Mechanical Engineering / Makina Mühendisliği
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Article Pompa Tasarımının Geliştirilmesinde Hesaplamalı Akışkanlar Dinamiğinin Kullanılması(TMMOB Makina Mühendisleri Odası, 2006) Karamanoğlu, Yılmaz; Mobedi, Moghtada; Ertöz, A. ÖzdenBilgisayar teknolojisindeki hızlı gelişmeler ve sağladığı kolaylıklar, ürün tasarım ve geliştirilmesinde yönelik hazırlanan bilgisayar yazılımların kullanılmasını kaçınılmaz hale getirmiştir. Hesaplamalı akışkanlar dinamiği (Computational Fluid Dynamics-CFD) yazılımları ise özellikle akışkan ve ısı transferi ile ilgili olan ürünlerin analizlerinin yapılmasına ve performansının sınanmasında kullanılmaktadır. Bu programlar kullanılarak, süreklilik, momentum ve enerji denklemleri bilgisayar ortamında sayısal olarak çözülmekte, sıcaklık ve akış ile ilgili verilere ulaşılmakta ve değişik parametrelerin dağılımlarının (örneğin hız, basınç, sıcaklık ve benzeri parametrelerin dağılımlarının) elde edilmesine olanak sağlamaktadır. Bu makalede, pompa tasarımı ve tasarlanan pompanın denetlenmesinde hesaplamalı akışkanlar dinamiğinin kullanılması ele alınmıştır. Pompa tasarımı safhasında, tasarımcıların en çok merak ettiği konu, tasarlanan pompadaki akışın formu ve akış ile ilgili veri (debi, basınç, tork, verim) değerleridir. Bu makalede, pompalardaki akış için geçerli olan süreklilik, momentum, türbülans denklemleri ve sınır koşulları ile ilgili özet bilgiler verilmekte, çözüm yöntemleri ve doğru çözümler elde edilebilmesi için dikkat edilmesi gereken hususlar üzerine durulmaktadır. Çözüm ağının oluşturulması, çözümün elde edilmesi safhasında karşılaşılan zorluklar ve edinilen tecrübeler aktarılmıştır.Article Citation - WoS: 120Citation - Scopus: 136Numerical Study on Latent Thermal Energy Storage Systems With Aluminum Foam in Local Thermal Equilibrium(Elsevier, 2019) Buonomo, Bernardo; Çelik, Hasan; Ercole, Davide; Manca, Oronzio; Mobedi, MoghtadaThe paper analyzes the behavior of a Latent Heat Thermal Energy Storage system (LHTES) with a Phase Change Material (PCM), with and without aluminum foam. A numerical investigation in a two-dimensional domain is accomplished to investigate on the system thermal evolution. The enthalpy-porosity method is used to describe the PCM melting. The open-celled aluminum foam is described as a porous medium by means of the Darcy-Forchheimer law. A hollow cylinder represents the considered thermal energy storage and it consists of the enclosure between two concentric shell tubes. The external surface of the internal tube is at assigned temperature with a value greater than the melting PCM temperature, while the other surfaces are adiabatic. Local thermal equilibrium (LTE) is numerically adopted for modelling the heat transfer between the PCM and the solid matrix in aluminum foam. In the case with metal foam, simulations for different porosities are performed. A comparison in term of liquid fraction, average temperature of the system, temperature fields, stream function and a performance parameter are made between the clean case and porous assisted case for the different porosities. A scale analysis is developed for evaluating the time and the melting zone in different regimes (i.e. conduction, mixed conduction-convective and convective) during the melting processes of the PCM in porous media. Numerical simulation shows that aluminum foam increases overall heat transfer by a magnitude of two, with respect to the clean case.Conference Object Citation - Scopus: 2Numerical Investigation on the Effect of Aluminum Foam in a Latent Thermal Energy Storage(ASME, 2016) Buonomo, Bernardo; Ercole, Davide; Manca, Oronzio; Çelik, Hasan; Mobedi, MoghtadaIn this paper, a numerical investigation on Latent Heat Thermal Energy Storage System (LHTESS) based on a phase change material (PCM) is accomplished. The geometry of the system under investigation is a vertical shell and tube LHTES made with two concentric aluminum tubes. The internal surface of the hollow cylinder is assumed at a constant temperature above the melting temperature of the PCM to simulate the heat transfer from a hot fluid. The other external surfaces are assumed adiabatic. The phase change of the PCM is modeled with the enthalpy porosity theory while the metal foam is considered as a porous media that obeys to the Darcy-Forchheimer law. The momentum equations are modified by adding of suitable source term which it allows to model the solid phase of PCM and natural convection in the liquid phase of PCM. Both local thermal equilibrium (LTE) and local thermal non-equilibrium (LTNE) models are examined. Results as a function of time for the charging phase are carried out for different porosities and assigned pore per inch (PPI). The results show that at high porosity the LTE and LTNE models have the same melting time while at low porosity the LTNE has a larger melting time. Moreover, the presence of metal foam improves significantly the heat transfer in the LHTES giving a very faster phase change process with respect to pure PCM, reducing the melting time more than one order of magnitude.Article Citation - WoS: 3Validation on of Local Thermal Equilibrium and Uniform Pressure Assumptions for an Isobaric Adsorption Process in an Adsorbent Bed(Türk Isı Bilimi ve Tekniği Derneği, 2016) Gediz İliş, Gamze; Mobedi, Moghtada; Ülkü, SemraBu çalışmanın amacı, adsorbent yatakta ısı ve kütle transferini analiz etmek için kullanılan yerel ısıl denge ve sabit basınç yaklaşımı varsayımların geçerliliğini araştırmaktır. İçerisinde silika jel partikülleri içeren bir yatak ile su kabı olan bir deney düzeneği tasarlanmış ve adsorpsiyon sürecinde yatağın içinde farklı yerlerde yerel sıcaklık ve basınç ölçülmüştür. Ayrıca, sabit basınç yaklaşımı ve yerel ısıl denge varsayımlara dayalı ısı ve kütle transferi denklemleri çözülmüştür. Sayısal sonuçlar, ilgili deneysel sonuçlarla karşılaştırılmış ve aralarında oldukça iyi bir uyum tespit edilmiştir. Gerçekleştirilen karşılaştırmaya dayanarak, incelenen yatak için iki önemli sonuç şu şekildedir: a) katı madde ve su buharı arasında yerel ısıl denge bulunmaktadır, b) bir yatak içinde parçacıklar arası kütle transferi direnci ihmal edilebilir düzeyde olup konsantrasyonunun ve sıcaklığın belirlenmesi için sabit basınç yaklaşımı geçerlidir. Ayrıca, bu çalışmada sunulan deneysel sonuçlar diğer araştırmacıların sayısal çalışmalarının geçerliliğini doğtulamak için değerli veriler sağlayacaktır.Article Citation - WoS: 12Citation - Scopus: 12Numerical Determination of Interfacial Heat Transfer Coefficient for an Aligned Dual Scale Porous Medium(Emerald Group Publishing, 2018) Sabet, Safa; Mobedi, Moghtada; Barışık, Murat; Nakayama, AkiraPurpose Fluid flow and heat transfer in a dual scale porous media is investigated to determine the interfacial convective heat transfer coefficient, numerically. The studied porous media is a periodic dual scale porous media. It consists of the square rods which are permeable in an aligned arrangement. It is aimed to observe the enhancement of heat transfer through the porous media, which is important for thermal designers, by inserting intra-pores into the square rods. A special attention is given to the roles of size and number of intra-pores on the heat transfer enhancement through the dual scale porous media. The role of intra-pores on the pressure drop of air flow through porous media is also investigated by calculation and comparison of the friction coefficient. Design/methodology/approach To calculate the interfacial convective heat transfer coefficient, the governing equations which are continuity, momentum and energy equations are solved to determine velocity, pressure and temperature fields. As the dual scale porous structure is periodic, a representative elementary volume is generated, and the governing equations are numerically solved for the selected representative volume. By using the obtained velocity, pressure and temperature fields and using volume average definition, the volume average of aforementioned parameters is calculated and upscaled. Then, the interfacial convective heat transfer coefficient and the friction coefficient is numerically determined. The interparticle porosity is changed between 0.4 and 0.75, while the intraparticle varies between 0.2 and 0.75 to explore the effect of intra-pore on heat transfer enhancement. Findings The obtained Nusselt number values are compared with corresponding mono-scale porous media, and it is found that heat transfer through a porous medium can be enhanced threefold (without the increase of pressure drop) by inserting intraparticle pores in flow direction. For the porous media with low values of interparticle porosity (i.e. = 0.4), an optimum intraparticle porosity exists for which the highest heat transfer enhancement can be achieved. This value was found around 0.3 when the interparticle porosity was 0.4. Research limitations/implications The results of the study are interesting, especially from heat transfer enhancement point of view. However, further studies are required. For instance, studies should be performed to analyze the rate of the heat transfer enhancement for different shapes and arrangements of particles and a wider range of porosity. The other important parameter influencing heat transfer enhancement is the direction of pores. In the present study, the intraparticle pores are in flow direction; hence, the enhancement rate of heat transfer for different directions of pores must also be investigated. Practical implications The application of dual scale porous media is widely faced in daily life, nature and industry. The flowing of a fluid through a fiber mat, woven fiber bundles, multifilament textile fibers, oil filters and fractured porous media are some examples for the application of the heat and fluid flow through a dual scale porous media. Heat transfer enhancement. Social implications The enhancement of heat transfer is a significant topic that gained the attention of researchers in recent years. The importance of topic increases day-by-day because of further demands for downsizing of thermal equipment and heat recovery devices. The aim of thermal designers is to enhance heat transfer rate in thermal devices and to reduce their volume (and/or weight in some applications) by using lower mechanical power for cooling. Originality/value The present study might be the first study on determination of thermal transport properties of dual scale porous media yielded interesting results such as considerable enhancement of heat transfer by using proper intraparticle channels in a porous medium.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: 12Citation - Scopus: 13A General Expression for the Stagnant Thermal Conductivity of Stochastic and Periodic Structures(The American Society of Mechanical Engineers(ASME), 2018) Bai, X.; Çelik, Hasan; Mobedi, Moghtada; Nakayama, AkiraA general expression has been obtained to estimate thermal conductivities of both stochastic and periodic structures with high-solid thermal conductivity. An air layer partially occupied by slanted circular rods of high-thermal conductivity was considered to derive the general expression. The thermal conductivity based on this general expression was compared against that obtained from detailed three-dimensional numerical calculations. A good agreement between two sets of results substantiates the validity of the general expression for evaluating the stagnant thermal conductivity of the periodic structures. Subsequently, this expression was averaged over a hemispherical solid angle to estimate the stagnant thermal conductivity for stochastic structures such as a metal foam. The resulting expression was found identical to the one obtained by Hsu et al., Krishnan et al., and Yang and Nakayama. Thus, the general expression can be used for both stochastic and periodic structures.Article Citation - WoS: 15Citation - Scopus: 15Enhancement of Heat Transfer in Partially Heated Vertical Channel Under Mixed Convection by Using Al2o3 Nanoparticles(Taylor and Francis Ltd., 2018) Çelik, Hasan; Mobedi, Moghtada; Manca, Oronzio; Buonomo, BernardoLaminar mixed convection in a two-dimensional symmetrically and partially heated vertical channel is investigated. The heaters are located on both walls and uniform temperature is applied on the heated sections. The number of heaters is considered as 1, 4, 8, and 10. Aluminum oxide/water nanofluid is considered as working fluid and the inlet velocity is uniform. The continuity, momentum and energy equations with appropriate boundary conditions are solved in dimensionless form, numerically. The study is performed for Richardson number of 0.01 and 10, Reynolds number of 100 and 500, and nanofluid volume fraction of 0% and 5%. Based on the obtained velocity and temperature distributions, the local and mean Nusselt number is calculated and plotted for different cases. The variation of the mean Nusselt number with the number of the heated portions is also discussed. It is found that the addition of nanoparticles into the base fluid increases mean Nusselt number but the rate of increase depends on Reynolds, Richardson numbers and number of heated portions. It is possible to increase mean Nusselt number 138% by increasing Reynolds number from 100 to 500, Richardson number from 0.01 to 10 and number of heated portions from 1 to 10 when volume fraction value is 5%.Article 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.