Validation of Hydrodynamic Stability of Supercritical Once-Through Boiler

2008 ◽  
Author(s):  
Bing Wei ◽  
Dong Zhou
Keyword(s):  
Flow Instability ◽  
Hydrodynamic Instability ◽  
Pressure Head ◽  
Operating Conditions ◽  
Hydrodynamic Characteristics ◽  
Boiler Operation ◽  
Calculation Results ◽  
Low Mass ◽  
The Stability ◽  
Heating Loads

Operating safety is one of the most important things to supercritical once-through boilers. To study the hydrodynamic characteristics of fluid in water walls of supercritical once-through boilers and to find out the instable factors will be of great significance to boiler operation. In this paper the mathematical models for hydrodynamic characteristics of fluid in water walls are established. With an example of 600MW boiler, by using the calculation program, the hydrodynamic characteristics curves without and with the throttles at the inlets of the water walls at different operating conditions are presented, the fluid flow instability and the reasons are analyzed. The calculation results show that the boiler operates stably and safely at 100% MCR (Maximum Continuous Rating) condition, the hydrodynamic instability exists at low heating loads of 30% MCR. The method of installing the throttles at the inlets of the water wall pipes will increase the parabola characteristics, help to improve the fluid instability to a certain stable extent, but due to the small curve slopes at low mass flowrates, still need to pay more attention to the low heating loads operation. The existence of gravity pressure head is good to the stability of the vertical upward flow.

Delay of Unsteady Flow in a Radial Diffuser

2008 ◽  
Author(s):  
Saad A. Ahmed
Keyword(s):  
Mass Flow ◽  
Flow Rate ◽  
Flow Instability ◽  
Rotating Stall ◽  
Operating Conditions ◽  
Flow Coefficient ◽  
Width Ratio ◽  
Flow Rates ◽  
Lower Flow ◽  
Low Mass

The operation of centrifugal compressor systems is limited at low-mass flow rates by fluid flow instabilities leading to rotating stall or surge. These instabilities limit the flow range in which the compressor can operate. They also lower the performance and efficiency of the compressor. Experiments were conducted to investigate a model of radial vaneless diffuser at stall as well as stall-free operating conditions. The speed of the impeller was kept constant at 2000 RPM, while the mass flow rate was reduced gradually to scan the steady and unsteady operating conditions of the compressor. The flow rate through the compressor was gradually decreased until flow instability is initiated at the diffuser. The flow rate was further reduced to study the characteristics of rotating stall. These measurements were reported for diffuser diameter ratios, Do/Di, of 2.0 with diffuser width ratio, b/Di, of 0.055. At lower flow rates than the critical, the rotating stall pattern with one stall cell was dominant over the pattern with two cells. In addition, the instability in the diffuser was successfully delayed to a lower flow coefficient when rough surfaces were attached to one or both sides of the diffuser with the lowest values achieved by attaching the rough surface to the shroud. Results show that the roughness has no significant effect on stall cell characteristics.

scholarly journals Thermal-hydraulic modeling of the steady-state operating conditions of a fire-tube boiler

2009 ◽  
Vol 24 (1) ◽  
pp. 29-37 ◽  
Author(s):  
Ahmed Rahmani ◽  
Ahmed Dahia
Keyword(s):  
Heat Transfer ◽  
Steady State ◽  
Hydraulic Modeling ◽  
Sensitivity Study ◽  
Operating Conditions ◽  
Saturated Steam ◽  
Operating Pressure ◽  
Operating Data ◽  
Boiler Operation ◽  
Calculation Results

In this work, we are interested to simulate the thermal-hydraulic behavior of three-pass type fire-tube boiler. The plant is designed to produce 4.5 tons per hour of saturated steam at 8 bar destined principally for heating applications. A calculation program is developed in order to simulate the boiler operation under several steady-state operating conditions. This program is based upon heat transfer laws between hot gases and the fire-tube internal walls. In the boiler combustion chamber, the heat transfer has been simulated using the well-stirred furnace model. In the convection section, heat balance has been carried out to estimate the heat exchanges between the hot gases and the tube banks. The obtained results are compared to the steady-state operating data of the considered plant. A comparative analysis shows that the calculation results are in good agreement with the boiler operating data. Furthermore, a sensitivity study has been carried out to assess the effects of input parameters, namely the fuel flow rate, air excess, ambient temperature, and operating pressure, upon the boiler thermal performances.

scholarly journals Hydrodynamic Instability of Solar Thermosyphon Water Heaters

1994 ◽  
Vol 116 (1) ◽  
pp. 53-62 ◽  
Author(s):  
S. C. Du ◽  
B. J. Huang ◽  
R. H. Yen
Keyword(s):  
Flow Instability ◽  
Hydrodynamic Instability ◽  
System Stability ◽  
Solar Irradiation ◽  
Linear Perturbation ◽  
Water Heater ◽  
Water Heaters ◽  
Stability Maps ◽  
The Stability ◽  
Solar Water Heaters

The flow instability of a solar thermosyphon water heater is studied analytically. A system dynamics model is derived by means of a one-dimensional approach and a linear perturbation method. The characteristic equation is obtained and the Nyquist criterion is used to examine the flow stability. The parameter M is a dimensionless parameter of system stability. The stability maps are plotted in terms of 14 parameters. The occurrence of hydrodynamic instability is determined by comparing the stability curves and the designed values of M. Flow instability is shown not to occur in most of solar water heaters commercially available, because the loop friction is relatively high in the design and because solar irradiation in field operation is still not high enough to cause flow instability.

Extremely Low Mass Flow at High Blade to Jet Speed Ratio in Variable Geometry Radial Turbines and its Influence on the Flow Pattern: A CFD Analysis

2017 ◽  
Author(s):  
José Ramón Serrano ◽  
Antonio Gil ◽  
Roberto Navarro ◽  
Lukas Benjamin Inhestern
Keyword(s):  
Mass Flow ◽  
Flow Instability ◽  
State Of The Art ◽  
Flow Behavior ◽  
Weight Ratio ◽  
Operating Conditions ◽  
Speed Ratio ◽  
Cfd Simulations ◽  
Wide Range ◽  
Low Mass

State of the art car engines are fed by compressed air, coming from a turbocharger compressor, to increase the power to weight ratio and to allow downsizing the combustion engine. The used compressor is driven by a radial turbine taking advantage of the hot and pressurized exhaust gases of the engine. Thus, the turbine acts under highly unsteady conditions, working at very different turbine map regions. In urban driving the turbine faces even higher changes due to frequent acceleration and deceleration so that extremely low mass flow can occur. However, the flow behavior in turbocharger turbines at these extreme off-design conditions is rather unknown. So the development of physically-based models for extrapolating the usually narrow experimental turbine maps and advanced measurements to increase the range of turbine maps has been in the focus of many researchers. To provide valuable information about those flow characteristics, this paper supplies a detailed analysis at low mass flow in a radial turbocharger turbine. The turbine has been experimentally characterized under steady flow from normal operating working conditions up to extreme off-design points, where the turbine could even work with negative efficiency. Since heat transfer significantly affects the turbine efficiency calculation when turbine power is low, the experiments have been executed under quasi-adiabatic conditions and residual heat fluxes have further been corrected. This paper takes advantage of these data to validate adiabatic CFD simulations in a wide operating range, from optimum efficiency point up to negative turbine power. Stationary and transient three-dimensional CFD simulations of the turbocharger turbine have been performed. The numerical campaign covers a wide range of operating conditions, providing different flow patterns. The obtained results show that the secondary flow field changes appreciably with mass flow rate. At low mass flows, a further backflow region develops over the entire circumference close to the hub, significantly constricting the effective turbine area and provoking mass flow instability. The highlighted flow phenomena will allow to improve state of the art extrapolation models and might help designers to understand turbine flow operating under extreme off-design conditions.

Coverage and Stability of NHx Terminated Cobalt and Ruthenium Surfaces: a First Principles Investigation

2019 ◽  
Author(s):  
Ji Liu ◽  
Michael Nolan
Keyword(s):  
Metal Surfaces ◽  
High Vacuum ◽  
Atomic Layer ◽  
Nitrogen Plasma ◽  
Operating Conditions ◽  
Ultra High Vacuum ◽  
Bridge Site ◽  
Stable Surface ◽  
Saturation Coverage ◽  
The Stability

<div>In the atomic layer deposition (ALD) of Cobalt (Co) and Ruthenium (Ru) metal using nitrogen plasma, the structure and composition of the post N-plasma NHx terminated (x = 1 or 2) metal surfaces are not well known but are important in the subsequent metal containing pulse. In this paper, we use the low-index (001) and (100) surfaces of Co and Ru as models of the metal polycrystalline thin films. The (001) surface with a hexagonal surface structure is the most stable surface and the (100) surface with a zigzag structure is the least stable surface but has high reactivity. We investigate the stability of NH and NH2 terminations on these surfaces to determine the saturation coverage of NHx on Co and Ru. NH is most stable in the hollow hcp site on (001) surface and the bridge site on the (100) surface, while NH2 prefers the bridge site on both (001) and (100) surfaces. The differential energy is calculated to find the saturation coverage of NH and NH2. We also present results on mixed NH/NH2-terminations. The results are analyzed by thermodynamics using Gibbs free energies (ΔG) to reveal temperature effects on the stability of NH and NH2 terminations. Ultra-high vacuum (UHV) and standard ALD</div><div>operating conditions are considered. Under typical ALD operating conditions we find that the most stable NHx terminated metal surfaces are 1 ML NH on Ru (001) surface (350K-550K), 5/9 ML NH on Co (001) surface (400K-650K) and a mixture of NH and NH2 on both Ru (100) and Co (100) surfaces.</div>

Slope Stability Analysis under the Condition of Seepage

2012 ◽  
Vol 204-208 ◽  
pp. 241-245
Author(s):  
Yang Jin
Keyword(s):  
Finite Element Method ◽  
Shear Strength ◽  
Slope Stability ◽  
Negative Impact ◽  
Strength Reduction ◽  
Soil Slope ◽  
Failure State ◽  
Calculation Results ◽  
The Stability ◽  
Shear Strength Reduction

The stability of soil slope under seepage is calculated and analyzed by using finite element method based on the technique of shear strength reduction. When the condition of seepage or not is considered respectively, the critical failure state of slopes and corresponding safety coefficients can be determined by the numerical analysis and calculation. Besides, through analyzing and comparing the calculation results, it shows that seepage has a negative impact on slope stability.

scholarly journals Performance of Iron-Functionalized Activated Carbon Catalysts (Fe/AC-f) on CWPO Wastewater Treatment

Catalysts ◽  
2021 ◽  
Vol 11 (3) ◽  
pp. 337
Author(s):  
Sara Mesa Medina ◽  
Ana Rey ◽  
Carlos Durán-Valle ◽  
Ana Bahamonde ◽  
Marisol Faraldos
Keyword(s):  
Wastewater Treatment ◽  
Activated Carbon ◽  
Landfill Leachate ◽  
Surface Composition ◽  
Treatment Plant ◽  
Reaction Medium ◽  
Pollutant Removal ◽  
Operating Conditions ◽  
Water Matrix ◽  
The Stability

Two commercial activated carbon were functionalized with nitric acid, sulfuric acid, and ethylenediamine to induce the modification of their surface functional groups and facilitate the stability of corresponding AC-supported iron catalysts (Fe/AC-f). Synthetized Fe/AC-f catalysts were characterized to determine bulk and surface composition (elemental analysis, emission spectroscopy, XPS), textural (N2 isotherms), and structural characteristics (XRD). All the Fe/AC-f catalysts were evaluated in the degradation of phenol in ultrapure water matrix by catalytic wet peroxide oxidation (CWPO). Complete pollutant removal at short reaction times (30–60 min) and high TOC reduction (XTOC = 80 % at ≤ 120 min) were always achieved at the conditions tested (500 mg·L−1 catalyst loading, 100 mg·L−1 phenol concentration, stoichiometric H2O2 dose, pH 3, 50 °C and 200 rpm), improving the results found with bare activated carbon supports. The lability of the interactions of iron with functionalized carbon support jeopardizes the stability of some catalysts. This fact could be associated to modifications of the induced surface chemistry after functionalization as a consequence of the iron immobilization procedure. The reusability was demonstrated by four consecutive CWPO cycles where the activity decreased from 1st to 3rd, to become recovered in the 4th run. Fe/AC-f catalysts were applied to treat two real water matrices: the effluent of a wastewater treatment plant with a membrane biological reactor (WWTP-MBR) and a landfill leachate, opening the opportunity to extend the use of these Fe/AC-f catalysts for complex wastewater matrices remediation. The degradation of phenol spiked WWTP-MBR effluent by CWPO using Fe/AC-f catalysts revealed pH of the reaction medium as a critical parameter to obtain complete elimination of the pollutant, only reached at pH 3. On the contrary, significant TOC removal, naturally found in complex landfill leachate, was obtained at natural pH 9 and half stoichiometric H2O2 dose. This highlights the importance of the water matrix in the optimization of the CWPO operating conditions.

scholarly journals A Unified Control Strategy of Distributed Generation for Grid-Connected and Islanded Operation Conditions Using an Artificial Neural Network

2021 ◽  
Vol 13 (11) ◽  
pp. 6388
Author(s):  
Karim M. El-Sharawy ◽  
Hatem Y. Diab ◽  
Mahmoud O. Abdelsalam ◽  
Mostafa I. Marei
Keyword(s):  
Neural Network ◽  
Artificial Neural Network ◽  
Control System ◽  
Distributed Generation ◽  
Control Strategy ◽  
Current Control ◽  
Operating Conditions ◽  
Pi Controllers ◽  
Artificial Neural ◽  
The Stability

This article presents a control strategy that enables both islanded and grid-tied operations of a three-phase inverter in distributed generation. This distributed generation (DG) is based on a dramatically evolved direct current (DC) source. A unified control strategy is introduced to operate the interface in either the isolated or grid-connected modes. The proposed control system is based on the instantaneous tracking of the active power flow in order to achieve current control in the grid-connected mode and retain the stability of the frequency using phase-locked loop (PLL) circuits at the point of common coupling (PCC), in addition to managing the reactive power supplied to the grid. On the other side, the proposed control system is also based on the instantaneous tracking of the voltage to achieve the voltage control in the standalone mode and retain the stability of the frequency by using another circuit including a special equation (wt = 2πft, f = 50 Hz). This utilization provides the ability to obtain voltage stability across the critical load. One benefit of the proposed control strategy is that the design of the controller remains unconverted for other operating conditions. The simulation results are added to evaluate the performance of the proposed control technology using a different method; the first method used basic proportional integration (PI) controllers, and the second method used adaptive proportional integration (PI) controllers, i.e., an Artificial Neural Network (ANN).

scholarly journals Centrifugal Compressor Stall Control by the Application of Engineered Surface Roughness on Diffuser Shroud Using Numerical Simulations

Materials ◽  
2021 ◽  
Vol 14 (8) ◽  
pp. 2033
Author(s):  
Amjid Khan ◽  
Muhammad Irfan ◽  
Usama Muhammad Niazi ◽  
Imran Shah ◽  
Stanislaw Legutko ◽  
...  
Keyword(s):  
Surface Roughness ◽  
Mass Flow Rate ◽  
Mass Flow ◽  
Flow Rate ◽  
Centrifugal Compressor ◽  
Control Method ◽  
Flow Instability ◽  
Passive Flow Control ◽  
Passive Flow ◽  
Low Mass

Downsizing in engine size is pushing the automotive industry to operate compressors at low mass flow rate. However, the operation of turbocharger centrifugal compressor at low mass flow rate leads to fluid flow instabilities such as stall. To reduce flow instability, surface roughness is employed as a passive flow control method. This paper evaluates the effect of surface roughness on a turbocharger centrifugal compressor performance. A realistic validation of SRV2-O compressor stage designed and developed by German Aerospace Center (DLR) is achieved from comparison with the experimental data. In the first part, numerical simulations have been performed from stall to choke to study the overall performance variation at design conditions: 2.55 kg/s mass flow rate and rotational speed of 50,000 rpm. In second part, surface roughness of magnitude range 0–200 μm has been applied on the diffuser shroud to control flow instability. It was found that completely rough regime showed effective quantitative results in controlling stall phenomena, which results in increases of operating range from 16% to 18% and stall margin from 5.62% to 7.98%. Surface roughness as a passive flow control method to reduce flow instability in the diffuser section is the novelty of this research. Keeping in view the effects of surface roughness, it will help the turbocharger manufacturers to reduce the flow instabilities in the compressor with ease and improve the overall performance.

Stability of Ring-Type MEMS Gyroscopes Subjected to Stochastic Angular Speed Fluctuation

2017 ◽  
Vol 139 (4) ◽  
Author(s):  
Samuel F. Asokanthan ◽  
Soroush Arghavan ◽  
Mohamed Bognash
Keyword(s):  
Angular Velocity ◽  
Degrees Of Freedom ◽  
Microelectromechanical Systems ◽  
Damping Ratio ◽  
Operating Conditions ◽  
System Stability ◽  
System Response ◽  
Ring Type ◽  
Mems Gyroscopes ◽  
The Stability

Effect of stochastic fluctuations in angular velocity on the stability of two degrees-of-freedom ring-type microelectromechanical systems (MEMS) gyroscopes is investigated. The governing stochastic differential equations (SDEs) are discretized using the higher-order Milstein scheme in order to numerically predict the system response assuming the fluctuations to be white noise. Simulations via Euler scheme as well as a measure of largest Lyapunov exponents (LLEs) are employed for validation purposes due to lack of similar analytical or experimental data. The response of the gyroscope under different noise fluctuation magnitudes has been computed to ascertain the stability behavior of the system. External noise that affect the gyroscope dynamic behavior typically results from environment factors and the nature of the system operation can be exerted on the system at any frequency range depending on the source. Hence, a parametric study is performed to assess the noise intensity stability threshold for a number of damping ratio values. The stability investigation predicts the form of threshold fluctuation intensity dependence on damping ratio. Under typical gyroscope operating conditions, nominal input angular velocity magnitude and mass mismatch appear to have minimal influence on system stability.

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