# Thermodynamic Analysis of Partitioned Combined Cycle using Simple Gases
## 4 Result and Discussion  
### Temperature and Viscosity Profiles  
Figure 6 presents the temperature profiles of the main and partitioned fluid in the combustion chamber. The distance between the green (maximum temperature gradient) and red (temperature gradient at highest main fluid temperature) vertical lines is a function of heat transfer and flame length. The higher the distance the better the heat transfer. As expected, the behavior of nitrogen was not very different from air (nitrogen is a main constituent of air). However, the highest temperature tended to increase. There were two main factors that controlled this temperature: fluid conductivity and the amount of fuel burned. In the case of nitrogen, the difference was because of the conductivity. Carbon dioxide showed the best heat transfer, despite the lower maximum temperature gradient. Another fact that favors carbon dioxide is its lower entering temperature because of its better compression efficiency. This is due to its higher molecular weight and density. Hydrogen showed a similar temperature profile but its pressure drop was lowest among the gases studied (Figure 7, provides the viscosity profiles). Furthermore, hydrogen has previously used as heat transfer medium in electrical generators but was gradually replaced by helium due to safety concerns. The behavior of helium is expected to be not much different because of almost same viscosities as those of hydrogen under the studied conditions. On the other hand, argon not only provided a huge pressure drop but also showed poor heat transfer properties. The pinch temperature remained 42 \({}^{\circ}\)C at 7 m of the exchanger length, the highest among other fluids. Flame length was a direct measure of the load being shared between the main and partitioned cycle, as more load on the main cycle injects more fuel and thus a higher flame length. The heat duties of the combustor for hydrogen, air, and nitrogen at different adiabatic efficiencies were comparable. As the adiabatic efficiency of the cycle components increased, it decreased the irreversible losses and thus the heat input. A comparison between the heat duties of carbon dioxide and argon provides a noteworthy observation. Argon duty was lower than carbon dioxide duty at higher adiabatic efficiencies but the reverse is true at lower adiabatic efficiencies. This is linked to the optimization algorithm based on Equation (2). At high adiabatic efficiencies, the argon Brayton cycle became more effective and took higher loads than carbon dioxide, while its pressure drop remained the same. The entropy loss because of carbon dioxide thus became higher than the combined losses (entropy and pressure drop) offered by the argon. This resulted in a better overall performance by argon at higher components of adiabatic efficiencies. The average temperatures after the combustor were 1429 \({}^{\circ}\)C, 1428 \({}^{\circ}\)C, 1429 \({}^{\circ}\)C, 1393 \({}^{\circ}\)C, and 1433 \({}^{\circ}\)C for nitrogen, hydrogen, air, argon, and carbon dioxide, respectively.  
Figure 6: Temperature profiles in the combustor/heat exchanger at a mean adiabatic efficiency of 92.7%. Solid and dotted lines represent the main and partitioned fluid temperatures as a function of length, respectively, with a maximum error \(\pm 1\)\({}^{\circ}\)C. Green and red lines represent the highest and maximum temperature gradient, respectively. Inset tables present the heat exchanged in the combustor; green, blue, and red represent the adiabatic efficiencies of 95.63%, 92.7%, and 89.9%, respectively.