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Item Open Access Capillary boosting for enhanced heat pipe performance through bifurcation of grooves: Numerical assessment and experimental validation(2022-10) Saygan, S.; Akkus, Y.; Dursunkaya, Z.; Cetin, BarbarosShow more In this study, an enhanced heat pipe performance for grooved heat pipes has been demonstrated through capillary boosting with the introduction of the bifurcation of grooves. Wider grooves regularly branch to narrower grooves such that the total cross-sectional liquid flow area remains approximately the same. Following the computational framework drawn by a recently developed heat pipe analysis toolbox (H-PAT), we develop a numerical model for the heat pipes with tree-like groove architecture. Then we utilize the model to design a flat-grooved heat pipe with one step groove bifurcation at the evaporator. To verify our numerical findings, two heat pipes with and without groove bifurcation are manufactured and experimented under the same conditions. Experimental results show that the numerical model can predict the thermal performance quite accurately. The results reveal that groove bifurcation can be a viable option for a better thermal performance than that of heat pipes with standard grooved heat pipes with straight grooves which leads to at least 25% higher maximum heat transport capacity. The effect of number of branching on the temperature flattening across the heat pipe is also demonstrated for different evaporator lengths.Show more Item Embargo Comprehensive three-dimensional hydrodynamic and thermal modeling of steady-state operation of a flat grooved heat pipe(Elsevier Ltd, 2023-12-22) Gökçe, G.; Kurt, Cem; Odabaşı, G.; Dursunkaya, Z.; Çetin, BarbarosShow more Mathematical modeling of grooved heat pipes is a challenging task since multiple coupled physical phenomena such as phase change, free-surface flow and heat transfer are involved. Moreover, the fact that the shape of the liquid–vapor interface in the heat pipe is unknown a priori requires simultaneous determination of the interface variation as a part of the solution procedure, a capability currently not addressed in commercially available engineering CFD tools. In this study, a multi-dimensional and multi-scale computational model is presented to gain a comprehensive understanding of the underlying physics of grooved heat pipes. The computational model is based on an iterative scheme for the solution of 3D heat transfer and liquid flow, interface phase change heat transfer (evaporation and condensation) and shape of the interface. The model is implemented using three different methodologies two of which utilize commercial engineering CFD software. The results are verified for a problem previously studied in the literature which indicates the robustness of our computational approach.Show more Item Open Access Effect of design and operating parameters on the thermal performance of aluminum flat grooved heat pipes(Elsevier, 2018-03-05) Alijani, Hossein; Çetin, Barbaros; Akkuş, Y.; Dursunkaya, Z.Show more Four aluminum flat grooved heat pipes with groove widths of 0.2, 0.4, 0.8 and 1:6 mm are fabricated and the effect of filling ratio on the thermal performance is experimentally studied for four different heat flux values of 2.1, 3.2, 4.2 and 5:3W=cm2. An optimum filling ratio corresponding to each heat flux is determined where the heat pipe has the best thermal performance. Thermal performance of the heat pipes are evaluated using three indicators; (i) the temperature difference between the heat source and heat sink surfaces, (ii) the temperature difference between the peak system temperature and the temperature of the cooling ambient and (iii) heat pipe effectiveness defined as a temperature difference ratio under dry and operating conditions. A flow and evaporative mass scaling model is developed to interpret the experimental findings. Experimental results reveal that at the optimum point the heat pipe with the 0:4 mm groove width has the best thermal performance, and the heat pipe with the smallest 0:2 mm groove operates under dryout conditions even for the lowest heat flux, the reason of which is discussed based on interpretation of underlying phase change physics. Experiments reveal the existence of two operating regimes; with and without dryout in the grooves. Although higher heat loads can be carried under dryout conditions, a limit exists for the maximum heat flux where the pipe operates without the onset of dryout for a specific groove density.Show more Item Open Access Effect of liquid-vapor interaction on the thermal performance of a flat grooved heat pipe(Begell House Inc., 2023-03) Derebaşı, B.; Saygan, S.; Çetin, Barbaros; Dursunkaya, Z.Show more Flat grooved heat pipes (FGHP) are predominantly used in electronics cooling due to their ability to transfer high heat loads with small temperature differences and superior reliability. Modeling the underlying physics is challenging due to the presence of multiple simultaneous physical phenomena, including phase change, free surface, two-phase flow and heat transfer. In this study, a recently developed modeling tool H-PAT [1] is extended by including the interaction at the interface between the two phases of the FGHP's working fluid. The vapor phase is assumed to be saturated, eliminating the need to solve the energy equation for the vapor. Analytical solutions of liquid and vapor flows are used, and the steady-state energy equation is solved via a thermal resistance network to get the temperature distribution. Interface heat transfer is modeled using the fundamental findings of kinetic theory. The model is exercised to quantify the effect of vapor spacing on the thermal performance of a flat grooved heat pipe. The results show that liquid-vapor interaction on the interface enhances the evaporation performance in the micro-region, resulting in a more uniform temperature distribution.Show more Item Open Access Experimental thermal performance characterization of flat grooved heat pipes(Taylor and Francis, 2019) Alijani, Hossein; Çetin, Barbaros; Akkuş, Y.; Dursunkaya, Z.Show more The thermal characterization of aluminum flat grooved heat pipes is performed experimentally for different groove dimensions. Three heat pipes with groove widths of 0.2 mm, 0.4 mm, and 1.5 mm are used in the experiments. The effect of the amount of the working fluid is extensively studied for each groove width. The results reveal that, although all three succeed in dissipating the heat input through the phase change of the working fluid by continuous evaporation and condensation, the effectiveness of the heat transfer increases with reduced groove width. Furthermore, it is observed that there exists an optimum operating point, where the temperature difference between the heating and cooling sections is at a minimum, and the magnitude of this temperature difference is a strong function of the groove width. To the best of the authors’ knowledge, the combined effects of groove dimensions and the amount of the working fluid, from fully flooded to dry, is reported for the first time for aluminum flat grooved heat pipes.Show more Item Open Access Interplay of transport mechanisms during the evaporation of a pinned sessile water droplet(American Physical Society, 2021-07-27) Akdag, O.; Akkus, Y.; Çetin, Barbaros; Dursunkaya, Z.Show more Droplet evaporation has been intensively investigated in past decades owing to its emerging applications in diverse fields of science and technology. Yet the role of transport mechanisms has been the subject of a heated debate, especially the presence of Marangoni flow in water droplets. This work aims to draw a clear picture of the switching transport mechanisms inside a drying pinned sessile water droplet in both the presence and absence of thermocapillarity by developing a comprehensive model that accounts for all pertinent physics in both phases as well as interfacial phenomena at the interface. The model reveals a hitherto unexplored mixed radial and buoyant flow by shedding light on the transition from buoyancy induced Rayleigh flow to the radial flow causing the coffee ring effect. Predictions of the model excellently match previous experimental results across varying substrate temperatures only in the absence of Marangoni flow. When thermocapillarity is accounted for, strong surface flows shape the liquid velocity field during most of the droplet lifetime and the model starts to overestimate evaporation rates with increasing substrate temperature.Show more Item Open Access An iterative solution approach to coupled heat and mass transfer in a steadily fed evaporating water droplet(American Society of Mechanical Engineers, 2019) Akkuş, Y.; Çetin, Barbaros; Dursunkaya, Z.Show more Inspired by the thermoregulation of mammals via perspiration, cooling strategies utilizing continuously fed evaporating droplets have long been investigated in the field, yet a comprehensive modeling capturing the detailed physics of the internal liquid flow is absent. In this study, an innovative computational model is reported, which solves the governing equations with temperature-dependent thermophysical properties in an iterative manner to handle mass and heat transfer coupling at the surface of a constant shape evaporating droplet. Using the model, evaporation from a spherical sessile droplet is simulated with and without thermocapillarity. An uncommon, nonmonotonic temperature variation on the droplet surface is captured in the absence of thermocapillarity. Although similar findings were reported in previous experiments, the temperature dip was attributed to a possible Marangoni flow. This study reveals that buoyancy-driven flow is solely responsible for the nonmonotonic temperature distribution at the surface of an evaporating steadily fed spherical water droplet.Show more Item Open Access Modeling of evaporation from a sessile constant shape droplet(ASME, 2017) Akkuş, Y.; Çetin, Barbaros; Dursunkaya, Z.Show more In this study, a computational model for the evaporation from a sessile liquid droplet fed from the center to keep the diameter of the droplet constant is presented. The continuity, momentum and energy equations are solved with temperature dependent thermo-physical properties using COMSOL Multi-physics. At the surface of the droplet, convective heat and evaporative mass fluxes are assigned. Since the flow field is affected by evaporative flux, an iterative scheme is built and the computation is automated using COMSOL-MATLAB interface. Correlations are implemented to predict the convective heat transfer coefficients and evaporative flux. Three different wall temperatures are used in simulations. The results show that the flow inside the droplet is dominated by buoyancy when the effect of the thermo-capillarity is neglected. The resulting flow generates a circulation pattern emerging from the entrance to the apex, along the surface of the droplet to the bottom heated wall and back to the entrance.Show more Item Open Access Performance assessment of commercial heat pipes with sintered and grooved wicks under natural convection(TIBTD, 2019) Atay, Atakan; Sarıarslan, Büşra; Kuşçu, Yiğit F.; Saygan, S.; Akkuş, Y.; Gürer, A. T.; Çetin, Barbaros; Dursunkaya, Z.Show more Heat pipes are widely used in thermal management of high heat flux devices due to their ability of removing high heat loads with small temperature differences. While the thermal conductivity of standard metal coolers is approximately 100–500 W/m.K, effective thermal conductivities of heat pipes, which utilize phase-change heat transfer, can reach up to 50,000 W/m.K. In industrial applications, commercially available heat pipes are commonly preferred by thermal engineers due to their low cost and versatility. Thermal performance of a heat pipe is functions of heat pipe type and operating conditions. Selection of the appropriate heat pipe complying with the operating conditions is critical in obtaining satisfactory thermal management. One key point for the utilization of heat pipes is to avoid dryout operation condition in which heat pipes operate no more at the desired heat transport capacity. In the current study, the performance of cylindrical heat pipes with sintered and grooved wick structures, which are among the most commonly used types, is experimentally tested at different heat loads, gravitational orientations and ambient temperatures. Dryout limits of the heat pipes are determined and the relationship between the dryout onset and operating conditions is elucidated. The results reported in the present study are expected to guide thermal engineers for the proper selection and operation of conventional heat pipes.Show more Item Open Access A theoretical framework for comprehensive modeling of steadily fed evaporating droplets and the validity of common assumptions(Elsevier, 2020) Akkuş, Y.; Çetin, Barbaros; Dursunkaya, Z.Show more A theoretical framework is established to model the evaporation from continuously fed droplets, promising tools in the thermal management of high heat flux electronics. Using the framework, a comprehensive model is developed for a hemispherical water droplet resting on a heated flat substrate incorporating all of the relevant transport mechanisms: buoyant and thermocapillary convection inside the droplet and diffusive and convective transport of vapor in the gas domain. At the interface, mass, momentum, and thermal coupling of the phases are also made accounting for all pertinent physical aspects including several rarely considered interfacial phenomena such as Stefan flow of gas and the radiative heat transfer from interface to the surroundings. The model developed utilizes temperature dependent properties in both phases including the density and accounts for all relevant physics including Marangoni flow, which makes the model unprecedented. Moreover, utilizing this comprehensive model, a nonmonotonic interfacial temperature distribution with double temperature dips is discovered for a hemispherical droplet having internal convection due to buoyancy in the case of high substrate temperature. Proposed framework is also employed to construct several simplified models adopting common assumptions of droplet evaporation and the computational performance of these models, thereby the validity of commonly applied simplifying assumptions, are assessed. Benchmark simulations reveal that omission of gas flow, i.e. neglecting convective transport in gas phase, results in the underestimation of evaporation rates by 23–54%. When gas flow is considered but the effect of buoyancy is modeled using Boussinesq approximation instead of assigning temperature dependent density throughout the gas domain, evaporation rate can be underestimated by up to 16%. Deviation of simplified models tends to increase with increasing substrate temperature. Moreover, presence of Marangoni flow leads to larger errors in the evaporation rate prediction of simplified models.Show more Item Open Access Two-dimensional computational modeling of thin film evaporation(Elsevier Masson, 2017) Akkuş, Y.; Tarman, H. I.; Çetin B.; Dursunkaya, Z.Show more A considerable amount of the evaporation originates from the close vicinity the three-phase contact line in an evaporating extended meniscus due to the low thermal resistance across the ultra thin film. Evaporation taking place within the thin film region is commonly modeled using the uni-directional flow assumption of the liquid following the lubrication approximation. Although the uni-directional flow based models may yield practically reasonable results in terms of the cumulative quantities such as total evaporation rate, the underlying physics of the problem cannot be explained solely by uni-directional flow, especially when the dominant transverse liquid motion is considered near the close proximity of the contact line. The present study develops a solution methodology to enable the solution of steady, incompressible, 2-D conservation of mass and linear momentum equations for the liquid flow in an evaporating thin film. Solution methodology includes the coupling of an uni-directional solver with high precision numerics, a higher order bi-directional spectral element solver and a finite element solver. The novelty of the present study is that steady, 2-D conservation of mass and linear momentum equations are considered in the modeling of thin film evaporation without the exclusion of any terms in the conservation equations. © 2017 Elsevier Masson SASShow more