# The Water-Enhanced Turbofan as Enabler for Climate-Neutral Aviation
## 3 The Water-Enhanced Turbofan  
The Water-Enhanced Turbofan (WET) concept is a dual-fluid, highly efficient, highly specific power gas turbine recovering exhaust heat. Unlike conventional heat-recuperating aero-engine concepts, the WET concept transfers significant parts of its exhaust heat back into a cycle-integrated, quasi-closed water circuit during cruise. The water humidifies the primary working fluid and allows the achievement of unmatched specific power featuring low turbomachinery weights and a significant reduction in emissions. To provide an initial introduction, the illustration shown in Figure 1 represents the basic WET concept architecture. Pressurized water is injected as superheated steam into the combustion process. The resulting heat capacity and mass flow rate of the humidified working gas increases significantly and allows the turbine to extract more specific work. The resulting increase is leveraged by the incompressibility of liquid water, so that pumping to a given pressure costs much less power than compressing an equal mass of gas. Thus, the engine-internal power demand to provide a mixed working fluid at high pressure decreases considerably. Compared to a conventional aero engine of the same technology and thrust requirements, the core size of the WET concept is less than half the size, allowing for substantially smaller and lighter turbo components. The WET concept uniquely features operations such as a closed circuit with water, enabled through its ability to recover water from the exhaust gas. The water is partially injected and partially produced during combustion of kerosene. By evaporating the recovered and pressurized liquid water in the vaporizer, the WET concept not only returns a large amount of exhaust heat back into its thermodynamic cycle, but also, above all, reduces the temperature of the humidified exhaust gas. To lower the temperature of the gas below its dew point and, thus, cause the water to condense, a second, colder heat exchanger is necessary. Similar to the vaporizer, the extracted heat from the condenser is also transferred back into the thermodynamic cycle. Figure 1: Basic schematic of the WET concept in a half-side arrangement supplemented with international engine station nomenclature [9]. The WET concept has evolved over multiple phases since the original publication [9] and first patents [20; 21; 22], as shown in Figure 2. During the first stage, the integration was improved: All WET components were integrated into the nacelle and pylon control volume. The condenser was placed into the bypass flow, which was used as the heat sink. From a practical standpoint, this reduces the interdependence between engine and aircraft design. In comparison, previous WET designs utilized the wing root for condensation and would have complicated aircraft integration. The added components in the nacelle increase the weight and volume of the nacelle. The added weight reduces the wing bending moment (wing bending relief). This concept served as the baseline for the following studies. In a second stage, measures for cycle improvement were investigated. The focus was placed on optimizing the usage of water and steam within the cycle and maximizing the heat recovery. Splitting the last turbine stage to increase the vaporizer temperature level was investigated, as well as splitting the vaporizer to optimize the temperature difference between the exhaust side and the water side. Moreover, sequential super-heating of the steam with an auxiliary combustor was investigated to maximize the steam work potential. In the compressor, intercooling via either an intercooler using water or direct water or steam injection was investigated. In a third stage, the concept was simplified based on the findings from the second stage to derive a concept with minimum viable complexity, which retains as much as possible of the thermodynamic benefit. The resulting concept is schematically shown in Figure 3. The configuration features a vaporizer to recover waste heat after the LPT.