E. Nino

Simultaneous visualization of flow field and evaluation of local heat transfer by transitional impinging jets

Abstract A combined approach has been employed to characterize the flow field and local heat transfer in jet impingement configurations, featuring a mass transfer experiment and a digital visualization technique. A jet velocity range is spanned to ensure flow regime transition. The well-known heat/mass transfer analogy has been used to infer on the local heat exchange on a infinite plate. In this experiment, a naphthalene film is ablated from a disk, due to jet exposure. Automated contact measurements of the variation of film depth in the stagnation region and beyond have been performed. From the local naphthalene loss rate the local heat transfer is then inferred. Coherent structures are created both at the interface between free jet and quiescent medium and upon impingement at plate, and need to be visualized in the vicinity of stagnation. To this end a particle image velocimetry system is exploited to extract the two components velocity instantaneous information. Ablation measurements confirm the non-monotonic progress of local heat transfer for small nozzle-to-plate spacings. The visualizations evidence that local heat transfer is strongly influenced by impingement structures: the maximum heat transfer coefficient offset which can be detected is due, even for laminar or transitional jet, to large-scale toroidal vortices impacting on the plate.

Flow Field Analysis in a Ventilated Large Enclosure Subject to Non-Homogeneous Boundary Conditions

Non-homogeneous thermal boundary conditions may play a relevant role in flow fields within ventilated enclosures. One such large-scale (23 m 3 ) test enclosure has been set in operation by termination jets under a combination of thermal unbalances in order to describe the dependence of secondary flow patterns on the bulk airflow. An integrated approach has therefore been attempted that features both measurements and calculations. An extensive and painstaking set of flow field measurements has been performed first, relative to different thermal and fluid dynamic working conditions, by means of a custom Laser Doppler Velocimetry apparatus. A commercial CFD software was then employed to perform the related calculations. A transitional regime is present (Re , 2000, based on grille diffuser equivalent diameter) at the given forcing inlet flow, as well as the competition between natural and forced convection. Some qualitative comparisons with respective experimental data are presented to delineate the role of thermal disturbances.

Experimental Investigation of Three Types of Clean Room Diffusers using Particle Image Velocimetry and Laser Doppler Velocimetry Techniques

Abstract The main purpose of this paper is to evaluate the main fluid mechanics parameters in the exit region of three different terminal systems commonly used for controlled environments such as clean rooms or operating theatres. Three different types of diffuser were tested i.e. a diffuser grid, a square ceiling diffuser and a swirling radial discharge diffuser. The paper presents results obtained experimentally which were carried out in order to study the flow characteristics of isothermal jets issuing from such devices. The study has been performed for four different air flow rates for each type of terminal tested. To characterise the flow with confidence and obtain high spatial resolution, a Particle Image Velocimetry technique and a Laser Doppler Velocimetry technique were used. For each case investigated, the results were plotted as velocity profile, velocity magnitude, turbulence intensity and vorticity. The study displays the particular behaviour of the flow generated by each terminal. Vorticity regions and turbulence spots were also detected and carefully localised for the investigated configurations.