Faculty of Engineering

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Acoustic analysis of an axial fan
(IEEE, 2017) Hashım, Hafız Muhammad; Dogruoz, M. B.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Hashım, Hafız Muhammad
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.
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Acoustics and heat transfer characteristics of piezoelectric driven central orifice synthetic jet actuators
(Taylor & Francis, 2022-09-19) Ikhlaq, M.; Yasir, M.; Ghaffari, O.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet
Growth in computational capacity combined with a decrease in the size of digital devices has led to increasing demand for more active and efficient cooling of electronics. In this study, an experimental investigation into two different sizes of central orifice Synthetic Jet Actuators (SJAs) is conducted to evaluate their heat transfer as well as noise generation characteristics. Two SJAs (40 mm and 20 mm) are examined, covering a distinct span of frequencies ranging from low to medium (<5500 Hz) in regards to the effect of SJA size over performance. The SJAs’ disk deflection, structural frequency, and jet exit velocity were measured to fully characterize the jet performance. The maximum Nusselt number for the largest SJA was 3 times more than the smallest SJA, where the evaluation of stroke length suggests no effective synthetic jet formation for the smallest SJA. The noise from the SJAs was measured in an anechoic chamber using three microphones, Fast Fourier Transform (FFT) of the sound pressure levels provide contributions to different tones in the resulting noise. 1/3 Octave Constant Percentage Bandwidth (CPB) analysis was performed to identify the frequency bands making the largest contribution to the noise. The largest SJA showed the highest heat transfer at acceptable noise levels during the operation below resonance frequency.
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Characteristics of low reynolds number steady air jet impingement heat transfer over vertical flat surfaces
(IEEE, 2012) He, X.; Lustbader, J. A.; Arık, Mehmet; Sharma, R.; Mechanical Engineering; ARIK, Mehmet
In this paper, heat transfer characteristics of single-slot steady-impinging air jets on a 25.4 mm × 25.4 mm vertical surface were experimentally investigated. The experiments were conducted with four different nozzles (length × width: 4 mm × 1 mm, 8 mm × 1 mm, 12 mm × 1 mm, and 15 mm × 1 mm). The parameters varied in the testing were Reynolds number (Re) (100 - 2,000) and dimensionless nozzle-to-plate spacing (H/Dh = 5, 10, 15, and 20). Correlations for average Nusselt numbers (Nu) were developed that accurately predict experimental data. The heat transfer coefficient over a vertical surface increases with increasing Re. For a small nozzle-to-plate spacing (H/Dh = 5), the average Nu correlation is not only a function of Re but also a function of nozzle length. For large nozzle-to-plate spacing (H/Dh ≥ 10) and nozzle length larger than 8 mm, the heat transfer coefficient is insensitive to H/Dh and nozzle length. A subset of this data was then compared to synthetic jet data in a separate study.
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A comparative study for the junction temperature of green light-emitting diodes
(IEEE, 2019-10) Özlük, Burak; Muslu, Ahmet Mete; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Özlük, Burak; Muslu, Ahmet Mete
Solid-state lighting devices offer a wide variety of color options applicable for general and automotive lighting, various display systems, and a number of niche applications. As they get smaller, generated heat fluxes become more intense and induce serious lifetime and performance issues. Although the light output from light-emitting diodes (LEDs) is the most efficient at a narrow optical spectrum compared with conventional lighting sources, they are still not adequate to satisfy consumer demands due to considerable amounts of lost energy and emerging thermal issues. On the other hand, it is possible to achieve effective thermal solutions if the junction temperature of LEDs is precisely determined. A number of techniques have been proposed for the junction temperature measurement of LEDs such as forward voltage change and infrared (IR) thermal imaging. In this study, green LEDs were studied to observe optothermal interactions using a number of proposed junction temperature measurement techniques. The effect of an LED lens on junction temperature and optical extraction was investigated by examining the change in the thermal and optical properties of an LED chip after the LED lens was removed. In addition, the results of the green LED were compared with a 450-nm blue LED and verified with numerical findings. As a result, it has been determined that the thermal behavior of LEDs is significantly affected by electrical conditions, since the junction temperature of green and blue LEDs has risen by around 45% after the operating current has been increased from 200 to 500 mA.
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A comparative study on the junction temperature measurements of LEDs with raman spectroscopy, microinfrared (IR) imaging, and Forward voltage methods
(IEEE, 2018-11) Tamdoğan, Enes; Pavlidis, G.; Graham, S.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; TAMDOĞAN, Enes; Tamdoğan, Enes
Energy efficiency, long life, exceptional color, and performance of solid-state light sources have resulted in a rapidly increasing trend in a number of practical applications especially for general lighting after a long history of incandescent lamps. Besides, light-emitting diodes (LEDs) have thermal limitations that are vital for device quality and lifetime. Specifically, to improve the heat dissipation, one major parameter used to evaluate the LED performance is thermal resistance (R). Reducing the resistance can improve the heat flow from the p-n junction to ambient during operation. To quantify this parameter, the LED junction temperature (TJ) must be determined. In this paper, the junction temperatures are first measured with forward voltage method (FVM), Raman spectroscopy, and infrared (IR) imaging for a 465-nm bare blue LED chip (without any phosphor coating). Then, the same samples have been coated with a phosphor-particles added epoxy mixture (%13, 4300 CCT) to convert blue to white light, and the junction temperatures were measured again experimentally with the previously mentioned three methods and compared to each other. While IR imaging shows better capability on capturing the possible hotspots over the surface, Raman method and FVM were in reasonably good agreement on measuring the junction temperature for 465-nm blue (uncoated) LED chip. However, the measurements performed after coating have shown slightly different results with IR imaging and Raman methods, while FVM has shown consistent results for coated chips.
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Comparison of synthetic and steady air jets for impingement heat transfer over vertical surfaces
(IEEE, 2012) Arık, Mehmet; Sharma, R.; Lustbader, J.; He, X.; Mechanical Engineering; ARIK, Mehmet
Natural convection air cooling is the method of choice for many low-power electronics applications due to cost, availability, and reliability considerations. This method is not only limited to low-power applications, but is also constrained by the buoyancy dependence of the flow. Therefore, further enhancement of natural convection is needed. Enhanced natural convection allows higher heat dissipation while maintaining the simplicity of passive cooling. Synthetic jet devices operating on the microfluidics principle provide unique cooling advantages for local cooling with high coefficients of performance. Synthetic jets used in the current study are piezoelectrically driven, small-scale, pulsating devices capable of producing highly turbulent jets formed by periodic entrainment and expulsion of the fluid through an orifice. The compactness of the jet actuator coupled with the high exit air velocities can significantly reduce the size of thermal management systems. In this paper, we present experimental results for impingement heat transfer for both steady and unsteady jets over a Reynolds number range of 100 to 3,000. A range of nozzle-to-plate surface distances is discussed. To mimic a comparable electronics component, we used a 25.4-mm square heated surface.
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A computational and experimental investigation of synthetic jets for cooling of electronics
(The American Society of Mechanical Engineers, 2015-06-01) Arık, Mehmet; Utturkar, Y. V.; Mechanical Engineering; ARIK, Mehmet
Seamless advancements in electronics industry resulted in high performance computing. These innovations lead to smaller electronics systems with higher heat fluxes than ever. However, shrinking nature of real estate for thermal management has created a need for more effective and compact cooling solutions. Novel cooling techniques have been of interest to solve the demand. One such technology that functions with the principle of creating vortex rings is called synthetic jets. These jets are mesoscale devices operating as zero-net-mass-flux principle by ingesting and ejection of high velocity working fluid from a single opening. These devices produce periodic jet streams, which may have peak velocities over 20 times greater than conventional, comparable size fan velocities. These jets enhance heat transfer in both natural and forced convection significantly over bare and extended surfaces. Recognizing the heat transfer physics over surfaces require a fundamental understanding of the flow physics caused by microfluid motion. A comprehensive computational and experimental study has been performed to understand the flow physics of a synthetic jet. Computational study has been performed via FLUENT commercial software, while the experimental study has been performed by using laser Doppler anemometry (LDA). Since synthetic jets are typical sine-wave excited between 20 and 60 V range, they have an orifice peak velocity of over 60 m/s, resulting in a Reynolds number of over 2000. Computational fluid dynamics (CFD) predictions on the vortex dipole location fall within 10% of the experimental measurement uncertainty band.
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A computational and experimental study on a harsh environment LED system for vehicle exterior lighting applications
(IEEE, 2014) Saati Khosroshahi, Ferina; Tüfekçi, C. S.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Saati Khosroshahi, Ferina
Effect of compact thermal packaging approaches and challenges are investigated for a next generation automotive LED lighting. A challenging three-purpose in one (3i1) highly packaged light emitting diodes (LEDs) lighting system has been studied computationally and experimentally. A tightly packed light engine printed circuit board (PCB) with both LEDs and electronics in a very hermetically sealed enclosure close to the vehicle engine department pose significant thermal and mechanical challenges for the thermal design as well as optical considerations. Challenge is due to local high ambient temperatures and aggressive operating conditions. Finite element based computational models have been developed first to study a single package thermal performance, and then followed by system level computational fluid dynamics (CFD) models. Later, a series of experiments and analytical studies have been performed for validation of computational results. It is found that interface layer at the package poses significant bottlenecks for meeting the design requirements. System level CFD models showed that having a two-sided flame retardant (FR4) based board causes local hot spots that need innovative system level cooling solutions.
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A computational study on the momentum and heat transfer distribution of a low frequency round impinging synthetic jet
(ASME, 2015) Ikhlaq, M.; Dogruoz, B.; Ghaffari, O.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet
Impinging synthetic jets have been considered as a possible solution for cooling miniature structures. 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 decreases with a reduction in nozzle-to-target plate distance. At low nozzle-to-target spacing, no detailed information about the momentum and temperature fields have been shown in prior literature, therefore further investigation is needed. In this study, a 3-D computational fluid dynamics model was constructed to determine the flow and temperature fields of a meso-scale synthetic jet at a nozzle-to-target surface spacing of H/D = 2, ReD,j= 1400 and f = 500 Hz. Unlike the majority of previous computational studies, rather than specifying the boundary conditions at the nozzle, the flow inside the synthetic jet device was solved by specifying the time dependent boundary conditions on the vibrating diaphragm and utilizing the moving mesh technique. Local surface pressure and heat transfer coefficient distributions were determined and discussed. It was found that the pulsating flow at the nozzle exit for a round jet generates vortex rings and these rings seem to have some considerable effects on the target surface profiles.
Open Access
Conduction-driven cooling of LED-based automotive LED lighting systems for abating local hot spots
(SPIE, 2018-02) Saati, F.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet
Light-emitting diode (LED)-based automotive lighting systems pose unique challenges, such as dual-side packaging (front side for LEDs and back side for driver electronics circuit), size, harsh ambient, and cooling. Packaging for automotive lighting applications combining the advanced printed circuit board (PCB) technology with a multifunctional LED-based board is investigated with a focus on the effect of thermal conduction-based cooling for hot spot abatement. A baseline study with a flame retardant 4 technology, commonly known as FR4 PCB, is first compared with a metal-core PCB technology, both experimentally and computationally. The double-sided advanced PCB that houses both electronics and LEDs is then investigated computationally and experimentally compared with the baseline FR4 PCB. Computational models are first developed with a commercial computational fluid dynamics software and are followed by an advanced PCB technology based on embedded heat pipes, which is computationally and experimentally studied. Then, attention is turned to studying different heat pipe orientations and heat pipe placements on the board. Results show that conventional FR4-based light engines experience local hot spots (ΔT>50°C) while advanced PCB technology based on heat pipes and thermal spreaders eliminates these local hot spots (ΔT<10°C), leading to a higher lumen extraction with improved reliability. Finally, possible design options are presented with embedded heat pipe structures that further improve the PCB performance.
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Controlled thermal design of LED SLIM TV systems
(IEEE, 2017) Özaydın, M. C.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet
The market trend of global TV industry is nourished due to recent advancements in optical component technology and user expectations. Due to strong market requirement for slimmer mechanical designs (<;10mm) and increased luminance (700nit), higher color gamut (DCI: 120%); Decreasing Backlight Unit (BLU) thickness while increasing the image quality resulted in a factor of 3 increase in volumetric heat generation rates. Heat generated by LED packages is the primary cause of defects and deformations in the LED TV optical components. TV manufacturers have then faced challenging thermal design issues. Therefore, the thermal design for a BLU must be well understood to develop a novel optic-thermal design. In this study, an approach for the thermal management of an LED TV system is developed to determine local temperature distribution over BLU and electronics. A hybrid analytical and CFD modeling approach followed by an experimental validation has been developed. First, an idealized one dimensional resistance network is created. Then, 3D CFD models of the LED TV are developed by utilizing commercial software. CFD results are then validated with the experimental data performed in a controlled thermal chamber. The test data is compared with CFD findings. A variation of 6.3% is found between numerical and experimental findings.
Open Access
A critical review on the junction temperature measurement of light emitting diodes
(MDPI, 2022-10) Cengiz, C.; Azarifar, M.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet
In the new age of illumination, light emitting diodes (LEDs) have been proven to be the most efficient alternative to conventional light sources. Yet, in comparison to other lighting systems, LEDs operate at low temperatures while junction temperature (Tj) is is among the main factors dictating their lifespan, reliability, and performance. This indicates that accurate measurement of LED temperature is of great importance to better understand the thermal effects over a system and improve performance. Over the years, various Tj measurement techniques have been developed, and existing methods have been improved in many ways with technological and scientific advancements. Correspondingly, in order to address the governing phenomena, benefits, drawbacks, possibilities, and applications, a wide range of measurement techniques and systems are covered. This paper comprises a large number of published studies on junction temperature measurement approaches for LEDs, and a summary of the experimental parameters employed in the literature are given as a reference. In addition, some of the corrections noted in non-ideal thermal calibration processes are discussed and presented. Finally, a comparison between methods will provide the readers a better insight into the topic and direction for future research.
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Design and development of a durable series elastic actuator with an optimized spring topology
(Sage, 2021-12) Yıldırım, M. C.; Şendur, Polat; Kansızoğlu, Mehmet Taha; Uras, U.; Bilgin, Onur; Emre, Sinan; Yapıcı, Güney Güven; Arık, Mehmet; Uğurlu, Regaip Barkan; Mechanical Engineering; ŞENDUR, Polat; YAPICI, Güney Güven; ARIK, Mehmet; UĞURLU, Regaip Barkan; Kansızoğlu, Mehmet Taha; Bilgin, Onur; Emre, Sinan
This paper aims to present the integrated design, development, and testing procedures for a state-of-the-art torsion-based series elastic actuator that could be reliably employed for long-term use in force-controlled robot applications. The main objective in designing the actuator was to meet weight and dimensional requirements whilst improving the long-term durability, ensuring high torque output, and containing its total weight. A four-fold design approach was implemented: (i) following recursive design-and-test procedures, an optimal torsional spring topology was unveiled with the help of SIMP (Solid Isotropic Material with Penalization) topology optimization method, (ii) the proposed spring was manufactured and multiple specimens were experimentally tested via a torsional test machine to validate linearity, loading rate response, and mechanical limits, (iii) the actuator’s thermal response was experimentally scrutinized to ensure that the generated heat was dissipated for long-term use, and (iv) the fatigue life of the spring was computed with the help of real-life experiment data. Having concluded the development and verification procedures, two different versions of the actuator were built, and preliminary torque control experiments were conducted. In conclusion, favorable torque tracking with a bandwidth of 19 Hz was achieved while peak-to-peak torque input was 20 Nm.
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Developing a standard measurement and calculation procedure for high brightness LED junction temperature
(2014) Arık, Mehmet; Kulkarni, K. S.; Royce, C.; Weaver, S.; Mechanical Engineering; ARIK, Mehmet
White light emitting diodes (LEDs) are appearing in general illumination applications. Clusters of such LEDs can replace an incandescent light bulb of equal luminosity on the merit of considerably low power consumption. However the optical performance and working life of these LED packages are strongly dependent on the temperature of the p-n junction of the LED. Hence it is very critical to determine the temperature of the junction. Three methods - forward voltage change, peak wavelength shift and infrared thermal imaging are employed to determine the junction temperature. Forward voltage change method is found to be the most accurate method (± 3 °C) for an optimized set of parameters. Analytical model is proposed for the thermal transient behavior of the LED junction and the predictions are compared with experimental results. A good agreement is observed between that of two experimental methods. Thermal resistance of the LED package is estimated analytically and experimentally. Experimental values show a larger variation than expected through material property variation.
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Development of a computational modeling and experimental validation approach for KSF LED packages in a 65” ultra thin LED TV system
(IEEE, 2018-07-24) Elibol, M.; Özaydın, M. Ç.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet
The customer expectations in global TV market have changed over the last decade significantly towards a thinner with a higher brightness and a wider color gamut (WCG) TV products. These requirements led to a drastic increase in the power consumption and constrained mechanical design affecting the optical components and reducing the volume for thermal management. The KSF (K2SiF6:Mn 4+ ) LED is one of the best solutions to reach a higher color gamut (gamut > 90% in CIE 1976). However, the brightness of a typical KSF LEDs is about 15% lower than the conventional LEDs. KSF LED is also a thermally sensitive chemical compound but it enhances the color gamut of a TV system. Therefore, the determination of the best suitable LEDs for ultra thin TVs (thickness <; 7.9 mm) is critical since they are the main heat source consuming over 70% of total TV power consumption. Identifying thermally sensitive optical and mechanical components is one of the most essential steps of combined thermal and optical designs in an LED TV. At the same time, it offers significant advantages in terms of cost and time. Design and implementation of LED light engines (i.e. LED bars) is critical to reach performance goals of a high-end TV system. In this study, various LED bars consisting of KSF LED arrays and KSF chip scale packages (CSP) LED arrays have been studied both theoretically and experimentally. Modeling is started with analytical models (1D resistance network) and expanded to CFD analysis providing more accurate results capturing 3D heat transfer behavior. CFD simulations have been performed with a commercial CFD package (ICEPAK), and an experimental validation study has been performed to understand the thermal and the optical performance comparing different type of KSF LEDs in a TV system.
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Development of figure of merits for energy efficient LED lighting systems
(IEEE, 2014) Inan, Muhammed Nasir; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Inan, Muhammed Nasir
Energy has been identified as one of the most important problems during the last few years. While energy generation and new energy resources are critical, energy efficiency in production, transportation and utilization are as important as well. One of those energy efficient technologies may reduce consumed energy in buildings from 20 percent to less than 5 percent. A novel lighting technology has recently been evolved as light emitting diodes (LEDs) that can be over 7-10 times more efficient than conventional-old incandescent lamps. Therefore, LED lighting systems have been rapidly replacing conventional energy-hungry lighting products like incandescent lamps and more recently environmentally hazardous fluorescent lamps. While LEDs may produce large amounts of lumens, they are solid state based technologies similar to computer chips so they have to be kept cool at certain chip junction temperature. The demand for high light output LED systems lead to significant heat generation rates, so that higher heat fluxes result in elevated junction temperatures on LED chips in SSL lighting systems. Moreover, the changes on the junction temperature strongly impact the reliability, lifetime, light output and quality of the light. Because of their simplicity, reliability, low cost and silent operation, passive air-cooling systems are preferred in LED lamps. Passive metal based heat sinks are the main cooling components of typical LED lamps serves for both LEDs and driver electronics. Heat is dissipated generally from finned surfaces to ambient air with primarily convection mechanism and partially radiation. But it requires a large surface area and weight under the limitations of the standardized lamps. Thus, the optimization of the heat sink in an LED system is crucial. Developing figure of merits (FOM) is very important for designers and researchers to fmd the most optimal solution accounting for critical metrics such as size, weight, cost and performance. In the present study, thermal, electrical and optical experimental results of various commercial A-line LED lamps are investigated and a nwnber of FOMs are proposed based on the performance, size and weight. Proposed FOMs aim to evaluate different aspects by combining a nwnber of performance metrics. Results show that one can combine and analyze multi-purpose design parameters for thermal, electrical and optical performances and manufacturing for engineers and consumers.
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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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Direct numerical simulation of synthetic jet coupled to forced convection cooling in a channel flow
(IEEE, 2023) Azarifar, M.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet
A synthetic jet (SJ) is a microfluidic device that uses the 'zero-net-mass-flux' concept to create a compact cooling solution and provide a net positive momentum flux to the local environment. SJs have been studied extensively for natural convection heat transfer, but there is a limited data available for SJs in cross flow regimes. This paper presents results based on direct numerical simulation of a SJ in a confined heat transfer channel with and without cross flow. Studied SJ had a deforming boundary that oscillated at 1000 Hz and was placed at a high orifice-to-plate distance ratio of 20. The flow field inside the device with a moving boundary was modeled in a coupled manner to the flow field outside of the device for 80 oscillation cycles. The coupled study of the flow fields inside and outside of the cavity revealed their interaction towards an unstable flow field. Moreover, comparison between SJ's and continuous jet's (CJ) cooling performance was performed with the same net mass flow rate and identical jet outlet temperatures. Without cross flow, CJ, and with cross flow, SJ outperformed in terms of heat removal. The remarkable difference in spatial evolution of CJ and SJ explains the better performance of SJ in cross flow regime. In the studied high orifice-to-plate distance, CJ stream was unable to penetrate effectively through the crossflow, while the vortical structures created by SJ were able to do so and impinge on the target surface with heat transfer augmentation at upstream. Furthermore, the SJ's cavity heating was found to be a limiting factor in its capability to achieve high heat transfer coefficients in confined channels, which needs to be addressed to maintain its reliable heat removal performance.
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Discrete phase analysis of self heating particles over an immersion liquid cooled high power blue light-emitting diode with suspended phosphor particles
(ASME, 2022-10) Cengiz, Ceren; Muslu, Ahmet Mete; Azarifar, Mohammad; Arık, Mehmet; Dogruoz, B.; Mechanical Engineering; ARIK, Mehmet; Cengiz, Ceren; Muslu, Ahmet Mete; Azarifar, Mohammad
In recent years, the interaction of unrestricted particles with dispersed multiphase flows has been linked to a number of important engineering applications. Among these applications, the novel idea of immersion-cooled phosphor particles, which has the potential of significantly increasing the thermal limits of phosphor converted white light-emitting diode (LEDs) (Pc-WLEDs), has yet to be thoroughly investigated. With this objective, this research utilizes the discrete phase modeling (DPM) technique for the characterization of phosphor location and movements within a buoyancy-driven flow, which is the determining factor in the optical behavior of the newly proposed Pc-WLED configuration. Two-phase flow analysis is conducted to characterize particle movement. Heat transfer, flow, and energy paths of self-heating phosphor particles are extracted, and the influence of particle sizes is analyzed in detail. The results show that with immersion liquid cooling, the highest phosphor particle temperature is recorded to be under 420 K, while larger size particles introduce higher heat transfer rates to the Pc-WLED package for the same number of particles. Moreover, depending on the particle size and position, individual phosphor particles can follow a different trajectory that can affect the probability of obtaining white light emission.
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Dynamic opto-electro-thermal characterization of solid state lighting devices: measuring the power conversion efficiency at high current densities
(IOP Publishing, 2022-09-22) Azarifar, Mohammad; Cengiz, Ceren; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Azarifar, Mohammad; Cengiz, Ceren
Solid state lighting devices with high power densities require accurate characterization, as the rise of chip temperature impacts its optical, electrical, and lifetime characteristics. A detailed experimental study is performed in order to analyze opto-electro-thermal behavior as new high-power devices like laser diodes (LDs) are becoming of interest. The outline of this article was inspired by two major issues identified in previous experimental approaches. First, there is some debate in the literature about the linearity of the temperature coefficient of the forward voltage. Second, there is a limited number of experimental reports on the temperature dependence of power conversion efficiency. We have shown that prior variations are the result of a diversity in the selection of electrical parameters during thermal calibrations, as both linear and non-linear relationships for temperature coefficient of voltage can be obtained. On the other hand, it was discovered that report scarcity for temperature-dependent studies can be related to the use of passive approaches for temperature-dependent measurements. In temperature-controlled environments, short pulses with high current densities may not ensure the thermal equilibrium of the device under test due to small thermal capacitance. This issue, as well as the lengthy process of passive measurements can be addressed by implementing a dynamic measurement method presented in this study. Finally, linear power conversion efficiency trends with junction temperature are demonstrated for both blue light emitting diodes and LDs at high current densities.

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