Qing-lin Cai

Bio: Qing-lin Cai is an academic researcher from Chinese Ministry of Education. The author has contributed to research in topics: Natural convection & Annulus (firestop). The author has an hindex of 1, co-authored 1 publications receiving 3 citations.

Numerical investigation on melting and energy storage density enhancement of phase change material in a horizontal cylindrical container

Abstract: Thermal energy storage (TES) plays a significant role in storing heat energy during the availability of a source. The phase change material (PCM) storage density is high, but its meager thermal conductivity leads to ineffective heat transfer. This mathematical modeling work simulates two different heat transfer tube (HTT) arrangements in a horizontal cylindrical container to melt the PCM using the enthalpy‐porosity method. Case‐1 with a single HTT at the centroidal axis and Case‐2 with multiple tubes spread radially. Cases‐1 and 2 are observed with melting and solidification duration of 39.8 and 184 minutes, and 69.83 and 292.5 minutes respectively. Case‐1 has a shorter melting and solidification time because of the optimal HTT location. Case‐2 is observed with a high heat storage density of 224.95 MJ/m3, compared to Case‐1 with 224.571 MJ/m3. Nusselt number is higher for Case‐2 (385.86) during charging and higher for Case‐1 (208) during discharging. Nusselt number initially varies for both cases due to effective heat convection at the start; however, Case‐1 has a shorter melting duration with its energy density lower than the Case‐2. Based on the obtained results, it could be suggested that Case‐1 could be applicable when simultaneous heat transfer requires less energy storage and a faster melting rate, while Case‐2 could be used in places that demand a higher storage density.

Investigation of the effects of shell geometry and tube eccentricity on thermal energy storage in shell and tube heat exchangers

Abstract: In this research, the combined effects of the shell geometry and downward eccentricity of the heat transfer tube on the melting behavior of the paraffin wax in shell and tube heat exchangers (HXs) are parametrically investigated. Different shell geometries with a wide range of eccentricity factors are studied while the mass of paraffin in all geometries is kept constant. Transient numerical simulations using the enthalpy-porosity method have been carried out to explore the melt interface evolution, streamlines, heat transfer rates, average velocities, as well as thermal energy storages for all the considered cases. Results reveal that the melting rate accelerates by increasing the eccentricity of the HTF tube. Nevertheless, there is an optimum eccentricity factor for each geometry beyond which the melting rate reduces. The maximum melting time reduction compared to the concentric tube HX with circular shell (base case) was 50.4% obtained by the HX with circular shell at an eccentricity factor of 0.5. It was also concluded that increasing the eccentricity factor prolongs the convection-dominated melting time and shortens the conduction-dominated melting, and results in a more uniform heat transfer rate over the whole charging process which is important for the industrial deployment of the thermal storage units.

Optimization of thermal energy storage: Evaluation of natural convection and melting-solidification time of eccentricity for a rotational shell-and-tube unit

Abstract: Changing the eccentricity between outer and inner tubes in horizontal shell-and-tube unit during phase change of the heat storage medium is an economical procedure to improve overall heat transfer. However, because of the reverse effect of a fixed eccentric structure (downward or upward) on melting and solidification processes, investigators in previous studies have only examined the potential of performance improvement separately. Finding a technical approach to ensure lower times for melting and solidification in the fixed eccentric structure can yield significant benefits. This study numerically evaluates the processes of melting and solidification in the eccentric design for a shell-and-tube unit concurrently based on the second law of thermodynamics and natural convection. Based on the second law of thermodynamics analysis confirms that natural convection can be analyzed using the fractional entropy generation. The results show that downward eccentric designs enhance natural convection during the process of melting in stationary units. Solidification results show the significance of natural convection in the primary stage of the process. Melting time decreases by 71 % in the case with the eccentric number of 0.4 whereas solidification time decreases in upper eccentric cases (eccentric number of <0.1 in the stationary units). Reduction in solidification time by 13 % is demonstrated with the eccentric number of 0.05 and the diameter ratio of 2.52. This study presents a novel and applicable method (180-degree rotation) to achieve a shorter melting-solidification time in eccentric configurations. Additionally, a rotational mechanism is investigated to reduce the total melting-solidification time.