相变材料在含翅片球形容器内的约束熔化传热过程

Constrained melting heat transfer of a phase change material in a finned spherical capsule

为了定量评估翅片对球形容器内相变材料储热性能的影响,本文采用数值模拟与试验相结合的方法研究了等温加热条件下添加不同高度环形翅片时球形容器内的约束熔化传热过程.数值模拟中采用焓模型描述相变过程,并利用有限容积法求解控制方程.与可视化试验结果的对比表明,该方法可以较好地预测熔化过程中固液相界面的演化趋势.通过对数值模拟得到的熔化过程中自然对流流形和温度场的分析可以发现,添加环形翅片不仅起到了增强导热的作用,还能够强化翅片附近特定区域的自然对流传热.在本文所研究的工况下,添加高度与球体半径之比为0.25,0.50和0.75的翅片可以使总熔化时间分别减少约10.6%,20.2%和28.7%,显著加快了球形容器的储热速率.

Spherical capsules are widely used as housing for phase change materials（PCMs） in heat exchangers for thermal energy storage applications. In view of the low thermal conductivity of common PCM candidates, extended surfaces, such as fins, are routinely added to PCM capsules to improve the thermal performance. In this paper, to quantitatively evaluate the influence of fins on the thermal performance of PCM-filled spherical capsules, constrained melting heat transfer of a PCM in a circumferentially finned spherical capsule with various fin heights was investigated both numerically and experimentally under constant-temperature boundary conditions. The enthalpy-based model was used in the numerical simulations to deal with phase change, while the control volume method was used to solve the governing equations for the melting problem with natural convection in the liquid phase. A spherical melting facility that allows for direct observation of the solid-liquid phase interface during melting was designed and constructed. The spherical capsule and fins were made of glass and aluminum, respectively. Octadecane with a nominal melting point at 28.2°C was used as the PCM. Qualitative agreement was obtained between the experimentally observed and numerically predicted results of solid-liquid interface evolution. The sources of the differences were identified to be the departure of the physical model from the real system, as well as simplifications of the numerical model. The evolution of the natural convective flow and temperature fields during melting are presented in the form of a series of snapshots of streamline and isotherm contours. The heat transfer mechanisms during melting were interpreted by these contour snapshots. The presence of circumferential fins not only enhances heat conduction, but also augments natural convection heat transfer in localized areas in the vicinity of the fins. The localized interactions between these two effects are significantly affected by the fin height. Under the specific conditions considered, fins with height-to-radius ratios of 0.25, 0.50, and 0.75 decrease the total melting time by 10.6%, 20.2%, and 28.7%, respectively, leading to a significant increase of the thermal energy storage rates in the spherical capsule. Although the presence of fins benefits heat transfer enhancement, a more comprehensive understanding of the effects of other important factors, such as the inclination angle of the spherical capsule, is required.