Forced Convective Heat Transfer and Fluid Flow Characteristics in Curved Ducts

Fluid flow through curved ducts is influenced by the centrifugal action arising from duct curvature and has behaviour uniquely different to flow within straight ducts. In such flows, centrifugal forces induce secondary flow vortices and produce spiralling fluid motion within curved ducts. Secondary flow promotes fluid mixing with intrinsic potential for thermal enhancement and, exhibits possibility of fluid instability and additional secondary vortices under certain flow conditions. Reviewing the published work on numerical and experimental studies, this chapter discusses the current knowledge-base on secondary flow in curved ducts and, identifies the deficiencies in analyses and fundamental understanding. The chapter then presents an extensive research study capturing advanced aspects of secondary flow behaviour and associated wall heat transfer processes for both rectangular and elliptical curved ducts.This study develops a new three-dimensional numerical model incorporating helicity approach and curvilinear mesh that is validated against published data to overcome current modelling limitations. Flow patterns and thermal characteristics are obtained for a range of duct aspect ratios, flow rates and wall heat fluxes. Results are analysed for parametric influences and construed for clearer physical understanding of the flow mechanics involved. The study formulates two analytical techniques whereby secondary vortex detection is integrated into the computational process with unprecedented accuracy and reliability. The vortex inception at flow instability is carefully examined with respect to the duct aspect ratio, duct geometry and flow rate. An entropy-based thermal optimisation technique is developed and tested for fluid flow through curved rectangular and elliptical ducts

Heat transfer enhancement in single impinging jets due to surface cavities

Paper presented at the 5th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 1-4 July, 2007.This paper presents an assessment of a novel technique that further enhances the heat transfer potential of a single impinging jet. The method entails a geometrical modification to the jet impingement surface wherein the jet is directed into a cylindrical cavity located coaxially beneath the jet orifice. A numerical study is performed to examine the parametric influence on heat dissipation and flow characteristics of this modified jet impingement process. The results indicate a very significant increase in heat transfer, which is primarily dependent on cavity depth and jet Reynolds number.cs201

Analysis of secondary flow characteristics and hydrodynamic instability in fluid flow through curved ducts

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.This paper presents an investigation on the unique flow characteristics associated with fluid flow through curved ducts, which are fundamentally different to those in straight fluid passages. In curved ducts, the flow is subjected to centrifugal forces that induce counter-rotating vortices in the main axial fluid stream and give rise to spiralling fluid motion, commonly known as secondary flow. The study develops a novel three-dimensional computational fluid dynamics analysis whereby the laminar developing fluid flow in a curved rectangular duct is modelled. The flow characteristics are identified for a range of flow rates and duct aspect ratios at several duct curvatures. The contours of secondary flow and axial velocities are obtained to recognise the influence of flow/geometrical parameters on the secondary flow. Comparisons are made between the numerical predictions and 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. The fluid flow structure is then significantly altered with the appearance of additional pair (or pairs) of vortices, termed as Dean Vortices, at the outer wall of the curved duct. This flow behaviour is also highly influenced by the duct aspect (height to width) ratio. The paper develops and presents a new approach for predicting the onset of Dean vortex generation.mp201

Multi-Phase fluid behavior and thermal characteristics in flow through heated curved ducts

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.Fluid flow and thermal characteristics in curved ducts are influenced by the secondary flow effects arising from duct curvature and are uniquely different to those in straight ducts. Such flows stimulate fluid mixing to promote wall heat transfer even in laminar flow and exhibit hydrodynamic instability under certain flow conditions. This paper provides an overview of the present knowledge-base in the field for analysing and designing technological systems with embedded curved flow geometries. It then discusses the details and outcomes of a research programme covering both single and two-phase flow behaviour in curved ducts. As a key contribution to the field, the study develops and validates a novel numerical process based on three-dimensional vortex structures (helicity) and a curvilinear mesh system that overcomes previous modelling limitations. For both single and two-phase flow situations, computations are performed to examine the flow characteristics within rectangular, elliptical and curved ducts and, to recognise parametric influences. These are effectively deployed for physical interpretation and illustration of unique features in single and two-phase flow processes within curved ducts. The study introduces a unified approach for identifying the onset of secondary flow instability directly within the computational process. An entropy-based analysis is appraised in the study for optimising curved duct thermal characteristics.dc201

Multimode heat transfer characteristics in airflow through a differentially heated rectangular duct

Paper presented at the 5th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 1-4 July, 2007.This paper presents a numerical investigation on airflow through a heated horizontal rectangular duct wherein the model considers the combined modes of natural and forced convection heat transfer and the surface thermal radiation from duct walls. The duct periphery is differentially heated with known temperature profiles imposed on the two opposite vertical sidewalls and the other two walls treated as adiabatic. The air enters the duct hydrodynamically fully developed and flows steadily under laminar conditions undergoing thermal development within the duct. Considering several temperature profiles on the two vertical sidewalls, the numerical simulation generates the heat transfer rates and associated fluid flow patterns in the duct for a range of airflow rates, duct aspect ratios and surface emissivity. The Nusselt number at duct wall and the fluid flow patterns are critically examined to identify thermal instabilities and the significance of wall surface radiation effects on the overall heat transfer rates.cs201