Giulio Lorenzini

Evolution in the Design of V-Shaped Highly Conductive Pathways Embedded in a Heat-Generating Piece

This paper presents the evolution of architecture of high conductivity pathways embedded into a heat generating body on the basis of Constructal theory. The main objective is to introduce new geometries for the highly conductive pathways, precisely configurations shaped as V. Four types of V-shaped inserts, evolving from “V1” to “V4,” have been comparatively considered. Geometric optimization of design is conducted to minimize the peak temperature of the heat generating piece. Many ideas emerged from this work: first of all, the numerical results demonstrated that the V-shaped pathways remarkably surpass the performance of some basic configurations already mentioned in literature, i.e., “I and X-shaped” pathways. Furthermore, the evolution of configurations from V1 to V4 resulted in a gradual reduction of the hot spot temperature, according to the principle of “optimal distribution of imperfections” that characterizes the constructal law.

Optimization of Pin-Fins for a Heat Exchanger by Entropy Generation Minimization and Constructal Law

Pin-fins are considered as one of the best elements for heat transfer enhancement in heat exchangers. In this study, the topology of pin-fins (length, diameter, and shape) is optimized based on the entropy generation minimization (EGM) theory coupled with the constructal law (CL). Such pin-fins are employed in a heat exchanger in a sensible thermal energy storage (TES) system so as to enhance the rate of heat transfer. First, the EGM method is used to obtain the optimal length of pin-fins, and then the CL is applied to get the optimal diameter and shape of pin-fins. Reliable computational fluid dynamics (CFD) simulations of various constructal pin-fin models are performed, and detailed flow and heat transfer characteristics are presented. The results show that by using the proposed system with optimized pin-fin heat exchanger the stored thermal energy can be increased by 10.2%.

Constructal Design of Complex Assembly of Fins

Constructal design is a method that conducts the designer toward flow (e.g., heat flux) architectures that have greater global performance. This numerical work uses this method to seek for the best geometry of a complex assembly of fins, i.e., an assembly where there is a cavity between the two branches of the T-Y-assembly of fins and two additional extended surfaces. The global thermal resistance of the assembly is minimized four times by geometric optimization subject to the following constraints: the total volume, the volume of fin material, the volume of the cavity, and the volume of the two additional extended surfaces. Larger amount of fin material improves the performance of the assembly of fins. The three times optimized global thermal resistance of the complex assembly of fins performs 32% better than the best T-Y-configuration under the same thermal and geometric conditions. The three times minimized global thermal resistance of the complex assembly of fins was correlated by power laws as a function of its corresponding optimal configurations.

Constructal Design of Convective Y-Shaped Cavities by Means of Genetic Algorithm

In the present work constructal design is employed to optimize the geometry of a convective, Y-shaped cavity that intrudes into a solid conducting wall. The main purpose is to investigate the influence of the dimensionless heat transfer parameter a over the optimal geometries of the cavity, i.e., the ones that minimize the maximum excess of temperature (or reduce the thermal resistance of the solid domain). The search for the best geometry has been performed with the help of a genetic algorithm (GA). For square solids (H/L = 1.0) the results obtained with an exhaustive search (which is based on solution of all possible geometries) were adopted to validate the GA method, while for H/L ≠ 1.0 GA is used to find the best geometry for all degrees of freedom investigated here: H/L, t1/t0, L1/L0, and α (four times optimized). The results demonstrate that there is no universal optimal shape that minimizes the thermal field for all values of a investigated. Moreover, the temperature distribution along the solid domain becomes more homogeneous with an increase of a, until a limit where the configuration of “optimal distribution of imperfections” is achieved and the shape tends to remain fixed. Finally, it has been highlighted that the GA method proved to be very effective in the search for the best shapes with the number of required simulations much lower (8 times for the most difficult situation) than that necessary for exhaustive search.

A Bejan’s Constructal Theory Approach to the Overall Optimization of Heat Exchanging Finned Modules With Air in Forced Convection and Laminar Flow Condition

Optimizing ever smaller heat exchangers determines two opposite needs: augmenting performances, on the one hand; removing heat in excess to reduce failures, on the other. This numerical study, modeled thanks to Bejans Constructal theory, researches the overall optimization of finned modules, differently shaped and combined, cooled by air in laminar flow and forced convection condition: Losses of pressure, together with heat removed, contribute to the final assessment made through a novel idea of performance based on the so called overall performance coefficient.

A CFD application to optimize T-shaped fins : Comparisons to the constructal theory's results

The problem of heat removal in energetic processes represents a hi , < challenge especially because of the enhanced requirements of the modern industry. The thermal exchange systems therefore have to guarantee better performances in correspondence to ever more severe dimensional constraints. This paper shows a numerical approach, based on computational fluid dynamics (CFD) software, for the evaluation of the heat exchange performances of finned (straight fins) surfaces made of highly heat conductive material. The same geometric constraints assumed in a reference work were adopted. This research attempts to develop an easy-to-use method to face what was previously solved by the powerful approach of Bejan Constructal theory. The results obtained show a good agreement between CFD and the Constructal theorys results, validating, therefore, the simplified approach proposed and encouraging its application to a broader variety of geometries.

CFD analysis of pulsatile blood flow in an atherosclerotic human artery with eccentric plaques

Atherosclerosis is a slow vascular degeneration. It thickens the internal walls of a blood vessel locally depositing an atherosclerotic plaque. Such reduced lumen increases the resistance to blood flow. Plaques can be punctual (eccentric, here considered) or circumferential (symmetrical). Stenoses do not have a typical shape: we hypothesised here a reference geometry (trapezium) with its possible evolutions (semi-ellipse, triangle). Two criteria (Equivalent Area and Equivalent Dimensions) were then defined to compare the results among the 35 case studies numerically analysed with a Computational Fluid Dynamics code (Comsol Multiphysics 3.3). Blood was considered a Cassonian fluid with modified viscosity equation. The artery was cylindrical, rigid and straight, interested by a pulsatile blood flow. Among the variables: shape and dimensions of the stenoses; number of stenoses (single or coupled pathologies); mutual locations (3 possibilities). The main results were that the length of the consequent flow disturbance is due to the stenotic shape and height; blood flow recirculation, downstream of the pathology, is due to the slope of the stenotic walls; and the peak velocities depend on the shape and height of stenosis. The differences from case to case diminish in diastole.

Numerical Prediction of Flow Structure and Heat Transfer in Square Channels with Dimples Combined with Secondary Half-Size Dimples/Protrusions

The present study employs square cross-section dimpled channels with different arrangements of upstream secondary half-size dimples or protrusions to determine the optimal configurations for augmenting heat transfer rates with minimized pressure drop penalties. Five dimpled channels with and without upstream secondary dimples or protrusions are investigated (simple dimpled channel [case A]; dimpled channels with secondary dimples upstream each dimple [cases B1 and B2, respectively]; and dimpled channels with secondary protrusions upstream each dimple [cases C1 and C2, respectively]). All turbulent fluid flow and surface heat transfer results are obtained using computation fluid dynamics with a k-ϵ RNG turbulence model. Numerical results are qualified using grid-independent predictions of experimental data for one baseline dimple array arrangement. The channel inlet Reynolds number ranges from 8,000 to 24,000. From this study, secondary protrusions can bring forward flow separations and reduce the scope of recirculating flows in adjacent primary dimples and then greatly improve averaged local heat transfer of primary dimple surface. The result does not apply to secondary dimples which hinder flow reattachment in primary dimples and go against heat transfer enhancement. For averaged heat transfer on all the middle heated surfaces, heat transfer enhancement by secondary protrusions is not evident especially at high Reynolds numbers and the uniformity of roughness arrangements as dimples and protrusions makes a dominant role in the averaged heat transfer efficiency, while the dimple structure exhibits heat transfer advantage over protrusions at high Reynolds numbers. For the studied cases, case C1 obtains the best overall thermal performance at low Reynolds numbers, while case B2 is the best one at high Reynolds numbers. It is also recommend that case A can be effectively designed to exhibit the relatively good overall thermal performance with minimizing the blade weight and stress.

Numerical Heat Transfer Optimization in Modular Systems of Y-Shaped Fins

This paper analyses the heat exchange behavior in systems characterized by Y-shaped fins through a numerical approach based on a CFD software. Starting from individual Y profiles, as optimized in a previous work in relation to the dimensionless conductance and to the performance parameter of efficiency, it has been here investigated the advantage of a modular use of profiles. The analysis has been performed by superimposing some dimensional constraints to make immediately comparable the results obtained in the different configurations faced. Each module considered has a number of fins depending on the angle a between the two arms of the Y profile. This number depends therefore also on the horizontal width occupied by the whole system and it is upperly limited by the value allocated to the best performing individual fin. The results showed a significant increase of the dimensionless conductance and therefore of the exchanged thermal power for those multifin configurations with low values of a. This result validates the new optimization criterion proposed.

Computational fluid-dynamics-based analysis of a ball valve performance in the presence of cavitation

In this paper, the ball valve performance is numerically simulated using an unstructured CFD (Computational Fluid Dynamics) code based on the finite volume method. Navier-Stokes equations in addition to a transport equation for the vapor volume fraction were coupled in the RANS solver. Separation is modeled very well with a modification of turbulent viscosity. The results of CFD calculations of flow through a ball valve, based on the concept of experimental data, are described and analyzed. Comparison of the flow pattern at several opening angles is investigated. Pressure drop behind the ball valve and formation of the vortex flow downstream the valve section are also discussed. As the opening of the valve decreases, the vortices grow and cause higher pressure drop. In other words, more energy is lost due to these growing vortices. In general, the valve opening plays very important roles in the performance of a ball valve.