Thermal and Hydraulic Effects of Dimples on Performance of Clamshell Heat Exchangers in Residential Gas Furnaces

Abstract

This paper investigates the thermal and hydraulic effects of dimples on clamshell heat exchangers in residential gas furnaces through experimental studies and by using one-dimensional and multidimensional numerical models. Calculated results from the one-dimensional thermal model show that six dimples in a clamshell heat exchanger improve the heat transfer of the clamshell heat exchanger by only 1.5%. In contrast, experimental results and one-dimensional calculations show that the dimples cause a 360% increase in the pressure drop. Multidimensional numerical calculations of flow and heat transfer in the channels show that a single dimple significantly increases total pressure drop (60 to 125%) but only minimally increases heat transfer (3 to 9%) for the Reynolds number range of 6 × 102 to 1.5 × 104. A new clamshell heat exchanger design without dimples is proposed. This new design has a reduced cross-sectional area but the same perimeter as the original dimpled clamshell heat exchanger. The new furnace design has better or equal heat transfer and less pressure drop than the original dimpled heat exchanger. Based on the results, it is concluded that dimples are not an efficient means to increase heat transfer in clamshell heat exchangers because the pressure drop penalty is high and heat transfer enhancement is minimal.