1,721,765 research outputs found
Electro-osmotic non-isothermal flow in rectangular channels with smoothed corners
Full text linkMicrochannel heat sinks are able to provide high cooling capabilities in terms of heat flux rates. This makes them particularly interesting for the thermal management of electronic components such as CPUs, which have high power density and small dimensions. Pressure drop of the coolant across the microchannels may, however, be significant and give rise to viscous heating, thereby preventing the practical use of these devices. When the coolant is a polar fluid and the channel walls possess a net electric charge, an alternative means of moving the fluid is through an applied external electric field. The flow which originates is called electro-osmotic (EOF). EOF does not require moving parts, is free of vibrations and does not need lubrication, but is subject to Joule heating of the fluid and has flow and heat transfer characteristics which differ from those of pressure-drive flows. In spite of several previous investigation on EOF, no attention has been paid to the changes in velocity and temperature distributions caused by modifying the base cross-section of the channels which may be circular, rectangular or polygonal, thanks to the current capabilities of microfabrication. This work investigates numerically the influence of smoothing the corners of the cross-section at fixed hydraulic diameter on the values of the Poiseuille and Nusselt numbers for the laminar, steady and fully developed, electro-osmotic flow in a rectangular channel subject to uniform heat flux and Joule heating. Several aspect ratios are considered, as are different values of the ratio of Joule heating to heat flux through the walls. The results highlight a very slight increase of the Poiseuille number with the radius of curvature, whereas the Nusselt number experiences a significant improvement. Correlations are obtained for both the Poiseuille and Nusselt number as a function of the radius of curvature, aspect ratio and Joule heating-to-heat flux ratio
Performance Evaluation Criteria for Triangular Microchannels with Smoothed Corners
Full text linkThe manufacturing capabilities available today for microchannels make it possible to produce in a comparatively easy, quick and cheap way several cross-sections, possibly allowing modifications of the macro-geometry. Another way is smoothing the corners of polygona cross-sections: this eliminates low-gradient areas and increases transport phenomena, i.e. frictional losses and heat transfer. Several ways of assessing the relative performance of the new shape in comparison to the original have been suggested over the years, among which are Performance Evaluation Criteria (PEC), as proposed by Bergles and Webb. PEC are based on a first-law analysis and aim at extremising the thermal power, heat exchange area, inlet temperature difference or pumping power under varying constraints. In this work equilateral triangular microchannels with uniform wall temperature are considered, through which a liquid flows in fully-developed, steady laminar regime. The cross-sectional area has its corners progressively rounded, and the velocity and temperature profiles are determined, in order to compute the Poiseuille, Nusselt and Stanton numbers, which are then employed in computing the objective functions for some PEC
Sensitivity to heat and mass transfer correlations of the model for an evaporative condenser
Full text linkEvaporative condensers have established themselves in refrigeration plants for food process and storage owing
to the combination of air and water cooling, with an overall reduction in energy (pumping) and fluid (water)
consumption. Given the steep rise of electricity prices, the need to develop conduction strategies at the
plant level which are less energy-intensive has driven the need for models to carry out simulations of the
new strategies without actually impacting normal plant operations. This paper describes the development
of a distributed-parameter model of an evaporative condenser with fill-pack, based on Merkel’s pioneering
works on evaporative cooling. Several correlations are available for the heat and mass transfer coefficients,
but none that satisfies the operating conditions of the case at hand, yet they are the most critical set of
parameters for the model, therefore a sensitivity study has been carried out, to ascertain which combinations
of transport coefficients out of a total of 288 would yield a total cooling power closest to the value declared
by the manufacturer for a real-life evaporative condenser under rating conditions. This work demonstrates the
applicability of the correlations examined, and also twenty of the examined combinations allow the model to
reach a good agreement with the reference data (within ±5%). Further, the distributed-parameter structure
of the model allows highlighting the reasons for good or bad performance of the correlations by determining
the spatial distribution in the cooling coil and fill-pack of quantities such as moist air enthalpy and water
temperature. The model and methodologies are a little cost-intensive in terms of computational time and can
be applied to the study and design of any similar piece of equipment
Control-oriented low-order models for the transient analysis of a domestic electric oven in natural convective mode
No full textAlmost 25% of the overall environmental impact of buildings can be attributed to appliances, among which
electric domestic ovens represent a low-efficiency category when considering apartments. In order to improve
their low efficiency, usually lying between 10% and 12%, the oven industry is often stimulated to develop more
efficient technologies, among which there is the design of suitable strategies for the control of the oven center
temperature, as required by the EN 60350-1 European standard, which regulates the energy consumption test.
This work describes the simulation of the transient thermal behaviour of a domestic electric oven in natural
convection heating mode by means of dynamic models suitable for control design purposes. A lumped-parameter
approach based on the thermoelectric analogy was chosen, in order to obtain a low-order dynamic model with a
proper level of discretization which allowed to reproduce the main temperatures within the cavity, in particular
at the oven’s centre and the temperature sensed by the probe used for oven control during normal operating
conditions. Eight lumped parameters were introduced to represent walls, glass door, cavity air and temperature
probe, and three parameters to simulate the thermal behaviour of the heaters were considered, resulting in an
11-th order model. Moreover, the influence of non-linearized radiative terms on model accuracy was investigated,
leading to two different lumped-parameter, semi-physical models. Two sets of capacities and conductances
were identified for both the models through an optimization procedure based on experimental data,
giving a grey-box nature to the models; subsequently the prediction potential of the models was verified at
different temperature set-points
Diaporthe rudis Associated with Berry Rot of Postharvest Grapes in Italy.
No full textGrapes for Italian passito wine production are subject to fungal decay during postharvest processing (withering). In January 2018, decayed berries of Vitis vinifera ‘Nosiola’ were observed in bunches stored in a fruit-drying room in Valle dei Laghi, Provincia Autonoma di Trento (Italy). The bunches displayed single or few berries with brown to dark brown discolorations and solitary or rarely aggregated, erumpent, black ascomata in the skin. The incidence of bunches with symptomatic berries was approximately 5%. Twelve small pieces (about 10 mm2) of epidermal and mesocarp berry tissues were sterilized in 0.5% NaOCl for 2 min, rinsed once in sterile distilled water, and then placed on potato dextrose agar (PDA). After 4 days at 25°C in the dark, fungal colonies showed similar morphology resembling cultures of Diaporthe spp. (Udayanga et al. 2014). Single-spore isolation was carried out to obtain pure cultures. Two representatives of these cultures (P19 [= CBS 145104] and P10) were identified by morphological and molecular analysis. On PDA, after 7 days at 25°C, colonies showed flat, entire edges, whitish to grayish, fluffy aerial mycelium. The reverse colony was light brown to gray, with dark brown pigmentation in the center. Mycelial PDA plugs (2.5 mm2) were transferred to alfalfa stems on water agar (15 g of agar/liter) as described by Udayanga et al. (2014). Pycnidia on alfalfa stems were black, globose, and erumpent at maturity. Conidiophores were hyaline, cylindrical, smooth, branched, ampulliform, straight to sinuous, 18 to 40 × 2 to 2.5 μm. Conidiogenous cells were cylindrical, terminal, with slight tapering toward apex (0.5 to 1 μm diameter). α-Conidia were hyaline, smooth, ellipsoidal, biguttulate, 5.7 ± 0.7 μm long, and 2.3 ± 0.4 μm wide (n = 40). β-Conidia were not observed. Species identification was performed by amplification of internal transcribed spacers, translation elongation factor 1-α, histone H3, and β-tubulin gene sequences using ITS1/ITS4, EF728/EF986, CYLH3F/H3-1b, and Bt2a/Bt2b primers, respectively (Guarnaccia et al. 2018). BLAST analysis and phylogenetic analysis, constructed with MEGA 7.0 software using the neighbor-joining method (evolutionary distances using the Tajima–Nei method) from combined multilocus databases, identified the isolates as Diaporthe rudis. The gene sequences were deposited in GenBank (accession nos. MH447355, MH447356, MH457708, MH457709, and MK205427 to MK205430). Pathogenicity testing was performed on 120 healthy partially dehydrated grape berries (cultivar Nosiola) with intact pedicel, surface sterilized by immersion in 0.5% NaOCl and rinsed three times in sterile distilled water. Each berry was inoculated with a 106 conidia/ml suspension of both isolates by piercing using a sterile tip (wounded berry, n = 50) or pipetting onto the berry surface (unwounded berry, n = 50). The control (20 berries) was inoculated with sterile water by piercing and pipetting. All inoculated withered berries were incubated at 25°C for 7 days at 95% relative humidity (RH). High RH values (>90%) are reached during the natural withering process depending on seasonal weather conditions. Disease index (DI), calculated for four disease severity classes in percentage, where DI = [sum (class frequency × score of rating class)]/[(total number of berries) × 4] × 100, of wounded and unwounded berries was approximately 26 and 18%, respectively. The frequent presence of microcracks and wounds on berry skin, owing to dehydration, handling, and insects that favor the fungal colonization, could explain the high DI values observed in unwounded berries. Symptoms on the infected berries were similar to those observed in the fruit-drying room. There were no symptoms on the control berries. The fungus was reisolated from the infected berries, thus completing Koch’s postulates. Although D. rudis has been recovered from asymptomatic and symptomatic grapevine (cane and leaf spots) (Guarnaccia et al. 2018), this is the first report of the fungus causing rot on grape fruit. Recently, D. eres, together with Pestalotiopsis biciliata and Diplodia seriata, has been associated with fruit rot in withered grapes (Lorenzini and Zapparoli 2018). This finding indicates that D. rudis may contribute, together with other pathogenic fungi, to the decay of grapes during the withering. Such decay is a major concern for producers because it could have a negative impact on wine quality
Occurrence and infection of Cladosporium, Fusarium, Epicoccum and Aureobasidium in withered rotten grapes during post-harvest dehydration
No full textFungi like Cladosporium, Fusarium, Epicoccum and Aureobasidium can occur on withered grapes causing spoilage of passito wine. There is little or no information on the pathogenic role of these fungi. This study describes the isolation, incidence and identification of several isolates from different withered rotten grapes. Representative isolates grouped in several phenotypes were identified by phylogenetic analysis of internal transcribed spacer, actin or elongation factor gene sequences. Isolates of Cladosporium and Fusarium were ascribed to different species, of these C. ramotenellum, C. halotolerans and F. graminearum were isolated from Vitis vinifera for the first time. All Epicoccum and Aureobasidium isolates belonged to E. nigrum and A. pullulans, respectively. Random amplified DNA polymorphism analysis showed high level of heterogenicity among Epicoccum and Fusarium isolates. Infection assays were carried out to evaluate infectivity in some strains under different withering conditions. Fusarium spp. strains had similar infectivity, while significant variability was observed among Cladosporium spp. and E. nigrum strains. A. pullulans resulted particularly infective. This study provided insights into the occurrence and infection of these fungi in fruit-drying rooms with important implications towards control management during the witherin
Thermal field measurements of heat sinks through a novel three-dimensional method based on acoustic waves
No full textNatural convection heat sinks are still important in many industrial applications thanks to their simple design, effectiveness and noise-free operation. In this paper we present some case studies of experimental determination of the thermal characteristics of such devices, carried out using an innovative three-dimensional method of position detection for the temperature transducer (thermocouple). The three-dimensional position detection system uses acoustic waves emitted by a small grid of four loudspeakers, which simultaneously emit pseudo-random noise sequences orthogonal to one another. A small microphone samples the sounds coming from the four loudspeakers and its position in the three-dimensional space is obtained through trilateration calculations and signal analysis. Microphone and thermocouple are mounted on the same movable support, so that by measuring the position of the microphone, the position of the thermocouple is also detected. In this way it is possible to measure the temperature in a cloud of points of known position in space and obtain the three-dimensional thermal field around the heat sink. From these measurements it is possible to evaluate the thermal field around the heat sink and to easily compare different construction geometries. The position detection system has good precision and is inexpensive, allowing this type of measurement to be carried out even in low-budget situations. It also differs from thermal imaging systems such as IR cameras, in that it allows measurement of the temperature in the fluid surrounding the heat sink and not on its surface only
Ontologie di applicazione ed informazione geografica: la ricognizione nel territorio di Siligo (Sassari)
No full textThermal Transport in the Entry Region of a Microchannel in the Presence of Electro-osmotic Flow
Full text linkThe motion of a polar fluid in microducts induced by an external electric field,
known as electro-osmotic flow, enables fluid circulation without the need for mechanical
devices. This feature makes it particularly appealing for the thermal management of electronic
components, as microchannels with almost any cross-sectional shape can be easily integrated
on the chips. Therefore, it is essential to evaluate how the channel’s geometry influences heat
transfer performance. This study investigates the thermal entry region and fully developed
electro-osmotic flow in a microchannel with a rectangular cross-section and smoothed corners,
with one adiabatic wall and uniform temperature elsewhere. The paper proposes correlations for
the Poiseuille and Nusselt numbers, taking into account the aspect ratio and non-dimensional
smoothing radius under fully developed thermal and hydrodynamic conditions, which can be
valuable for practical design purposes. The study also emphasizes how Joule heating may lead
to the reversal of heat flow and how the thermal entry length is linearly dependent on the
logarithm of the non-dimensional Joule heating in thermally developing flow. Additionally, it
demonstrates that smoothing the corners may increase the local Nusselt number over sharp
corners, but it may also shorten the thermal entry length
Numerical Model of an Evaporative Condenser for the Food Refrigeration Industry
Full text linkEvaporative condensers play a key role in refrigeration plants for industrial applications, as they are needed to dispose of the thermal and electric power required by the process. Modelling them as lumped capacitance systems is very expedient when trying to simulate e. g. the steady-state and dynamic behaviour of the whole refrigeration plant, but at the expenses of loss of information and often oversimplifying assumptions. This paper analyses two models for heat and mass transfer in cooling equipment, Poppe's and its earlier, simpler version, Merkel's. The two approaches are applied to describe the transport phenomena in an evaporative condenser and the predicted cooling power compared to the data declared by the manufacturer under certain operating conditions. It is demonstrated that predictions are accurate within less than 10%
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