# Performance of a Supercritical CO2 Bottoming Cycle for Aero Applications
## 3 Results
### Main Heat Exchanger  
The three most influential parameters on the performance of the MXH are the hot-side inlet Mach number, the number of columns of tubes per pass and the length of the tube major axis. Note that the tube minor axis changes pro rata with the major axis, as the aspect ratio of the tubes is kept constant. The plots in Figure 6 illustrate the studies of those parameters. The investigation of the hot-side inlet Mach numbers emphasizes, that the band of feasible values is relatively small. Although reducing Mach numbers results in higher effectiveness values for the MHX, it results in an exponential increase in weight. Reducing the inlet Mach number from 0.1 to 0.05 only increases the temperature effectiveness by about 1.2 percentage points, which does not justify almost doubling the weight. At the other end of the scale, designs featuring higher inlet Mach numbers are not only penalised through lower effectiveness values, they also have very high hot-side pressure losses. That drop in hot-side pressure ratio can greatly diminish the fuel burn benefits of the RBC, even though the RBC enables smaller core engine mass flows. The observed trends led to a narrowing of the investigated Mach numbers to between 0.1 and 0.2. The cold-side pressure losses are relatively stable at less than 2%, but increase slightly with lower hot-side inlet Mach numbers, due to the longer heat exchanger tubes.  
Increasing the number of columns per pass clearly increases the effectiveness and temperature rise of the MHX, as is visible in Figure 6b. It is therefore likely that the most fuel efficient combined cycle engine will feature four or more columns per pass in the MHX. However, component weight and hot-side pressure losses rise simultaneously, and the asymptotic nature of the effectiveness implies that having more than 10 columns may not be feasible. The S-CO\({}_{2}\) pressure ratio is relatively stable across the whole range of simulations, but the higher flow Mach numbers, which arise from smaller tube counts in the MHX, do lead to slightly higher pressure losses for those designs.  
Figure 6: _Cont._The length of the tube's major axis appears to be less influential than initially assumed (see Figure 6c). Most performance parameters of the MHX appear almost constant, with the exception of component weight. The scatter is due to the MHX component code, which rounds the number of rows of tubes in each module to an integer and recalculates the inner annulus diameter. The range of tube major axis lengths was reduced to between 5 and 13 mm to minimise weight.  
The thermal boundary conditions applied during the full design-space study of the MHX and the parameter values that were identified as the most fuel burn efficient are listed in Table 7. The fuel-burn value indicates that the design does not improve the overall engine performance. The main reason is that the initially assumed values for the pre-cooler were too aggressive, especially the air-side inlet Mach number of 0.15. The pre-cooler's poor performance therefore considerably penalises the combined cycle's performance.