Medição de densidade, calor específico e condutividade térmica em líquidos baseada em redes de Bragg

Abstract

The monitoring of thermal properties in liquids is an important and complex task, widely applied in industrial processes. Which highlights this importance is the fact that thermal properties of fluids are usually related with the efficience of these industrial processes. In crude oil refining operations, for example, parameters such as temperature, viscosity and thermal conductivity are directly related to the volumetric efficiency of the oil. In these operations, optical solutions, such as fiber sensors, have been widely developed due to their intrinsic operation, in addition to characteristics such as: immunity to electromagnetic interference, compactness, multiplexing capacity and chemical stability. For the thermal measurements assessment, stationary and transient methods are mostly designed for electrically-powered components. Despite this, some of these methods, such as Modified transient plane source (MTPS), admit physical modifications in their design, depending on the application demands. Thus, this work presents an optical system for measuring density, specific heat and thermal conductivity in liquids, based on MTPS and fiber Bragg gratings. For the task, a methodology of characterization of FBGs was developed, which allows an automatic control of data collection, in addition to the simultaneous characterization of several FBGs simultaneously. Thereafter, an optical diaphragm-based system is proposed for the task of measuring density. The sensor (with sensitivity of 0,.025 nm/kPa) is constructed with two FBGs, in which one is embedded between the diaphragms and the other is used to compensate the liquid temperature variation In addition, the diaphragm material (nitrile rubber) is corrosion resistant and chemically stable, which enables operations in liquids such as crude oil. For the estimation of specific heat and thermal conductivity, an FBG (with sensitivity of 11,5 pm/oC and determination coefficient R2 = 0, 9999) is applied in three experiments, named tank, beaker and test tube. The results show the repeatability and reproducibility of the system, in addition to the possibility of accuracy adjustments through a calibration constant. Finally, an analysis of thermal power and flexibility of the methodology are performed, describing the heat distribution in the setup and the possibility of constructing a liquid detection system.