1,720,996 research outputs found
Effects of two lobe wave squeeze film damper in the support of an unbalanced rigid rotor
No full textA possible improvement of the performances of squeeze film damper (SFD) for supporting the rotors of high-speed turbomachinery has been sought adopting a two lobe, wave (2LW) geometry of the bearing bore. A statically unbalanced, symmetrical, rigid rotor supported with 2LW-SFD has been theoretically examined through numerical continuation, assuming laminar, isoviscous oil flow within the damping film and incomplete centering action of the retainer springs mounted parallel to the film. Despite nonlinearity which still affects the system, the obtained results highlight the potential of the unconventional geometry as a mean for a possible reduction of the typical drawbacks in the response with conventional SFD, mainly consisting in undesired whirling motions with too large journal orbits and/or nonsynchronous character, so as to assure more safe conditions for the rotor operation. Yet, further theoretical and experimental investigation is desirable in order to confirm such an outcome
Effect of the switch strategy on the performance and stability of reactor networks for methanol synthesis
No full textA network of catalytic reactors with a periodic switching of the feed and discharge position (also called loop reactor) is studied for reversible exothermic reactions like methanol synthesis. The aim of the study is the comparison of two different switching strategies in overcoming the conversion limits imposed by the thermodynamic equilibrium. The first strategy, that is the only considered in previous works, consists of changing at each switch time the feed/discharge positions, so that the first reactor of the NTW is moved to the last place. The second strategy consists in moving the last reactor in the first place each switch time. We show through numerical simulations that the second forcing strategy is more convenient in terms of yield and methanol conversion, because it is able to create a conversion/temperature path closer to the optimal one. © 2010 Elsevier B.V
A nonlinear approach to the design of gain-scheduled controllers
No full textGain scheduling is a method widely applied in industrial practice to control processes where large changes of the operating conditions can occur. In its standard implementation, this technique requires to compute a family of steady states covering the operating region of interest and then to design a family of linear feedback controllers ensuring stability and desired output behavior about the selected steady states. In this contribution, a novel approach to design gain-scheduled controllers of nonlinear processes is presented. Parametric continuation and optimization techniques are implemented to compute a parameterized family of steady states covering the output range of interest and, at the same time, fulfilling a prescribed set of control requirements. Then, bifurcation analysis is performed to design a family of linear feedback controllers guaranteeing desired output behavior around the selected steady states and preventing the occurrence of state multiplicity. The method is validated on the problem of controlling a continuous exothermic reactor exhibiting state and input multiplicity. © 2010 Elsevier B.V. All rights reserved
Formation of Thermal Wave Trains in Loop Reactors: Stability Limits and Spatiotemporal Structure for Reversible Reactions
No full textNetworks of fixed bed reactors with periodically switched inlet and outlet sections are studied with reference to equilibrium limited reactions. The methanol synthesis is selected as representative reaction example and the mechanisms governing the emergence of periodic regimes corresponding to trains of traveling thermal waves are analyzed. Analytical approximations, accounting for the influence of the implemented switching strategy and in satisfactory agreement with numerical simulation, are derived for the switch time stability limits in terms of temperature fronts velocities. Large and sudden variations in the switch time stability range of any wave train solution are demonstrated to occur when the sum of the velocities of declining temperature fronts exceeds the sum of the velocities of rising temperature fronts. The illustrated results provide indications on how to design and operate the network so as to generate thermal wave trains with desired number of waves and stability limits
Multiplicities of temperature wave trains in periodically forced networks of catalytic reactors for reversible exothermic reactions
No full textNetworks of catalytic reactors with periodically switched inlet and outlet sections offer a competitive technological solution to the operation of reversible exothermic reactions. Traditionally, this operation mode is implemented by periodically shifting inlet and outlet sections so as to jump a single reactor unit in the flow direction. Here, a network of four catalytic reactors carrying on the methanol synthesis process is considered and the effect of varying the number (n s) of reactor units jumped by inlet and outlet sections on network stability and performance is investigated. Increasing n s, a greater variety of periodic regimes giving rise to trains of temperature waves characterized by spatial periodicity are detected as the switching velocity varies. These regimes well reproduce the inter-stage cooling effect of multistage fixed bed reactors and, hence, guarantee in general large conversion values. Moreover, an intriguing coexistence between T-periodic and multi-periodic temperature wave trains is revealed, T being the period needed for the system to recover its initial configuration. A T-periodic symmetric wave train characterized by k waves always coexists with a number of k-1 stable symmetric kT-periodic regimes, except when symmetry breaking is encountered. The k-1 coexisting regimes correspond to wave trains with a number of waves ranging between 1 and k-1. Bifurcational analysis is performed to characterize the stability range of periodic regimes and to systematically analyze multiplicities and bifurcations as the switching velocity is varied and at different n s. © 2011 Elsevier B.V
Temperature and conversion patterns in a network of catalytic reactors for methanol synthesis with different switch strategies
No full textA network of connected catalytic reactors with periodically switched inlet and outlet sections is numerically studied for reversible exothermic reactions. The methanol synthesis is selected as representative process example and two different switch strategies are compared with the objective of overcoming the conversion limits imposed by thermodynamic equilibrium. The first strategy, which is the most considered in literature, consists of periodically switching the feed to the second reactor of the current reactor sequence while the second strategy is implemented by periodically switching the feed to the last reactor of the current sequence. Periodic regimes corresponding to single square-like temperature waves travelling over the catalytic bed and characterized by comparable methanol conversion values are detected for both the considered strategies. These regimes exhibit, however, a significantly larger domain of existence for the second strategy. Moreover, the second strategy gives rise to other periodic regimes corresponding to spatiotemporal temperature patterns characterized by different spatial and temporal periodicity. These patterns arise in the form of temperature wave trains and ensure methanol conversion values significantly larger than those found under periodic regimes characterized by single temperature waves. © 2010 Elsevier Ltd
Nonlinear analysis of a network of three continuous stirred tank reactors with periodic feed switching: Symmetry and symmetry-breaking
No full textThe spatiotemporal symmetry properties of a periodically forced network of three continuous stirred tank reactors are assessed. The symmetry is induced to the system by a cyclic switching of the feed and discharge positions. The symmetry properties imply that the Poincare map is the third iterate of another non-stroboscopic map. This feature is here used to characterize the symmetry of the regime solns., and to carry out bifurcation anal. Possible bifurcation scenarios and, in particular, symmetry-breaking bifurcations are discussed. As the switch time is varied, different transitions were identified: among the others, an important role in the birth of asym. regimes is played by frequency-locking phenomena. In addn., different routes to chaotic regimes (both sym. and asym.) are reported
Control of thermal runaway via optimal bifurcation tailoring aided gain-scheduling feedback Symposium on Process Systems Engineering
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