Visualization of the microscopic flow profile of state-of-the-art absorption heat pump working pairs under operational conditions

• We successfully used an innovative optical method in a lab-scale falling-film AHP.
• We measured simultaneous film thickness and velocity profiles during operation.
• LiBr/water and an additive enhanced LiBr/water solution were used as working pairs.
• The additive enhanced solution is more often deflected from the main flow direction than the pure one.
• We discussed the main limitations of the method and gave ideas to improve it.

The research and development of novel working pairs and highly efficient heat exchangers for absorption heat pump applications require a detailed analysis of the microscopic flow profile at the absorber heat exchanger during the absorption process. A recently developed Single Aperture Defocussing Micro Particle Tracking method, based on a conventional Particle Image Velocimetry system, demonstrates the possibility of simultaneous measurements of velocity profiles and film thicknesses in falling film absorbers at steady-state flows. We show first results of velocity profile measurements of the state-of-the-art working pair LiBr/water, with and without a 2-Ethyl-1-Hexanol additive, during operation in a lab-scale absorption heat pump. Based on the results of the measurements, the advantages and disadvantages of the developed measurement technique and their potential application in the characterization of novel working pairs is discussed.