This paper shows experimental results obtained from a T100 microturbine connected with different volume sizes. The activity was carried out with the test rig developed at the University of Genoa for hybrid system emulation. However, these results apply to all the advanced cycles where a microturbine is connected with an additional external component responsible for volume size increase. Even if the tests were performed with a microturbine (for laboratory scale and for the related research interest in innovative cycles), similar analyses can be extended to on large size turbines. The main power systems including the effect of an additional volume connected with a turbine are: fuel cell based hybrid plants, humid cycles, externally fired layouts and innovative systems including high temperature thermal storage devices. Since in this case a 100 kW turbine was used, the volume was located between the recuperator outlet and the combustor inlet as in the typical cases related to small size plants. A modular vessel was used to perform and to compare the tests with different volume sizes. To highlight the volume size effect, preliminary experimental results were carried out considering the transient response due to an on/off bleed valve operation. So, the main differences between system parameters obtained for a bleed line closing operation are compared considering three different volume sizes. The main results reported in this paper are related to surge operations. This analysis was carried out to extend the knowledge about this risk condition: the systems equipped with large volume size connected with the machine present critical issues related to surge prevention especially during transient operations. For instance, if the T100 machine is operating with large volume components, the standard shutdown procedure can produce surge condition. This behavior is due to a slow depressurization rate in comparison with a standard microturbine. So, to produce surge conditions in this test rig, a valve operating in the main air path was closed to generate unstable behavior. It was possible to compare the effect of different volume sizes on main properties of the system using a modular vessel. Particular focus was devoted to the operational curve plotted on the compressor map. The system was equipped with different dynamic probes to measure the vibrations during normal and surge operations. The frequency analysis showed significant vibration increase not only during surge events but also close to the unstable condition. In details, possible surge precursor indicators were obtained to be used for the detection of risky machine operations. Since these surge precursors are considered important parameters for the control system point of view, an extensive experimental analysis was carried out considering the influence of volume size. These precursors were defined to produce control data (e.g. an on/off signal for a bleed valve) for surge prevention. The experimental data collected during these tests are analyzed with the objective of designing control systems to prevent surge conditions.

Compressor behaviour analysis in critical working conditions, such as incipient surge, represents a significant aspect in the turbomachinery research field. Turbines connected with large-size volumes present critical issues related to surge prevention especially during transient operations. Investigations based on acoustic and vibrational measurements appear to provide an interesting diagnostic and predictive solution by adopting suitable quantifiers calculated from microphone and accelerometer signals. For this scope a wide experimental activity has been conducted on a T100 microturbine connected with different volume sizes. A machine dynamical characterisation has been useful for better interpretation of signals during its transient to the surge. Hence, different possible methods of incipient surge identification have been developed through the use of different signal processing techniques in time, frequency and angle domain. These results will be useful for control system development to prevent compressor failures. Response to Reviewers: Reviewer #1 •The paper should deal with the identification of surge precursors (as stated from the author) nevertheless it only reports the techniques already known in literature. We have tried to better explain the innovative aspects of our work including the following sentence in the introduction: "After a preliminary machine vibration response analysis, based on standard techniques, most of this paper is devoted to the development of different surge precursors for application in advanced gas turbine cycles. Considering that no surge prevention techniques are implemented in commercial machines, this innovation is very important for systems based on the coupling of microturbines with large-volumesize components." Thank you.

This paper shows experimental results obtained from a T100 microturbine connected with different volume sizes. The activity was carried out with the test rig developed at the University of Genoa for hybrid system emulation. However, these results apply to all the advanced cycles where a microturbine is connected with an additional external component responsible for volume size increase. Even if the tests were performed with a microturbine, similar analyses can be extended to large size turbines. A modular vessel was used to perform and to compare the tests with different volume sizes. To highlight the volume size effect, preliminary experimental results were carried out considering the transient response due to an on/off bleed valve operation. So, the main differences between system parameters obtained for a bleed line closing operation are compared considering three different volume sizes. The main results reported in this paper are related to surge operations. To produce surge conditions in this test rig, a valve operating in the main air path was closed to generate unstable behavior for the three different volume sizes. Particular focus was devoted to the operational curve plotted on the compressor map. The vibration frequency analysis showed significant amplitude increase not only during surge events but also close to the unstable condition. In details, possible surge precursor indicators were obtained to be used for the detection of risky machine operations. The experimental data collected during these tests are analyzed with the objective of designing control systems to prevent surge conditions

Dynamic compressors operating region is mainly constrained by fluid-dynamic instabilities occurring at low mass flow rate conditions, such as surge and rotating stall. This work presents a vibro-acoustic experimental investigation on a centrifugal compressor of an automotive turbocharger aimed to identify and confirm some surge precursor values in correspondence of its inception conditions. The experimental campaign was carried out on a turbocharger equipped by a vaneless diffuser compressor exploited for the pressurization of an innovative solid oxide fuel cell (SOFC) emulator. The investigated turbocharger is coupled with a pressure vessel for a former emulation activity on a pressurized SOFC. In such kind of plants, the joint effect of large volume size downstream the compressor makes more complex the dynamic behavior of the whole system during transients, thus significantly increasing surge onset risk. The main goal is to define a suitable quantitative indicator to detect in advance surge inception by relying only on system vibro-acoustic response. Several transient from a compressor stable condition to surge instability onset were performed by progressively closing specific valves in the air line. When moving close to surge , vibro-acoustic signals were acquired at a high sampling rate to investigate blade passage phenomena which might interact with rotating stall inception. Meanwhile, pressures, temperatures and mass flow rates measured at a lower sampling rate to correlate compressor vibro-acoustic and performance behavior. Cyclostationary analysis was performed on vibro-acoustic acquired signals to provide innovative diagnostic and predictive solutions (precursors) for early surge detection.

This work aims to present the development and testing of an innovative tool for surge prevention in advanced gas turbine cycles. The presence of additional components, such as a saturator in humid cycles, a heat exchanger for an external combustor, a solar receiver or fuel cell stack in a hybrid system, implies the presence of larger size volumes between compressor outlet and recuperator or expander inlet. This large volume increases the risk of incurring in surge instability, especially during dynamic operations. For these reasons, at the University of Genoa, the Thermochemical Power Group (TPG) has implemented four surge precursors in a new diagnostic real-time software which can recognise a surge incipience condition comparing the precursor values with a set of moving thresholds. The most innovative aspects of this work are: (i) operational range extension and safer management of advanced gas turbine systems for energy generation, (ii) positive impact in energy efficiency due to this range extension of high efficiency systems, (iii) development of a new diagnostic tool for surge prevention using standard probes, (iv) small impact of this tool on the control and sensor costs, (v) software flexibility for adaptation to different conditions and machines. This very important final aspect is obtained with thresholds able to change automatically to adapt themselves to the plant and machine operational regime. From the cost point of view, the utilization of standard measurements is an essential requirement to equip commercial machines without significant impact on the capital costs. The software performance has been demonstrated using experimental data from a test rig composed of a T100 microturbine connected with a modular vessel, which permits to generate the effect of additional components (especially from the volume size point of view). Vibro-acoustic data, collected during machine transients from a stable operative condition to surge, were used to tune all the software parameters and to obtain a good surge predictivity.

The aim of this work is the demonstration of a surge prevention technique for advanced gas turbine cycles. There is significant surge risk in dynamic operation for turbines connected with large volume size additional components, such as a fuel cell stack, a saturator, a solar receiver or a heat exchanger for external combustion. In comparison with standard gas turbines, the volume size generates different behaviour during dynamic operations (with significant surge risk), especially considering that such additional components are including important dynamic constraints. In order to prevent the surge events, a vibration analysis was carried out to develop precursors which are able to highlight the approach of this unstable operative zone. Since the subsynchronous content of the measured vibrations is significantly increasing approaching the surge line, special attention was devoted to this parameter. The demonstration of a surge prevention system based on the sub-synchronous vibration content was carried out at the Innovative Energy Systems Laboratory of the University of Genoa. In this laboratory, a recuperated microturbine connected with a large size vessel was used. Starting from the stable operation, closing a valve in the main air line or increasing the compressor inlet temperature produced operative conditions with significant surge risk. The increase in sub-*Revised Manuscript with No Changes Marked Click here to download Revised Manuscript with No Changes Marked: Manuscript (Ferrari) CLEAN.doc Ferrari 2 synchronous vibration content detected by the control system was used to perform an active operation (bleed valve opening) to avoid the approaching surge event.

Multi-disk bladeless turbines, also known as Tesla turbines, are promising in the field of small-scale power generation and energy harvesting due to their low sensitivity to down-scaling effects, retaining high rotor efficiency. However, low (less than 40%) overall isentropic efficiency has been recorded in the experimental literature. This article aims for the first time to a systematic experimental characterization of loss mechanisms in a 3-kW Tesla expander using compressed air as working fluid and producing electrical power through a high speed generator (40krpm). The sources of losses discussed are: stator losses, stator-rotor peripheral viscous losses, end wall ventilation losses and leakage losses. After description of experimental prototype, methodology and assessment of measurement accuracy, the article discusses such losses aiming at separating the effects that each loss has on the overall performance. Once effects are separated, their individual impact on the overall efficiency curves is presented. This experimental investigation, for the first time, gives the insight into the actual reasons of low performance of Tesla turbines, highlighting critical areas of improvement, and paving the way to next generation Tesla turbines, competitive with state of the art bladed expanders.

The aim of this work is to describe the design of an innovative test rig for investigating the expansion of saturated fluids in the two-phase region. The experimental test rig was thought up and built by TPG of the University of Genoa. It will be equipped by probes and some optical accesses that permit high speed video recording and laser measurements. It will be useful for the study of the quality ratio, vapour and liquid droplet thermodynamic properties and their speed.