## 4 Conclusions  
The two-dimensional ANSYS FLUENT CFD model has been set up to investigate the oscillatory flow across the parallel-plate heat exchanger at the low drive ratio of 0.3%. The flow, as represented by the velocity profiles, is well predicted by the model. For example, the velocity amplitudes at the centre of the heat exchanger channel have a maximum discrepancy of 1.8% compared to the experimental  
Figure 31: (**a**) The effect of the device’s orientation on viscous dissipation and (**b**) the enlarged view for viscous dissipation at the open area next to the heat exchangers. Figure 30: (**a**) The effect of the heat exchanger wall temperature on viscous dissipation and (**b**) the enlarged view for dissipations at the open area next to the heat exchangers. data. On the other hand, the numerically calculated temperatures have the maximum discrepancies of 6% compared to the experimental values. The differences noted in the temperature profiles and the resulting heat fluxes are likely to be the result of the combination of factors related to experimental uncertainties and the inability of the model to capture all the complexities of the real experimental setup. Overall, the current model under-predicts the space-time-averaged wall heat transfer by up to 30% in comparison to the experimental values. The investigation of the effect of initial temperature on the flow and heat transfer helps explain the selection of an appropriate initial temperature that best suits the model. The current approaches of prescribing the initial temperature fields showed that the initialisation procedure influences both the magnitude of heat flux and velocity profiles within the structures. The investigation suggests that the current model is best initialised at 300 K. The model initialised with a temperature field defined more closely to the experimental conditions resulted in a change of the flow character, incongruent with the real flow behaviour. It can be speculated that the reason for this is the type of boundary conditions imposed on the model, which may not fully represent the reality of the experiment with its long time-history (hours), far beyond the capabilities of the current CFD model. The imposed temperature boundary conditions (on the heat exchanger plates) is shown to cause asymmetry to the flow seemingly related to the nonlinearity caused by the combined effects of the temperature-dependent density and viscosity and the temperature-driven flow (buoyancy). The change in fluid density with respect to the temperature results in lower velocity amplitudes during the first part of the cycle and relatively higher magnitudes of velocity for the second part of the cycle. Here, an \"asymmetrical boundary\" creates asymmetries in the temperature and velocity fields. In addition, the temperature-driven buoyancy flow creates further \"distortions\" to the already asymmetrical features of the velocity and temperature fields. The results indicate that there are \"net flows\" through channels (or in other words \"streaming\"). These effects are well illustrated through changing the orientation of the device relative to the gravity field. Temperature-dependent gravity effects influence the magnitudes of velocity profiles and temperature profiles between the plates depending on the direction of the flow and the locations of the CHX/HHX. Consequently, the heat fluxes also change. The presence of the imposed temperature field is shown to influence the velocity profile and gas displacement across the plates, hence breaking the symmetry of the flow. The vortex structures at the end of the plates, and the shear layer within the area bounded by the plates, change with temperature. Device orientation influences the flow and heat transfer due to temperature-driven buoyancy effects. The viscous dissipation is also affected.