Diego Micheli

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.

Extensive study on the control of centrifugal compressor surge

Abstract This paper reports the results of an extensive study concerning surge instability in an industrial compression system and its dynamic suppression. Both numerical simulations and experiments have been used to analyse the system behaviour under unstable operating conditions and to compare different control devices for surge suppression. First, the characterization of surge is performed over the whole unstable operating range of a compression system based on a four-stage centrifugal compressor. The unsteady energy associated with the surge cycle is employed as an index of the surge intensity to allow for a quantitative approach to the problem. Subsequently, the behaviour and performance of a passive-type control system and of an active-type one are analysed and compared. The former consists of an innovative device based on an oscillating water column, whereas the latter is a high-gain type of feedback device implemented digitally. Experimental data show that the proposed passive device is effective not only in inhibiting surge development, but also in suppressing fully developed instability. The active device turns out to be more effective than the passive one, but under severe conditions it may fail because of actuator limitations and noise amplification. Finally, the relative merits and limitations of the two control systems are discussed with reference to their effectiveness range, flexibility, steady-state performance, installation, maintenance, and costs.

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.

Performance Analysis and Modeling of Different Volumetric Expanders for Small-Scale Organic Rankine Cycles

In the last years one of the main research topics in energy field is represented by Organic Rankine Cycles (ORCs), due to their applicability in energy recovery from waste heat and in distributed combined heat and power (CHP) generation, particularly in small and micro scale systems. One of the key devices of the cycle is the expander: it must have a limited cost (like all the other components, in order to ensure the economic feasibility), but also a high efficiency, since the temperature of the heat source is often low and then the cycle efficiency is inherently scarce. In the first part of this paper a literature review on various positive-displacement expanders is presented, in order to outline their performances and their application field. Then, the numerical model of a volumetric reciprocating expander is implemented. This model, and another one previously developed to simulate scroll expanders, is combined with a thermodynamic model of the whole ORC system, so that a comparison between the two technologies can be carried out. The results confirm the possibility of realizing small scale energy recovery and cogeneration (CHP) systems with acceptable electrical efficiency also adopting low-cost components, directly derived from large scale industrial components.

Experimental Evaluation of a High-Gain Control for Compressor Surge Suppression

The present paper considers the suppression of surge instability in compression systems by means of active control strategies based on a high-gain approach. A proper sensoractuator pair and a proportional controller are selected that, in theory, guarantee system stabilization in any operating condition for a sufficiently high value of the gain. Furthermore, an adaptive control strategy is introduced that allows the system automatically to detect a suitable value of the gain needed for stabilization, without requiring any knowledge of the compressor and plant characteristics. The control device is employed to suppress surge in an industrial compression system based on a four-stage centrifugal blower, An extensive experimental investigation has been performed in order to test the control effectiveness in various operating points on the stalled branch of the compressor characteristic and at different compressor speeds. On one hand, the experimental results confirm the good performance of the proposed control strategy; on the other, they show some inherent difficulties in stabilizing the system at high compressor speeds due to the measurement disturbances and to the limited operation speed of the actuator.

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

An Integrated Design Approach for Micro Gas Turbine Combustors: Preliminary 0-D and Simplified CFD Based Optimization

The present work presents a novel approach for the optimised design of small gas turbine combustors, that integrates a 0-D code. CFD analyses and an advanced game theory multi-objective optimization algorithm. The output of the 0-D code is a baseline design of the combustor, given the required fuel characteristics, the basic geometry (tubular or annular) and the combustion concept (i.e. lean premixed primary zone or diffusive processes). For the optimization of the baseline design a parametric CAD/mesher model is then defined and submitted to a CFD code. Free parameters of the optimization process are position and size of the liner holes arrays, their total area and the shape of the exit duct, while different objectives are the minimisation of NOx emissions, pressure losses and combustor exit Pattern Factor. As a first demonstrative example, the integrated design process was applied to a tubular combustion chamber with a lean premixed primary zone for a recuperative methane-fuelled small gas turbine of the 100 kW class.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.

Modeling and Performance Analysis of an Integrated System: Variable Speed Operated Internal Combustion Engine Combined Heat and Power Unit–Photovoltaic Array

The paper presents the model of a combined heat and power (CHP) unit, based on a variable speed internal combustion engine (ICE) interfaced with a photovoltaic (PV) system. This model is validated by means of experimental data obtained on an 85 kWe CHP unit fueled with natural gas and a PV system with a rated power of 17.9 kW. Starting from daily load profiles, the model is applied to investigate the primary energy saving (PES) of the integrated CHP + PV system in several operating conditions and for different sizes of PV array. The results demonstrate the dependence of the CHP performance on the operating mode and a limited convenience of the variable speed strategy. The integrated system operation leads to performance improvements, which depend on the size of the PV component.