Identification of flow patterns in an opaque condenser tube by distributed fiber Bragg gratings

Two-phase loops with phase change heat transfer are pivotal for cooling electronics under high heat flux and for thermal control in spacecraft. Numerous studies aim to clarify the mechanisms behind these loops to enhance cooling capacity and stability, which are closely tied to flow patterns. However, conventional flow pattern identification methods, requiring transparent tubes for direct observation, are impractical for simultaneous heat transfer measurement, especially during condensation. This paper introduces a novel approach for identifying flow patterns within an opaque, vertical condensation tube using embedded distributed fiber Bragg gratings (DFBGs) for R134a. The method involves a “fingerprint” derived from temperature profile derivatives and additional criteria based on temperature variation characteristics. These include the relative deviation from saturation temperature, the relative spatial temperature gradients, and the relative amplitude of temperature fluctuations. Validation against high-speed camera images confirms the method’s efficacy. It enables the determination of flow pattern lengths, the endpoint of condensation, and the construction of a flow regime map. Additionally, it allows for the measurement of vapor velocity in slug flow, facilitating the calculation of slip ratio and void fraction, which correlates reasonably with the Zuber-Findlay model, with systematic positive deviations up to 30%. This method also paves the way for simultaneous measurement of in-tube condensation heat transfer characteristics for various flow patterns.