Evaluation of the hydrodynamic behavior and the heat transfer mechanisms during flow boiling in microchannels under conditions close to the surface dryout

Abstract

The present proposal is inserted in the context of a FAPESP thematic grant (Process nº 2016/09509-1) and aims the conversion of a master FAPESP research project (Process nº 2019/01755-1) into a direct doctorate by means of the expansion of the scope and continuity of the original research. It deals with the simultaneous study of the liquid film hydrodynamic behavior during annular flow, and its effects on the temperature field along the heated surface, evaluated through infrared thermography, for convective boiling under conditions close to the wall dryout. The annular flow pattern prevails in channels of reduced dimensions and is characterized by high Heat Transfer Coefficients (HTC) that drastically decrease as dryout conditions are established. This behavior is influenced by the liquid film characteristics (´) and its effects are determinant on the heated wall temperature field. Therefore, methods to estimate the liquid film thickness are frequently incorporated into models to predict HTC, dryout vapor quality (xdry) and pressure drop. Due to intrinsic difficulties associated to measurements in reduced scales, most of these methods are based on speculations about the behavior of ´ and the heat transfer mechanisms. It is a fact that the wall dryout is a dynamic phenomenon with time-dependent characteristics, behavior neglected by most of the models, which are based on empirically adjusted parameters from time-averaged experimental data. This brings the fact that these models do not include the actual physical mechanisms, turning them only suitable to the experimental conditions for which they were adjusted. In this context, the present proposal involves the development of a microsensor for the instantaneous evaluation of ´, activity already under execution, which signal is based on the local electrical conductance of the liquid film. It also involves the design of a heated test section that allows optical access for the IR camera to the internal wall and, consequently, to the surface temperature distribution. Once the devices are developed, calibration procedures for the signal associated with ´ and the incident irradiation on the camera, related to the measured temperature, will be proposed and implemented. From that, experimental data for ´ and for the heated surface temperature field will be gathered for flow boiling under conditions close to the wall dryout in a rectangular channel with water as the test fluid, using an experimental test bench available at the Heat Transfer Research Group at EESC-USP. Based on those results, yet unavailable in the literature, the heat transfer mechanisms are expected to be identified and incorporated to models for the prediction of HTC and the establishment of wall dryout conditions which includes the effects of ´. Hence, this proposal stands out for gathering unique experimental results 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 fabrication steps 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. This doctoral proposal also includes a Research Internship abroad under supervision of Professor Yuji Suzuki from the University of Tokyo. (AU)