## 3 Results and Discussions
### The Effect of the Heat Exchanger Wall Temperature on the Flow Field
#### 3.2.2 The Effect of Wall Temperature on the Flow and Heat Transfer
On the other hand, focusing on one location, Figure 20b shows that the  
Figure 19: The effect of the imposed temperature gradient (dT) on the velocity profile taken at 15 mm from the joint above the cold plate (CHX). Figure 18: Velocity profiles from CFD (black and grey lines) and the experiment (red and black symbol) for 20 phases of a flow cycle with the influence from heat exchanger wall temperature. velocity profiles at the selected point of the CHX in analogous phases of the two flow directions (i.e., \(\phi\)9 vs. \(\phi\)19 here), are also affected by the presence of temperature effects. For ease of comparison, the axial velocity of \(\phi\)19 in Figure 20b is presented as a negative (\(-\)ve) value. The presence of temperature effects results in a lower centreline velocity at \(\phi\)9 but bigger magnitudes in the reversed flow direction at \(\phi\)19. The results indicate the changes in the viscous boundary layer due to the presence of temperature. The asymmetry of the flow structure can be further illustrated through the vorticity contours at the open area next to the heat exchanger for three different wall temperature conditions (Figures 21-23). The vorticity is plotted for two phases corresponding to the opposite flow direction (\(\phi\)1 and \(\phi\)11). For each phase, the contour is shown with two views of the two ends of the heat exchanger assembly. Figure 21: Vorticity contour for the case of the heat exchanger set as the adiabatic wall at the (a) hot and (b) cold plates. (1 bar in nitrogen gas, 0.3% drive ratio, 13.1 Hz). Figure 20: Velocity profile; (a) for \(\phi\)9 at CHX and HHX, both at a location 15 mm away from the joint; (b) at CHX only at the different flow direction. Figure 21 illustrates the case of adiabatic conditions and it is clear that the flow around two ends of the heat exchanger assembly is symmetrical, without any noticeable buoyancy driven convective patterns. Figure 22 shows the structure of the vortices when there is a temperature difference imposed: 200 \({}^{\circ}\)C at the hot plate, and 30 \({}^{\circ}\)C at the cold plate. The symmetry of the flow is broken: as the viscosity of the gas increases with temperature, the thicknesses of the viscous penetration depth increases, as seen, for example, in different separations of the vorticity contours on the right and left ends of the channel, which is also congruent with the velocity profiles in Figure 19a. Consequently, the vortex structures rolling up on the two sides of the plate assembly differ in size and strength. However, in addition, the temperature-driven flow at the open area at the left introduces some differences in the vortex patterns between the top and bottom side of the plate--note the hot plume observed in Figures 9 and 13 which adds a \"cross-flow\" (vertically upward) component relative to the hot fluid ejected from the channels. Interestingly Figure 22a, for phase \(\phi\)11, shows the artefacts of a vortex structure on the top of the hot plate, but not on the bottom. These flow features are strengthened in Figure 23, for temperature conditions of 300 \({}^{\circ}\)C at the hot, and 30 \({}^{\circ}\)C at the cold plates. There are two more effects that could be speculated on: First, that the large vortex structures created on the left side of hot plates may provide additional resistance to the flow when it reverses and starts to move from left to right by \"bottlenecking\" the channel. The same effect may be at play on the right side of the CHX when the flow reverses and starts moving to the left, but such flow would only need to handle much smaller vortices. Second, the temperature driven flow in the open area to the left of the hot plates may lead to setting up a variety of convective \"cells\" (loops), which could be contained on one side of the plate assembly, but equally induce a local streaming current between the plates (some of these being also adiabatic plates above and below the heated/cooled channels).