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We investigated the local frost growth in a louvered fin heat exchanger using a scaled-up model and analyzed the characteristics of frost behaviors and variation in the heat transfer rate according to frost formation compared to a flat-fin heat exchanger. In contrast to a uniform frost layer of the flat-fin heat exchanger, highly non-uniform frost layer between the front and rear sides was observed on the louvered fin heat exchanger. The heat transfer rate of the louvered fin heat exchanger more rapidly decreased with the frost formation than that of the flat-fin heat exchanger. The blocking of the spaces between the louvers on the front side by frost mainly induced the significant decrease of the heat transfer rate of the louvered fin heat exchanger. On the louvered fin front side, the spaces between the louvers near the redirection region were first blocked by frost formation before those near the air inlet region due to the higher mass transfer driving force around the redirection region. Once the louvered fin front side was totally blocked by frost, the louvered fin rear side did not effectively work anymore. [ABSTRACT FROM AUTHOR]

We investigate cross-cut cylindrical heat sink to improve cooling of a light-emitting diode (LED) bulb compared with conventional plate-fin cylindrical heat sinks using various installation angles. In contrast to plate-fin heat sinks, whereby the fins in the upper heat sink do not provide effective cooling at large installation angles, flow formed between the cross-cut fins and the air reached the upper region of the heat sink. As the installation angle increased, the length of the flow path increased. Accordingly, the flow efficiency of the cooling air improved, and the dependence of the cooling on orientation was reduced. The improvement of the cross-cut heat sink was more noticeable in the shorter cross-cut length. We suggest a correlation to predict the degree of improvement in the thermal resistance compared with a plate-fin heat sink as a function of design parameters and installation angle, and develop a contour map describing the optimum heat sink shape for various installation angles. [ABSTRACT FROM AUTHOR]

Effective de-icing technologies are important to the ships and offshore structures in cold regions and air bubble de-icing method is expected to have great potential for application. To deeply understand the ice melting process under point-source bubble flows, especially the heat transfer characteristics, an experimental set-up is designed to explore the influence of the flow rate. According to the shape of the ice bottom surface and the distribution characteristics of bubbles, the ice melting process is divided into flat, concave, and holed ice stages. The ice melting efficiency decreases with the increasing flow rate and when the flow rate increases from 0.5 L/min to 2.0 L/min, the ice melting efficiency decreases by 58.0 %. The stage-averaged heat transfer coefficients of the flat, concave, and holed ice stages decrease successively and all of them increase with the increase of flow rate. Correlations for central and global heat transfer coefficients are established to describe the ice melting process, which fits well with the experimental data with a deviation less than ±15 %. The findings are helpful to the design and optimization of the air bubble de-icing system. [ABSTRACT FROM AUTHOR]

Abstract: The thermal performance and uniformity of frost growth between the front- and rear-sides of microchannel heat exchangers was investigated as a function of the depth of the heat exchanger, the pitch of the fins, and the pitch of the channels. The microchannel heat exchangers exhibited frost formation that was concentrated on the front-side, resulting in significant frost growth non-uniformity between the front- and rear-sides, which led to rapid performance degradation. During the defrosting cycle, compared with natural convection, forced convection defrosting resulted in the unit retaining less water between the fins, leading to improved thermal performance. However, after a number of frosting and defrosting cycles, the relative improvement of forced convection became smaller. Thinner heat exchangers (i.e., with a smaller coil depth) exhibited more uniform frost growth between the front- and rear-sides of heat exchangers, and exhibited approximately 5% increase in the heat transfer rate per unit volume. A larger fin pitch showed less airflow blockage due to larger air pathway. In addition, heat exchangers with a larger fin pitch showed more uniform frost growth, as well as approximately 6% increase in the average heat transfer rate. Heat exchangers with a larger channel pitch exhibited a larger heat transfer rate during the early stages of the experiment; however, the frost growth was less uniform between the front- and rear-sides, leading to a rapid reduction in the heat transfer rate mid-way through the experiment. [Copyright &y& Elsevier]

We experimentally investigate the frosting and defrosting performance of slippery surfaces which have low sliding angles of water droplets and low ice adhesion strengths. The frosting and defrosting characteristics of slippery surfaces are compared with those of bare aluminum, hydrophilic, and superhydrophobic surfaces. The enhanced sliding properties of water droplets on the slippery surfaces effectively promote the drainage of the condensate on their surfaces, which not only leads to significant frost retardation under frosting conditions, but also substantially reduces the mass of the retained water on the surfaces after defrosting by heat. In addition, when mechanical vibration is applied together with heating during defrosting process, the low ice adhesion strengths of the slippery surfaces enable the effective detachment of the lumped frost layer from their surfaces, thereby significantly reducing the defrosting time. [ABSTRACT FROM AUTHOR]

• 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. [ABSTRACT FROM AUTHOR]