177,033 research outputs found
Transient energy growth modulation by temperature dependent transport properties in a stratified plane Poiseuille flow
No full textWe investigate the effect of temperature dependent thermal conductivity λ and isobaric specific heat c_P on the transient amplification of perturbations in a thermally stratified laminar plane Poiseuille flow. It is shown that for decreasing thermal conductivity the maximum transient energy growth is amplified with respect to the λ=1 case, while the opposite occurs for increasing λ. A reversed mechanism is induced by a variable c_p. Substantial maximum growth enhancement/suppression is found in the range of Prandtl numbers Pr which encompasses most fluids of practical interest. The relative growth modulation shows an optimum Pr under spanwise perturbations. For energy amplifying property distributions a speed-up of the transient to reach the maximum energy growth is observed at low Pr, while a slow-down is found at large Pr. The opposite is true when the property variations suppress the growth of perturbations
Effect of viscosity and density gradients on turbulent channel flows
No full textWe perform Direct Numerical Simulations (DNS) of a turbulent channel flow with temperature dependent density and viscosity. The Navier-Stokes equations are solved using their low Mach number formulation. In the simulations performed, the fluid is internally heated and the temperature at the walls is fixed. The friction Reynolds number based on half channel height and wall friction velocity is Reτ = 395. The modulation of turbulence, which is caused by the density and viscosity gradients, is characterized using the semi-local scaling of Huang et al. [1995, JFM]
Design and Optimization of Volumetric Solar Receivers based on Nanoparticles with Supercritical Carbon Dioxide
No full textEnergy TechnologyProcess and EnergyMechanical, Maritime and Materials Engineerin
Physical modeling of cavitation using an enthalpy based model
No full textCavitation is defined as the process of formation and disappearance of a vapor phase in a liquid when this liquid is subjected to reduced pressures, followed by an increase of pressure. One of the main challenges in the design and application of centrifugal pumps is the ability to control and limit the development of cavitation. It is generally unlikely that a pump will operate across its entire operating range without any cavitation. Computational Fluid Dynamics (CFD) is used extensively to model cavitation in pump impellers. These models are almost always governed by empirical relations, which is no problem for cold water that has ample test data to be validated with, but makes the prediction of cavitation for hydrocarbons or amine solutions impossible. In the present work the cavitation development of water, butane and propane is described using a barotropic model assuming an isenthalpic expansion in the two-phase region. This barotropic relation should only be governed by the fluid properties, no empiricism should be involved. In order to validate the model, it is implemented in both 1-dimensional and 2-dimensional situations. In the 1-dimensional situation the Euler equations are solved for a single dimension in combination with source terms that model the varying area distribution of a Venturi-like nozzle. By forcing all three liquids through the nozzle at different velocities and pressures, insight is gained into the general qualitative performance of the model, both physical and numerical. In order to validate the model quantitatively with test data, the model is implemented into a 2-dimensional situation. A circular rod with a hemispherical head is pointed into the flow to obtain the pressure distribution over the head and part of the rod. This pressure distribution is then compared with data from experiments performed by Rouse \& McNown, in order to perform a simple quantitative validation of the model. Based on the work done in this thesis it is concluded that an isenthalpic barotropic model is a suitable approach to describe cavitation. Due to the independence from empirical relations, the development of cavitation for three different fluids can be modeled, based solely on fluid properties taken from a thermodynamic library. The general results from the 2D implementation are encouraging, but require more work to remedy the numerical instabilities and extremely slow convergence of the solution. The general recommendation for future work is to further develop and improve the numerics to make the solver more efficient and stable, thus viable for bigger simulations.Energy TechnologySustainable Process and Energy TechnologyMechanical, Maritime and Materials Engineerin
Numerical simulation of dense gas turbulent flows
No full textDense gas turbulent flows, of great interest for a wide range of engineering applications, exhibit physical phenomena that are still poorly understood and difficult to reproduce experimentally. In this work, we study for the first time the influence of dense gas effects on the structure of compressible turbulence by means of numerical simulations. The fluid considered is PP11, a heavy fluorocarbon, whose thermodynamic behavior is described by means of different equations of state to quantify the sensitivity of solutions to modelling choices.
First, we considered the decay of compressible homogeneous isotropic turbulence. Temperature fluctuations are found to be negligible, whereas those of the speed of sound are large because of the strong dependence on density. The peculiar behavior of the speed of sound significantly modifies the structure of the turbulence, leading to the occurrence of expansion shocklets. The analysis of the contribution of the different structures to energy dissipation and enstrophy generation shows that, for a dense gas, high expansion regions play a role similar to high compression ones, unlike perfect gases, in which the observed behaviour is highly asymmetric. Then, we carried out numerical simulations of a supersonic turbulent channel flow for several values of Mach and Reynolds numbers. The results confirm the validity of the Morkovin' hypothesis. The introduction of a semi-local scaling, taking into account density and viscosity variations across the channel, allow to compare the wall-normal profiles of turbulent quantities (Reynolds stresses, anisotropy, energy budgets) with those observed in ideal gases. Nevertheless, the thermodynamic variables exhibit a different evolution between
perfect and dense gases, since the high specific heats of the latter lead to a decoupling of dynamic and thermal effects, and to a behavior close to that of variable property incompressible fluids
Turbulence Modeling of Wall Jets using the Algebraic Structure Based Model
No full textReport PRE-2628. The Algebraic Structure Based Model (ASBM) of Langer and Reynolds (2003) provides an innovative approach for modeling the turbulent stresses, while incorporating information on the structure of turbulence and providing closure for Reynolds-averaged Navier-Stokes equations. The normal turbulent stresses, for which the ASBM has shown superior results, are difficult to replicate using the conventional Boussinesq hypothesis that forms the backbone of common eddy-viscosity models. The results for mean velocities and diagonal turbulent stresses, and the computational cost are kept at an acceptable level to allow the model to compete effectively with common eddy-viscosity models. In this work, the ASBM has been applied to two new validation cases; the plain wall jet of Eriksson et al. (1998) and the slot impinging jet of Zhe and Modi (2001). Encouraging results are obtained for the normal turbulent stresses, while the mean velocities and turbulent shear stress are comparable to the v2f eddy-viscosity model of Lien and Durbin (1996). Also drawbacks of the ASBM are pointed out that emerge from the fact that the model is algebraic and hence relies only on local flow properties.Energy TechnologyProcess and EnergyMechanical, Maritime and Materials Engineerin
Supercritical CO2 Power Cycle for Solar Applications: Thermodynamic Analysis and 1D Turbomachinery Design
No full textSupercritical CO2 (s-CO2) Brayton cycle is indicated as one of the possible substitutes of the traditional steam cycle, that is currently used to produce the biggest share of electricity. Specifically, one of main candidates is the Brayton recompression cycle, that has been investigated for a wide range of operating conditions. This work is originated from the need to develop a tool that integrates the cycle design to a more detailed turbomachinery design. The results of the cycle design confirm its high thermodynamic efficiency and allow to calculate the mass flow needed to match the desired power output. Then, a preliminary and mean line design integrated code is developed for compressors and expanders. The preliminary design is based on the Baljé diagrams and it allows to estimate the number of stages and the diameter recommended to achieve the optimum efficiency. The mean line design provides a more advanced sizing and performance estimation, that is based on traditional relations as well as on the most recently improved correlations available in literature for supercritical fluids. The models are validated with the results of designs found in literature and with the few experimental data available. The methodology developed is applied to a case study: the Brayton recompression cycle is compared to the Rankine cycle in the solar thermal power plant PS10, a 11 MW solar tower located near Seville, Spain. The tool developed is accurate in the prediction of the cycle operating conditions and it provides a robust design tool for the turbomachinery equipment.Mechanical, Maritime and Materials EngineeringProcess and EnergySustainable process and energy technology (SPET
Numerical Modelling of Non-Equilibrium Condensing Steam Flows
No full textTwo-phase condensing flows are very common in many technical applications, such as rotating machinery operating with steam and nuclear reactors. The occurrence of condensation can lead to a degradation of a component's performance. Thus, the physical understanding and accurate numerical modeling of the condensation process can be of great help in the design process. The present work is focused on the numerical modeling of non-equilibrium condensing steam flows in 1-D Laval nozzles. The fluid dynamic equations for an inviscid and adiabatic flow (Euler equations) are solved using a quasi-1-D finite volume code, which accounts for the nozzle area variation. The model for homogeneous nucleation and the droplet growth rate in high-speed supersonic nozzle flow and also applicable to the wet stages of a steam turbine, is implemented in the present work. In order to assess the accuracy of the condensation model implemented, experimental data of various 1-D supersonic nozzle is compared to the numerical results.Sustainable Process and Energy TechnologyProcess and EnergyMechanical, Maritime and Materials Engineerin
Appropriate Similarity Measures for Author Cocitation Analysis
No full textWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Turbulence modelling for flows with strong variations in thermo-physical properties
No full textThis paper presents a novel methodology for improving eddy viscosity models in predicting wall-bounded turbulent flows with strong gradients in the thermo-physical properties. Common turbulence models for solving the Reynolds-averaged Navier–Stokes equations do not correctly account for variations in transport properties, such as density and viscosity, which can cause substantial inaccuracies in predicting important quantities of interest, for example, heat transfer and drag. Based on the semi-locally scaled turbulent kinetic energy equation, introduced in [Pecnik and Patel, J. Fluid Mech. (2017), vol. 823, R1], we analytically derive a modification of the diffusion term of turbulent scalar equations. The modification has been applied to five common eddy viscosity turbulence models and tested for fully developed turbulent channels with isothermal walls that are volumetrically heated, either by a uniform heat source or viscous heating in supersonic flow conditions. The agreement with results obtained by direct numerical simulation shows that the modification significantly improves results of eddy viscosity models for fluids with variable transport properties.Energy Technolog
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