Enhancement of heat transfer by an air/water spray in a transformer oil cooler

The use of water to enhance heat transfer and cool overheated transformers has been practiced by the power industry for many years. However, the application of the water has often been "crude" and wasteful. This has motivated the present research to investigate the effects of water spray cooling on a transformer cooler.; A survey of water cooling literature is presented, highlighting the improvement in performance of heat exchangers with various geometries. The reported studies usually focus only on the global heat transfer performance, and do not consider the local behavior. The development of the experimental test facility is delineated. It consists of a full-scale oil transformer cooler and uses a 150-kW circulation heater to simulate the transformer load. An experimental program was carried out to evaluate the performance of a finned-tube transformer cooler under air-cooled conditions and deluged-water conditions. Significant performance enhancement was found by spraying relatively small amounts of water onto the heat exchanger. Tests were performed for air-cooled and air/water-cooled conditions. Various air velocities, oil flow rates, and water spray flow rates were tested. From the air-cooled data, individual air-side heat transfer coefficients were "backed-out" for each row of tubes. An analytical model was developed to predict the global air-cooled performance. In addition, a model was developed to predict the local air/water-cooled performance and incorporates the heat and mass transfer analogy to determine the water evaporation rates. For the "wet" model, the water distribution is a required input. Visual observations are made on the distribution of the water film on the tube bundle. For both air-only and air/water conditions, a comparison of the predicted and experimental data is made. The results show good agreement between the data, and, thus, confirm the reliability of both models.