Cezar O.R. Negrão

Integration of computational fluid dynamics with building thermal and mass flow simulation

Abstract Advanced heat and flow models (as employed within Building Thermal and Mass Flow Simulation (BSim) and Computational Fluid Dynamics (CFD)) with different degrees of detail were investigated and their modelling deficiencies identified. The CFD technique, which defines the fluid flow on a ‘micro’ scale, was integrated into BSim in which fluid flow is described in a larger scale. The resulting combined approach strengthens the modelling potential of each methodology by overcoming their specific deficiencies. BSims inability to predict air flow property gradients within a single space was surmounted and the difficulty of estimating CFD boundary conditions are now supplied by BSim. The conflated methodology allows the analysis of part of the building via a detailed CFD technique where air property gradients are judged to be crucial while in the rest of the building, where the air can be considered mixed, less resolved BSim approach is employed. The BSim environment, ESP-r, was elected to perform the current work. Two air flow situations encountered within buildings are discussed to demonstrate the combined methods applicability when compared with the BSim approach. Finally, general conclusions are presented showing that the developed methodology is very promising.

A view of energy and building performance simulation at the start of the third millennium

The use of computer-based models for performance predictions has become almost ubiquitous in the design, operation and management of buildings and the systems that service them. This special issue bears witness to the fact that the building performance simulation field is rapidly evolving. The techniques and applications of building performance simulation are undergoing rapid change. Dramatic improvements in computing power, algorithms, and physical data make it possible to simulate physical processes at levels of detail and time scales that were not feasible only a few years ago. Applications that were not attainable or practicable some years ago are now commonplace.

The yielding and the linear-to-nonlinear viscoelastic transition of an elastoviscoplastic material

Elastoviscoplastic materials present a transition from a gel-like to a liquidlike state induced by shearing: While the first is primarily elastic, the second is predominantly viscous. The point that characterizes this transition is usually known as the yield point, which is associated to critical quantities such as yield stress and/or yield strain. Another characteristic of elastoviscoplastic materials is the transition from linear to nonlinear viscoelasticity. In the current work, a commercial hair gel, which is an elastoviscoplastic material, was tested in two rotational rheometers in order to evaluate these two transition points. Stress oscillatory amplitude sweeps at different frequencies were performed and a Fourier-Transform analysis was applied to the results in order to determine the linear viscoelastic limit. The linear viscoelastic limit stresses and strains at different frequencies were then compared to quantities that are usually associated to the yield point: The extrapolated zero-shear-rate ...

A Simplified Model with a Hybrid Analytical-Numerical Solution for Predicting the Unsteady Conjugate Heat Transfer Process in Pipelines

This work presents a new, simplified and yet efficient model for studying the transient conjugate heat transfer process in pipelines. The simplified model consists of assuming the thermal transport by fluid flow in bulk form and the pipe–wall heat conduction as two-dimensional. The scale analysis of the dimensionless equations resulting from the simplified model allows for the identification of two predicting parameters useful in determining when axial diffusion in the pipe wall is important, ψ = Pe Γ/ξ < 1 and λ = 4 Nu E 2/ξ < 1, and when radial diffusion is important, ψ > 1 and λ > 1. When these criteria are satisfied, in which case the existing analytical solution becomes invalid, the simplified model proposed here becomes an efficient alternative to the full-scale numerical simulations required for solving the problem. The scale analysis predictions and the hybrid analytical-numerical solutions of the simplified model, involving the Laplace transform and the finite-volume method, are validated first by comparison against the existing analytical solution, and then by comparison against experimental results of turbulent flow, showing excellent agreement in both cases.

Non-monotonic response of waxy oil gel strength to cooling rate

The oil at high temperatures in the reservoir loses heat to the surroundings and is submitted to different shear stresses while it is produced and transported. Thermal and shear histories have great influence on the rheological characteristics of waxy oils at low temperature. Wax crystals precipitate during cooling, building up a percolated matrix that entraps the oil and consequently, forms a gel-like structure. One of the main parameters that affect the crystals’ morphology and therefore the gel strength is the cooling rate. Although the oil static cooling has been widely studied in the literature, many questions are still open. The current work analyzes the influence of the cooling rate on the gel strength and on the dynamic moduli (G′ and G″) of a waxy model oil. Microscopic images of wax crystals were obtained and a hypothesis to explain the non-monotonic response of the rheological parameters as a function of the cooling rate is proposed based on the crystals’ morphology.

A Numerical Approach on New Constitutive Model for Thixotropic Substances

The thixotropic substances can be found in different industrial sectors, such as chemical, biomedical, manufacturing and oil. These substances show a rheological time-dependent behavior, dependent of their structural level. Generally, a constitutive model for the thixotropic substances is composed by a pair of coupled equations: the constitutive equation (based on viscoelastic models) and the rate equation (that describes the structural evolution). In many works presented in the specialized literature, the shear modulus and viscosity dependencies with the structural nature are not formally considered in the dynamical principles from that the constitutive equation is originated. In the present work, a new, thermodynamically consistent, constitutive model for thixotropic substances, where such dependences are considered, is presented and some rheological tests are analyzed in a numerical simulation point of view (code developed in MATLAB). The constitutive model is based on Jeffreys’ model and the coagulation theory of Smoluchowsky.

The Effect of Elastic Modulus Rate in Stress Controlled Tests on the Modified Jeffrey’s Model for Structured Fluids

Structured fluids are known to be dependable of their structural level. Examples of such fluids may be found in different industries as chemical, biomedical, manufacturing, food and oil. The mathematical models to describe structured fluids are normally composed by a coupled system: one constitutive equation (based on viscoelastic models) and one kinetics equation (an equation which describes the structural level evolution in time of the material). The works found in the literature use linear viscoelastic constitutive equations which do not account the dependence of the elastic modulus with the microstructural level in their fundamental hypothesis. In this sense, the present work aims to evaluate, through numerical simulation, the effect of a new constitutive equation in rheological tests and compare its results to those of the model developed by Souza Mendes and Thompson (2013), in which those considerations are not made.