Frost formation on a super-hydrophobic surface under natural convection conditions

This study reviewed the defrosting techniques applicable for the heating, ventilation, air-conditioning, and refrigeration industry, including passive, active, and system techniques. The passive methods normally use treated surfaces by changing the surface morphology through micro-grooved, anti-frost coating, hydrophilic, hydrophilic, or superhydropholic coating. For passive defrosting techniques, the microgrooved surfaces can improve the drainage of the frost melt effectively. It is generally agreed that the superhydropholic coating can delay the initialization of frosting and provides less water adhesion during defrosting. Yet defrosting performance for hydrophobic surfaces outperforms hydrophilic and uncoated surfaces. Active and system techniques, including electrohydrodynamic (EHD), low-frequency oscillation, and ultrasonic vibration methods, hot gas reverse cycle, electric heater, desiccant dehumidifiers and controlling strategies are reviewed. The EHD defrosting method is proved to be comparatively effective in natural or laminar flow operation. Test results also indicate that utilization of alternative current source is superior to the direct current source. The electrode with negative polarity is better than positive polarity as far as frost accumulation is concerned. The low-frequency oscillation is ineffective in defrosting while ultrasonic vibration provides effective frost removal and can delay the frost growth appreciably. Test results indicate that the ultrasonic oscillation poses considerable positive influence on defrosting either operated continuously or intermittently, in direct contact or not. For system defrosting, the hot gas reverse cycle is comparatively expensive to install but the efficiency, COP, and energy consumption are superior to the other system methods. Desiccants, either solid or liquid, can be employed in association with the system defrosting methods to lower energy consumption. There were various controlling strategies to detect the frost formation and to decide the best time to initiate defrosting. However, many of them were applicable to some specific systems and environments and require further investigations to test the relevant reliability, stability, and repeatability.

Review of defrosting methods

Review on the measurement and calculation of frost characteristics

https://ntrs.nasa.gov/api/citations/20110009969/downloads/20110009969.pdf

Frost Growth and Densification in Laminar Flow Over Flat Surfaces

Frost formation on a plate with different surface hydrophilicity

An experimental study has been conducted to investigate the effects of surface energy on frost formation. Test samples with three different surfaces of which dynamic contact angles (DCA) were 23, 55, and 88 deg were installed in a wind tunnel and exposed to a humid airflow. The airflow Reynolds number, humidity, the air and the cold plate temperatures were maintained at 9000, 0.0042 kg/kg', +12 and -22°C, respectively. The thickness and the mass of frost layer were measured and used to calculate frost density while heat flux and temperature profile were measured to obtain thermal conductivity. Exact positions of thermocouple junctions were verified by means of visualization system in order to increase accuracy. Results showed that frost density and thermal conductivity increase with time. The surface with a lower DCA showed a higher frost density and thermal conductivity during a two-hour test, but minor differences have been found after two hours of frost generation. Empirical correlations for thickness, mass deposition, density and thermal conductivity were proposed as the functions of test time and surface energy.

Experimental Study on Frost Structure on Surfaces With Different Hydrophilicity: Density and Thermal Conductivity

A system has been created for extracting and transporting light radiation from the irradiation chamber and the protective box of the INURA accelerator. Light outside the protective box is recorded using compact spectrometers, photomultipliers, photodetectors in the range from 300 nm to 2.1 µm. At present, the accelerator operates in the ion acceleration mode; the number of problems to be solved should increase when switching to the electron acceleration mode. The system was tested in the registration of light radiation from the interelectrode gap of the diode assembly. Lines of a hydrogen atom and an oxygen atom associated with the decomposition of absorbed water vapor and the discharge over the dielectric surface at the anode of the high-current accelerator diode have been registered.

/pdf/optical-diagnostics-system-at-the-inura-accelerator-302wyjo4.pdf

Optical diagnostics system at the inura accelerator

Warmetauscher fur ein Kuhlgerat, umfassend: wenigstens zwei einander benachbarte senkrechte Saulen aus Kuhlrohren (10), wobei jedes Kuhlrohr (10) eine Vielzahl von geraden Stucken (11) und eine Vielzahl von gekrummten Stucken (12) umfasst, welche die geraden Stucke (11) verbinden; eine Vielzahl von Rippen (20), wobei jede Rippe (20) eine Vielzahl von darin ausgebildeten Durchgangslochern (21) aufweist, und zwar an einer oder mehr als einer Saule entlang einer Langsrichtung, zur Kopplung mit den geraden Stucken (11) der Kuhlrohre (10); und ein Paar von Verstarkungsplatten (30), welche mit den geraden Stucken (11) des Kuhlrohres (10) auf gegenuberliegenden Seiten der Rippen (20) gekoppelt sind, wobei die geraden Stucke (11) der Kuhlrohre (10) in den einander benachbarten Saulen zueinander versetzt angeordnet sind, dadurch gekennzeichnet, dass die Rippen (20) kontinuierlich gerade flach plattenformig sind, und S T = D/N und S T /S L = 1 ist, wobei: S L den Abstand zwischen Zentren...

Wärmetauscher für ein Kühlgerät

Efforts on the development of frost control technique consider the important influences of surface energy on frost crystal structure. The present study is focused on frost map for hydrophilic surface with low advanced dynamic contact angle(ADCA) obtained by plasma treatment. An objective of this study is to determine the effects of main parameters on the formation of frost structures on hydrophilic surface. For each test run airflow parameters are maintained at airflow temperature within the range of 10~20℃, humidity within the range of 2.64~9.36 g/㎏, Re within the range of 7,000~17,000 and plate surface temperature within the range of -11.6~-28.4℃. Total deposition time is 30 minutes, while frost images are taken at 5, 10, 20 and 30 minutes. The results of investigations are summarized in a form of the frost map. The entire area of tested parameters is divided into regions with different frost crystal structures. It is found that the density of the frost formed on a surface with lower ADCA is higher, than that generated on bare aluminum surface.

Experimental Study on Frost Formation on a Plate Part 2 : Frost Map for Hydrophilic Surface

Investigations to determine the ranges of main parameters responsible for the formation of certain crystal structures have been conducted in the present study. Frost grows on a plate under the following conditions: airflow temperature is 10 ~ 20℃, humidity is 2.64 ~ 9.36 g/㎏, Re number is 7,000 ~ 17,000 and plate temperature is -11.6 ~ -28.4℃. Bare aluminum and lacquer coated surfaces show similar advanced dynamic contact angle, which in present study is 88°. All measurements are accompanied by visual observations. The results are summarized in a form of the frost map. The entire area of tested parameters is divided into regions with different frost crystal structures. Frost has highest thickness when formed on a plate with the temperature below -20℃ from the moist air with absolute humidity within 4.0 ~ 7.0 g/㎏, while most dense frost is observed at humidity smaller than 3.0 g/㎏ and higher than 8.0 g/㎏.

Alexei V. Tikhonov

Papers

Experimental Study on Frost Structure on Surfaces With Different Hydrophilicity: Density and Thermal Conductivity

Optical diagnostics system at the inura accelerator

Wärmetauscher für ein Kühlgerät

Experimental Study on Frost Formation on a Plate Part 2 : Frost Map for Hydrophilic Surface

Experimental Study on Frost Formation on a Plate Part 1 : Frost map for Aluminum Surface