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

• Final Report Year: 2023

Final Report Abstract

Within the DFG project, flow boiling of CO2 in horizontal evaporator tubes with different periodic, open-cell structures (POCS, here specifically cubic cells, Kelvin cells) was investigated. For both structures, experiments were carried out to determine the two-phase pressure loss in the diabatic and adiabatic state. Furthermore, a circumferentially resolved determination of the heat transfer at the tube wall was carried out as well as an optical characterisation of the prevailing flow forms on the basis of high-speed images. Based on the experimental results, a new pressure loss model was developed. The model is based on the single-phase pressure loss model of periodic, open-cell structures by Klumpp et al. By extending the model to the geometry of Kelvin cells and combining it with a suitable twophase method, all data points of the Kelvin cells and the cubic cells could be described with a MAPE of 11 % and 14 %, respectively. Consequently, a high robustness of the correlation for the prediction of the pressure drop in the two-phase flow of POCS with different cell geometries is assumed. In the second part of the project, the heat transfer during convective flow boiling was determined experimentally. Subsequently, a separate model of convective flow boiling was developed for each cell type investigated. The models describe the measured data of the Kelvin cells with a MAPE of 20 % and those of the cubic cells with a MAPE of 19 %. It should be noted here that, in accordance with solid sponge structures, the heat transfer of the upper half of the tube can be better represented with the homogeneous model and that of the lower half of the tube with the drift-flux model. Here, too, a high reliability of the developed correlations for the prediction of the convective heat transfer in POCS with two-phase flow is shown. In addition to the experimental investigations, a simulation routine was set up in the open-source software OpenFOAM, with which the heat transfer of POCS materially bonded to the pipe wall was investigated. Simulation studies with varying strut diameter and constant porosity show that, in addition to the bonding surface of the structures, the position in which the unit cells are cut on the pipe wall has a significant influence on the heat transfer. In addition to an increasing bonding surface, the heat transfer is primarily improved by struts that are orthogonally bonded to the pipe wall. Finally, based on all simulation data, a model was developed to predict the utilization efficiency (ratio of conjugate heat transfer to heat transfer at the POCS surface). The model is based on the fin efficiency of a cylindrical strut and describes the simulation data with a MAPE of 12 % for square tube geometries and 7 % for round tube geometries.

Publications

• Flow-pattern based prediction of flow boiling heat transfer in horizontal tubes with circumferentially varying heat flux. International Journal of Heat and Mass Transfer, 148(2020, 2), 119018.
  Weise, Sonja; Dietrich, Benjamin & Wetzel, Thomas
  
• Influence of porous inserts on flow boiling heat transfer in horizontal tubes. International Journal of Heat and Mass Transfer, 171(2021, 6), 121087.
  Weise, Sonja; Wetzel, Thomas & Dietrich, Benjamin
  
• Characterization of Heat Transfer and Pressure Drop during Steady State Flow in Periodic Open Cellular Structures 16th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Online, 08.-10. August 2022 (Presentation)
  Dubil, Konrad; Wetzel, Thomas & Dietrich, Benjamin
  
• Development of a generalized thermal resistance model for the calculation of effective thermal conductivities in periodic open cellular structures International Journal of Heat and Mass Transfer, 183(2022, 2), 122083.
  Dubil, Konrad; Wolf, Hannah; Wetzel, Thomas & Dietrich, Benjamin
  
• Numerical investigations of coupled solid and fluid heat transfer in periodic open cellular structures Annual Meeting on Reaction Engineering and ProcessNet Subject Division Heat and Mass Transfer, Würzburg, 18.-20. July 2022 (Poster)
  Dubil, Konrad; Wetzel, Thomas & Dietrich, Benjamin
  
• HYDRODYNAMICS AND HEAT TRANSPORT IN HIGHLY POROUS OPEN-CELLED STRUCTURES. Proceeding of International Heat Transfer Conference 17 (2023), 14. Begellhouse.
  Dietrich, Benjamin