Browsing by Author "Azarifar, M."

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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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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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Effect of phase change materials on the optical path of LEDs for opto-thermal enhancement
(IEEE, 2023-05) Azarifar, M.; Arık, Mehmet; Xie, B.; Luo, X.; Mechanical Engineering; ARIK, Mehmet
A novel concept based on the encapsulation of transparent phase change materials (PCMs) into the optical packaging structure of light-emitting diodes (LEDs) is presented in this article. The concept was initiated by challenges of thermal management of photoluminescent particles in high-power optical systems. LEDs, white LEDs (WLEDs), and porous/network-based photoluminescent matrices can achieve improved thermal networks by embedding PCMs. In this article, paraffin is selected as a suitable PCM encapsulant, and aside from thermal perspectives, an unexpected optical benefit with melted paraffin after surface wetting of the chip was observed. Immersing an LED chip in a melted paraffin pool showed up to an 8% increase in light extraction efficiency and a 1.5% increase in power conversion efficiency (PCE). An accurate dynamic opto-electro-thermal monitoring of studied devices was used to support the proof of concept. This viable method can be integrated into current industrial packaging processes.