Flow field and heat transfer in swirling impinging jets
Ianiro, Andrea (2011) Flow field and heat transfer in swirling impinging jets. [Tesi di dottorato] (Inedito)
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The high heat transfer rate obtainable with impinging jets is widely recognized and explained in scientific literature and the use of jets is very popular in many industrial applications like paper drying, glass tempering and turbine blades cooling. A huge quantity of data is available for single, rows and arrays of jets with also correlations for heat and mass transfer. A major disadvantage of impinging isothermal and flame jets, however, is that the local heat flux can be highly non-uniform (Viskanta, 1993). For some applications, like electronic cooling or chemical vapour deposition, high values of heat and mass transfer with radial uniformity are requested. The swirling impinging jets, characterized by tangential velocity components that cause a spiral-shaped motion and the broadening of the jet, could be a possible solution to achieve both high heat transfer and radial uniformity. The purpose of this work is to study the flow field and the heat transfer in swirling impinging jets. For the present study the swirling jets are obtained with helical inserts based on the concept of the cross swirling strips inside the nozzle and two experimental techniques are used: Tomographic Particle Image Velocimetry (Elsinga et al., 2006) for the three-dimensional three-components flow field measurements and IR thermography along with the “heated thin foil” heat transfer sensor (Carlomagno and Cardone, 2010) for the heat transfer measurements. In chapter one the literature about free and impinging jets is reviewed in attempt to explain how the flow field influences the heat transfer distribution on the wall and to describe the state of the art in the field of swirling impinging jets and swirl flows. This literature review motivates the study of swirling impinging jets in order to understand both the fluid mechanics characteristics and the heat transfer performances. So far, has not yet been formulated a comprehensive framework where all the results about vortex dynamics in swirl flows can be satisfactorily explained, and the modelling of rotating and swirling flows is still considered a perpetual challenge (Jakirlic et al. 2002). Most of the quantitative experimental studies on swirling flows have been limited to examining flow details using single-point measurement techniques, such as Laser Doppler Velocimetry (LDV), or planar techniques, such as PIV in particular. The inability to make instantaneous volumetric measurements often leads to ambiguities in the interpretation of the data, which necessitates various assumptions to link these reduced dimensional representations to the three-dimensional instantaneous structure of the flow. At the same time, while numerical simulation such as Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES) in particular, has been instrumental in elucidating the three-dimensional dynamical features of swirling flows (see e.g. García-Villalba et al. 2006, Ranga Dinesh and Kirkpatrick 2009), they have to face with the difficulty of obtain reliable velocity profiles at the nozzle exit as boundary conditions (Ortega-Casanova et al. 2010). On the heat transfer side, almost all data related to swirling impinging jets reported in literature are presented as radial distribution even though, to assess the behavior of the flow field on the wall, two dimensional measurements are required. Furthermore, is not available in literature a quantitative analysis for the concept of heat transfer uniformity. In chapter two, the basic working principles and the state of art of research about Tomographic PIV are described in order to understand the main parameters involved in the design of the experiments for the flow field measurements. In chapter three, the basic working principles and the state of art of research on IR thermography for convective heat transfer measurements are described in order to understand the main parameters involved in the design of the experiments for the heat transfer measurements. In chapter four, the flow field measurements are presented. A first study is performed by means of time resolved tomographic PIV on the three dimensional flow field of free swirling jets (at Reynolds number equal to 1,000) in order to analyze the main features of this complex flow. The development of the jets, the effect of swirl and the growth and interactions of coherent structures is discussed describing flow topology in swirling jets. Swirling impinging jets (at Reynolds number equal to 10,000) are then studied at low nozzle to plate distance analyzing both instantaneous measurements and flow statistics. This study allows to understand how the impinged plate influences the development of the jets and the lifetime of its coherent structures; a special attention is put on the development of the jet in the “wall jet” region. In chapter six, are presented experimental two-dimensional measurements of convective heat transfer between a flat plate and a swirling air jet impinging on it. This work is performed at a fixed Reynolds number (Re = 28,000) for different nozzle-to-plate distances and for different Swirl numbers. The heat transfer performances of swirling jets are also compared with those of a circular jet in order to account for both effects of the swirl and of the cross strips in the nozzle. Data are reported as Nusselt number surface maps, surface averaged Nusselt number and surface standard deviation percentage of the Nusselt number, in the attempt to quantify heat transfer rate and uniformity. In particular, this work represents the first effort to quantify non-uniformity in convective heat transfer coefficients in case of swirling, multichannel and circular jets. Moreover, the author proposes the use of the standard deviation percentage of the Nusselt number as a quantitative estimator for the heat transfer uniformity.
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