*3.2. Effect of θ on the Channel Flow and Heat Transfer Performance*

In this part of the analysis, four different *θ* of the channel (100◦, 115◦, 140◦, and straight) are considered for this comparative study with the inlet mass flux *G* = 200 kg/m2-s, outlet pressure *Pout* = 8 MPa, and wall heat flux *Qw* <sup>=</sup> ±12 kw/m2-k. *dh* and *Lp* of the channels remain unchanged with the values 2 mm and 7.75 mm, respectively. The bulk temperature *Tb* of the CO2 varies between 280 K and 360 K to cover the pseudocritical temperature *Tm*.

**Figure 6.** Velocity vector along the zigzag channel with *Lp* of 3 mm, 4.5 mm, and 6 mm.

**Figure 7.** Local heat transfer coefficient of the zigzag channel with *Lp* of 3 mm, 4.5 mm, and 6 mm.

As can be seen from Figure 8, *h* and Δ*P* both decrease with the increase of *θ*, and better thermal performance for all of the zigzag channels is demonstrated, compared with the straight channel. The variation trend of *h* and Δ*P* of zigzag channel with *Tb* is the same as that of the straight channel. This provides an approach for defining the flow and heat transfer correlations in zigzag channels, as there have been several studies on the correlations of *Nu* of the sCO2 semicircular straight channel [29,30].

Figure 9 shows the comparison of the velocity vector along the channel with different bend angles of the zigzag channel. A sharper bending angle will significantly increase the local fluid velocity at the turning position and aggravate the separation of the boundary layer, which will result in more violent mixing of fluid between the wall area and the core region. It means that the decrease of *θ* enhances the channel convective heat transfer under the geometric parameters of the current study. Therefore, *h* increases with the decrease of *θ*. As *θ* =180◦, the channel becomes a straight channel, and *h* is smaller than any of the zigzag channels with bending angles.

**Figure 8.** Effect of *θ* 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 9.** Velocity vector along the zigzag channel with *θ* of 100◦, 115◦, 140◦, and straight.

Figure 10 shows the contour plots of local convective heat transfer coefficient in zigzag channel under different *θ*. It can be seen that the convective heat transfer coefficient of the wall surface of the zigzag channel is significantly higher than that of the straight channel. The region with the highest local convective heat transfer coefficient appears on the windward side of the turning angle of zigzag channel. It is because this area is washed by the mainstream and has a locally thinner boundary layer.

**Figure 10.** Local heat transfer coefficient of the zigzag channel with *θ* of 100◦, 115◦, 140◦, and straight.
