Transient Performance Research Articles

Numerical investigation of the transient fluid flow and heat transfer behavior in self-driven natural circulation cooling loop

An innovative open natural circulation system is proposed in this work, which utilizes the difference of the heat flux loaded on different parts of the pipe to form a self-circulation flow of working fluid. Via CFD simulation established by embedding self-programmed code into commercial software, the transient performance of a three-dimensional natural circulation loop is studied. The results indicate that: applying uniform heat flux on the wall of the loop, the fluid flow state successively experiences three stages, i.e., liquid expansion, partial circulation, and phase change, and finally a vapor plunger blocks the pipe, leading to local heat transfer deterioration and temperature increase; applying non-uniform heat flux, the fluid can be driven to form self-circulation flow, which can not only cool the structure via convection, but also prevent bubbles from accumulating, stabilizing the maximum temperature of the system around the boiling point; further for the loop with a more complex configuration, the self-circulation can be also started but with different start modes. Therefore, the new natural circulation system can achieve effective and adaptive cooling according to the external thermal environment, avoid fluctuation and vapor block phenomenon caused by violent phase change, and reduce the driving equipment and control unit. This work provides a reference for the design of the natural circulation system applied in a non-uniform heating environment. Through transient numerical simulation of the start-up stage, the fluid flow, phase change, and heat transfer characteristics in the natural circulation loop are studied, and the start mode and mechanism are analyzed.

Computational study on transient thermal performance enhancement of phase change materials in triplex tube heat exchanger through novel annular-finned configurations

The introduction of highly conductive media to enhance the thermal performance of phase change materials (PCM) is an efficient method to counter the inherently low thermal conductivity of PCMs while retaining its benefits. This article reports the transient thermal performance of PCM in novel finned configurations based on Triplex Tube Heat Exchangers (TTHX). The shape, size, and placement of the constituent fins in the PCM domain are varied, and their effect on the transient melting rate of PCM is studied. Two PCM formulations, RT-82 and RT-35 are evaluated. Pockets within the encapsulation geometry wherein a vortex-like melting pattern of PCM can be facilitated, are identified as factors for expediting the melting process. The best performing configuration resulted in ∼17% (RT-82) and ∼9% (RT-25) reduction in melt duration compared to the baseline configuration. The effects of gravity conditions on the PCM melt process of the best performing and baseline configurations were also studied.

Thermal performance assessment of exterior building walls under intermittent air-conditioning operation in China's hot summer and cold winter zone

Optimizing the design and accurately assessing the performance of building envelopes are critical for substantially reducing energy use in the building sector. Existing studies on methods for evaluating the thermal performance of building envelopes have primarily focused on periodic unsteady conditions (often associated with continuous air-conditioning operation). This study aims to investigate the initial transient thermal performance of exterior building envelopes. Specifically, we examined varying insulation distributions considering the hot summer and cold winter zone of China as an example. To simulate real-world conditions, we developed a dynamic hot box facility that mimics both outdoor and indoor environments, enabling the measurement of key parameters of the dynamic thermal process of the wall. We evaluated the thermal response, energy performance, and heat storage metrics of the wall to understand the impact of the insulation distribution and air-conditioning operation modes on the thermal performance of opaque building walls. This assessment covered six intermittent operation stages—four construction structures and two types of building usage—for both summer and winter conditions. The proposed methodology provides a quantitative approach for assessing the dynamic thermal processes of the wall, with the results indicating that the heat transmission of walls during intermittent operation exhibited initial transient characteristics. As a result, thermal performance indices suitable for periodic unsteady conditions could not be directly applied to initial transient conditions. To address this issue, the metrics of daily mean dynamic thermal transmittance and daily mean dynamic thermal resistance were proposed and validated. These metrics accurately reflected the actual fluctuations in total transmission loads, thus providing an accurate rapid index-oriented method for assessing the energy-saving performance of walls. Overall, this study lays the groundwork for evaluating dynamic thermal behaviour of building envelopes.

Adaptive neural network fault-tolerant control of hypersonic vehicle with immeasurable state and multiple actuator faults

This paper proposes a fault-tolerant control scheme for tracking and controlling hypersonic vehicles with unknown dynamics, actuator failures, and unmeasurable states. The approach involves using a radial-based neural network to approximate the unknown dynamics and reconstruct the entire system model. Additionally, a neural network state observer is proposed to estimate the unmeasurable state of the system. To address the impact of actuator faults, a nonlinear observer is designed to estimate and compensate for the approximation error of the neural network system and fault values. Furthermore, a prescribed performance function is introduced to ensure both transient and steady-state performance of the system. The bounded stability of the closed-loop system is demonstrated through Lyapunov stability analysis.

Disturbance Observer-Based Adaptive Fault Tolerant Control with Prescribed Performance of a Continuum Robot

This paper studies an adaptive fault tolerant control (AFTC) scheme for a continuum robot subjected to unknown actuator faults, dynamics uncertainties, unknown disturbances, and prescribed performance. Specifically, to deal with uncertainties, a function approximation technique (FAT) is employed to evaluate the unknown actuator faults and uncertain dynamics of the continuum robot. Then, a nonlinear disturbance observer (DO) is developed to estimate the unknown compounded disturbance, which contains the unknown disturbances and approximation errors of the FAT. Furthermore, the prescribed error bound is treated as a time-varying constraint, and the controller design method is based on an asymmetric barrier Lyapunov function (BLF), which is operated to strictly ensure the steady-state and transient performance of the continuum robot. Afterwards, the simulation results validate the effectiveness of the proposed AFTC in dealing with the unknown actuator faults, uncertainties, unknown disturbances, and prescribed performance. Finally, the effectiveness of the proposed AFTC scheme is verified through experiments.

LN: A Flexible Algorithmic Framework for Layered Queueing Network Analysis

Layered queueing networks (LQNs) are an extension of ordinary queueing networks useful to model simultaneous resource possession and stochastic call graphs in distributed systems. Existing computational algorithms for LQNs have primarily focused on mean-value analysis. However, other solution paradigms, such as normalizing constant analysis and mean-field approximation, can improve the computation of LQN mean and transient performance metrics, state probabilities, and response time distributions. Motivated by this observation, we propose the first LQN meta-solver, called LN, that allows for the dynamic selection of the performance analysis paradigm to be iteratively applied to the submodels arising from layer decomposition. We report experiments where this added flexibility helps us to reduce the LQN solution errors. We also demonstrate that the meta-solver approach eases the integration of LQNs with other formalisms, such as caching models, enabling the analysis of more general classes of layered stochastic networks. Additionally, to support the accurate evaluation of the LQN submodels, we develop novel algorithms for homogeneous queueing networks consisting of an infinite server node and a set of identical queueing stations. In particular, we propose an exact method of moment algorithms, integration techniques for normalizing constants, and a fast non-iterative mean-value analysis technique.

Output feedback adaptive neural network control for uncertain nonsmooth systems with application

The output feedback adaptive neural network control problem is addressed for uncertain nonsmooth systems with network communication constraints. First, a funnel function is provided to ensure that the tracking error restrains the pre-specified performance and improves the transient and steady-state performance simultaneously. Second, an event-triggered controller including the control input, a fixed threshold and the tracking error is designed to alleviate the communication burden. It is rigorously mathematically proved that there exists a positive lower bound to avoid the Zeno behavior. With the designed observer, the control algorithm assures that all the signals are semi-globally uniformly ultimately bounded. Finally, the validity of the proposed scheme is demonstrated through an application example.

Decentralized asymptotic tracking control for time‐varying interconnected nonlinear systems with prescribed performance

AbstractThis article studies decentralized adaptive asymptotic tracking control of time‐varying interconnected nonlinear systems. To address the obstacles caused by interactions and these unknown time‐varying parameters, a smooth function and a bound estimation approach are introduced. Additionally, by means of a class of funnel functions and barrier Lyapunov function approach, a prescribed performance control strategy is constructed, allowing tracking accuracy and the settling time to be predetermined in accordance with control requirements, without relying on initial conditions or design parameters. Particularly, reasoned proofs have been given to confirm that the presented control method not only guarantees the prescribed transient performance, but also achieves the asymptotic tracking without requiring knowledge of high‐order derivatives of the reference signal. Ultimately, the efficiency of our approach is verified by simulation results.

Study and transient neutronics analysis of a 10kWth space heat pipe reactor based on a new dual protection control scheme with control annulus and movable radial reflector

This paper mainly focusses on a new dual protection control scheme based on the control annulus and movable radial reflector for a 10kWth space heat pipe reactor. Through the radial reflector, reactive coarse adjustment can be achieved, while reactive fine adjustment can be achieved through B4C control annulus. Both control schemes allow for independent control. Firstly, the steady-state neutron analysis method is conducted to evaluate the critical safety performance of different core control schemes when the reactor falls into sand or water. The first control scheme uses a movable radial reflector, which can still enable the reactor to maintain a keff below 0.85 when the reactor is soaked into the sand or water. The second control scheme uses an independent B4C control annulus, which enables the reactor to maintain a keff below 0.95 when the reactor is soaked into the sand or water. Through the dual protection control scheme, it can ensure the critical safety for the reactor when the launch or drop accident occurs. Moreover, a transient neutronics analysis is conducted to evaluate the operating characteristics and transient neutronics performance for the reactor. The results showed that compared to the control scheme of Kilopower, when the positive reactivity is introduced, the transient neutron dynamics parameters of this dual protection control scheme are superior to that of Kilopower. And it showed that the dual protection control scheme provides an improved control design for the further design and optimization of control scheme for the space reactors.

Hybrid air conditioning and seawater desalination system assisted by solar energy: thermoeconomic investigation and optimization

The demand for freshwater has become progressively critical due to population growth and water scarcity. The current work investigates the energetic performance of a hybrid absorption refrigeration and desalination system. A parametric study for the system operating conditions has been performed to get the optimal conditions. Also, economic analysis and a multi-objective optimization method are used with the aim of maximizing the energy utilization factor of the system and minimizing total costs. The specific cost of desalinated water from the multi-objective optimization is calculated as (1.728 Cent/L) with the greatest energy utilization factor of (2.214). It turns out that, the system performs optimum at condensation and generation temperatures of 30 and 65 °C respectively, with resultant COP and GOR of 0.84 and 1.37. Using the optimum conditions, TRNSYS has been used to investigate the transient performance of the system for Marsa–Matrouh City in Egypt. The cooling and freshwater demands are 10 kW and 24 L/h. It was concluded that the input energy required to run the hybrid system decreased by about 65% less than that required for separate systems. With the assistance of solar energy, it became clear that the energy required was 60% less than the hybrid system as well.

Disturbance-Observer-Based Adaptive Prescribed Performance Formation Tracking Control for Multiple Underactuated Surface Vehicles

This study proposes a new disturbance-observer-based adaptive distributed formation control scheme for multiple underactuated surface vehicles (USVs) subject to unknown synthesized disturbances under prescribed performance constraints. A modified sliding mode differentiator (MSMD) is applied as a nonlinear disturbance observer to estimate unknown synthesized disturbances, which contain unknown environmental disturbances and system modelling uncertainties, thus enhancing the robustness of the system. Based on this, we impose the time-varying performance constraints on the position tracking error between the neighboring USVs. A novel differentiable error transformation equation is embedded in the prescribed performance control, and an adaptive prescribed performance controller is constructed by employing the backstepping method to ensure that the position tracking error remains within the prescribed transient and steady performance, and each USV realizes collision-free formation motion. Furthermore, a novel second-order nonlinear differentiator is introduced to extract the derivative information of the virtual control law. Finally, the numerical simulation results verify the effectiveness of the proposed control scheme.

Adaptive command filter control for switched time‐delay systems with dead‐zone based on an exact disturbance observer

AbstractThe focus of this article lies in the adaptive command filter tracking control problems for a class of switched time‐delay systems with an asymmetric dead‐zone and external disturbance under arbitrary switching. First, the external disturbance is estimated via an exact disturbance observer. Then, an echo state network (ESN) was adopted to approximate unknown function in system without tuning the weights between a reservoir and an input layer. Second, the time‐varying input of the dead‐zone model is considered as uncertain physical quantity of the system based on the dead‐zone slope boundary information. Third, an adaptive command filtered controller is employed to conquer the matter of “explosion of complexity” encountered in the traditional backstepping method. The filtering errors present in a dynamic surface control (DSC) approach are effectively eliminated by means of error compensation signals. A prescribed performance control (PPC) is resorted to confine the system output to meet prescribed steady‐state and transient tracking performances. The time delay terms are compensated by a Lyapunov–Krasovskii functional. The semi‐globally ultimately uniformly boundedness of all states in the closed‐loop system is ensured by the Liapunov's stability criterion. Finally, the validity of the developed control scheme is further verified through a simulation example.

Performance prediction and regression analysis of scroll expander based on response surface methodology

The Organic Rankine Cycle (ORC) plays a pivotal role in the domain of renewable energy, adeptly harnessing thermal energy from sources such as solar and geothermal to generate electrical power. Central to this system is the scroll expander, whose operational efficacy directly influences the ORC's overall efficiency. This study initiates with a detailed numerical analysis of the impact of various operational parameters on both the steady and transient output performance of the scroll expander. Following this, a sophisticated performance prediction model is developed employing the response surface methodology. The model boasts high coefficients of determination, R2,values of 0.9967 and 0.9916, respectively, underscoring its superior predictive accuracy. Further, this model delves into the complex interactions among operational parameters and their consequent effects on the expander's performance. Validation of this predictive model is achieved through rigorous testing on an experimental platform designed for the ORC low-temperature waste heat power generation system. The discrepancies between predicted and actual performance measurements fall within a commendable margin of 15 %, affirming the model's robustness in forecasting scroll expander performance. The culmination of this research furnishes a novel and efficacious optimization strategy, establishing a robust theoretical foundation and offering fresh insights for the enhancement and prediction of scroll expander performance. This not only underscores the innovation of the approach but also its practical applicability in advancing the efficiency of renewable energy systems.

Virtual synchronous generator control with parameters optimization based on new WaOA algorithm for grid-feeding inverters

The widespread integration of renewable energy sources (RESs) within traditional power systems causes a considerable impact, such as a decrease in total inertia, damping properties, and large frequency deviation. To tackle this issue, numerous research have been proposed to introduce the virtual synchronous generator (VSG) control strategy as an effective solution applied to power electronics inverters operating in grid-forming or grid-feeding control modes. However, the proper tunning of the inertia and damping parameters is considered as a significant challenge to ensure the system stability and reliability. In this paper, an efficient meta-heuristic Walrus Optimization Algorithm (WaOA) -based VSG control intended for grid-feeding inverters within a weak grid is proposed. The objective of this work is to offer a proper tuning of the VSG parameters, thereby improving the dynamic characteristics of the microgrid (MG) and guaranteeing a stable operation. In addition, it highlights the adopted optimization technique which is introduced on VSG control while ensuring minimum objective function value. Through offline simulations in MATLAB/Simulink, the superiority of the suggested WaOA method is first confirmed, in which it offers the lowest Integral Square Error (ISE) fitness function value of 0.026, compared to the Mountain Gazelle Optimizer (MGO) and Tunicate Swarm Algorithm (TSA) with 0.027 9 and 0.0281, respectively. Second, the performance of the WaOA-based VSG control is compared to the ones based on MGO and TSA techniques under active and reactive power variations. The obtained results demonstrate that the proposed VSG control ensures a better transient performance, especially in terms of under and overshoots.

Integral global fast terminal sliding mode control for LLC converters

Abstract The complex characteristics of full-bridge LLC converters and PFM mode of operation result in a variety of operating modes and characteristic changes at different operating frequencies, thus increasing the difficulties in small signal characteristic analysis and good compensator design. In terms of control strategy, the traditional linear controller has limited response performance under various operating conditions such as input voltage ripple perturbation, input voltage mutation, and load mutation. To obtain greater transient response performance, stronger disturbance capability, and higher input stability of the system under different operating conditions, this paper proposes a new integral global sliding mode control law for LLC converters. In contrast to conventional control methods, the proposed control strategy exhibits enhanced dynamic response capabilities in the presence of load variations. It effectively mitigates jitter issues during steady-state operation, demonstrating a superior control performance.

Fixed-Time Adaptive Neural Network-Based Trajectory Tracking Control for Workspace Manipulators

This paper proposes a novel neural network-based control algorithm with fixed-time performance constraints for manipulator systems in workspaces. The algorithm efficiently controls the manipulator’s trajectory tracking by tuning a preset performance function, thereby optimizing both speed and accuracy within a fixed timeframe. Initially, a tangent-type error transformation, applied through homogeneous embryonic transformation, ensures rapid convergence of tracking errors to a specific region. Subsequently, integrating a predetermined control strategy into the fixed-time stability framework ensures the system’s state reaches a defined boundary within a finite period. Lastly, neural networks are employed to approximate dynamic parameters and adjust the controller, achieving optimal parameter approximation and significantly enhancing trajectory tracking robustness. Simulation analyses and comparisons confirm the controller’s effectiveness and superiority in enhancing both the transient and steady-state performance of the control system.

Optimized backstepping‐based finite‐time containment control for nonlinear multi‐agent systems with prescribed performance

AbstractIn this article, a finite‐time optimal containment control method is proposed for nonlinear multi‐agent systems with prescribed performance. First, a neural network‐based reinforcement learning algorithm is developed under the optimized backstepping framework. The algorithm employs an identifier‐critic‐actor architecture, where the identifiers, critics and actors are used to estimate the unknown dynamics, evaluate the system performance, and optimize the system, respectively. Subsequently, in order to guarantee the transient performance of the tracking error, the original system is converted into an equivalent unconstrained system. Then, the tracking errors are allowed to converge to a prescribed set of residuals in finite time by combining prescribed performance control and finite‐time optimal control techniques. Furthermore, by using the Lyapunov stability theorem, it is verified that all signals are semi‐globally practical finite‐time stable, and all followers can converge to a convex region formed by multiple leaders. Finally, the effectiveness of the proposed scheme is demonstrated by a practical example.

Influence of mass conservation cavitation boundary on transient performance of water-lubricated bearings

Given the current research gap regarding cavitation phenomena and startup conditions in water-lubricated bearings (WLBs), this study offers an innovative approach by integrating the mass conservation boundary condition proposed by Jakobsson, Floberg, and Olsson (known as the JFO boundary condition) with a transient mixed lubrication model of WLBs. It also considers the impact of elastic deformation and surface roughness peaks, thus establishing a transient startup model of WLBs that incorporates the cavitation effect. Furthermore, the dynamic behaviors of this boundary condition are contrasted with the commonly used Reynolds boundary condition during the startup process of the bearings, as well as under eccentric and step load impacts. The findings demonstrate that the JFO boundary condition, accounting for cavitation effects, significantly influences the transient performance of WLBs. The presence of a cavitation region diminishes the damping of the bearing system following step load impacts, leading to increased overshoot and adjustment time while weakening the system's self-adjusting ability against step loads. This study offers valuable insights for theoretical analysis of ship propulsion systems operating under complex conditions.

An Output-Capacitorless LDO regulator with fast transient response and high PSRR

Abstract A fast transient response and high supply voltage rejection ratio (PSRR) output-capacitor-less low-dropout (OCL-LDO) regulator with a dual power transistor structure is proposed in this paper. The designed flip voltage follower (FVF) structure and the proposed back-gate control loop improve the transient performance of the system. The proposed PSRR enhancement circuit transmits the power supply voltage fluctuation 1:1 to the gate of the power transistor so that the influence of the power supply voltage fluctuation on the output voltage of the LDO is offset by the difference between the gate and source voltages of the power transistor. The simulated results have shown that the proposed OCL-LDO regulator achieves full range stability from 100 μA to 50 mA. The output voltage undershoot is reduced by more than 100 mV when the load current is stepped from 1 to 50 mA in 500 ns without the load capacitor. The PSRR enhancement circuit increases the PSRR of the LDO from 1 KHz to 10 MHz by more than 20 dB.

A compensator design and stability margin monitor for digitally controlled DC-DC converters

ABSTRACT In this paper, an automatic direct digital design approach for compensators is presented to achieve a lookup table proportional – integral – derivative controller. Additionally, a correlation-based stability margin monitor with adaptive sliding window smoothing is proposed to increase the identifiable range for bandwidth detection. The proposed automatic compensator design approach meets the expected phase margin and crossover frequency and considers limit-cycle oscillation conditions throughout the design process. The digital controller with the direct digital design was implemented on a field programmable gate array to validate the approach and alternative design capabilities. Experimental results show that the direct digital design yields a higher crossover frequency than digital redesign. The best transient response performance can be obtained if a proper integral gain is designed. The digital controller with ASWS was fabricated using the TSMC 1P6M 0.18-μm standard CMOS process. Further experimental results show that the proposed ASWS provides a significantly more accurate identified frequency response by using narrow windows at low frequencies and expansive windows at high frequencies.