Enhancement of thermofluid characteristics via a triple-helical tube heat exchanger

An investigation of the thermal and hydraulic performance of a novel triple-helical tube heat exchanger, the THTHE, is presented. The novel design is a modified design of a DHTHE created by adding a third tube to a DHTHE tube. The third passage is expected to enhance the thermal performance of the DHTHE as a result of an increase in the temperature gradient between the hot and cold fluids. The effects of the coil radius, inner annulus spacing, water inlet temperature, direction of flow arrangement, and Dean number were explored. Five test samples with different coil radii of 150 mm, 125 mm, and 90 mm and different inner annulus spacings of 6.2 mm, 9 mm, and 12 mm were examined. The test samples were designed, fabricated, and tested to demonstrate the influence of design parameters on the thermal and hydraulic performance of the triple-helical tube heat exchanger. The experimental runs were conducted on the hot water side with the water inlet temperature T_h,i ranging from 50:80 °C. Moreover, at Dean number 400 ≤ De_h ≤ 5500, corresponding to Reynolds number 2700 ≤ Re_h ≤ 31,000. Compared with the double-helical tube heat exchanger, the THTHE resulted in a higher Nusselt number by 146.1% and 109.3% for both the counterflow and parallel-flow arrangements, respectively. Furthermore, lowering the hot water source temperature from 80 to 50 °C results in a 60.6% increase in the Nusselt number of 60.6%, with no increase in the pumping power. Additionally, with a decreasing coil radius from 150 to 90 mm and inner annulus spacing from 12 to 6.2 mm, a significant increase in Nu occurs by 58.2% and 130.4%, respectively. A general correlation was presented for predicting Nu, f, and ε.