GEOHEX: The Test Bench for Boiling Heat Transfer
In the GEOHEX project, one of the most advanced objectives was the study of boiling heat transfer, a complex phenomenon that is fundamental for numerous geothermal and ORC applications. To understand how the innovative materials and coatings developed by the consortium could enhance heat exchanger performance, Spike designed and built a test bench dedicated to the direct observation of low-surface-tension fluid boiling on treated plates. The facility was designed to reproduce controlled conditions and analyze the mechanisms of bubble nucleation, growth, and detachment as a function of surface morphology.
Spike’s Role in Test Bench Design and Construction
Spike developed the test bench using an integrated approach that included the design of the thermo-fluidic circuit, definition of operating conditions, and engineering of the boiling chamber. The test rig was designed to operate with R134a in evaporation at 35 °C, heated by an 80 °C hot water circuit. The configuration included an observation option through a backlit optical window, enabling high-quality video recording to analyze bubble dynamics and identify the effects of surface treatments on boiling behavior.
Boiling Chamber and Optical Observation
The core of the system is a boiling generator equipped with a transparent window and integrated lighting, designed to show the bubble formation stages in detail. The chamber geometry allowed the treated plate to be positioned at the center of the visual field, while surrounding instrumentation ensured precise control of pressures, temperatures, and flow rates. The goal was to observe the transition between nucleate boiling and film boiling, a critical phenomenon in determining thermal stability and operational safety of heat exchangers.
Instrumentation and Operating Conditions of the Test Bench
The bench was designed to operate up to 9 bar working pressure, with a maximum pressure of 14.5 bar and a design pressure of 16 bar. Instrumentation included temperature, pressure, and flow sensors, all connected to a PLC for managing startup sequences, monitoring, and data acquisition. The system also featured a modular structure, allowing rapid replacement of plates for subsequent experimental campaigns on different materials.
Numerical Support with FEM Simulations
During the design phase, Spike used Comsol Multiphysics to analyze the structural behavior of the boiling chamber under high-pressure conditions. FEM simulations showed stress distribution and confirmed the safety and robustness of the chosen design. This numerical work was essential to ensure that the optical window and flanges could withstand operational conditions during testing without compromising visibility or experiment safety.
A Key Contribution to GEOHEX Material Validation
The boiling test bench represented a key element of GEOHEX development, allowing study of the effect of advanced coatings on bubble nucleation and growth, liquid film removal, and boiling process stability. The data collected provided essential guidance for selecting materials with the best performance, contributing to the design of more efficient and reliable heat exchangers for the geothermal sector. The facility built by Spike thus played a decisive role in transferring research results toward solutions applicable in real operational contexts.