Browsing by Author "Doğruöz, M. B."

Now showing 1 - 6 of 6

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Direct liquid cooling of high flux LED systems: hot spot abatement
(ASME, 2013) Tamdoğan, Enes; Arık, Mehmet; Doğruöz, M. B.; Mechanical Engineering; ARIK, Mehmet; TAMDOĞAN, Enes; Tamdoğan, Enes
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.
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An investigation into momentum and temperature fields of a meso-scale slot synthetic jet for a small jet-to-surface spacing
(Begell House Inc., 2014) Ghaffari, Omidreza; Doğruöz, M. B.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Ghaffari, Omidreza
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.
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An investigation into momentum and temperature fields of a meso-scale synthetic jet
(IEEE, 2014) Ghaffari, Omidreza; Doğruöz, M. B.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Ghaffari, Omidreza
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.
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An investigation into performance characteristics of an axial flow Fan using CFD for electronic devices
(ASME, 2015) Hashim, Hafiz Muhammad; Doğruöz, M. B.; Arık, Mehmet; Parlak, M.; Mechanical Engineering; ARIK, Mehmet; Hashim, Hafiz Muhammad
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.
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An investigation of performance of synthetic jets emanating from circular, elliptical and rectangular nozzles
(IEEE, 2018-07-24) Işıl, Onuralp; Doğruöz, M. B.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Işıl, Onuralp
Impinging synthetic jets have been considered as a future solution for cooling miniature structures. Synthetic jet performance is sensitive to a number of parameters such as orifice size, orifice shape and nozzle-to-target spacing. For the former, it has been shown that synthetic jet performance is sensitive to the distance between the jet nozzle and the target surface where enhancement of heat transfer usually decreases with a reduction in nozzle-to-target plate distance. For the latter, different nozzle shapes are responsible for generating different vortex fields which affect the temperature distribution over the target plate. However, no detailed information about the momentum and temperature fields has been shown, therefore further investigation is needed. In this study, three different meso scale synthetic jets were fabricated with three different nozzle shapes, namely, circular, elliptical and rectangular with each having an identical hydraulic diameter. A wave form generator was used to create a sine wave on the enclosure's diaphragm to form the jet action. Deflection on the diaphragm and local heat transfer coefficients on the heated target plate were determined as a function of the jet frequency and relevant discussions were made. It was found that the nozzle shape significantly affects the heat removal rates on the target plate. In addition, it was shown that the conventional hydraulic diameter definition used to compare synthetic jets with non-circular orifices may not be the correct length scale to evaluate the thermal/hydraulic performance of such devices. A short discussion on the jet fabrication techniques was also included.
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A numerical study of a single unsteady laminar slot jet in a confined structure
(ASME, 2013) Ghaffari, Omidreza; Doğruöz, M. B.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Ghaffari, Omidreza
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.