Transient Operation Research Articles

Predicting the dynamic behavior of a magnetocaloric cooling prototype via artificial neural networks

Although magnetocaloric cooling is considered a promising long-term alternative to vapor compression, recent prototype developments have not yet made this technology commercially competitive, primarily due to its high energy consumption and lack of cost-effective, long-term mechanically-chemically stable materials. To address the first issue and understand how the efficiency of magnetocaloric systems can be improved, dynamic models can offer valuable insights into their transient operation. This work focuses on the development of an artificial neural network with experimental data to model the dynamic operation of a magnetic refrigeration system. Through a design of experiments approach, we propose excitation signals for the identification experiment, involving five manipulated variables and one selected disturbance as inputs, with the output temperature of the cold manifold and power consumption as the target parameters. We chose a nonlinear autoregressive artificial neural network with exogenous inputs to model the transient operation of the system. The temperature model achieved R2 values of 0.995 and 0.955 for the 1-step and 90-step ahead predictions, respectively. Similarly, the power consumption model achieved R2 values of 0.988 and 0.949 for the 1-step and 90-step ahead predictions, respectively. These performance metrics were evaluated on the test sets that were not used for training the models, highlighting the robustness and accuracy of the models in both short-term and long-term predictions.

Thermal and hydraulic characteristics analysis of horizontal lead-bismuth reactor assembly based on sub-channel analysis code

In this paper, the self-developed subchannel analysis program SACOS-PB was used to analyze the thermal hydraulic characteristics of 127 rods in horizontal lead-bismuth reactor under steady state and typical ocean transient conditions. The results of the steady-state analysis showed that the coolant flow and temperature of the vertical assembly were symmetrically distributed, the coolant flow of the horizontal assembly gradually increased along the gravity direction, the temperature rose first and then fell, and the peak coolant temperature of the cross-sectional channel was about 365 °C. Three typical ocean transient operating conditions, inclined, undulating and swing, were also carried out. The results showed that the maximum temperature of the coolant at the exit of the assembly increased with the increase of the transverse heeling angle and the decrease of the trim angle after the incline motion changes the gravity component. Under the undulating and swing conditions, the coolant hydrothermal parameters fluctuated periodically, and the parameter fluctuation period is 10s in line with the oceanic conditions. In the above transient conditions, the peak coolant temperature at the assembly outlet increases and exceeds 365 °C. Compared with the pitching motion on the Y-axis, the rolling and tumbling motion on the X-axis had a significant effect on the coolant flow rate and temperature distribution. The calculation results can provide support for the analysis study of the safety characteristics of lead-bismuth power units under marine conditions.

Research on the Optimal Design Approach of the Surface Texture for Journal Bearings

Aiming to improve the comprehensive performance of the journal bearing system, this paper presents a multi-objective adaptive scale texture optimization design approach. A mixed lubrication model for the textured journal bearing system is established by considering the effects of cavitation and roughness. The geometrical parameters of the textures were co-optimized using a multi-objective grey wolf optimizer to obtain the optimal texture schemes that are suitable for different operating conditions. Through this approach, the influences of different texture schemes under transient operating conditions can be investigated. According to the results, it was found that different texture schemes result in different friction reduction effects. Proper surface texture is beneficial in increasing the minimum oil film thickness and reducing the possibility of asperity contact. The adaptive scale texture exhibits strong adaptability and achieves significant hydrodynamic effects. Therefore, the developed approach provides valuable insights for the optimization design of journal bearing systems.

Experimental investigation of time-dependent electrical load effects through multipoints in-situ measurement of temperature and relative humidity of PEMFC bipolar plate under transient operation

In this study, multiple micro-relative humidity and temperature (RH/T) sensors were installed in the anode and cathode channels of a proton exchange membrane fuel cell (PEMFC) bipolar plate to simultaneously observe and analyze the variation in RH/T behaviors during transient operation. In order to simulate the acceleration and deceleration process, the electrical load was ramped from 10 to 40 A and then back to 10 A, with time intervals of 0, 2, and 5 s. The data corresponding to different acceleration/deceleration time intervals were compared and analyzed to assess the time-dependent effects of load changes. The analyzed data provide visual information, including the transient voltage response, transient behavior of temperature and water distribution, and evolution of RH/T within PEMFC. The results indicate that the acceleration and deceleration interval times significantly influence the temperature and water distribution within PEMFC during transient operation. Shorter acceleration times lead to more rapid changes in temperature and humidity within the PEMFC active area, resulting in higher transient temperature and water concentration. This finding explains why the output voltage decreases more when the load changes with a short time interval. This research enhances our understanding of transient temperature and water behavior, providing a foundation for further research and development of effective operational strategies and vehicle PEMFC designs.

RELAP5-based thermal-hydraulic assessment of the STEAM facility for DEMO WCLL balance of plant analysis

DEMO power station aims to demonstrate the generation of hundred MWs of electrical power from fusion reactions, then transmitted from the Tokamak reactor to the grid through the Balance of Plant (BoP). The design approach for the Water Cooled Lithium Lead (WCLL) Breeding Blanket (BB) Primary Heat Transfer Systems (PHTSs) leverages nuclear industry expertise but faces challenges due to DEMO pulsed operation and low-load periods. To assess the feasibility of these components, ENEA Experimental Engineering Division at Brasimone R.C. is designing STEAM, a facility investigating water technologies applied to the DEMO BB and BoP systems and components. STEAM is mainly composed of a primary system reproducing the DEMO WCLL BB PHTS thermodynamic conditions (15.5 MPa, 328–295 °C) and a secondary two-phase (liquid/steam) loop reproducing the DEMO power conversion system conditions (6.4 MPa, 238–300 °C). Experimental validation will reproduce steady-state and transient operation under DEMO-relevant conditions, including dedicated tests on the DEMO once through steam generator mock-up.STEAM objectives and description are presented in this paper, together with the RELAP5/Mod3.3 nodalization of the facility. The latter is used to, thermal-hydraulically characterize the facility behaviour. The outcomes of the steady-state qualification supported the optimization of the system layout appraising the performances of key components under the specified operating conditions.

Comparative analysis of different FeCrAl alloys in pressurized water reactors

Discussing and researching issues related to their safety is essential to guarantee the continuity of fission systems in generating energy for existing and new electrical matrices. Commercial fission reactors currently in operation have undergone a series of redesigns. One aspect much discussed in recent years is related to research about new fuels and cladding materials. Therefore, in this work, the feasibility of replacing Zircaloy-4 with FeCrAl-based alloys (C06M, C35M, C36M, Kanthal APMT) was evaluated considering these materials' physical–chemical, thermal, and neutronic properties. Moreover, thermal–hydraulic and neutronic simulations have been presented to compare the cladding behavior in a PWR core in steady state and transient operation conditions. The results showed, in a general way, that the analyzed alloys have equivalent properties from a physical–chemical and thermal point of view. Despite the similarity, the Kanthal APMT alloy presented the worst performance, and the C35M alloy showed more advantageous characteristics. Considering the similarities in the composition of the C06M, C35M, and C36M alloys, which have the same elements in their constitution, the alloy with the best thermal performance has the lowest aluminum content. Considering the neutronic behavior, the Kanthal APMT alloy, with more Cr percentile, presents the lowest neutron effective multiplication factor, keff. Axial and radial power distribution did not change significantly after the cladding replacement. Nevertheless, modifications must be made to keep the criticality during the fuel cycle burnup for all FeCrAl alloys.

Prediction of mechanical efficiency of spur gear pairs based on tribo-dynamics model of multi-tooth meshing

The prediction of load-dependent losses of gear pairs is a topic of constant concern, which depends largely on the modelling of friction coefficient in tooth meshing. Although the current load-sharing based models can handle the friction coefficient in mixed elastohydrodynamic lubrication regimes, dynamics behaviour of each tooth pair or the time variation of direction of normal contact load is not taken into consideration. This study proposes a new method for modelling efficiency of spur gear pairs in transient operating conditions by coupling the multi-tooth meshing model with the friction coefficient model. The friction coefficient is modelled through load-sharing function, and the gear dynamics model is established with regard to alternate meshing of single-double tooth pair and time variation of direction of normal load and mesh stiffness. The model is validated by comparing the calculated results of friction coefficients and load-dependent losses with experimental data in published literatures, showing good agreement. At the end, the efficiency model is applied to two types of gear pairs to investigate the influence of tooth shapes and operating parameters on the load-dependent losses of gear pairs.

Adoption of a fast dynamic model to simulate transient operations of MNSR

After developing a thermal–hydraulic model, it should be coupled to a point kinetic model to completely simulate transient behavior of a miniature neutron research reactor (MNSR). In this study introduction and validation of such a dynamic model which could be used as computing source of a micro-simulator, is presented. A time dependent point kinetic model in association to a simple control algorithm and reactivity feedbacks calculation (poisons and temperature) is adopted. In order to validate the model, it is compared against MNSR operational data in different scenarios such as normal fixed power operations, and reactivity insertion operations, and post shutdown scenarios. Result indicates that the coupled model could estimate the critical rod position for different power levels with an error less than 5%. Although the control algorithm is simple, it could predict the actual trend of control rod position accordance to the actual data. The coupled model simulates different reactivity insertion accident similar to the reactor operations in such a way that the error between estimated power peak, peak time, and long-term power is less than 5%.

Comparison of the transient response of a compression ignition engine running with gasoline, naphtha, and methanol as fuels

This study conducted a detailed investigation to evaluate the transient operation of a compression ignition engine using three low-reactivity fuels. The fuels tested were gasoline, methanol, and naphtha. This research gives valuable insight into how these alternative fuels behave under dynamic operating conditions compared to stationary conditions. Employing a comprehensive two-part methodology, the study firstly undertakes experimental procedures involving parametric sweeps to optimize the engine performance across three operating conditions, and a dedicated calibration was achieved for the engine operation for each of the three fuels. With this calibration, two transient cycles were proposed for evaluating the engine response regarding the injection system, air management system, and emissions. The transient response was assessed through load variations at fixed engine speeds and combined cycles of speed and load transitions. The second part of the work covered a numerical assessment using GT-Power to simulate global emissions in a Class 3 WLTC. The findings reveal that naphtha outperforms other LRFs like methanol and gasoline in torque response and stability in fixed-speed cycles. Methanol, while exhibiting almost negligible soot emissions, encountered significant combustion stability challenges under low-load conditions, rendering it less suitable for exclusive use in real vehicle scenarios. On the other hand, naphtha showed promising results as a diesel alternative in the WLTP cycle, achieving similar torque and speeds, with a substantial reduction of 40 % in NOx emissions, a slight increase of 9 % in soot emissions, and an important reduction of 10 g CO2/km in carbon impact. However, the rise in unburned hydrocarbon species poses a challenge for meeting tailpipe emissions regulations.

Overview of Fourth Generation Reactors Studied at the Universidade Federal de Minas Gerais (Brazil)

This paper presents the results of the studies about the Fourth Generation Reactors developed at DENU-UFMG, such as the Very High Temperature Reactor (VHTR), Molten Salt Breeder Reactor (MSBR), Sodium Fast Reactor (SFR) and Gas-cooled Fast Reactor (GFR). These studies evaluate neutronic parameters of the nuclear system at steady state and during the burnup using computer models developed at DENU-UFMG using SCALE 6.0 (KENO-VI/ORIGENS), MCNPX 2.6.0 and MCNP5. The results show that the adopted methodologies confirm the capabilities of the codes to simulate specific situations in steady state or transient operating conditions.

Fatigue life and transmission efficiency prediction of marine timing gears under 3D mixed lubrication state

The timing gears are the core transmission components of marine diesel engine, whose working conditions are complex and harsh, transient and coupled with the microscopic lubrication state, meshing in the mixed lubrication environment with lubrication-contact coexistence, local asperity contact brings stress concentration, further affects the fatigue life and friction loss. In this study, based on a three-dimensional (3D) line-contact mixed lubrication model, dynamic subsurface stress calculation, fatigue life prediction, and friction loss power study were carried out on timing gears, taking into account transient operating conditions and actual surface roughness. For the established simulation model, an equivalent simulation test of line-contact friction of timing gears was carried out, which can realize the measurement of friction coefficient under the condition of different slip-roll ratios, and make up for the deficiency that the traditional test equipment can only carry out the test under the fixed slip-roll ratio. Although the gear meshing process can only be simulated by the disc specimen due to the limitation of the specimen form, this limitation is similar to the simplified form of the simulation model, which can meet the needs of the verification test. The influence of structure and material on the lubrication state, fatigue life and friction power consumption are studied, which provide theoretical guidance for the tribological optimization design and reliability analysis for the marine timing gears.

Low Temperature Combustion Modeling and Predictive Control of Marine Engines

The increase of popularity of reactivity-controlled compression ignition (RCCI) is attributed to its capability of achieving ultra-low nitrogen oxides (NOx) and soot emissions with high brake thermal efficiency (BTE). The complex and nonlinear nature of the RCCI combustion makes it challenging for model-based control design. In this work, a model-based control system is developed to control the combustion phasing and the indicated mean effective pressure (IMEP) of RCCI combustion through the adjustments of total fuel energy and blend ratio (BR) in fuel injection. A physics-based nonlinear control-oriented model (COM) is developed to predict the main combustion performance indicators of an RCCI marine engine. The model is validated against a detailed thermo-kinetic multizone model. A novel linear parameter-varying (LPV) model coupled with a model predictive controller (MPC) is utilized to control the aforementioned parameters of the developed COM. The developed system is able to control combustion phasing and IMEP with a tracking error that is within a 5% error margin for nominal and transient engine operating conditions. The developed control system promotes the adoption of RCCI combustion in commercial marine engines.

Phototriggered Equilibrated and Transient Orthogonally Operating Constitutional Dynamic Networks Guiding Biocatalytic Cascades.

The photochemical deprotection of structurally engineered o-nitrobenzylphosphate-caged hairpin nucleic acids is introduced as a versatile method to evolve constitutional dynamic networks, CDNs. The photogenerated CDNs, in the presence of fuel strands, interact with auxiliary CDNs, resulting in their dynamically equilibrated reconfiguration. By modification of the constituents associated with the auxiliary CDNs with glucose oxidase (GOx)/horseradish peroxidase (HRP) or the lactate dehydrogenase (LDH)/nicotinamide adenine dinucleotide (NAD+) cofactor, the photogenerated CDN drives the orthogonal operation upregulated/downregulated operation of the GOx/HRP and LDH/NAD+ biocatalytic cascade in the conjugate mixture of auxiliary CDNs. Also, the photogenerated CDN was applied to control the reconfiguration of coupled CDNs, leading to upregulated/downregulated formation of the antithrombin aptamer units, resulting in the dictated inhibition of thrombin activity (fibrinogen coagulation). Moreover, a reaction module consisting of GOx/HRP-modified o-nitrobenzyl phosphate-caged DNA hairpins, photoresponsive caged auxiliary duplexes, and nickase leads upon irradiation to the emergence of a transient, dissipative CDN activating in the presence of two alternate auxiliary triggers, achieving transient operation of up- and downregulated GOx/HRP biocatalytic cascades.

CO2 capture from flue gases in a temperature swing moving bed

This paper presents a test stand for the capture of CO2 from flue gases arising due to firing pulverised hard coal. The stand, financed from the 2014–2021 Norway Grants, is installed at a Polish power plant. The innovation of the proposed CO2 capture method, developed by the Norwegian partner in the project (SINTEF Industry), lies in the use of activated carbon in the process of temperature swing adsorption in a moving bed. The paper also presents preliminary results of numerical simulations performed using the General PROcess Modelling System (gPROMS) software. The simulations concerned the operation of a supercritical power unit combined with a system for capturing CO2 from flue gases. Transient operation of the system was analysed, assuming rapid changes in the power unit load. Special attention was paid to the CO2 capture process energy consumption at an increase in load by 5% of the power unit nominal capacity in 30 s. It is found that the proposed CO2 capture method “keeps up” with such rapid load changes at the method energy consumption smaller than 2 MJ/kg CO2.

Mathematical modeling of dynamic processes of agricultural mobile energy vehicles on an electric drive

The article considers the issues of modeling the processes of agricultural mobile energy vehicles (MEV) with electric drive (ED). A review of modern literature on the problem under study as well as questions on modeling the functional properties of mobile machines and improving the quality indicators of MEV work are presented. A mathematical model of the motion of the MEV with an electric motor is presented, as well as a description of the method used and a sequence of actions for conducting research. Preliminary theoretical studies of motion in different modes of operation have been carried out. The proposed model is convenient for implementation and calculation in any of the applied software products that support the modeling of dynamic systems with an electromechanical drive. The proposed model, the solution of which is based on the methods of numerical integration of systems in the Matlab Simulink software environment, made it possible to simulate the dynamic processes of electromechanical power transmission of MEV during various agricultural operations. With the help of this model, the analysis of electromechanical processes in transient and steady-state operating modes, as well as dynamic processes in the power transmission were carried out. Graphs of changes in the studied parameters of the MEV power transmission are obtained and elastic moments in the joints of the 5-mass design scheme are determined. The use of the model allows you to track the change in statistical characteristics when the conditions of the experiment change. The model has shown its operability when performing simulation of agricultural operations: fertilization, cultivation and sowing, and it can be used at the design stage to study the characteristics of dynamic processes of small-class traction MEV power transmissions with an electromechanical drive. With different parameters of the model, there is a change in the mathematical expectation of the angular velocity of the ED from 147.89 to 156.87 rad/s, a change in the mathematical expectation of the speed of the MES from 4.51 to 4.79 m/s.

Adiabatic Compressed Air Energy Storage system performance with application-oriented designed axial-flow compressor

Medium and long-duration energy storage systems are expected to play a critical role in the transition towards electrical grids powered by renewable energy sources. ACAES is a promising solution, capable of handling power and energy ratings over hundreds of MW and MWh, respectively. One challenge with ACAES is achieving the required highly efficient operation in the compressor over the range of conditions encountered in the system as the pressure in the air store changes. In this paper, an application-oriented axial-flow compressor is designed, aiming towards efficient operation throughout the operation range, whilst also associating the performance prediction to a practical compressor geometry. A two-step design methodology based on inviscid, axisymmetric flow conditions has been implemented, leading to the flowtrack, blade-row geometries and the compressor performance map. The compressor model is integrated into an ACAES model, including two compression spools, two expansion stages with preheat, a constant volume high pressure storage operating between 5.5 and 7.7 MPa and two separate Thermal Energy Storage units. While the existing ACAES literature either ignores the transient off-design operation or uses generic numerical correlations (which are not associated to a particular geometry), the key novelty of this paper is the application of a detailed design method for turbomachinery to ACAES. The results indicate that the designed compressor requires 33 stages over the two spools, and is able to operate efficiently over the storage pressure range, showing that if the application-oriented design procedure is applied to the compressor, it does not stop ACAES reaching 70% round-trip efficiency, outputting 35MW for approximately 15 h. Importantly, the specific ACAES requirement of conserving heat at higher temperatures has been fulfilled by decreasing the number of intercoolers. Finally, it is recommended that a similar level of scrutiny is applied to the other components (i.e. expanders, heat exchangers and TES units), keeping in mind the unique set of operational requirements of ACAES. This work is an important step towards removing the common misconception that off-the-shelf components can be easily be used in typical ACAES designs.

Design of Multi-Input Multi-Output Non-linear Model Predictive Control for Main Steam Temperature of Super Critical Boiler

Flexible operation of coal-fired power plants is becoming increasingly necessary for successful integration of large-scale renewable power generation into the power grid. The maximum ramp rate and the number of load cycles are generally limited by the thermal stress experienced by the boiler pressure parts, turbine metallurgy and creep and fatigue of critical thick-walled components Main steam temperature is a critical operating parameter that must be controlled within acceptable limits for safe operation. Main steam temperature deviation beyond acceptable limit has impact on boiler pressure parts and turbine material of construction due to creep and fatigue effect. Base load operating units do not require steep ramp rate and hence recommended ramping rates are kept low within the safe operating zone in comparison to the flexible operation of the units with wide range load change width. Thermal stresses are caused by the temperature changes inside the thick-walled components and turbine steam admission parameters. Hence, the quality of main steam temperature control plays a vital role in flexible operation of the coal fired units. Conventional cascaded PID temperature control loop architecture performs well at steady state condition within a limited variation of load change at low ramp rate but it acts slowly and performs poorly at transient operating conditions of flexible operation of the boiler turbine with wide range load variation and load cycle with high ramp rate and remains far from rated conditions. In this paper, a Multi-Input Multi-Output (MIMO) Non-linear Model Predictive Control (MPC) design for regulation of the main steam temperature of a Once-Through supercritical Boiler is proposed. The controller is based on a non-linear dynamic model which incorporates dynamics of the variables of interest. It has the capability to operate effectively across a wide load range while maintaining main steam temperature within acceptable limits. A notable advancement in this design of MPC is the incorporation of coal flow demand and feedwater flow demand as additional control inputs alongside primary and secondary spray flows. In simulation test cases, the MPC controller demonstrates satisfactory performance and computational efficiency.

Formation and evolution of vortex breakdown consequent to post design flow increase in a Francis turbine

Draft tube flow instability encountered under off-design operating conditions in hydraulic turbines significantly limits their operational flexibility. The instability arises consequent to a higher than threshold swirl content in the runner outflow and leads to vortex breakdown phenomenon in the draft tube cone. At high load condition, the phenomenon presents as an enlarged vortex core counter-rotating with respect to the runner. The flow situation is known to compromise the turbine efficiency besides the generation of unwanted effects such as power swings and large-scale pressure fluctuations. The present paper is the first to encapsulate a thorough numerical investigation on the formation and evolution of the enlarged vortex core alongside the consequent effects. A transient operating sequence between best efficiency and high load operating points in a model Francis turbine is simulated. Turbulence closure has been attained using the shear stress transport-scale adaptive simulations turbulence model. Dynamic meshing based on a Laplacian smoothing scheme has been used to account for mesh deformation arising from guide vane motion during load change. The pressure and velocity fields have been determined and analyzed to elucidate the physics of vortex breakdown, the phenomenon underlying the formation of the enlarged vortex core. Furthermore, pressure fluctuations at salient points in the domain have been analyzed using Fourier and short-time Fourier transforms. Finally, the enlarged vortex core formed in the draft tube has been visualized through the λ2 criterion. The core takes the shape of a cork-screw like compactly wound spiral structure extending up to the draft tube elbow.

Multiphysics modeling and experimental validation of heat and mass transfer for the vacuum induction melting process

Vacuum induction melting is crucial in casting nickel-based superalloy components, ensuring excellent properties for aero-engine applications. Precise melting temperatures are vital for achieving optimal metallurgical quality before casting. Hence, a multiphysics numerical model is developed to simulate the induction melting process for the Inconel 718 superalloy. The proposed holistic model integrates magnetic fields, induced currents, and heat and momentum transfer phenomena in a single coupled model. A moving mesh approach reproduces the magneto-hydrodynamic behavior of the free surface, simulating the oscillations of the melt. The stabilized deformed surface profile is correlated with experimental measurements, reporting a proper correlation. Then, the flow field and recirculation effect are modeled with a Low Reynolds Number turbulence approach and coupled with the melt convective heat transfer, developing a complete magneto-thermo-hydrodynamic model. In a laboratory-scale vacuum induction melting furnace, transient melting operation variables are characterized and introduced to the numerical model to compute the temperature evolution. An accurate reproduction of the transient melt temperature variations is reported with a relative error of less than 5%. The influence of the crucible thermal insulating capacity is assessed, emissivity dependence evaluated, and melt homogenization is reported at different process stages. This comprehensive numerical and experimental approach offers valuable insights for enhancing vacuum induction melting for Ni-based superalloys.

Modeling and Control of Micro Grid Powered by Maximum Power PV Array and Fixed Speed Wind Energy Conversion System

In this paper, the dynamic modeling of the studied micro grid (MG) is presented for steady state and transient operation conditions. Moreover, control strategies to obtain the maximum converted power and stabilize the voltage under different operating conditions are derived. The integrated system under study has four key subsystems to supply the required electric loads. The first and second subsystems include the layout of the studied MG and the renewable generation sources from Wind turbine and PV array. The third represents the boost converter between the PV and the DC collection bus as well as the interfaced inverter to the point of common coupling (PCC) with the MG. The fourth subsystem comprises mainly the required PI controllers to regulate the modulation index of the inverter and the duty cycle of the boost converter for PV maximum in addition of regulating the boost inverter DC link voltage. The proportional and integral parameters of these controllers are tuned off-line to enhance the grid performance. The integrated system and the associated subsystems are fully validated for efficient operation under normal or fault conditions using the Matlab/Simulink/Sim Power software.