Combined effects of upward eccentricity and volume fraction of graphene nanoparticles on the melting performance of a horizontal double-tube latent heat storage unit

This paper investigates the effects of the eccentricity of an inner tube and the addition of graphene nanoparticles to a phase change material (PCM) on the melting performance of horizontal double-tube latent heat storage units. To study the effect of the eccentricity (ec), the inner tube is shifted upwards in three levels (e), relative to the inner radius of the inner tube (r_i). Thus, four geometric configurations having eccentricities of (i.e., ec = e / r_i = 0, 0.25, 0.5, and 1.0) are investigated with three volume fractions (i.e., 1%, 2%, and 3%) of graphene nanoparticles for the graphene-PCM composites. Paraffin wax with a melting point of 309 K is adopted as the PCM. Based on the enthalpy-porosity approach, a transient, three-dimensional, numerical model is developed to study the melting dynamics and performance of the four configurations. Liquid fraction, average temperature, velocity distribution, charging and storage capacity, and thermal effectiveness are used to evaluate the performance. The results show that the concentric case has the highest melting rate, while the case with ec = 1.0 has the lowest melting rate, when the pure PCM is used. The addition of graphene nanoparticles to the PCM enhances the melting rate and thermal effectiveness of all cases, and the enhancement is strengthened for the eccentric cases. Compared to the pure PCM cases, adding 3% of graphene to the PCM reduces the melting time of the concentric case and the eccentric case with ec = 1.0 by 36.6% and 44.3%, respectively. Concomitantly, the daily storage capacities increase by 43.5% and 64.9%, respectively. At 3 vol% of graphene nanoparticles, the concentric case shows an increase in the daily charging capacity, daily storage capacity, and average effectiveness by 50.6%, 49.8%, and 36.5%, respectively, while for the case with ec = 1.0, they increase by 68.8%, 64.9%, and 54.3%, respectively, compared with those of the corresponding pure PCM cases.