## 3 Results and Discussion
### Freezing Completion Effect in Aquatic Products at Different Stacking Modes  
The freezing completion effect is assessed by the decrease of the highest product temperature within a residence time of 24 h. The larger is the decrease of the highest product temperature, the greater is the freezing completion effect. The refrigerated room temperature is maintained at 255.15 K. The temperature distributions in eight stacking modes were analyzed. In a vertical direction, the XY plane with z = 2.4 m was considered. In the horizontal direction, the XZ planes with y = 0.35, 1.05 and 1.75 m were studied. 1 & 256.1 & 256.155 & 0.945 & 0.37 & 17 & 255.7 & 255.199 & 0.541 & 0.21 \\
2 & 256.3 & 255.154 & 1.146 & 0.45 & 18 & 255.4 & 256.154 & 0.10 \\
3 & 256.2 & 255. Four temperature distributions for NSMs (vertical direction) are shown in Figure 4. These are for one-pile, two-pile, three-pile and six-pile arrangements. The temperature distributions (vertical direction) for the corresponding PSMs are demonstrated in Figure 5.  
As shown in Figure 4, the highest product temperatures for the four NSMs are 259.42, 259.26, 258.57 and 258.47 K, while the temperature reductions are 1.73, 1.89, 2.58 and 2.68 K, respectively. In addition, as indicated in Figure 5, the highest product temperatures for the four PSMs are 255.79, 255.60, 258.37 and 258.33 K, with temperature reductions of 5.36, 5.55, 2.78 and 2.82 K, respectively. The temperature differences between NSMs and their corresponding PSMs are 3.63, 3.65, 0.2 and 0.14 K, respectively. The freezing completion effect at PSMs is better than the effect at NSMs for a residence time of 24 h. More gaps among package piles can enhance the freezing completion effect because of the increased heat exchange areas [18,40]. Moreover, the freezing completion effects at UPF-PSMs are much better than that at UPT-PSMs. As compared with UPT-PSMs, the use of UPF-PSMs enlarges the areas of return air and wider the gaps between package piles. Larger space for the return air and wider gaps between package piles facilitate the forced convection in the refrigerated room [16]. Four temperature distributions in NSMs (horizontal direction) are shown in Figure 6. These NSMs are for one-pile, two-pile, three-pile and six-pile arrangements. The temperature distributions (horizontal direction) in the corresponding PSMs are illustrated in Figure 7.  
Figure 4: Temperature distributions in NSMs (vertical direction), (**a**) NSM with one-pile, (**b**) NSM with two-piles, (**c**) NSM with three-piles, (**d**) NSM with six-piles. Figure 5: Temperature distributions in PSMs (vertical direction): (**a**) UPF-PSM with one-pile, (**b**) UPF-PSM with two-piles,(**c**) UPT-PSM with three-piles, (**d**) UPT-PSM with six-piles. Figure 6: Temperature distributions in NSMs (horizontal direction): (**a**) NSM with one-pile, (**b**) NSM with two-piles, (**c**) NSM with three-piles, (**d**) NSM with six-piles. Figure 7: Temperature distributions in PSMs (horizontal direction): **(a)** UPF-PSM with one-pile, **(b)** UPF-PSM with two-piles, **(c)** UPT-PSM with three-piles, **(d)** UPT-PSM with six-piles. As shown in Figures 6 and 7, the isothermal surfaces have the form of concentric ellipses. Cold air is blown from the outlets of two refrigerating units, and circulated through the top and the bottom of the refrigerated room. Afterwards, the exhausted air is collected back via the return outlets of the refrigerating units. The cold air moves around each package, so the freezing completion process is performed by gradually increasing the food ice fraction from the surface to the centre of each package. In temperature cloud diagrams, red areas (some10% of the highest temperatures) are defined as relatively high temperature zones (RHTZs). For NSMs and UPT-PSMs, the RHTZs are in the center of each package pile. However, as shown in Figure 7a,b, in case of UPF-PSMs with two-piles, RHTZs are offset in two opposite directions, which is due to wider gaps between two package piles.