EXPERIMENTAL INVESTIGATION OF THE TRANSPORT PHENOMENA ON A FALLING FILM ABSORBER WITH MICRO-STRUCTURED TUBES

摘要

One of the most important tasks in engineering is to optimize the energy efficiency of all processes due to limited natural resources. In this context, the energy demand of refrigeration and HVAC applications must be considered, because a significant amount of the primary energy consumption is used in these applications. A higher efficiency can be achieved by the replacement of conventional compression chillers by absorption chillers which may be driven by waste heat instead of electric energy. This leads to a lower consumption of primary energy. Thus, research on heat driven absorption chillers is important to gain higher energy efficiency. The absorber is still the limiting component of those cycles and the influence of many design parameters on the heat and mass transfer is still not completely understood, although many investigations are documented in literature. In this paper, the influence of micro-structured surface modifications on the heat and mass transfer on a tubular absorber made of copper is discussed. A drawn and a polished surface structure are used in the experiments. The microstructure of the tubes is analyzed with a new three dimensional contactless optical roughness measurement technique based on the focus variation. An aqueous lithium-bromide solution is used as working fluid and various parameters like solution flow rate, solution concentration and coolant flow rate are investigated during the experiments. The obtained results are discussed and compared with those published in literature and the effect of micro-structured surface modifications on the heat and mass transfer are evaluated. The tubular absorber with a drawn surface shows significantly higher heat and mass transfer coefficients compared with the polished one. One of the reasons is the lower wettability of the polished surface and the resulting lower surface area which is available for the transport process.