Gianmario L. Arnulfi

Multistage Centrifugal Compressor Surge Analysis: Part II—Numerical Simulation and Dynamic Control Parameters Evaluation

This paper describes, from a theoretical point of view, the behavior of compression systems during surge and the effect of passive and active control devices on the instability limit of the system. A lumped parameter model is used to simulate the compression system described in Part I of this work (Arnulfi et al., 1999), based on an industrial multistage centrifugal compressor. A comparison with experimental results shows that the model is accurate enough to describe quantitatively all the features of the phenomenon. A movable wall control system is studied in order to suppress surge in the compressor. Passive and active control schemes are analyzed, they both address directly the dynamic behavior of the compression system to displace the surge line to lower flow rates. The influence of system geometry and compressor speed is investigated; the optimum values of the control parameters and the corresponding increase in the extent of the stable operating range are presented in the paper.

An Innovative Device for Passive Control of Surge in Industrial Compression Systems

The present paper reports a numerical-experimental study on the dynamic behavior of a compression system based on a multistage centrifugal blower and fitted with an innovative device for the dynamic suppression of surge instability. The control device is of passive type and is based on the aeroelastic coupling of the basic compression system with a hydraulic oscillator. The controlled system is modeled at first by using a nonlinear lumped parameter approach. The simulated system dynamics within a wide range of operating conditions allows a parametric analysis to be performed and the optimal values of the control parameters to be singled out. Such optimal values are then used to design the hydraulic oscillator, which results in a technically feasible and very simple configuration. Finally, experimental tests are carried out on the compression plant with and without the passive control device, which demonstrate the effectiveness of the proposed control system in suppressing surge instabilities, at least within the limits predicted by the numerical simulation.

Multistage centrifugal compressor surge analysis : Part I. Experimental investigation

This paper reports an experimental investigation on centrifugal compressor surge. The compression system consists of a four-stage blower with vaned diffusers and a large plenum discharging into the atmosphere through a throttle valve. Measurements of unsteady pressure and flow rate in the plant, and of instantaneous velocity in the diffusers of the first and fourth compressor stage, are performed during deep surge, at several valve settings and three different rotation speeds. Additional tests have been carried out on a different system configuration, i.e., without plenum, in order to obtain the steady-state compressor characteristics and to collect reference dato on stall in surge-free conditions. In this configuration, a fully developed rotating stall was detected in the compressor diffusers, while during surge it affects only a limited part of the surge cycle. The goal of the present experimental work was to get a deeper insight into unstable operating conditions of multistage centrifugal compressors and to validate a theoretical model of the system instability to be used for the design of dynamic control systems.

EXPERIMENTAL INVESTIGATION ON ROTATING STALL IN A CENTRIFUGAL BLOWER WITH TWO AND FOUR STAGES AND VANELESS DIFFUSERS

The paper presents the results of an experimental investigation on a multistage centrifugal blower, during rotating stall. The test plant allows to change the turbomachine characteristics; in this research the blower has been tested in two different configurations: two-stage and four-stage, with vaneless diffusers.The unsteady flow field inside the blower has been measured by means of hot-wire anemometers. Three single hot-wire probes have been utilised to measure the development of the rotating stall, while a crossed hot-wire probe has been utilised to obtain the instantaneous flow field behind the impellers.The measurements have been done at different flow rate values, including stall inception.Copyright

Analysis of Transient Performance of a Compressed Air Energy Storage Plant

Electrical energy storage might become a strategic topic if distributed generation will be matched with stochastic sources as wind or sun. Compressed Air Energy Storage (CAES) is one of the most promising options today: energy is stored as pressurized air in a cavern. Transient phenomena, occurring during the charging process, are analyzed in this paper. Two kinds of systems are considered with or without pressure compensation; in fact a water column can be used to link the cavern to a pond in order to compensate the pressure oscillations. A lumped parameter model has been adjusted by the authors to simulate the initial charging and the subsequent surge. The obtained results supply some insight about the safe working conditions and also the surge operation.Copyright

Velocity Measurements Downstream of the Impellers in a Multistage Centrifugal Blower

The paper presents the results of an experimental investigation on a four-stage centrifugal blower, having the aim of obtaining an accurate description of the flow field behind the impellers in several operative conditions and for different geometric configurations. Actually, the test plant allows one to change the turbomachinery characteristics assembling one, two, three, or four stages and three different types of diffuser. In this first research step, the blower has been tested in the four-stage vaneless diffuser configuration. The unsteady flow field behind the impellers and in the diffusers has been measured by means ofa hot-wire anemometer. A phase-locked ensemble-averaging technique has been utilized to obtain the relative flow field from the instantaneous signals of the stationary hot-wire probes. Several detailed measurement sets have been performed using both single and crossed hot-wire probes, to obtain the velocity vectors and turbulence trends, just behind the blower impellers and in several radial positions of the vaneless diffusers. These measurements have been done at different flow rate conditions, covering unsteady flow rate phenomena (rotating stall) also. The results obtained allowed us to get a detailed flow field analysis in the multistage centrifugal blower, in relation to the geometric configuration and to the differing operating conditions.

Performance Analysis of a Wind Powered Gas Storage System

All over the world huge masses of gas are compressed in a number of storage stations to compensate seasonal fluctuations of the users’ demand versus the methane extraction from geological deposits. In the great majority of such plants, turbo-compressors are used, namely centrifugal machines. Since in this kind of machines compression is essentially adiabatic, gas temperature rises up even to dangerous values. Natural gas cannot be injected into the reservoirs too hot without risk of geological damage, so often an after-cooler has to be provided. Natural gas compressors are driven by gas turbines (GT), fuelled by part of the gas flowing through the station; otherwise electric motors connected to the general grid are used. In the paper the exploitation of the renewable energy of the wind to drive the compressors of the system is proposed. The matching with the driving wind turbine is different from the matching with a gas turbine or an electric motor. However whereas the stochastic character of the wind source affects power generation seriously, in the proposed use it is not a real problem: the only constraint consists of having enough wind energy to complete a charge all over a season. An in-house code, based on the lumped parameter approach and a quasi-steady dynamics, has been developed in order to simulate the system performance during a complete charge for a known wind distribution. The turbo-compressor is modeled through its characteristic maps. Similarly the wind turbines, that drive the storage station, and the fans, that counterbalance the friction losses of the after-cooler, are replaced with their characteristic curves. The after-cooler, which is a gas-air compact heat exchanger, is modeled by means of the overall heat transfer coefficient and the total pressure losses. Finally the reservoir is supposed omothermal and isothermal. In order to investigate the plant performance, different kinds of wind distributions have been considered and the corresponding operation paths as well as power and pressure evolutions are shown and discussed.© 2010 ASME

Performance of a Water Compensated Compressed Air Energy Storage System

In a growing energy scenario, electric utility companies have to take into account new managing strategies. The increasing seasonal gap in energy demand, the penetration of stochastic sources (wind and sun) and of combined heat and power plants are making more and more difficult to schedule power production. Energy storage can balance supply and demand over different time scales, with technical and economical benefits. The two options for large size plants are pumped storage hydro and Compressed Air Energy Storage (CAES). In the present paper, a CAES plant both with and without water compensation, is considered. The time window is an entire year as there is a remarkable difference between the seasons. Indeed in winter and summer the price fluctuation amplitude can be profitably exploited while between seasons are less suitable in a storage perspective because of the relative flatness of the daily price pattern. The adopted strategy is based on two price thresholds: below the former, a single charging step is carried out at night, above the latter, one or more steps of electricity production are carried out at peak hours. Finally, amid the thresholds, the plant works as a mere gas turbine or is shut off. Of course the mere GT working is available only if turbo compressor and expander are consistent and this affects the performance of each machine during charge or discharge phases. The shape of the daily price pattern strongly impacts on the cash flow. The proposed model is applied to the present Italian scenario as the energy market, taxes and services are concerned. The water compensated plant attains a storage density nearly twice higher than without compensation.Copyright

Parametric Analysis of Thermal Energy Storage for Gas Turbine Inlet Air Cooling

Gas turbine efficiency and power output are strongly dependent on the inlet air condition. Thus, several authors proposed the use of different inlet air cooling systems. Such systems include, as examples, spraying water in the inflow air stream or air cooling through a chiller during GT operation. In the latter case, it is possible to operate the chiller at night time, taking advantage of the remarkable price gap between peak and off-peak hours. A parametric analysis of such a system is presented, focusing on the effect of price gap, chiller and storage design parameters and climatic conditions on the optimal sizing of the plant. Both the gas turbine performance changes, due to the different inlet conditions, and thermal losses related to the storage system are taken into account. The economic return of the system is evaluated through the year-round integral of gas turbine fuel consumption and chiller electricity requirements, for given scenarios of electricity price tag, ambient temperature and humidity profile. For different boundary conditions (market constraints and climate) the optimal configurations are identified and discussed.Copyright

Combined Heat and Power Plants Based on Mirror Heat Exchange Brayton Cycles

As gas turbine exhaust gases leave the turbine at high temperature, heat recovery is often carried out in a combined heat-and-power system or in the steam section of a combined-cycle plant. An interesting alternative is a mirror cycle, which involves coupling together a direct Brayton top cycle and an inverted Brayton bottom cycle; this results in significantly higher power output and efficiency, though at the expense of added complexity. The research illustrated in the present paper was based on two in-house codes and aimed to analyze different plant configurations, one of which was a heat recovery (regenerative) top cycle with the heat exchanger hot side located between the top and bottom cycle turbo-expanders. The authors call this configuration a distorting mirror, as the hot side may not be at atmospheric pressure. A parametric analysis was carried out in order to optimize plant performance vs. pressure levels. Simulation showed that, at the design point, very good performance is obtained: efficiency close to 0.50 with plant cost (per megawatt) about half vs. combined-cycle plants. An off-design analysis showed that the mirror plant is a little more sensitive to changes in load than a simple Brayton, single-shaft GT.Copyright