•Frost growth behavior at ultra-low temperature were investigated.•Three unique frosting phenomena were observed.•Frost growth mechanism was classified by frost surface temperature.•The effect of ...frosting factors on the frost growth behavior was investigated.
An experiment was conducted to investigate the frost growth mechanism and its behavior under forced convection and ultra-low temperature (−160°C <Tw<−100°C) conditions. In ultra-low temperature frosting, peculiar phenomena such as formation of thread-shaped frost and a frost hill was observed. The point at which the frost growth mechanism changes was identified, and two stages of the frost growth mechanism were defined based on this point. The effect of cooling surface temperature, absolute humidity, air velocity, and air temperature on frost growth were investigated. As the cooling surface temperature decreased, the frost thickness increased, whereas the density was less affected. The results show that the frosting mechanism was dependent on the frost surface temperature, and it will be a useful guideline to solve the problems caused by frost generated under ultra-low temperature conditions.
Frost modeling under cryogenic conditions Byun, Sungjoon; Jeong, Haijun; Kim, Dong Rip ...
International journal of heat and mass transfer,
November 2020, 2020-11-00, 20201101, Letnik:
161
Journal Article
Recenzirano
•Frosting experiment was conducted under cryogenic forced convection condition.•Frosting model for cryogenic conditions was developed and validated.•Frost growth behavior was numerically analyzed ...under cryogenic conditions.•Frost properties were discussed qualitatively and quantitively.
A frost model for predicting frosting behavior under cryogenic conditions was developed. The frosting behavior was visualized through numerical analysis, and the results were compared to experimental data to confirm the validity of the model. It was shown that under cryogenic temperatures, a considerably low-density frost layer was formed owing to the frosting mechanism, which was sublimation-dominant frosting, rather than conventional condensation-freezing. It was qualitatively confirmed through experiments that low-density frost formed near the frost surface, and high-density frost formed near the cryogenic cold plate. Furthermore, the frost density distribution of the interior frost was quantitatively analyzed through a numerical analysis. The surface temperature of frost was seen to rise rapidly during the initial stage of frosting. Thus, it is possible to estimate the time at which the frosting mechanism changes from sublimation to condensation-freezing.
According to the American Association for the Study of Liver Diseases (AASLD) treatment guidelines for hepatocellular carcinoma (HCC), the role of surgery has been expanded beyond the Barcelona ...Clinic Liver Cancer (BCLC) algorithm. We compared primary hepatectomy (PH) with transarterial chemoembolization (TACE) in patients with intermediate‐ to advanced‐stage (BCLC stage B/C) HCC to determine the current evidence. Through a database search, we included 18 high‐quality studies (one randomized controlled trial RCT, five propensity‐score matching nonrandomized comparative trials NRCTs, and 12 NRCTs) that compared survival outcomes of 5,986 patients after PH and TACE. We found significant survival benefits for PH over TACE in BCLC stage B/C patients (hazard ratio HR, 0.59; 95% confidence interval CI, 0.51‐0.67; P < 0.00001; I2 = 84%). According to the BCLC, both stage B and stage C patients showed significantly better overall survival (OS) for PH compared to TACE (HR, 0.53; 95% CI, 0.43‐0.65; P < 0.00001; I2 = 77%; HR, 0.67; 95% CI, 0.59‐0.77; P < 0.00001; I2 = 79%, respectively). Five‐year survival rates for PH were significantly higher than those for TACE in BCLC stage B/C, stage B, and BCLC stage C patients (odds ratio OR, 2.71, 2.77, and 3.03, respectively; all P < 0.00001). Survival benefits persisted across subgroup, sensitivity, and metaregression analyses; interstudy heterogeneity remained constant. Conclusion: This meta‐analysis suggests that surgical resection provides survival benefits in patients with intermediate‐ to advanced‐stage HCC. The evidence found herein may assist in the choice of treatment modality based on diverse definitions of operability. (Hepatology 2018).
•Frost shapes were microscopically observed using a laser.•The frost growth under cryogenic conditions were classified into four periods.•The frost coverage factor was defined to investigate the ...surface area covered with the frost.•The heat flux was measured to estimate the heat transfer rate of the test surface.
Experimental research was conducted to observe the frost growth under forced convection and cryogenic conditions (−180 °C). A laser was installed as a light source to investigate the frost microscopically, and the frost film and frost seed were observed on the test surface. The process of frost growth under cryogenic conditions was classified into four periods and it was compared with those under general-low temperature conditions. The frost coverage factor was defined based on the pixel intensity of the frost image, and the shape of the frost layer could be visualized using the frost coverage factor. The heat flux of the test surface was compared with the frost coverage factor, and it decreased by 40% when the frost coverage factor was 1. Therefore, the frost coverage factor can be used as a useful index to evaluate the heat transfer performance of the heat exchanger operating under cryogenic conditions.
•Frost growth behavior according to the cold surface inclination angle was observed.•Frost thickness increased and the density decreased as the angle increased.•Scattering of the frost particles and ...initial shape of the frost crystals were observed.•Frost layer was strongly bound as the cold surface temperature increased.
Frosting experiments were performed to analyze the frost growth behavior according to the inclination angle of a cold surface. The thickness and density of frost on an inclined cold surface were analyzed with respect to a horizontal cold surface. The scattering of the frost particles and the initial shape of the frost crystals were observed. The frost thickness increased and the frost density decreased as the cold surface inclination angle increased, even with variable operating conditions (humidity and cold surface temperature). In the case of varying humidity, the initial frost crystals exhibited feather shapes under relatively high humidity conditions, and needle and pole shapes under relatively low humidity conditions. The decrease in frost density owing to frost particle scattering was significant when feather-shaped crystals were observed. As the cold surface temperature increased, the frost layer was strongly bound, and frost particle scattering occurred less frequently.
•Frost accumulation on an evaporator was measured using meltwater collectors.•Heat distribution of an evaporator was calculated by measuring the heat transfer from the heater.•Power optimization of ...the defrosting heaters was conducted.•The defrosting performance was improved at the optimum heater power.
Power optimization was conducted for the defrosting heaters (conductive and radiant heaters) in a freezer to enhance their performance. The distribution of frost in the evaporator was measured experimentally, and the required defrosting energy corresponding to frost accumulation was calculated. The influence of defrosting heaters on frost was evaluated by measuring the heat quantity of the evaporator. The optimization was carried out such that the two heaters distribute the valid energy to the evaporator minimizing the excessive waste heat. At the optimum heater power, the maximum surface temperature of the evaporator was reduced, and the temperature distribution of the evaporator became uniform. The heater power and defrosting time were reduced through power optimization, thereby increasing the defrosting efficiency up to 6.7%. The effective heater power distribution could be further promoted to other heat exchangers with various heat transfer technologies.
• Frosting and fogging were investigated through experiments.• Frost and fog modeling at ultra-low temperatures was presented.• Frosting and fogging behaviors were analyzed experimentally and ...numerically.• The period of fog formation could be predicted using the model.
When liquefied natural gas (LNG) is vaporized by an ambient air vaporizer (AAV), frost and fog appear, which reduce energy efficiency and cause environmental issues. A model for simulating frost growth behavior and fog formation was developed for evaluating an AAV. The mechanisms of frost and fog formation and their mass transfer rates were defined, and they were characterized numerically. Experimental tests were performed at ultra-low temperatures (below −100°C) to quantitatively verify the frost thickness and density and qualitatively verify the fog formation. Despite the irregular characteristics of the fog, it was found that fog generation was dominantly influenced by the frost surface temperature. Therefore, the period of fog formation could be predicted through frost and fog modeling, and the model would be useful in managing AAVs.
A guide vane was installed on a heat sink to enhance the cooling effect of light-emitting diode (LED) lights. The validity of the numerical analysis was verified against the experimental results and ...the result of the previous studies. The effect of the guide vane on the heat dissipation performance of the heat sink was identified. The effect of the guide vane on the heat sink was qualitatively studied using the streamline and temperature contour. The cooling effect of the heat sink was enhanced by increased air supplement to the center-bottom part. A parametric study was conducted to determine the thermal resistance according to the guide vane angle, installation height, and vane length. Optimization was performed to minimize the thermal resistance using the Kriging model and micro-genetic algorithm (MGA). The cooling performance of the heat sink was enhanced by a maximum of 17.2% when the guide vane was installed.
•Heat and mass transfer analogy was proposed under cryogenic conditions.•A dimensionless correlation was developed for the frost density.•The correlation for thermal conductivity was derived via ...regression analysis.
An experimental study on the phase change occurring on a cryogenic cooling surface was conducted under forced convection conditions. The mass and surface temperature of frost and the heat flux at the cooling surface were measured in the experiment. The heat and mass transfer analogy was investigated under cryogenic conditions. The mass estimated under various experimental conditions using the modified analogy differed by up to 8% from the experimentally measured mass. The density and thermal conductivity of frost at cryogenic temperatures were lower than those of general-low temperature frost; the dimensionless correlations of density and thermal conductivity of frost were derived via regression analysis. The calculated frost density and thermal conductivities were differed from the measured value by up to 9% and 11%, respectively. Therefore, a model was built to predict the heat and mass transfer and improve the thermal performance on the cooling surfaces of various cryogenic devices using the presented frost density and thermal conductivity correlations.
Experimental research was conducted to study the frost formation on superhydrophobic surfaces at ultra-low temperatures, which might broaden the insight into frost growth behavior on the heat ...exchangers. The frost properties were examined over time, and the frost retardation according to the frosting factor was identified using the frost reduction ratios. The ablimation-dominant (desublimation) mechanism was observed at ultra-low temperatures in which almost no water droplets were generated. Consequently, the influence of the superhydrophobic surface on frost retardation was insignificant within −5% owing to the frosting mechanism. It is difficult to expect an increase in thermal performance of heat exchangers by applying superhydrophobic surfaces; using superhydrophobic surfaces for the purpose of frost retardation at ultra-low temperatures is not recommended.
•Frosting behavior on superhydrophobic surfaces at ultra-low temperatures was investigated.•Frost thickness and density were analyzed over time.•Frost retardation was identified using the frost reduction ratios.•The influence of the superhydrophobic surface on frost retardation was insignificant.
