Experimental investigations and numerical modeling of 2D temperature fields in flow boiling in minichannels

• Flow boiling heat transfer in a rectangular minichannel is examined experimentally.
• Temperature distribution on the wall is measured due to liquid crystal thermography.
• The flow patterns are observed and the void fraction is determined.
• The flow boiling heat transfer model is formulated.
• The Trefftz method is applied for solving heat conduction problems in flow boiling.

The experimental purpose of the study was to simultaneously measure and record two-dimensional temperature distributions on the heating surface and the images of the corresponding two-phase flow structures. The study was conducted for FC-72 flow boiling in rectangular, vertical and asymmetrically heated minichannels with a high aspect ratio of 40, 20 and 13.3 (0.5, 1.0, 1.5 mm deep, 20 mm wide and 360 mm long), for heat fluxes ranging from 58 to 132 kW m−2, absolute pressure from 1.16 to 1.84 bar and the mass flux from 185 to 1139 kg m−2 s−1. The flat heating surface was made of a thin rolled plate. The inlet temperature of the liquid was kept constant at 288 K. The CCD camera, mounted on the heated side of the channel, produced color images of liquid crystals. High-speed camera images, taken through the glass window on the opposite side of the channel, were used to measure void fraction for selected cross-sections as a function of variable thermal and flow parameters. The ranges of characteristic two-phase flow structures were determined relative to the hydraulic diameter dh and local void fraction φ(x). The best-agree correlation was found for the relationship between the local void fraction and the local vapor quality in high aspect ratio minichannels.The theoretical part of the study focused on the development of flow boiling heat transfer model based on the Trefftz method. It was formulated to include the minimum number of experimental constants and to numerically solve the manifold inverse problems. The numerical procedure included solving two sequential inverse problems (in the heating foil and in the flowing liquid) and the accompanying direct problem (in the protecting glass pane). To obtain two-dimensional temperature distributions in the boiling liquid for bubbly and bubbly–slug flows, a triple coupled inverse problem was solved using the Trefftz method. The numerical solution was compared with the simplified one, assuming that the entire heat generated in the foil was transferred to the flowing refrigerant. Both solutions gave similar results.