Measurement 222 (2023) 113644 Available online 11 October 2023 0263-2241/© 2023 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Measurement journal homepage: www.elsevier.com/locate/measurement Advances on the detection and measurement of bubble contours during subcooled boiling in microgravity Xenophon Zabulis a,∗ , Polykarpos Karamaounas a , Ourania Oikonomidou b , Sotiris Evgenidis b , Margaritis Kostoglou b , Martin Schinnerl c , Axel Sielaff c , Peter Stephan c , Thodoris Karapantsios b a Institute of Computer Science, Foundation for Research and Technology - Hellas, N. Plastira 100 Vassilika Vouton, Heraklion, 70013, Crete, Greece b Laboratory of Chemical and Environmental Technology, School of Chemistry, Aristotle University of Thessaloniki, University Campus, Thessaloniki, 54124, Greece c Institute for Technical Thermodynamics, Technische Universität Darmstadt, Alarich-Weiss-Str. 10, Darmstadt, 64287, Germany ARTICLE INFO MSC: 62H35 76Txx Keywords: Multi-phase imaging Bubble detection Subcooled boiling Boiling Microgravity Image processing ABSTRACT A method for the detection of bubble contours in images of subcooled boiling in microgravity is proposed. The method refines an initial, rough contour approximation obtained from conventional background subtraction. This refinement entails the warping of an elliptical ring image region into a polar image, where bubble contour detection becomes simpler. The proposed method exhibits robustness to a wide variety of visual phenomena that typically hinder the detection of bubble contours. This is achieved by the grouping of continuous image edges. A ground truth annotated data set is provided that enables the quantitative and comparative evaluation of the proposed methods. 1. Introduction The study of boiling phenomena is complex because it involves the combined action of heat and mass transport, which depend on multiple factors. The measurement of bubble size and shape is important for investigating the underlying physical phenomena. This work pertains to the domain of subcooled boiling, where the liquid bulk average temperature is less than the saturation temperature and, thus, a phase change occurs only on the walls of the heated substrate. The study of multiphase phenomena is aided by image processing methods, operating on high-speed video recordings of those [1,2]. This work studies the special case of subcooled boiling in microgravity, following the reference in [1], where it is explained how measurements in microgravity aid the understanding of boiling in general. Pertaining to this work is that the absence of buoyancy leads to different bubble formations than already studied in terrestrial experiments (e.g., [3–5]). The dominant difference is the continuous bubble growth instead of detachment from the substrate. Another difference is brought by the growth of bubble size, which gives rise to practical inconveniences, such as the occurrence of the bubble in low-contrast areas or the reflections of other bubbles. ∗ Corresponding author. E-mail address: zabulis@ics.forth.gr (X. Zabulis). 1.1. Challenges Image analysis in boiling conditions is challenging as multiple phenomena occur rapidly and simultaneously. This work provides a method to study bubble growth in microgravity for varying ex- perimental conditions, including pressure, heat flux, and subcooling temperature, through high-speed video. These conditions give rise to challenging artifacts inter-bubble reflections and lack of contrast. As such artifacts are present in other, more general bubble measurement problems, e.g. [3,4,6], their treatment by this work can contribute to other types of bubble measurements. Some works simplify contour tracing, by carefully configuring the imaging apparatus. The working principle is to configure the imaging conditions to guarantee high contrast between the bubble and the background. This way, edges provide reliable boundary localization tools. Background subtraction and edge detection methods can then be used to trace bubble contours in images, e.g. [7–10]. However, this principle is hard to reach because our scene involves multiple interacting bubbles, giving rise to complex illumination artifacts, inter-, and intra-bubble reflections, as well as minute or zero image contrast. Moreover, due to boiling, these phenomena include evaporation, vis- cous flow, and non-equilibrium effects near the vapor–liquid interface that give rise to additional artifacts and noise [1]. https://doi.org/10.1016/j.measurement.2023.113644 Received 18 February 2023; Received in revised form 17 August 2023; Accepted 29 September 2023