Tilak Chandratilleke

Numerical prediction of secondary flow and convective heat transfer in externally heated curved rectangular ducts

A numerical simulation is presented to describe the secondary flow characteristics in the flow through curved rectangular ducts that are heated on the outer (concave) wall. The buoyancy body forces due to external heating are incorporated in the formulation. The governing equations of momentum, mass continuity and energy of the flow are non-dimensionalised and transformed into vorticity-streamfunction. The equations are discretised using a finite volume method and the dynamical variables are defined on a staggered grid. The resulting penta-diagonal system of linear algebraic equations is solved using the Strongly-Implicit-Procedure. Numerical computations are performed for the flow through rectangular ducts of aspect ratios 1 to 8 and for the Dean number lying in the range 20 to 500. The external wall heat fluxes of 0 to 200 W·m−2 are considered. The computation accurately captures the influence of external heating on the secondary vortex motion and, predicts the occurrence of Dean vortices in non-isothermal flow through the curved ducts. With wall heating, the flow pattern rapidly deviate from the symmetrical vortex motion observed under isothermal conditions. The formation of Dean vortices still takes place in the flow although it is postponed to higher flow rates where the flow symmetry is gradually re-established. The number of Dean vortices formed in the flow is strongly influenced by the aspect ratio of ducts. Convective heat transfer is significantly enhanced by the secondary flow particularly when the Dean vortices emerge at the outer wall. The predicted characteristics of secondary flow and heat transfer conform to and agree well with the available experimental data.

Analysis of a Synthetic Jet-Based Electronic Cooling Module

This article presents a numerical study of an electronic cooling module using a periodic jet flow at an orifice with net zero mass flux, known as a synthetic jet. The two-dimensional time-dependant numerical simulation models the unsteady synthetic jet behavior, the flow within the cavity and the diaphragm movement while accounting for fluid turbulence using the shear-stress-transport (SST) k-ω turbulence model. Computations are performed for a selected range of parameters and the boundary conditions to obtain the heat and fluid flow characteristics of the entire synthetic jet module. The numerical simulation aptly predicts the sequential formation of the synthetic jet and its intrinsic vortex shedding process while accurately illustrating the flow within the cavity. It is indicated that the thermal performance of the synthetic jet is highly dependant on the oscillating diaphragm amplitude and frequency. At the heated surface, this jet impingement mechanism produces a very intense localized periodic cooling effect that reaches a peak with a time lag relative to the top displacement position of the diaphragm. The overall heat transfer rate of the synthetic jet module is about 30% better than an equivalent continuous jet. When compared to pure natural convection the enhancement varies from 20 to 120 times in the range of parameters studied. The study clearly identifies the outstanding thermal potential of the synthetic jet module for intense electronic cooling applications and its ability to operate without additional fluid circuits.

Laminar Convective Heat Transfer in a Microchannel With Internal Fins

A numerical study was conducted to investigate the fluid flow and heat transfer characteristics of a square microchannel with four longitudinal internal fins. Three-dimensional numerical simulations were performed on the microchannel with variable fin height ratio in the presence of a hydrodynamically developed, thermally developing laminar flow. Constant heat flux boundary conditions were assumed on the external walls of the square microchannel. Results of local Nusselt number distribution along the channel length were obtained as a function of the fin height ratio. The analysis was carried out for different fin heights and flow parameters. Interesting observations that provide more physical insight on this passive enhancement technique, and the existence of an optimum fin height is brought out in the present study.Copyright

Performance analysis of a synthetic jet-microchannel hybrid heat sink for electronic cooling

This paper proposes a novel concept for enhancing the heat removal potential in a microchannel-based heat sinks and presents a study of its thermal performance. This technique utilises a jet mechanism that injects into the heat sinks flow passage a strong periodic fluid jet without any net mass outflow through the discharge orifice, hence termed “synthetic jet”. The flow within this microchannel-synthetic jet hybrid heat sink is modelled as a 2-dimensional finite volume simulation with unsteady Reynolds-averaged Navier-Stokes equations while using the Shear-Stress-Transport (SST) k-ω turbulence model to account for fluid turbulence. For a range of conditions, the characteristics of this periodically interrupted micro fluid flow are identified while evaluating its convective heat transfer rates. The results indicate that this pulsed jet micro-heat sink can deliver heat removal rates 4.3 times higher than an equivalent heat sink without jet mechanism. The thermal enhancement is first seen to grow gently and then rather rapidly beyond a certain flow condition to reach a steady value. The proposed strategy has the unique intrinsic ability to deliver unprecedented thermal performance in micro-heat sinks without additional fluid circuits or pressure drop. The technique has application potential in miniature electronic devices where intense localised cooling is desired over a base heat load.

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.

Thermal performance evaluation of a synthetic jet heat sink for electronic cooling

This paper presents a performance investigation on a highly effective heat removal technique for heat sinks in electronic cooling applications. This arrangement utilises a pulsating fluid jet mechanism known as synthetic jet, which is characterised by zero net fluid discharge through the jet orifice. The study uses an experimental rig comprising a high-frequency pulsating air jet that impinges on a heated surface to emulate the heat sink operation attached to an electronic device. The cooling characteristics of this jet are examined for a range of parametric conditions, including jet-impinging distance while evaluating the heat removal rates. The results indicate that the pulsating jet produces outstanding cooling performance at the heated surface with significant dependency of it on the jet-impinging distance. The study also assesses the interaction of a cross-flow fluid stream on the pulsed jet operation. It is observed that the cross-flow somewhat impedes the pulsed jet thermal performance. However, the pulsed jet, with or without cross flow, delivers an overall cooling ability that supersedes the standard flow-through heat sink performance. This technique provides highly enhanced surface cooling potential without incurring increased fluid pressure drop or requiring additional fluid circuit, which are significant advantages for high-powered heat sink design.

Convective heat transfer in airflow through a duct with wall thermal radiation

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 thermal radiation from duct walls. The duct periphery is differentially heated with known temperature profiles imposed on the two opposite vertical sidewalls while the other two walls are treated as adiabatic. The air enters into 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 variation of local Nusselt number at duct walls and the fluid flow patterns are critically examined to identify thermal instabilities and the significance of wall thermal radiation effects on the overall heat transfer rates.

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.

A synthetic jet heat sink with cross-flow for electronic cooling

This paper presents an investigation on the operational characteristics and thermal effectiveness of a pulsed (or synthetic) jet mechanism that periodically cools at a heated surface while acting in cross-flow fluid stream. The study uses a test rig having a variable-frequency pulsed air jet impinging at a heated surface that emulates an electronic device to be cooled. The cooling characteristics of the jet are observed over a wide parametric range. The results show that the pulsed jet mechanism delivers outstanding cooling performance that is primarily dependent on the jet-impinging distance and operating frequency. Without cross flow, the pulsed jet provides about 11 K temperature reduction and 7 times more heat removal rate compared to natural convection at the heated surface. The jet impingement height indicates a strong dependency with an optimum on the heat removal rate. With cross-flow fluid stream, the pulsed jet cooling is enhanced. This combined fluid action achieves about 13 K temperature drop and delivers 2.2 times more cooling compare to pulsed jet operating alone. The pulsed jet operation is numerically simulated to understand the associated flow characteristics leading to thermal enhancement. It is recognised that the pulsed jet arrangement has the unique surface cooling ability without additional fluid circuits, making it particularly desirable for high-capacity electronic cooling applications.

Thermal performance of a double-action pulsed jet CPU cooler

More efficient and compact thermal management techniques are critical for the development of Central Processing Units (CPU) embedded in complex and powerful modern computer systems. Introducing a technological alternative to conventional fan-cooled systems, this paper presents an experimental investigation of a double-action CPU cooler based on the pulsed (or synthetic) jet principle. The study develops a prototype of this new CPU cooler and tests it for a range of operating conditions to ascertain its cooling capabilities. The performance of this device is compared with a conventional fan CPU heat sink design for evaluating the relative thermal advantages of the new configuration. It is observed that the pulsed-jet CPU cooler achieves about 1.5 times more heat removal rate than a comparable fan CPU cooler. Whilst thermal optimisation is feasible, it is recognised that this pulsed jet arrangement has unique surface cooling ability without additional fluid circuits, making it particularly desirable for high-capacity electronic cooling applications.