## 1 Introduction  
Air conditioning, refrigeration and heat pump systems have been widely used in the civil, military and industrial fields, for example, the application of heat pump systems in the cold climate of northern China [1, 2]. Building energy consumption accounts for 23% of global primary energy and 30% of global electricity. To improve the energy efficiency of refrigeration/heat pump systems, phase change materials (PCMs) for thermal energy storage have attracted more and more attention [3, 4], as they can help smooth energy supply and consumption. In particular, it has been pointed out that organic solid-liquid PCMs have great practical application value, and can be used in building air conditioning, underground heating and refrigeration systems [5, 6]. However, the thermal conductivity of organic solid-liquid PCMs is low and needs to be increased by adding thermal conductive particles. In the preparation processes of PCM mixture, the key is to ensure a degree of homogeneity throughout the vessel, prevent solid accumulation and maximize the contact area between solid and liquid phases [7, 8, 9]. Thus, most of the methods for the mixing of thermal conductive particles and PCMs needs high efficiency stirring. For example, the melt blending method, one of the most favorable manufacturing methods for solid-liquid PCMs due to its potential in large-scale production, is to directly add the thermally conductive filler to the molten polymer, mix it evenly in the stirred tank, and then solidify it to form the product (Figure 1). The mixing quality in the stirring process is directly related to the performance of solid-liquid PCMs, which affects the energy utilization efficiency. Atpresent, various small batches production of solid-liquid PCMs have been widely adopted, however, large-scale production is still in its infancy. The most important reason is the lack of manufacturing technology and equipment suitable for mass production. Therefore, a study of stirring equipment for mixing with high efficiency, rapidity, and homogeneity is important, and can also be helpful for solid-liquid mixing in other fields. Pitched blade turbine (PBT) has been extensively applied in solid-liquid mixing systems [9; 10; 11; 12], however, some undesirable problems, such as flow stagnant regions, the settling of solids and the accumulation of solids near the wall, still occur when using PBT, which seriously affects the output, quality and process cost of the PCMs. In order to obtain good solid-liquid suspension quality, the most commonly used method is to increase the impeller speed, but this means high power consumption and cost. Thus, the mixing equipment requires further technical optimization to reach economic viability. To address the above issues, numerous recent works have been centered on mixing processes, which indicates that the modification of impeller design can enhance mixing behavior [13; 14]. The effects of a Rushton turbine with streamlined or perforated blades on power, turbulent dispersion, gas holdup and mass transfer rate performance were studied by [PERSON] et al. [15]. [PERSON] et al. [16] evaluated the turbulent mixing efficiency of a perforated paddle impeller using computational fluid dynamics (CFD). [PERSON] et al. [17] investigated the effects of blade shape on trailing vortices, indicating that trailing vortices could provide a source of turbulence and have a potential benefit for mixing in a stirred tank. [PERSON] et al. [18] developed a novel zigzag punched impeller with promising outcomes. Another alternative is based on cut blade surfaces. [PERSON] et al. [19] reported that the power consumption of the Rushton turbine was reduced when V-shaped or rectangular cuts were made in the blade tips. [PERSON] et al. [20] and [PERSON] et al. [21] studied the mechanism of fractal shaped blades in a Rushton turbine stirred tank reducing the power consumption through experiments and simulations.