Marco Cioffi

Robust design of high field magnets through Monte Carlo analysis

An effective approach to the optimal design of electromagnetic devices should take into account the effect of mechanical tolerances on the actual devices performance. A possible approach could be to match a Pareto optimality study with a Monte Carlo analysis by randomly varying the constructive parameters. In this paper it is shown how such an analysis can be used to allow an expert designer to select among different Pareto optimal designs.

Stochastic handling of tolerances in robust magnets design

Due to construction tolerances, the performances of actual electromagnetic devices differ from those of the nominal design. A strategy for the search of design solutions robust against construction tolerances is presented. The approach is based on the statistical analysis of the tolerances impact on the design objective function to be evaluated. The application of the proposed strategies to the optimal design of magnetic resonance imaging magnets by a genetic algorithm shows the possibility to direct the search toward robust solutions.

Increasing design robustness in evolutionary optimisation

The role of the parameters uncertainness in the optimal design of electromagnetic devices is discussed and an efficient strategy to look for robustness of feasible solutions is proposed. A suitable modification of the objective function (OF) is used to rank different device configurations on the basis of their ability to maintain the required performances against small parameters modifications due to construction tolerances. In the frame of a genetic algorithm approach, the modified OF has been able to address the evolutionary optimisation towards more robust solutions.

Improvement of MRI magnet design through sensitivity analysis

Superconducting magnets for MRI are designed to provide high level of magnetic flux density in a wide testing volume with the greatest level of field homogeneity. The design of such magnets is usually performed using optimization techniques able to tune the geometrical parameters of the magnets, taking also into account constructional issues and dimensions and packaging factors of the wires for each coil and, in addition, technical and physical constraints, such as the critical current of the superconductor to prevent the quench phenomenon. Unfortunately due to manufacturing tolerances, the actual geometrical parameters of the magnet differ from the design ones, affecting the field homogeneity. In this paper the effects of the manufacturing tolerances on the field homogeneity are investigated for an MRI magnet by means of a statistical Monte Carlo analysis.

Three-dimensional quench propagation in HTS coils

The behavior of high temperature superconductor (HTS) coils is ruled by thermal and electromagnetic interacting phenomena. In this paper, a three-dimensional modeling of the quench propagation in HTS coils is discussed and numerically modeled. As a matter of fact, in spite of the axially symmetric design, a simple two-dimensional modelization may reveal too rough, particularly from the thermal point of view. In addition, the possibility of representing the complex structure of the coil composite material by means of a simplified homogeneous and anisotropic model is discussed and the advantages in terms of computational burden are evaluated

Distributed niching concept for electromagnetic shape optimization by genetic algorithm

Generic algorithms are becoming a common tool for optimal design applications, where, due to multiple solutions, global search techniques are required. When dealing with real problems, involving several degrees of freedom, the computing power restricts the global search ability. New genetic techniques have been proposed for parallel architectures, allowing one to deal with real problems. One of these techniques, called the niching approach, can be implemented by dividing the population into subgroups, and letting each group to evolve on one of the processors, interacting only when scheduled. The authors discuss the niching approach in the optimal design of electromagnetic applications. As an example, some preliminary results on SMES (superconducting magnetic energy storage) devices are proposed.

Statistical analysis in robust design of superconducting magnets

Purpose – To present a robust optimal design technique in the presence of system parameters uncertainties.Design/methodology/approach – The properties of normally distributed random variables are exploited, together with surface response fitting techniques, with the aim to reduce the computational cost in assessing the effect of uncertainties.Findings – A fast approximate method for computing statistical average is presented together with its implementation for the design of magnets for magnetic resonance imaging.Research limitations/implications – Future research will be focused to multi‐dimensional problems and to the best choose of closed form expressions to evaluate statistical moments fitting.Practical implications – Robust optimal design methodologies are receiving an increasing interest in both academic and industrial research, due to their capability of coping with construction uncertainties and tolerances.Originality/value – The effectiveness of the simplified method has been demonstrated for an ...

Unsteady Operative Conditions of Bleeding Lines in Heavy Duty Axial Gas Turbine

The proper design and operation of air bleeding pipes (blow-off lines) from axial compressors in heavy duty gas turbines is relevant to protect the compressor during start-ups and shut-downs by avoiding dangerous flow instabilities in the first stages. The blow-off lines are usually equipped by valves, which are closed during normal gas turbine operation and opened at low rotor speed. During gas turbine shut-downs the blow-off valves open instantaneously.In this paper the unsteady flow behavior in blow-off lines following the valve opening is presented together with numerical results based on available field data. The paper main scope is to address and to help the design of experimental activities on production gas turbines and to make available some simple numerical tools to be adopted during the industrial design of an axial compressor and its auxiliary systems. The performed analysis results have been used to define the structural requirements and the correct positioning of the measuring probes installed in blow-off lines. In addition the presented models are part of the compressor design loop, used to compute a fast evaluation of the limiting mass flow rate, which characterizes the blow-off pipes as gas turbine safety devices.Copyright

Minimum Environmental Load Reduction in Heavy Duty Gas Turbine by Bleeding Lines

The power generation and energy market scenarios are requiring the power generation plants to fulfill more flexible operations respect to the recent past. One of the main concerns of plant operators is the lowering of minimum load at which the machines can be exercised while respecting the pollution limits. A strategy to improve minimum turndown capability by reducing the minimum environmental load of heavy duty axial gas turbines is here presented: it is based on the use of the compressor air bleeding lines (blow-off lines).The described technical development activities are based on the numerical modeling of blow-off lines and bleeding compressor sections; these preliminary tasks have been followed by on-field plant testing.The blow-off lines modeling reserves a particular regard, due to the somehow non-usual fluid dynamics involved. A Fanno flow 1D approach has been adopted to properly model the bleeding lines fluid flow whereas full 3D numerical solutions have been developed to get a better insight of the bleeding plenums and of the line sector including the valve. In addition, the gas turbine components off-design behavior and the overall performances are computed by the Ansaldo modular simulation code. Numerical analysis and performed field tests are here presented and results are compared, showing a good agreement, in accord to the simplified model adopted. Additional comparisons with different alternative strategies are finally presented in terms of gas turbine power and excess air variation.The described technique by blow-off lines opening shows to be able to fulfill the required task by incrementing the plant operative flexibility and guaranteeing safe plant operation. The technique drawbacks are a gas turbine slightly lower efficiency and the lower output flue gas temperature, whose relative importance have to evaluate by the plant operators. At present the long term sustainability of the new operative condition is the object of a deeper and longer field testing phase.Copyright

FANNO DESIGN OF BLOW-OFF LINES IN HEAVY DUTY GAS TURBINE

ABSTRACT In typical heavy duty gas turbines the multistage axial compressor is provided with anti-surge pipelines equipped with on-off valves (blow-off lines), to avoid dangerous flow instabilities during start-ups and shut-downs. Blow-off lines show some very peculiar phenomena and somewhat challenging fluid dynamics, which require a deeper regard. In this paper the blow-off lines in axial gas turbines are analyzed by adopting an adiabatic quasi-unidimensional model of the gas flow through a pipe with a constant cross-sectional area and involving geometrical singularities (Fanno flow). The determination of the Fanno limit, on the basis of the flow equation and the second principle of thermodynamics, shows the existence of a critical pipe length which is a function of the pipe parameters and the initial conditions: for a length greater than this maximum one, the model requires a mass-flow reduction. In addition, in the presence of a regulating valve, so-called multi-choked flow can arise. The semi-analytical model has been implemented and the results have been compared with a three-dimensional CFD analysis and cross-checked with available field data, showing a good agreement. The Fanno model has been applied for the analysis of some of the actual machines in the Ansaldo Energia fleet under different working conditions. The Fanno tool will be part of the design procedure of new machines. In addition it will define related experimental activities.