Transient Start-up Research Articles

Parametric studies on startup transients in multiple parallel channels of a natural circulation boiling system

The nature of startup transients in a natural circulation boiling system with multiple parallel channels are governed by the interplay between numerous geometric and operating parameters. Present work focuses on numerically ascertaining the relative influence of such variables, with primary objective being to identify possible options of suppressing concerned oscillations. A scaled-down version of AHWR, comprising of three parallel channels, has therefore been simulated using RELAP5. Both geysering and type-I instabilities have been observed in all three channels on the initiation of two-phase operation, commonly separated by a span of near-stable reversed flow in one of the channels. Adoption of higher initial pressure has been found to have a strong stabilizing effect, as it can completely eliminate geysering and can substantially shorten the duration of type-I instability. Increase in inlet loss coefficient and decrease in either core outlet or riser outlet loss coefficients can also subside type-I oscillations, without having any noteworthy effect on geysering. Channels with smaller diameter can also reduce both amplitude and duration of type-I fluctuations, albeit at the expense of a deteriorated steady-state flow rate. The effects of wall roughness and thermal capacitance have also been explored. The influence of initial pressure has been found to be significantly stronger than the others, particularly in removing the geysering instabilities. An augmented initial pressure is capable of neutralizing the system even when the channels are subjected to unequal power, thereby identifying itself as a very potent option for practical implementation.

Start-up timing behavior of single-loop oscillating heat pipes based on the second-order dynamic model

Oscillating heat pipes have the potential to combine with the grinding wheel to enhance heat transfer in grinding process to avoid thermal damage of workpiece and grinding wheel. Since the time of grinding process is short, it raises a requirement for a good start-up behavior of oscillating heat pipes inside grinding wheel. In order to understand the start-up behavior, a new method is proposed based on the second-order dynamic theory to represent and evaluate start-up process. Several parameters (i.e., rising and settling time, percent-overshoot) are defined to value quantitatively the start-up process. Accordingly, three start-up modes are discovered with respect to the evaporator temperature, which are over-damped, under-damped and transient start-up modes. The start-up behavior is discussed with both pure fluids and nanofluids. Results show that as the heating power increases, the start-up speed enhances, and over-damped start-up mode develops to under-damped or transition start-up mode. The start-up speed of DI water is faster at low heating power, while acetone and nano-diamond solution without dispersant show faster start-up speed at high heating power. This methodology will help to compare the start-up timing of the OHP experiments in literatures, providing a more robust way to analyze start-up data.

Vibration Analysis For Reducing Excessive Vibration Level on Gas Turbine Generator (GTG) 100 MW in Cogeneration Power Plant

The goal of this research was to gather knowledge and to analyze the problem of excessive vibration level of Gas Turbine Generator (GTG) 100 MW system installed in cogeneration power plant in Sumatra Island, Indonesia which supports one of biggest oil and gas industries in Indonesia. The case research related to vibration problem were presented to diagnosis the main causes of excessive vibration that occur in the gas turbine generator during operation. Vibration analysis is one of the most important activities in predictive maintenance. Vibration monitoring system and machinery diagnostic technical specification are presented. The vibration data of this research were collected using online vibration monitoring system Bently Nevada 3500 series and system 1® display software at different bearing locations during transient (shutdown & start-up and steady state (on-line) condition. Assessment on overall vibration levels shall refer to Original Equipment Manufacturer (OEM) alert & danger set points, as well as relevant ISO 20816-2 standard. Finally, recommendation of reducing excessive vibration level is provided to ensure safe and reliable operation of the GTG unit.

A novel method to predict the transient start-up time for natural draft dry cooling towers in dispatchable power plants

To predict the time taken for the start-up process of NDDCTs with different geometric parameters and initial conditions under windless condition, dimensional analysis is used to first determine the significant variables governing the transient start-up process. Buckingham Pi theorem is subsequently adopted to identify the relationship between the air mass flow rate through the tower and the time-taken during the start-up process of NDDCTs. Then, numerical tests are carried out to investigate the start-up process of NDDCTs with different geometric parameters including the tower height H, (H = 20 m, 40 m, 60 m and 80 m) and base diameter D, (D = 8 m, 10 m and 12 m) over a range of initial condition, i.e., when the temperature difference between the heat exchanger and the ambient air ΔT, varies from = 20 K to 40 K. The numerical results demonstrate that the base diameter will not affect the time taken during the start-up process of NDDCTs in the absence of cold air inflow, i.e., when no cold air intrudes into the tower from the top. However, the time taken for the start-up process of an NDDCT rises with the increase of the tower height, but is shortened with the increase of air-heat exchanger temperature difference. Finally, an accurate mathematical model is established based on both dimensional analysis and numerical tests to predict the start-up time leading to t=αHΔT20.3.

Demodulation Technique for Broken Rotor Bar Detection in Inverter-Fed Induction Motor Under Non-Stationary Conditions

Transient motor current signature analysis has become a mature technique for fault detection in induction motors. By using start-up transients, the whole range of slip in the machine is exploited to generate well defined fault-frequency patterns. However, in the inverter-fed motor case, the fault patterns are always close to the supply frequency and often of low amplitude. Therefore, it is difficult to distinguish and localize the fault-patterns. In this paper, a novel method is proposed to create a new fault pattern; the proposed technique can concentrate the fault harmonic in a specific frequency bandwidth and avoid the spectral leakage by reducing the supply frequency amplitude. The methodology has been validated through experimental tests carried out to detect broken rotor bar in an induction motor started through a voltage source inverter.

A Push–Pull Parallel Resonant Converter-Based Bidirectional IPT System

Bidirectional inductive power transfer (BD-IPT) systems facilitate wireless grid integration of electric vehicles to offer vehicle-to-grid (V2G) services. This paper presents a novel push–pull parallel resonant converter-based BD-IPT system to provide a more efficient and cost-effective solution to wireless V2G technologies. The proposed system integrates the functionality of bulky passive elements into a double-D type magnetic coupler to achieve high power density. A phase-delay control technique is proposed to reduce the startup transients as well as to regulate bidirectional power flow between the primary and secondary sides of the push–pull converters. A zero-voltage switching zone is identified to maintain safe operation of the system. Both the operating principles and control philosophy of the converters are described, and a detailed mathematical analysis is presented. The effectiveness of the proposed system is verified by building a 3.3-kW prototype system. The experimental results show that the peak power transfer efficiency reaches 96.5%, and the efficiency remains high under a wide range of operating conditions.

Diagnosis of three-phase induction motor and the impact on the kinematic chain using non-destructive technique of infrared thermography

The diagnostics failures of monitoring the condition of rotation machinery in the industry are considered very significant to ensure the efficiency and safety of production. However, many serious malfunctioning to the machine could easily occur because of these failures, thus, a huge shortage in production. In this domain, In fact, the most important installation faults lead to an increase of temperature in specific areas. In this paper, we describe an experimental device based on an infrared thermography technic, which allows observing thermal phenomena. The three-phase induction motor, used in this work, is mounted on a base plate with an alignment device. During the start-up transients, from standstill to steady state, thermal images of the motor trunk are captured every second by an infrared camera connected to a computer equipped with analysis software. These images are used first of all to build the thermal model other induction motor. Then, the results obtained by the building model are compared with those obtained by the faulty machines, in order to study the applicability of thermographic data for diagnostic purposes. Results show that this method represents a powerful diagnostic tool, which can be used to complete the information provided by techniques based on other quantities, in cases where they are inconclusive.

Starting characteristics of a novel high temperature flat heat pipe receiver in solar power tower plant based of“Flat-front”Startup model

In this study, a novel high temperature two-phase closed flat heat pipe receiver is proposed and investigated experimentally. In order to simulate the transient startup from frozen state, a transient analysis code for the novel flat heat pipe receiver has been developed. Closed-form analytical solutions for the temperature distribution along the heat pipe length are obtained and experimental tests are undertaken. These closed-form analytical solutions are in good agreement with the experimental data. The theoretical and experimental studies prove that the flat heat pipe receiver with sodium as working fluid has a well startup performance. Experiments were carried out to investigate the temperature uniformity of FHPR under the normal conditions. It is found that the FHPR has well feature of uniformity and stability through startup process experiments with constant heat flux. This research provides guidance for the research and development of flat heat pipe solar receiver, and has important significance for broadening its application in the field of heat utilization of solar energy at high temperature.

Large eddy simulation of an axial pump with coupled flow rate calculation using the sharp interface immersed boundary method

PurposeTurbomachinery, including pumps, are mainly designed to extract/produce energy from/to the flow. A major challenge in the numerical simulation of turbomachinery is the inlet flow rate, which is routinely treated as a known boundary condition for simulation purposes but is properly a dependent output of the solution. As a consequence, the results from numerical simulations may be erroneous due to the incorrect specification of the discharge flow rate. Moreover, the transient behavior of the pumps in their initial states of startup and final states of shutoff phases has not been studied numerically. This paper aims to develop a coupled procedure for calculating the transient inlet flow rate as a part of the solution via application of the control volume method for linear momentum. Large eddy simulation of a four-blade axial hydraulic pump is carried out to calculate the forces at every time step. The sharp interface immersed boundary method is used to resolve the flow around the complex geometry of the propeller, stator and the pipe casing. The effect of the spurious pressure fluctuations, inherent in the sharp interface immersed boundary method, is damped by local time-averaging of the forces. The developed code is validated by comparing the steady-state volumetric flow rate with the experimental data provided by the pump manufacturer. The instantaneous and time-averaged flow fields are also studied to reveal the flow pattern and turbulence characteristics in the pump flow field.Design/methodology/approachThe authors use control volume analysis for linear momentum to simulate the discharge rate as part of the solution in a large eddy simulation of an axial hydraulic pump. The linear momentum balance equation is used to update the inlet flow rate. The sharp interface immersed boundary method with dynamic Smagorinsky sub-grid stress model and a proper wall model is used.FindingsThe steady-state volumetric flow rate has been computed and validated by comparing to the flow rate specified by the manufacturer at the simulation conditions, which shows a promising result. The instantaneous and time averaged flow fields are also studied to reveal the flow pattern and turbulence characteristics in the pump flow field.Originality/valueAn approach is proposed for computing the volumetric flow rate as a coupled part of the flow solution, enabling the simulation of turbomachinery at all phases, including the startup/shutdown phase. To the best of the authors’ knowledge, this is the first large eddy simulation of a hydraulic pump to calculate the transient inlet flow rate as a part of the solution rather than specifying it as a fixed boundary condition. The method serves as a numerical framework for simulating problems incorporating complex shapes with moving/stationary parts at all regimes including the transient start-up and shut-down phases.

On-off cycling losses of reversible air-to-water heat pump systems as a function of the unit power modulation capacity

In this paper, the impact of on-off cycling losses on the seasonal energy performances of three reversible air-to-water heat pumps having equivalent thermal performance at full load but different power modulation capacity is discussed by means of a series of dynamic simulation models. The analysis of the heat pump performance at partial load, the development and comparison of different control algorithms for heat pumps and, finally, the modeling and evaluation of the degradation effect due to the heat pump on-off cycles at partial load conditions are numerically investigated by means of a series of innovative TRNSYS models not available in the software standard library. The models are able to take into account the variation of the heat pump performance at partial load conditions for multi-stage, inverter-driven and single-stage devices. The findings of this paper point out that the energy losses introduced during the start-up transient of the heat pump has a significant impact on the overall energy performance of the system only for single-stage units, which perform on-off cycles during the whole season, with a decrease of the seasonal performance factor (SPF) up to 12%. On the other hand, modulating heat pumps (i.e. multi-stage and inverter-driven units) avoid carrying out on-off cycles and, of course, cycling losses for a significant part of the season according to their enhanced modulation capacity. It is shown how the accurate dynamic simulation of heat pump-based systems is a useful tool for HVAC designers in order to optimize their control system parameters, with the aim to reduce the compressor maximum hourly number of cycles.

Transient Analysis of CVCS Malfunction in Large Passive PWR

The present work focuses on a simulation of increase of coolant inventory due to malfunction of chemical and volume control system (CVCS) in advance passive pressure water reactor 1000 (AP1000) operating at a 102% nominal power before transient start-up. The objective is to verify if this transient may induce any over pressure in the reactor coolant pressure boundary due to inadequate residual heat removal. RELAP5 system code was used to simulate the transient and the simulation result was compared with the simulation result by LOFTRAN code. The RELAP5 simulation result is well comparable with the simulation result by LOFTRAN and this transient does not endanger the AP1000 pressure boundary system.

Application of transient CHI plasma startup to future ST and AT devices

Employment of non-inductive plasma start-up techniques would considerably simplify the design of a spherical tokamak fusion reactor. Transient coaxial helicity injection (CHI) is a promising method, expected to scale favorably to next-step reactors. However, the implications of reactor-relevant parameters on the initial breakdown phase for CHI have not yet been considered. Here, we evaluate CHI breakdown in reactor-like configurations using an extension of the Townsend avalanche theory. We find that a CHI electrode concept in which the outer vessel wall is biased to achieve breakdown, while previously successful on NSTX and HIT-II, may exhibit a severe weakness when scaled up to a reactor. On the other hand, concepts which employ localized biasing electrodes such as those used in QUEST would avoid this issue. Assuming that breakdown can be successfully attained, we then apply scaling relationships to predict plasma parameters attainable in the transient CHI discharge. Assuming the use of 1 Wb of injector flux, we find that plasma currents of 1 MA should be achievable. Furthermore, these plasmas are expected to Ohmically self-heat with more than 1 MW of power as they decay, facilitating efficient hand-off to steady-state heating sources. These optimistic scalings are supported by Tokamak Simulation Code simulations.

CFD simulation of transient startup for a low specific-speed pump-turbine

Hydropower plants are very well suited for the modern electricity market which depends on increased flexibility and storage capabilities. Nevertheless, increased dynamic operation leads to new challenges in the design as well as the mechanical integrity of the equipment. In particular, the number of start and stops has significantly increased in recent years. The startup of the machine is a transient procedure with considerable load-oscillations which can affect the lifespan of the machine. Detailed understanding of transient effects in pump-turbines is therefore useful for improved prediction of loads and an important tool in pump-turbine layout and design. This study discusses the application of unsteady RANS CFD to fully transient pump-turbine operations. A low specific-speed pump-turbine during startup is investigated and results are discussed accordingly. The boundary conditions are imposed from 1D hydro-acoustic results of the power plant, using time dependent quantities like guide-vane angle, rotational speed and flow rate. Previous studies of the same authors contain simulations such as runaway or load-rejection, where the same method has been successfully applied. Turbine-startup results in terms of head and torque can be compared to the 1D model data for the validation of the CFD procedure. Instantaneous fluctuation of the same quantities can be observed as runner speed increases. A flow feature analysis shows that partial pumping takes place in all individual runner passages, resulting in significant global pressure fluctuations. This phenomenon can be associated with unsteady vortex formation as the characteristic reaches the speed-no load curve close to the S-shape of the characteristic.

The crosswind effects on the start-up process of natural draft dry cooling towers in dispatchable power plants

To investigate the suitability of natural draft dry cooling towers (NDDCTs) for PV/CST hybrid power plants in practice, the crosswind effects on transient start-up of NDDCTs are analyzed first using a simplified theoretical approximation and then numerical computations. The study covers crosswind speeds from 1 to 15 m/s. Simplified model identifies two distinct flow patterns that correspond to two ends of the wind speed range. The intersection of asymptotes technique is adopted to find when the flow switches from one pattern to the other. The switching wind speed also corresponds to the longest start-up time. Results of numerical analysis agree with these observations and provide more detailed insights. It is found that natural draft dominates the air flow at wind speeds below the cross-over and crosswind is dominant at higher wind speeds. The cold air incursion at the top plays an increasingly important role as the wind speed is increased and finally shuts off the tower top completely preventing any outflow from the top. The uniformity index is introduced as a single parameter to describe the flow distribution characteristics. The flow field is uniform if only a single mechanism becomes dominant: either natural draft or forced convection caused by crosswind. The start-up time is shorter when the flow is uniform and the longest when the flow field is least uniform. All results have been validated against experimental data collected on the University of Queensland natural draft dry cooling tower Gatton test rig. This research provides a practical perception for the crosswind effects on the transient start-up of NDDCT for dispatchable thermal power plants, guaranteeing a better cooperation between PV and CST part in PV/CST hybrid power generation system.

On-line process monitoring during transient operations using weighted distance Auto Associative Bilateral Kernel Regression

In this paper, a new data-driven auto associative bilateral kernel regression (AABKR) method based on weighted distance is proposed for the on-line monitoring of transient process operations. A bilateral approach to the kernel regression formulates a representative model that uses both the spatial and temporal information in the data, and a new weighted-distance algorithm captures temporal information. Moreover, an adaptive approach is proposed to dynamically compensate for faulty process inputs in the bilateral kernel evaluations, providing a robust model with little spillover. The proposed weighted-distance AABKR is first implemented using numerical process examples and then applied to the transient start-up operation of a nuclear power plant. Monte Carlo simulation results are provided by randomly assigning fault sensors and fault magnitudes. The results demonstrate the feasibility and efficiency of the proposed method.

A shear model for unsteady state for viscoelastic fluids, based on the correction of torque transfer lag

In this study, a mathematical model for the magnetic coupling transmission process was set up to solve the problem of torque transfer lag in magnetic-coupled rheological testing systems. This model was developed on the basis of torque balance in a magnetic coupling rotatory rheometer test system, which considered friction loss for the jewel bearing, as well as the inertia of both the motor and fixture. The reliability of the mathematical model was fully confirmed by a constant stress no-load start-up experiment conducted using a HAAKE-MARS60 high pressure rheometer. Based on the mathematical model for simultaneous magnetic coupling transmission and a constitutive equation of viscoelastic thixotropic fluid, a model, put forward for the unsteady mathematical shear of viscoelastic fluids, was designed to correct torque transfer lag. Changes in real stress and strain on the rotator surface were then obtained under the initial unsteady-state shear phase of the rheometer. The results showed that magnetic coupling intensifies the unsteady shear flow of fluids. The bigger the elastic modulus and viscosity of the material, the larger the deviation of test results between rotational rheometers and magnetic couple rheometers will be. The results obtained in this study could facilitate a better understanding of the rheological properties of materials in the transient start-up process under high pressure conditions in later research.

Screen Compliance Experiments for Application of Liquid Acquisition Device in Space

The purpose of a liquid acquisition device (LAD) is to separate liquid and vapor phases inside a spacecraft propellant storage tank in the reduced gravity and microgravity conditions of space so that vapor-free liquid can be extracted to the transfer line. A popular type of LAD called a screen channel LAD or gallery arm, uses a fine porous screen and surface tension forces of the liquid to allow pure liquid to flow through the screen while blocking vapor penetration. To analyze, size, and optimize the design of LADs for future in-space propellant transfer systems, models and data are required for the four fundamental influential factors for LAD systems, including bubble point, flow-through-screen pressure drop, wicking rate, and screen compliance for a wide variety of screen meshes. While there is sporadic data available for three of these parameters, there is no published quantitative data for screen compliance. During the transient startup of propellant transfer, the liquid must be accelerated from rest to the steady flow demand velocity, which causes the screen to deform or comply, so compliance data is required for accurate transient LAD analyses; most design codes only consider steady state analysis. This paper presents screen compliance experiments on 14 different screens, examining the effects of fineness of mesh, open area, and screen metal type on compliance. A basic equation of state is also developed and validated against the data which can be easily integrated into any transient LAD flow code to model propellant transfer.

An MPC Approach to Transient Control of Liquid-Propellant Rocket Engines

Abstract The current context of launchers reusability requires the improvement of control algorithms for their liquid-propellant rocket engines. Their transient phases are generally still performed in open loop. In this paper, it is aimed at enhancing the control performance and robustness during the fully continuous phase of the start-up transient of a generic gas-generator cycle. The main control goals concern end-state tracking in terms of combustion-chamber pressure and chambers mixture ratios, as well as the verification of a set of hard operational constraints. A controller based on a nonlinear preprocessor and on linear MPC (Model-Predictive Control) has been synthesised, making use of nonlinear state-space models of the engine. The former generates the full-state reference to be tracked while the latter achieves the aforementioned goals with sufficient accuracy and verifying constraints for the required pressure levels. Robustness considerations are included in the MPC algorithm via an epigraph formulation of the minimax robust optimisation problem, where a finite set of perturbation scenarios is considered.

A Non-Stationary Service Curve Model for Estimation of Cellular Sleep Scheduling

While steady-state solutions of backlog and delay have been derived for wireless systems, the analysis of transient phases still poses significant challenges. Considering the majority of short-lived and interactive flows, transient startup effects, as caused by sleep scheduling in cellular networks, have, however, a substantial impact on the performance. To facilitate reasoning about the transient behavior of systems, this paper contributes a notion of non-stationary service curves. Models of systems with sleep scheduling are derived and transient backlogs and delays are analyzed. Further, measurement methods that estimate the service of an unknown system from observations of selected probe traffic are developed. Fundamental limitations of existing measurement methods are explained by the non-convexity of the transient service and further difficulties are shown to be due to the super-additivity of network service processes. A novel two-phase probing technique is devised that first determines the shape of a minimal probe and subsequently obtains an accurate estimate of the unknown service. In a comprehensive measurement campaign, the method is used to evaluate the service of cellular networks with sleep scheduling (2G, 3G, and 4G), revealing considerable transient backlog and delay overshoots that persist for long relaxation times.

Numerical investigation of the influence of local effects on the transient start-up process of natural draft dry cooling towers in dispatchable power plants

In exploring the suitability of natural draft dry cooling towers (NDDCTs) for dispatchable thermal power plants, the transient start-up process of NDDCTs is investigated numerically. It is an established fact that the draft caused by the density difference between the interior and outside air drives the tower flow at steady-state conditions. The study finds that, starting from cold, the tower flow goes through three stages before the steady-state is reached. In the first stage, natural convection is dominant. The stagnation of thermal plumes before pinch-off contributes to a sharp increase in the inlet air temperature and the merger of rising caps further increases the rate at which the interior is heated. As the inside air warms up, an inside-outside density difference emerges and the draft starts contributing to the flow development in addition to the plumes. This is the second stage characterized by mixed convection. The distribution of air temperature inside tower becomes horizontally uniform and vertically linear through the mixed convection stage. In the third stage, the draft finally becomes the dominant mechanism and is sufficient to explain subsequent transition to steady-state and onwards. The dimensionless number, Richardson number (Ri) is introduced to identify the boundaries between these three stages. The cold air incursion is observed as a parallel cyclical phenomenon with impact on the tower flow development. A general criterion for steady state is proposed that accounts for cold air incursion. The results are validated against the experimental data from the University of Queensland natural draft dry cooling tower Gatton test rig and compared with the results of a simplified theoretical model. A better understanding of the start-up process through the numerical investigation presented in this paper provides insights to how to reduce the NDDCT transient response and thereby improving the dispatchability of the CST system employing the NDDCT.