Search

The present work features post-processing methods applied to vibro-acoustic data acquired from a T100 micro gas turbine (mGT) plant coupled with different volume interposed plenums. Such experimental campaign was conducted by relying on a test bench developed at the University of Genoa for hybrid systems emulation. Nonetheless, the obtained results can be generalized to all advanced cycles in which a mGT is coupled with further external elements which cause an increase of plant overall volume size. Since in this case a 100 kW mGT was employed, the interposed vessel was placed between heat recovery system outlet and combustor inlet, such as in common cases relevant to small size plants. Post-processing techniques carried out on acoustic and vibrational measurements can make available innovative diagnostic tools and predictive solutions by relying on appropriate instability indicators which are defined basing only on microphone and accelerometer experimental data. The main results presented in this work are relevant to rotating stall and incipient surge proper identification. Such investigation has been performed to increase the knowledge about such dangerous compressor working conditions; indeed, energy systems characterized by significant interposed volumes coupled with centrifugal compressors feature issues relevant to structural damaging due to surge and rotating stall.

Tesla bladeless expanders are promising in energy harvesting and small-scale power generation applications due to their lower cost and simplicity in design. Although such expanders exhibit very high rotor efficiency (analytical total to static efficiency ~ 90%), it shows poor performance when coupled with a stator (experimental total to static efficiency ~30%) due to losses present in the stator and stator-rotor interaction. This paper presents the design and experiment of a novel, high-efficient Tesla bladeless expander concept. The concept arises from the loss phenomena in the stator-rotor interaction in conventional bladeless expanders, which are among the main causes of the low performance. This concept is believed to bring the bladeless expanders to the same performances as the traditional ones with vanes, compared to which however the bladeless machines boast greater simplicity, robustness, and the absence of performance decay as the size decreases, competing even in the contexts for traditional turbomachinery. The high-efficient bladeless expander prototype with water as a working fluid is designed and developed, representing the similitude case for a liquid butane heat pump. The available isentropic power across the throttling process in the butane case is 3.3 kW @10000 rpm. The turbine consists of 24 nozzles and 150 disks separated by 0.1 mm spacers. The turbine shaft is connected to the high-speed electric generator. The performance test on the expander is carried out at rotational speeds ranging from 3000 rpm to 6200 rpm and with differential pressure across the expander up to 14 bar. Experimental ventilation loss is characterised and its effect on the performance of the expander is discussed. The preliminary results of the expander under investigation showed satisfactory production of power with an acceptable efficiency range. It is also shown that the present concept is promising and able to address the major i.e., stator-rotor interaction which is the major source of loss in the traditional bladeless expander.

It is well known that bladeless or Tesla turbomachinery, which was invented by Nikola Tesla in 1913, has several distinct features, such as reversibility of operation, which includes expander as well as compressor operation, just by reversing the rotational speed, provided that the statoric channels are purposely designed. Despite their potential application to a variety of fields, such as energy harvesting, automotive, light aircraft, and food processing, especially for low volumetric flows, Tesla machines have not found yet a specific market niche. In fact, at small size, it is estimated that the Tesla machinery does not change performance significantly, while conventional bladed machines are subject to significant efficiency reduction because of mechanical tolerances, thus matching the Tesla relatively low performance. Therefore, Tesla machines can become the fluid machinery of choice for small-size applications, thanks to their competitive performance at that size, simple construction, and reversible operation. A key objective of this paper is to numerically study Tesla devices in both expander and compressor modes with air as the working fluid. As a consequence of the high losses due to rotor and stator interactions, statorless (volute) configurations are investigated here, showing superior performance in both direct and indirect modes of operation. With reference to a laboratory prototype under construction, this paper presents the numerical design results, which predict the peak isentropic efficiencies of 63.5% and 57.5%, for the expander and compressor mode of operation, respectively. Actual prototype is expected to match those performance, apart from leakage and ventilation losses, not included in the numerical analysis.

High-speed centrifugal compressors are commonly exploited to pressurize fuel cell-based hybrid energy systems. In such complex plants, because of significant interposed volumes due to fuel cells, dynamic compressor response can induce severe vibrations caused by low mass flow rates instability. In particular, surge strongly limits centrifugal compressors stable working region when moving towards low mass flow rate due to a change in system operating point. Consequently, a complete system identification is performed in order to adequately characterize compressor dynamic response for early surge detection. To this goal, a tailored experimental activity has been carried out at the Thermochemical Power Group of the University of Genoa on a vaneless diffuser compressor turbocharger used for the pressurization of an innovative solid oxide fuel cell (SOFC) emulator plant. Several post-processing methods have been performed on system vibro-acoustic responses to better predict and classify compressor status as stable or unstable. The obtained results provide original diagnostic insights for monitoring systems capable of preventing surge and other low mass flow unstable phenomena, such as rotating stall cells inception. Low mass flow rate fluid-dynamic instabilities prevention can extend compressor operating range, performance, and reliability to allow better integration with other plant components.

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.

Technology is not good or bad, neither humanizing or dehumanizing per se: what truly qualifies its meaning and impact is defined by designers and users, and by their complex interaction, thereby showing how this matter assumes deep ethical implications. The following pages assume as a premise that the purpose and the way technological tools are designed highly determines how they interact, impact and affect human users. This is remarkable in the context of a publication aimed at critically reflecting on the ethical and legal implications of the Metaverse, whose pervasive and immersive character will likely play a strong role in shaping people's lives already in the near future. After some philosophical considerations on the problematic relationship between human beings and the technique, my reflections will briefly focus on some ways in which technology may result in creating or enhancing discriminations between and among human beings, thereby negatively impacting their lives and relationships. From this conceptual platform, I will seek to show that a restorative and mediative approach can offer important conceptual and ethical standpoints for legal designers and technology developers in order to design digital technology around users, their interests and their needs rather than imposing such tools «on», or «notwithstanding», them. My final aim is to underline how this approach matches a "critical openness" towards technological innovations with the issue of ethically safeguarding the protection of their human users, without missing the point of facilitating users in their ability of consciously and successfully accessing the potentials offered by the digital world. [ABSTRACT FROM AUTHOR]

In the current energy scenario, it is necessary to reduce fossil fuel consumption to achieve the far-sighted and stringent decarbonization goals. To date, heat is mainly produced through fossil fuels. Alternatively, electrically driven heat pumps can exploit renewable power to recover environmental and waste heat, offering energy efficient and environmentally friendly heating and cooling for applications ranging from domestic and commercial buildings to process industries. For this reason, they are expected to play a primary role in complementing or displacing natural gas boilers in the residential and industrial sectors in the near future. Centrifugal compressors are already used as prime movers of the working fluid in heat pumps, thanks to their industrial replicability, compact size, affordable costs, and good performance in terms of efficiency and low noise. However, they are subject to instabilities such as surge and stall like any other dynamic compressor and these phenomena develop quite differently than in classic open-loop systems such as gas turbines. In fact, such peculiarity is mainly due to the closed-loop configuration with real gases in two-phase conditions, occurring in typical heat pump cycles. In addition, heat exchangers also contribute to make these phenomena different from what is commonly studied. Compressor surge in closed-loop heat pump systems has received lower attention than other applications by the engineering community, lacking dedicated experimental characterization, and clear exposition of the phenomenon. The aim of this paper is to experimentally investigate the behavior of a centrifugal compressor installed into an innovative close loop heat pump system under stable and unstable conditions from both vibrational and fluid-dynamic points of view. The impact of the main process parameters on the evolution of the instability is shown, highlighting how surge cycles change by varying system operating conditions. The energy contents of surge cycles and pressure fluctuations are highlighted, using data postprocessing techniques such as fast Fourier transform and phase locked average. The vibro-acoustic analysis enrich the comprehension of the phenomena. The experimental results shown in this paper can be a basis for the future development of validated mathematical models of closed-loop heat pumps systems equipped with dynamic compressors operating under stable and unstable operating conditions. [ABSTRACT FROM AUTHOR]

Heat pumps are expected to play a primary role in electrification of thermal users in the residential and industrial sectors. Dynamic compressors are widely used in large size heat pumps, thanks to their industrial replicability, compact size, affordable costs, and good performance in terms of efficiency and low acoustic emissions. The instability, which may occur in a compressor installed in closed-loop cycle such as in a heat pump, is quite different from the classic open loop configuration involving dynamic compressors. This is mainly due to the complexity of the compressor instability mechanism coupled to a vapor compression system connected with two-phase heat exchangers, having different thermal and fluid dynamic capacitance properties, under a physical feedback loop. The aim of this paper is to investigate the behavior of a dynamic compressor installed in an innovative heat pump prototype, of laboratory scale, under stable and unstable conditions. A preliminary simple dynamic model of the compression system consisting of a radial compressor, condenser, and evaporator is developed to represent the heat pump compression system, aiming at its time-dependent representation during compressor instable behavior. The evaporator and condenser are modeled using empirical correlations representing the heat exchange and phase change phenomena and including the thermal capacitances due to refrigerant mass and heat exchanger pipes. The preliminary validation of the dynamic model results is done through a dedicated experimental campaign, under different operating conditions. Results show the complexity of the interaction between the centrifugal compressor and the heat pump loop, discerning the different contributions to the time-dependent response of the system. Future steps will encompass a more detailed modeling of the heat pump loop and the use of updated field measurements, including liquid-level meters in the heat exchangers. [ABSTRACT FROM AUTHOR]

In the present energy scenario, heat pumps (HPs) are assuming an important role to improve energy efficiency in reversible cooling systems. In particular, in case of industrial size plants, centrifugal compressors are preferred with respect to other solutions; nonetheless, they are often subjected to variable operation and therefore they must withstand off-design conditions. For studying unstable operation of refrigerant closed-loop systems, Carrier provided the University of Genoa with a small size chiller rig equipped with an innovative high speed centrifugal compressor driven by a variable speed motor. This paper presents the vibro-acoustic signature analysis of such a closed-loop rig from a system perspective, mainly focusing on mechanical response of compressor casing. First, its vibro-acoustic characterization is conducted at on-design and stable operation. Afterwards, some surge transients are obtained by progressively closing some valves in plant feeding lines, and meanwhile vibro-acoustic signals are acquired at relevant plant locations to characterize system response just before instability onset. In this way, suitable surge precursors can be defined in incipient surge conditions for early surge detection in this kind of complex plants. Indeed, system dynamics is significantly affected by interposed volumes, and therefore the effect of heat exchangers in system response may be relevant. To this aim, system dynamics is analyzed in detail both in subsynchronous frequency range and in high frequency region to assess how its vibro-acoustic response varies when moving from stable conditions toward surge. Surge precursors are obtained by relying only on vibro-acoustic signals both in low and high frequency ranges in order to perform early surge detection in such chiller pilot system. The main advantage of the proposed approach is to exploit nonintrusive probes, which allow to define diagnostic indicators without interacting directly with the working fluid, therefore preserving system integrity and reliability. [ABSTRACT FROM AUTHOR]