Effects of more electric systems on fuel tank thermal behaviour

With the advent of more electric airframe systems and ultra-high bypass ratio turbofan engines, there is growing interest in the associated thermal implications. In this research project, an aircraft level model that is appropriate to enable investigations into novel thermal management solution on future aircraft is developed. In this paper, an investigation into the effects of more electric systems on the thermal behaviour of fuel tanks in civil transport aircraft is presented. Specifically, the influence of the heat generated by conventional and more electric systems on the fuel tank was modelled and simulated. A fuel thermal model was developed, which consists of a tank geometry representation, coupled to a module that calculates remaining mission fuel mass. The systems architectures are represented by connected thermal component models. Standard approaches were then employed to estimate convection and conduction heat transfer coefficients at the tank interfaces. The model solves 1-D transient heat equations, coupling heat transfer and material heat capacity via heat flux balances. The thermal and systems models were integrated into a baseline aircraft performance model, which was used to dynamically simulate the tank thermal behaviour during representative missions. The initial results indicate that switching to more electric environmental control and iceprotection systems likely have negligible thermal impact on the bulk fuel temperature. However, some benefits may be obtained regarding safety and certification, but this requires further study. Introduction Thermal management in aircraft is becoming an increasingly important topic. There are several reasons for this, many relating to new and higher localised heat loads associated with advances in propulsion and the electrification of subsystems (i.e. the rise of the ‘more electric aircraft’). Simultaneously, there is also a general decline in the availability of heat sinks. For example, the increase in the use of composite airframe skins, which have higher thermal resistance than Aluminium alloys, weakens the prospects of using the external atmospheric air for cooling purposes. Furthermore, additional increases in engine bypass ratios, (i.e. higher ratios of air mass flow through the bypass duct to engine core mass flow), are likely to further exacerbate this heat management problem. This is because increasing the bypass ratio leads to a reduction in engine core compartment ventilation (due to lower fan pressure ratios); additional heat sources arising from, for example, the incorporation of a fan power gearbox; as well as the need for thinner, “slimline” nacelles, which leave less room for heat dissipating equipment, such as heat exchangers, amongst many others. The purpose of the work presented in this paper was to investigate the thermal effects of more electric systems on one of the most important heat sinks on civil transport aircraft – the fuel. This was necessary to understand how the ability of the fuel to act as a heat sink, especially in cooling down hot engine oil associated with higher bypass ratio engines, would be influenced. The work forms part of a larger multidisciplinary effort, the ‘UHBR Thermals’ Programme (Ultra-High Bypass Ratio). This Innovate UK and Meggitt PLC funded research programme investigates engine and airframe heat exchange concepts for future UHBR geared turbofan engines and quantifies their benefit on the system and aircraft level. In particular, it was hypothesised that the electrification of the environmental control system (ECS) and ice protection system (IPS) would lead to reduced thermal loads on the fuel tank. This may enable the fuel to accept more heat from the engine, by means of several approaches, such as recirculation. It was therefore required to model the thermal behaviour of the fuel tank, the ECS and IPS (in both their conventional and more electric forms) in an integrated manner, along with the overall performance of the aircraft on representative missions. This paper reports on the modelling and simulation approach, the results obtained, and the possible implications of these results.