Fluid Machinery, Energy Systems and Power Generation

Abstract

The chapter summarizes the research activities and main outcomes of the fluid machinery, energy systems and power generation groups, occurred during the 2013–2023 decade. The focus is on the theoretical, experimental, and numerical analysis of power generation systems either from hydrocarbons or from renewables, and more in general, on fluid machinery devices and components.

The research activities of the group cover a wide spectrum of topics belonging to the energy conversion family based on fluid flows. Main emphasis is on power generation systems either from hydrocarbons or from renewables, as well as on fluid machinery in general. Heat pumps and refrigerators are also of interest. Cutting edge technologies at plant or component level are typically faced with a thermo-fluid-dynamic approach of various complexity and fidelity, both from a design and analysis perspective. Operational and performance aspects of steam, gas turbine, combined cycle, piston engine power plants and fluid power systems are fully within the research scope of the group. Likewise, design and analysis issues of power and CHP systems, accounting for efficiency, pollutant emission, size and cost constraints are also covered in detail. Finally, pumps, compressors, turbines specific features, performance assessment, as well as all key aspects related to the design and part load functioning remain within the scope of the group.

1 Internal Combustion Engines

1.1 Introduction

The research groups operating in the energy and mechanical fields of Industrial Engineering Department (University of Naples Federico II) have undertaken a series of funded research projects [1,2,3,4] in collaboration with several Universities, Research centers and Industrial partners. In this framework, the activities mainly focused on the analyses of innovative powertrain technologies and alternative fuels to improve the efficiency and noxious emissions of internal combustion engines (ICEs). The primary objective of research studies is to contribute to the progression of environmentally friendly and highly efficient powertrain systems, thus driving the transition towards a sustainable mobility in the transport sector.

1.2 Research Areas

Research collaborations have been conducted through the development of both experimental and numerical methodologies. The experimental analyses mainly concern the effects of alternative fuels on the performance and emissions of ICEs, while the numerical methodologies are oriented to the simulation of flows in the intake/exhaust pipes and of the in-cylinder processes. A particular emphasis has been devoted to the simulation of combustion process in ICEs, employing both zero-dimensional (0D), one-dimensional (1D) and three-dimensional (3D) computational fluid dynamics (CFD) approaches.

1.3 Results and Discussion

The main findings of research efforts are detailed by the published papers [5,6,7,8,9,10,11,12,13,14], listed in the reference section. In the following, some valuable outcomes of both experimental and numerical activities will be briefly discussed. The experimental analyses on ICEs have been carried out with the main purpose to decrease the exhaust emissions while reducing the dependency on fossil fuels. In this regard, ethanol is considered a clean and renewable alternative fuel for Spark Ignition (SI) engines when used in blends with gasoline. Researchers performed experimental studies to investigate the effect of bioethanol-gasoline blends on the exhaust emissions of Euro 3 large-size four-stroke motorcycles, operated on the chassis dynamometer for emission measurements [5]. Experiments were realized without change the engine design, under the original fuel injection system and employing bioethanol/gasoline blends (range of bioethanol 5% vol. to 30% vol.). Regulated and unregulated emissions and fuel consumption were quantified over the execution of chassis-dynamometer tests. The combustion analysis, realized by acquiring the pressure cycle inside the cylinder, highlighted the auto adjustment of the engine control unit and guaranteed use within the same parameters of several tested fuels, with the except of fuel injection time, which increases with increasing ethanol percentage. A significant reduction in carbon monoxide CO and particle number (PN) was associated with well-defined percentages of ethanol content in gasoline fuel blends. Volatile organic compounds, mainly alkanes and aromatics, were not substantially influenced by the bioethanol content of the fuel, while the contribution of carcinogenic benzene ranged between 2 and 5%. Additional measurements were performed to investigate the effect of ethanol/gasoline blends on both fuel consumption and CO and HC emissions during the cold start transient [7]. Results of the experimental tests and the application of a new calculation procedure, which is designed and optimized to model the cold transient behavior of SI engines using different ethanol-gasoline blends, indicated that CO and hydrocarbons (HC) cold start emissions decrease compared to commercial gasoline, with the 20% v/v ethanol blend achieving the highest emission reduction. More in detail, the reduction of CO and HC cold emission factors is associated to the high oxygen content in the ethanol molecule, and the high volatility of gasoline-20% ethonol (E20) fuel which enhances fuel evaporation during cold operating conditions. Furthermore, the addition of ethanol in gasoline fuel blends (20 and 30% v/v ethanol content) produced lower flame temperature which led to lower exhaust temperature, thereby producing a lower amount of nitrogen oxides ${\text {NO}}_{\text {x}}$ emission. Referring to numerical studies, 0D/1D modelling has been adopted to simulate the turbulent combustion in SI engines, by developing a phenomenological combustion model, based on fractal schematization of flame front. A 0D turbulence sub-model has been also developed and coupled to the combustion one to consider the turbulence-induced enhancement of burn rate. The reliability of combustion and turbulence models can be appreciated in Fig. 1, which reports some results related to the activities realized for National Operative Programs (PON) with Stellantis (ex Fiat Chrysler Automobiles) [1, 2] on a downsized turbocharged Variable Valve Actuation (VVA) SI engine [8]. Figure 1a shows that turbulence model is capable to reproduce the 3D-derived evolution of in-cylinder turbulence intensity at varying the engine speed for the early intake valve closure (EIVC) strategy. A proper description of spark event and characteristic combustion angles has been achieved for different operating points (Fig. 1b). Combustion model accurately reproduces the increase of burn duration at very low loads (Fig. 1b), and it also shows a good prediction of the in-cylinder pressure trace and burn rate at full load (Fig. 1c).

Fig. 1

0D/3D comparison of turbulence intensity for EIVC and different speeds (a); numerical/experimental comparison of combustion angles in a load sweep at 1800 rpm and EIVC (b); numerical/experimental comparison of in-cylinder pressure cycle and burn rate at full load and 1800 rpm (c)

The cycle-to-cycle variation (CCV) phenomenon has been modelled basing on the measurements derived from the collaboration with Lamborghini Auto, considering a high-performance twelve-valve (V12) naturally-aspirated SI engine [9]. To this aim, the burn fraction of average cycle, predicted by fractal combustion model, has been stretched to reproduce the measured pressure peak variability, deriving the “faster-than-average” and “lower-than-average” pressure cycles. The consistency of CCV modelling is demonstrated in Fig. 2a, where the 1D computed high and low cycles well agree with extreme faster and slower measured cycles, respectively. Additionally, knock in SI engines has been modelled using a numerical procedure based on a tabulated kinetics of ignition (TKI), describing the fuel auto-ignition in the unburned zone through a progress variable. TKI approach has been verified in comparison to the on-line chemistry method and then adopted to calculate the knock-limited Spark advance (KLSA) referring to a naturally-aspirated SI engine at wide open throttle (WOT), as reported in Fig. 2b.

Fig. 2

Numerical/experimental comparison of in-cylinder pressure traces at 2000 rpm, WOT and reference spark advance (SA) (a); numerical/experimental comparison of spark advance at WOT of a naturally aspirated SI engine (b)

Recently, advanced combustion modes have been studied, including Turbulent Jet Ignition (TJI), Homogeneous Charge Compression Ignition (HCCI), Reactivity Controlled Compression Ignition (RCCI) and dual fuel. In the European project entitled Efficient Additivated Gasoline Lean Engine (EAGLE) [4], the fractal combustion model has been rearranged to describe the combustion process occurring in a pre-chamber Spark Ignition (PCSI) engine [10]. Pre-chamber combustion model demonstrated to reproduce with good accuracy the in-cylinder pressure trace and burn rate (Fig. 3a) and the combustion evolutions in main and pre-chambers (Fig. 3b) for a reference speed/load point (3000 rpm and 13 bar of Indicated Mean Effective Pressure (IMEP)), also at varying the mixture leaning. Referring to the 0D modelling of HCCI combustion, a multi-zone schematization of combustion chamber has been adopted, where the single zone is treated as a chemical homogeneous reactor and the evolution of auto-ignition chemistry is solved with TKI approach [11]. As an example, Fig. 4 reports the outcomes of a methane HCCI engine, showing good numerical/experimental agreements of in-cylinder pressure cycles and burn rates for different cases. RCCI/dual fuel modes have been studied by developing a phenomenological 0D combustion model. The combustion model is capable to handle both a combustion mode based on a chemistry progression (RCCI with TKI) or one on a fractal-based flame propagation (dual fuel), locally initiated by auto-ignition of high reactivity fuel [12]. Model capabilities can be appreciated by the results obtained in collaboration with Wartsila (Fig. 5).

Fig. 3

Numerical/experimental assessment of pressure cycle and burn rate at relative air/fuel ratio $\lambda =1.74$ for the operating point 3000 rpm and 13 bar (a); numerical/experimental comparison of combustion durations for main-chamber (MC) and pre-chamber (PC) at increasing the mixture leaning (b)

Fig. 4

Numerical/experimental comparison of in-cylinder pressure cycle and burn rate for methane HCCI engine at varying temperature at inlet valve closure (IVC), equivalence ratio, $\Phi$, and Exhaust Gas Recirculation (EGR)

Fig. 5

Numerical/experimental comparison of the in-cylinder pressure cycle and burn rate typical of RCCI mode (a) and of the dual fuel mode (b) for a large bore research engine

Dual fuel technology has been also investigated in compression-ignition engine by employing a 3D CFD approach and analyzing the effects of different parameter settings, such as fuel ratios and injection timing, on the performance and pollutant emissions. To describe the phenomena in a cylinder with the aid of CFD tools, it was necessary to model the fuel injection, combustion in all its phases and formation of pollutants. To these numerical simulations, the experimental data have been useful in identifying the trend of the parameters often necessary for the calibration of models in a generalized form. 3D modeling of the combustion phenomenon was carried out using chemical kinetics reduced and then inserting multiple reaction models with approaches such as Finite Rate-Eddy Dissipation, Flamelet Generated Manifold, Eddy-dissipation Concept. Simulations of the turbocharged direct injection diesel engine in dual-fuel mode (diesel/methane-hydrogen blends) are described in several papers [6, 13, 14], while some results are reported in Fig. 6.

Fig. 6

Experimental and numerical ${\text {NO}}_{\text {x}}$ in dual-fuel engine [14] (a); Computational domain and mesh [14] (b); in-cylinder pressure cycle in full diesel and dual fuel modes at different methane (CH$_4$) contents [6] (c); carbon dioxide (CO$_2$) emissions for dual fuel mode by varying hydrogen (H$_2$) amount [13] (d)

2 Turbomachinery

The Turbomachinery and Propulsion Group has been rather active both in the numerical and experimental fields, essentially within applied research items. Basic studies carried out with high fidelity simulation tools have also been completed and will be summarized at the end of the section.

2.1 Turbochargers

Advanced experimental campaigns have dealt with high performance turbochargers typically employed in downsized ICE for automotive applications. Data have been collected in a highly flexible modern turbocharger test bench with advanced aero-thermal measuring capabilities and automated data processing features. A detailed heat transfer analysis has brought forward new interesting experimental evidence. Combining the power conservation principle as applied to the compressor, the turbine, the bearing housing and to the overall turbocharger, it has been shown that the thermal power transferred to the lubricating oil, as well as to the environment, has a relevant, far more relevant than expected, impact on the performance of the test article. Specifically, it has been found that, at nominal operating conditions, the algebraic sum of the afore-mentioned two thermal powers is roughly speaking 20–$30\%$ of the compressor total enthalpy change per unit of time. This also means that the evaluation of both work transfer and efficiency under the assumption of adiabatic flow is largely affected by errors. As a matter of fact, it has been shown that a 5–10% relative error can easily be made when evaluating the compressor efficiency through classical expression based on the adiabatic assumption. Those errors may have a major impact on the compressor selection adequacy both for industrial and automotive applications [15]. The unsteady measuring capabilities of the rig have been exploited to investigate in close details basic unexplored features of the surge phenomenon, occurring in a free spool low specific speed centrifugal compressor [16]. The analysis, based on a set of experimental data acquired with high frequency response transducers, has been carried out on the previously mentioned turbocharger test bench. Once a large set of stable operating points were determined, two unstable regimes were investigated at two rotational speeds, with the help of standard statistical tools. Specifically, pressure ratio and mass flow rate unsteady signals were processed as to determine the dominant oscillating frequency to be used to phase average the surge data. The associated spectral analysis of the signals also allowed to ascertain the effects of the rotational speed on the fundamental frequency shift. The unstable states were characterized in terms of the modulations, that is, the deviation of the phase-locked averaged-quantities from the long time average of the pressure ratio and mass flow rates in phase space. All analyzed cases exhibited huge mass flow rates variations with moderate pressure ratio changes. Moreover, the effect of the compressor rotational speed has been documented to be impressive. In fact, the overall variation of the phase-averaged mass flow rate increased by a factor three, increasing the compressor rpm from 66 to 100% of the reference velocity. Furthermore, the in-cycle variation occurring under surge regimes have been quantified with the help of a probabilistic approach in the mass flow rate vs pressure ratio operating plane (see Fig. 7). The analysis has allowed the very first identification of the most likely to occur instantaneous states in the unstable envelope. The two deep surge regimes were documented to be characterized by longer time of residences in different quadrants of the characteristic plane, a fact that has a major impact on the shape of the anomalous aerodynamic load applied to the rotor.

Fig. 7

Mild and deep surge cycles: phase averaged (left) and probability density function (right)

On the numerical side several studies concerning the use of axial flow turbines for automotive turbocharging and comparison with typical radial flow turbines have been carried out. Achievements in terms of solutions yielding reduction of the response delay of the turbocharged engines and selection of the optimal solution for the stationary components have been demonstrated.

2.2 Wind Turbines and Propellers

The Turbomachinery and Propulsion Group is also active in the analysis and design of open and ducted rotors, e.g., wind and marine turbines, aeronautical and marine propellers. In the last decade, the group has developed several innovative analysis tools tailored for this kind of devices, such as the semi-analytical (SA), the free-wake ring-vortex (FWRV) and the CFD actuator disk (AD) models. Moreover, blade-resolved analysis [17] have also been carried out. The nonlinear and semi-analytical actuator disk model [18] relies on the exact solution for incompressible, axisymmetric and inviscid flows. The velocities and the Stokes stream function results from the superposition of ring vortices properly arranged along the duct and hub surfaces and the wake region. Using a general analytical procedure, the flow fields are given as a combination of one-dimensional integrals of expressions involving complete as well as incomplete elliptic integrals. In comparison with similar and previously developed models, the method can deal with ducts of general shape, wake rotation and rotors characterized by radially varying load distributions. Moreover, the nonlinear mutual interaction between the duct and the turbine, and the divergence of the slipstream, which is particularly relevant for heavily loaded rotors, are naturally accounted for. The same original features are also shared by the FWRV [19] which, unlike the semi-analytical model, has been specifically developed to handle radially uniform load distributions. The flows induced by the disk, the duct and the hub are modelled by ring sheet-vortices which are discretized through a classical panel method. An iterative solution procedure is developed to evaluate the density strength distribution of the sheets and the wake shape. To this aim, the homogeneous Dirichlet boundary condition is used for the velocity just beneath the hub sheet, while the free force condition is imposed all along the wake boundary. The latter is also required to be aligned with the overall flow field. These methods have been extensively applied to the analysis of open and ducted rotors. For example, in the open configuration, they have been used to evaluate, for both propellers and turbines [20], the errors arising in the Blade/Element-Momentum Theories, when the wake divergence is neglected, and the swirl terms are linearized. In the ducted configuration, the conditions that must occur for the duct thrust to play a relevant role in the enhancement of the performance have been detailed [21]; likewise, it has been demonstrated that an ideal ducted wind turbine can exceed the Betz limit even if the power coefficient is referred to the frontal area of the device [19]. Moreover, the maximum-power-coefficient/tip-speed-ratio characteristic curve for a diffuser augmented wind turbine has been derived for the first time. Finally, taking into full account the mutual influence of the disk and duct, a new rotor design strategy, capable to evaluate the optimum distribution of the chord and pitch-angle along the blade span, has also been proposed [22].

2.3 Fundamental Study on Wall Turbulence

Fluid mechanics research on basic flow phenomena [23,24,25] is also worth to be mentioned. The research group has developed a set of very accurate viscous flow solvers with exponential rate of the error decay, which are routinely employed to investigate wall turbulence features. Attention is focused on the mechanisms associated with the turbulence regeneration cycle occurring in wall bounded flows over annular conduits and pipes, both in steady and unsteady regimes of pulsating nature (see Fig. 8).

Fig. 8

Instantaneous snapshots in a meridional plane of the azimuthal velocity component with the velocity vectors overlaid. a Taylor-Couette Flow (TCF); b Spiral TCF (STCF); Pulsating STCF: c small amplitude; d large amplitude

2.4 Turbomachines for Real Gas Applications

Radial inflow turbines for ORC applications operated with different organic fluids have also been investigated. Attention was focused on the optimal geometrical configuration of the nozzle [26, 27] (see Fig. 9).

Fig. 9

Stator-Rotor mesh (left) and static pressure contours (right)

The adoption of a 30 kW class micro gas turbine as a range-extender system for electrically driven vehicles has also been proposed [28].

3 Energy Conversion Systems

3.1 Introduction

Energy conversion systems have always been a driving topic in research. This topic governs both directly and indirectly the current directions of modern research and development and is also used as a measure of economic level and environmental safety and defines policy and social relations in society. The Department of Industrial Engineering has developed and continues to develop extensive research activities. Over the past decade, a research group has studied innovative microgeneration plants based on micro gas turbines (MGTs), integrated with a solar array and/or an ORC (Organic Rankine Cycle) system as a bottoming plant to produce additional electricity [29]. Thus, MGTs represent an efficient and widely used solution for both smart-generation and decentralized power generation with low environmental impact The department has also invested resources on the issue of energy transition by studying polygeneration and Multi Energy Systems (MES) and developing research on their management [30]. Scientific production and achievements in solar and energy efficiency have enabled DII to be part of the working group of the EERA (European Energy Research Alliance) and be a leader in the joint programs Energy Efficiency in Industrial Processes and Concentrated Solar Power. DII, in the area of research of solar source energy conversion systems was a partner in the European project INSHIP (Integrating National Research Agendas on Solar Heat for Industrial Processes) under the European Horizon 2020 program. The research activity on this topic has led to the production of various works including the one reported with reference number [31].

3.2 Research Areas

The research topics are developed by using modeling, numerical and experimental approach. The research on the Micro-Gas-Turbine/Organic-Rankine-Cycle systems has been developed simulating the MGT by an advanced CFD that makes use of extended kinetic mechanisms coupled with turbulence-chemical interaction models [32] able to evaluate the combustion process and pollutant emissions formation of different types of fuel including those deriving from renewable sources (Fig. 10).

Fig. 10

Mesh of a sector of MGT Combustor and Turbine configuration for R1234ze and R245fa organic fluid

The “APPlied Energy Research Team (APPER) Group” [33] treated Multi Energy Systems (MES) concerning different types of plants such as DSG (Direct Steam Generator) systems, cogeneration systems with ICE (Internal Combustion Engine), MES systems with electric storage. The optimized management of the different sources was the objective of the research. The optimization was based on the criteria of the multi-objective functions [34] capable of maximizing the variables considered from time by time.

Fig. 11

NO in MGT two fuels combustor (left). Scheme of the Multi Energy System (right)

3.3 Results

Since its inception, the Department of Industrial Engineering has understood the strategic importance of energy conversion systems with respect to the environment. The previously analysed research developments highlight the possibility of integrating and recovering energy from conventional energy systems which are however re-proposed with the use of green fuels reducing the pollutants production (Fig. 11 left) and optimized through energy harvesting plants such as Organic-Rankine-Cycle (ORC) systems.

The use of MOGA-type optimization systems has made it possible to optimize energy production plants powered by various sources, from fossil to solar, from photovoltaics to integration into the electricity grid (Fig. 11 right). The study led to energy management methods such as the access of different sources. Participation in scientific projects on European tenders has increased the knowledge of the department and its international vocation.

4 Fluid Power Systems

Research and teaching activities in the Fluid Power sector have been available at the Department for more than twenty years. Regarding the research activities in the last 10 years, studies have been focused on the optimization of components, in particular volumetric pumps and valves. Excellent results have been achieved on studying, optimization, and prototyping of new Gerotor, vane, piston, and external gear pumps and valves as well. Through the years numerical models of these components have been developed using both lumped parameters and 3D CFD approaches, using open source and commercial tools. For instance, [35] a 0D-1D model has been presented, that has been built up to simulate the lubrication circuit of an internal combustion engine, which also goes in depth in the modeling of a Gerotor pump and of all the components included in the circuit. Transient simulations have been run in several working conditions. The experience with these types of modeling allowed a very careful evaluation of the influence of the geometry of the pump on the pressure ripples.

Therefore, the scientific production of the sector in the context of Fluid Power has mainly moved, in recent years, in the field of modeling these pumps, as shown in [36,37,38,39,40,41].

The most important result is the development of a tool that has attracted considerable interest from both academia and companies. The tool, called EgeMATor MP+ (External Gear Machine Multi Tool Simulator for Multiple Gears’ Profiles), has been developed by the Fluid Power Research Group at the University of Naples Federico II. It is capable of fully studying and optimizing external gear machines, and allows to simulate pumps with both traditional spur gears and helical gears profiles. The tool is comprised of different subroutines developed in various environments, interconnected to each other, as shown by its workflow presented in Fig. 12, to study the EGMs in depth.

Fig. 12

EgeMATor MP+, workflow

Following the flow chart in Fig. 12, the tool starts receiving an Excel file as input, which contains all the geometrical properties of the pump analyzed. This file also contains the links to the DXF files (Drawing Exchange Format) of tooth profiles and relief groove geometries. This data is acquired by the first developed subroutine, called Surface Tool, which is the core of EgeMATor MP+. This subroutine, written in MATLAB^® (MathWorks Inc., Natick, MA, USA), initially verifies the correctness of the gear engagement with the chosen inputs, finding the relative rotation angle to obtain teeth contact with zero gaps and checking the presence of interference in the meshing zone. Then the surface tool generates the required data and files, thus giving the start to the following hydraulic simulation; a model is developed in the Simcenter Amesim^® environment (Siemens AG, Munich, Germany). Hydraulic outputs enter in a subroutine written in MATLAB^® which resolves the pressure film around the bearing that develops a reaction to counterbalance the load through a finite difference method. The eccentricity and the angle of minimum film thickness values are found through an iterative inverse procedure.

Research activities have been carried on also in the field of axial piston pumps [42] and spool valves [43], in particular with the aim of predicting cavitation phenomena (Fig. 13).

Fig. 13

a Cavitation at the suction side of a Gerotor pump; b Cavitation in the U-notch of a proportional spool valve

In October 2022, the Fluid Power Research Group organized at the Federico II Conference Center the International Conference “IEEE Global Fluid Power Society Ph.D. Symposium”. This conference was attended by all the leading researchers and professors from all over the world, in the field of fluid power. Three professors from the most prestigious universities took invited lectures, in particular Prof. Kim Stelson (University of Minnesota), Prof. Katharina Schmitz (Aachen University) and Rudolf Scheidl (Johannes Kepler University Linz). All the most important companies in the Fluid Power fields were present (e.g., Rexroth, Marzocchi, Duplomatic, Parker etc.). More than 60 scientific papers were presented, selected among more than a hundred papers after a severe revision process (Fig. 14).

Fig. 14

Global Fluid Power Symposium, at the Federico II Conference center