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Ultrasonic measurements are common techniques in industrial liquid analysis for level detection or monitoring the concentration, the distribution of particles or the flow velocity in pipes [1]. Especially the noninvasive clamp-on configuration is an interesting approach due to its non-destructive nature and easy handling. Such a sensor can be mounted on existing reactors or pipes without either stopping the process or rebuilding the system. However, such a technique often needs a sophisticated signal analysis in order to discriminate the measurement effect from disturbances caused by the multimodal sound propagation. The disturbing signals occur from wave propagation within the metallic or plastic pipe wall, the ringing due to low damping and from mode conversions at the solid-liquid-interfaces. Up to now the multiple reflections and the wave travelling in the pipe wall are handled by optimizing the sensor position with empirical calibration, only. However, in cases where the sensors position can not be varied and the multimodal sound propagation is not known, the signal containing the medium information can not be extracted without a media dependent signal pre-processing. Precisely it can be superposed by the pipe wall waves, which sometimes are reduced with damping mats [2], and the focusing wave occurring due to mode conversion at the curvilinear inner pipes interface towards the liquid. Especially the disturbing mode converted waves travelling at the solid liquid interface depend on the properties of the liquid media. Hence, all systems have in common, that they do not handle multiphase applications because of the changing propagation conditions within the pie (varying acoustic impedances along the acoustic path). As consequence the clamp-on liquid analysis is carried out for a few cases only, e.g. for concentration measurements of homogeneous media or homogeneous distributed particles [2]. Nonetheless, all systems need a calibration of sensor position and time windowing. To enhance the performance and reliability of clamp-on ultrasonic applications towards analysis tasks [3], a sophisticated signal analysis [4] and knowledge of the wave behaviour, the mode conversion and the influence of dispersion within the large scale complex technical structure with a high precision is required. In order to design such a robust sensor system a tool is needed that estimates the behaviour of multimodal sound propagation within a pipe prior to the sensor setup and measurement process. The preestimation of the sound phenomena based on the aforementioned techniques could save engineering time and costs compared to conventional and empirical calibrations. Accordingly this paper discusses the multimodal sound wave propagation in small up to large scale pipes filled with multiphase liquid systems. A first estimation of all expected wave propagation paths, corresponding travel time and multiple reflections is performed using the Mason-theory [5]. It will be shown that this linear modelling is adequate to estimate the travel time with certain precision for optimizing the sensors position or choosing proper time windows in the acquired ultrasonic signals where no significant disturbing effects occur. The paper further presents simulations in combination with experimental studies that are used to investigate the geometric aspect of the mode conversion. Exemplarily a clamp-on configuration with varying sensor position, different wideband excitation in the range of f = [0.2, 0.5, 2.25] MHz, the transmission, the reflection from a liquid interface and the disturbing signals are discussed.

A6.4 - Wave Propagation Phenomena in Case of Clamp-On Ultrasonic Systems Applied on Liquid Filled Pipes