Direct numerical simulation of turbulent forced convection in a wavy channel at low and order one Prandtl number

Turbulent forced convection in a channel with one planar wall and one wall of sinusoidal shape is investigated by Direct Numerical Simulation. The flow is fully developed and the Reynolds number based on the mean bulk velocity and the average hydraulic diameter is Re ≈ 18,900; in this weakly turbulent flow regime three different Prandtl number values are investigated, Pr = 0.025, 0.20, 0.71. The fluid is in contact with the colder channel walls at an equal, uniform temperature. The main statistical quantities, like the root-mean-square of temperature fluctuations and the turbulent heat fluxes, the local heat transfer coefficient and turbulent Prandtl number values are reported. Effects of flow separation and reattachment on the local heat transfer rate and turbulent Prandtl number distribution are also presented and discussed. An a priori analysis of the behavior of the simple gradient diffusion model of turbulent heat fluxes is performed in the low Prandtl number, separated flow conditions of the present work. While the low Prandtl number effect can be accounted for by an appropriate selection of the turbulent Prandtl number value to be provided to the model, deviations form the expected behavior of turbulent heat fluxes are seen to occur in the flow separation region and downstream reattachment

Thermal management of a Formula E electric motor: Analysis and optimization

The thermal analysis of a high performance brushless synchronous electric motor with permanent magnets and water jacket cooling is presented. The analysis is carried out following a lumped parameter thermal network approach which allows to identify the most important thermal paths in the motor and the main parameters influencing them. Thanks to its simplicity, the solution of such a thermal network model is very fast, allowing a large number of what-if scenarios to be computed over a short amount of time. For this reason, the model is coupled with external tools for performing systematic sensitivity analyses and optimizations. Goal of the investigation is the reduction of the windings temperature being this temperature inversely proportional to the efficiency and the power delivered by the motor. The sensitivity analysis, performed over a series of material, geometric, and operational factors, leads to the identification of the most relevant parameters influencing the thermal behaviour of the motor. A series of optimizations, focusing on these parameters and including suitable constraints granting the well-posedness of the problem and the feasibility of the solution, bring to the definition of an optimum layout of the water jacket and of the stator geometries. The optimized geometry allows a significant reduction of the windings temperature to be achieved

A fast algorithm for Direct Numerical Simulation of natural convection flows in arbitrarily-shaped periodic domains

A parallel algorithm is presented for the Direct Numerical Simulation of buoyancy-induced flows in open or partially confined periodic domains, containing immersed cylindrical bodies of arbitrary cross-section. The governing equations are discretized by means of the Finite Volume method on Cartesian grids. A semi-implicit scheme is employed for the diffusive terms, which are treated implicitly on the periodic plane and explicitly along the homogeneous direction, while all convective terms are explicit, via the second-order Adams-Bashfort scheme. The contemporary solution of velocity and pressure fields is achieved by means of a projection method. The numerical resolution of the set of linear equations resulting from discretization is carried out by means of efficient and highly parallel direct solvers. Verification and validation of the numerical procedure is reported in the paper, for the case of flow around an array of heated cylindrical rods arranged in a square lattice. Grid independence is assessed in laminar flow conditions, and DNS results in turbulent conditions are presented for two different grids and compared to available literature data, thus confirming the favorable qualities of the method

Comparison between cooling strategies for power electronic devices: fractal mini-channels and arrays of impinging submerged jets

Power electronic devices like Insulated Gate Bipolar Transistors (IGBTs) and diodes are often characterized by power densities and dimensions that could result in very high heat flux densities. In order to guarantee the expected performance and lifetime for these components, dedicated active cooling devices are usually adopted. In the present paper, the comparison between two different cooling strategies for power electronics is presented: fractal channel design and submerged impinging jets. Each cooling strategy is tested on two different geometrical configurations. Water is used as coolant in all cases. Assessment of the considered cooling methods is done through application of the selected configurations in a simplified system composed by a rectangular chip (heat source) separated from the coolant by a solid block. Three-dimensional conjugated heat transfer simulations are performed by using RANS solver implemented in OpenFOAM and two-equations turbulence models, resolving also the viscous sublayer. Numerical results allow to compare the cooling strategies in terms of maximum chip temperature, overall chip-to-coolant thermal resistance, and pumping power required. In summary, the fractal-channel design shows limitations in guaranteeing low chip temperatures at an affordable pumping power. The submerged impinging jets approach shows very high local heat transfer coefficient by which it is possible to tailor the cooling expect on specific hot spots

Experimental assessment and predictive model of the performance of Ti-based nanofluids

The need for innovative propulsion technologies (e.g., fuel cells) in the mobility sector is posing a higher-than-ever burden on thermal management. When low operative temperature shall be ensured, dissipation of a significant amount of heat is requested, together with limited temperature variation of the coolant; mobile applications also yield limitations in terms of space available for cooling subsystems. Nanofluids have recently become one of the most promising solutions to replace conventional coolants. However, the prediction of their effectiveness in terms of heat-transfer enhancement and required pumping power still appears a challenge, being limited by the lack of a general methodology that assesses them simultaneously in various flow regimes. To this end, an experiment was developed to compare a conventional coolant (ethylene glycol/water) and a TiO2-based nanofluid (1% particle loading), focusing on heat transfer and pressure loss. The experimental dataset was used as an input for a physical model based on two independent figures of merit, aiming at an a priori evaluation of the potential simultaneous gain in heat transfer and parasitic power. The model showed conditions of combined gain specifically for the laminar flow regime, whereas turbulent flows proved inherently associated to higher pumping power; overall, criteria are presented to evaluate nanofluid performance as compared to that of conventional coolants. The model is generally applicable to the design of cooling systems and emphasizes laminar flow regime as promising in conjunction with the use of nanofluids, proposing indices for a quantitative a priori evaluation and leading to an advancement with respect to an a posteriori assessment of their performance

Compact finite volume schemes on boundary-fitted grids

The paper focuses on the development of a framework for high-order compact finite volume discretization of the three dimensional scalar advection–diffusion equation. In order to deal with irregular domains, a coordinate transformation is applied between a curvilinear, non-orthogonal grid in the physical space and the computational space. Advective fluxes are computed by the fifth-order upwind scheme introduced by Pirozzoli [S. Pirozzoli, Conservative hybrid compact-WENO schemes for shock turbulence interaction, J. Comp. Phys. 178 (2002) 81] while the Coupled Derivative scheme [M.H. Kobayashi, On a class of Pade´ finite volume methods, J. Comp. Phys. 156 (1999) 137] is used for the discretization of the diffusive fluxes. Numerical tests include unsteady diffusion over a distorted grid, linear free-surface gravity waves in a irregular domain and the advection of a scalar field. The proposed methodology attains high-order formal accuracy and shows very favorable resolution characteristics for the simulation of problems with a wide range of length scales

Direct numerical simulation of low-Prandtl number turbulent convection above a wavy wall

Turbulent forced convection is investigated by Direct Numerical Simulation in a channel with one sinusoidal wavy wall and one flat wall. Fluid flow and heat transfer are periodically fully developed, the simulated Reynolds number of the bulk velocity and the hydraulic diameter is Re = 18, 880 while three Prandtl numbers are considered, i.e. Pr = 0.025, Pr = 0.2, and Pr = 0.71. The fluid flow is characterized by separation, reattachment and a shear layer downstream the wave peak, these are conditions relevant for turbulent heat transfer and passive scalar transport applications. In the range of Péclet numbers investigated, the most important heat transfer mechanism is by mean flow advection. Accordingly, the peak heat transfer region is in the upslope part of the domain. The separation bubble instead acts as a barrier to convection and the heat transfer rate is minimum close to separation. An a priori analysis is performed in order to assess the accuracy of turbulent heat transfer models based on the Generalized Gradient Diffusion Hypothesis

Direct Numerical Simulation of Heat Transfer Over Riblets

Riblets are well-known as a passive mean for drag reduction in turbulent flow conditions, but their effectiveness for heat transfer is quite controversial. In this paper we present the numerical results for fully developed laminar and turbulent flow and heat transfer in a channel with triangular riblets. The turbulent study is performed by means of direct numerical simulation at a Reynolds number Re_\tau =180 based on the wall-shear velocity, for a fluid with a Prandtl number Pr=0.71. Four different ribbed channels are considered, under a constant heat flux boundary condition, and correspond to ridge angle a \alpha = 45 and 60 degrees, and riblet spacing s^+ = 20 and s^+ = 40. The results obtained, for the flow and turbulent quantities, are in good agreement with past experimental and numerical studies, and correctly reproduce drag reduction over the smaller s^+ = 20 riblets and drag increase over the larger s^+ = 40 riblets. The predicted heat transfer efficiency of riblets do not agree with some experimental results, and is below that of a flat plate for all the configurations. The conditions for heat transfer enhancement are discussed

Experimental Investigation on Three-Phase Oil/Water/Air Flow

none2G. Sotgia; E.StalioSotgia, GIORGIO MARCO; E., Stali