C-MAST - Center for Mechanical and Aerospace Science and Technologies
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Historically, the research unit “CAST – Centre for Aerospace Science and Technologies” was created in 1994 by a small group of aerospace engineers. Now we became a Reasearch Center developing studies in Energy, Mechanical and Aerospace Engineering with a wide spectrum of areas, from Astrodynamics to Technological Forecasting.
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Browsing C-MAST - Center for Mechanical and Aerospace Science and Technologies by Author "Aguiar, Martim"
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- Current status and future trends of computational methods to predict frost formationPublication . Aguiar, Martim; Gaspar, Pedro Dinis; Silva, Pedro Dinho daNowadays, the increasing energy prices and associated environmental concerns lead the refrigeration systems’ developers and manufacturers to develop more energy efficient and sustainable equipment and devices. On the most demanding systems, intense usage results in the fast accumulation of ice on the evaporator fins that reduces the efficiency and may even clog the system. These systems often have time-controlled defrost cycles, that heat the evaporator, melting the ice and allowing the system to keep working normally after the defrost cycle. This cycle consumes extra energy and causes a thermal imbalance on the refrigerated space, that may result in a worst refrigeration quality. If it was possible to avoid the defrosting cycle passively (without energy consumption) its efficiency would greatly increase, and the refrigeration temperature would be more stable. Currently defrost cycles cannot be avoided in an economically viable way, although new designs, materials and configurations show promising results, and are currently being investigated. These studies require experimental tests that may become expensive as several geometries, topologies, materials and surface treatment combinations should be evaluated. To access the efficiency before these experimental tests, computational models that simulate frost formation could predict with some accuracy which of the most promising configurations should be then tested experimentally. The present paper aims to review the computational methods to predict frost formation and compare them for possible usage in the computational study of evaporators. Additionally, the future trends of the simulations are discussed, taking into account physical and mathematical models, numerical procedures and the accuracy of the dynamic pattern of the predictions.
- Fast return path planning for agricultural autonomous terrestrial robot in a known fieldPublication . Cernicchiaro, Carlo; Gaspar, Pedro Dinis; Aguiar, MartimThe agricultural sector is becoming more critical than ever in view of the expected overpopulation of the Earth. The introduction of robotic solutions in this field is an increasingly researched topic to make the most of the Earth's resources, thus going to avoid the problems of wear and tear of the human body due to the harsh agricultural work, and open the possibility of a constant careful processing 24 hours a day. This project is realized for a terrestrial autonomous robot aimed to navigate in an orchard collecting fallen peaches below the trees. When it receives the signal indicating the low battery, it has to return to the docking station where it will replace its battery and then return to the last work point and resume its routine. Considering a preset path in orchards with tree rows with variable length by which the robot goes iteratively using the algorithm D*. In case of low battery, the D* algorithm is still used to determine the fastest return path to the docking station as well as to come back from the docking station to the last work point. MATLAB simulations were performed to analyze the flexibility and adaptability of the developed algorithm. The simulation results show an enormous potential for adaptability, particularly in view of the irregularity of orchard field, since it is not flat and undergoes modifications over time from fallen branch as well as from other obstacles and constraints. The D* algorithm determines the best route in spite of the irregularity of the terrain. Moreover, in this work, it will be shown a possible solution to improve the initial points tracking and reduce time between movements.
- Frost measurement methods for demand defrost control systemsPublication . Aguiar, Martim; Gaspar, Pedro Dinis; Silva, Pedro Dinho daIt is widely known that defrosting operation on commercial refrigerators is one of the main causes of inefficiency on these systems. Several defrosting methods are used nowadays, but the most commonly used are still time-controlled defrosting, usually by either electric resistive heating or reverse cycle, as most demand defrost methods are usually complex, expensive or unreliable. Demand defrost can work by either predicting frost formation by processing measured conditions (fin surface temperature, air humidity and air velocity) and/or frost accumulation symptoms such as pressure drop and refrigerant properties. Other way of knowing when to defrost is to directly measure the frost formation using sensors such as photoelectric, capacitive or resistive. This review gathers some of the methods that can be used for directly measuring frost accumulation on the evaporator fin surface.
- Frost Measurement Sensors for Demand Defrost Control Systems: Purposed Applications in EvaporatorsPublication . Aguiar, Martim; Gaspar, Pedro Dinis; Silva, Pedro Dinho daIt is widely known that defrosting operation on commercial refrigerators is one of the main causes of inefficiency on these systems. Several defrosting methods are used nowadays, but the most commonly used are still time-controlled defrosting, usually by either electric resistive heating or reverse cycle, as most demand defrost methods are usually complex, expensive or unreliable. Demand defrost can work by either predicting frost formation by processing measured conditions (fin surface temperature, air humidity and air velocity) and/or frost accumulation symptoms such as pressure drop and refrigerant properties. Other way of knowing when to defrost is to directly measure the frost formation using sensors such as photoelectric, capacitive or resistive. This review gathers some of the methods that can be used for directly measuring frost accumulation on the evaporator fin surface.
- Frost Measuring and Prediction Systems for Demand Defrost ControlPublication . Aguiar, Martim; Gaspar, Pedro Dinis; Silva, Pedro Dinho daIt is widely known that the defrosting operation of evaporators of commercial refrigeration equipment is one of the main causes of inefficiency on these systems. Several defrosting methods are used nowadays, but the most commonly used are still time-controlled defrosting systems, usually by either electric resistive heating or reverse cycle. This happens because most demand defrost methods are still considered complex, expensive, or unreliable. Demand defrost can work by either predicting frost formation by processing measured conditions (fin surface temperature, air humidity, and air velocity), operative symptoms of frost accumulation (pressure drop and refrigerant properties), or directly measuring the frost formation using sensors (photoelectric, piezoelectric, capacitive, resistive, etc.). The data measured by the sensors can be directly used by the system but can also be processed either by simple algorithms or more complex systems that use artificial intelligence and predictive methods. This chapter approaches frost sensing and prediction for command of demand defrost systems.
- Further development and experimental testing of a resistive sensor for monitoring frost formation in refrigeration systemsPublication . Aguiar, Martim; Gaspar, Pedro Dinis; Silva, Pedro Dinho daRefrigeration systems are widely accepted as a mean to prevent food degradation. These systems usually rely on fin-and-tube heat exchangers to lower the air temperature. Due to long operation times and surface temperatures below dew point, frost may form and accumulate on the evaporators fin surface. This frost accumulation reduces and may clog the airflow, decreasing the system efficiency. Usually, most systems use a time-controlled system (stopping the compressor with or without powering a heating resistor inserted among fins to heat) to melt the ice. This method does not measure or predict the amount of frost formed, thus it is not reliable. Ice sensors that are cheap, reliable and easy to implement in commercial systems are studied. A resistive sensor is purposed, that measures the voltage between two electrodes with the water phase change (condensation and solidification -freezing-), as these states have different resistive values. Different electrode materials, morphologies and configurations are studied in order to obtain the best design.
- Image recognition method for frost sensing applicationsPublication . Aguiar, Martim; Gaspar, Pedro Dinis; Silva, Pedro Dinho daFrost formation in the heat exchangers of refrigeration systems is a well-documented phenomenon. This frost accumulation creates a thermally insulating barrier that can restrict, or even block, the airflow between fins, resulting in decreased efficiency and degradation of the food products. Several methods of frost detection and defrosting have been developed, although there is not an efficient mainstream method to measure and control frost formation. In previous works, the results of a small low-cost resistive sensor for frost detection were shown to be promising. This paper extends that research using computer vision to compare the results of this sensor with the frost formed on the heat exchanger, allowing for a better study of the sensor. This method allowed to trace and plot a frost formation curve of the sensor detected values.
- Low-Cost IoT Remote Sensor Mesh for Large-Scale Orchard MonitorizationPublication . Varandas, Leonor Cristina Pinheiro dos Santos; Faria, João; Gaspar, Pedro Dinis; Aguiar, MartimPopulation growth and climate change lead agricultural cultures to face environmental degradation and rising of resistant diseases and pests. These conditions result in reduced product quality and increasing risk of harmful toxicity to human health. Thus, the prediction of the occurrence of diseases and pests and the consequent avoidance of the erroneous use of phytosanitary products will contribute to improving food quality and safety and environmental land protection. This study presents the design and construction of a low-cost IoT sensor mesh that enables the remote measurement of parameters of large-scale orchards. The developed remote monitoring system transmits all monitored data to a central node via LoRaWAN technology. To make the system nodes fully autonomous, the individual nodes were designed to be solar-powered and to require low energy consumption. To improve the user experience, a web interface and a mobile application were developed, which allow the monitored information to be viewed in real-time. Several experimental tests were performed in an olive orchard under di erent environmental conditions. The results indicate an adequate precision and reliability of the system and show that the system is fully adequate to be placed in remote orchards located at a considerable distance from networks, being able to provide real-time parameters monitoring of both tree and the surrounding environment.
- Modeling the heat transfer and the pressure loss in flat fin-and-tube compact heat exchangersPublication . Aguiar, Martim; Gaspar, Pedro Dinis; Silva, Pedro Dinho daFin-and-tube heat exchangers (HX) have wide application in the industry. The study of HXs with reduced tube diameter has allowed the development of more compact and equally efficient equipment. Most of the HXs of this type are composed of flat fins, not only due to the ease of construction but also to their reliability in long-term operation and reduced friction characteristics. The formulation of mathematical equations that predict the effect of the geometric characteristics on the heat transfer and friction factors of the HXs lead many researchers to try to establish correlations that approximate the experimental results. In this paper is developed a computational tool based on literature correlations is used to evaluate the Colburn and friction factors for flat fin-and-tube compact HXs. The results of predictions show the Colburn and friction factors behavior for different geometrical parameters (fin pitch, tube diameter, tube number) and typical Reynolds number.
- Standalone docking station with combined charging methods for agricultural mobile robotsPublication . Varandas, Leonor Cristina Pinheiro dos Santos; Gaspar, Pedro Dinis; Aguiar, MartimOne of the biggest concerns in the field of agriculture is around the energy efficiency of robots that will perform agriculture’s activity and their charging methods. In this paper, two different charging methods for agricultural standalone docking stations are shown that will take into account various variants as field size and its irregularities, work’s nature to which the robot will perform, deadlines that have to be respected, among others. Its features also are dependent on the orchard, season, battery type and its technical specifications and cost. First charging base method focuses on wireless charging, presenting more benefits for small field. The second charging base method relies on battery replacement being more suitable for large fields, thus avoiding the robot stop for recharge. Existing many methods to charge a battery, the CC CV was considered the most appropriate for either simplicity or effectiveness. The choice of the battery for agricultural purposes is if most importance. While the most common battery used is Li-ion battery, this study also discusses the use of graphene-based new type of batteries with 45% over capacity to the Li-ion one. A Battery Management Systems (BMS) is applied for battery balancing. All these approaches combined showed to be a promising method to improve a lot of technical agricultural work, not just in terms of plantation and harvesting but also about every technique to prevent harmful events like plagues and weeds or even to reduce crop time and cost.