Autologous hair transplantation has become an established therapy for irreversible alopecia. With the advent of CO2 lasers, new procedures have developed. However, CO2 lasers are associated with some disadvantages, and the introduction of the erbium YAG laser has led to further improvements. A case illustration of this technique is given here, and a study of 50 patients has been initiated to further evaluate the potential of this laser.
In this manuscript, off-set strip fin structures are presented which are adapted to the possibilities of additive manufacturing. For this purpose, the geometric parameters, including fin height, fin spacing, fin length, and fin longitudinal displacement, are varied, and the Colburn j-factor and the Fanning friction factor are numerically calculated in the Reynolds number range of 80–920. The structures are classified with respect to their entropy production number according to Bejan. This method is compared with the results from partial differential equations for the calculation of the irreversible entropy production rate due to shear stresses and heat conduction. This study reveals that the chosen temperature difference leads to deviation in terms of entropy production due to heat conduction, whereas the dissipation by shear stresses shows only small deviations of less than 2%. It is further shown that the variation in fin height and fin spacing has only a small influence on heat transfer and pressure drop, while a variation in fin length and fin longitudinal displacement shows a larger influence. With respect to the entropy production number, short and long fins, as well as large fin spacing and fin longitudinal displacement, are shown to be beneficial. A detailed examination of a single structure shows that the entropy production rate due to heat conduction is dominated by the entropy production rate in the wall, while the fluid has only a minor influence.
In diesem Beitrag werden zwei verschiedene additiv herstellbare Metallstrukturen hinsichtlich der Entropieproduktionszahl bewertet. Die beiden Strukturen werden bei zwei Temperaturniveaus, 200/80 °C und 700/580 °C, untersucht. Bei dem niedrigen Temperaturniveau ist bei beiden Strukturen insbesondere die Wärmeleitung innerhalb der Struktur der Haupttreiber für die Entropieproduktion. Werden die Temperaturen angehoben, so steigt die gesamte Entropieproduktionszahl stark an, wobei die Dissipation durch Fluidreibung nun die Hauptursache für die auftretenden Verluste in beiden Strukturen ist.
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