J. Phys. IV France 125 (2005) 339-341  EDP Sciences, Les Ulis DOI: 10.1051/jp4:2005125080 Determination of velocity profiles in pipes by a pulsed photoacoustic method F. Hernández 1 , J. Morales 1 , M. Navarrete 2 and M. Villagrán-Muniz 3 1 Posgrado de la Facultad de Ingeniería, UNAM, A.P. 70-258, CP 04511, Mexico, DF 2 Instituto de Ingeniería, Edificio 12, Sección de Ing. Mecánica Térmica y Fluidos, UNAM, A. P. 70-472, CP 04510, Mexico, DF 3 Centro de Ciencias Aplicadas y Desarrollo Tecnológico, Laboratorio de Fotofísica, UNAM, A.P. 70-186, Mexico, DF Abstract. The present work describes the development of a pulsed photoacoustic (PA) flowmeter. An acoustic wave in a flowing fluid is produced by absorption of a laser pulse focused over a path flow line. The acoustic propagations, along and against the flow, are monitored by two cw probe beams. In the interaction, the probe beam undergoes a transient deflection that is detected by a fast response photodiode (PD). Measurements of the acoustic arrival time through the pipe cross section are acquired. The velocity distribution data profile of a square pipe is obtained applying the cylindrical shockwave model developed by Vlasses and the velocity equation established in an earlier work by Zapka and Tam [9]. The profiles determined with this method are fixed through two turbulent pipe flow models, showing good agreement. 1. INTRODUCTION Hot wire anemometry (HWA) [1], laser Doppler anemometry (LDA) [2] and particle imaging velocimetry (PIV) [3] are relative well-established tools for the flow studies, but each of them possesses certain limitations. Digital particle imaging velocimetry (DPIV) [4] and Holographic particle imaging velocimetry (HPIV) [5], are now more attractive to fluid dynamists due to its three- dimensional (3-D) full-field measuring capability and because it enables an instantaneous flow pattern to be captured in a pseudo real-time manner. There are others methods that use nonlinear optical effects, such as: coherent anti-Stokes Raman spectroscopy (CARS) [6] or stimulated Raman gain spectroscopy (SRGS) [7]. All LDV methods require the presence of light-scattering particles, and so are not applicable for pure gases. CARS and SRGS methods have comparatively poor velocity resolution (30 m/s). Thus, Zapka and Tam [8, 9] showed that the flow velocity of a pure particle-free gas could be measured by means of a laser-induced acoustic source with a resolution of 5cm/s. Such non-contact measurements were not possible previously by other laser scattering methods known and, because the pipe flow in fluid mechanics is 2-D, it is well suited for investigation of fluid phenomena in internal flows. In many technological systems, thermal-stratified flow in a cylindrical tube is often encountered and it is important to investigate the heat and momentum transfer mechanism in such flow geometry [10]. 2. EXPERIMENTAL SETUP The schematic diagram of the experimental setup is shown in Fig 1. The acrylic (transparent) pipe is about 40 cm long and it presents an internal square section of 5x5 cm 2 and 4 mm of thickness. The test section is located 1.8 diameters from the inlet in order to achieve a fully steady state flow. A Q- Article published by EDP Sciences and available at http://www.edpsciences.org/jp4 or http://dx.doi.org/10.1051/jp4:2005125080