Abstract
The aim of this paper is to expose the main involved physical phenomena underlying the alteration of convective heat transfer in a heat exchanger subjected to imposed vibrations. This technique seems to have interesting features and industrial applications, such as for efficiency increases, heat transfer rate control and cleanliness action. However, a clear description and comprehension of how vibrations may alter the convective heat transfer coefficient in a heat exchanger has still not been reached due to the complexity of the involved physical mechanisms. For this reason, after a presentation and a schematization of the analyzed thermodynamic system, the fundamental alterations of the thermo-fluid dynamics fields are described. Then, the main involved physical phenomena are exposed for the three cases of gaseous, monophasic liquid and boiling liquid mediums. Finally, on the basis of the characteristics of these described phenomena, some considerations and indications of general validity are presented.
Highlights
The influence that the acoustic field has on convective heat transfer has attracted the interest of the scientific and technical community since the 1930s, with the pioneering works of important authors such as Martinelli and Boelter [1]
Considering the results exposed in the different papers it is possible to observe that the increase of convective heat transfer coefficient can reach maximum values close to 1000%, but this value is largely dependent on the experimental conditions and can reach minimum values under 10%
It is possible to observe that the liquid, in particular in subcooled conditions, is the medium for which the maximum increments are reached, though not negligible results have been gained for air and boiling; the lower the speed of the fluid, the higher the measured increments; the highest increments are reached giving vibrational energy directly to the heating surface rather than to the fluid in convection
Summary
The influence that the acoustic field has on convective heat transfer has attracted the interest of the scientific and technical community since the 1930s, with the pioneering works of important authors such as Martinelli and Boelter [1]. It is possible to observe that the liquid, in particular in subcooled conditions, is the medium for which the maximum increments are reached, though not negligible results have been gained for air and boiling; the lower the speed of the fluid, the higher the measured increments (so that the best conditions are for, in the order, natural convection, laminar regime and turbulent regime); the highest increments are reached giving vibrational energy directly to the heating surface rather than to the fluid in convection Even though this achieves good results, some problems are still present: there is a great variability in results among the different articles considered in the paper [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28], indicating that similar experimental conditions may lead to very different convective heat transfer enhancements; there is not agreement on what is the fundamental physical mechanism of alteration, while each author tries to give their own explanation of involved physics; no mathematical model able to quantitatively predict the convective heat transfer has been developed yet. Based on the presented physical phenomena, some general considerations and indications are given for each of the three fluid mediums
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