper regards the design and testing of control systems for a 30-kW turbocharged solid oxide fuel cell system fuelled with biogas. The adoption of a turbocharger, instead of a micro gas turbine, for the fuel cell stack pressurisation, is an innovative solution that is expected to decrease the capital cost of such systems and to facilitate their penetration into the energy market. However, not being connected to an electric generator, the turbocharger rotational speed, and thus the air mass flow, cannot be directly controlled as in microturbines. The control of turbocharged solid oxide fuel cell systems is a novel topic, characterised by many technical challenges that have not been addressed before. To regulate the stack temperature, a cold bypass valve is included, connecting the compressor outlet to the turbine inlet. A dynamic model of this system was developed in Matlab-Simulink® to analyse the response of the turbocharged solid oxide fuel cell system to a cold bypass valve opening step change. System information obtained from this analysis was used to design and tune four controllers: a conventional proportional integral controller and three different cascade controllers. The controller performance was evaluated under two different scenarios, considering quite aggressive power ramps. The best results were obtained with a cascade controller, where the feedback loop was complemented by a feed-forward contribution based on power demand. This analysis demonstrated that such a control system effectively tracks the fuel cell maximum temperature target, complying with all the system operative constraints.

System optimization of thermal management performance of fuel cell system for automobile

Vehicle fuel cell systems release a large amount of heat while generating electricity. The suitable thermal management system must be built to ensure system performance and reliability. Based on the analysis of the working principle of the vehicle fuel cell thermal management system, the paper establishes a control-oriented fuel cell thermal management. The stack, air cooler, hydrogen heat exchanger, bypass valve, heat sink, and cooling water circulating pump model are taking into account. System model, and the relationship between stack current, coolant flow rate, fin surface wind speed, bypass valve opening, and fuel cell temperature are in established in simulation experiments. The paper discusses their effects on system as a whole, air coolers, hydrogen heat exchangers, and the influence of the temperature difference between the inlet and outlet of the radiator. The simulation results can provide guidance and help to design the fuel cell thermal management control system.

Application of Multi-Parameter Fuzzy Optimization to Enhance Performance of a Regulated Two-Stage Turbocharged Diesel Engine Operating at High Altitude

Air intake and fuel supply conditions are the major factors that affect diesel engine performance at plateau. In a regulated two-stage turbocharged diesel engine, these parameters are reflected as the adjustment of fuel injection mass (mfuel), fuel injection advance angle, and bypass valve opening of a high-pressure stage (HP) turbine. Due to the strongly nonlinear nature and complexity of the diesel engine, it is difficult to find the proper parameter combinations. That is why a model-based optimization method is adopted in this paper. The simulation model of a six-cylinder two-stage turbocharged diesel engine is built on the GT-SUITE platform. According to the analysis of diesel engine operation characteristics at high altitude, a fuzzy optimization algorithm is proposed based on a fuzzy logic controller and is realized in a MATLAB/simulink (MATLAB 2014, Mathworks, Natick, MA, USA) environment. Joint optimization of air intake and fuel supply parameters is then performed on the GT-MATLAB co-simulation platform. Results show that engine torque at full load is significantly increased. At the full load point of 2100 r/min, engine power is increased from 256.5 to 319.6 kW, and brake specific fuel consumption (BSFC) is reduced from 243.1 to 222.3 g/(kW·h). Peak torque is increased from 1944.8 to 2173.2 N·m.

Preliminary design and part-load performance analysis of a recompression supercritical carbon dioxide cycle combined with a transcritical carbon dioxide cycle

Nuclear energy can be efficiently converted to electrical energy in the supercritical carbon dioxide (sCO2) power system. In this paper, a combined cycle comprising a topping sCO2 cycle and a bottoming transcritical CO2 cycle (tCO2) is investigated. Multi-objective optimization by means of a genetic algorithm is carried out to obtain better thermodynamic and economic performance in the design stage. The methodology for part-load operation of the combined sCO2-tCO2 cycle is proposed and the quantitative performance analysis is conducted for the utilization of nuclear energy. The results indicate that there exists optimal values for the maximum system exergetic efficiency and the minimum total product unit cost. A composite control strategy by adjusting the rotational speed of compressors and the opening of bypass valve is proposed for the topping sCO2 cycle operation from the point of view of both efficiency and operation range. The bottoming tCO2 cycle is well adapt to the changes of parameters in the topping sCO2 cycle and heat sink temperature by using the sliding pressure control strategy. The combined sCO2-tCO2 cycle can operate under 10–100% normalized generator load when the variation scope of heat sink temperature is 5–25 ℃.The corresponding exergetic efficiency of combined sCO2-tCO2 cycle ranges from 24.5% to 65.7%.

Experimental study on transient flow fluctuation characteristics in a 3 × 3 rod bundle under rolling condition

Abstract Experimental investigation on adiabatic transient flow fluctuation characteristics in a 3 × 3 rod bundles subjected to rolling motion in normal pressure and temperature was carried out. Single-phase flow as well as gas parameters under stagnant liquid condition were concerned. Ranges of liquid Reynolds number and gas flow rate were 657–3 328 and 0.079–0.2 m3/h, respectively. Rolling period and amplitude range from 8 s to 16 s and from 5° to 15°, respectively. Fluctuation amplitudes of transient parameters such as flow rate, pressure drop, void fraction and gas rising velocity were focused on this study. The results indicate that flow rate and frictional pressure drop present obvious fluctuations after rolling motion, and the fluctuation amplitudes decrease with the decrease of bypass valve opening. Flow rate reaches to peak at the positive rolling amplitude and it reaches to trough at the negative rolling amplitude, and there is no obvious phase difference between the fluctuation and rolling angle. Periodical fluctuation of gas flow rate was weakened with the increase of gas inlet pressure and valve opening under experimental conditions, while the variations of void fraction and gas rising velocity were limited. Transient parameter fluctuations correspond to the rolling period at low gas flow rate, whereas their periodical fluctuations are inconspicuous in relative high gas flow rate. Amplitudes of void fraction and gas rising velocity increase with increasing the rolling amplitude while the amplitudes of void fraction and gas rising velocity are nearly invariable with the rolling period no matter if gas flow rate oscillates.

Dynamic Effect of Cold-Air Bypass Valve for Compressor Surge Recovery and Prevention in Fuel Cell Gas Turbine Hybrid Systems

A large volume between compressor and turbine is present in fuel cell gas turbine hybrid systems. The substantially larger compressor plenum volume modifies the dynamic behaviour of these systems, increasing the risk of compressor surge during transients and subsequent destruction of both turbomachinery and fuel cell components. Diverting part of the compressor inlet flow directly to the turbine inlet through a cold-air bypass valve, bypassing the fuel cell stack, has been proven to be an effective method to increase the surge margin during normal operation and also to recover the machine from fully developed surge. This study investigates the dynamic effect of different cold-air bypass valve opening/closing procedures, especially steps and ramps changing the valve fractional opening. This analysis was carried out with reference to the Hybrid Performance (Hyper) facility: a hybrid system emulated using hardware and a cyber-physical fuel cell system at the National Energy Technology Laboratory (NETL), U.S. Department of Energy (DOE). Simulations performed on a Matlab®-Simulink® dynamic model of the system based on Greitzer’s theory showed a different behaviour varying the valve fractional opening with steps or ramps. Many experimental tests were performed on the Hyper facility to confirm the trends obtained from the simulations results. From the outcomes of this study, it has been possible to determine how to maximize the surge recovery effect of the cold-air bypass valve opening and to minimize surge related risks during the valve closure.

Neural-network based boost pressure prediction for two-stage turbocharging system of diesel engine

Abstract A boost pressure prediction research based on neural network is carried out for a diesel engine with two-stage adjustable turbocharging system. First, based on the co-simulation using GT-POWER and MATLAB/Simulink model, the dynamic influence of cycle fuel injection quantity and bypass valve opening of the turbine on boost pressure of diesel engine is studied. The boost pressure shows strong nonlinear relationship with the two aforementioned affecting parameters. Second, a nonlinear autoregressive neural network prediction model (NARXNN) with external input is designed to predict the boost pressure of the diesel engine. The cycle fuel injection quantity, bypass valve opening of turbine, and current boost pressure are used as the inputs of the neural network in order to predict the future boost pressure. Then the neural network prediction model is identified based on the mean absolute percentage error (MAPE). The identification results show that the minimum MAPE (3.52%) is achieved when the number of hidden layer nodes is 15. At last, the neural network prediction model for boost pressure is verified by simulation. The one-step prediction simulation results of boost pressure show that the established prediction model can accurately predict the boost pressure, and the MAPE of the verification data is 3.98%. And the multistep prediction simulation results show that the pressure prediction error in the 50-step prediction domain is within 0.05bar, which indicates that the prediction model has high accuracy.

Study of Energy-Saving Potential of Electronically Controlled Turbocharger for Internal Combustion Engine Exhaust Gas Energy Recovery

To promote the energy utilization efficiency of internal combustion engine, the approach of electronically controlled turbocharger (ECT) for IC engine exhaust gas energy recovery was investigated by the method of test coupling with numerical simulation. First, the tests for turbocharged gasoline engine and high-speed motor were conducted so as to provide experimental data for numerical simulation. Then, the simulation model of ECT engine was built and calibrated, and the working processes of ECT engine were simulated. The results show that the recovered exhaust gas energy by ECT increases with the decrease of by-pass valve opening due to the rising of exhaust gas mass flow rate, but the pumping loss also ascends; limited by the original engine turbocharger map, the engine working points are beyond turbine map when the by-pass valve opening increases to a certain degree. To further improve the energy recovery potential of ECT, a larger turbine was rematched, and the working processes of ECT engine under the whole operating conditions were resimulated. The results indicate that engine exhaust gas energy cannot be recovered by ECT in low-load and low-speed area due to the low exhaust gas pressure. In the effective working area, as the load and speed ascend, both the recovery efficiency of ECT and the utilization efficiency of exhaust gas energy increase, and their maximum values reach 8.4% and 18.4%, respectively. All those demonstrate that ECT can effectively recover engine exhaust gas energy.

Modeling and Model-based Analysis of a Solid Oxide Fuel Cell Thermal-Electrical Management System with an Air Bypass Valve

For stand-alone solid oxide fuel cell (SOFC) power systems without grid connection, delivering demanded power, maintaining system temperature constraints, and achieving high system efficiency are the key design objectives. A typical SOFC system consists of an SOFC stack and a blower, a heat exchanger, an exhaust-burner as the primary components in its balance of the plant. In this work, a novel SOFC system design with an air bypass valve and a target operating range of 1 to 5kW is considered. A system dynamic model, which captures the spatial electrical and thermal distributions of stack, is developed and optimal operating points to maintain the maximum steady state system efficiency are explored while enforcing four constraints on temperature and temperature gradient. Furthermore, the effects of the control variables, namely the bypass valve opening ratio (BP), voltage (U), fuel utilization (FU), and air excess ratio (AR) in improving system performance and expanding feasible operating range are quantified. In addition, the open-loop optimal operation strategy is obtained. The analysis demonstrates that the design of the stand-alone SOFC system is feasible and the bypass valve plays an important role in improving the overall system performance and operation ability.

Test Results of Pressure Head Mitigation of Supercritical Helium Across Cold Circulators at KSTAR for the Justification of the ITER Central Solenoid Cooling Circuit Design

The pressure head necessary for the forced convection of supercritical helium (SHe) across the ITER superconducting (SC) magnet system is generated by centrifugal-compressor type cold circulators. From the viewpoint of the cryogenic system, simulation results indicate that the main challenge of the ITER Central Solenoid (CS) operation is the peaking pressure head of SHe across the cold circulator induced during the rapid ramping of electrical current on the coils. Excess variation in pressure head will push the operation point of the rotating machine beyond the surge line in the compressor map which can trip the circulator and in the worst case result in damages. Therefore, in order to avoid the occurrence of a surge, it has been proposed to open adaptively a cryogenic valve, installed in parallel with the CS cooling channels, during the high pressure head instances. Such an action creates an additional flow channel which consequently suppresses the pressure head peak. The proposed strategy has been numerically benchmarked and tests have been performed in existing cryogenic facilities. In this paper, we will present the test results of pressure head mitigation across the cold circulator dedicated to the KSTAR CS and Poloidal Field (PF) coil cooling circuit with the main purpose of demonstrating the safe operation of the ITER CS circulator as well as cooling circuit during plasma shots. The tests were performed by progressively increasing the coil by-pass valve opening during identical PF/CS current shots and monitoring the tendency of the maximum pressure head variation across the cold circulator. Also, preliminary results of pressure head mitigation during real plasma experiments will be introduced.

고온가스 및 액체 바이패스 적용 용량가변 히트펌프의 성능특성에 관한 실험적 연구

A small air-conditioner or chiller for a constant temperature bath normally uses a constant speed compressor. The constant speed compressor is relatively inexpensive, but it uses on/off control for capacity modulation. The on/off control has several disadvantages, specifically energy loss and large temperature fluctuation. Continuous operation with a bypass system can be an alternative to on/off control, for capacity modulation. In this study, a heat pump system having a hot-gas bypass and a liquid bypass was adopted. The performance of the bypass-type heat pump was measured, by varying the bypass valve opening. The differences of the COP between the hot-gas bypass and the liquid bypass, in the cooling and heating operations, were within 2% and 1%, respectively. The liquid bypass showed a wider range of capacity control in the cooling operation but the hot-gas bypass showed a wider range of capacity control in the heating operation.

Effect of hydrogen addition on combustion and emissions performance of a spark-ignited ethanol engine at idle and stoichiometric conditions

Regarding the limited fossil fuel reserves, the renewable ethanol has been considered as one of the substitutional fuels for spark ignition (SI) engines. But due to its high latent heat, ethanol is usually hard to be well evaporated to form the homogeneous fuel–air mixture at low temperatures, e.g., at idle condition. Compared with ethanol, hydrogen possesses many unique combustion and physicochemical properties that help improve combustion process. In this paper, the performance of a hydrogen-enriched SI ethanol engine under idle and stoichiometric conditions was investigated. The experiment was performed on a modified 1.6 L SI engine equipped with a hydrogen port-injection system. The ethanol was injected into the intake ports using the original engine gasoline injection system. A self-developed hybrid electronic control unit (HECU) was adopted to govern the opening and closing of hydrogen and ethanol injectors. The spark timing and idle bypass valve opening were governed by the engine original electronic control unit (OECU), so that the engine could operate under its original target idle speed for each testing point. The engine was first fueled with the pure ethanol and then hydrogen volume fraction in the total intake gas was gradually increased through increasing hydrogen injection duration. For a specified hydrogen addition level, ethanol flow rate was reduced to keep the hydrogen–ethanol–air mixture at stoichiometric condition. The test results showed that hydrogen addition was effective on reducing cyclic variations and improving indicated thermal efficiency of an ethanol engine at idle. The fuel energy flow rate was reduced by 20% when hydrogen volume fraction in the intake rose from 0% to 6.38%. Both flame development and propagation periods were shortened with the increase of hydrogen blending ratio. The heat transfer to the coolant was decreased and the degree of constant volume combustion was enhanced after hydrogen addition. HC and CO emissions were first reduced and then increased with the increase of hydrogen blending fraction. The acetaldehyde emission from the hydrogen-enriched ethanol engine is lower than that from the pure ethanol engine. However, the addition of hydrogen tended to increase NO x emissions from the ethanol engine at idle and stoichiometric conditions.

Analysis of a BWR Turbine Trip Experiment by Entire Plant Simulation with Spatial Kinetics

An analysis of a boiling water reactor turbine trip was performed with the THYDE-NEU code. In spatial kinetics, reactivity was not used since the three-dimensional transient diffusion equation was solved with the implicit direct integration method. The plant was treated as a closed coolant system, and hence, it was necessary to cope with thermal-hydraulic behaviors at pressures as low as the atmospheric pressure. At low pressures, nonlinearity of the thermal-hydraulic equation is enhanced, and hence, a thermal nonequilibrium model is required. To satisfy the measured initial pressure distribution within the reactor, it was necessary to have the moisture separator model and to account for a reversible pressure drop at a junction with a flow area change. Among the parameters in THYDE-NEU is γ in the thermal nonequilibrium model in addition to C1 and C2 regarding the manner in which to express the coolant density used in the table look-up of cross sections. For a pair of C1 and C2, it is possible to find parametrically a value of γ, namely, γC, so that THYDE-NEU can reproduce the experimental fact that the core-averaged local power range monitor output RAPRM reached 0.95 at 0.63 s to generate a scram signal. One of the calculations with γC was compared with the experiment. It was shown that the spatial kinetics results are sensitive to the temporal behavior of the bypass valve opening. Among the assumptions in use, those to be scrutinized before further performing sensitivity calculations were indicated.

A study on water level control of PWR steam generator at low power and the self-tuning of its fuzzy controller

The water level control system of the steam generator in a pressurized water reactor and its control problems are analysed. In this work, a stable control strategy during low power operation and transient states is studied. The control strategy employs substitutional information using the bypass valve opening instead of incorrectly measured signal at the low flow rate as the fuzzy variable of the flow rate during low power operation, and includes the flexible scale adjusting method for fast response at a large transient. A practical self-tuning algorithm based on the control performance and the descent method is also suggested and applied to tuning the membership function scale of the flow error.