DIMENSIONLESS PHYSICAL-MATHEMATICAL MODELING OF TURBULENT NATURAL CONVECTION

Natural convection heat transfer is present in the most diverse applications of Thermal Engineering, such as in electronic equipment, transmission lines, cooling coils, biological systems, etc. The correct physical-mathematical modeling of this phenomenon is crucial in the applied understanding of its fundamentals and the design of thermal systems and related technologies. Dimensionless analyses can be applied in the study of flows to reduce geometric and experimental dependence and facilitate the modeling process and understanding of the main influence physical parameters; besides being used in creating models and prototypes. This work presents a methodology for dimensionless physical-mathematical modeling of natural convection turbulent flows over isothermal plates, located in an “infinite” open environment. A consolidated dimensionless physical-mathematical model was defined for the studied problem situation. The physical influence of the dimensionless numbers of Grashof, Prandtl, and Turbulent Prandtl was demonstrated. The use of the Theory of Dimensional Analysis and Similarity and its application as a tool and numerical device in the process of building and simplifying CFD simulations were discussed.

CORRELATION BETWEEN LEIDENFROST TEMPERATURE, COOLING CAPACITY AND MACHINING TEMPERATURE: AN EXPERIMENTAL STUDY OF CUTTING FLUIDS

Much of the energy consumed by machining materials is converted into heat, which causes several technical and economic problems for the process. The cutting fluid application by the conventional method forms a vapor film of low thermal conductivity, which prevents direct contact between the fluid and the heated surface; the so-called Leidenfrost effect, which reduces cooling efficiency. Studies of this phenomenon applied to the machining of materials are still very restricted and scarce. In this sense, the present work experimentally studied the correlation of parameters Leidenfrost temperature, cooling capacity and machining temperatures, applied to the SAE 52100 steel turning process with conventional lubri-coolant, with different cutting fluids. The thermal properties of the studied cutting fluids were taken from the technical-scientific literature. An experimental apparatus was developed for measuring and acquiring machining temperatures. Synthetic cutting fluids were shown to have a better cooling capacity, followed by semi-synthetic fluids and emulsions. There is no relationship between Leidenfrost temperature, cooling capacity, and machining temperatures for the different types of cutting fluids studied.

ENERGY RECOVERY FROM MUNICIPAL SOLID WASTE: OPPORTUNITIES AND CHALLENGES IN THE BRAZILIAN SCENARIO

Municipal solid waste has always been an undesirable asset in society, and its generation grows every year. Inadequate waste disposal may cause many problems, either by the contamination of the environment or by its capacity to serve as a vector for a series of pathogenic elements. The COVID-19 pandemic drew the world’s attention to these challenges and made it clear how they impact society in an unprecedented way. The higher amount of waste and safety supplies discarded, such as masks and facial shields, require an analysis of the current situation of solid waste management along with solutions to increase the capacity for resource recovery. Methods of treating, collecting, transporting, and disposing of municipal solid waste must be integrated with the other levels of the waste hierarchy (prevention, reuse and preparing for reuse, recycling, other recovery (including energy recovery), and disposal). The scientific literature on this subject was verified in this paper, serving as a subsidy for the implementation of possible processes to be used in companies in the area of basic sanitation and city halls, which can benefit from investments that will incur in the generation of products of added value, creating a new link in its business chain. The production and application of integrated municipal solid waste management systems, including energy recovery from refuse derived fuel, can reduce the volume and expenses of municipal administrations with inadequate waste disposal in landfills and promote more sustainable practices in the circular economy scenario. Therefore, this paper sought to highlight the main activities related to municipal solid waste management with an aim to energy recovery.

STUDY OF MATERIALS AND WELDING USED IN THE CONSTRUCTION OF RACKS FOR STORAGE OF FUEL ELEMENTS AT NUCLEAR PLANTS

This paper showed the technological innovations and the necessary requirements for the welding of ASTM A240 TP316L Austenitic Stainless Steel in the construction of racks used in the storage of Fuel Elements inside nuclear power plants. It presents the development of welding processes using coated electrode, SMAW and as addition metal rods EAS 2-IG / ER 308L. This study is divided into two stages, the preparation of technical documentation and the development of methods and manufacturing processes used in the qualification of welding processes. A sequence was outlined based on real situations used by nuclear component manufacturing companies, meeting the physical and mechanical properties required by the nuclear classification standards and their regulations. The results showed that the welding processes were satisfactory, that the destructive and non-destructive tests showed that there was no discontinuity in the surface and defects in the volume of the welding and that, in the present study, safety in the project for the operation of a Nuclear Power Plant was demonstrated.

ALTERNATIVES OF DOMESTIC WASTEWATER SLUDGE DRYING PROCESSES FOR ENERGY RECOVERY : A REVIEW

As communities grow, cities need to increase their capacity to collect and treat domestic wastewater. The need of larger domestic wastewater treatment plants and proper disposal of its solid waste has attracted the scientific community to research about new technologies that will use those systems and waste as a way to generate energy. The moisture content of a fuel effects the combustion products and the energy released by the reaction. Therefore, in order to make biomass to be a viable fuel option, the technological and scientific challenges of the drying process of wastewater sludge must be faced and overcome so the lowest moisture content level is achieved. Conventional drying processes as for example, direct and indirect thermal drying, are commonly used. However, other processes using renewable energy as for example solar drying are also being studied by the scientist around the world. Moisture content, physical-chemical properties as for example, heating values, composition, ash fusibility are all relevant properties taken into consideration when choosing a fuel for a specific application. The herein review is intended to present some existing domestic wastewater drying processes, alternative ways of improving the efficiency of those processes.

INFLUENCE OF THERMOPLASTIC PROCESSING PARAMETERS ON THEIR INTERNAL TEMPERATURE

With technological advances, polymers are increasingly used to manufacture various components that were previously exclusively manufactured with metals. One of the significant challenges in polymer processing is its relatively low thermal resistance, since relatively small temperature variations, especially when compared to metals and ceramics, lead to significant changes in material properties and in the final component geometry. This paper investigated how the internal temperature of polymers, subjected to an intermittent particulate jet deposition process in conjunction with a continuous flow of hot air, is affected by variation in surface roughness, polymer type, and air pressure. As the main result, low efficiency in heat transfer was caused by the combination of the convective nature of the heat exchange with the low thermal conductivity of the polymers. The variables with the most significant influence on the process were the intermittence and pressure of the particulate jet.

EDITORIAL

The editorial of Thermal Engineering of this issue continues the discussion on scientific research needs in vital areas in which thermal engineering has important participation. The main goal is to motivate the readers, within their specialties, to identify possible subjects for their future research. Natural Convection is present in the most diverse applications of Thermal Engineering, such as controlling and reducing temperatures in electronic systems, reducing the thermal efficiency of cooling in machining processes by the Leidenfrost effect and even in biological systems. With the increasing technological evolution and the development of industrial automation, microelectronics, quantum computing, signal processing, mobile telephony, etc., transmission systems operate increasingly with smaller spacing and higher integration rates between components, with greater power density and heat generation. As a result, there is a growing demand for cooling systems with greater safety, reliability, and efficiency. Therefore, natural convection cooling systems are viable alternatives due to their characteristics of: (A) protection and safety of the transmission system, especially in cases of mechanical and/or electrical failures of the forced cooling system; (B) high reliability and safety of operation; (C) low maintenance costs and (D) no noise. However, due to their low thermal efficiency, such cooling systems are still limited to applications with the low power density and/or combined with forced convection cooling systems. In this sense, the natural convection area is increasingly being researched to create and enable even smaller and more robust high power density transmission systems, with greater economic feasibility (lower costs of acquisition, manufacturing, and maintenance) and exclusively refrigerated (or with minimal use of forced cooling components) by natural convection; all without reducing the efficiency or reliability of these systems. One of the main technologies for thermal optimization of cooling systems researched is the inclusion of geometric surface modifications, through fins (extended surfaces) or corrugated surfaces. The use of corrugated surfaces has been gaining more space in the academic community and industry, standing out for: (A) increasing the area of exposure to the heated surface and the transfer of energy to the circulating fluid; (B) induce changes in the flow in the vicinity of the heated surface, such as the formation of vortices, recirculations, and zones of rarefaction and stagnation; and (C) anticipate and facilitate the flow transition process for the turbulent regime. The study of natural convection – in its most diverse applications and areas of theoretical, applied, and experimental investigation – has been widely explored by Thermal Engineering, arousing more and more the academic community's interest and motivating further research in this area. The mission of Thermal Engineering is to document the scientific progress in areas related to thermal engineering (e.g., energy, oil and renewable fuels). We are confident that we will continue to receive articles’ submissions that contribute to the progress of science. Sílvio Aparecido Verdério JúniorProfessor of Mechanical Engineering

STUDY OF THE INFLUENCE OF THE DIAMETER OF MODIFIED TOOLS WITH INTERNAL COOLING CHANNELS UNDER DIFFERENT THERMAL FLOWS

In this work, the effect on heat generation at the chip-machine tool interface was studied by varying the diameter of internal grooves of a tool for the turning process. This tool is modified with internal channels that circulate water as a coolant through a closed system. As an output parameter, the maximum cutting temperature at the chip-tool interface was studied. The input parameters were the thermal flux present at the chip-tool interface and the diameter of the internal channels present in the cutting tool. All the analysis of variation of the internal channels of the tools and also of the thermal flow exerted on the chip-tool interface were carried out using the finite element method by the Ansys® Workbench 19.2 software. The main one was that the variation in the diameter of the tool's internal grooves does not expressly impact the machining specifications.

CASCADE REFRIGERATION SYSTEM FOR LOW TEMPERATURES USING NATURAL FLUIDS

Cascade refrigeration systems work with two or more serial disposed cycles and can obtain internal temperatures below -60°C, which is necessary for several activities in medicine and scientific research. This paper presents a thermodynamic analysis of cascade system refrigeration using natural refrigerant fluids for ultra-low temperatures. These fluids are environmentally friendly refrigerant and are an alternative to hydro chlorofluorocarbons (HCFCs) and to hydrofluorocarbons (HFCs). Energy and exergy analyses were performed using a thermodynamic model based on the law of conservation of mass and also on the first and second laws of thermodynamics. A simulator was developed to assess the technical practicability of this system, considering it running as a real refrigeration cycle. Natural fluids have best performance energetically and environmentally.

EDITORIAL

The editorial of Thermal Engineering of this issue continues the discussion on scientific research needs in vital areas in which thermal engineering has important participation. The main goal is to motivate the readers, within their specialties, to identify possible subjects for their future research. Mathematical modeling is a powerful tool used in engineering when one wants to design and optimize and equipment and/or processes. The main idea behind a mathematical model is to be able to simulate, as accurately as possible, the behavior of any physical system, real or virtual. However, the task of creating a mathematical model is far from simple. For the sake of comparison, disciplines of physics also propose mathematical models, but these models need to be universal. In engineering, this degree of universality is practically impossible. In reality, a good mathematical model for engineering is the one that is a good tool, assisting the design and optimization of equipment and systems. But which model should we choose when we find ourselves in real life engineering problems? Unfortunately, mathematical models for engineering are quite subjective. It takes into account what each author of the model saw and what problem he/she wanted to solve. That is why for the same system, for example, a shell and tube heat exchangers, there are multitude mathematical models proposed with different features, from highly complex models that use computational fluid dynamics to the simplest models used in undergraduate courses. This makes creativity and analytical skills very important when designing and optimizing a real system in view of the fact that design failures can generate monetary and human life losses. Choosing the right mathematical model is not a simple task, especially in unconsolidated areas, which are generally those where there is a great interest in applied research. Due to the very large number of existing mathematical models, engineers generally choose to propose their own mathematical model. A classification system for mathematical models would be very useful to find what mathematical models are most useful for a given situation.The mission of Thermal Engineering is to document the scientific progress in areas related to thermal engineering (e.g., energy, oil and renewable fuels). We are confident that we will continue to receive articles’ submissions that contribute to the progress of science.