Time-averaged and time-resolved laser optical temperature measurements in water with Filtered Brillouin Scattering combined with LDV

Abstract

A laser measuring system was developed and built that allows to optically measure temperature in water using the method of Filtered Brillouin Scattering (FBS). First time-resolved optical temperature measurements were demonstrated. Furthermore, the FBS-system was combined with an LDV to enable simultaneous measurement of flow velocity and therefore the system is also capable to measure the heat flow. Time-averaged temperature values were determined with good accuracy and, as a special highlight, also timeresolved temperature measurements have been demonstrated with temporal resolution in the order of approximately 10 ms, validated by comparison with fast thermocouple measurements. The overarching goal of the research project was to prepare the market introduction of a laser-optical measuring system for spatially point-based and time-resolved measurement of the heat flow in liquids, especially in water. In order to realize this, it was necessary to measure the local velocity and the local temperature in a liquid. The optical measurement of the local velocity has long been possible using the established method of laser Doppler velocimetry (LDV). Therefore, the heat flow measurement method to be developed should be based on this technology. Until now, there was no suitable optical method for measuring the temperature. In recent years, however, the physical phenomenon of Brillouin scattering has become one focus of measurement technology development. If a small volume of liquid is irradiated with light, the molecules in the liquid scatter back part of the light, which is known as Brillouin scattering. The spectrum of the scattered light depends on the local temperature in the liquid; and it turns out that this physical relationship can be exploited to develop a highly accurate, fast, and non-contact method for measuring temperature. In this paper, we explain the Filtered Brillouin Scattering (FBS) method, show a setup for measuring temperature and velocity in water flows and thus a method for determining the heat flow, and demonstrate the measurement accuracy using a calibration test bench. The temperature measurement accuracy achieved is in the order of 1 K.