Enhanced heating surface application in a minichannel flow and the use of the FEM and Trefftz functions for the solution of inverse heat transfer problem

The paper presents results of flow boiling in a rectangular minichannel of 1.0 mm depth and 40 mm width, vertically oriented. The heating element for the working fluid (FC-72) that flows along the minichannel is a single-sided enhanced alloy foil made from Haynes-230. Micro-recesses were formed on the selected area or entire heating foil by laser technology. The observations of the flow structure were carried out through a piece of glass. Simultaneously, owing to the liquid crystal layer placed on the opposite side of the enhanced foil surface, it was possible to measure temperature distribution on the heating wall through another piece of glass. The first objective is to calculate the void fraction for some cross-sections of selected images for increasing heat fluxes supplied to the heating surface. The flow structure photos were processed using Corel graphics software and binarized. The analysis of phase volumes was developed in Techystem Globe software. The second objective of the calculations is to evaluate a heat transfer model and numerical approach to solving the inverse boundary problem, and to calculate the heat transfer coefficient at boiling incipience. This problem has been solved by means of the finite element method in combination with Trefftz functions (FEMT). Trefftz functions are used to construct basic functions in Hermite space of the finite element.

► Flow boiling heat transfer in a rectangular minichannel examined experimentally.
► Single-sided enhanced alloy foil with micro-recesses as the heating wall.
► Temperature distribution on the wall measured by liquid crystal thermography.
► Flow pattern observed on the opposite side of the wall.
► Heat transfer coefficients (FEM, Trefftz functions) and void fractions calculated.