Design and Evaluation of Lab-Scale, Heat Exchanger Prototypes to Provide Thermal Refugia for Trout

Abstract

In recent years, some streams and fisheries have been experiencing higher surface water temperatures, which negatively impact cold-water trout species. One potential solution is to provide localized refugia of colder water produced via active cooling. The present work focuses on the design and testing of a small-scale prototype heat exchanger, for such a cooling system. An effectiveness-number of transfer units (NTU) approach was used to model the heat exchanger and guide the initial development of the prototypes. Based on the model, a staggered, tube-bundle heat exchanger was used. Various prototypes of the heat exchanger were tested in a 1/10th–scaled model of a section of the creek. The prototypes consisted of just the heat exchanger placed directly in the scaled-stream model and of the heat exchanger placed inside an enclosure with a downstream aperture. The results show that, without the enclosure, the average temperature difference is 0.64 °C, with a corresponding heat transfer requirement of 1.63 kW/°C of cooling. However, with an enclosure, the average temperature difference is 1.95 °C, which required 0.59 kW/°C of cooling. Modifications to the enclosure impact the average temperature difference and the temperature distribution within the enclosure, e.g., decrease the standard deviation of the temperature difference. Thus, the results demonstrate the need for an enclosure to achieve a meaningful cooling effect and to reduce the power requirement for the refugia. The enclosure design can be used to balance the requirements of obtaining the desired temperature difference with a relatively low spatial variation in that temperature difference.