Thermal and geometrical investigation of an original double-pipe helical coil heat storage system with kock snowflake cross-section containing phase-change material

Abstract

In our study, a double-pipe helical coil heat storage system for water heaters containing Phase-change Material (PCM) with a Koch snowflake cross-section of the inner pipe is numerically investigated. Instead of using a simple geometry as a cross-section and fins in order to improve the heat transfer efficiency, as a novelty, the Koch snowflake cross-section with angled sides and sharp corners increases the effective surface area and also acts like fins. Water in the inner pipe is Heat Transfer Fluid (HTF), and RT-35 is located in the annulus as PCM. Hypotheses such as the insulation of the outer pipe surface and the incompressibility of the Newtonian fluid have been applied to solve the equations in the simulation. Validation has been done by comparing the results with a previous study in the literature. Important geometrical and fluid parameters, such as the first three iterations of Koch snowflake for the inner pipe cross-section, coil diameter, helical pitch, Reynolds number, and operating fluid inlet temperature for this heat storage system, were discussed in detail. The outcomes showed that the second iteration is the optimum iteration in the first three iterations of the Koch snowflake for heat exchanging. As a matter of fact, the second iteration enhances the PCM's melting rate by 5% at the maximum state (liquid fraction changes from 0.79 to 0.83), while the third iteration, compared to the second iteration, maximum decreases it by 1.2% (liquid fraction changes from 0.83 to 0.82). Furthermore, the coil diameter parameter has an inverse relationship with melting; thus, with a 33% increase in the size of the coil diameter (90 to 120 mm), the melting decreases by 1.1%. Also, increasing the helical pitch by 100% (40 to 80 mm) raises the melting by 3.2%. The Reynolds number change does not influence the melting in the range of laminar flow; however, rising the inlet temperature of heat transfer fluid by 2.9% (340 to 350 K) increases the liquid fraction by about 23%.