Design of a test rig for the investigation of falling film flows with counter-current gas flows

Geothermal phase change probes employ falling film evaporation to efficiently harness geothermal heat for space heating applications. Despite successful applications in research, commercial use remains limited due to the lack of validated models capable of accurately representing the complex flow phenomena within these probes. To address this issue, a test rig was developed, replicating the flow phenomena within these probes and facilitating the validation of future models. This paper presents the results of the first measurement campaign conducted with water and humid air as working fluids. For the gas phase, velocity profiles were measured with a hot-wire anemometer, and for the liquid phase, high temporal resolution film thickness measurements were conducted with a confocal chromatic imaging sensor. The results of both measurement systems were qualitatively consistent with existing literature. The time-averaged film thickness h̄film in the flow direction, without gas flow, revealed an increase of h̄film along the flow direction for Reliq = 500. In contrast, for Reliq ≥ 980, an opposing trend was observed. In addition, the influence of the gas flow on h̄film was investigated in the flow direction. The results show no noticeable influence below a threshold gas Reynolds number near the liquid film inlet (x = 0.1 m), while an increase appeared beyond this Reliq-dependent value. Observations further downstream (x = 0.6 m) indicated an h̄film increase toward the threshold and a decrease beyond it. The latter observation aligns with findings in the literature, suggesting the onset of flooding. This observation was further strengthened by a film flow transition captured through high-speed camera imaging, indicating the test rig's capability of investigating flooding conditions.