This study investigates the integration of solid oxide fuel cells (SOFC) with proton exchange membrane fuel cells (PEMFC) to improve the system efficiency and minimise exergy losses from unused hydrogen. The paper offers new insights into the efficiency-power density trade-off of SOFC+PEMFC combined systems by simultaneously evaluating the systems’ efficiency trends and their overall volume and mass. The SOFC+PEMFC is thermodynamically analysed and compared for the first time against an SOFC stand-alone system with anode off-gas recirculation (AOGR), another approach to increase efficiency by maximising the direct conversion of fuel into power. Simulations are run to reveal the impact of varying stack operating parameters, fuel utilisation, cell voltage, and operating temperature on the system efficiency, shape of the system’s operational envelope, and overall volume and mass. An exergy analysis identifies major loss sources in the system and proposes pathways for improvement. The results demonstrate that integrating an SOFC with a PEMFC increases system efficiency to 55%, comparable to AOGR, while reducing the total system volume and mass by 20% and 23%, respectively. This study provides new insights into the potential use of SOFCs in volume and mass-limited applications such as long-distance transportation to reduce pollutant emissions.

Low emissions and fuel flexibility are two important criteria required for gas turbine combustors to facilitate the energy transition to low-carbon fuels for propulsion and power applications. A jet-stabilized combustor, having both these characteristics, was operated with CH ₄–H ₂ fuel mixtures with H ₂ varying from 0 to 100 % and with varying equivalence ratios (ϕ). Comprehensive measurements were carried out of the velocity field using Particle Image Velocimetry (PIV), temperature and gas composition by traversing probes in the chamber, and flame topology using chemiluminescence imaging. The flow field in this combustor consists of a jet that undergoes recirculation, generating Central and Peripheral Recirculation Zones (CRZ and PRZ). The recirculation ratio in the PRZ is found to be twice that of the CRZ. Increasing H ₂ % for the same ϕ leads to higher NO _x. Ultra-low ϕ flames could be stabilized only at H ₂≥50 %, which in turn leads to low NO _x due to low adiabatic flame temperatures. The combination of temperature, gas composition (CO/NO), and chemiluminescence images is used to identify the extent and location of the reaction zone. Distributed reaction zones, stabilizing at around 30 % of the length of the chamber, are achieved at lean conditions, whereas an increase in H ₂ % makes the reaction zone more compact and shifts upstream towards the burner head. Flame kernels are extracted from the instantaneous chemiluminescence images, and probability distribution functions for their aspect ratio and axial location are constructed. It is seen that reducing ϕ leads to low aspect ratio kernels that tend to occur further downstream, whereas increasing H ₂ % leads to higher aspect ratio kernels, stabilizing upstream. These flame kernel statistics are also used to identify ignition modes (autoignition/flame propagation) for varying fuel H ₂ % and inlet ϕ based on a hypothesis of flame stabilization mechanisms.

Flameless Combustion is an interesting low NO_x combustion technology for gas turbine engines. In order to design systems for stringent performance standards, it is important to understand emission formation in this regime. To this end, the characteristics of a combustor capable of operating in the Flameless regime are studied. Particle Image Velocimetry and thermocouple measurements were performed to obtain the velocity field and gas temperatures respectively, in the combustor under reacting conditions. Results from experiments were used to generate an "informed" chemical reactor network (CRN) model from which, temperature and species distributions were obtained. As such, this paper presents measured data and a methodology to combine it with CRN modelling to obtain gas composition and temperature. The temperature, NO_x, CO and O₂ mole fractions obtained at three different operating conditions shall be validated with gas composition measurements in the future.