Thermal/frictional performance of spiral pipe with ring-shape depression used as in-pond heat exchanger

In the present numerical study the ring-shape depressions were created on the wall of spiral pipe that can be employed as a heat exchanger in the solar ponds. Unlike available literature that focused on the optimization of solar ponds through its nature recognition, present study tried to propose an efficient passive method for maximizing heat extraction. This is a simple and cost-efficient technique for enhancing the overall performance of salinity-gradient solar ponds through augmentation of turbulence inside the modified piping system and yielding higher heat extraction accordingly. Indeed, the introduced practical technique not only prevents the increment of size and volume of the piping system but also makes it needless to use additional supplementary equipment like turbulator or even air injectors. Moreover, this is a good way to prevent fouling inside piping system which bears huge costs. 3D numerical simulation was done based on Finite Volume Method (FVM) and turbulent flow simulated by RNG k-epsilon model. Due to the steady-state nature of this problem pressure-velocity coupling algorithm was adopted. The effect of several decisive parameters including Dean, Pr (fluid type and inlet temperature), groove distance, and groove depth on Nu, f, j-factor, and TPF was studied. Interestingly, it was turned out that the values of thermal performance factor (TPF) for all cases were above 1 which shows the justifiability of made modifications. For all flow rates water has the highest Nu and the lowest friction factor compared to Theminol-55 and Ethylene-glycol. Groove pitch of 20 mm makes the highest values of Nu so that 168, 195, and 216 were the maximum obtained Nu for cases with groove depth of 0.7 mm, 1 mm, and 1.3 mm, respectively (working fluid was water and Dean = 1615). Said statement is true for f where its maximum values were 0.1, 0.14, and 0.2, respectively, for cases with groove depth of 0.7 mm, 1 mm, and 1.3 mm (working fluid was water and Dean = 269). The highest Nu of Therminol-55, Ethylene-glycol, and water were 100, 114, and 216, respectively for groove pitch of 20 mm.