Natural convection characterization during melting of phase change materials: development of a simplified front tracking method

This study presents the development of a front tracking method for melting of phase change materials (PCMs) inside horizontal shell and tube heat exchangers. Two numerical models, i.e. pure conduction (PC) model as well as combined conduction and natural convection (CCNC) model, are used to develop the method. Governing equations are numerically solved by ANSYS Fluent v17.2. The PC model benefits from simplicity but its prediction is far from reality, whereas CCNC model’s prediction is more realistic but its modeling is complicated. Generally, during the melting process, the upper half of the system is affected by the upward buoyancy-driven melted PCM motion. To consider this phenomenon, the front tracking method assumes that the upper and lower halves of the system have two separate melting fronts. Therefore, it is assumed that the natural convection contributes only to the upper half until the upper half liquid fraction value reaches unity. Meanwhile, the lower half melting front is assumed to be the same as that of the PC model. Once the upper half is totally melted, the method attributes the rest of the natural convection to the lower half of the system. Using three different PCMs and three different geometries, two correlations have been developed for each half based on two dimensionless numbers; i.e. the shell-to-tube radius ratio and PC model liquid fraction. The method is then verified using another PCM, which has not been included during the correlation development stage to guarantee the methods validity. These correlations provide results within ± 15% discrepancy range

Numerical investigation of a triplex tube heat exchanger with phase change material: simultaneous charging and discharging

Thermal energy storage by phase change materials (PCMs) has received considerable attention in recent years. Its potential application is due to the issue of energy supply and demand time mismatch management. Thus, at the time of energy availability at supply side, it is stored in PCMs so as to be extracted later on when it is needed. However, in order to provide continuous operation, there might be some times that a system needs to be simultaneously charged and discharged. Most studies focused either on charging, discharging, or consecutive charging and discharging process, while limited work has been conducted for the case of simultaneous processes namely: simultaneous charging and discharging (SCD). This study presents the development of a numerical model to study the performance of a triplex tube heat exchanger equipped with a PCM under SCD. Governing equations were developed and numerically solved using ANSYS Fluent v16.2. Based on the solid-liquid interface evolution over time, the effect of natural convection on the heat transfer was investigated, and internal heating/external cooling mode was compared with the internal cooling/external heating for a triplex tube heat exchanger. According to the mode of heating and initial condition of the storage (fully melted or fully solidified), four different cases were compared based on the steady solid-liquid interface. The results indicated that the upward melted PCM motion had a great impact on the process. Interestingly, depending upon the initial PCM condition, different final solid-liquid interfaces were found. Finally, the pure conduction model was compared with combined conduction and natural convection to identify the dominant heat transfer mechanism. It was found that the former could be applied to the initially fully melted PCMs under SCD with small error, but for the initially solidified PCMs neglecting the natural convection would result in unacceptably large error.</p

Simultaneous charging and discharging of phase change materials: Development of correlation for liquid fraction

In this study, a method to track the melting front of phase change materials (PCMs) is developed under simultaneous charging and discharging (SCD) inside horizontal triplex tube heat exchangers. For SCD, the heat transfer mode of internal heating and external cooling is investigated. According to the heat transfer mechanism within the PCMs, two models of pure conduction (PC) and combined conduction and natural convection (CCNC) are developed and solved using ANSYS Fluent (version 17.2). Generally, during charging (i.e. melting), the buoyancy forces induce the liquid PCM to move upwards; therefore, the upper section of the storage is affected. However, this motion is limited during SCD due to the simultaneous cooling from the outer tube. Therefore, a front tracking method is developed assuming that natural convection only contributes melting to the upper section of the storage unit. On the other hand, it is assumed that the melting front for the lower section behaves similar to the PC model. In this study, three PCMs are used in three geometries to develop the correlations using two dimensionless numbers; i.e. liquid fraction of the PC model and the storage radius ratio. Another PCM is then used to verify the method. The correlations can provide results in the range of ±15% discrepancy

Experimental investigation of multiple tube heat transfer enhancement in a vertical cylindrical latent heat thermal energy storage system

Thermal energy storage in phase change materials (PCMs) received considerable attention due to the capability of tackling the time mismatch between energy supply and demand, especially for renewable energy sources. Nevertheless, PCMs suffer from some drawbacks preventing their widespread commercialization. In this study, a geometrical heat transfer enhancement technique was investigated to increase the rate of heat transfer from the heat transfer fluid (HTF) to the PCM in a shell-and-tube heat exchanger. The performance of two designs of single and multiple (five) tube heat exchangers (i.e. STHX and MTHX, respectively) were experimentally investigated and compared in terms of average PCM temperature, liquid fraction and stored heat during a complete charging and discharging cycle. It was found that the MTHX out-performed the STHX in terms of phase change duration and stored heat. Furthermore, the validity of a common simplifying assumption in numerical investigation of MTHXs which is considering an artificial cylindrical boundary around each tube was experimentally investigated. This assumption was found to result in inaccuracy meaning that it should no longer be considered in future numerical studies