Hydrodynamic investigations of a two-phase geothermal pipe for building heating

This study reports on the hydrodynamics of a falling liquid film and an upward gas flow in the pipe core in a vertical plain pipe representing a section of a two-phase-closed geothermal probe, also known as a thermosyphon used for building heating. The flow was examined experimentally in two different sized test-rigs and supported by numerical simulations. The background of this seemingly simple study is the analysis of the thermohydraulic behaviour of a thermosyphon used as a geothermal probe to extract validated design correlations. Thermosyphons offer the advantage that, compared to conventional geothermal brine probes, they do not require a pump to transport the heat transfer fluid. It was shown that for a single-phase gas flow, the deviation between the measured differential pressures, the simulated values and the theoretically calculated values is less than 5%. For the countercurrent gas-liquid flow, the differential pressures and entrainment limit for the given diameter could be determined. Experimentally determined differential pressures are given as a function of film thickness and inner diameter of the pipe. • Investigations in a 3-metre test rig with two different pipes for CO 2 thermosyphons. • Experimental investigation and DNS of waves in a liquid falling film. • Single-phase air flow: experiments show good agreement with simulation. • Measured friction factors: plain pipe vs helically corrugated pipe vs literature data.