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A good estimation of the fin surface temperature was achieved with the methodology developed in the present study. Experimental data of frost weight, local frost thickness, 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 the frost growth rates depend mainly on the local fin surface temperature while water retention and surface coatings could have secondary and minor effects. Some geometries performed better in frosting condition than others such as geometries with lower fin density that delayed the blockage of the air flow. The fin length and fin depth had minor effects on frosting performance. It was found that air humidity has a significant effect on rate of frost formation while air velocity seemed to have a small effect on frost formation.
The book contains research results obtained by applying Bejan's Constructal Theory to the study and therefore the optimization of fins, focusing on T-shaped and Y-shaped ones. Heat transfer from finned surfaces is an example of combined heat transfer natural or forced convection on the external parts of the fin, and conducting along the fin. Fin's heat exchange is rather complex, because of variation of both temperature along the fin and convective heat transfer coefficient. Furthermore possible presence of more fins invested by the same fluid flow has to be considered. Classical fin theory tried to reduce the coupled heat transfer problem to a one-dimensional problem by defining an average temperature of the fin and writing equations using this parameter. However, it was shown that this approach cannot be used because of the effects of two-dimensional heat transfer, especially in the presence of short fins. CFD codes offer the possibility to consider bi-dimensional (and more generally, three-dimensional) effects and then a more real approach to the physic phenomena of finned surface's heat exchange. A commercial CFD code was used to analyse the case of heat exchange in presence of T-shaped fins, following an approach suggested by Bejan's Constructal Theory. The comparative results showed a significant agreement with previous research taken as a reference, and this result allows for the application of this approach to a wider range of systems. T-shaped optimized fin geometry is the starting point for further research. Starting from the optimal results (T-shape optimized fins), we show the trend of the assessment parameter (the dimensionless conductance) in function of the angle a between the two horizontal arms of the fin. A value for a, 90°
The book contains research results obtained by applying Bejan's Constructal Theory to the study and therefore the optimization of fins, focusing on T-shaped and Y-shaped ones. Heat transfer from finned surfaces is an example of combined heat transfer natural or forced convection on the external parts of the fin, and conducting along the fin. Fin's heat exchange is rather complex, because of variation of both temperature along the fin and convective heat transfer coefficient. Furthermore possible presence of more fins invested by the same fluid flow has to be considered. Classical fin theory tried to reduce the coupled heat transfer problem to a one-dimensional problem by defining an average temperature of the fin and writing equations using this parameter. However, it was shown that this approach cannot be used because of the effects of two-dimensional heat transfer, especially in the presence of short fins. CFD codes offer the possibility to consider bi-dimensional (and more generally, three-dimensional) effects and then a more real approach to the physic phenomena of finned surface's heat exchange. A commercial CFD code was used to analyse the case of heat exchange in presence of T-shaped fins, following an approach suggested by Bejan's Constructal Theory. The comparative results showed a significant agreement with previous research taken as a reference, and this result allows for the application of this approach to a wider range of systems. T-shaped optimized fin geometry is the starting point for further research. Starting from the optimal results (T-shape optimized fins), we show the trend of the assessment parameter (the dimensionless conductance) in function of the angle α between the two horizontal arms of the fin. A value for α, 90°
Heat transfer and pressure drop data for nine different fin geometries are presented, and a decrease in the overall heat transfer coefficient and an increase in the pressure drop are observed as frost accumulates on the surfaces. A reduction in air-side flow rate and bridging of louver gaps by frost are identified as the factors most important to the reduced heat transfer performance. Correlations are presented for predicting the thermal performance of these heat exchangers under frosting conditions.
Descripción del editor: "This volume is concerned with the heat transfer from extended surfaces, such as fins attached to a primary transfer surface. These are used extensively within heat exchanges and on heat transfer equipment to ensure that a specified rate of heat transfer is achieved between a heat source and sink. All of the chapters come from invited contributors and follow a unified outline and presentation.Contents: Overview of Extended Surface Heat Transfer ââ,¬â€œ Fins; Coupled Forced Convection, Conduction and Thermal Radiation of a Rectangular Fin in a Confined Space; Mechanistic Investigation of the Performance of a Triangular Fin; Conjugate Free and Mixed Convection Heat Transfer from a Vertical Fin Embedded in a Porous Medium; About Fin Performance and Optimization; Two-Dimensional Effects in Extended Surface Assessment; Steady-State Heat Transfer and Performance Assessment; Multi-Louvred Fin Surfaces; Methodology for the Design of Multi-Stream Plate-Fin Heat Exchangers; Incorporation of a Consideration of Operability into the Design of Multi-Stream Heat Exchangers." (WIT Press).
Theoretical investigations show that the presence of frazil ice in water can be quantitatively measured by its effect on the conductivity and the permittivity of the water. At frequencies much lower than 10 7 Hz, the frazil ice effect on conductivity predominates, and at frequencies much higher than 10 7 Hz, the ice effect on permittivity is predominant. An experimental instrument based on the resistance (i.e. conductivity) principle has been constructed and tested. Experiments were performed in a cold room with frazil ice produced in a beaker. The experiments showed that the laboratory instrument could quantitatively sense the presence of ice. The detailed circuit of the experimental instrument is shown. Some modification of the circuit and the probe for improving the instrument are suggested as a consequence of the laboratory experiments and further theoretical investigations. The concentrations of frazil ice measured by the experimental instrument were by average about three times the concentration calculated theoretically. If the frazil ice crystals are assumed to have elongated spheroidal shapes with a length to breadth ratio of 10:1, the measured concentrations are about twice the calculated concentration. Further theoretical investigation is needed to explain this puzzle. The puzzle, however, is beneficial because it decreases the sensitivity of the instrument. This report describes phase I of an instrument development project. The design and production of and improved and manufacturable instrument will be phase II of the project.