*3.1. Effect of Lp on the Channel Flow and Heat Transfer Performance*

To study the effect of *Lp* on thermal and hydraulic performance of zigzag channel, three different *Lp* (3, 4.5, and 6 mm) are considered for this comparative study. The other geometric factors *dh* and *θ* take the valve 1.17 mm and 115◦, respectively, and remain unchanged to avoid coupling effects. The channel has an inlet mass flux of *G* = 200 kg/m2 s, an outlet pressure of *Pout* = 8 MPa, and a wall heat flux of *Qw* <sup>=</sup> ±12 kw/m2-k. The bulk temperature *Tb* of CO2 varies between 280 K and 360 K, covering the pseudocritical temperature *Tm*.

As shown in Figure 5, *h* of the three channels gradually increases and reaches the maximum value as the *Tb* of the fluid approaches *Tm*. This is due to the surge of the specific heat and thermal conductivity of CO2 near the pseudocritical point. Δ*P* of the three types of channels decrease with the increase of *Tb*. This is mainly because the density of CO2 decreases with the rising of *Tb*. It can be seen from the comparison results of the three channels that the heat convection coefficient *h* and pressure drop Δ*P* both decrease with the rising *Lp*.

Figure 6 shows the velocity vector along the zigzag channel with different *Lp*. The flow-field distribution possesses certain periodicity. A large velocity gradient occurs at the channel corner and the maximum velocity in the channel also appears in this area. Boundary layer separation occurs at the corner of the channel and the local velocity increases due to the appearance of vortex. As the velocity direction is different from the wall direction of the next pitch, it can strengthen the mixing of the mainstream and the fluid near the wall, which is conducive to the heat transfer enhancement. As can be seen from the partial enlarged view, the local velocity increases with the reduction of channel *Lp*, which means that the fluid at the boundary region is mixed with the fluid in the core region more sufficiently. As a result, the reduction of *Lp* enhances the channel heat transfer, and improves the channel total heat convection coefficient *h*. The local flow resistance Δ*P* also rises with the reduction of *Lp* as the wall separation increases.

**Figure 5.** Effect of *LP* on flow and heat transfer performance: (**a**) *h* of cooling case; (**b**) Δ*P* of cooling case; (**c**) *h* of heating case; (**d**) Δ*P* of heating case.

Figure 7 represents the local convective heat transfer coefficient in zigzag channel with different *Lp*. The local heat transfer coefficient increases significantly on the windward side of the corner area. It is because the boundary layer is locally thinner under the direct flushing of the incoming flow. As mentioned above, the local fluid velocity increases with the decrease of channel *Lp*, which also leads to the local heat transfer coefficient increasing.
