Investigation of the internal flow behavior in active chilled beams
Aachen / E.ON Energy Research Center, RWTH Aachen University (2020) [Book, Dissertation / PhD Thesis]
Page(s): 1 Online-Ressource (xii, 135 Seiten) : Illustrationen, Diagramme
Active chilled beams (ACBs) are terminal devices of a corresponding ACB system which provides ventilation as well as air cooling or heating in an indoor environment. Unlike in classical all-air systems, water is the essential medium to transport the required thermal energy. This results in considerable advantages regarding volume specific thermal power as well as energy demand for air movement. Thus, in applications with high thermal loads and reasonable air exchange rates, ACBs can offer superior energy efficiency and compactness. In the context of the present trend of a growing worldwide primary energy demand for heating, ventilation and air conditioning, it must be a permanent interest to improve components and systems with regard to their energy efficiency and total life cycle energy demand. Until today, the development of ACBs is predominantly based on empirical testing of full-scale prototypes and the practical experience of the manufacturer. A comprehensive consideration of the internal flow phenomena and the interrelated physical processes in ACBs, which may support a goal-driven development, is lacking in the scientific literature and shall therefore be treated in this work. This dissertation analyzes the internal air flow of an ACB and related effects of entrainment, heat transfer and acoustic behavior inside an active chilled beam. Using both an experimental approach as well as computational fluid dynamics (CFD) tools, flow characteristics of different geometries representing varying levels of geometrical abstraction or functional complexity with regard to an ACB are investigated with the aim of gaining a better understanding of the internal air flow. In the experiment, global entrainment ratios were measured using dedicated secondary air supply ducts in which the mass flow was controlled with regard to set values of the static pressure. Quantifiable velocity information for a general, isothermal case was obtained with the aid of two-dimensional two-component particle image velocimetry (2D2C PIV), which also enabled the identification of characteristic flow patterns. Two-dimensional, time-averaged velocity data was gained for the axisymmetric, turbulent jet using two-component laser Doppler anemometry (LDA). Further experimental investigations were conducted with regard to flow noise and heat exchanger characteristics. Results from measurements using different methods were found to be consistent, showing similarities between flow patterns of different ACB geometries. Direct, isolated correlations as well as interconnected correlations for the entrainment ratio were found for the primary air static pressure, nozzle height, secondary air opening area, heat exchanger flow resistance and mixing air duct diffuser angle. Using a modern commercial CFD software, three-dimensional simulation data were generated for different Reynolds-averaged Navier-Stokes turbulence models and compared to the measurement data. The comparative study revealed the k-omega based turbulence models as most promising regarding the prediction of global flow quantities. However, the additional prediction of characteristic flow patterns was only achieved satisfactorily by the k-omega SST turbulence model. Good agreement with the measurement data regarding local and global flow quantities was found for results of large eddy simulations (LES).
Melikov, Arsen Krikor