132,610 research outputs found
Frost Formation on Fan-Supplied Tube-Fin Evaporators: A Visual and Numerical Analysis
No full textThis paper presents an experimental and numerical study to evaluate the thermal-hydraulic performance of light commercial capacity evaporators operating under frosting conditions. A tube-fin heat exchanger was tested on a closed-loop wind-tunnel considering the fan performance. A visual analysis of the frost formation processes was carried out to compare the frost layer morphology in different conditions. A mathematical model was developed and the numerical results were compared with the experimental accumulated mass of frost, air flow rate and cooling capacity, with all the predictions falling within the experimental band of uncertainties. Based on this comparison, a frost density correlation was proposed for a typical range of light commercial refrigeration applications. The results show evaporator cooling capacity reductions up to 40% due to frost formation process. Moreover, the effects of air flow reduction and low conductivity frost layer on the overall thermal resistance were analyzed, when it was found that the former is the main cause of the cooling capacity reduction under frosting conditions
Avaliação de métodos para quantificar geada em evaporadores de refrigeradores frost-free
No full textDissertação (mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia Mecânica, Florianópolis, 2015.Em evaporadores no-frost o acúmulo de geada é inevitável. Por degenerar a capacidade de refrigeração do sistema, degelos periódicos são necessários. Esse processo normalmente ocorre através do acionamento de um resistor elétrico e tem impacto negativo para o sistema, pois aumenta a carga térmica e causa variação da temperatura no refrigerador. Portanto, em um refrigerador é normalmente desejada a ocorrência de degelo somente quando necessário, para manter a eficiência do sistema de refrigeração. Entretanto, o atual controle desse processo permite que degelos desnecessários ocorram, não atuando de forma inteligente. Em vista disso, este trabalho tem por objetivo estudar possibilidades de identificação da quantidade de geada presente nos evaporadores para que o degelo possa ser controlado pela demanda. Para isso, investigaram-se experimentalmente diversas possibilidades. Deu-se enfoque às possibilidades que não exigem inserção e utilização de mais transdutores nos refrigeradores. Para os ensaios, construiu-se bancada própria e um refrigerador de modelo comum foi adaptado permitindo a visualização do processo. Testaram-se soluções baseadas na perda de carga do escoamento de ar sobre o evaporador, na eficiência da troca térmica do evaporador e também na evolução das temperaturas do evaporador e suas consequências, tais como a variação de resistência elétrica do resistor de degelo e a do enrolamento do motor do ventilador. Alguns transdutores capacitivos, indutivos e ópticos também foram avaliados. Os melhores resultados foram obtidos nas soluções baseadas na evolução de temperaturas. Um transdutor de fluxo de calor instalado permitiu analisar a influência da geada. Os resultados mostraram a degradação do coeficiente de transferência de calor devido à presença da geada, porém de maneira insuficiente para indicar a quantidade de geada. A possibilidade baseada na análise da dinâmica de variação das temperaturas frente à perturbação do sistema foi validada. Porém, o conceito encontrou obstáculo para diferenciar quantidades menores de geada depositada. Ao final deste trabalho não foi possível encontrar um método eficiente para o presente problema.Abstract : In no-frost evaporator, frost accumulation is unavoidable. Since this accretion depletes the cooling capacity of the refrigeration system, defrosts must be periodically carried out. This process is usually made by electrical heaters but it has a negative impact for the system because it increases the thermal load, and also causes temperature variation in the refrigerator. For this reason, in a refrigerator, it is normally desirable to defrost only as necessary to maintain an efficient cooling system. However, the current control of this process allows unnecessary defrosts, and not intelligent actions. Therefore, the present work aims to study possibilities to identify amount of frost accumulated in evaporators, so that process of melting can be controlled by demand. Because of this, some solutions were experimentally investigated. The main focus were the possibilities that do not require insertion and use of more transducers in refrigerators. In order to test, an experimental apparatus was assembled; a common model of refrigerator was adapted allowing the visualization of the process. Solutions based on the pressure loss of the airflow over the evaporator, the heat exchange efficiency of the evaporator; and the evolution of evaporator temperatures and its consequences, such as the variation of the electrical resistance of the heater and the fan motor winding, were tested. Some sensors, based in capacitive, inductive and optical principle, were also evaluated. The best results were obtained in solutions based on the temperature development. A heat flow transducer installed in coil allowed analysis of the frost. The results showed the degradation of heat transfer coefficient due to the presence of frost, but insufficiently to indicate the amount of frost. The option based on the analysis of the dynamics of temperatures variation by a system disturb was validated. However, the concept encountered an obstacle in real operation condition of refrigerator. At the end of this work was not possible to find an efficient method to this problem
Calculating maximum frost depths at Mn/ROAD : winters 1993-94, 1994-95 and 1995-96
No full text75209This effort involved calculating maximum frost penetration depths for each of the 40 test cells at Mn/ROAD, the Minnesota Department of Transportation's pavement testing facility, for the 1993-94, 1994-95, and 1995-96 winters. The report compares results with measured maximum frost penetration depths for the same three winters. Generally, calculated depths were within plus or minus 15% of measured depths, but differences were much greater for the four test cells underlain by the granular subgrade. Researchers conducted sensitivity tests to determine the influence of the n-factor, soil moisture content, material density, layer thickness, thermal conductivity, mean annual soil temperature, and volumetric latent heat of fusion. Conclusions included the following: (1) Small variation in layer thickness will have a very minor effect on computed frost depths; (2) Reasonable variations in moisture content and density of the various base course, subbase course, and subgrade layers will have a minor effect on calculated frost penetration depths; (3) Large n-factors caused deeper calculated frost penetration depths, and the use of n-factors of .90 and .95, respectively, for flexible and rigid pavements provided the most reasonable estimates of frost depths; (4) Increasing the thermal conductivity of the materials by 25% resulted in closer calculated agreement with measured frost depths; and (5) Using a mean annual soil temperature of 9.4 deg C rather than 11.1 deg C resulted in better agreement between calculated and measured data. Tables, figures. 1879k, 70p
Robert Frost and the dramatic monologue
No full textvoce enciclopedica su Robert Frost e il dramatic monologueencyclopaedia entry on Robert Frost and the Dramatic Monologu
Robert Frost and Ralph Waldo Emerson
No full textvoce enciclopedica su Robert Frost e Ralph Waldo Emersonencyclopaedia entry on Robert Frost and Ralph Waldo Emerso
Correcting Frost Diagram Misconceptions Using Interactive Frost Diagrams
No full textFrost diagrams provide
convenient illustrations of the aqueous reduction potentials and thermodynamic
tendencies of different oxidation states of an element. Undergraduate textbooks
often describe the lowest point on a Frost diagram as the most stable oxidation
state of the element, but this interpretation is incorrect because the
thermodynamic stability of each oxidation state depends on the specific redox
conditions in solution (i.e., the potential applied by the environment or an
electrode). Further confusion is caused by the widespread use of different,
contradictory conventions for labeling the y-axis of these diagrams as either nE°
or −nE°, among other possibilities. To aid in discussing and correcting
these common mistakes, we introduce a series of interactive Frost diagrams that
illustrate the conditional dependence of the relative stabilities of each
oxidation state of an element. We include instructor’s notes for using these interactive
diagrams and a written activity for students to complete using these diagrams.</p
The Frost Multidimensional Perfectionism Scale revisited: More perfect with four (instead of six) dimensions
No full textThe Frost Multidimensional Perfectionism Scale (FMPS; Frost, Marten, Lahart & Rosenblate, 1990) provides six subscales for a multidimensional assessment of perfectionism: Concern over Mistakes (CM), Personal Standards (PS), Parental Expectations (PE), Parental Criticism (PC), Doubts about actions (D), and Organization (O). Despite its increasing popularity in personality and clinical research, the FMPS has also drawn some criticism for its factorial instability across samples. The present article argues that this instability may be due to an overextraction of components. Whereas all previous analyses presented six-factor solutions for the FMPS items, a reanalysis with Horn's parallel analysis suggested only four or five underlying factors. To investigate the nature of these factors, item responses from N = 243 participants were subjected to principal component analysis. Again, parallel analysis retained only four components. Varimax rotation replicated PS and O as separate factors, whereas combining CM with D as well as PE with PC. Consequently, the present article suggests a reduction to four (instead of six) FMPS subscales. Differential correlations with anxiety, depression, parental representations and action tendencies underscore the advantage of this solution
Modelagem semi-empírica de um refrigerador frost-free sujeito à abertura de portas
No full textDissertação (mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia Mecânica, Florianópolis, 2013.O projeto de refrigeradores domésticos é focado não somente na redução do consumo de energia em condições normalizadas, mas também em condições reais de operação, quando as portas dos compartimentos refrigerados são abertas periodicamente. Nesse caso, há transporte de umidade para os compartimentos refrigerados com consequente formação de geada sobre o evaporador, redução da vazão de ar e elevação no tempo de funcionamento do compressor. O efeito de tais fenômenos deve ser considerado para o adequado projeto dos componentes assim como para a definição de estratégias de controle do produto. A robustez do produto à formação de geada é geralmente estudada através de ensaios experimentais complexos, demorados e dispendiosos. Tal cenário favorece o uso de modelos matemáticos, embora nenhum dos modelos disponíveis na literatura seja capaz de simular o comportamento de um refrigerador doméstico submetido a ciclos de aberturas de portas e a consequente formação de geada sobre as superfícies do evaporador. Para eliminar essa lacuna, desenvolveu-se um modelo matemático para simular o comportamento transiente de um refrigerador doméstico sujeito a condições típicas de um teste de robustez à formação de geada. Para tanto, adotou-se uma abordagem semi-empírica, onde os componentes do refrigerador foram modelados com base em trabalhos disponíveis na literatura. Alguns dos parâmetros empíricos associados aos modelos desses componentes foram derivados de resultados experimentais obtidos com o próprio refrigerador. As estimativas do modelo foram devidamente validadas contra dados experimentais. O desvio máximo observado na previsão da evolução temporal da vazão mássica de refrigerante, da potência consumida e das pressões de evaporação e condensação foi de ±10%, da perda de carga no evaporador de ±20%, da massa de geada acumulado no evaporador de ±30% e das temperaturas dos compartimentos refrigerados de ±2ºC. Além disso, o modelo foi utilizado para estimar o comportamento de variáveis não facilmente mensuráveis tais como a evolução temporal da posição da fronteira entre as regiões bifásica e superaquecida no evaporador, da densidade e espessura da camada de geada e da distribuição da massa de geada e da perda de carga em cada fileira do evaporador.Abstract : Modern refrigerator design is aimed not only at energy savings but also at product robustness to evaporator frosting. Whilst the former is evaluated by means of standardized test procedures, the latter is assessed by testing the refrigerator under real usage conditions, when the doors are opened in a regular basis, allowing moisture to enter the refrigerated compartment and, consequently, frost to accumulate on the evaporator surface. As the frost layer grows, the air-side pressure drop rises, reducing the air flow rate and, therefore, the evaporator capacity. As a consequence, the compressor is driven to run longer cycles, thus increasing the energy consumption. The laboratory procedures required for product assessment and development rely on costly and time consuming experiments. Albeit it has been advocated that the adoption of simulation models may aid the product development process, there is no model available in the open literature which is capable of simulating the refrigerator performance under door-opening and frost build-up conditions. Therefore, a mathematical model for simulating the transient behavior of a domestic refrigerator in such conditions has been advanced in this work. A first-principles simulation model was put forward for the refrigeration loop, whereas a semi-empirical approach was adopted for the refrigerated compartment, in such a way that the key empirical parameters were obtained by testing the refrigerator in an environmental chamber. The model was validated against experimental data, when it was found that its predictions for power consumption and refrigerant mass flow rate fell within a ±10% error band, its predictions for accumulated frost mass were within a ±20% error band, and its predictions for compartment air temperatures were within a ±2K error band. The model is also capable of predicting the behavior of some non-measurable variables, such as the time evolution of the liquid-vapor to vapor transition point of the refrigerant in the evaporator, the density and thickness of the frost layer over the different rows of the evaporator and also the contribution of each of these rows on the air side pressure drop on the evaporator
Análise teórico-experimental da formação de geada em refrigeradores domésticos
No full textDissertação (mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico. Programa de Pós-Graduação em Engenharia Mecânica.Para que um refrigerador seja classificado como frost-free, a geada deve se formar apenas sobre o evaporador, de onde esta é periodicamente removida por resistências elétricas. No entanto, existem situações onde as condições ambientais e de operação provocam o sobre as superfícies internas de gabinetes refrigerados, o que exige a intervenção do usuário para a sua remoção. Em face deste problema, empresas do setor têm recebido um número significativo de chamadas de assistência técnica e, e em situações extremas, de devoluções de produto. Os custos dessas operações geram um grande prejuízo para os fabricantes, que, por sua vez, procuram desenvolver produtos cada vez mais robustos à formação de geada, geralmente elevando o custo e penalizando a eficiência energética do equipamento. Apesar do grande número de trabalhos existentes na literatura com foco nos fenômenos físico-químicos associados com o processo de formação da geada, nenhum deles se concentrou nas condições psicrométricas e de superfície tipicamente observadas em refrigeradores domésticos. Com o objetivo de preencher essa lacuna, este trabalho se propõe a estabelecer uma metodologia para analisar o processo de formação de geada em gabinetes refrigerados com base nas condições de operação e nas características das superfícies. Neste contexto, as condições de operação de um refrigerador típico foram avaliadas em uma câmara com controle da temperatura e umidade do ar. Amostras do material das superfícies internas do refrigerador foram extraídas e submetidas à medição de ângulo de contato e rugosidade. As condições mínimas necessárias para o início do processo de nucleação foram avaliadas em uma bancada especialmente projetada e construída para essa finalidade. Os resultados foram então empregados para validar um critério teórico para prever se ocorre ou não formação de geada sobre uma determinada superfície. Adicionalmente, um modelo matemático foi desenvolvido com base nos princípios de conservação da massa e da energia para prever a espessura e a massa da camada de geada ao longo do tempo. O modelo foi validado contra dados experimentais, quando se observou que a espessura da geada, em diferentes condições de operação, era prevista dentro de uma faixa de erro de ±10%. Embora a metodologia tenha sido elaborada para um refrigerador doméstico específico, ela se aplica a quaisquer compartimentos refrigerados onde exista formação de geada. Household refrigerators are labeled as frost-free when frost forms on the evaporator coil only, from where it is periodically removed by the action of electrical heaters. However, when certain environmental and operating conditions take place, frost can also be formed on the cabinet inner liners, increasing the field call rates and consequently the maintenance and quality costs. In order to avoid this inconvenience, manufacturers have been trying to improve their refrigerator designs, but that increases costs and penalizes the energy performance. Albeit several distinguishing publications in the field of frost formation can be found in the open literature, none of them actually focused on the typical working and geometric conditions of household refrigerators and freezers. Therefore, this dissertation aimed to devise a methodology for analyzing the frost formation on the inner liners of frost-free refrigerators focused on the most influencing factors that drive the frost nucleation, growth and densification processes, such as psychrometric (temperature and humidity), flow (velocity), and surface conditions (roughness and contact angle). In order to accomplish that, a refrigerator was tested inside an environmental air temperature and humidity controlled chamber using an automatic door-opening device. In addition, samples of different liner materials were extracted from the refrigerated cabinet in order to have their contact angles and surface roughness measured. These samples were tested using a purpose-built testing facility for measuring both the mass and thickness of the frost deposited over a flat horizontal surface. The results were used to devise a criterion that establishes whether the frost nucleation would start. In addition, a mathematical model based on the mass and energy conservation principles, was developed to predict the mass and thickness of the frost layer over time. The model was validated against experimental data, showing deviations for the predicted frost thickness within a ±10% error band. Although the methodology was applied to a specific household refrigerator, it can be easily extended to any kind of refrigerated room subject to frost formation
frost n; frostburn cpd
No full textfrostThe severity of the cold winds coupled with low temperatures often caused 'frost burn', extreme cases when the flesh, benumbed with cold, was rendered without feeling.See cited quotationPRINTED ITEM DNE SupG. M. Story FEB. 9 1988WK FEB 9 1988Used I and SupUsed I and SupUsed sup[Add to DNE frost n, to 1916 cite
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