95,928 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
Frost resistance of F-concrete
No full textFreeze-thaw tests and frost-salt tests were carried out for evaluating the frost resistance of F-concrete. Porosity and microstructural studies were included in the lest programme. The frost resistance and frost-salt resistance of F-concrete are good whenever the water-cement ratio is smaller than or equal to 0.4. The air content appears to have virtually no influence on the frost resistance or frost-salt resistance of F-concrcte. In the frost-salt test, however, rapid primary scaling takes place and increases concomitantly with the age of concrete. This phenomenon is probably related to carbonation at the surface of test specimens. Inner cracking of F-concrete is probably due to hydration shrinkage. Cracking, however, does not have a clearly detrimental effect on the frost resistance of F-concrete
Robert Frost papers
No full textAmerican poet Robert Frost (1874-1963) was born in San Francisco, California. After the death of his father, the family moved to New England, which provided the backdrop for Frost's trademark regional poetry. Frost briefly attended both Dartmouth College and Harvard University but did not earn a degree. Frost initially encountered difficulties in establishing himself as a published poet in American newspapers and literary journals. After little success in America, Frost and his family moved to England for three years, which proved to be more fertile publishing ground. By the time he returned to the United States in 1915, Frost had published two well-received full-length collections, A Boy's Will (1913) and North of Boston (1914), which solidified his reputation on both continents. By the 1920s, he was the most celebrated poet in America. Frost was awarded the Pulitzer Prize for literature four times: for New Hampshire (1923), Collected Poems (1930), A Further Range (1936), and A Witness Tree (1942). Frost received more than forty honorary degrees from colleges and universities, including Oxford and Cambridge Universities, Amherst College, and the University of Michigan. He also had the honor of participating in President John F. Kennedy's inauguration ceremonies, 1961, by reading his poems "Dedication" and "The Gift Outright." This collection includes manuscripts of "Closed for Good" and "The Middleness of the Road"; page proofs of the verse drama A Masque of Reason (1942) and galley proofs of Steeple Bush (1947); art work, photographs, correspondence, work papers, an award, sheet music, serials, and audio recordings of Frost. The collection is unprocessed, but a preliminary inventory is available
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
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
Medium-resolution multispectral data from sentinel-2 to assess the damage and the recovery time of late frost on Vineyards
No full textIn a climate-change context, the advancement of phenological stages may endanger viticultural areas in the event of a late frost. This study evaluated the potential of satellite-based remote sensing to assess the damage and the recovery time after a late frost event in 2017 in northern Italian vineyards. Several vegetation indices (VIs) normalized on a two-year dataset (2018-2019) were compared over a frost-affected area (F) and a control area (NF) using unpaired two-sample t-test. Furthermore, the must quality data (total acidity, sugar content and pH) of F and NF were analyzed. The VIs most sensitive in the detection of frost damage were Chlorophyll Absorption Ratio Index (CARI), Enhanced Vegetation Index (EVI), and Modified Triangular Vegetation Index 1 (MTVI1) (-5.26%,-16.59%, and-5.77% compared to NF, respectively). The spectral bands Near-Infrared (NIR) and Red Edge 7 were able to identify the frost damage (-16.55 and-16.67% compared to NF, respectively). Moreover, CARI, EVI, MTVI1, NIR, Red Edge 7, the Normalized Difference Vegetation Index (NDVI) and the Modified Simple Ratio (MSR) provided precise information on the full recovery time (+17.7%, +22.42%, +29.67%, +5.89%, +5.91%, +16.48%, and +8.73% compared to NF, respectively) approximately 40 days after the frost event. The must analysis showed that total acidity was higher (+5.98%), and pH was lower (-2.47%) in F compared to NF. These results suggest that medium-resolution multispectral data from Sentinel-2 constellation may represent a cost-effective tool for frost damage assessment and recovery management
New Correlations for the Air-Side Heat Transfer Coefficient of Microchannel Heat Exchangers Under Quasi-Steady State Frosting Operating Conditions
No full textThis study experimentally investigated the frost growth on louvered folded fins in outdoor microchannel heat exchangers used in air-source heat pump systems. The effects of surface temperature and fin geometries on the performance of the microchannel heat exchangers under frosting condition were studied. 7 fin samples with various fin width, fin height and fin density were tested in controlled laboratory conditions that replicated those of actual heat pump systems in winter season. The fin surface temperature was experimentally estimated with the novel methodology developed in the present study. Experimental data of local frost thickness, air pressure drop across the coils, time of frost-defrost cycles and heat transfer rates were recorded for heat exchangers operating in actual transient frosting conditions. Data showed that the frosting time and the frost growth rates were depended mainly on the local fin surface temperature. A set of empirical correlations were developed to predict the frost thickness on fin leading edge and the reduction of air face velocity due to air pressure drop across the frosted coil during frosting operation. The correlations aid to calculate the instantaneous air-side Reynolds numbers during frosting operation of the fin samples. These are critical for predicting the heat transfer rates of the microchannel coils in quasi-steady state frosting operating conditions
Bacterial blight (Pseudomonas pisi Sackett) of peas in South Africa, with special reference to frost as a predisposing factor
No full textIn the beginning of the nineteen fifties bacterial blight caused much damage to pea crops in South Africa, particularly to those grown for seed production. A study has been made of the causal organism and the conditioning factors of the disease, special attention being paid to frost as a predisposing factor.The symptoms of the disease vary according to weather conditions during and after infection. In dry weather with occasional frost they usually start on the stem near the soil and extend upwards to stipules and leaflets, where a characteristic fan- like pattern develops. In rainy weather they appear as scattered spots on the stems and leaves. The bacterium that causes the disease was identified as Pseudomonas pisi Sackett.Although frost is not essential to infection, the susceptibility of pea plants increased considerably when they were frosted before inoculation. There was no correlation between the increase in susceptibility and the amount of permanent frost injury. In freezing experiments with pods alone, it was confirmed that frost is a predisposing factor for infection, but frost cracks in the tissue were not a prerequisite. A second effect of frost is that the disease spreads more rapidly in tissue that has been frosted. Stem lesions did not spread far on the stems of plants in experimental fields as long as the temperature remained above freezing point. After a frost, however, the disease spread for a considerable distance on the stems and to the stipules, on which the typical fan-like symptoms developed. These results were confirmed in experiments with artificially frosted plants. A possible explanation is that the pathogen spreads in the temporarily enlarged, water-filled intercellular spaces of the stems and leaves that have been exposed to frost.Because frost is a predisposing factor and because the optimum temperature for the development of symptoms is relatively high (about 25°-30°C), the winter climate in the seed producing areas of South Africa where frost in the early winter morning is followed by high day temperatures, is very conducive to the disease.The time of planting had a considerable effect on the occurrence of the disease; it was much more severe in crops planted in April than in those planted in the second half of May or later. The difference in susceptibility between crops planted at different times is probably caused by the fact that early planted peas are more sensitive to frost than those that are planted later. Furthermore it was found in glasshouse experiments that pea plants that were grown at high temperatures were more susceptible to blight than those grown at low temperatures. The differences in susceptibility were correlated with the stomatal frequency. These results may also partially explain the differences in susceptibility between early and late plantings. Varietal resistance investigated in cultivar trials was not consistent over the years. Pea plants suffering from lack of water were more sensitive to frost than plants that were regularly irrigated, but not much difference in the occurrence of blight was found between peas grown at different irrigation levels.Bacterial blight is a seed-borne disease and the pathogen remains viable in infected seed from one season to the next. Only if moderate frost occurs when the seeds are swelling in the pods and if there is a source of inoculum nearby, the pods and seeds may become infected in normal dry winters. Because the South African isolates of the pathogen do not easily move in the wood vessels, it is unlikely that seeds from apparently healthy pods will be infected, even if they are borne on diseased plants.In South Africa no other natural host plant for Ps. pisi was found. In a pea field the spread of the disease was erratic, no obvious connection was found between new and already existing infections.The following preventive measures are recommended: (a) plant at the right time of the year, (b) avoid frost pockets and (c) plant government certified seed.Experiments were done to investigate how frost renders the plants more susceptible to infection. One of the effects of frost is that the plant tissue becomes water-soaked on thawing. Water-soaking, induced either by injecting the stems with sterile water or by bruising the leaflets, rendered pea plants considerably more susceptible to infection, probably because the pathogen penetrated the water- soaked tissue more easily. In experiments to study the effect of frost on the establishment of the pathogen it was found that the growth rate of Ps. pisi in pea plants, that had been exposed to frost, did not differ much from the growth rate in non-frosted plants. Neither did prolonged watersoaking of plant tissue have a favourable effect on the multiplication of Ps. pisi . Saprophytes did, however, multiply rapidly in water-soaked leaves while they remained stationary in normal plant tissue. During the first day of incubation the growth of Ps. pisi in intercellular fluid from non-frosted pea plants, which had been infiltrated with distilled water, showed a lag phase, even though the fluid was inoculated with actively dividing bacteria. In one case a decrease in the concentration of Ps. pisi was observed. In intercellular fluid from plants which had been frosted, the lag was less distinct than in fluid from infiltrated plants of the same planting, or it was entirely absent. After the lag phase little difference was observed between the growth rate of Ps. pisi in intercellular fluid from plants that had been frosted and that in intercellular fluid from infiltrated, non-frosted plants.The results described above show that the effect of frost on infection is primarily, that by inducing water congestion of the tissue it facilitates the invasion of the plants by the pathogen; and in the second place compounds, exuding into the intercellular spaces from cells which have been exposed to frost, favour the establishment of the bacteria.The Ps. pisi strain tested did not produce polygalacturonase, pectinlyase or the cellulolytic Cx enzyme in artificial media or in intercellular fluid. Ps. pisi did, however, produce caseinolytic enzymes, but it is doubtful whether they are important as a virulence factor, because they were also found in sterile intercellular fluid
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