Scopus İndeksli Yayınlar Koleksiyonu / Scopus Indexed Publications Collection
Permanent URI for this collectionhttps://hdl.handle.net/11147/7148
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Article Citation - WoS: 8Citation - Scopus: 10Spray Analysis of Biodiesels Derived From Various Biomass Resources in a Constant Volume Spray Chamber(American Chemical Society, 2022) Ulu, Anılcan; Yıldız, Güray; Rodriguez, Alvaro Diez; Özkol, ÜnverThis research aimed to analyze the spray characteristics of various biodiesels, which have rarely been investigated in terms of spray analysis in the literature compared to fossil diesel. For this purpose, four different methyl ester-type biodiesels were produced from canola, corn, cottonseed, and sunflower oils. These feedstocks were selected due to their wide availability in Turkey and being among the significant resources for biodiesel production. Measured physical properties of biodiesel samples showed that biodiesel fuels had, on average, 1.7 to 1.9 times higher viscosities, 5.3 to 6.6% larger densities, and 37 to 39.1% higher contact angle values than the reference diesel fuel. Spray characteristics of all fuels were experimentally examined in a constant volume spray chamber under chamber pressures of 0, 5, 10, and 15 bar and injection pressures of 600, 800, and 1000 bar. All tested biodiesels performed, on average, 3 to 20% longer spray penetration lengths, 5 to 30% narrower spray cone angles, and 5-18% lesser spray areas than the reference diesel fuel under chamber pressures of 5 and 10 bar. No significant differences occurred at 15 bar ambient pressure between biodiesels and diesel. In addition, analytical and empirical predictions showed that biodiesels had around 21.2-35.1% larger SMD values and approximately 7% lower air entrainment.Article Citation - WoS: 9Citation - Scopus: 11Experimental Investigation of Spray Characteristics of Ethyl Esters in a Constant Volume Chamber(Springer, 2022) Ulu, Anılcan; Yıldız, Güray; Özkol, Ünver; Rodriguez, Alvaro DiezAbstract: Biodiesels are mainly produced via the utilization of methanol in transesterification, which is the widespread biodiesel production process. The majority of this methanol is currently obtained from fossil resources, i.e. coal and natural gas. However, in contrast with methanol, biomass-based ethanol can also be used to produce biodiesels; this could allow the production line to become fully renewable. This study aimed to investigate the spray characteristics of various ethyl ester type biodiesels derived from sunflower and corn oils in comparison to methyl esters based on the same feedstocks and reference petroleum-based diesel. Spray penetration length (SPL) and spray cone angle (SCA) were experimentally evaluated in a constant volume chamber allowing optical access, under chamber pressures of 0, 5, 10 and 15 bar and injection pressures of 600 and 800 bar. Sauter mean diameter (SMD) values were estimated by using an analytical correlation. Consequently, ethyl esters performed longer SPL (2.8–20%) and narrower SCA (5.1–19%) than diesel under ambient pressures of 5 and 10 bar. Although the SMD values of ethyl esters were 48% higher than diesel on average, their macroscopic spray characteristics were very similar to those of diesel under 15 bar chamber pressure. Moreover, ethyl esters were found to be very similar to methyl esters in terms of spray characteristics. The differences in SPL, SCA and SMD values for both types of biodiesels were lower than 4%. When considering the uncertainty (± 0.84%) and repeatability (±5%) ratios, the difference between the spray characteristics of methyl and ethyl esters was not major.Article Citation - WoS: 11Citation - Scopus: 11A Numerical Study on Determination of Volume Averaged Thermal Transport Properties of Metal Foam Structures Using X-Ray Microtomography Technique(Taylor & Francis, 2018) Çelik, Hasan; Mobedi, Moghtada; Nakayama, Akira; Özkol, ÜnverVolume averaged thermal transport properties of two metal foams with 10 and 20 PPI are obtained by using microtomography technique. The digital 3D structures of samples are generated in computer environment. The governing equations are solved for the entire domain and the volume averaged technique is used to determine interfacial heat transfer coefficient, longitudinal and transverse thermal dispersion conductivity. The study is performed for the pore scale Reynolds number from 100 to 600. The obtained results are within the ranges of the suggested correlations in literature. The present study supports the correlations suggested by Calmidi and Mahajan (2000) and Zhang et al. (2016).Article Citation - WoS: 6Citation - Scopus: 6A Study on Numerical Determination of Permeability and Inetia Coefficient of Aluminum Foam Using X-Ray Microtomography Techniques: Focus on Inspection Methods for Reliability (permeability and Inertia Coefficient by Tomography)(Begell House, 2019) Mobedi, Moghtada; Nakayama, Akira; Özkol, Ünver; Çelik, HasanThe volume-averaged (i.e., macroscopic) transport properties such as permeability and inertia coefficient of two aluminum foams with 10 and 20 pores per inch (PPI) pore density are found using microtomography images. It is shown that a comparison between the numerical values and the experimental results may not be sufficient to prove the correctness of the obtained results. Hence, in addition to traditional validation methods such as grid independency and comparison with reported results in literature, further inspections such as (a) checking the development of flow, (b) inspection of Darcy and non-Darcy regions, (c) conservation of flow rate through the porous media, (d) sufficiency of number of voxels in the narrow throats, and (e) observation of transverse velocity gradients in pores for high and low Reynolds numbers can be performed to further validate the achieved results. These techniques have been discussed and explained in detail for the performed study. Moreover, the obtained permeability and inertia coefficient values are compared with 19 reported theoretical, numerical, and experimental studies. The maximum deviation between the present results and the reported studies for 10 PPI is below 25%, while for 20 PPI it is below 28%.Article Citation - WoS: 1Citation - Scopus: 2Off-Design Analysis of Transonic Bypass Fan Systems Using Streamline Curvature Through-Flow Method(Walter de Gruyter GmbH, 2019) Acarer, Sercan; Özkol, ÜnverThe two-dimensional streamline curvature through-flow modeling of turbomachinery is still a key element for turbomachinery preliminary analysis. Basically, axisymmetric swirling flow field is solved numerically. The effects of blades are imposed as sources of swirl, work input/output and entropy generation. Although the topic is studied vastly in the literature for compressors and turbines, combined modeling of the transonic fan and the downstream splitter of turbofan engine configuration, to the authors' best knowledge, is limited. In a prior study, the authors presented a new method for bypass fan modeling for inverse design calculations. Moreover, new set of practical empirical correlations are calibrated and validated. This paper is an extension of this study to rapid off-design analysis of transonic by-pass fan systems. The methodology is validated by two test cases: NASA 2-stage fan and GE-NASA bypass fan case. The proposed methodology is a simple extension for streamline curvature method and can be applied to existing compressor methodologies with minimum numerical effort.Article Citation - WoS: 6Citation - Scopus: 7Aerodynamic Optimization of Through-Flow Design Model of a High By-Pass Transonic Aero-Engine Fan Using Genetic Algorithm(SAGE Publications Inc., 2018) Kor, Orçun; Acarer, Sercan; Özkol, ÜnverThis study deals with aerodynamic optimization of a high by-pass transonic aero-engine fan module in a through-flow inverse design model at cruise condition. To the authors’ best knowledge, although the literature contains through-flow optimization of the simplified cases of compressors and turbines, an optimization study targeting the more elaborate case of combined transonic fan and splitter through-flow model is not considered in the literature. Such a through-flow optimization of a transonic fan, combined with bypass and core streams separated by an aerodynamically shaped flow splitter, possesses significant challenges to any optimizer, due to highly non-linear nature of the problem and the high number of constraints, including the fulfillment of the targeted bypass ratio. It is the aim of this study to consider this previously untouched area in detail and therefore present a more sophisticated and accurate optimization environment for actual bypass fan systems. An in-house optimization code using genetic algorithm is coupled with a previously developed in-house through-flow solver which is using a streamline curvature technique and a set of in-house calibrated empirical models for incidence, deviation, loss and blockage. As the through-flow models are the backbone of turbomachinery design, and great majority of design decisions are taken in this phase, such a study is assessed to result in significant guidelines to the gas turbine community.Article Citation - WoS: 7Citation - Scopus: 8An Extension of the Streamline Curvature Through-Flow Design Method for Bypass Fans of Turbofan Engines(SAGE Publications Inc., 2017) Acarer, Sercan; Özkol, ÜnverThe two-dimensional through-flow modeling of turbomachinery is still one of the most powerful tools available to the turbomachinery industry for aerodynamic design, analysis, and post-processing of test data due to its robustness and speed. Although variety of aspects of such a modeling approach are discussed in the publicly available literature for compressors and turbines, not much emphasis is placed on combined modeling of the fan and the downstream splitter of turbofan engines. The current article addresses this void by presenting a streamline curvature through-flow methodology that is suitable for inverse design for such a problem. A new split-flow method for the streamline solver, alternative to the publicly available analysis-oriented method, is implemented and initially compared with two-dimensional axisymmetric computational fluid dynamics on two representative geometries for high and low bypass ratios. The empirical models for incidence, deviation, loss, and end-wall blockage are compiled from the literature and calibrated against two test cases: experimental data of NASA two-stage fan and three-dimensional computational fluid dynamics of a custom-designed transonic fan stage. Finally, experimental validation against GE-NASA bypass fan case is accomplished to validate the complete methodology. The proposed method is a simple extension of streamline curvature method and can be applied to existing compressor methodologies with minimum numerical effortArticle Citation - WoS: 15Citation - Scopus: 16A Pore Scale Analysis for Determination of Interfacial Convective Heat Transfer Coefficient for Thin Periodic Porousmedia Undermixed Convection(Emerald Group Publishing Ltd., 2017) Çelik, Hasan; Mobedi, Moghtada; Manca, Oronzio; Özkol, ÜnverPurpose - The purpose of this study is to determine interfacial convective heat transfer coefficient numerically, for a porous media consisting of square blocks in inline arrangement under mixed convection heat transfer. Design/methodology/approach - The continuity, momentum and energy equations are solved in dimensionless form for a representative elementary volume of porous media, numerically. The velocity and temperature fields for different values of porosity, Ri and Re numbers are obtained. The study is performed for the range of Ri number from 0.01 to 10, Re number from 100 to 500 and porosity value from 0.51 to 0.96. Based on the obtained results, the value of the interfacial convective heat transfer coefficient is calculated by using volume average method. Findings - It was found that at low porosities (such as 0.51), the interfacial Nusselt number does not considerably change with Ri and Re numbers. However, for porous media with high Ri number and porosity (such as 10 and 0.51, respectively), secondary flows occur in the middle of the channel between rods improving heat transfer between solid and fluid, considerably. It is shown that the available correlations of interfacial heat transfer coefficient suggested for forced convection can be used for mixed convection for the porous media with low porosity (such as 0.51) or for the flow with low Ri number (such as 0.01). Originality/value - To the best of the authors' knowledge, there is no study on determination of interfacial convective heat transfer coefficient for mixed convection in porous media in literature. The present study might be the first study providing an accurate idea on the range of this important parameter, which will be useful particularly for researchers who study on mixed convection heat transfer in porous media, macroscopically.Article Citation - WoS: 100Citation - Scopus: 106Determination of Kozeny Constant Based on Porosity and Pore To Throat Size Ratio in Porous Medium With Rectangular Rods(Taylor and Francis Ltd., 2014) Özgümüş, Türküler; Mobedi, Moghtada; Özkol, ÜnverKozeny-Carman permeability equation is an important relation for the determination of permeability in porous media. In this study, the permeabilities of porous media that contains rectangular rods are determined, numerically. The applicability of Kozeny-Carman equation for the periodic porous media is investigated and the effects of porosity and pore to throat size ratio on Kozeny constant are studied. The continuity and Navier- Stokes equations are solved to determine the velocity and pressure fields in the voids between the rods. Based on the obtained flow field, the permeability values for different porosities from 0.2 to 0.9 and pore to throat size ratio values from 1.63 to 7.46 are computed. Then Kozeny constants for different porous media with various porosity and pore to throat size ratios are obtained and a relationship between Kozeny constant, porosity and pore to throat size ratio is constructed. The study reveals that the pore to throat size ratio is an important geometrical parameter that should be taken into account for deriving a correlation for permeability. The suggestion of a fixed value for Kozeny constant makes the application of Kozeny-Carman permeability equation too narrow for a very specific porous medium. However, it is possible to apply the Kozeny-Carman permeability equation for wide ranges of porous media with different geometrical parameters (various porosity, hydraulic diameter, particle size and aspect ratio) if Kozeny constant is a function of two parameters as porosity and pore to throat size ratios.Article Citation - WoS: 20Citation - Scopus: 26Effect of Urban Geometry on Pedestrian-Level Wind Velocity(Taylor and Francis Ltd., 2014) Arkon, Çelen Ayşe; Özkol, ÜnverThe orientation of the streets and the height of continuous buildings cut off summer breezes and the prevailing wind in Izmir, Turkey. Compared with the northern parts of Turkey, the summer period in Izmir is relatively hot, humid and long. Due to the dense urban structure and the expansion of hard surface materials, the temperature in the city centre is higher than this centre's surroundings and this effect is called the urban heat island. Consequently, pedestrian comfort in the city drops dramatically especially in locations where the wind flow is obstructed by buildings. In addition, natural ventilation through the building façades is weakened due to the low average wind speed in the streets. For better outdoor and indoor comfort the citizens in Izmir should benefit from the prevailing wind and summer breezes locally named imbat in the sea-land direction. Therefore, the existing situation is examined through the field study in order to understand the natural ventilation potential at the pedestrian level in the selected main streets in Izmir.