Frost Conditions Research Articles

Effects of surface coating and water retention on frost formation in microchannel evaporators (ASHRAE RP-1589)

This study experimentally investigated frost growth on louvered folded fins in outdoor microchannel heat exchangers when used in air-source heat pump systems. The effects of five surface coatings on the thermal and hydraulic performance of the microchannel heat exchangers under frosting conditions were studied. The overall aim was to provide some guidelines about frosting behavior of microchannel heat exchangers with hydrophilic and hydrophobic coatings. The approach taken was to perform laboratory experiments on small-scale coils and several sample fins that were cut out from commercially available heat exchangers. The sample fins were tested in controlled laboratory conditions that replicated those in actual heat pump systems in the winter season. A better estimation of the fin surface temperature was achieved with the methodology developed in the present study. The experimental value of 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 frost growth rates depended mainly on the local fin surface temperature. Five surface coatings were investigated, and the data show that the hydrophobic coating had slightly lower frosting performance after a few defrost cycles with respect to hydrophilic surface coating.

Water and nitrate movements in an agricultural field with different soil frost depths: field experiments and numerical simulation

AbstractTo evaluate water and anion movement in an agricultural field in different frost conditions, a paired-plot field experiment was conducted at an agricultural site in northern Japan where a significant decrease in the frost depth has occurred during the past 20 years. Snow cover was removed to enhance soil freezing in one plot (treated plot), whereas natural conditions were maintained in a control plot. The maximum frost depth was 0.43 m in the treated plot and 0.11 m in the control plot, which induced substantial differences in water movement throughout the winter. A substantial amount of water moved upward before the onset of snowmelt. However, nitrate did not move markedly before the snowmelt period in either plot. The amount of snowmelt infiltration in the control plot was larger than in the treated plot. Correspondingly, the peak of nitrate content in the control plot was deeper than that in the treated plot after the snowmelt period. Soil freezing, snow accumulation and snowmelt processes were simulated reasonably well using a one-dimensional numerical model: Simultaneous Heat and Water (SHAW). Nevertheless, the model performed poorly for simulating soil thawing and soil water movement, suggesting a need for improvement.

Cold Region Geomorphology in Japan

This paper reviews glacial and periglacial geomorphology and associated studies on physical geography and Quaternary geology, undertaken by Japanese researchers during the last three decades. Major progress has been achieved in two aspects. First, overseas fieldwork in a wide range of polar and alpine regions has promoted under - standing of present-day glacier and permafrost landforms, processes and sediments. The outcomes have greatly in- fluenced domestic research, improving the interpretation of past glaciation and periglaciation in Japan. Second, Japa- nese studies have made significant contributions to the international research communities in applying tephrochro- nology to dating tills and periglacial structures, proposing a distinct feature of glacial advances possibly reflecting climate conditions in eastern Asia, promoting comprehensive monitoring of periglacial processes and understanding periglacial processes in seasonal frost and marginal periglacial conditions.

Прогнозирование температурного режима грунтов оснований зданий и сооружений в условиях линзовой мерзлоты

Прогнозирование температурного режима грунтов оснований зданий и сооружений в условиях линзовой мерзлоты

Characteristics and performance evaluation of surface-treated louvered-fin heat exchangers under frosting and wet conditions

Hydrophilic, hydrophobic, and dual (hydrophilic and hydrophobic) coatings were applied to louvered-fins typically used for heat pump heat exchangers, and the characteristics and performance of the heat exchangers under both frosting and wet conditions were compared according to surface treatment. The hydrophilic heat exchanger had the highest air-side pressure drop under frosting conditions. The hydrophobic unit had a lower air-side pressure drop than the others (hydrophilic and dual), due to frost retardation, and the reduction of the heat transfer rate was also smaller. The dual-fin heat exchanger exhibited frost retardation only in the early stage of the experiment, and the heat transfer rate was slightly greater than that of the hydrophilic unit. In wet-condition experiments for evaluating the evaporating performance of surface-treated heat exchangers, the hydrophilic unit had a lower air-side pressure drop than the others, due to the thin film of water condensation on the fins. The differences in the heat transfer rates of the heat exchangers were not significant.

Characteristics of an air source heat pump with novel photoelectric sensors during periodic frost–defrost cycles

To avoid mal-defrost phenomenon, an innovative photoelectric sensor is developed and presented in this paper. It is referred to as “Tube Encircled Photoelectric Sensor” (TEPS). Experiments are carried out in a controlled environmental chamber under standard frosting conditions. Ten TEPSs in 4 different models are tested on a commercial size air source heat pump with the nominal heating capacity of 60 kW. The characteristics of the air source heat pump, together with the performance of the TEPSs are investigated during 9 periodic frost–defrost cycles. Compared with the original defrosting control strategy equipped by the manufacturer, the proposed TEPS sensor reveals its potential ability to accurately control the defrosting process. Experimental results demonstrate that TEPSs can substantially prolong defrost intervals from 28.8min to 52min under the experimental conditions, and the number of defrost cycles can be reduced from 9 to 5. The performance improvement is found to be 6% to the heating efficiency, and 5% to the COP.

A new method for preventing air-source heat pump water heaters from frosting

This paper proposes a frost-free air-source heat pump water heater (ASHPWH) system with integrated solid desiccant, in which frosting can be retarded by dehumidifying air before it enters the ASHPWH evaporator. The system consists of a conventional heat pump water heater and an extra heat exchanger coated by a solid desiccant. During the desorption process of the desiccant, regeneration air is cycled between the desiccant-coated heat exchanger and the evaporator to recover the sensible heat and latent heat of the regeneration air. A numerical simulation is carried out at a dry-bulb temperature of −7 °C–5.5 °C and a relative humidity of 60%–80% depending on the frosting conditions. The results show that the coefficient of performance (COP) is within the range 3.3–3.8, which is 5–30% higher than that of the ASHPWH system using the hot-gas defrosting method.

Spatial variations of surface water chemistry and chemical denudation in the Erdalen drainage basin, Nordfjord, western Norway

This study conducted over an investigation period of six years (2004–2010) focuses on spatial variation in chemical components of drainage water from sub-areas within a confined drainage basin of 79.5km2 (Erdalen drainage basin, inner Nordfjord, western Norway) for detection of spatial diversity and relation to environmental factors. The Erdalen drainage basin is homogenously composed of gneisses, which is reflected in a homogenous relative (as a percentage) chemical composition of surface water across the entire drainage basin system. At the same time, the mean annual TDS values of surface water sampled in creeks draining defined subsystems within Erdalen show a rather high spatial variability. The main controls of this spatial variability are (i) differences in slope deposit/regolith thickness, (ii) differences in slope angle, (iii) differences in areal regolith cover, (iv) differences in vegetation cover, (v) differences in snow cover and ground frost conditions, and (vi) differences in elevation (ma.s.l.). Altogether, the mean annual TDS values in Erdalen are rather low, which can be explained by (i) the shallow thickness of regolith across the very steep drainage basin, (ii) the small percentage of surface areas showing a significant cover of regolith, (iii) the cool climate in the fjord landscape of western Norway and (iv) the weathering resistance of the predominant gneisses within Erdalen. The annual chemical denudation rates in Erdalen are in a similar range of magnitude to rates published for numerous other cold environment catchments worldwide. Despite the rather low mean annual TDS values, chemical denudation is a comparably important and spatially very variable denudational process and should therefore not be neglected when studying slope development as well as slope- and catchment-wide denudation rates and mass budgets in this kind of steep, mountainous cold climate drainage basins.

A new method of defrosting evaporator coils

A new method is presented to defrost evaporator coils of heat pumps using air as a heat source. At low outdoor temperatures the evaporation temperature can drop below the freezing point of water, the water vapor in the air then freezes on the outer surface of the coil. This increases air side pressure drop and reduces the heat transfer capability of the evaporator coil, leading to a decrease in system efficiency. Long frost build-up times would lead to a partly or totally blocked evaporator coil, rendering the system inoperable. To maintain the functionality of the system it is therefore necessary to remove the frost regularly. For a reversible air conditioning system this is typically done by reversing the flow of the system. In the reversed mode the outdoor coil serves as a condenser, hereby melting the frost on the coil surface. Each of these defrost cycles however further reduces the system efficiency substantially. The new method uses an actively distributing valve which is able to feed parallel evaporator passes individually. With this valve single evaporator circuits are regularly shut off. While no refrigerant is evaporated in a closed circuit, the coil surface temperature increases and the flow of the ambient air is sufficient to defrost this part of the evaporator as long as the air temperature is above 0 °C. Experimental results show that under standard frost conditions the evaporator can be kept frost-free and even under severe conditions most of the highly inefficient system reversals can be avoided. Thereby system efficiency is increased significantly.

Combination of air-source heat pumps with liquid desiccant dehumidification of air

This paper proposes a frost-free air source heat pump system with integrated liquid desiccant dehumidification, in which frosting can be retarded by dehumidifying air before entering an outdoor heat exchanger. And the water removed from the air is used to humidify a room. Simulation is carried out at a dry-bulb temperature of −7 to 5.5°C and a relative humidity of 80% depending on the frosting conditions. The results show that the coefficient of performance (COP) is in the range of 2.6–2.9, which is 30–40% higher than that of heat pump heating integrated with an electric heater humidifying system. And it is found that the optimum value of the concentration of lithium chloride aqueous solution is 37% for the frost-free operation mode. Experiments are conducted for liquid desiccant system under low air temperature and high relative humidity conditions. Experimental results show that the dew point of the dehumidified air is decreased by 8°C and the humidity ratio of the humidified air is kept at 8.1gkg−1, which ensures the frost-free operation of the heat pump evaporator and the comfortable level of room humidity simultaneously. The heating load of solution is 3–4.5 times larger than cooling load of solution, which agrees with the assumption given at the part of the simulation. Furthermore, the deviations between the calculated COPLHRU and the experimental results are within 33%.

Numerical Model on Frost Height of Round Plate Fin Used for Outdoor Heat Exchanger of Mobile Electric Heat Pumps

The objective of this study is to provide the numerical model for prediction of the frost growth of the round plate fin for the purpose of using it as a round plate fin-tube heat exchanger (evaporator) under frosting conditions. In this study, numerical model was considering the frost density change with time, and it showed better agreement with experimental data of Sahin (1994) than that of the Kim model (2004) and the Jonse and Parker model (1975). This is because the prediction on the frost height with time was improved by using the frost thermal conductivity reflecting the void fraction and density of ice crystal with frost growth. Therefore, the developed numerical model could be used for frosting performance prediction of the round plate fin-tube heat exchanger.

COMPARISON OF FROST AND DEFROST PERFORMANCE BETWEEN MICROCHANNEL COIL AND FIN-AND-TUBE COIL FOR HEAT PUMP SYSTEMS

This paper presents a comparison of frost and defrost cycling performance between a microchannel heat exchanger with louvered fin and a fin-and-tube heat exchanger with straight fins employed as outdoor coils of a 14 kW (48 000 Btu/h) heat pump system. In addition to temperature, pressure and flow rate measurements taken at various locations of the systems, the fin-base and tube wall surface temperature were also recorded by using fine-gauge precalibrated thermocouples on the coils. Further, load cells were used to measure the mass of frost accumulation during heating tests. Data showed that the frosting time of the microchannel heat exchanger is more than 50% shorter than for the fin-and-tube heat exchanger, which is chosen as the baseline system. The average heating capacity and system performance over a frost–defrost cycle are also lower for the system with microchannel heat exchangers. Higher frost growth rate was mainly due to augmented temperature difference between air and the surface of the heat exchanger, and preferential frost nucleation sites on the louvered fins and microchannel tubes. Removal of residual water in the microchannel heat exchanger did not improve the frost performance significantly. Blowing nitrogen on the microchannel coil after defrost removed any visible water retained in the coil after the defrost cycle but the cycle time increased only by 4% with respect to wet and frost conditions. The cycle time of the same microchannel coil starting with dry conditions was about 60% longer than the cycle time in wet and frost conditions.

Modelling temperature, moisture and surface heat balance in bare soil under seasonal frost conditions in China

Soil heat and moisture processes are interconnected, especially during low temperatures. To examine the interaction between soil temperature and moisture under freeze-thaw cycles, a physical process-based model (CoupModel) coupled with uncertainty analysis was applied to 3-year measurements under seasonal frost conditions from a site in the black soil belt of northeast China. The uncertainty in parameters and measurements was described by general likelihood uncertainty estimation (GLUE). To identify the degree of linkage between soil temperature and moisture, three criteria were applied to them separately or together. The most sensitive parameters among 26 site-specific parameters were closely related to soil heat, soil evaporation and freeze-thaw processes. Soil temperature was simulated with less uncertainty than soil moisture. Soil temperature measurements had the potential to improve model performance for soil water content, whereas soil moisture measurements demonstrated a trade-off effect when finding a model with good performance for both temperature and moisture. During winter conditions the uncertainty ranges of soil temperature were most pronounced, probably because of the greater complexity of soil properties during the freeze-thaw process and the uncertainty caused by snow properties. The largest uncertainty ranges of both soil water content and soil water storage were found mainly in the deep soil layers. The simulated surface heat fluxes are an important output of the model and it is of great value to compare them with the results from regional climate models and micrometeorological measurements.

냉동탑차용 냉장시스템의 착상 및 무착상 상태에서의 성능변화에 관한 해석적 연구

ABSTRACT:To analyze the cooling performance in the refrigerator truck according to frost growth, the analytical model of refrigeration system was developed under frost and non-frost condition using EES. The system performance was analyzed with outdoor temperature, storage temperature, outdoor front air velocity and compressor speed in order to investigate the system performance characteristics with operating conditions. Besides, the system performance under frost condition was compared with that under non-frost condition. As a result, the frost thickness was 0.9 mm when the refrigerating capacity of frost condition was decreased by 30%. The maximum of the system COP was shown at compressor speed of 1500 rpm for non-frost and frost condition, simultaneously. The performance under frost condition was more sensitive to the operating con-dition compared to that under non-frost condition.Keywords:Refrigerator truck(냉동탑차), COP(성능계수), Frost(서리), R404A †Corresponding author Tel.: +82-62-230-7050; fax: +82-62-230-7055 E-mail address: hhcho@chosun.ac.kr

A generalized simple model for predicting frost growth on cold flat plate

With the aim to develop a generalized model for predicting the frost growth on cold flat plate, experiments were conducted in a suction-type open-loop wind tunnel under variant frosting conditions. Based on the Hayashi correlation, a modified correlation of initial frost density was developed by the experimental results. After validating this proposed correlation, dynamic frosting characteristics, such as the frost thickness, density, frost surface temperature and heat flux were predicted under a wide range of frosting conditions. Compared with 25 groups of experimental results taken from this paper or previous open literatures, the average simulating errors were found to be limited within ±10%. The validation results indicate the proposed model is suitable to predict the dynamic heat and mass transfer process of the frost growth under a wide range of frosting conditions.

Experimental analysis of frost formation with the presence of an electric field on fin and tube evaporator

This paper deals with an experimental study carried out to investigate the influence of DC electric fields on the performance of a fin and tube evaporator under frosting conditions. Several experiments are carried out measuring frost mass, air-side pressure drop and cooling capacity for different applied voltage, air velocity and evaporation temperature. Results show that the electric field reduces the frost mass and air-side pressure drop whereas it increases the cooling capacity. Finally, with a superimposed electric field, the overall energy consumption of the refrigerating unit reduces when referred to the same refrigerating effect.

Modeling the airside dynamic behavior of a heat exchanger under frosting conditions

A general distributed model with a non-steady-state heat exchanger model coupled with a frost model was developed to study the dynamic behavior of an airside heat exchanger in an air-to-water heat pump heater/chiller unit. The effects of water vapor diffusion and uneven fin temperature distribution were considered. The model was found to agree well with reported experimental results. Compared with the routine model, the present model has higher precision of frost layer thickness especially on the fin surface. Results include the propagation of frost formation along the tube and its effect on the dynamic characteristics of refrigerant, air, and tube sides. According to the results, the temperature difference between air and tube surface temperature was proposed to be the main driving force of frosting. Tube surface temperature is the most important factor affecting frosting when there is little variation in air humidity. Frost at the fin base was found to be thicker than that at the fin tip due to the fact that the frost layer grows faster with lower tube surface temperature.

Effect of seasonal frost conditions on the permanent strain behaviour of compacted unbound granular materials used as base course

Frost action affects permanent strain behaviour of unbound granular materials' performance by causing an increase in saturation degree as well as a decrease in density. This research aims to improve the understanding on how the base granular materials' permanent strain behaviour changes through seasons. In order to characterise this behaviour, eight permanent strain tests were performed at various water contents, densities and test conditions. Freeze and thaw cycles, for drained and undrained testing conditions, were applied to samples to measure the contribution of frost action to the plastic deformation mechanisms. From the obtained results, it is concluded that the effect of the freeze–thaw cycle on the permanent strain behaviour of unbound granular materials used as pavement base is mostly explained by the increase of water content and, to a lesser extent, by the decrease of dry density.

Severe soil frost reduces losses of carbon and nitrogen from the forest floor during simulated snowmelt: A laboratory experiment

Considerable progress has been made in understanding the impacts of soil frost on carbon (C) and nitrogen (N) cycling, but the effects of soil frost on C and N fluxes during snowmelt remain poorly understood. We conducted a laboratory experiment to determine the effects of soil frost on C and N fluxes from forest floor soils during snowmelt. Soil cores were collected from a sugar maple (Acer saccharum)–American beech (Fagus grandifolia) and a red spruce (Picea rubens)–balsam fir (Abies balsamea) forest at the Hubbard Brook Experimental Forest in New Hampshire, U.S.A. Soils were exposed to one of three temperature treatments, including severe (−15 °C), mild (−0.5 °C), and no soil frost (+5 °C) conditions. After one week the soils were incubated at +5 °C and snow was placed on top of the soils to simulate spring snowmelt. NO3− losses were up to 5.5 mg N kg−1 soil greater in the mild soil frost treatment than the severe soil frost treatment. Net losses of NH4+ and DON in leachate were up to 19 and 18 mg N kg−1 soil greater in the no soil frost and mild soil frost treatments, respectively, than the severe soil frost treatment. In contrast, soil frost did not have a significant impact on dissolved organic C or cumulative gaseous fluxes of C and N throughout the snowmelt period. However, the total cumulative flux of C (i.e. dissolved organic C + CO2 + CH4) and N (i.e. dissolved organic N + NH4 + NO3 + N2O) in the severe soil frost treatment were between one quarter and one half that observed in the no soil frost treatment for both forest types. Together, the results of this study show that total fluxes of N in leachate, as well as total cumulative C and N fluxes (gases + leachate), were significantly reduced following severe soil frost. We conclude that the extent to which C and N cycling during snowmelt is altered in response to changes in winter climate depend on both the presence and severity of soil frost.

과부하 및 착.제상 조건에서 히트펌프 칠러의 난방성능에 관한 실험 연구

In the present study, the performance characteristics of the heat pump chiller under heating conditions were experimentally investigated. Capacity, input power and COP under overload, frost and defrost conditions were obtained. The experimental data for the heat pump chiller were measured using the air-enthalpy calorimeter and the constant temperature water bath. At overload condition, the heating capacity and COP increase about 25.7% and 19.1%, respectively. The variations of the evaporator, the compressor outlet and the condenser temperature were obtained under frost and defrost conditions. The frost and defrost period of the heat exchanger decreases about 36.0~56.1%.