According to the heat release characteristics of the thermal storage tank (or the mixing characteristics inside the thermal storage tank), it can be divided into three categories: fully extruded flow, fully mixed flow, and partially mixed flow. If υ represents the water flow velocity, L represents the length of the water tank, and E represents the mixing diffusion coefficient, then the above three categories can be classified based on the degree of mixing of the water temperature inside the tank or the magnitude of the mixing characteristic M=υ L/(2E) value. 
(1) Fully extruded flow 
Also known as piston flow, it refers to the complete piston flow inside the water tank. There are two water bodies, hot and cold, inside the tank, and the boundary between the two is very clear, indicating almost no mixing. At this point, it can be considered as E → 0 or M →∞. When the hot water storage tank releases heat (cold), water flows in from the bottom (top), and all the heat can be utilized. This is an ideal state, as shown in Figure 2-11. Assuming there is 100L of hot water with a temperature of 80 ℃ in the hot water storage tank, then slowly inject cold water with a temperature of 20 ℃ from the bottom inlet A, while all the hot water flowing out from outlet B is 80 ℃. But when the outflow of water exceeds 100L, the water temperature immediately drops to 20 ℃. 
(2) Fully mixed flow 
The temperature inside the water tank is completely uniform, indicating that the mixing is very thorough. At this point, E →∞ or M → 0 can be considered. Usually, this can only be achieved by installing a powerful mixer inside the hot water storage tank, which slowly injects cold water while stirring. At the beginning, the water temperature flowing out from outlet B was 80 ℃, and then as time passed, the water temperature decreased exponentially. When the outflow reached exactly 100L, the water temperature had dropped to around 80 × e ≈ 29.3 ℃. 
(3) Partial mixed flow 
Also known as temperature stratification flow, it indicates that the temperature distribution inside the water tank is uneven and stratification occurs. This can be considered as a finite E value, i.e. 0<E<∞, and therefore the M value is also finite, i.e. 0<M<∞. Under normal circumstances, the situation inside a general hot water storage tank is mostly like this.