MS Thesis Presentation by Vishwanath Subramaniam
Tuesday, July 6, 2004
(Dr. Srinivas Garimella, Chair )
"Design of Air-Cooled Microchannel Condensers for Mal-Distributed Airflow Conditions"
Air-cooled condensers are routinely designed for a variety of applications, including residential air-conditioning systems. Recent attempts at improving the performance of these heat exchangers have included the consideration of microchannel tube, multilouver fin heat exchangers instead of the more conventional round tube-plate fin designs. In most packaged air-conditioning systems, however, the condenser is part of an outdoor unit, in which the condenser surrounds the compressor and other auxiliary parts, with an induced draft fan at the top of this enclosure supplying the airflow. Such a configuration results in significant mal-distribution of the airflow arriving at the condenser, with a commensurate decrease in performance over the design case. This work attempts to address this issue of mal-distribution by adapting the air-side geometry to the expected air flow distribution. A variety of tube-side pass arrangements are also considered to help tailor the pass-wise performance to the realistic airflow patterns. Flow regime-based models from the literature were adapted to model condensation of refrigerant blend R410A in the microchannel tubes under consideration. Similarly, correlations for air flow over multilouver fins are used to predict the air-side behavior. The resulting computer program is capable of predicting the performance of the condenser for a variety of candidate airflow distributions across the heat exchanger. For the base (uniform air flow case) as well as several mal-distributed cases, tube-side pass arrangements, and more importantly, features of the air-side geometry such as fin pitch, height and louver details are varied. This ensures that the air-side surface area is distributed in accordance with the air flow variation to yield the required heat duty with the lowest possible tube and fin material requirement. Through the analysis of several cases, it is shown that material savings can result through judicious choice of surface area and tube-side flow area allocations. The results from this study can used to guide the practical design of air-cooled condensers under realistic flow conditions.