Condensation Heat Transfer for Zeotropic Mixtures on Horizontal Tubes and in Tube Bundles

• Applicant: Professor Dr.-Ing. Andreas Paul Fröba
• Subject Area: Technical Thermodynamics
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 570287051

Due to their low global warming potential, binary zeotropic mixtures based on hydrocarbons are promising alternatives to commonly used hydrofluorocarbon-based working fluids in refrigeration, heat pump, and organic Rankine cycles. For the efficient design and optimization of condensation heat exchangers integrated in the aforementioned cycles, such as the commonly applied shell-and-tube condenser, the maximization of the condensation heat transfer coefficient (HTC) on the outside of condensation tubes and in corresponding tube bundles is necessary. Due to the zeotropic character of most of the relevant hydrocarbon mixtures, which leads to a preferential condensation of the high-boiling component on the condensation tube wall and, therefore, an enrichment of the low-boiling component in the vapor phase limiting the heat transfer, the HTC is usually much smaller for such mixtures than that for their pure components. The proposed research project should contribute to an improved understanding of the heat transfer during the condensation of zeotropic mixtures on single horizontal tubes with different characteristics and in corresponding tube bundles. This knowledge should form the basis for an efficient design of shell-and-tube condensers applied for zeotropic mixtures in the future. For this purpose, binary zeotropic mixtures consisting of hydrocarbons of varying structures should be investigated over a wide range of compositions. This allows to study not only the influence of the so-called temperature glide, i.e. the difference between the dew point and the boiling point of a zeotropic mixture of given composition, but also of further thermophysical properties, such as the enthalpy of vaporization, viscosity, and surface tension, on the HTC. To examine how the condensation tube geometry influences the HTC, condensation tubes with a systematic variation of the fin density should be investigated experimentally in combination with binary zeotropic mixtures and, for comparison purposes, with their pure components in an existing setup. To study how the preferential condensation of one mixture component affects the vapor composition in the condenser, the experimental setup should be modified to allow the determination of the vapor composition in the vicinity of the condensation tube by Raman spectroscopy. Besides the influences on the HTC for single horizontal tubes, also the tube bundle effect describing the impact of liquid condensate impinging on the tubes in the lower rows of the bundle should be investigated systematically. The experimental results from this work should be used as the main database for the development of an analytical model for the prediction of the HTC for the condensation of zeotropic mixtures on single finned tubes. After validation of this model, it enables to study the influences of the individual thermophysical properties on the HTC by their exclusive variation, which is not possible in experiments.

• DFG Programme: Research Grants