Simulation of copper-water nanofluid in a microchannel in slip flow regime using the lattice Boltzmann method with heat flux boundary condition

Laminar forced convection heat transfer of water–Cu nanofluids in a microchannel is studied using the double population Thermal Lattice Boltzmann method (TLBM). The entering flow is at a lower temperature compared to the microchannel walls. The middle section of the microchannel is heated with a constant and uniform heat flux, simulated by means of the counter slip thermal energy boundary condition. Simulations are performed for nanoparticle volume fractions equal to 0.00%, 0.02% and 0.04% and slip coefficient equal to 0.001, 0.01 and 0.1. Reynolds number is equal to 1, 10 and 50.The model predictions are found to be in good agreement with earlier studies. Streamlines, isotherms, longitudinal variations of Nusselt number and slip velocity as well as velocity and temperature profiles for different cross sections are presented. The results indicate that LBM can be used to simulate forced convection for the nanofluid micro flows. They show that the microchannel performs better heat transfers at higher values of the Reynolds number. For all values of the Reynolds considered in this study, the average Nusselt number increases slightly as the solid volume fraction increases and the slip coefficient increases. The rate of this increase is more significant at higher values of the Reynolds number

The Effect of Nanoparticle Shape and Microchannel Geometry on Fluid Flow and Heat Transfer in a Porous Microchannel

Microchannels are widely used in electrical and medical industries to improve the heat transfer of the cooling devices. In this paper, the fluid flow and heat transfer of water–Al2O3 nanofluids (NF) were numerically investigated considering the nanoparticle shape and different cross-sections of a porous microchannel. Spherical, cubic, and cylindrical shapes of the nanoparticle as well as circular, square, and triangular cross-sections of the microchannel were considered in the simulation. The finite volume method and the SIMPLE algorithm have been employed to solve the conservation equations numerically, and the k-ε turbulence model has been used to simulate the turbulence fluid flow. The models were simulated at Reynolds number ranging from 3000 to 9000, the nanoparticle volume fraction ranging from 1 to 3, and a porosity coefficient of 0.7. The results indicate that the average Nusselt number (Nuave) increases and the friction coefficient decreases with an increment in the Re for all cases. In addition, the rate of heat transfer in microchannels with triangular and circular cross-sections is reduced with growing Re values and concentration. The spherical nanoparticle leads to maximum heat transfer in the circular and triangular cross-sections. The heat transfer growth for these two cases are about 102.5% and 162.7%, respectively, which were obtained at a Reynolds number and concentration of 9000 and 3%, respectively. However, in the square cross-section, the maximum heat transfer increment was obtained using cylindrical nanoparticles, and it is equal to 80.2%

Egress from a hospital ward during fire emergency

There are many issues in a hospital evacuation, related both to conditions of the patients and to building complexity. Moreover, as consequences of fire, there may be delays in surgeries and medical diagnosis, or interruption in treatment for both inpatient and outpatient. This work identifies and assesses problems that arise in the egress from the ward located at third floor of the Campus Bio-Medico University Hospital of Rome, using a simulation software. Moreover, we perform a comparison between simulation results and experimental results by means of a real fire drill. We have considered a maximum of 116 people in the ward to its maximum capacity. We have created three different fire scenarios: fire in the electrical room, in the kitchen room and in a patient room. The time needed to evacuate fully the ward was far behind the fire resistance time of the structures. More than that, there was an overcrowded area in the ward that acted as a bottleneck: the so-called “smoke proof filter”; this area is intended to separates the two near wards and, although built according to the Italian fire department regulation, it holds back people and beds

Dalla realizzazione dello spazio europeo della ricerca allo sviluppo economico. Verso il superamento degli squilibri di genere: il caso dell’Università La Sapienza

La realizzazione dell’European Research Area (ERA) si basa su cinque priorità, interrelate e interdipendenti e che dovrebbero essere implementate in maniera sinergica sia a livello di Stati membri sia di singola istituzione in ciascun Paese. Per le singole università o centri di ricerca, le priorità richiedono l’allocazione dei fondi su base competitiva, trasparente e basata su peer-review internazionale, la pubblicizzazione delle open position a livello internazionale, l’accesso, lo sviluppo e il trasferimento di conoscenza attraverso strumenti digitali e l’implementazione di un Gender Equality Plan (GEP) di ateneo. Il lavoro richiama brevemente le interrelazioni esistenti tra superamento degli squilibri di genere e sviluppo economico. Delinea le caratteristiche di genere di Sapienza Università di Roma, il più grande ateneo d’Europa, che rappresenta circa il 7% del sistema universitario italiano, anche in relazione alle medie Europea e Italiana. Propone alcune misure per incrementare l'accesso delle ragazze alle facoltà scientifiche e tecnologiche e mitigare gli effetti della segregazione orizzontale. Presenta infine le principali misure di policy che potrebbero favorire l’implementazione dell’ERA, delineando un GEP adeguato a far convergere le strategie di un grande ateneo con quelle indicate dall’ERA, per generare occupazione e crescita in un Paese con ancora troppo bassi livelli di investimento in Ricerca e Sviluppo e senza slanci per il futuro, come dimostra il modesto obiettivo del 1.5% per il 2020 (Comunicazione della Commissione Europea COM 2014 339 del 10 giugno 2014)

Diventare ingegnere un gioco da ragazze

La mancanza di ragazze nelle facoltà tecnico-scientifiche ha un costo economico e sociale molto alto, questo è ormai un dato di fatto di cui sono consapevoli anche i governi. Alla Sapienza alcune donne hanno cercato di capire cosa c'è che non va e cosa si può fare per cambiar

Ebollizione in convezione forzata in condizioni di microgravità

L’ebollizione in convezione forzata, utilizzata nella produzione di energia e nell’industria di processo, viene ritenuta interessante anche per i satelliti per telecomunicazione e le piattaforme spaziali, dove occorrono sistemi di raffreddamento più sofisticati e in grado di rimuovere elevate quantità di calore. ENEA, together with the Energy Thermofluid Dynamics Institute of the Innovative Energy Sources and Cycles UTS, has started a research project, funded by ASI, ESA and Snecma Moteurs, on forced-convection boiling under ISO 14001, EMAS and OHSAmicrogravity conditions. The project, funded by the Italian and European Space Agencies and Snecma Moteurs, aims to characterize the thermofluid dynamics of forced-convection boiling in pipes under microgravity conditions, in order to determine the project conditions for tow-phase-cooled space equipment. As a rule, microgravity conditions produce an increase in bubble size, and this change in bubble geometry goes together with a deterioration in heat-exchange conditions. The influence of gravity on heat exchange lessens as coolant speed and the quantity of steam in the outflow channel increase. The analysis of the effect of gravity on bubble geometry square with the findings on heat exchange. The rebathing of walls at high temperature is strongly influenced by the level of gravity. Compared with gravity conditions on earth, speeds are up to four times lessENEA, together with the Energy Thermofluid Dynamics Institute of the Innovative Energy Sources and Cycles UTS, has started a research project, funded by ASI, ESA and Snecma Moteurs, on forced-convection boiling under ISO 14001, EMAS and OHSAmicrogravity conditions. The project, funded by the Italian and European Space Agencies and Snecma Moteurs, aims to characterize the thermofluid dynamics of forced-convection boiling in pipes under microgravity conditions, in order to determine the project conditions for tow-phase-cooled space equipment. As a rule, microgravity conditions produce an increase in bubble size, and this change in bubble geometry goes together with a deterioration in heat-exchange conditions. The influence of gravity on heat exchange lessens as coolant speed and the quantity of steam in the outflow channel increase. The analysis of the effect of gravity on bubble geometry square with the findings on heat exchange. The rebathing of walls at high temperature is strongly influenced by the level of gravity. Compared with gravity conditions on earth, speeds are up to four times les

Corso di Impianti Ospedalieri: Appunti dalle lezioni – Parte I

Scopo dell’Azienda Ospedale è offrire ricovero e cura della migliore qualità possibile al maggior numero possibile di persone, al minor costo possibile: tutte le soluzioni tecnologiche (e impiantistiche) devono di ciò tenere conto; inoltre, la tecnologia presente in ospedale impatta, in differente grado, sulla persona umana e ciò costituisce un vincolo (principalmente dal punto di vista della sicurezza) ben più stringente che in altre situazioni. Le macchine che si interfacciano con la persona umana fanno parte di due grandi categorie tecnologiche: la Strumentazione, per così dire al di qua del muro, e gli Impianti Tecnologici, al di là del muro. Gli Impianti Tecnologici sono presenti anche in altri sistemi, ma all’interno dell’ospedale possono trovarsi soluzioni specifiche per l’ospedale stesso. Gli impianti di cui si tratta in questa parte di corso sono gli impianti di produzione, trasporto ed utilizzazione dell’energia termica, dedicati al condizionamento e alla climatizzazione (cioè al mantenimento del benessere termoigrometrico e della qualità dell’aria)

Premio ESSO Miglior tesi di dottorato

This thesis presents results of a Ph.D. research in Energetics, carried out at the Department of Mechanics and Aeronautics of the University of Rome ”La Sapienza” and at the ”Istituto delle Applicazioni del Calcolo Mauro Picone” (National Research Council) of Rome. The main topic of the research has been focused on the analysis development and applications of a thermal model in the context of the kinetic schemes. In the last decade lattice kinetic theory, and most notably the Lattice Boltzmann Method (LBM), have met with significant success for the numerical simulation of a large variety of fluid flows, including real-world engineering applications. The Lattice Boltzmann Equation (LBE) is a minimal form of the Boltzmann kinetic equation, which is the evolution equation for a continuous one-body distribution function f(~x; ~v; t), wherein all details of molecular motion are removed except those that are strictly needed to represent the hydrodynamic behaviour at the macroscopic scale. The result is a very elegant and simple evolution equation for a discrete distribution function, or discrete population fi(~x; t) = f (~x; ~ci; t), which describes the probability to find a particle at lattice position ~x at time t, moving with speed ~ci. In a hydrodynamic simulation by using the LBE, one solves the only-two-steps evolution equations of the distribution functions of fictitious fluid particles: they move synchronously along rectilinear trajectories on a lattice space and then relax towards the local equilibrium because of the collisions. With respect to the more conventional numerical methods commonly used for the study of fluid flow situations, the kinetic nature of LBM introduces several advantages, including fully parallel algorithms and easy implementation of interfacial dynamics and complex boundaries, as in single and multi-phase flow i porous media. In addition, the convection operator is linear, no Poisson equation for the pressure must be resolved and the translation of the microscopic distribution function into the macroscopic quantities consists of simple arithmetic calculations. However, whereas LBE techniques shine for the simulation of isothermal, quasi incompressible flows in complex geometries, and LBM has been shown to be useful in applications involving interfacial dynamics and complex boundaries, the application to fluid flow coupled with non negligible heat transfer, turned out to be much more difficult. The LBE thermal models fall into three categories: the multispeed approach, the passive scalar approach and the doubled populations approach. The so-called multi-speed approach, which is a straightforward extension of the LBE isothermal models, makes theoretically possible to express both heat flux and temperature in terms of higher-order kinetic moments of the particle distribution functions fi(~x; t). It implies that higher-order velocity terms are involved in the formulation of equilibrium distribution and additional speeds are required by the corresponding lattices. The latter is arguably the major source of numerical instabilities of thermal lattice kinetic equations; in addition, it can seriously impair the implementation of the boundary conditions, a vital issue for the practical applications. The passive scalar and the doubled populations approaches are based on the idea of dispensing with the explicit representation of heat flux in terms of kinetic moments of the particle distribution function f(~x; ~v; t). A successful strategy consists of solving the temperature equation independently of LBE, possibly even with totally different numerical techniques. If the viscous heat dissipation and compression work done by the pressure are negligible, the temperature evolution equation is the same of a passive scalar and this approach enhances the numerical stability; the coupling to LBE is made by expressing the fluid pressure as the gradient of an external potential. Clearly, this strategy represents a drastic departure from a fully kinetic approach, and lacks some elegance. A more elegant possibility is to double the degrees of freedom and express thermal energy density and heat flux still as kinetic moments, but of a separate ’thermal’ distribution g(~x; ~v; t). Two sets of discrete distribution functions are used, dedicated to density and momentum fields, and temperature and heat flux fields, respectively. The advantage of this latter approach is that no kinetic moment beyond the first order is ever needed, since heat flux (third order vector moment of f) is simply expressed as the first order vector moment of g: as a result, disruptive instabilities conventionally attributed to the failure of reproducing higher-order moments in a discrete lattice are potentially avoided/mitigated. With respect to the previous approaches, the method is able to include viscous heating effects, and the boundary conditions are easily implemented because both f and g live in the same lattice, where additional speeds are not necessary. The price to pay is doubling of the storage requirements. As far as the thermal boundary conditions are concerned, LBE techniques usually handle the Dirichlet-type constraints; in contrast, the Neumann-type constraints are either limited to insulated walls or obtained imposing the temperature gradient at the wall through a strategy of transfer to a Dirichlet-type condition. For a wide class of real phenomena, the fixed temperature condition is clearly inadequate. Examples are represented by the cooling of devices, where the problem is characterized by an imposed thermal power to be removed, or by the air behavior in building rooms, where the temperature of the external walls is a direct consequence of the heat flux administered to the walls. In this framework, a General Purpose Thermal Boundary Condition (GPTBC) has been proposed, discussed and validated for an existing double population model. This thermal boundary condition is based on a counterslip approach as applied to the thermal energy. The incoming unknown thermal populations are assumed to be equilibrium distribution functions with a counterslip thermal energy density, which is determined so that suitable constraints are verified. The GPTBC proposed here can simulate explicitly either imposed wall temperature (Dirichlet-type constraint) or imposed wall heat fluxes (Neumann-type constraint), which allows LBM to be used for successful simulation of many types of heat transfer and fluid flows applications. Thus, the method can become an effective and alternative easy-to-apply tool, as well as the athermal LBE counterpart, especially for all those situations wherein the use of the usual theoretical approaches may fail, e.g., due to the complexity of the geometry. The validity of the developed GPTBC is demonstrated through its application to different flow configurations. With regard to channel flows, thermal Couette and Poiseuille flows has been simulated. The results obtained in case of Couette flows, show the model, together with the GPTBC, working over a wide range of physical parameters and allowing strong temperature gradients and heat dissipation effects to be detected. With regard to applications of the scheme to pressure gradient driven flows (Poiseuille flow), two different set-up are discussed. In LBE techniques, the most common set-up to simulate (nearly) incompressible flows consists of driving the flow with a constant force (i.e. a forcing term acting on the discrete populations), representing the constant pressure gradient, and applying periodic boundary conditions at inlet and outlet of the channel. In practical applications, one is often confronted with open flows, with prescribed inlet flow speed, and outlet pressure, or both prescribed inlet and outlet pressure values. In this case the common solution in LBE techniques, in which not pressure but only density values can be handled, is to force the flow by means of a density difference, between inlet and outlet sections, or by imposing velocity and density profiles. This strategy proves viable for athermal flows, so long as relative density changes (¢½=½) can be kept within a few percent, because the velocity profile maintains a parabolic behaviour. If heat transfer takes place, the temperature profile can change, in virtue of the nonuniform density along the channel; more specifically, one simulates the energy equation, taking in account the contribution of the term ¡p@xiui. In this case, the model has been shown to capture the expansion cooling effect, which gradually increases along the stream wise direction, and the opposing viscous heating effect. In order to come closer to the request of handling nearly incompressible flow and prescribed inlet/outlet boundary conditions, a different arrangement has been investigated. The idea is to impose boundary conditions in terms of inlet profile, with outlet variables left free to assume values coming from the run, still using a suitable amount of forcing. This hybrid formulation provides results in excellent agreement with theoretical solutions, for velocity, temperature and heat flux fields, as well as for Nusselt number behaviour, for a hydrodynamically fully developed flow; it also captures the effect of the coexistence of both a hydrodynamically and thermally developing flow, in the near inlet region, with an entry-length region depending on Prandtl number. With regard to applications to flows in enclosed spaces, the scheme has been used to simulate different cases of natural convection flow, which today represents an active subfield in heat transfer research. This great interest is due to the several fields in which natural convection is involved and to its importance in many engineering applications, e.g. heat transfer in buildings, solar energy collection, heat removal in micro electronics, cooling of nuclear reactors, dispersion of fire fumes in buildings and tunnels, ventilation of rooms. Compared with this great applicative interest, natural convection research is characterized by several theoretical and practical issues. The buoyancy-induced heat and momentum transfer in enclosures, also in simple geometries, strongly depends on geometric and physical conditions. Several regimes and complex phenomena of successive transitions can take place. Standard simulation techniques CFD cannot predict the behaviour of natural convection systems with high geometric complexity, or where viscous heating effects and/or non trivial conditions, related to the rheological law, are non negligible. As said, alternative approaches can be useful and required. Two flow configurations has been investigated for a wide range of Rayleigh number. Firstly, laminar flows in a square cavity, with vertical walls differently heated, have been discussed and results have been found in excellent agreement as compared with benchmark solutions, for both motion and heat transfer aspects. Then, laminar flows in a square cavity, with vertical walls heated and cooled by means of a constant uniform heat flux, which is a flow configuration never investigated by means of lattice Boltzmann methods, have been simulated; results have been found in excellent agreement as compared with those of previous works, obtained from a theoretical analysis. The study shows that the double population model provides reliable results over a wide range of physical parameters and in different situation of engineering interest. The new GPTBC provides good results for both imposed wall temperature and imposed wall heat fluxes conditions, beyond the adiabatic condition of previous schemes. These significant improvements, in the context of the kinetic schemes, can be added to the advantages specific to these methods, and primarily to Lattice Boltzmann Models, which make them competitive tools, with respect to the usual theoretical approaches and to the standard numerical techniques, for the simulation of complex hydrodynamic phenomena, from fully developed turbulence to phase transitions to granular flows. The thermal lattice Boltzmann method can become an effective and alternative tool, as well as the athermal counterpart, for successful simulation of many types of heat transfer and fluid flow processes, especially for all situations where complex phenomena take place