267 research outputs found
Experimental investigation of the effects of compressor types on the performance of an automobile air conditioning system using R1234yf
No full textIn this study, the performance of R1234yf as an alternative to R134a has been experimentally investigated in an experimental automobile air conditioning (AAC) system with variable (VCC) and fixed (FCC) capacity compressors. The AAC system was equipped with original components and instruments for mechanical measurements. The experiments were conducted for both types of compressors and refrigerants at various compressor speeds, air temperatures entering the evaporator and condenser and air speeds passing over them. The performance parameters of the AAC system have been evaluated by performing energy and exergy analysis based on the experimental data, and the results were presented in the form of comparative graphics. The experimental results show that the VCC compressor limits the refrigerant flow rate when the capacity control system is activated due to the decrease in the evaporator and condenser air inlet temperatures and the increase in the compressor speed. It was found that the coefficients of performance (COPs) of the R1234yf AAC system with VCC and FCC were approximately 13.6% and 20.1% lower than those of the R134a system with VCC and FCC, respectively. For both refrigerants, in the AAC system with VCC, the total exergy destruction rate per unit cooling capacity was 18.2% − 47.5% lower compared to the system with FCC. The R1234yf AAC system experiences lower exergy destruction in the evaporator and condenser compared to the R134a system, while it practices higher exergy destruction in the compressor and expansion device in comparison to the R134a system © 2023 Elsevier Ltd and II
Experimental study on the impact of indoor unit airflow velocity on the performance of an automotive heat pump system with a suction line heat exchanger
No full textThis study aimed to investigate the effect of changing the indoor unit air flow rate on the performance of an automobile heat pump with a suction line heat exchanger. Using a four-way valve, the automotive heat pump system was developed by reversing the refrigerant direction in the automobile air conditioning system, excluding the compressor. A suction line heat exchanger was added to the test system to enhance heat transfer between the liquid and suction lines of the automotive heat pump system. Performance comparisons were first performed for R134a and R1234yf by disabling the suction line heat exchanger. Then, the suction line heat exchanger was activated for R1234yf, and the tests were repeated. Performance comparisons were made for two different compressor speeds and three different indoor unit airflow speeds. It was found that using the heat exchanger in R1234yf operations improved the heating capacity, compressor discharge temperature and coefficient of performance by approximately 1.8 %, 5.1 % and 5.9 %, respectively. The heating capacity of the heat pump system using R134a, R1234yf, and R1234yf with the suction line heat exchanger was determined to be in the range of 2.46–3.29 kW, 2.35–3.04 kW, and 2.39–3.11 kW, respectively. An increase in the airflow speed of the indoor unit from 1.4 m s−1 to 3.2 m s−1 resulted in an average decrease of approximately 12.3 % in the compressor discharge temperature. In contrast, the heating capacity and coefficient of performance increased by approximately 11.8 % and 14.4 % on average, respectively, for R1234yf operations with the heat exchanger. This study revealed that by optimizing the air flow rate in the R1234yf heat pump system with a suction line heat exchanger, improvements in the heating capacity and coefficient of performance can be achieved, thus providing better thermal comfort in the passenger compartment
Otomobil klima sisteminde R134a yerine R1234yf kullanımının teorik ve deneysel incelenmesi
No full text06.03.2018 tarihli ve 30352 sayılı Resmi Gazetede yayımlanan “Yükseköğretim Kanunu İle Bazı Kanun Ve Kanun Hükmünde Kararnamelerde Değişiklik Yapılması Hakkında Kanun” ile 18.06.2018 tarihli “Lisansüstü Tezlerin Elektronik Ortamda Toplanması, Düzenlenmesi ve Erişime Açılmasına İlişkin Yönerge” gereğince tam metin erişime açılmıştır.Bu çalışmada, R134a soğutucu akışkanına alternatif olan R1234yf soğutucu akışkanın bir otomobil klima sisteminde kullanımı teorik ve deneysel olarak araştırılmıştır. Deneysel kısımda, R1234yf ve R134a soğutucu akışkanı ile çalışabilen orijinal otomobil klima sistemi parçalarından oluşan deneysel otomobil klima sistemi laboratuvar ortamında kurularak çeşitli mekanik ve elektriksel ölçüm cihazlarıyla donatılmıştır. Sistemin soğutma çevrimi, evaporatör, kondenser, sıvı tankı, kompresör, genleşme elemanı ve iç ısı değiştiriciden oluşmaktadır. Oluşturulan deneysel otomobil klima sistemi anlık olarak kontrol yapabilen ve veri alabilen sistemler ile donatılmıştır. Deneysel sistemde karşılaştırma deneyleri, kompresör devrine, kondenser ile evaporatör giriş hava akım sıcaklıkları ve hava akım geçiş hızlarına bağlı olarak gerçekleştirilmiştir. Yapılan deneyler sonucu elde edilen verilere enerji ve ekserji analizleri uygulanarak R1234yf ve R134a soğutucu akışkanlı sistemlerin çeşitli performans parametreleri belirlenmiş ve grafikler halinde karşılaştırmalı olarak sunulmuştur. Elde edilen sonuçlara göre, kompresör devri ile evaporatör ve kondenser hava akım sıcaklıklarının artması sonucu her iki sistem için de soğutma kapasitesi, kompresör gücü, kondenserden atılan ısı, soğutucu akışkan kompresör çıkış sıcaklığı, soğutucu akışkan debisi ve çevrimde yok edilen toplam ekserji değerleri artar iken soğutma tesir katsayısının (STK) azalmakta olduğu görülmüştür. Evaporatör hava akım hızının artması ile her iki sistem için de soğutma kapasitesi, kompresör gücü, kondenserden atılan ısı, STK, soğutucu akışkan debisi, soğutucu akışkan kompresör çıkış sıcaklığı ve çevrimde yok edilen toplam ekserji değerlerinin artmakta olduğu görülmüştür. Kondenser hava akım hızının artması ile her iki sistem için de soğutma kapasitesi, kondenserden atılan ısı ve STK artmakta iken kompresör gücü, soğutucu akışkan debisi, soğutucu akışkan kompresör çıkış sıcaklığı ve çevrimde yok edilen toplam ekserji değerlerinin azalmakta olduğu görülmüştür. R1234yf soğutucu akışkanlı sistemin R134a soğutucu akışkanlı sisteme göre %1–13 arasında daha az soğutma kapasitesine, %7–12 arasında daha düşük STK ile 4–16 C arası daha düşük kompresör çıkış sıcaklığına sahip iken %8–18 arasında daha fazla soğutucu akışkan debisine sahip olduğu görülmüştür. Her iki sisteme eklenen iç ısı değiştiricisi soğutma kapasitesi, kondenserden atılan ısı, STK, soğutucu akışkan debisi, soğutucu akışkan kompresör çıkış sıcaklığını genel olarak azaltır iken çevrimde yok edilen toplam ekserji değerlerini arttırmakta olduğu görülmüştür. Bu çalışmanın teorik kısmında öncelikli olarak R1234yf ve R134a soğutucu akışkanların soğutma çevriminin çeşitli noktalarındaki termodinamik özellikleri polinom denklemler halinde modellenerek soğutma çevrimi termodinamik simülasyon programı oluşturulmuştur. Daha sonra, deneysel veriler ve soğutma çevrimi elemanlarının özelliklerinden faydalanılarak bu program ilerletilmiştir. Sonuçta, kompresör devri ile evaporatör ve kondenser hava akım sıcaklıkları ile hızlarına bağlı olarak R1234yf ve R134a kullanan sistemlerin performans değerlerini ortaya koyan bir simülasyon programı elde edilmiştir. Simülasyon sonuçları ile deneysel sonuçlar arasında yaklaşık % 2-10 arasında birbirine yakın sonuçlar elde edilmiştir.In this study, the use of R1234yf refrigerant in an automobile air conditioning system, which is an alternative to R134a refrigerant, was investigated both theoretically and experimentally. In the experimental part of the study, an experimental R134a and R1234yf automobile air conditioning system made up from original components has been set up in the laboratory and equipped with various mechanical and electrical instruments. The refrigeration circuit of the system consists of evaporator, condenser, liquid tank, compressor, expansion valve and internal heat exchanger. The experimental automobile air conditioning system has been equipped with systems that can acquire data and control the operation of the system continually. In the experimental system, comparative tests were performed by changing the compressor speed, condenser-evaporator inlet air flow temperatures and speeds of the air streams. By applying energy and exergy analyses to the experimental system, various performance parameters of the systems using R134a and R1234yf have been determined and presented in graphics comparatively. For both systems, the results show that the cooling capacity, compressor power, condenser heat rejection rate, compressor discharge temperature, refrigerant flow rate and the rate of total exergy destruction in the refrigeration circuit increase whereas coefficient of performance (COP) decreases on rising the compressor speed and temperatures of the evaporator-condenser air streams. Furthermore, the cooling capacity, compressor power, condenser heat rejection rate, COP, refrigerant flow rate, compressor discharge temperature and the rate of total exergy destruction in the circuit increase with the speed of the evaporator air stream for both systems. The cooling capacity, condenser heat rejection rate and COP increase with the speed of the condenser air stream while compressor power, refrigerant flow rate, compressor discharge temperature and the rate of total exergy destruction in the circuit decrease with it. It was determined that R1234yf system has a less cooling capacity (in the range of 1-13 %), less COP (in the range of 7-12 %) and less compressor discharge temperature (in the range of 4-16 oC) while it has more refrigerant flow rate (in the range of 8-18 %) than R134a system. When an internal heat exchanger is added, the cooling capacity, condenser heat rejection rate, COP, refrigerant flow rate and compressor discharge temperature decrease while the rate of total exergy destruction in the circuit increases for both refrigerant cases. In the theoretical part of the study, thermodynamic properties of each refrigerant at various points of the refrigeration circuit have been modelled as polynomial equations, and a thermodynamic simulation program of the refrigeration circuit was developed. Then, utilising experimental data and specifications of the refrigeration circuit components, this program was advanced. Consequently, a simulation program yielding the performance of R1234yf and R134a systems as functions of compressor speed as well as speeds and temperatures of the evaporator and condenser air streams has been obtained. The simulation and experimental results in about 2-10% of similar results were obtained
Designing preservation. Integrating the architectural project to UNESCO tools to tackle territorial fragility: the Tivoli case as a pilot experience
No full textThe research investigates the potential contribution of a design-based approach to the current institutional UNESCO safeguard tools concerning the relation core/territory, challenging the conservative role of buffer zones that lead to stalemate situations in terms of territorial development, especially in the Italian context. Through the analysis of case studies, a methodological approach is developed that focuses on the one hand on the use of design tools for the assessment of heritage-related territories (cognitive framework), and on the other hand on the use of design actions in the management strategies for valorisation (interpretation framework). These tools are tested on the relevant experimental case of Tivoli, seat of two major World Heritage sites, as a pilot experience of integration of design tools in safeguard and management strategies for the solution of territorial fragility in complex heritage-related contexts
On the relations between subgroups of a group and submodules of modules over group rings
No full textLet R be a commutative ring and G a finite group. In [1], using a structure on an R module M to make it an RG module, we find some relations between RG submodules and a subgroups. We also prove that for each normal subgroup H of G with an invertible IHI in R, there is a direct summand RG submodule of M
Retinal Vascular Changes in Patients with Chronic Obstructive Pulmonary Disease: An Optical Coherence Tomography Angiography Study
No full textÖzel okullardaki zaman çizelgeleme problemi için memetik algoritmalar
No full textThe aim of this study is to introduce a real-world timetabling problem that exists in some private schools in Turkey and to solve such problem instances utilizing memetic algorithms. Being a new type of problem and for privacy reasons, there is no real data available. Hence for benchmarking purposes, a random data generator has been implemented. Memetic algorithms (MAs) combining genetic algorithms and hill-climbing are applied to solve synthetic problem instances produced by this generator. Different types of recombination and mutation operators based on the hierarchical structure of the timetabling problem are proposed. A modified version of the violation directed hierarchical hill-climbing method (VDHC), introduced by A. Alkan and E. Ozcan, coordinates the process of 12 different low-level hill-climbing operators grouped in two distinct arrangements that attempt to resolve corresponding constraint violations. VDHC is an adaptive method advocating cooperation of hill-climbing operators. In addition, MAs with VDHC are compared with different versions of multimeme algorithms and pure genetic algorithms. Experimental results on synthetic benchmark data set indicate the success of the proposed MA.M.S. - Master of Scienc
Modeling of plastic deformation due to slip-its implications in characterizing fatigue crack growth thresholds and non-Schmid behavior in transforming alloys
No full textThis study will be divided into two main pillars in interrogating the microstructural barriers prevalent at the nanoscale: (i) the effect of twin and grain boundaries along with their contribution on the irreversibility of the crack-tip emitted slip under cyclic fatigue loading, (ii) the atomistic scale lattice resistance against glide motion of dislocations in a perspective of laying out the dislocation core-crystal structure and applied loading interplay in particular embracing the plastic behavior in ordered shape memory alloys. The primary goal of the current work is to provide physical insight for the implications of the slip-mediated plasticity in fatigue crack growth and non-Schmid behavior on both experimental and theoretical grounds.
In first part of the current study, the near threshold fatigue crack growth behavior of nanocrystalline Ni-2.89% wt. Co (Ni-Co) alloy with nanotwinned microstructure will be characterized in particular based on the contribution of microstructural variables such as the on-going crack-tip emitted dislocation and twin/grain boundary (i.e. of , and types) interactions, the pre-existing dislocation density and the characteristic dimensions of grain size, twin thickness and spacing. In order to accomplish this task, we architectured the microstructure of nanocrystalline Ni-Co alloy by conducting annealing treatments at various temperatures and promoted grain-twin coarsening as well as varying the pre-existing slip density. Furthermore, we conducted experiments on these engineered microstructures under uniaxial tension and cyclic fatigue loading employing Digital Image Correlation technique at different length scales. The monotonic tension experiments enables to characterize the contribution of microstructural variables on the mechanical response of Ni-Co alloy, such as ductility and strength. On the other hand, the cyclic fatigue crack growth experiments help identify the variation of crack growth behavior and threshold levels of Ni-Co alloy along with the architectured microstructures. The experimental measurements show that nanotwins hierarchically embedded in the microstructure of Ni-Co alloy promotes ductility and fatigue threshold in a profound fashion with decreasing characteristic dimensions.
Meanwhile, the primary focus is on nanocrystalline Ni-Co alloy, the current work has been put forward to establish a physical model informed by the multi-scale microstructural parameters which is capable of predicting the fatigue threshold levels of metallic materials devoid of empiricism. To that end, we simulated the interaction of crack-tip emitted slip and the grain/twin boundaries within the framework of Molecular Dynamics and characterized the on-going dislocation reactions as well as the crystalline resistance at the boundary against the glide of dislocations participating in these reactions. Subsequently, the effective threshold stress intensity factor range metric is predicted on theoretical grounds by incorporating physical parameters such as the friction stresses both inside the pristine crystal and at the grain/twin boundaries along with the glide geometry associated with the prevailing dislocation reactions into the continuum scale dislocation motion equations. The modelling efforts for cyclic glide motion of crack-tip emitted dislocations as a function of applied stress factor range , , provided a quantitative basis to determine the microstructure-sensitive crack threshold levels on theoretical grounds. The results indicate that coherent twin boundaries ( type ) impart superior fatigue properties to Ni-Co alloy compared to the less-coincident grain boundaries of and types. As a distinguishing finding of the present study, the increasing frequency of the grain and twin boundaries-linked with the grain size and twin spacing & thickness- are determined to promote the fatigue threshold levels in Ni-Co alloy.
In the second part of the study, the mechanisms governing the slip-mediated plasticity of the ordered shape memory alloys, particularly Fe3Al and NiTi, are focused on both experimental grounds (via Digital Image Correlation technique) and employing theoretical atomistic scale dislocation core simulations. The non-Schmid character of the plastic response profoundly governs on the functional performance of this class of alloys imparting tension-compression asymmetries and anisotropic glide resistance as a function of crystal orientation. To accomplish this task, the dislocation core structures are calculated employing Molecular Statics-Dynamics simulations and subsequently the interaction mechanisms of the non-planar dislocation core structure with applied stress tensor components are identified considering the corresponding crystal symmetries involved. The dislocation core shape that is governed by the atomistic scale disregistry distributions under applied loading is demonstrated to play a decisive role on the anisotropic glide resistance which results in deviations from the Schmid law. The theoretical predictions for the anisotropic glide resistance are demonstrated to be in well agreement with the high magnification experiments conducted on single crystals of these alloys. The current methodology followed enables us to build a comprehensive understanding for the non-Schmid glide behavior of dislocations in austenitic phase of Fe3Al and NiTi shape memory alloys considering the effects of both glide and non-glide stress components. Furthermore, generalized yield criteria for these materials are established extending atomistic scale core mechanics to the macro-scale crystal plasticity.Submission published under a 24 month embargo labeled 'U of I Access', the embargo will last until 2019-12-01The student, Sertan Alkan, accepted the attached license on 2017-12-05 at 13:43.The student, Sertan Alkan, submitted this Dissertation for approval on 2017-12-05 at 14:02.This Dissertation was approved for publication on 2017-12-06 at 10:46.DSpace SAF Submission Ingestion Package generated from Vireo submission #11864 on 2018-03-13 at 09:56:57Made available in DSpace on 2018-03-13T15:25:27Z (GMT). No. of bitstreams: 3
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Experimental study on the impact of indoor unit airflow velocity on the performance of an automotive heat pump system with a suction line heat exchanger
No full textThis study aimed to investigate the effect of changing the indoor unit air flow rate on the performance of an automobile heat pump with a suction line heat exchanger. Using a four-way valve, the automotive heat pump system was developed by reversing the refrigerant direction in the automobile air conditioning system, excluding the compressor. A suction line heat exchanger was added to the test system to enhance heat transfer between the liquid and suction lines of the automotive heat pump system. Performance comparisons were first performed for R134a and R1234yf by disabling the suction line heat exchanger. Then, the suction line heat exchanger was activated for R1234yf, and the tests were repeated. Performance comparisons were made for two different compressor speeds and three different indoor unit airflow speeds. It was found that using the heat exchanger in R1234yf operations improved the heating capacity, compressor discharge temperature and coefficient of performance by approximately 1.8 %, 5.1 % and 5.9 %, respectively. The heating capacity of the heat pump system using R134a, R1234yf, and R1234yf with the suction line heat exchanger was determined to be in the range of 2.46–3.29 kW, 2.35–3.04 kW, and 2.39–3.11 kW, respectively. An increase in the airflow speed of the indoor unit from 1.4 m s − 1 to 3.2 m s − 1 resulted in an average decrease of approximately 12.3 % in the compressor discharge temperature. In contrast, the heating capacity and coefficient of performance increased by approximately 11.8 % and 14.4 % on average, respectively, for R1234yf operations with the heat exchanger. This study revealed that by optimizing the air flow rate in the R1234yf heat pump system with a suction line heat exchanger, improvements in the heating capacity and coefficient of performance can be achieved, thus providing better thermal comfort in the passenger compartment
Prospective teachers' views and experiences with e-portfolios
No full textThe use of electronic portfolios (e-portfolios) is a burgeoning area ofresearch, with many teacher education programs utilising as botha product of and a process of reflective practice. This case studyaimed to explore prospective primary teachers’ (students) percep-tions, perspectives and experiences of the use of e-portfolios, focus-ing on the affordances and limitations of using e-portfolios in InitialTeacher Education (ITE). Eleven second-year undergraduate stu-dents from secondary and primary ITE programs were interviewed.A reflexive thematic analysis was conducted on the transcripts, inwhich the module tutors’ experiences were also incorporated.Findings suggest that e-portfolios can be valuable agentic andreflective pedagogical tools that can scaffold learning, althoughwith a number of important caveats for future implementations.This paper speaks to the paucity of research that extends the viewof e-portfolios as simply repositories for content.ARTICLE HISTORYReceived 11 December 2023Accepted 27 August 2024KEYWORDSE-portfolio; reflection; initialteacher education; ScotlandIntroductionThe definition and application of reflection, especially in initial teacher education, is bothcontested and complex. Typically, Dewey’s (1933) book called How We Think and Schön’s(1983) work on becoming a reflective practitioner are regarded as the foundational textsin this area. Dewey defined reflection as ‘an operation in which facts on one side andmeaning on the other are elicited through constant interaction with each other’ (1933,p. 165). Schön (1983) further developed this idea, emphasising the importance of thedevelopment of ‘feeling for practice’ within specific contexts. He highlighted the signifi-cance of uncertainties in his discussion on reflection, noting that ‘it is our capacity to see-as and do-as that allows us to have a feel for problems that do not fit existing rules’ (1983,p.140). Similarly, LaBoskey (1993), who argued that reflection should clearly be an objec-tive for all teachers, stated that ‘reflection begins when an individual is perplexed oruncertain about an idea or situation and ends with a judgement’ (p.10). The process bywhich these judgements are formed and can be documented was further examined byCONTACT Sinem Hizli Alkan [email protected] School of Education, Faculty of Arts, Humanities,Education and Social Sciences, Anglia Ruskin University, Bishop Hall Lane, Chelmsford CM1 1SQ, UKREFLECTIVE PRACTICEhttps://doi.org/10.1080/14623943.2024.2398800© 2024 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited. The terms on which this article has been published allow the posting of the Accepted Manuscript in a repository by the author(s) orwith their consent.</p
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