The Stability of Pumping Systems—The 1980 Freeman Scholar Lecture

1981 ◽  
Vol 103 (2) ◽  
pp. 193-242 ◽  
Author(s):  
E. M. Greitzer
Keyword(s):  
Steady State ◽  
Dynamic Instability ◽  
Transient Behavior ◽  
Operating Conditions ◽  
System Stability ◽  
Hydraulic Systems ◽  
Two Phase ◽  
Pumping System ◽  
Basic Concepts ◽  
Steady State Performance

A review is presented of the types of instabilities which are encountered in pumping systems of technological interest. These include axial and centrifugal compression systems, pumping systems involving cavitation, systems with two-phase flow, systems with combustion, hydraulic systems, and systems which have two or more pumping elements in parallel. All of the above will be seen to exhibit instabilities under certain operating conditions, although the mechanism of instability, as well as the particular system element that is responsible for the instability, will be quite different in the different systems. However, several basic concepts, such as the idea of negative damping which is associated with dynamic instability, will be seen to be common to the different systems. The review is organized around the different types of systems that are discussed, and includes descriptions of the steady-state performance, the regimes in which one would expect instability, and the mechanisms of instability. An idealized pumping system is first examined to illustrate some of the basic concepts. More realistic systems are then treated in the same manner of showing steady-state performance, regimes of instability and mechanisms. In the review attention is given mainly to those areas in which there is high current engineering interest, and an attempt is made to describe those areas of research which can be most fruitfully pursued. In general, it is suggested that efforts should be directed toward obtaining an improved understanding of the transient behavior of the active (instability causing) elements within the system, since it is lack of knowledge of this aspect that currently limits the accuracy of system stability predictions.

The Role of Dissolved and Nascent Air in Oil-Hydraulic Systems

2000 ◽  
Author(s):  
Hansjoerg Stern
Keyword(s):  
Steady State ◽  
High Performance ◽  
Transient Behavior ◽  
Operating Conditions ◽  
Phase System ◽  
Hydraulic Systems ◽  
Three Phase ◽  
Significant Damage ◽  
Water Hydraulic

Abstract In modern, high performance hydraulic systems the transient behavior of dissolving, dissolved and nascent air under changing dynamic conditions of pressure and temperature is emerging as an increasingly important factor, capable of creating undesirable operating conditions. The paper discusses the question of how to predict the performance of pump inlets and valve discharges, where we have known for some time that cavitation and cavitation-like conditions exist and can cause significant damage. The steady state conditions at which nascent air evolves from saturated air-in-oil solutions is normally one or two orders of magnitude above the vapor pressure of the system fluid. To what extent, therefore, is “cavitation” in these systems an air-oil problem? Or is it an oil-vapor problem that is analogous to cavitation in water hydraulic pumps and turbines? Or have we created the combination of the two, a three-phase system of liquid, gas and vapor?

Steady State Performance Characteristics of a Tilting Pad Thrust Bearing

2000 ◽  
Vol 123 (3) ◽  
pp. 608-615 ◽  
Author(s):  
Sergei B. Glavatskikh
Keyword(s):  
Steady State ◽  
Film Thickness ◽  
Thrust Bearing ◽  
Operating Conditions ◽  
Performance Characteristics ◽  
Shaft Speed ◽  
Oil Film ◽  
Tilting Pad ◽  
Bearing Load ◽  
Steady State Performance

The paper reports results of the experimental investigation into the steady state performance characteristics of a tilting pad thrust bearing typical of design in general use. Simultaneous measurements are taken of the pad and collar temperatures, the pressure distributions, oil film thickness, and power loss as a function of shaft speed, bearing load, and supplied oil temperature. The effect of operating conditions on bearing performance is discussed. A small radial temperature variation is observed in the collar. A reduction in minimum oil film thickness with load is approximately proportional to p−0.6, where p is an average bearing pressure. It has also been found that the oil film pressure profiles change not only due to the average bearing load but also with an increase in shaft speed and temperature of the supplied oil.

Transient and Steady State Vibration Analysis of a Wavy Thrust Bearing

2005 ◽  
Vol 128 (1) ◽  
pp. 139-145 ◽  
Author(s):  
H. Zhao ◽  
F. K. Choy ◽  
M. J. Braun
Keyword(s):  
Steady State ◽  
Vibration Analysis ◽  
Thrust Bearing ◽  
Numerical Procedure ◽  
Operating Conditions ◽  
System Stability ◽  
Thrust Bearings ◽  
External Perturbations ◽  
Vibration Signature ◽  
Transient And Steady State

This paper describes a numerical procedure for analyzing the dynamics of transient and steady state vibrations in a wavy thrust bearing. The major effects of the wavy geometry and the operating parameters on the dynamic characteristics of the bearing had been discussed in a previous paper; the present paper thus concentrates on examining the relationships between the development of the transient and steady state vibrations when operating conditions are parametrically varied. Special attention is given to the development of steady state vibrations from initial transients with comparisons and consequences to the overall system stability. Numerical based vibration signature analysis procedures are then used to identify and quantify the transient vibrations. The conclusions provide general indicators for designing wavy thrust bearings that are less susceptible to transients induced by external perturbations.

scholarly journals Dynamic Modeling of HVDC for Power System Stability Assessment: A Review, Issues, and Recommendations

Energies ◽  
2021 ◽  
Vol 14 (16) ◽  
pp. 4829
Author(s):  
Tarek Abedin ◽  
M. Shahadat Hossain Lipu ◽  
Mahammad A. Hannan ◽  
Pin Jern Ker ◽  
Safwan A. Rahman ◽  
...  
Keyword(s):  
Steady State ◽  
Dynamic Modeling ◽  
Direct Current ◽  
Current Source ◽  
Computational Cost ◽  
Operating Conditions ◽  
System Stability ◽  
Voltage Source ◽  
Voltage Source Converter ◽  
Stability Assessment

High-voltage direct current (HVDC) has received considerable attention due to several advantageous features such as minimum transmission losses, enhanced stability, and control operation. An appropriate model of HVDC is necessary to assess the operating conditions as well as to analyze the transient and steady-state stabilities integrated with the AC networks. Nevertheless, the construction of an HVDC model is challenging due to the high computational cost, which needs huge ranges of modeling experience. Therefore, advanced dynamic modeling of HVDC is necessary to improve stability with minimum power loss. This paper presents a comprehensive review of the various dynamic modeling of the HVDC transmission system. In line with this matter, an in-depth investigation of various HVDC mathematical models is carried out including average-value modeling (AVM), voltage source converter (VSC), and line-commutated converter (LCC). Moreover, numerous stability assessment models of HVDC are outlined with regard to stability improvement models, current-source system stability, HVDC link stability, and steady-state rotor angle stability. In addition, the various control schemes of LCC-HVDC systems and modular multilevel converter- multi-terminal direct current (MMC-MTDC) are highlighted. This paper also identifies the key issues, the problems of the existing HVDC models as well as providing some selective suggestions for future improvement. All the highlighted insights in this review will hopefully lead to increased efforts toward the enhancement of the modeling for the HVDC system.

scholarly journals Evaluation of the steady state performance of a squirrel cage induction motor using per-phase equivalent circuit

2022 ◽  
Vol 5 (1) ◽  
pp. 001-014
Author(s):  
Okafor Augustine ◽  
Olubiwe Matthew ◽  
Akukuegbu Isdore
Keyword(s):  
Steady State ◽  
Equivalent Circuit ◽  
Induction Motor ◽  
Operating Conditions ◽  
System Stability ◽  
Squirrel Cage ◽  
Squirrel Cage Induction Motor ◽  
Vital Effect ◽  
Cage Induction Motor ◽  
To Receive

The performance evaluation of cage induction motor continues to receive tremendous attention because of its vital effect on the overall system stability. The model has predicted the behavior of cage induction motor under different operating conditions and in selecting the appropriate motor for a specific load application. There is often a challenge when a squirrel cage induction motor is connected to a time-varying load, particularly when the motor is selected without considering the effects of pulsating torques. The usual method used for steady state analysis of induction motors is the equivalent circuit method. Using the per phase equivalent circuit of the induction motor, stator current and referred rotor current were computed using simple circuit analysis. Once the currents are available, then power can be computed because the voltage is already known.

Steady and Transient Flow of a Non-Newtonian Chemically Reactive Fluid in a Twin-Screw Extruder

2001 ◽  
Author(s):  
W. Zhu ◽  
Y. Jaluria
Keyword(s):  
Steady State ◽  
Response Time ◽  
Transient Flow ◽  
Transient Behavior ◽  
Operating Conditions ◽  
Twin Screw Extruder ◽  
Screw Extruder ◽  
Twin Screw ◽  
Calculated Velocity ◽  
Screw Extruders

Abstract The flow of chemically reactive non-Newtonian materials, such as bio-polymers and aciylates, in a fully intermeshing, co-rotating twin-screw extruder is numerically investigated. A detailed study of the system transient behavior is carried out. The main transient aspects, including response time, variation of system variables, and instability of operation, are studied for both single- and twin-screw extruders. The effect of a time-dependent variation in the boundary conditions is studied. The coupling due to conduction heat transfer in the screw barrel is found to be very important and is taken into account for single-screw extruders. In the absence of this conjugate coupling, the response time is much shorter. Several other interesting trends are obtained with respect to the dependence of the transient response on the fluid, materials, and operating conditions. Steady state results are obtained at large time. The calculated velocity distributions in the screw channel are compared with experimental results in the literature for steady state flow and good agreement has been obtained. The numerical results show that not all desired operating conditions are feasible. The calculated results for transient transport agree with the few experimental observations available on this system. These results will be useful in the design, control and optimization of polymer extrusion processes.

Squeeze Film Dynamics of Two-Phase Seals

1992 ◽  
Vol 114 (2) ◽  
pp. 236-246 ◽  
Author(s):  
J. A. Yasuna ◽  
W. F. Hughes
Keyword(s):  
Steady State ◽  
Dynamic Analysis ◽  
Operating Conditions ◽  
Squeeze Film ◽  
Transient Responses ◽  
Two Phase ◽  
Thermal Transients ◽  
Face Seals

A dynamic analysis of two-phase face seals including squeeze film effects and thermal transients is presented. Axial responses to perturbations from equilibrium for various sets of typical seal operating conditions are examined, and the sensitivity of these responses to certain parameters is discussed. Sample calculations indicate damped transient responses which often decay as steady state is approached asymptotically. In some cases, however, stable and unstable oscillations are observed.

scholarly journals The Influence of Stator Winding Turns on the Steady-State Performances of Line-Start Permanent Magnet Synchronous Motors

Energies ◽  
2019 ◽  
Vol 12 (12) ◽  
pp. 2363 ◽  
Author(s):  
Qiu ◽  
Zhang ◽  
Hu ◽  
Yang ◽  
Yi
Keyword(s):  
Steady State ◽  
Permanent Magnet ◽  
State Power ◽  
Operating Conditions ◽  
Stator Winding ◽  
Permanent Magnet Synchronous Motors ◽  
Synchronous Motors ◽  
Variation Mechanism ◽  
Torque Angle ◽  
Steady State Performance

The variation of stator winding turns will directly affect the key parameters of a motor, such as winding resistance and winding reactance, which further affect the steady-state performance of the motor. In order to get excellent steady-state performance from line-start permanent magnet synchronous motors (LSPMSMs) under different load powers, taking an 11 kW LSPMSM as an example, the finite element method (FEM), combined with the steady-state phasor diagram and torque angle characteristic, are used in this paper for the optimal design of the stator winding turns of the prototype. The correctness of the model is verified by comparing the experimental data with the calculated data. First, the influences of different stator winding turns on the no-load, back-induced electromotive force (EMF), as well as on inductance and overload ability are studied, and the variation mechanism is obtained. In addition, from the perspective of the torque angle characteristic, the influence of the change in synchronous inductance caused by the number of turns on the steady-state power angle is studied. Second, the variation of the current and power factors with turn number is obtained by studying the steady-state power angle and end voltage. Based on the coupling relationship between the no-load back EMF and the power angle, the mechanism of non-linear variation of current and power factor is revealed. Finally, the variation of the number of turns on the core loss and eddy current loss is analyzed under various operating conditions, and the variation mechanism is revealed, based on the armature reaction theory.

Two-Phase Pressure Drop Characteristics During Low Temperature Transients in PEMFCs

2014 ◽  
Author(s):  
Rupak Banerjee ◽  
Satish G. Kandlikar
Keyword(s):  
Fuel Cells ◽  
Steady State ◽  
Pressure Drop ◽  
Two Phase Flow ◽  
Transient Behavior ◽  
Proton Exchange ◽  
Phase Flow ◽  
Two Phase ◽  
Four Levels ◽  
Phase Pressure

Proton Exchange Membrane fuel cells are being considered as the powertrain of choice for automotive applications. Automotive fuel cells experience transients during start-up, shut-down and changing load conditions, which constitute a significant part of the drive cycle. Transient behavior of PEMFCs can be classified into three categories: electrochemical, thermal and two-phase flow. Two-phase transients require a longer time to return to steady state than the electrochemical transient (which typically requires less than 1 second). Experiments have shown two-phase transients to be more prominent at the lower temperatures due to the increased presence of liquid water. Overshoot / undershoot behavior of current and voltage has been observed during investigations of electrochemical transients. This study investigates similar overshoot / undershoot behavior in the two-phase pressure drop in the reactant channels. An increase in the current drawn from the PEMFC is accompanied by larger air flow rates and greater water generation. An in situ setup is utilized to measure the pressure drop in the reactant channels across the length of the channel, when the electrical load drawn from the PEMFC is changed. This pressure drop measurement along the length of the reactant channels is used to characterize the overshoot / undershoot behavior. A parametric study is conducted to identify the factors which influence the overshoot / undershoot in two-phase flow pressure drop. The transient behavior is explored at the temperatures of 40, 60 and 80°C. Transient behavior is more pronounced at the lower temperature. Five different ramp rates have been used to show that faster ramp rates results in larger overshoot. The effect of magnitude of current change is investigated using four levels of load change. It was observed that increased magnitude of change results in increased overshoot behavior. However, no direct relationship has been observed between the magnitude of overshoot and the time required to return to steady state.

scholarly journals Experimental Study on the Stability and Transient Behavior of a Closed-Loop Two-Phase Thermosyphon (CLTPT) Charged with NOVEC 649

Energies ◽  
2021 ◽  
Vol 14 (23) ◽  
pp. 7920
Author(s):  
Ana Larrañaga ◽  
Miguel A. Gómez ◽  
David Patiño ◽  
Jacobo Porteiro
Keyword(s):  
Heat Flux ◽  
Steady State ◽  
Stability Analysis ◽  
Closed Loop ◽  
Transient Behavior ◽  
Power Input ◽  
Heat Transfer Analysis ◽  
Two Phase ◽  
Flow Circulation ◽  
The Stability

Currently, the growing need for efficient refrigeration resources in the industrial sector has led to an increasing interest in finding technologies with a higher heat removal potential and better cooling performance. Along these lines, two-phase liquid cooling appears to be a very interesting solution, with the CLTPT (closed-loop two-phase thermosyphon) being one of the leading alternatives. Most works in the scientific literature study loop thermosyphons that work in flow boiling conditions in steady state. The present paper analyzes the transient thermal behavior of a pool boiling CLTPT gravitational channel as a passive cooling system using NOVEC 649 as working fluid. The evaporator works with two submerged cylindrical heaters that represent different heat sources located in different positions. The initial transient behavior and consequent instabilities of a laboratory-scale facility were studied, followed by a stability analysis for various power inputs. Parameters such as temperature and pressure along the experimental setup were monitored, and the effects of internal pressure and room conditions were also tested. The results show some instabilities in the process to start the flow circulation and a relative stability and quick adaptation to changes when circulation is reached. The temperature in the evaporator chamber was highly homogeneous during the whole process; however, the temperature changes in the riser and the loop top were delayed with respect to the evaporator zone. The analysis shows several pressure and temperature raises before the vapor flux reaches the condenser. When the flow circulation is established, the system becomes highly stable and thermally homogeneous, decreasing the thermal resistance when increasing the power input. The stability analysis also showed that, when the system reaches the steady state, the changes in the power input produce a transient increase in the pressure and temperature of the fluid, followed by a quick decrease of the previous steady state values. The heat transfer analysis in the evaporator shows a higher heat flux on the upper heater caused by the buoyancy flow that rises from the lower heater. It was also observed that the lower heater reaches the CHF point with a lower heat flux.

Sign in / Sign up

Export Citation Format

Share Document