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The present work studies the enhancement of heat transfer on a cylinder impinged by a rectangular jet in presence of a metallic grid on the slot exit. The experiments are carried out to cool a smooth cylinder, electrically heated, with a rectangular jet of air, at three Reynolds numbers in turbulent flow, i.e. Re = 5100, 10,600 and 15,300, where the Reynolds number is defined with the cylinder diameter. Three metallic grids, identified by the French classification as NF06-10-20, are employed with the solidity ratio ranging from 24.3% to 67.7%, and the same diameter of the mesh filament (0.6 mm). Fluid dynamic measurements, performed with a hot film anemometer, are done to justify the thermal results. Heat transfer measurements are presented as local and mean Nusselt numbers. The comparison with the case without grid, indicated as NF00, is done at the same mass flow rate of the jet and the same power consumption of the wind tunnel to move the jet. The main goal of the paper is to evaluate how the solidity ratio of the metallic grids influences the heat transfer. [ABSTRACT FROM AUTHOR]

Abstract: A new equation for the dissipation rate of turbulent kinetic energy is derived exactly in conservative form for a Generalized Newtonian Fluid (GNF). The transport equations for mass, momentum, and turbulent kinetic energy are written along to the transport equation for the shear rate. A new transport equation for the apparent viscosity is derived assuming the viscosity as dependent only on the shear rate. The assumption is of incompressible two-dimensional GNF flow. [Copyright &y& Elsevier]

The present work investigates mass conservation equations in turbulent flow between parallel plates with variable mass diffusivity. Species conservation equations are relative to the average concentration, as well as to the concentration variance. The product of fluctuating mass diffusivity and space gradient of concentration fluctuation is appearing in the equation of mean and concentration variance. A physical interpretation is given to the different terms. The assumption of a relation between mass diffusivity and concentration allows writing expressions for average and fluctuating mass diffusivity, which can be simplified on the basis of theoretical considerations. The new mass flux is expressed as a function of mass diffusivity and a gradient of concentration variance. Further considerations make it possible to model the new terms appearing in the concentration variance equation. The mass conservation equation can be solved when coupled to the equation of concentration variance. The equations are solved numerically for flow between parallel plates in order to evaluate the influence of the new terms. [ABSTRACT FROM AUTHOR]

Experimental results for the Sherwood number variation near the lateral edge of the active surface of a smooth, finite-width flat plate in turbulent boundary layer flow are presented. Using naphthalene sublimation, local mass transfer rates are found for two different free stream velocities. A semi-empirical correlation of the experimental data is presented, allowing calculation of the increase of mass transfer near the edge and the size of the region affected by the lateral edge. The effect is shown to scale more so with the diffusion thickness than the boundary layer thickness. [ABSTRACT FROM AUTHOR]

The present work investigates the average heat transfer on a finned cylinder cooled by a rectangular jet of height H . The diameter of the external cylinder, without fins, is D and the cylinder to slot ratio D / H is equal to 1. Numerical simulations are carried on with two turbulent models, RNG k – ɛ and SST k – ω . The jet flow can be characterized by the Reynolds number, defined with the diameter of the external cylinder without fins, D , or with the hydraulic diameter of the rectangular slot, D hydr ≈ 2 H . Two turbulent flows are investigated at Re D = 7100, or Re D hydr = 13,200, and at Re D = 19,700, or Re D hydr = 36,700. The main goal of the paper is to evaluate the optimal spacing, s , between the fins, and the optimal slot-to-cylinder distance, S / H , which maximize the average heat transfer on the finned cylinder. The optimization process is performed for a pitch of 3 mm and at a given value of the fin volume. The numerical simulations show that an optimal configuration is obtained when the ratio s / l of the spacing, s , to fin height, l , is maximum, and the cylinder is set at the slot-to-cylinder distance, S / H = 3. [ABSTRACT FROM AUTHOR]

The passive scalar spreading of fluids with laminar Prandtl or Schmidt number, Pr , Sc , equal to 1 in turbulent rectangular submerged free jets is analyzed by means of numerical simulation and theoretical analysis in the Reynolds number range 5000–40,000. The numerical investigation is carried out by means of a three-dimensional (3D) Large Eddy Simulation (LES) approach with the dynamic Smagorinsky model. A new mathematical model allows to obtain a simplified description of the passive scalar spreading in the largest area of the flow field, the Fully Developed Region (FDR). The present three-dimensional (3D) investigation shows that the passive scalar spreading follows a self-similarity law in the Fully Developed Region (FDR), as well as in the mean Undisturbed Region of Flow (URF) and in the Potential Core Region (PCR), similarly to what found in the Near Field Region (NFR) of rectangular submerged free jets, investigated with a two-dimensional (2D) approach. The turbulent Prandtl or Schmidt number is evaluated numerically and is found to be inversely proportional to the mean velocity gradient in the PCR. The present 3D numerical results show that the turbulent Prandtl or Schmidt number is zero in most part of the mean URF, and PCR, while it assumes different values outside. In the FDR the turbulent Prandtl or Schmidt number is constant and approximately equal to 0.7, in agreement with the literature, showing that turbulence affects momentum and passive scalar in a different way. [ABSTRACT FROM AUTHOR]

The present paper reports new numerical results of the mean mass transfer in a turbulent submerged round jet. A series of Large Eddy Simulations (LES) are presented at four Reynolds (Re) numbers (Re = 2492, Re = 4491, Re = 9994, Re = 19,988) and laminar Schmidt (Sc) number equal to Sc = 10. The numerical results are specially focused on their patterns in the Undisturbed Region of Flow (URF), Potential Core Region (PCR) and Fully Developed Region (FDR). The present results are compared with the previous literature concerning a two-dimensional and a three-dimensional jet, issuing from an infinitely wide slot, with the conclusion that the turbulence moments decay faster in a round jet configuration. [ABSTRACT FROM AUTHOR]

The paper investigates the mean mass transfer/passive scalar spreading in turbulent submerged round jets. Two regions of flow are present in the jet evolution: the Near-Field Region (NFR) and the Fully Developed Region (FDR). This group of research investigates from some years the mean evolution of turbulent rectangular jets with the new physical finding that two sub-regions (not a single one) are present in the NFR. The first region of the two is the newly discovered Undisturbed Region of Flow (URF), while the second one is the known Potential Core Region (PCR). In a recent paper we showed that the flow evolution of turbulent round jets, as far as momentum spreading is concerned, is self-similar also in the NFR. Literature shows that mass transfer spreading is self-similar only in FDR. The present paper presents new mean mass transfer results of the numerical Large Eddy Simulation (LES) in turbulent round jets. Four Reynolds numbers, from 2492 to 19,988, and two laminar Schmidt numbers, 1 and 10, are investigated. The first novel result of this paper is that mass transfer is self-similar in the NFR. The second result is that two new analytical models describe the passive scalar spreading in the URF and PCR. The third result is that two new self-similar laws describe the passive scalar spreading in the FDR. The fourth result states that the well-known power-law relationship, between passive scalar and axial momentum in the FDR, holds regardless of the modeling of turbulent viscosity and turbulent Schmidt number. [ABSTRACT FROM AUTHOR]

• Self-similarity of momentum in the near-field region of turbulent round jets. • One self-similar solution for mean velocity in undisturbed region flow, URF. • Two self-similar solutions for mean velocity in potential core region PCR. • Self-similar solutions fit LES results at all Reynolds numbers investigated. • NFR of turbulent round jets is self-similar if split into two URF and PCR. Self-Similarity in turbulent round jets has been the object of investigation from several decades. The evolution of turbulent submerged jets is characterized by the presence of two regions: the region of flow establishment, or near field region (NFR) and the fully developed region (FDR), or far-field region (FFR). The momentum spreading in the FDR is known to be self-similar and few mathematical models have been presented in the past to describe it. The flow evolution in the NFR has been rarely studied since there is a certain consensus on the idea that the flow in the NFR is not self-similar. In this work, we study the flow evolution of a turbulent submerged round jet by means of large eddy simulation (LES) at several Reynolds numbers ranging from 2492 to 19,988. Three new self-similar laws are proposed to describe the flow evolution in the NFR, one for the initial region, called Undisturbed Region of Flow, (URF), and two for the final region, the potential core region (PCR). The numerical results presented in this work are also validated with the self-similar laws for the FDR proposed by Tollmien (1926) and Görtler (1942), and the experimental data of Hussein et al. (1994), and Panchapakesan and Lumley (1993), in the FDR; those of Davies et al. (1963), in the PCR; and van Hout et al. (2018), in the URF. The conclusion is that previous inability to find the self-similarity law in the NFR is due to the attempt to find a unique self-similar variable to describe the momentum spreading in both the URF and the PCR. [ABSTRACT FROM AUTHOR]