M. Baris Dogruoz

A Model for Flow Bypass and Tip Leakage in Pin Fin Heat Sinks

A model for the pressure drop and heat transfer behavior of heat sinks with top bypass is presented. In addition to the characteristics of a traditional two-branch bypass model, the physics of tip leakage are taken into consideration. The total flow bypass is analyzed in terms of flow that is completely diverted and flow that enters the heat sink but leaks out. Difference formulations of the momentum and the energy equations were utilized to model the problem in the flow direction. Traditional hydraulic resistance and heat transfer correlations for infinitely long tube bundles were used to close the equations. Tip leakage mechanisms were modeled by introducing momentum equations in the flow normal direction in both the pin side and bypass channel, with ad hoc assumptions about the static pressure distribution in that direction. Although the model is applicable to any kind of heat sink, as a case study, results are presented for in-line square pin fin heat sinks. Results were compared with the predictions from a two-branch bypass model and previous experimental data. It is shown that tip leakage effects are important in setting the overall pressure drop at moderate and high pin spacing, but have only minor influence on heat transfer.

Using state-space models for accurate computations of transient thermal behavior of electronic packages

Under a given set of boundary conditions, thermal performance of an electronic system is generally evaluated based on its steady state response. It is a conventional practice that actual time dependent power cycles and thermal boundary conditions are time-averaged to estimate the steady state characteristics. This approach may produce accurate results provided that the time dependency of the power cycles and/or thermal boundary conditions is small. In general, time-dependent thermal analyses with actual time-dependent boundary conditions and power cycles should be performed in order to determine the steady state behavior. Whether an experimental or numerical approach is taken, accurate prediction of transient thermal behavior of complex electronic systems is time consuming. While being less overwhelming compared to the actual laboratory experiments, fully time dependent Computational Fluid Dynamics (CFD) analysis still requires large amount of CPU time. In order to prevail over this large computational cost, a number of approximate models were developed, such as Resistor-Capacitor (R-C) thermal network approaches which have produced reasonably accurate results, and therefore have been popular in determining system transient response. It is worth noting that these approaches require some rigorous curve-fitting effort followed by an optimization process and are applicable to relatively simple systems. The present study uses state-space model to determine transient and steady state thermal behavior of complex systems as accurately as possible without compromising the speed. The aforementioned technique is applied to an SOC package placed on a printed circuit board and the results are compared to those from the fully transient CFD model computations. With a dramatic reduction in the overall CPU times, the temperature histories obtained from the state-space approach agree well with the transient CFD simulations.

Accurate Prediction of Transient Thermal Behavior of Electronic Systems With State-Space Models

Accurate prediction of transient thermal behavior of electronic systems with the use of Computational Fluid Dynamics (CFD) requires large amount of CPU time. Even though steady state response of such systems is mostly of interest, transient thermal simulations with actual time dependent power cycles should be carried out to determine the steady-state behavior. Traditionally, power dissipation values are time-averaged to obtain steady-state characteristics, which may or may not be accurate. To overcome the large associated computational cost, several different approximate models were suggested and published in the literature. Among these models, Resistor-Capacitor (R-C) thermal network approaches have been popular in obtaining the transient response for the past few decades. These approaches require rigorous curve-fitting effort followed by an optimization process and are applicable to relatively simple systems. This study presents a state-space approach to determine transient and steady state behavior of electronic systems as accurately as possible without compromising the speed. This technique is applied to a sample graphics card system and the comparisons are made with the fully transient CFD model computations. It is shown that the temperature histories obtained from the state-space approach agree very well with those from the fully transient CFD simulations, where the CPU time for the former is radically (three to four orders of magnitude) smaller compared to that of the latter.Copyright

Computations with the multiple reference frame technique: Flow and temperature fields downstream of an axial fan

ABSTRACT In numerical computations, axial fans are typically abstracted as two-dimensional surfaces, and this forms the basis for the “Lumped Fan” (LF) model. The LF-model relies on experimentally derived P–Q (fan) curves which should conform to the published test codes. Despite its simplicity, the LF-model’s accuracy depends on the application and acceptable error margin. Therefore, with decreasing error margins in thermal engineering, there has been an interest in accurate fan modeling techniques such as “Multiple Reference Frame” (MRF) model. The current effort provides a two-part validation of the MRF model results for an axial fan against the relevant experiments: comparison of the (i) P–Q curves, and (ii) temperature distribution in an electronic enclosure. In the former, both fan models exhibit good agreement with the experiments, however the flow structures determined by the two models exhibit substantial differences. Given these flow structures’ strong influence on the temperature field, the MRF model demonstrates good correlation and significantly better agreement with the measured temperature distribution, compared to those of the LF-model. In light of these findings, benefits, limitations, and possible applications of both models are also discussed.

An investigation into momentum and temperature fields of a meso-scale synthetic jet

Thermal management has become a critical part of advanced micro and nano electronics systems due to high heat transfer rates. More constraints such as compactness, small footprint area, lightweight, high reliability, easy-access and low cost are exposed to thermal engineers. Advanced electronic systems such as laptops, tablets, smart phones and slim TV systems carry those challenging thermal needs. For these devices, smaller thermal real estates with higher heat fluxes than ever have created issues that current thermal technologies cannot meet those needs easily. Therefore, innovative cooling techniques are necessary to fulfill these aggressive thermal demands. Synthetic jets have been studied as a promising technology to satisfy the thermal needs of such tight electronics devices. The effect of nozzle-to-surface distance for a synthetic jet on its cooling performance has neither been studied extensively nor been well-understood. In a few available experimental studies, it was reported that synthetic jet performance is very sensitive to this distance and when the jet gets closer to the hot surface its performance degrades. Therefore, a computational study has been performed to understand the flow physics of a small-scale synthetic jet for a jet-to-surface spacing of H/Dh=5. Spatial discretization is implemented via a second order upwind scheme and a second order implicit scheme is used for temporal discretization to ensure stability. It is found that pulsating flow at the nozzle exit generates vortices and these vortices seem to have minimal effect on the target surface profiles. Local surface pressure, velocity, turbulence profiles and heat transfer coefficient distributions are determined, then the effects of jet frequency as well as near-wall vortices are discussed.

Sensitivity analysis in conjugate heat transfer for electronics cooling

This study presents sensitivity analysis in a conjugate heat transfer problem for electronics cooling. An algorithmic differentiation technique shown by Jemkov and Mathur was utilized to obtain the directional derivatives accurately in the sensitivity analysis. For a pin fin heat sink geometry, results of thermophysical property and flow parameter sensitivity analyses were shown and comparisons were made with the single-point simulations. The computed values indicate that the algorithmic differentiation technique used in this study lead to accurate results while accelerating design optimization applications significantly.

Advances in Fan Modeling: Issues and Effects on Thermal Design of Electronics

Forced convection air-cooled electronic systems consist of fans to provide fluid flow through the enclosure. Typically axial flow fans, radial impellers, and centrifugal blowers fall into this category. In numerical computations of flow fields in electronic enclosures, axial fans have most commonly been abstracted as planar (2-D) rectangular or circular surfaces. In some cases, these abstract or lumped models may be used to mimic impellers and centrifugal blowers as well. All of these models rely on an experimentally derived “pressure head-flow rate” (P-Q) curve (also called “fan curve”). The experiments to obtain the fan curve should conform to the test codes published by ASME and/or AMCA.Convenience and accuracy of abstract fan models are dependent on the specific application/cooling method and the acceptable error margin. The latter for the thermal design of electronics has recently diminished considerably which led to the need of using more accurate and robust fan modeling techniques such as Multiple Reference Frame (MRF) model. The authors validated this method for different types of fans against relevant experimental data previously [1,2]. As a continuation of this earlier effort, an attempt is made to examine the thermal field computed by various fan modeling techniques including MRF for air-cooled enclosures in the present work. The results show that the temperature values obtained from lumped fan model and the MRF technique differ considerably.© 2012 ASME

An experimental and computational investigation of a thin piezofan cooler

Recent trends in electronic cooling systems are targeted towards a reduction in size, therefore small form factor/miniature cooling devices are of interest to various applications. Among these devices are piezoelectric fans which are simply made of vibrating plates and shed vortices from their leading edge and enhance heat transfer from nearby target surfaces. This paper investigates the flow and temperature fields produced by a piezoelectric fan. An experimental study is performed to determine the temperature distribution of a vertically heated surface under various fan tip-to-target surface distances and driving conditions of the piezoelectric device (frequency). 2-D numerical simulations are carried out to predict the momentum and temperature fields in the domain of interest under the same boundary conditions of the experimental effort. The numerical results are in reasonably good agreement with the measured experimental data. The relevant dimensionless parameters such as Nusselt, Strouhal, and Keulegan-Carpenter numbers are determined. With a maximum Nusselt number of 20 and 57 for mylar and metallic piezo fans, respectively, the corresponding Strouhal, and Keulegan-Carpenter numbers suggest that a vortex formation occurs at the blade tip, however these vortices are weak such that they are neither able to approach the target surface as high strength structures nor improve heat removal significantly for the range of measurements.

Acoustic analysis of an axial fan

Axial fans are often used in cooling electronic enclosures where low noise levels are highly demanded. Therefore, methods for predicting the noise emitted by an application including single or multiple fans are desirable to improve, stimulate and reduce the cost of low-noise design. The prediction of sound generated from fluid flow has been difficult due to the non-linear form of the governing equations, however, recent developments in computational fluid dynamics (CFD) and computational acoustics allow us to determine sound pressure levels (SPL) in a fluid flow. In this study, time dependent flow field produced by an axial fan is computed via Large Eddy Simulations (LES), and the consequent sound pressure map is determined using the Ffowcs Williams-Hawkings (FW-H) model. Since an axial fan is a complex source of sound, for engineering design purposes, simplifications are needed when modelling its acoustic characteristics, therefore, the sound radiation of an axial fan in free space is examined by expanding the generated sound pressure field into spherical harmonics. In addition, acoustic measurements are carried out in a semi-anechoic chamber to validate the aforementioned computational models and make necessary comparisons. Comparison of the numerical results against the experimental data shows that, despite some discrepancies, the former is able to capture the trends observed in the measurements.

Transient thermal behavior of SOIC packages — an optimization study

Under a given set of boundary conditions, thermal performance of an electronic system is generally evaluated based on its steady state response. It is a conventional practice that actual time dependent power cycles and thermal boundary conditions are time-averaged to estimate the steady state characteristics. This approach may produce accurate results provided that the time dependency of the power cycles and/or thermal boundary conditions is small. In general, time-dependent thermal analyses with actual time-dependent boundary conditions and power cycles should be performed in order to determine the steady state behavior. Whether an experimental or numerical approach is taken, accurate prediction of transient thermal behavior of complex electronic systems is time consuming. While being less overwhelming compared to the actual laboratory experiments, fully time dependent Computational Fluid Dynamics (CFD) analysis still requires large amount of CPU time. In order to prevail over this large computational cost, a number of approximate models were developed, such as Resistor-Capacitor (R-C) thermal network approaches which have produced reasonably accurate results, and therefore have been popular in determining system transient response. It is worth noting that these approaches require some rigorous curve-fitting effort followed by an optimization process and are applicable to relatively simple systems. The present study uses state-space model to determine transient and steady state thermal behavior of complex systems as accurately as possible without compromising the speed. The aforementioned technique is applied to a “Small Outline Integrated Circuit” (SOIC) package placed on a printed circuit board (PCB) and the results are compared to those from the fully transient CFD model computations. With a dramatic reduction in the overall CPU times, the temperature histories obtained from the state-space approach agree well with the transient CFD simulations. Furthermore, an optimization scheme is defined so that the maximum temperature on the package is kept under a specified value by changing the power cycles on individual die elements which may be of great benefit to the chip designers especially at the early stages of design.