Development of a microsensor and its use to investigate the liquid film characteristics during flow boiling in microchannels under conditions close to the surface DRYOUT

Abstract

The present proposal concerns a master research project in a context of a FAPESP Thematic Grant (Process nº 2016/09509-1), that deals with the study of convective boiling in reduced diameter channels focusing on solar energy applications, among other topics. Specifically, the proposal concerns an evaluation of the liquid film thickness for annular flow pattern under conditions of microscale diabatic flows of water, focusing on conditions near to the surface dryout. A microsensor based on the electrical conductivity of the film will be developed and applied to the film thickness measurements. The annular pattern prevails in channels of reduced dimensions and is characterized by high heat transfer coefficients (HTC) that drastically decreases as the surface becomes dry. This behavior, as well as the high pressure drop under such conditions, is highly influenced by the liquid film characteristics. Therefore, methods to estimate the liquid film thickness are frequently incorporated into models to predict HTC, dryout vapor quality (xdry) and pressure drop. Most of these methods are based on results obtained under adiabatic conditions (in general air-water flows) and speculations about the behavior of the liquid film, due to intrinsic difficulties associated to the measurement of the film thickness in reduced scales. This brings the fact that the models for HTC, dryout vapor quality and pressure drop are based on the adjustment of experimental parameters, turning them only suitable to the experimental conditions for which the parameters were adjusted. In this context, the present proposal involves gathering unique experimental data that will improve the understanding of the physical phenomena, as well as, enable their incorporation into prediction methods applied to the design of heat exchangers and heat sinks based on flow boiling in microchannels. The development of the microsensor will be based on microfabrication processes and, therefore will benefit from the partnership with Semiconductors and Nanotechnology Components Center (CCSNano) at UNICAMP established by means of the FAPESP Thematic Project. Finally, two fundamental aspect that will contribute for the success of the proposed research are highlighted: (i) the expertise of the Heat Transfer Research Group on the study of heat transfer processes with phase change; and (ii) the high capacity and mature of the candidate. This last aspect is based on the following facts: he obtained the best academic record among the graduates in Mechatronics Engineering at EESC-USP in 2018, has already attended two compulsory courses of the Mechanical Engineering graduate program of EESC-USP (Classical Thermodynamics and Fluid Mechanics), obtaining the maximum marks in both, and is co-author of a paper in a high impact factor journal that was a result of his undergraduate research project sponsored by FAPESP (Process nº 2016/11657-9). (AU)