Flow patterns characterization in horizontal microchannels under temperature conditions close to the thermodynamic critical point

• Grant number: 23/17573-5
• Support Opportunities: Scholarships in Brazil - Scientific Initiation
• Start date: February 01, 2024
• End date: January 31, 2025
• Field of knowledge: Engineering - Mechanical Engineering - Thermal Engineering
• Principal Investigator: Gherhardt Ribatski
• Grantee: Dannilo Belo Cardoso
• Host Institution: Escola de Engenharia de São Carlos (EESC). Universidade de São Paulo (USP). São Carlos , SP, Brazil

Abstract

This research proposal is within a context of a broad study currently in execution at the Heat Transfer Research Group of the São Carlos School of Engineering, USP, which is focused on the study of two-phase flow and flow boiling at temperatures close to the thermodynamic critical point. Investigations concerning two-phase flow characterization in both macro and microscale were extensively performed over the last few decades, in order to understand the heat transfer and pressure drop mechanisms. Most research involving organic refrigerants is restricted to low saturation temperatures (5G5`5N5a), typical of refrigeration and air-conditioning applications. Recently, there is a growing interest in systems that operate at high evaporation temperatures, such as Organic Rankine Cycles and high temperature heat pumps. As a consequence, it is necessary to develop new predictions methods to be employed in the project of heat exchangers operating under these conditions, since the currently available methods for estimating heat transfer coefficient (HTC) and pressure drop (Dp) do not incorporate experimental results for high saturation temperatures in their conceptions. Given the recognized relationship between flow topology and heat transfer and pressure drop, the investigation of the flow characteristics at high saturation temperatures is essential in the development of phenomenological methods to predict the HTC and Dp. In this context, this proposal deals with the evaluation of the two-phase flow characteristics during flow boiling of R245fa refrigerant at saturation temperatures from 60 to 150 °C, corresponding to reduced temperatures of 0.78 to 0.99, inside a horizontal 1.2 mm diameter channel. A program developed in Matlab®, based on the evaluation of the pixel's light intensity, will be used to treat flow images obtained through a high-speed camera to characterize the flow patterns. Furthermore, signals obtained through a pair of diodes laser-sensors attenuated by the flow topology will be also used in the flow characterization. Based on this data, results for velocity, length and frequency of vapor bubbles will be obtained, the eccentricity of the vapor core will also be evaluated, and an objective method for the flow pattern characterization will be developed.