Physics-Based Thermal Model for Power Gearboxes in Geared Turbofan Engines

Abstract

Ultra-High Bypass Ratio Geared (UHBRG) turbofan technology allows a significant reduction in fuel burn, noise and emissions — key metrics for aircraft engine performance. However, one of the main challenges in this technology is the large amount of waste heat generated by the Power Gearbox (PGB). Therefore, having a practical tool for precise prediction of the PGB-generated thermal loads in UHBRGs is becoming a necessity. Such a tool would assist in analyzing engine performance, as well as ensuring that engine physical limitations/restrictions are not breached (e.g. over-temperature in fuel and oil, cocking, etc.). This paper presents a methodological approach to mathematically model the waste heat generated by a PGB on a UHBRG for different points on a typical flight profile. To do this, the total power loss in a PGB system is firstly defined as the summation of load-dependent and load-independent losses. Physics-based equations for each heat loss mechanism are introduced and, through a combination of the associated equations, a simulation model for the thermal loads calculation in PGBs is developed. In addition, the heat losses and efficiency of the PGB has been analyzed across a simulated flight. The developed PGB model calculates the main power losses generated in a gear reduction system of a turbofan engine. It is found that in a typical flight, the heat loss generated by the PGB can reach about 80% of the total waste heat generated by the engine. The values of the mechanical efficiency calculated by the tool at different flight points are above 97% which is in good agreement with publicly available data for planetary gearboxes. This tool is intended to be utilized by engine thermal management system designers to predict and analyze the heat loads generated by the PGB at different flight conditions.

Heat Load Development and Heat Map Sensitivity Analysis for Civil Aero-Engines

“…The results of the physics-based model developed for the TMS system of this engine show that the power gearbox is the main source of generated heat loads in geared turbofan engines. Although the efficiency of planetary gearboxes used in this type of engine is normally very high (above 97%), the heat load generated in this component is still very high due to the huge value of the transferred power by this component [6].  For the third case study, one of the versions of the UltraFan engine (an Ultra High Bypass Turbofan with a take-off thrust of 280kN) has been simulated.…”

“…In comparison with other heatproducing mechanisms, the latter is usually ignored since its contribution is very small. Fluiddynamic effects, bearing viscous losses, and trapped fluid between gear teeth all contribute to these losses [6]. In summary, the total power loss can be the sum of non-load dependent and loaddependent losses.…”

“…The procedure of design and simulation of an aero-engine thermal management system could be started with physics-based modelling of engine heat sources in which a mathematical thermal model for engine bearings, gearboxes, pumps, etc., will be developed to calculate the values of the thermal loads generated in each source in various operating conditions. There is an in-house developed software at Cranfield University for this purpose [6][7][8], and a brief description of the models, how they can be developed and validated, and the related pseudo codes and equations are provided in the below section.…”

Aircraft thermal management: Practices, technology, system architectures, future challenges, and opportunities

“…However, with the advent of large power gearboxes to reduce fan speed on geared turbofans and the even larger gearboxes destined for the approaching adoption of ultra-high bypass geared turbofans (UHBR GTF), it is projected the waste heat produced would be substantial and would require intensive thermal management. Jafari et al [47] presented a method for estimating the heat losses produced by planetary gearboxes for UHBR GTF engines. Using lubrication oil for cooling, as described previously under heat soakage, is the most common way of removing waste heat from mechanical transmission equipment.…”

“…Figure 18: UHBR GTF engine (with 110 kN Sea-Level Static Thrust) waste heat transferred to oil [47]. Note that AGB = accessory gearbox, PGB = power gearbox.…”