This paper presents the design and evaluation of a thermal nozzle for small-scale turbojets. Historically, small-scale turbojets have suffered from poor thermal efficiency and thrust specific fuel consumption (TSFC). This is primarily because their small geometry limits the compressor pressure ratio from reaching higher values, which directly impacts TSFC. An alternative approach to improving TSFC is to preheat the fuel before it enters the combustion chamber. This will add enthalpy to the flow, thus reducing the amount of fuel required to the reach the same turbine inlet temperature. The thermal nozzle presented in this study achieves this by acting as a heat exchanger between the hot exhaust gas and the fuel before it enters the engine.

Computational fluid dynamics (CFD) and preliminary experiments were used to guide the design of the nozzle, where key considerations included pressure drop, fuel temperature limits, and manufacturability. Parameters such as channel geometry and thickness were varied and analyzed to arrive at a final design that met the design goals. A 70 lbf KingTech K320 turbojet was chosen to evaluate the performance of the thermal nozzle. Analytical evaluation was performed using an ε-NTU heat exchanger model calibrated from CFD results, parametric cycle analysis, and engine performance analysis. Analysis shows a TSFC improvement of 2.07% at full throttle. However, the heat that is recuperated in the nozzle slightly reduces the thrust output of the engine, where analysis for specific thrust shows a decrease of 0.48% at full throttle.