Flow boiling in solid sponges

• Applicant: Professor Dr.-Ing. Thomas Wetzel
• Subject Area: Technical Thermodynamics ; Chemical and Thermal Process Engineering
• Term: from 2013 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 245385027

With ongoing efforts to avoid climate change, the efficiency requirements for refrigeration systems and their components are also increasing. Our previous research project therefore looked into the effects of integrating metal sponges in cylindrical evaporator tubes on pressure loss, flow pattern and heat transfer during flow boiling. The metal sponges - highly porous, irregular network structures with continuous solid and liquid phase - were used here without being cohesively bonded to the tube wall. On the basis of the knowledge gained in this first project phase, the following objectives are formulated for the proposed follow-up project:The comparison of the measured pressure loss of the multi-phase flow with a combination of the homogeneous model with the Darcy approach for the single-phase flow in porous media developed within the framework of the project showed good agreement for vapor qualities below 50%. In the follow-up project, a validated model for higher vapor qualities is to be found by e.g. variation of the structural parameters. By means of a heat transport model also developed in the course of the project, it can already be estimated that a further possible improvement of the heat transfer coefficient by a factor of 1.3 to 2.6 is to be expected due to directly bonding the porous structures to the tube wall. However, in order to complete and validate the model and the underlying physical considerations, the description of the influence of the flow modification on the heat transfer at the pipe wall must be improved. On the other hand, the heat transfer directly at the struts of the structure needs to be described as a function of the operating parameters and geometric properties for the directly wall-bonded structure. In the previous project it was shown that the exact knowledge of the structure parameters specific surface area, porosity and cell density is of great importance for the modelling of the pressure loss as well as the local heat transfer coefficient at the struts. Therefore, the planned further investigations are to be carried out additionally on regular structures that can be produced in 3D printing technique. For such structures, the variation and determination of the mentioned parameters is simpler and can be done with substantially improved quality compared to the stochastic sponges.As a result, the models developed within our previous projekt for predicting the heat transfer coefficient and pressure loss in evaporator tubes equipped with solid sponges will be completed in such a way, that a quantitative evaluation of the efficiency of evaporator with integrated porous structures tubes will be possible.

• DFG Programme: Research Grants
• Co-Investigator: Dr.-Ing. Benjamin Dietrich