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Abstract. The efficiency of temperature end energy separation is compared for Ranque tubes of circular and square cross section, with the square side equal to the circle diameter. The "square" tube demonstrates approximately two times less efficiency, yet the separation effect still being evidently presented.Experimental study of the vortex effect efficiency in Ranque-Hilsh tubes is presented. The insufficience of the available experimental data and theoretical approaches to the understanding of processes inside a vortex tube are well-known [1]. Systematic and detailed experiments are now strongly needed for verification of theoretical models, the techniques being preferable that do not or just slightly disturb the flow. So, the use of optical diagnostic has allowed a considerable progress in the study of inside swirling flow [2][3][4][5]. In [2] by means of Gilbert optics the existence of a large-scale vortex structure in the form of a double spiral in the field of optical phase density was for the first time diagnosed. The spiral arises on a plane end cover at the hot exit and propagates along the axis in the cold exit direction. In [3][4] an attempt was made to diagnose this structure by means of LDA. The sufficient nonstationarity of temperature and velocity was showed. While in [5] a circular tube is investigated, in [2-4] the tube was studied with square cross section all along for the convenience and delicacy of optical diagnostic. It was found that in spite of such a modernization the Ranque effect is distinctly manifested though the tube does not show any record features.At this stage, the authors proceed to investigate the velocity fields inside the same tube by means of optical field diagnostic -PIV [6]. At that the first step is a detailed investigation of tube efficiency in a wide region of regime parameters for the square tube and comparison of results with a circular tube with the diameter defined as that of inscribed circle.It should be noted that a significant amount of new publications has appeared in recent times with detailed description of certain vortex tube designs and corresponding efficiencies, such as, for example, [7][8]. The data is supposed to help not only to optimize the technical use but also to verify theoretical models. In our case the detailed data is needed as a road map guiding the field measurements inside the tube.

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