**4.5 Effect of ribs on stratification**

**Figure 11** shows the velocity vector diagram for a tank with three grids (case 4). It is evident that the ribs create disturbance in the flow field, creating wake regions in the space between successive ribs. It affects the formation of boundary layer throughout the tank wall surface. Ultimately the presence of ribs enhances mixing in the bulk liquid which may reduce the formation of stratified layer.

The nature of flow velocities over three different configurations is depicted in **Figure 12**. The number of ribs affects the flow velocity inside the tank. Case 6 is having maximum number of ribs (7) which possess less velocity among the three cases. More number of ribs results in less flow velocity which indicates better mixing of bulk fluid and delayed stratification.

**Figure 11.** *Velocity vector diagram of a tank with rib structure.*

*Effect of Roughness Elements on the Evolution of Thermal Stratification in a Cryogenic… DOI: http://dx.doi.org/10.5772/intechopen.98404*

**Figure 12.** *Comparison of velocity of flow for cases 4–6.*

#### **Figure 13.** *Comparison of temperature of liquid for cases 4–6.*

**Figure 13** shows the comparison of temperature over three cases. Apart from interface, local heating zones were created on both sides of the rib surface. Since the presence of ribs causes obstruction of flow to a large extend, the locally heated fluid cannot travel to the interface effectively. The presence of recirculation zones enhances the mixing of the fluid. As a result, stratification develops very slowly. The formation of stratified layer is entirely different for three cases. There is more resembles between case 4 and smooth wall case. As the number of ribs increases, formation of local hotspots adjacent to the rib wall were visible and it is not transported to the interface. **Figure 14** shows the volume fraction under these cases.

**Figure 14.** *Comparison of liquid volume fraction for cases 4–6.*

**Figure 15.** *Comparison of pressure evolution of a tank with different number of rib elements.*

*Effect of Roughness Elements on the Evolution of Thermal Stratification in a Cryogenic… DOI: http://dx.doi.org/10.5772/intechopen.98404*


**Table 5.**

*Influence of rib structure on the evolution of self-pressurization.*

Similar to temperature profile, the tank with more number of ribs has less phase change.

The evolution of pressure under three different cases is shown in **Figure 15**. As the number of ribs increases, the value of pressure inside the tank decreases and its value is well below that of smooth wall tank. The major change in pressure is noticeable after a time period of 70 seconds. As the number of grids increases from 3 to 7, the heated surface area increases by 24% and 56%. At the same time, a maximum reduction of 41.63% in volume to surface area ratio occurs for case 6. Ultimately the self-pressurization rate decreases by 32.89% for case 6. The decrease in self-pressurization rate due to presence of ribs is shown in **Table 5**.
