Numerical and experimental methods for heat transfer enhancement for finned-tube heat exchangers with oval tubes

摘要

This paper presents the results of an experimental and numerical study of forced convection heat transfer in a narrow rectangular duct fitted with an oval tube. The duct was designed to simulate a single passage in a fin-tube heat exchanger. The objective of this paper is to determine the suitability of numerical methods as a tool for predicting the effectiveness of oval tubes and winglets for heat transfer enhancement. Heat transfer measurements were obtained experimentally using a transient technique in which a heated airflow is suddenly introduced to the test section. High-resolution local fin-surface temperature distributions were obtained at several times after initiation of the transient using an imaging infrared camera. Corresponding local fin-surface heat transfer coefficient distributions were then calculated from a locally applied one-dimensional semi-infinite inverse heat conduction model. Heat transfer was predicted numerically using a three-dimensional computational model of the test section that was developed to accurately predict fin-surface heat transfer coefficient distributions. Comparisons of numerical and experimental results are made at Reynolds numbers based on channel height, Re{sub}H, of 500 and 1240, the normal range of operation of air-cooled heat exchangers in geothermal applications. These Re{sub}H values correspond to an air frontal velocity of 1.0 m/s and 2.5 m/s respectively. The elliptical tube had a major axis to minor axis aspect ratio, a/b of 3:1 and a/H equal to 4.33. Results presented in this paper reveal visual and quantitative details of local fin-surface heat transfer distributions in the vicinity of an oval tube obtained both numerically and experimentally. Comparisons of numerical and experimental results show good agreement, however the numerical results are consistently higher in value. Several possible explanations for this difference are identified that will be investigated further.