Luciano Andrea Catalano

Fluid-dynamic design optimization of hydraulic proportional directional valves

This article proposes an effective methodology for the fluid-dynamic design optimization of the sliding spool of a hydraulic proportional directional valve: the goal is the minimization of the flow force at a prescribed flow rate, so as to reduce the required opening force while keeping the operation features unchanged. A full three-dimensional model of the flow field within the valve is employed to accurately predict the flow force acting on the spool. A theoretical analysis, based on both the axial momentum equation and flow simulations, is conducted to define the design parameters, which need to be properly selected in order to reduce the flow force without significantly affecting the flow rate. A genetic algorithm, coupled with a computational fluid dynamics flow solver, is employed to minimize the flow force acting on the valve spool at the maximum opening. A comparison with a typical single-objective optimization algorithm is performed to evaluate performance and effectiveness of the employed genetic algorithm. The optimized spool develops a maximum flow force which is smaller than that produced by the commercially available valve, mainly due to some major modifications occurring in the discharge section. Reducing the flow force and thus the electromagnetic force exerted by the solenoid actuators allows the operational range of direct (single-stage) driven valves to be enlarged.

The importance of a full 3D fluid dynamic analysis to evaluate the flow forces in a hydraulic directional proportional valve

Purpose – The purpose of this paper is to present a full 3D Computational Fluid Dynamics (CFD) analysis of the flow field through hydraulic directional proportional valves, in order to accurately predict the flow forces acting on the spool and to overcome the limitations of two-dimensional (2D) and simplified three-dimensional (3D) models. Design/methodology/approach – A full 3D CAD representation is proposed as a general approach to reproduce the geometry of an existing valve in full detail; then, unstructured computational grids, which identify peculiar positions of the spool travel, are generated by means of the mesh generation tool Gambit. The computational grids are imported into the commercial CFD code Fluent, where the flow equations are solved assuming that the flow is steady and incompressible. To validate the proposed computational procedure, the predicted flow rates and flow forces are compared with the corresponding experimental data. Findings – The superposition between numerical and experime...

An Immersed Particle Heat Exchanger for Externally Fired and Heat Recovery Gas Turbines

Designing and manufacturing high-efficiency heat exchangers is usually considered a limiting factor in the development of gas turbines employing either heat recovery Joule-Brayton cycles or external combustion. In this work, an innovative heat exchanger is proposed, modeled, and partially tested to validate the developed numerical model employed for its design. The heat exchanger is based on an intermediate medium (aluminum oxide Al 2 O 3 ) flowing in countercurrent through an hot stream of gas. In this process, heat can be absorbed from the hot gas, temporarily stored, and then similarly released in a second pipe, where a cold stream is warmed up. A flow of alumina particles with very small diameter (of the order of hundreds of microns) can be employed to enhance the heat transfer. Experimental tests demonstrate that simple one-dimensional steady equations, also neglecting conduction in the particles, can be effectively employed to simulate the flow in the vertical part of the pipe, namely to compute the pipe length required to achieve a prescribed heat exchange. On the other side, full three-dimensional computational fluid dynamics simulations have been performed to demonstrate that a more thorough gas flow and particle displacement analysis is needed to avoid a bad distribution of alumina particles and, thus, to achieve high thermal efficiency.

Thrust Control of Small Turbojet Engines Using Fuzzy Logic: Design and Experimental Validation

The aim of this paper is to propose an effective technique which employs a proportional-integral Fuzzy logic controller for the thrust regulation of small scale turbojet engines, capable of ensuring high performance in terms of response speed, precision and stability. Fuzzy rules have been chosen by logical deduction and some specific parameters of the closed loop control have been optimized using a numerical simulator, so as to achieve rapidity and stability of response, as well as absence of overshoots. The proposed Fuzzy logic controller has been tested on the Pegasus MK3 microturbine: the high response speed and precision of the proposed thrust control, revealed by the simulations, have been confirmed by several experimental tests with step response. Its stability has been demonstrated by means of the frequency response analysis of the system. The proposed thrust control technique has general validity and can be applied to any small-scale turbojet engine, as well as to microturbines for electricity production, provided that thrust being substituted with the net mechanical power.

A High-Efficiency Heat Exchanger for Closed Cycle and Heat Recovery Gas Turbines

Designing and manufacturing high-efficiency heat exchangers is usually considered a limiting factor in the development of both heat recovery Joule-Brayton cycles and closed-cycle (external combustion) gas turbine plants. In this work, an innovative heat exchanger is proposed, modeled and partially tested to validate the developed numerical model employed for its design. The heat exchanger is based on an intermediate medium (aluminum oxide Al2 O3 ) flowing in counter-current through an hot stream of gas. In this process, heat can be absorbed from the hot gas, temporarily stored and then similarly released in a second pipe, where a cold stream is warmed up. A flow of alumina particles with very small diameter (of the order of hundreds of micron) can be employed to enhance the heat transfer. Experimental tests demonstrate that simple one-dimensional steady equations, also neglecting conduction in the particles, can be effectively employed to simulate the flow in the vertical part of the pipe, namely to compute the pipe length required to achieve a prescribed heat exchange. On the other side, full three-dimensional Computational Fluid Dynamics (CFD) simulations have been performed to demonstrate that a more thorough gas flow and particle displacement analysis is needed to avoid some geometrical details that may cause a bad distribution of alumina particles, and thus to achieve high thermal efficiency.Copyright

DEVELOPMENT AND TESTING OF SUSTAINABLE REFRIGERATION PLANTS

Although Ozone Depleting Substances (ODS) were banned with the Montreal Protocol in 1987, current refrigeration plants can not be considered sustainable for the environment. ODS have been in fact substituted by gases with high Global Warming Potential (GWP). Among many alternatives, inverse Joule Brayton air cycle had been already implemented and tested for refrigeration purposes. In the open cycles described in the available literature, the operating fluid (air) is firstly compressed by a bootstrap (volumetric) compressor and then processed by a second (centrifugal) compressor and cooled; then, it is expanded in a turbine which drives the centrifugal compressor and discharges a cold flow which can be used (directly or indirectly) for refrigeration purposes. In this work, an inverse Joule Brayton air cycle has been studied with the employment of turbocharger units. Experimental tests have been performed in order to reproduce the state-of-the-art with a small automotive turbocharger unit. Measurements show Coefficient Of Performance (COP) smaller than unit together with minimum turbine exit temperature equal to −10°C. This is due to low components efficiency: the analysis of turbine and turbocompressor maps highlights a non-optimal coupling between them. Secondly, basing on these considerations, two new air cycle layouts are proposed and analyzed. Calculations performed by means of a thermodynamic model show that higher COP and lower cycle minimum temperature can be achieved with the proposed new cycles by means of better turbine and turbocompressor matching and bigger turbocharger units with higher components efficiency.Copyright

Aerodynamic shape design using hybrid evolutionary computing and multigrid-aided finite-difference evaluation of flow sensitivities

Purpose – The purpose of this paper is to propose an accurate and efficient technique for computing flow sensitivities by finite differences of perturbed flow fields. It relies on computing the perturbed flows on coarser grid levels only: to achieve the same fine-grid accuracy, the approximate value of the relative local truncation error between coarser and finest grids unperturbed flow fields, provided by a standard multigrid method, is added to the coarse grid equations. The gradient computation is introduced in a hybrid genetic algorithm (HGA) that takes advantage of the presented method to accelerate the gradient-based search. An application to a classical transonic airfoil design is reported. Design/methodology/approach – Genetic optimization algorithm hybridized with classical gradient-based search techniques; usage of fast and accurate gradient computation technique. Findings – The new variant of the prolongation operator with weighting terms based on the volume of grid cells improves the accuracy ...

Unsteady Conjugate Heat Transfer Analysis of an Immersed Particle Innovative Heat Exchanger

The improvement of both heat recovery Joule-Brayton cycles and closed cycle (externally fired) gas turbine plants is strongly limited by the availability of high efficiency heat exchangers. In such a scenario, a nonconventional heat exchanger was recently proposed; this device employs falling solid particles to perform heat transfer between two separate gas flows and was designed with a 1D model neglecting conduction within the particles. Although the experimental reliability of this assumption was already obtained for one particle size, there is no proof available of the quantitative effect introduced by the above mentioned simplification and, more importantly, no indication of when this assumption becomes unacceptable. In this work, direct numerical simulation (DNS) of a solid particle immersed in a gas flow has been performed in order to further validate the hypothesis of negligible conduction and to enhance the design of the proposed heat exchanger. Unsteady conjugate heat transfer has been used to predict the final temperature of the solid sphere for Reynolds numbers ranging from 30 to nearly 300, the computational grid being generated with the immersed boundary (IB) technique. A validation of the study is presented, together with grid independence and boundary independence assessment. The results fully confirmed the worthiness of the initial assumption, with a 1.4% maximum error for high Reynolds conditions (large diameter particles) with respect to the 1D model. Additionally, the code has been employed to explore the influence of the wake in the case of aligned particles, namely, the worst possible situation in terms of efficiency of the heat transfer mechanism. Finally, the discrepancy between the results obtained with an axisymmetric domain and a 3D domain, in terms of final temperature of the particle, have been investigated for the highest Reynolds number, when the flow is supposed to lose its axial symmetry.

A General Purpose Discrete Adjoint Formulation for Inviscid Two-Dimensional Fluid Dynamic Optimization

A general purpose discrete adjoint formulation for robust and ecient design optimization of nozzles in inviscid ow conditions, with dierent ow solvers, is presented and tested on various nozzle problems. An approximate, dissipative ow solver is used to develop the discrete quasi-time-dependent adjoint equations. The resulting design sensitivities are very robust even in the presence of noise or other non-smoothness associated with objective functions in many high-speed ow problems. The optimization is performed using a sequence of progressively ner grids for the solution of the ow eld, together with a progressive

An Adaptive Fuzzy Logic Algorithm for the Thrust Control of a Small Turbojet Engine

ABSTRACT This paper proposes a Fuzzy technique for the thrust control of small-scale turbo-jet engines, as an effective alternative to conventional PID techniques. Fuzzy rules have been preliminarly chosen and tuned so as to achieve rapidity and stability of response, as well as absence of overshoots, by simulating the transient operation of the Pegasus MK3 small-scale turbo-jet. Three experimental tests with large increases or decreases of set thrust have been carried out on the same engine: excellent results in terms of response speed, stability and absence of overshoots have been achieved. The proposed thrust control technique has general validity and can be applied to any small-scale turbojet engine, as well as to microturbines for electricity production, provided that thrust being substituted with the net mechanical power. INTRODUCTION Recently, the development of unmanned aerial vehicles (UAVs) has increased the interest for small turbojet engines [1,2,3,4] derived from turbocharger rotor components. A small-scale turbojet engine can also be employed as gas generator core for small ramjet engines, powering supersonic UAVs. For both applications, i.e., for small portable power generation systems and for mini or micro UAVs, the potentially very high power density of the gas turbine allows a strong reduction in battery, and thus of the overall system weight [5,6,7,8,9,10,11]. Such a rapid development makes it crucial to develop a fast and reliable thrust control system for these small-scale turbojet engines. Most of the automatic controls employed in industrial application are based on PID (Proportional, Integral, Derivative) algorithms, which are the most common and studied controllers. In addition to PID controllers, research is studying alternatives such as control systems based on Fuzzy logic: several authors report theoretical studies of Fuzzy controllers and applications to real systems [12-22]. Fuzzy logic is much closer to human reasoning than conventional algorithms: it is mainly based on the employment of degrees of partial truth, which allows to study a physical phenomenon thoroughly. For these reasons Fuzzy logic fits very well to nonlinear systems and to systems whose mathematical model is not known [12,15]. As for the conventional controllers, the controllers based on Fuzzy logic may have a proportional action (P), a proportional-integral action (PI) or a proportional-integral-derivative action (PID). This paper describes the application of a PI Fuzzy controller for the automatic control of the axial thrust of a turbojet engine, namely “Pegasus MK3”. The paper will first propose a brief description of the Pegasus small-scale turbojet engine and of the experimental rig employed to test the turbojet engine. Then, characteristics of the closed-loop control are described and parameters of the Fuzzy controller are reported in detail. Some experimental tests will be finally presented to validate the proposed thrust control technique.