Browsing by Author "Emir, Tolga"

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Experimental investigation on pool boiling heat transfer over micro textured, additive manufactured surfaces, and impact of extended exposure to boiling heat transfer
Emir, Tolga; Arık, Mehmet; Arık, Mehmet; Başol, Altuğ Melik; Çelik, M.; Department of Mechanical Engineering
The ever-increasing population and rising living standards require the development of high-performance, efficient, and safe technologies. Compact thermal systems face challenges due to limited space availability, while waste heat reduction and efficient cooling become crucial. Traditional thermal management methods have already reached their limits, and alternative technologies utilizing phase-change materials, heat pipes, or miniature/mini chambers are required to overcome heat removal limitations and achieve effective cooling in compact systems. Pool boiling heat transfer offers high-performance cooling opportunities for thermal problems especially electronics limited with high heat fluxes, and have been extensively studied over the last eight decades. This sensitive cooling method is influenced by a vast number of parameters, while the interplay of these parameters introduces a significant level of intricacy, making accurate predictions challenging. The predictive expressions for a broader spectrum of operating conditions, fluid chemical composition, or even surface topography are still inefficient in order to forecast the fundamental heat transfer measures of critical heat flux (CHF) and heat transfer coefficient (HTC). Indeed, this study aims to enhance boiling heat transfer (BHT) by incorporating artificial cavities through microdrill manufacturing and additive manufactured (AM) microchannel surfaces. Additionally, the development of oxide layers and their impact on surface performance are investigated. The boiling performance of surfaces with micro-drilled artificial cavities is analyzed under different operating conditions. In addition, the study examines the effect of microchannel geometries, specifically rectangular, V-shape, and inverse V-shape channels fabricated using laser powder bed fusion, on BHT. The influence of surface roughness on heat transfer is compared between additive manufactured surfaces and conventionally polished surfaces. Oxide layer development and its effect on heat transfer are explored on copper substrates under various operating scenarios. In order to conduct the experiments, an in-house manufactured and fully automated boiling setup, which resist to high temperature and pressure was constructed. De-ionized (DI) water and 3M™ Novec™ hydrofluoroether (HFE-7100) Engineered Fluid were preferred as working liquids to be suitable for the cooling of electronic systems priorly. In this study, the tests were conducted at between 0 - 10 K subcooling temperatures and 1 - 1.5 bar pressures. Experimental analysis includes contact angle measurements, confocal microscopy, scanning electron microscopy, X-ray diffraction, and high-speed imaging for bubble formation. These investigations contribute to understanding and improving heat transfer performance in boiling systems.
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Impact of oxidation on pool boiling heat transfer performance over flat plates exposed to extended operating conditions
(IEEE, 2023) Emir, Tolga; Budaklı, Mete; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Emir, Tolga; Budaklı, Mete
Phase change heat transfer is utilized in a number of high heat flux applications. In order to ensure reliable functionality at moderate temperatures, one has to guarantee a stable long-term operation. In this experimental study, boiling heat transfer (BHT) performances of several substrates under continuous operation at pre-determined heat fluxes were studied. Tests were performed on bare copper surfaces in saturated deionized (DI) water and HFE-7100 under atmospheric conditions. Measurements were conducted at heat fluxes of 30 W/cm2 and 60 W/cm2 for DI water, whereas at 6 W/cm2 and 12 W/cm2 for HFE-7100. In order to identify the temporal change in surface conditions for each substrate, subsequently repeated heating tests were conducted before a 24-hour operation at constant heat flux. Besides the computation of heat transfer coefficients, contact angle (CA) measurements, high-resolution microscopic images, and scanning electron microscope (SEM) analyses were carried out to characterize the impact over surfaces. Microscopic images showed that the use of DI water leads to an intensified oxidization on the test surface. HFE-7100 does not allow oxide layer formation on the copper surfaces. Critical heat flux (CHF) at the surface operated only at 60 W/cm2 in DI water increased, while the boiling curves shifted to the left by decreasing surface temperatures over time. The surfaces immersed in HFE-7100 showed a great consistency with preliminary tests on heat transfer and repeatability tests.
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Parametric effects on pool boiling heat transfer and critical heat flux: A critical review
(ASME, 2022-12) Emir, Tolga; Ourabi, H.; Budaklı, M.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Emir, Tolga
Pool boiling heat transfer offers high-performance cooling opportunities for thermal problems of electronics limited with high heat fluxes. Therefore, many researchers have been extensively studying over the last six decades. This paper presents a critical literature review of various parametric effects on pool boiling heat transfer and critical heat flux (CHF) such as pressure, subcooling, surface topography, surface orientation, working fluid, and combined effects. To achieve an optimal heat removal solution for a particular problem, each of these parameters must be understood. The governing mechanisms are discussed separately, and various options related to the selection of appropriate working fluids are highlighted. A broad summary of correlations developed until now for predicting CHF is presented with their ranges of validity. While proposed correlations for predicting CHF have been quite promising, they still have a considerable uncertainty (625%). Finally, a correlation proposed by Professor Avram Bar-Cohen and his team (thermal management of electronics (TME) correlation) is compared with the experimental dataset published in previous studies. It shows that the uncertainty band can be further narrowed down to 612.5% for dielectric liquids by using TME correlation. Furthermore, this correlation has been enhanced to predict CHF values underwater above 50 W/cm2 by applying a genetic algorithm, and new perspectives for possible future research activities are proposed.
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Pool boiling heat transfer in dielectric fluids and impact of surfaces on the repeatability
(ASME, 2022) Emir, Tolga; Yazıcı, Yakup Gökalp; Budaklı, M.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Emir, Tolga; Yazıcı, Yakup Gökalp
This study presents an experimental investigation on the nucleate boiling heat transfer (NBHT) in deionized (DI) water and HFE-7100 on bare copper surfaces. The experiments were performed under atmospheric condition at 0 and 10 K subcooling levels. The primary objective was to understand the effect of fluid property on critical heat flux (CHF) and heat transfer performance, where the occurrence of surface oxidation over the entire set of experiments were investigated for a range of operating conditions. In order to determine the onset and development of the latter phenomenon, experiments for the complete boiling process have been repeated three times under similar conditions. A detailed visualization study with a highspeed camera has been utilized to capture the dynamics of bubble formation and departure. Additionally, high-resolution microscopic images were captured, and contact angle measurements were used to express the experimental results conveniently. Microscopic images showed that using DI water leads to an intensified oxidization on the heater surface, while HFE-7100 yields a minor occurrence of oxide layer on the copper surfaces. The results indicated that CHF values remain constant for water at 0 K; however, a remarkable increase was observed for 10 K subcooling from the first to third run of successive measurements.