Linear Temperature Distribution Sensor Using FBG in Liquids—Local Heat Transfer Examination Application

Measuring the rate and direction of heat flow is essential for several scientific applications and industrial processes. However, the existing heat flux sensors face many drawbacks, including low resolution, slow response and nonlinear response. In contrast, fibre-optic sensors present a more attractive alternative, being immune to electromagnetic interference, robust, compact and highly sensitive. In this work, we experimentally demonstrate, for the first time, a fibre-optic heat flux sensor. Here we show the measurement of the magnitude as well as direction of conductive heat flux in liquids via a Fibre Bragg Grating (FBG). We obtain a sensitivity of 5.22121 pm/(kWm
^-2
) for outward heat flux and 6.33794 pm/(kWm
^-2
) for inward heat flux. To measure heat flux, temperature difference between hot and cold fluids was measured initially by a single, uniform FBG. Two cases were considered: Outward heat flux, wherein the hot fluid surrounded by cold fluid, and inward heat flux, in which the cold fluid being placed in the middle of the hot fluid container. The reflected wavelength showed a linear response in both cases, with sensitivities of 2.39 pm/°C in the first case, and 4.44 pm/°C in the second. These results are expected to be of significance in the optimization of fibre-optic heat transfer local liquid heating based sensors and in developing new applications of such systems.