ISBN: 978-1-56700-537-0
ISBN Online: 978-1-56700-538-7
ISSN Online: 2377-424X
International Heat Transfer Conference 17
EXPERIMENTAL SETUP FOR THE MEASUREMENT OF CONDENSATION HEAT TRANSFER COEFFICIENTS ON STRUCTURED TESTING SURFACES
Resumo
Plate heat exchangers feature flexible designs in combination with outstanding thermohydraulic power density. Therefore, they find application in a wide range of technical processes and are not only used as single-phase heat exchangers but also as evaporators and condensers. The already efficient devices can be improved regarding condensation by functionalizing the condensate facing surface of the plates for example by means of femtosecond laser pulses (FSLP). The surface changes that can be achieved by applying FSLP are on the one hand of a geometric character in the nano- and micrometer range, and on the other hand of a chemical character due to the incorporation of selected foreign atoms. The aim of functionalization is to control the wettability of the surface. By producing a hydrophobic surface, droplet condensation can be forced, which shows significantly higher heat transfer coefficients.
In order to investigate the thermal behaviour of such structured surfaces during condensation, an experimental setup is developed to provide direct metrological accessibility to the two-phase heat transfer coefficient. The main part of the experimental setup is a measurement section with a testing surface, on which condensation occurs. The measurement section is instrumented with temperature sensors arranged in several levels so that heat flux density and wall temperature can be measured. By determining these quantities, the heat transfer coefficient can be calculated directly from the defining equation, thus providing the determination with low uncertainties. In the context of this work an investigation of the film condensation of R365mfc on an unmodified smooth surface of copper in a range of Reynolds number from 15 to 200 is performed. Already at a Reynolds number of 20, wave formation occurs on the condensate film. The measured heat transfer coefficient rises from 1.46 kWm2 K⁄ to 3.07 kWm2 K⁄ within the range of Reynolds numbers. Due to the early onset of turbulence, the measurement results show a low agreement with the analytical solution for the heat transfer coefficient. Furthermore, it is shown that the measurement of the heat transfer coefficient can be performed with a comparatively low measurement uncertainty. Within the scope of the measurements performed, the standard measurement uncertainty of the heat transfer coefficient is a maximum of 19 %.
In order to investigate the thermal behaviour of such structured surfaces during condensation, an experimental setup is developed to provide direct metrological accessibility to the two-phase heat transfer coefficient. The main part of the experimental setup is a measurement section with a testing surface, on which condensation occurs. The measurement section is instrumented with temperature sensors arranged in several levels so that heat flux density and wall temperature can be measured. By determining these quantities, the heat transfer coefficient can be calculated directly from the defining equation, thus providing the determination with low uncertainties. In the context of this work an investigation of the film condensation of R365mfc on an unmodified smooth surface of copper in a range of Reynolds number from 15 to 200 is performed. Already at a Reynolds number of 20, wave formation occurs on the condensate film. The measured heat transfer coefficient rises from 1.46 kWm2 K⁄ to 3.07 kWm2 K⁄ within the range of Reynolds numbers. Due to the early onset of turbulence, the measurement results show a low agreement with the analytical solution for the heat transfer coefficient. Furthermore, it is shown that the measurement of the heat transfer coefficient can be performed with a comparatively low measurement uncertainty. Within the scope of the measurements performed, the standard measurement uncertainty of the heat transfer coefficient is a maximum of 19 %.