The Effect of Pitch on Heat Transfer to an Immersed Heat Exchanger in a Solar Thermal Storage Tank With and Without a Baffle

Abstract

The effect of the pitch of a copper coil heat exchanger immersed in a hot water storage tank on heat transfer from the storage tank to the heat exchanger working fluid is investigated. The storage tank is initially quiescent and full of hot water. The heat exchanger located at the top of the tank has a coil diameter just under the tank diameter and has a pitch of 2, 3, 4, 6, or 12 times the heat exchanger diameter, D. The effect of the pitch is explored both with and without a cylindrical baffle, which creates an annular region with the tank wall that has a width of 1.5D and within which the heat exchanger is located. In experiments without the baffle, increasing the heat exchanger pitch improves the rate of heat transfer to the working fluid. The improved heat transfer is attributed to the increased thermal stratification generated by the larger pitches. In experiments with the baffle, the results for experiments with pitches of 2D, 3D, 4D, and 6D are all very similar, with 3D slightly outperforming the others. Heat transfer to the heat exchanger with a 12D pitch was significantly lower than the others. In experiments both with and without the baffle, the larger pitches resulted in more variation in experimental results, despite strict standards for initial and operating conditions. As in the prior work, the presence of the baffle resulted in significantly higher heat transfer rates compared to respective experiments without the baffle.