Frost Conditions Research Articles

Experimental methods for detecting frosting in cross-flow air-to-air energy exchangers

Frost formation in air-to-air heat/energy exchangers is a challenging problem in regions with very cold weather conditions, when energy recovery is most needed. Membrane-based energy exchangers may assist in overcoming frosting. In this paper, an experimental facility was developed to enable researchers to test air-to-air exchangers under frosting conditions. The test setup, sensors and data acquisition system, uncertainty bounds for major parameters, and experimental procedure are described. Two geometrically identical plate exchangers: one with a water vapor permeable membrane (energy exchanger), and one with an impermeable plate (heat exchanger) were tested under frosting conditions. The first step in frosting experiments is to find a reliable and cost-efficient method to detect frosting in the exchangers. Four methods were used to determine the onset of frost growth: visual inspection, change in the effectiveness (Δε), change in the pressure drop across the exchanger (Δp), and change in the outlet temperature (ΔT). The main contributions of the paper are the detailed comparison of these methods and the introduction of a new frost detection method based on temperature measurements only (ΔT method).It is concluded that the Δp and ΔT methods were more reliable and practical than the other methods for both heat and energy exchangers. The ΔT method detected frosting sooner and with lower uncertainties than the other methods. Furthermore, the ΔT method was least affected by the operating conditions. On the other hand, the ΔP method gave a better indication of the severity of frosting in the exchanger as more frost resulted in a higher pressure drop across the exchanger.

Frosting characteristics and heating performance of a direct-expansion solar-assisted heat pump for space heating under frosting conditions

Direct expansion solar-assisted heat pump system (DX-SAHP) is promising in energy saving applications, but the performance of DX-SAHP under frosting conditions is rarely reported in the published literatures. In this paper, a DX-SAHP system with bare solar collectors for space heating is designed and experimentally investigated in the enthalpy difference lab with a solar simulator. The system is tested under a range of frosting conditions, with the ambient temperatures from 7°C to −3°C, the relative humidities of 50%, 70% and 90% and the solar irradiances of 0W/m2, 100W/m2, 200W/m2 and 300W/m2. The conditions when the DX-SAHP system frosts are studied. Results show that solar irradiance as low as 100W/m2 can totally prevent frosting when the ambient temperature is above −3°C and the relative humidity is 70%. Besides, the frosting process is observed to be slower than that of fin-and-tube heat exchangers. The evaporator is not seriously frosted and the system performance is not significantly influenced after 360min of continuous operating. Moreover the effects of ambient parameters, including the ambient temperature and the relative humidity, especially solar irradiation, on the system performance are studied and analyzed. Solar irradiation can effectively prevent or retard frosting, and also improve the heating performance of the DX-SAHP system. The DX-SAHP system is proved to be applicable under frosting conditions.

Patterns of climber distribution in temperate forests of the Americas

We propose a standard protocol at the landscape to continental scale for examining to what extent the range of ecological conditions found in temperate latitudes explains the variations in climber species richness and traits. The protocol was tested in forests of the two Americas. The data set included 151 climber species. We selected four categorical traits and grouped these species into six clusters with regard to these traits. Floristic records of American forests were first gathered into alliances, second combined with bioclimatic indices (rainfall, temperature, continentality). We obtained a total of 59 vegetational units in which we calculated values of climber species richness and proportion of clusters. Vegetational units were ultimately gathered into five forest formations (characterized by leaf longevity). Wetlands and uplands were considered separately. Our results emphasize clear trends in large-scale patterns of climber distribution, independently of taxonomy. Climber species richness (in particular woody climbers) peaks in moist and warm upland forests with oceanic climates, and where conifers are rare. In flooded areas, climber richness is also very high and peaks in seasonally flooded large floodplains. In ecological conditions of frost, dryness or lack of nutrients, climber species richness, abundance and trait diversity decline, resulting in the dominance of small, twining and deciduous life traits.

Modeling and performance analyses of fin-and-tube heat exchanger operating under frost conditions

As far as the frost accumulation in fin-and-tube heat exchangers is concerned (regarding this process as a quasi-dynamic), a time-dependent distributed parameter model of fin-and-tube heat exchangers in the presence of frosting is presented. The model consists of two different sub-models: frosting and heat exchanger. The model results are compared with the experimental data, and an average error between them of ∼20% was found. In addition, the performances of the fin-and-tube heat exchangers under frost conditions, obtained from the experimental results, have been analyzed. These experimental results reveal that the inlet air velocity, temperature and relative humidity strongly affect frost thickness, frost mass accumulation, evaporating pressure and pipe wall temperature.

A fossil giant tortoise from the Mehrten Formation of Northern California

Author(s): Biewer, Jacob; Sankey, Julia; Hutchison, Howard; Garber, Dennis | Abstract: Hesperotestudo is a genus of giant tortoise that existed from the Oligocene to the Pleistocene of North and Central America. Recorded occurrences in the United States are plentiful; however, California seems to be an exception. Literature on Hesperotestudo in California is limited to faunal lists in papers, with few detailed descriptions. Here we review the literature on the genus, describe and identify specimens found in the upper Mehrten Formation (late Miocene-early Pliocene) exposed in the Central Valley of California at Turlock and Modesto Reservoirs, Stanislaus County, and address their implications for early Pliocene California biogeography and climate. All fossils described are from the collections of the University of California Museum of Paleontology (UCMP). The largest specimen from the Mehrten is a peripheral from an animal with an estimated carapace length over one meter. The specimens were compared first to modern material of Gopherus, the only other tortoise genus from the late Miocene-early Pliocene of California, and then to measurements from the literature of the three species of Hesperotestudo to which it could most likely be referred: H. osborniana, H. orthopygia, and H. campester. Based on characteristics and measurements of the carapace and plastron, these specimens are assigned to H. orthopygia. Hesperotestudo orthopygia is a species known primarily from the Great Plains region, so its presence in California during the late Miocene-early Pliocene indicates that it expanded west into California at this time. Large tortoises are not very tolerant of frost conditions, possibly indicating a relatively frost free climate for this area at the time. This agrees with previous estimates of annual temperature records based on plant fossils from the upper Mehrten Formation, in particular the presence of Persea, an avocado relative, which is also frost sensitive.

Performance evaluation of a novel method of frost prevention and retardation for air source heat pumps using the orthogonal experiment design method

In this study, a novel technology of frost prevention and retardation for air source heat pump (ASHP), which reduces thermal discomfort, is introduced. It utilizes auxiliary electric heaters (AEH) on tubes before (B-AEH) and (or) after (A-AEH) the outdoor evaporator to retard frost formation or prevent frost from accumulating on the outdoor heat exchanger. This enables the supply of hot air into the interior space without interruption. This new method differs from the common high pressure hot gas bypass defrosting (HGBD) methods and the reverse cycle defrosting (RCD), both of which could result in significant temperature fluctuations and thermal discomfort. The orthogonal experiment design (OED) was applied in this study to evaluate the performance of the air source heat pump (ASHP) with different magnitudes of AEH power under a range of ambient frosting conditions. The L25(56) orthogonal array was selected for the experiment and data were analyzed by means of the analysis of range (ANORA) and the analysis of variance (ANOVA). The optimum parameter combination affecting the performance of the ASHP was determined and the most significant parameters were identified. It was demonstrated that the proposed ASHP design is effective for frost prevention and retardation in real applications.

Assessing some economic and biological characters of introduced varieties of genus Astilbe Buch.-Ham ex D.Don

Purpose. To study flowering productivity and reproductive ability of multiple-aged plants of introduced Astilba varieties, assess varieties for these characters and investigate the effectiveness of vegetative propagation. Methods. Field experiment, introduction and statistics evaluation. Results. The analysis of published data and author’s own observations revealed that Astilba varieties are highly tolerant for winter and frost conditions as well as pest and disease resistant, therefore we defined flowering productivity and reproductive ability as main economic and biological characters that are subject to assessment. According to the results of flowering efficiency study, it was found that the highest value of this index for the most studied varieties was observed on the fourth year of cultivation. Whereas the majority of varieties rating was relatively low, Astilbe flowering productivity on the whole was sufficiently high. In order to arrange collection plots, flower beds and implement the selection process, the most effective method of vegetative propagation can be used – the division of rhizomes into pieces with three to five renovation buds, and for mass reproduction – with one or two buds. It turned out that for Astilbe the indicator of reproductive ability is specific. For most of the investigated five-year varieties it was quite high. We have noted a high positive correlation between the index of reproductive ability and flowering productivity. Varieties with optimal flowering had high value of planting material yield. Conclusions. Based on the experimental data, the flowering productivity and reproductive ability of multiple- aged plants of Astilbe introduced varieties was studied, varieties was assessed according to these characters, and the effectiveness of vegetative propagation was investigated.

Heat and mass transfer in cross-flow air-to-air membrane heat exchanger in heating mode

The main objective of the paper is numerical simulation and analysis of heat and mass transfer, condensation and frost formation conditions in cross-flow membrane-based heat exchanger. To get the desired results, the following problems were solved: numerical models for heat transfer under uniform wall temperature for parallel plates channel without/with fins were developed; numerical models were validated using literature data; local Nusselt number equations were obtained for parallel plates channel without/with fins; numerical models for heat and mass transfer in cross-flow membrane-based heat exchanger were developed incorporating local Nusselt numbers for finite difference method; numerical models were validated using ε-NTU method and experimental data from laboratory; effectiveness, condensation and frosting conditions in cross-flow membrane-based heat exchanger were numerically studied for different water vapour permeability coefficients, outside air temperature and inside air relative humidity.

Geomorphological processes and frozen ground conditions in Elephant Point (Livingston Island, South Shetland Islands, Antarctica)

Elephant Point is an ice-free area in the SW corner of Livingston Island (Maritime Antarctica). The retreat of Rotch Dome glacier during the Holocene has exposed a land area of 1.16km2. Up to 17.3% of this surface has become ice-free between 1956 and 2010. A detailed geomorphological mapping of this ice-free environment was conducted in late January 2014. A wide range of active periglacial landforms show that periglacial processes are widespread. From the glacier to the coast four different geomorphological areas are identified: proglacial environment, moraine complex, bedrock plateaus and marine terraces. In situ measurements of the thawed soil depth show evidence of the widespread frozen ground conditions in the area. Field observations of permafrost exposures suggest that these frost conditions may be related to a soil permafrost regime, almost down to sea level. The activity of penguin colonies and elephant seals has created minor geomorphological features in the raised marine terraces. Here, several archaeological sites related to early human colonization of Antarctica were also found in natural shelters.

Visualization study of the effect of surface contact angle on frost melting process under different frosting conditions

Frost melting is closely related to solid surface characteristics and initial frost layer. In this paper, visualization study of frost melting processes on different surfaces, from hydrophilic to superhydrophobic, was carried out under different frosting conditions. On the hydrophilic and hydrophobic surfaces, the whole frost layer is divided into several blocks accompanying with melting, and the frost-water mixed blocks finally shrink into water droplets because of the surface tension and surface free energy. However, frost layer directly sheds from the superhydrophobic surface and only several spherical water droplets remain on the surface. The surface coverage ratio and frost-melting water mass all decrease with the increase in surface contact angle, which are smallest on the superhydrophobic surface. The frosting time and multiple frosting cycles show significant effects on frost melting of the hydrophilic and hydrophobic surfaces while frosting time and multiple frosting cycles have no influence on that of the superhydrophobic surface.

Rule-based Mamdani-type fuzzy modelling of thermal performance of fintube evaporator under frost conditions

Frost formation brings about insulating effects over the surface of a heat exchanger and thereby deteriorating total heat transfer of the heat exchanger. In this study, a fin-tube evaporator is modeled by making use of Rule-based Mamdani-Type Fuzzy (RBMTF) logic where total heat transfer, air inlet temperature of 2 °C to 7 °C and four different fluid speed groups (ua1=1; 1.44; 1.88 m s-1, ua2=2.32; 2.76 m s-1, ua3=3.2; 3.64 m s-1, ua4=4.08; 4.52; 4.96 m s-1) for the evaporator were taken into consideration. In the developed RBMTF system, outlet parameter UA was determined using inlet parameters Ta and ua. The RBMTF was trained and tested by using MATLAB® fuzzy logic toolbox. R2 (%) for the training data and test data were found to be 99.91%. With this study, it has been shown that RBMTF model can be reliably used in determination of a total heat transfer of a fin-tube evaporator.

English

In the present study, the CO 2 frost phenomenon of CH 4 -CO 2 mixtures has been observed for the rational design of CO 2 removal from natural gas using a controlled freeze out area. The CO 2 frost conditions were estimated using the ZNE method and process simulation software (Aspen HYSYS® v7.3). The experiment was carried out using a double pipe heat exchanger (DPHE) with the concentration of CO 2 in the gas mixture at 5 and 10% and pressure of the gas mixture from 1 to 20 bar. The equilibrium temperature predictions of the ZNE method and the process simulation software only had a slight difference, with a magnitude deviation of less than 1% for pressures below 20 bar and 3% for pressures in the range of 20-30 bar, respectively. In the experimental study, CO 2 frost formation was detected at pressures of 1, 5, 10 and 20 bar. The locations of the initial CO 2 frost formation were determined using a pressure drop indicator associated with the predicted frost temperatures obtained from the ZNE method and the process simulation software. For all studied variables, the locations of initial CO 2 frost formation were found at 0.887-1.531 m from the inlet.

Frosting behaviors and thermal performance of louvered fins with unequal louver pitch

We created heat exchanger louvered fin designs in which the louver pitch successively increased or decreased by 20% from the air inlet region to the redirection region. The frost behavior and thermal performance of the unequal louver pitch design were compared with those of an equal louver pitch design, under both dry and frost conditions. When unequal louver pitch design was used, frost blocking of the spaces between louvers at the front side was delayed and the thermal performance was improved by 21%, compared with the equal louver pitch design. The frost blocking ratios of the two unequal louver pitch designs were compared; the design in which the louver pitch successively decreased from the air inlet region to the redirection region provided more uniform frost growth and improved thermal performance, compared to the design in which the louver pitch successively increased.

Antifogging and Frost-Resisting Polyelectrolyte Coatings Capable of Healing Scratches and Restoring Transparency

Polymeric antifogging/frost-resisting coatings are suitable for use on flexible substrates but are vulnerable to accidental scratches and cuts. To solve this problem, we present the fabrication of healable, highly transparent antifogging and frost-resisting polymeric coatings via the layer-by-layer assembly of poly(ethylenimine) (PEI) and a blend of hyaluronic acid and poly(acrylic acid) (HA-PAA). Due to their remarkable water-absorbing capability, the highly transparent and flexible (PEI/HA-PAA)*50 coatings show excellent antifogging and frost-resisting capabilities even under aggressive fogging and frosting conditions. Meanwhile, these coatings can conveniently and repeatedly heal scratches and cuts several tens of micrometers deep and wide in the same region upon exposure to water because of the dynamic nature of the PEI/HA-PAA coatings. The healability of the (PEI/HA-PAA)*50 coatings provides a new way to design transparent antifogging/frost-resisting polymeric coatings with high flexibility, enhanced...

Field test study of a novel defrosting control method for air-source heat pumps by applying tube encircled photoelectric sensors

To improve the defrosting accuracy of air source heat pumps (ASHPs), this paper proposes a novel defrosting control method by applying tube encircled photoelectric sensors (TEPSs). A field test was conducted for two heating seasons in Beijing, China, to verify the feasibility and practicality of the novel TEPS method. The test results revealed that irrespective of the environmental conditions, the TEPS method can initiate defrosting in similar situations: most of the heat exchanger surface had been covered by frost; the compressor suction temperature decreased by ~9 °C; the compressor discharge temperature increased by ~16 °C; and the heating capacity decreased by ~30%. Furthermore, the TEPS method was verified to make more accurate and more reasonable defrosting decisions than the traditional T-T method under both frosting and non-frosting conditions. The results indicate that the TEPS method is a competitive defrosting control method that can be used for ASHPs.

Deciduous forest responses to temperature, precipitation, and drought imply complex climate change impacts

Changes in spring and autumn phenology of temperate plants in recent decades have become iconic bio-indicators of rapid climate change. These changes have substantial ecological and economic impacts. However, autumn phenology remains surprisingly little studied. Although the effects of unfavorable environmental conditions (e.g., frost, heat, wetness, and drought) on autumn phenology have been observed for over 60 y, how these factors interact to influence autumn phenological events remain poorly understood. Using remotely sensed phenology data from 2001 to 2012, this study identified and quantified significant effects of a suite of environmental factors on the timing of fall dormancy of deciduous forest communities in New England, United States. Cold, frost, and wet conditions, and high heat-stress tended to induce earlier dormancy of deciduous forests, whereas moderate heat- and drought-stress delayed dormancy. Deciduous forests in two eco-regions showed contrasting, nonlinear responses to variation in these explanatory factors. Based on future climate projection over two periods (2041-2050 and 2090-2099), later dormancy dates were predicted in northern areas. However, in coastal areas earlier dormancy dates were predicted. Our models suggest that besides warming in climate change, changes in frost and moisture conditions as well as extreme weather events (e.g., drought- and heat-stress, and flooding), should also be considered in future predictions of autumn phenology in temperate deciduous forests. This study improves our understanding of how multiple environmental variables interact to affect autumn phenology in temperate deciduous forest ecosystems, and points the way to building more mechanistic and predictive models.

착상조건에서 R134a와 R1234yf를 적용한 핀-관 형태의 증발기 성능 비교

Abstract The low temperature distribution of the refrigerated and frozen food has been increased gradually. Refrigeration industry is using R134a refrigerant, which GWP is 1300. R1234yf is an alternative refrigerant of R134abecause GWP of R1234yf refrigerant is just 4. Evaporator used in refrigeration truck refrigeration system is operatedon low temperature condition. Accordingly, evaporator is formed frost and the formation of frost is rapidly decreasedperformance of evaporator. In this study, the performance of evaporator using R134a and R1234yf refrigerant was analyzed with operating conditions under frost condition. As a result, the performance of R134a evaporator accordingto air inlet temperature, relative humidity and evaporating temperature was more sensitive than R1234yf evaporator.Besides, the frost growth of R134a evaporator is steeper than that of R1234yf one. Keywords : Evaporator, Frost thickness, Refrigeration capacity ratio, R134a, R1234yf * Corresponding Author : Honghyun Cho(Chosun Univ.)Tel: +82-62-230-7769 email: hhcho@chosun.ac.krReceived June 3, 2015Accepted September 11, 2015Revised August 31, 2015Published September 30, 2015

Impacts of snow season on ground-ice accumulation, soil frost and primary productivity in a grassland of sub-Arctic Norway

Europe’s and the World’s northernmost agriculture is very vulnerable to harsh overwintering conditions. It is important from both an economic and societal standpoint to have accurate methods of predicting the severity and impact of the current snow season. Technology has advanced to enable such measurements to be regularly recorded but despite this, a detailed assessment, involving remote sensing , of the impacts of various types of snow season on agricultural yields in northernmost Europe has not previously been undertaken. Here we characterize variation in snow types and concomitant soil frost and ground-ice accumulation at a Norwegian sub-Arctic, maritime-buffered site (Tromsø, Troms County, 69 °N) during the period 1989/90 to 2013/14 and analyse how winter conditions affect agricultural productivity (both measured in the field and using remote sensing). These data were then used to build important predictive modelling approaches. In total, five contrasting types of snow season were identified, from snow-rich with no soil frost and no ground-ice to low snow and considerable soil frost and ground-ice. Conditions of low snow and low soil frost and ground-ice that result from numerous warming events were rare within the time period studied but are predicted to become the dominant snow season type. Agricultural productivity was lowest and claim settlements paid to farmers were highest after winters with high accumulation of plant-damaging, hermetic ground-ice. Deep soil frost per se did not affect primary productivity. Overall, our results together with information from other sources, suggest that icy, low snow conditions are the most challenging of all seasonal types for both the environment and livelihoods in sub-Arctic Norway. Winters with extremely deep snow also cause considerable problems. As winters are expected to warm more than summers, it is likely that the winter climate will become an even stronger regulator of northern primary productivity. To better understand the physical and biological effects of the changing winter climate, there is a requirement for continued and increasing monitoring of winter processes, especially related to frost and ice in the rhizosphere, as this is currently not well covered in national monitoring programs. Continued monitoring will enable further refinement of predictions and will support the better community planning for greatest agricultural benefit.

Adfreeze Forces on Lightly Loaded Pile Foundations of Solar PV Farms in Cold Regions

Renewable energy generation through utility scale ground mounted solar photo-voltaic systems has gained steady popularity with increasing number of such facilities being constructed in various regions worldwide. Solar PV systems are very popular in the province of Ontario in Canada and strong growth in this sector is led by the popular initiatives of the Government of Ontario which offers extremely attractive rates for generation of renewable energy through Ontario Hydro’s popular Feed-In Tariff (FIT) Program. Many other countries offer incentives on such generation of renewable energy while many governments aim at increasing the percentages of renewable energy in their systems tremendously. Most ambitious plan has recently been launched by the state of Hawaii to deploy 100% of renewable energy in their grid by 2045. Solar PV systems are a cheap source of renewable energy as the energy released by the sun is harnessed as electricity by the solar photo-voltaic panels which is fed to the main transmission systems after raising its voltage. The costs of solar photo-voltaic panels meanwhile have also kept downward trends while the manufacture of various types of solar panels has multiplied rapidly. These renewable energy generation facilities are fully sustainable being completely recyclable on completion of their design/ contract period. Typical utility scale ground mounted solar PV facilities usually comprise of solar PV panels mounted on series of racking tables supported on foundations mostly comprising of partially embedded steel pipes. The governing loads for the foundations of these lightly loaded solar PV structures are usually frost loads in areas facing extremely cold winters. In fine grained soils like silty/ clayey soils, large adfreeze stresses develop due to penetrating frost deep into the soil resulting into uplift of foundation piles. Typical winter conditions in Ontario are harsh with extreme frost conditions in most areas which poses unique issues for design and construction of such foundations. Being a relatively newer technology, codes and standards for design and testing of such lightly loaded solar PV structures are still in the formulation stages. Frost heaving and its effects often create adverse conditions for these structures thereby affecting the production and continuous supply of renewable energy. Due to larger depths of frost penetration in extreme winter conditions, understanding the action of frost and related development of adfreeze stresses on these lightly loaded pile foundations is extremely important. Calculating reasonable frost depths and thereby the design loads is an important part of pile design for such facilities while the contractors tend to save on pile lengths to save on costs and compromising the structural design. Many such Solar PV facilities have experienced frost uplift of foundation piles either during the construction phase or during its lifetime. Since frost heave is more of a serviceability related issue, unfactored adfreeze loads without any factor of safety is a usual tendency by the EPC contractors. This paper investigates the frost depths and adfreeze stress related issues with the foundation piles of solar PV facilities hence the governing design forces on these piles and suggests appropriate frost related design stresses for the foundation piles. The authors have been heavily involved in design/ design reviews, pile selection/ design and pile load testing in the majority of the solar PV farms in Canada and US along with rehabilitation of piles affected by frost [1,2,3].

Experimental investigation of the performance of microchannel heat exchangers with a new type of fin under wet and frosting conditions

The performances of three different types of microchannel heat exchangers were experimentally investigated and compared under wet and frosting conditions. In all contrast tests, the heat exchanger with a new type of fin with drainage design exhibited the best performance. Under wet conditions, the capacity of the sample with a new type of fin and that of the sample with a wavy fin were 56.7% and 25.8% higher than that of the sample with a louver fin, whereas the air pressure drops were 63% and 35% lower at 1.0 m-s−1. Under frosting conditions, the new sample exhibited stable performance, whereas the performance of the wavy fin sample worsened over time because water retention on the heat exchanger increased as the cycles progressed. After 5 frost-defrost cycles, the maximum capacity of the wavy sample decreased by 26%, the air pressure drop increased by 69%, the frosting time decreased by 35%. The new sample was also tested under different frosting conditions to study their effects on frost formation. From the results, the air humidity and evaporator inlet temperature had a significant effect on frosting time and maximum capacity, whereas the air superficial velocity had a small effect on the rate of the frost formation.