Nima Nadim

Analysis of Secondary Flow Instability and Forced Convection in Fluid Flow Through Rectangular and Elliptical Curved Ducts

This article examines the unique fluid flow characteristics and associated forced convection in curved ducts where the flow behavior is typified by counterrotating secondary flow vortices arising from the centrifugal forces due to flow curvature. For laminar developing fluid flow through curved heated ducts, the study formulates a novel three-dimensional computational fluid dynamics model based on vortex structures (or helicity). The fluid and thermal characteristics are examined using the helicity contours in duct cross sections for a range of flow rates, wall heat fluxes, and duct aspect ratios at selected duct curvatures. Curved ducts of rectangular and elliptical cross section are analyzed to identify and compare the fundamental differences in flow characteristics for each duct type. The study also presents a new technique using dimensionless helicity for detecting the onset of hydrodynamic instability in curved ducts. Numerical predictions are validated with the available experimental data. It is observed that with increased duct flow rate, the secondary flow intensifies and beyond a certain critical flow condition leads to hydrodynamic instability in both types of ducts. However, the overall fluid flow structure, hydrodynamic instability, and forced convection are significantly dependent on the type of duct, while these aspects are also significantly influenced by the duct aspect ratio and wall heating.

Secondary Flow Vortex Structures and Forced Convection Heat Transfer in Fluid Flow through Curved Elliptical Ducts

Fluid flow through curved ducts is essentially characterised by the secondary flow effects due to duct curvature and cross-sectional flow geometry. Such flows produce vortex structures making the fluid behaviour vastly different than those in straight ducts while intrinsically promoting forced convection through fluid mixing. Examining the unique features of secondary flow and wall heat transfer, this paper presents a numerical simulation on the fluid flow through curved elliptical ducts, including circular geometry. The study develops and validates a novel numerical model based on three-dimensional vortex structures (helicity) and a curvilinear mesh system to overcome previous modelling limitations. Considering several duct aspect ratios, flow rates and wall heat fluxes, computations are performed to obtain the flow patterns and thermal characteristics. Parametric influences on flow features and forced convection are described through physical interpretation. The onset of vortices due to secondary flow instability is carefully examined in relation to the duct aspect ratio and flow rate. Appraising their merits, two techniques are developed for accurate detection of secondary flow instability and integrated into the computational process, which was not previously feasible. An approach based on the Second Law irreversibility is evaluated for thermal optimisation of fluid flow through curved elliptical ducts.

Enhanced Thermo-Fluid Dynamic Modelling Methodologies for Convective Boiling

Analytical tools embedded in current thermal design practice for convective boiling systems are traditionally built upon correlated empirical data, which are constrained by the thermo-fluid dynamical complexities associated with stochastic and interactive behaviour of boiling fluid mixtures. These methodologies typically overlook or under-represent key characterising aspects of bubble growth dynamics, vapour/liquid momentum exchange, boiling fluid composition and local phase drag effects in boiling processes, making them inherently an imprecise science. Resulting predictive uncertainties in parametric estimations compromise the optimal design potential for convective boiling systems and contribute to operational instabilities, poor thermal effectiveness and resource wastage in these technologies. This book chapter first discusses the scientific evolution of current boiling analytical practice and predictive methodologies, with an overview of their technical limitations. Forming a foundation for advanced boiling design methodology, it then presents novel thermal and fluid dynamical enhancement strategies that improve modelling precision and realistic processes description. Supported by experimental validations, the applicability of the proposed strategies is ascertained for the entire convective boiling flow regime, which is currently not possible with existing methods. The energy-saving potential and thermal effectiveness underpinned by these modelling enhancements are appraised for their possible contributions towards a sustainable energy future.

LBM-LES Modelling of Low Reynolds Number Turbulent Flow Over NACA0012 Aerofoil

Contemporary evolution of numerical methods in fluid dynamics includes a growing application of lattice Boltzmann modelling (LBM) for turbulent flows. Large eddy simulation, implemented on LBM framework, is established as a competent alternative for the finite volume turbulence modelling method owing to enhanced feasibility of parallelism and the transient nature of LBM equations. This work utilises a simple Smagorinsky SGS model to investigate fundamental characteristics of a turbulent flow over NACA0012 aerofoil for a range of low Reynolds number in a turbulent flow.

Forced Convective Heat Transfer and Fluid Flow Chracteristics in Curved Ducts

© 2012 Chandratilleke and Nadim, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Forced Convective Heat Transfer and Fluid Flow Characteristics in Curved Ducts

Fluid and thermal characteristics of flow through rectangular and elliptical curved ducts under different gravity conditions

Active interaction between centrifugal and buoyancy forces in curved channel has been investigated numerically in different gravity condition. Given interaction makes the flow patterns fundamentally different with straight channels; nevertheless, either of these forces domination leads to a different vortices structure and heat transfer quality. Threedimensional, incompressible, laminar model has been used to compare rectangular and elliptical cross-sections in different gravity conditions as local and average heat transfer discussed regarding vortices structure effect. Based on the discussion different cross section has been briefly compared and concluded in term of stability whereas gravity is subjected to variation.