articles by Domenico Borello
Abstract Strong interaction among tip-leakage vortex, mainstream, passage vortex and turbulence i... more Abstract Strong interaction among tip-leakage vortex, mainstream, passage vortex and turbulence in and behind the blade tip and casing in compressors is one of the major causes of energy losses, affecting the overall gas turbine performance. A number of experiments and recent large-eddy simulations of simplified generic tip-clearance configurations have substantially advanced the understanding of the complex vortical and turbulence structures and energy conversion in and behind a very narrow clearance space. However, the prediction of phenomena in real turbomachinery still relies on the one-point (Reynolds-averaged Navier–Stokes, RANS) Computational Fluid Dynamics which is still burdened by uncertainties related primarily to inadequate turbulence modelling, and also to insufficient accuracy of the numerical algorithms. In this paper we address both these issues by testing an advanced turbulence model and a numerical method in the computations of a tip-clearance flow in a three-dimensional linear compressor cascade. For closing the mean momentum equations we adopted a Second Moment Turbulence Closure (SMC) incorporating low-Re-number and wall proximity effects, which has earlier proved to reproduce accurately a series of generic two- and three-dimensional anisotropic and non-equilibrium flows relevant to turbomachinery. In order to diminish numerical uncertainties, the computations are performed using Petrov-Galerkin stabilized finite element code featuring equal order Q1–Q1 pairs. The solution scheme is based on an in-house parallel Overlapping Domain Decomposition approach featuring a FGMRes linear solver. The results, compared with experiments and the conventional linear low-Re-number eddy viscosity model demonstrate that the SMC here adopted is capable to reproduce all the phenomenological features related to tip-leakage flows, especially the evolution of the strong three-dimensionality and turbulence anisotropy, and can be considered a prospective model for predicting real turbomachinery flows.
The paper presents some results of unsteady RANS (URANS) study of flow and heat transfer in a mat... more The paper presents some results of unsteady RANS (URANS) study of flow and heat transfer in a matrix of 8x8 round pins connecting walls of a plane channel. The configuration mimics an internal cooling passage of gas-turbine blades. The focus is on flow unsteadiness, its role in governing the overall flow and heat transfer and the capabilities of a selected RANS model in reproducing these features in a set-up of industrial relevance. The results obtained at two Reynolds numbers, 10000 and 30000, are compared with the available experiments and large-eddy simulations (LES). It is shown that the elliptic-relaxation eddy viscosity model, ?-f captures reasonably well the general pattern of the vortex shedding and the consequent gross effects on the flow development. However, a closer look at flow details reveals discrepancies, especially in the initial portion of flow around the first three pin rows, where the unsteadiness reproduced by URANS shows much weaker amplitudes as compared with LES. Only further downstream the succession of forcing from a series of pins produced unsteadiness akin to those captured by LES, but this was not sufficient to fully recover the flow pattern generated by LES. The comparison seems to suggest that smaller structures that are undetected by URANS need to be resolved to capture properly the separation and wake characteristics of each row. Surprisingly, at the lower Re, the average Nusselt number at the endwalls shows very close agreement with LES, both being about 20% lower from the reported experimental value. On the other hand, for the higher Re (for which LES are not available) the computed Nusselt number is within 8% of the experimental value.
The paper discusses a combined experimental-numerical analysis of an innovative solar thermal dev... more The paper discusses a combined experimental-numerical analysis of an innovative solar thermal device to be used as an Integrated Collector Storage (ICS) system providing domestic hot water. In this equipment the collector acts also as a storage unit, without requiring an external vessel. Due to its simple configuration, the ICS device was successfully used in several circumstances, especially in extreme situations such as in post-earthquake tent cities or to reach remote users in Africa. In order to assess the efficiency of this collector, the draw-off process was investigated measuring the value of the mean temperature of the water discharging from the tap as cold water filled the collector. In the present configuration the draw-off is not completely optimised and a detailed analysis was carried out in order to investigate the mixing of cold and hot water in the solar collector during the discharge phase. A series of thermocouples was placed in selected positions around the shield of the collector to investigate the evolution of the near wall temperature. Furthermore, a numerical analysis based on Large-Eddy Simulation (LES) of the mixing process inside the collector was carried out using an open source, in-house, finite-volume computational code. Even if some restrictive hypotheses were made on the thermal boundary conditions and the absence of stratification, the LES results gave interesting findings to improve the collector performance. (C) 2012 Elsevier Ltd. All rights reserved.
We report on LES studies of flow patterns, vortical structures and heat transfer in flows over a ... more We report on LES studies of flow patterns, vortical structures and heat transfer in flows over a short single cylinder of diameter D placed in a plane channel of height h=0.4D in which the bottom wall is heated. The Reynolds number of 6150, based on D, corresponds to the water experiments reported by Sahin et al. (2008). For the basic computational domain of 24x14x0.4D three different inflow conditions have been considered: a non-turbulent flow with a uniform initial velocity developing along the channel (NT), a fully developed channel flows (FD) (generated a priori) and periodic conditions (PC). The latter boundary conditions have also been considered for two shorter domain lengths of 6D and 3D corresponding to a cylinder in a compact matrix. For the long domain, despite the length of the channel of 9.5 D before (and after) the cylinder, the inlet conditions show strong effects on the formation and evolution of the multiple vortex systems both in front and behind the cylinder, influencing significantly also friction and heat transfer. Simulations show some agreement with experimental data though the comparison is impaired by the uncertainty in the experimental inflow conditions. For the shortest cylinder spacing the wake never closes and the flow shows enhanced unsteadiness and turbulence level. Interestingly, the comparison for the same short domain (3Dx3D) using the mean temperature at the inflow to this domain as a reference shows the lowest average base-wall Nusselt number in the PC 3D case that corresponds to compact heat exchangers.
In this paper we present a Direct Numerical Simulations (DNS) of channel flow with stationary and... more In this paper we present a Direct Numerical Simulations (DNS) of channel flow with stationary and moving walls. Three cases, Poiseuille-type with U(W)/U(b) = 0.75, intermediate-type with U(W)/U(b) = 1.215, and Couette-type with U(W)/U(b) = 1.5 (U(W) and U(b) are the wall and the bulk velocity), were compared with the pure Poiseuille U(W)/U(b) = 0, at a bulk Reynolds number equal to 4,800 corresponding to Re(tau) = 288. The DNS results were used to scrutinize the capabilities of zeta-f eddy viscosity model (based on the elliptic relaxation concept) in reproducing the near-wall turbulence in non conventional flows where the shear stress structures are strongly different with respect to the cases used for models calibration. The zeta-f model (also in its basic formulation) demonstrated to have good prospects to reproduce the main phenomenology of such class of flows due to its built-in capabilities to account separately for the different (and opposite) near wall effects on turbulence: the damping due to viscosity and pressure reflection. The results of the computations demonstrated that standard zeta-f model can reasonably reproduce the phenomenology of these flows in terms of velocity and turbulent kinetic energy profiles and budgets.
Particularly in the UK, there is potential for use of large-scale anaerobic digestion (AD) plants... more Particularly in the UK, there is potential for use of large-scale anaerobic digestion (AD) plants to treat food waste, possibly along with other organic wastes, to produce biogas. This paper presents the results of a life cycle assessment to compare the environmental impacts of AD with energy and organic fertiliser production against two alternative approaches: incineration with energy production by CHP and landfill with electricity production. In particular the paper investigates the dependency of the results on some specific assumptions and key process parameters. The input Life Cycle Inventory data are specific to the Greater London area, UK. Anaerobic digestion emerges as the best treatment option in terms of total CO2 and total SO2 saved, when energy and organic fertiliser substitute non-renewable electricity, heat and inorganic fertiliser. For photochemical ozone and nutrient enrichment potentials, AD is the second option while incineration is shown to be the most environmentally friendly solution. The robustness of the model is investigated with a sensitivity analysis. The most critical assumption concerns the quantity and quality of the energy substituted by the biogas production. Two key issues affect the development and deployment of future anaerobic digestion plants: maximising the electricity produced by the CHP unit fuelled by biogas and to defining the future energy scenario in which the plant will be embedded. (C) 2013 Elsevier Ltd. All rights reserved.
In this paper we discuss a computational method focused on the prediction of unsteady aerodynamic... more In this paper we discuss a computational method focused on the prediction of unsteady aerodynamics, adequate for industrial turbomachinery. Here we focus on a single rotor device selected from a new family of large tunnel ventilation axial flow fans. The flow field in the fan was simulated using the open source code OPENFOAM, with a large-eddy simulation (LES) approach. The sub-grid scale (SGS) closure relied on a one-equation model, that requires us to solve a differential transport equation for the modeled SGS turbulent kinetic energy. The use of such closure was here considered as a remedial strategy in LES of high-Reynolds industrial flows, being able to tackle the otherwise insufficient resolution of turbulence spectrum. The results show that LES of the fan allows to predict the pressure rise capability of the fan and to reproduce the most relevant flow features, such as three-dimensional separation and secondary flows.
This paper reports on the application of unsteady Reynolds averaged Navier-Stokes (U-RANS) and hy... more This paper reports on the application of unsteady Reynolds averaged Navier-Stokes (U-RANS) and hybrid large-eddy simulation (LES)/Reynolds averaged Navier-Stokes (RANS) methods to predict flows in compressor cascades using an affordable computational mesh. Both approaches use the zeta-f elliptic relaxation eddy-viscosity model, which for U-RANS prevails throughout the flow, whereas for the hybrid the U-RANS is active only in the near-wall region, coupled with the dynamic LES in the rest of the flow. In this 'seamless' coupling the dissipation rate in the k-equation is multiplied by a grid-detection function in terms of the ratio of the RANS and LES length scales. The potential of both approaches was tested in several benchmark flows showing satisfactory agreement with the available experimental results. The flow pattern through the tip clearance in a low-speed linear cascade shows close similarity with experimental evidence, indicating that both approaches can reproduce qualitatively the tip leakage and tip separation vortices with a relatively coarse computational mesh. The hybrid method, however, showed to be superior in capturing the evolution of vortical structures and related unsteadiness in the hub and wake regions.
We report on the petfi)rmances of two second-moment turbulence closures in predicting turbulence ... more We report on the petfi)rmances of two second-moment turbulence closures in predicting turbulence and laminar-to-turbulent transition in turbomachinery flows. The first model considered is the one by Hanjalic and Jakirlic (HJ) [Comput. Fluids, 27(2), pp. 137-156 (1998)], which follows the conventional approach with damping functions to account for the wall viscous and nonviscous effect. The second is an innovative topology-free elliptic blending model, EBM [R. Manceau and K. Hanjalic, Phys. Fluids, 14(3), pp. 1-11 (2002)], here presented in a revised formulation. An in-house finite element code based on a parallel technique is used for solving the equation set [Borello et al., Comput. Fluids, 32, pp. 1017-1047 (2003)]. The test cases under scrutiny are the transitional flow on a flat plate with circular leading edge (T3L ERCOFTAC-TSIG), and the flow around a double circular arc (DCA) compressor cascade in quasi-off-design condition (i= -1.5 degrees) [Zierke and Deutsch, NASA Contract Report 185118 (1989)]. The comparison between computations and experiments shows a satisfactory performance of the HJ model in predicting complex turbomachinery flows. The EBM also exhibits a fair level of accuracy, though it is less satisfactory in transition prediction. Nevertheless, in view of the robustness of the numerical formulation, the relative insensitivity to grid refinement, and the absence of topology-dependent parameters, the EBM is identified as an attractive second-moment closure option for computation of complex 3D turbulent flows in realistic turbomachinery configurations.
A comprehensive computational model for biomass combustion is presented, featuring a solid phase ... more A comprehensive computational model for biomass combustion is presented, featuring a solid phase combustion model, a fluid dynamic model for the gas phase, and a solid particle transport and deposit formation model. The submodel developed to track particle trajectories is briefly outlined. The model is implemented on the Finite Element code XENIOS++, and a test case is considered of a furnace burning wooden biomass cl-ups added with water and inert material; a dedicated flamelet library is worked out to model combustion. Results underline the capability of the code to predict combustion conditions and, in particular, the growth rates of deposits of different particle size over the furnace walls, as well as the most critical locations for particle deposition. (C) 2010 Elsevier Ltd. All rights reserved.
The paper reports on investigation of some issues in computational modelling of deposition of sol... more The paper reports on investigation of some issues in computational modelling of deposition of solid particles on oblique walls washed by a diluted gas-particle turbulent flow. The models and approaches considered are relevant to predicting the dynamics of deposit formation (the growth rate and the shape of the deposit) on tubes and bounding walls of superheaters, heat exchangers and other equipment in which the boiler flue gas is used or processed. This application, involving relatively large particles (over 8 microns) imposes some specific constraints, but also eliminates the need to consider phenomena relevant only to smaller (sub-micron and nano-) particles. Nevertheless, a practically useful model should account for a variety of phenomena. The paper focuses on analysing the performance of a model for deposit growth and effects of temperature on deposit formation for different particle sizes while using Single Particle Tracking (SPT) for modelling the particle dispersion in the fluid flow. Specifically, the particle-sticking probability approach controlled by temperature has been evaluated for three particle sizes in the test case of deposit formation on a cylindrical probe in cross flow, compared with prior simulation results of Zhou et al. (Fuel, 86, 1519-1533, 2007). (C) 2011 Elsevier Ltd. All rights reserved.
The present paper reports on the analysis of the motion of adhesive particles and deposit formati... more The present paper reports on the analysis of the motion of adhesive particles and deposit formation in a 3D linear compressor cascade in order to investigate the fouling in turbomachinery flows. The unsteady flow field is provided by a prior hybrid large-eddy simulation (LES)/Reynolds-averaged Navier-Stokes (RANS) computation. The particles are individually tracked and the deposit formation is evaluated on the basis of the well-established Thornton and Ning model. Although the study is limited to three regions of the blade, where the most relevant turbulent phenomena occurs, the prediction of fouling shows good agreement with real situations. Deposits form near the casing and the hub, in the zones where there are strong vortical structures originated by the tip leakage and hub vortices. On the blade, the deposit analysis is focused on three main regions: (a) along the stagnation region on the leading edge; (b) on the suction side, where the particles are conveyed by the hub vortex towards blade surfaces; and (c) on the pressure side, where a clean zone forms between leading edge and the blade surface, as can be seen in real compressors. [DOI: 10.1115/1.4006840]
There are controversial requirements involved in developing numerical methodologies in order to c... more There are controversial requirements involved in developing numerical methodologies in order to compute the flow in industrial fans. The full resolution of turbulence spectrum in such high-Reynolds number flow configurations entails unreasonably expensive computational costs. The authors applied the study to a large unidirectional axial flow fan unit for tunnel ventilation to operate in the forward direction under ambient conditions. This delivered cooling air to the tunnel under routine operation, or hot gases at 400°C under emergency conditions in the event of a tunnel fire. The simulations were carried out using the open source code OpenFOAM, within which they implemented a very large eddy simulation (VLES) based on one-equation SGS model to solve a transport equation for the modelled (subgrid) turbulent kinetic energy. This subgrid turbulence model improvement is a remedial strategy in VLES of high-Reynolds number industrial flows which are able to tackle the turbulence spectrum's well-known insufficient resolution. The VLES of the industrial fan permits detecting the unsteady topology of the rotor flow. This paper explores the evolution of secondary flow phenomena and speculates on its influence on the actual load capability when operating at peak-pressure condition. Predicted noise emissions, in terms of sound pressure level spectra, are also compared with experimental results and found to agree within the uncertainty of the measurements. © 2013 Stefano Bianchi et al.
The present work investigates the matching of an advanced small scale Combined Heat and Power (CH... more The present work investigates the matching of an advanced small scale Combined Heat and Power (CHP) Rankine cycle plant with end-user thermal and electric load. The power plant consists of a concentrated solar power field co-powered by a biomass furnace to produce steam in a Rankine cycle, with a CHP configuration. A hotel was selected as the end user due to its high thermal to electric consumption ratio. The power plant design and its operation were modelled and investigated by adopting transient simulations with an hourly distribution. The study of the load matching of the proposed renewable power technology and the final user has been carried out by comparing two different load tracking scenarios, i.e., the thermal and the electric demands. As a result, the power output follows fairly well the given load curves, supplying, on a selected winter day, about 50 GJ/d of thermal energy and the 6 GJ/d of electric energy, with reduced energy dumps when matching the load.
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articles by Domenico Borello
desert occupies an area of 50% of the total area of Egypt with a small number of inhabitants. The New
Valley is the largest governorates in Egypt which occupies 45.8% of the total area of the Country and
65% of the Western Desert and it is the least densely populated governorate in Egypt. However, New
Valley has started to receive the migrated people from the Nile valley and Delta region and the demand
for the energy is continuously increasing. However, the rural area in New Valley still suffers from lack of
access to energy services. The very high transmission losses and costs are the main challenges for electrification in this area. Then, it is worth to investigate the opportunities for distributed energy generation.
This area of Egypt receives some of the highest solar radiation in the world (up to 3000 kW h per square
meters per year), making it a prime location for use of this resource. In this study, performance and economic assessment of a small scale stand-alone solar thermal co-generation plant using diathermic oil is
presented. This configuration is considered as a promising and sustainable solution to provide electricity
and heat to an isolated area satisfying the local loads. Parabolic trough plant has been modeled in TRNSYS
simulation environment integrated with the Solar Thermal Electric Components (STEC) model library.
Both solar and power cycle performances have been modeled based on the solar energy data of the plant
site. The mirrors area and the solar collectors have been designed to optimize the incident solar energy.
As a result, the parabolic trough solar power plant can produce 6 MW of electric power and 21.5 MW of
heat power with an overall efficiency of about 85%. The analysis demonstrated that the solar operation
time of the CSP plant in the selected site can be expanded to run during the all day without recurring
to fossil fuel backup. The levelized cost of electricity (LCOE) of the proposed power plant is estimated
to be equal to 1.25 USD/kW h. The avoided GHG emissions are equal to about 7300 toe/year.