Modulated flow patterns for vertical upflow by the phase separation concept

•A phase separation concept was proposed to modulate flow patterns for vertical upflows.•The tube was divided into an annular region and a core region.•The gas bubbles flow in the annular region and liquid majorly flows in the core region.•Thin liquid film and pulsating flow have the potential to enhance the heat transfer. The passive phase separation concept was proposed to create thin liquid film on the wall surface. The inserted mesh cylinder made of a single layer of mesh pore surface divides the tube cross section into an annular region near the tube wall and a core region. Gas bubbles are prevented from entering the core region and liquids can be sucked towards the core region. Thus, the two-phases are majorly flowing in two different regions. The concept is expected to be used for multiphase (boiling/evaporation and condensation) heat transfer enhancement. Air–water two-phase flow experiments were performed with vertical upflows. Miniature bubbles are modulated to flow in the annular region. Cap/slug/plug bubbles are modulated to form the elongated-ring-slug bubble in the annular region to generate thin liquid films on the wall. For all the cases, the core region is full of liquid. Due to the large density difference between liquid in the core region and gas in the annular region, pulsating flow is self-sustained in the core region. The mesh pore surface promotes the mass and momentum exchange between the annular region and core region. For ring-slug bubbles such as longer than 10cm, miniature bubbles are emitted from the slug bubble front. The proposed concept is being verified by the phase change heat transfer experiment in our laboratory.