An investigation into momentum and temperature fields of a meso-scale slot synthetic jet for a small jet-to-surface spacing

Impinging synthetic jets have been identified as a promising technology for cooling miniature structures. Recognizing their thermal performance on the target surface requires a fundamental understanding of the momentum field produced by the pulsating coolant flow which is dependent on the distance between the nozzle exit and the wall. It was earlier reported that the cooling performance of a synthetic jet is highly sensitive to this distance, i.e. as the nozzle-to-plate distance is reduced the jet performance degrades, however the fundamental mechanism for this behavior has not been well-understood. Therefore, a computational study is performed to investigate the flow and thermal fields of a meso-scale slot synthetic jet for a small jet-to-surface spacing of H/Dh = 2. Spatial discretization is implemented via a second order upwind scheme and a second-order implicit scheme is used for temporal discretization to ensure stability. The results show that the pulsating flow at the nozzle exit generates vortices and these vortices seem to have effect on the target surface profiles before they get dissipated. Mean surface profiles are also determined and their applicability at various frequencies is discussed.TÜBİTA

A numerical study of a single unsteady laminar slot jet in a confined structure

Due to copyright restrictions, the access to the full text of this article is only available via subscription.With the inherit advantages of air cooling, jet impingement can produce a factor of two or higher heat transfer than conventional fan flow over bodies. Therefore, impinging jets can solve a number of electronics thermal issues. Those jets produce complex flow and thermal structures leading to non-uniform and non-monotonic profiles on target surfaces. A numerical study is performed to investigate the flow and heat transfer characteristics of an unsteady laminar impinging jet emanated from a single high-aspect ratio rectangular (slot) nozzle in a confined arrangement. The spacing between the target plate and the nozzle is such that the jet would still be in its potential core length as it was in a free axial jet. Following the initial transients, flow and heat transfer parameters still vary considerably in time that the instantaneous and time-averaged values of surface profiles are not identical. Instantaneous surface pressure distributions exhibit that the stagnation point translates periodically around the initial jet-symmetry line and the surface profiles demonstrate off-center (non-stagnation point) peaks

A numerical study of a single unsteady laminar slot jet in a confined structure

Due to copyright restrictions, the access to the full text of this article is only available via subscription.With the inherit advantages of air cooling, jet impingement can produce a factor of two or higher heat transfer than conventional fan flow over bodies. Therefore, impinging jets can solve a number of electronics thermal issues. Those jets produce complex flow and thermal structures leading to non-uniform and non-monotonic profiles on target surfaces. A numerical study is performed to investigate the flow and heat transfer characteristics of an unsteady laminar impinging jet emanated from a single high-aspect ratio rectangular (slot) nozzle in a confined arrangement. The spacing between the target plate and the nozzle is such that the jet would still be in its potential core length as it was in a free axial jet. Following the initial transients, flow and heat transfer parameters still vary considerably in time that the instantaneous and time-averaged values of surface profiles are not identical. Instantaneous surface pressure distributions exhibit that the stagnation point translates periodically around the initial jet-symmetry line and the surface profiles demonstrate off-center (non-stagnation point) peaks

Direct liquid cooling of high flux LED systems: hot spot abatement

Due to copyright restrictions, the access to the full text of this article is only available via subscription.With the recent advances in wide band gap device technology, solid-state lighting (SSL) has become favorable for many lighting applications due to energy savings, long life, green nature for environment, and exceptional color performance. Light emitting diodes (LED) as SSL devices have recently offered unique advantages for a wide range of commercial and residential applications. However, LED operation is strictly limited by temperature as its preferred chip junction temperature is below 100 °C. This is very similar to advanced electronics components with continuously increasing heat fluxes due to the expanding microprocessor power dissipation coupled with reduction in feature sizes. While in some of the applications standard cooling techniques cannot achieve an effective cooling performance due to physical limitations or poor heat transfer capabilities, development of novel cooling techniques is necessary. The emergence of LED hot spots has also turned attention to the cooling with dielectric liquids intimately in contact with the heat and photon dissipating surfaces, where elevated LED temperatures will adversely affect light extraction and reliability. In the interest of highly effective heat removal from LEDs with direct liquid cooling, the current paper starts with explaining the increasing thermal problems in electronics and also in lighting technologies followed by a brief overview of the state of the art for liquid cooling technologies. Then, attention will be turned into thermal consideration of approximately a 60W replacement LED light engine. A conjugate CFD model is deployed to determine local hot spots and to optimize the thermal resistance by varying multiple design parameters, boundary conditions, and the type of fluid. Detailed system level simulations also point out possible abatement techniques for local hot spots while keeping light extraction at maximum

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

Due to copyright restrictions, the access to the full text of this article is only available via subscription.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.TÜBİTA

An investigation into performance characteristics of an axial flow Fan using CFD for electronic devices

Due to copyright restrictions, the access to the full text of this article is only available via subscription.Rotating fans are widely utilized in thermal management applications and their accurate characterization has recently become even a more critical issue for thermofluids engineers. The present study investigates the characterization of an axial fan computationally and experimentally. Using the three-dimensional CAD models of the fan, a series of computational fluid dynamics (CFD) simulations were performed to determine the flow and pressure fields produced by the axial mover over a range of flow rates. In order to validate the computational model findings, experiments were conducted to obtain the pressure drop values at different flow rates in an AMCA (Air Movement and Control Association) standard 210-99, 1999 wind tunnel. These data sets were also compared with the fan vendor’s published testing data. A reasonably good agreement was obtained among the data from these three separate sources. Furthermore, an attempt was made to understand the overall fan efficiency as a function of the volumetric flow rate. It was determined that the maximum overall fan efficiency was less than 27% correlating well with the computational results

Postverbal elements in immigrant Turkish: Evidence of change?

The research reported on was made possible with a grant from NWO (Netherlands Organizatio

The Effect of Desulfovibrio sp. Biofilms on Corrosion Behavior of Copper in Sulfide-Containing Solutions

This study aims to detect the effect of Desulfovibrio sp. on copper in terms of biofilm formation and corrosion in 722 h. In that way, appropriate strategies to inhibit microbiological corrosion in copper systems with Desulfovibrio sp. can be evaluated. For this purpose, experiments were performed in 1 L glass model system containing 28 copper coupons and pure culture of the sulfate-reducing bacteria (SRB) strain Desulfovibrio sp. in Postgate's medium C. Also, a control system with copper coupons but without Desulfovibrio sp. containing sterile Postgate's medium was studied concurrently with the test system. The test coupons were collected from systems at certain time intervals, namely 24, 168, 360, and 720 h. The samples were then subjected to several characterization analyses such as measurement of Desulfovibrio sp. numbers, corrosion resistance, EPS extraction, carbohydrate analysis, SEM, and EDS. During the experiments, the maximum Desulfovibrio sp. count in biofilm samples was found at 360 h. Carbohydrate and copper concentrations in biofilm were increased over time. EDS analysis revealed Cu, S, C, O, and Cl peaks on the surface of the samples. For the control coupons, only Cu peaks were observed. The results obtained from this study showed that copper was corroded by Desulfovibrio sp. in the model system under laboratory conditions

Effect of Mixed-Species Biofilm on Copper Surfaces in Cooling Water System

This study aimed to investigate the formation and effect of a biofilm on copper heat exchangers in full-scale system conditions. A modified Pedersen device with copper coupons was installed in parallel to a heat exchanger system to investigate several physico-chemical parameters, such as bacterial enumeration, carbohydrate content of exopolymeric substances, weight loss of test/control coupons, Cu concentrations, and corrosion products over ten months. Findings of this study showed that planktonic bacterial cells attach to each other and form a mixed-species biofilm on the copper coupon surface even though copper is toxic to a variety of microorganisms. These results also revealed that the mixed-species biofilm has a corrosive effect on copper surfaces used in cooling water systems despite the presence of biocide and the corrosion inhibitor. Additionally, it was demonstrated that a shock-dosed biocide application increased the corrosion rate on copper surface in a real system. Preventing risk of microbiologically influenced corrosion entails appropriate material selection and proper/regular chemical treatment of cooling systems. The current study provides useful insights through the evaluation of corrosion of materials with microbiological techniques