''Energy separation" is the re-distribution of the total energy in a fluid flow without external work or heat, and has potential to heat or cool a fluid without using a conventional heating or cooling system. However, currently obtainable heating and/or cooling effects are not big enough for practical applications. It is required to understand the mechanism of energy separation and investigate its enhancement methods. In the present study which consists of two parts, energy separation in a free jet is investigated with instantaneous velocity and total temperature measurements. As a method to enhance energy separation, acoustic excitation with various frequencies is examined. In this first part, an experimental study is performed to investigate the motion of the coherent vortical structure and its response to acoustic excitation in free jets whose Reynolds numbers are 8000 and 120,000. For the low Reynolds number jet, spectral analysis of instantaneous velocity and flow visualization by a schlieren system are performed to characterize the motion of the large scale coherent vortical structure. The frequency of dominant fluctuation, which represents the vortex passing frequency at a given axial location, is around Sr-D approximate to 0.65 at z/D = 1 and moves to Sr-D approximate to 0.4 at z/D = 4. When acoustic excitation is applied, the coherent structure develops more rapidly than in the absence of excitation regardless of the excitation frequency, but very regular and strong vortex pairing is observed only when acoustic excitation with Sr-ex = 0.9. The flow characteristics of the high Reynolds number jet are also investigated with spectral analysis. The result shows that the velocity fluctuation level is generally elevated, but the frequency of dominant fluctuation is still within 0.4 less than or equal to Sr-D less than or equal to 0.6. The response to acoustic excitation is also very similar to the low Reynolds number jet. (C) 2003 Elsevier Ltd. All rights reserved.