Predicting the air-side thermal-hydraulic performance characteristics of flat-tube louver-fin heat exchangers under dry and wet conditions.

摘要

The air-side performance of flat-tube louver-fin heat exchangers for conditions typical to air-conditioning applications is studied experimentally. Using a calorimetric wind tunnel, heat transfer and pressure drop are measured for a wide range of fin geometry and operating conditions, including conditions leading to dry, partially wet, and fully wet fin surfaces. Transient and steady-state condensate retention data are obtained using the wind tunnel, and novel dynamic drainage data are obtained using a newly developed experimental method.; A simple area-partitioning method for analyzing the wind-tunnel data is developed for flat-tube heat exchangers under partially wet conditions, and new air-side performance correlations are presented for both dry- and wet-surface conditions. Correlations between the Colbum j or f factors and the Reynolds numbers are developed in the conventional way and compared to multivariate non-parametric regressions. The conventional correlations significantly extend prior work by expanding the parameter space and establishing a physical basis for the form of the fits. However, it is found that multivariate non-parametric regressions can dramatically reduce the RMS errors if a large, well-structured dataset is used. Furthermore, this method provides statistical indicators that can be used to prevent over-fitting when the dataset is limited. Important condensate retention and drainage mechanisms are identified and modeled using analytical and numerical methods. When condensate obstructs the louver gap, boundary layer re-starting does not occur, and the most important heat transfer enhancement mechanism in these flows lost. It is shown that surface tension can maintain inter-louver condensate bridges against gravity and flow-generated shear and pressure forces, even at air-side Reynolds numbers much higher than are typical to the applications of interest. Thus, maintaining wet-surface heat transfer performance depends on avoiding the initial formation of inter-louver bridges.