Nevin ÇELİK, Daniel W. BETTENHAUSEN, Ryan D. LOVIK

Formation of co-axial jets and their downstream development

Eş-eksenli bir jetin oluşumu ve düzlem jet şeklinde yayılımının akışkan mekaniği ile ilgisini ortaya çıkarmakiçin detaylı bir sayısal çalışma hazırlanmıştır. Akışkan akışı türbülanslı ve Reynolds sayısı jet oluşum bölgesinde10000, 20000 ve 50000 olacak şekilde modellenmiştir. Model , iç içe iki borudan oluşmuş bir boru düzeneğininhem iç hem dış borusundan tam gelişmiş akım geçmesi temeline dayandırılmıştır. Modellemede değişkengeometrik parametreler, içteki merkezi borunun kesit alanının ve jetin oluştuğu boru uzunluğunun değiştirilmesişeklindedir. Hem jet oluşumu hem de serbest jet gelişimi nedeniyle ortaya çıkan ortalama basınç düşüşününsürtünmeden kaynaklandığı görülmüştür. Jet çıkışında, hız profili boru cidarlarının mevcudiyetinden dolayısüreksizdir. Bu mevcudiyet aynı zamanda jet merkezinden uzaklaştıkça yavaşça kaybolan çift-yükseltili hızprofillerine sebep olur. Jetten yeterince uzak mesafede jetin genişliği hem geometrik parametrelerden hem deReynolds sayısından bağımsız hale gelmiştir.

Eş-eksenli jetlerin oluşumu ve akımlarının gelişimi

A comprehensive numerical simulation has been performed to inter-relate the fluid mechanics of theformation of co-axial jets and their development downstream of the plane of jet emergence. The fluid flow wasmodeled as being turbulent, with corresponding Reynolds numbers in the jet-formation section of 10000, 20000, and50000. The model was based on a fully developed pipe flow encountering a double-pipe arrangement and splittingbetween the two pipes. Subsequent to flow development in the central pipe and in its enveloping annulus, the flowexited into a large free space and became a co-axial free jet. The simulations were performed for geometricparameters which varied the relative cross sections of the central pipe and of the annulus and also varied the length ofthe jet-formation section. The overall pressure drop responsible for both the jet formation and for the subsequent free- jet development was found to be due to friction in contradistinction to inertial losses. At the jet exit, the velocityprofile is discontinuous because of the intrusive presence of the walls of the pipes which bound the jet-formationsection. These intrusions cause a double-humped velocity profile which disappeared with decreasing downstreamdistance from the jet origin. At sufficient downstream distances, the width of the jet was found to be independent ofboth the geometrical parameters and the Reynolds number.

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