Experimental Dynamic Analysis on a T100 Microturbine Connected With Different Volume Sizes

Abstract

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.

Surge prevention for gas turbines connected with large volume size: Experimental demonstration with a microturbine

“…The nominal main properties of this recuperated turbine are: 100 kW electrical power, 30% electrical efficiency, and 70,000 rpm rotational speed. Since the turbine was modified for the external connections and the volume has resulted in significant temperature and pressure drop (99 K and 142 mbar, respectively), the T100 machine cannot achieve the nominal performance [36]. Hence, the test rig is able to produce 73.5 kW maximum net electrical power with compressor inlet temperature at 300 K and Turbine Outlet volume size was not accounted in [13] because it was not included in the fuel cell emulator).…”

“…The location of the probes used in this work are reported in Fig.2, considering that more details on the measurement system and performance have been discussed in previous works, such as [13] for the anodic side and [14] for the microturbine and the modular vessel. In addition to the previously presented sensors [14], a tri-axial accelerometer (ACC in Fig.2) was installed on the top of the T100 generator case (x, y, z axes were oriented in the microturbine axial, tangential and radial direction, respectively) for investigating the frequency range up to 10 kHz [36].…”

“…Considering the trend of the RMS amplitude for the sub-synchronous vibration content (as shown in [36]), a surge prevention technique was developed and demonstrated with the T100 machine connected to the 2. In order to consider the influence of different operating conditions (e.g.…”

“…the rotational speed change), instead of using a fixed value, the threshold was calculated as the average vibration RMS amplitude at normal operating conditions (far from the surge line) multiplied by 1.6. This multiplication factor was obtained from the previous experiments [36] and to maintain a good surge margin (Kp>1.1 considering the definition of Eq.1) during all operations. This margin was considered representative of advanced cycles where, in some cases, a surge event can be extremely dangerous for the components (e.g.…”

“…for a fuel cell). In case of different volume sizes connected to the machine (as shown in [36] for this T100 microturbine), the same approach can be used even if the average vibration RMS amplitude at normal operating conditions is different; the threshold value can be calculated by applying the same multiplication factor to a different reference value.…”