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FP7

ACOC-TH Sintesi della relazione

Project ID: 255881
Finanziato nell'ambito di: FP7-JTI
Paese: Belgium

Periodic Report Summary 2 - ACOC-TH (Acoustic and thermal instrumentation, tests and modelling of engine surface coolers in representative aerodynamic conditions)


Project Context and Objectives:

Summary description of project context and objectives

The recent technological developments in the aeronautical domain and the continuous search for more efficient engine architectures demand a parallel investigation on advanced oil cooling strategies. The usual cold sources like the inlet air stream and the fuel circuit are approaching their limits as new engine designs are exploited. The higher level of complexity on the mechanical systems requires an adequate thermal management of the systems. The heat removal by the aircraft structure will be limited by the use of composite materials with lower operational temperature and thermal conductivity properties. Furthermore, the limitation on the maximum fuel temperature decreases the viability of the fuel tank as a cold source.

The present work is included in the research frame of novel engine cooling strategy. It has the objective to quantify the thermal performance of an Air Cooled Oil Cooler (ACOC) heat exchanger assembled on the inner wall of the secondary duct of a turbofan.

The goal of such design is to use the available surface as a heat exchanger between the air and the oil. In order to increase the thermal performance, the wet area is increased by adding longitudinal fins, reaching the required heat dissipation power. Such a design implies a strong compromise between the aerodynamic penalties, (introduced by the increased drag) and the thermal performance of the heat exchanger. The developed research presents an innovative aero-thermal study by testing the new heat exchanger concept in a 3D shaped transonic wind tunnel capable of reproducing the flow condition within the bypass of an engine. Innovative data processing approaches, based on inverse heat conduction methods (IHCM), were developed and employed during the course of this work.

Project Results:

Work performed during the second reporting period of the project

During the second reporting period (01/07/2011 to 28/02/2013) of the project the

following work was performed:

• The experiments on the transonic stream tube wind tunnel facility were performed

for various flow conditions over the ACOC. A through thermal characterization of the ACOC was performed with detailed thermocouple measurements.

• Numerical simulations of ACOC were performed in order to have a detailed insight of the heat transfer phenomena observed over the heat exchangers.

• A novel inverse heat conduction model (IHCM) has been developed and validated. The model was then used for the advanced heat transfer analysis of the ACOC.

• Thermal performance of the oil circulation underneath ACOC was investigated for various configurations.

Potential Impact:

Thermal performance of the ACOC (air side)

The developed research presents an innovative aero-thermal study by testing the new heat exchanger concept in a 3D shaped transonic wind tunnel capable of reproducing the flow condition within the bypass of an engine. Innovative data processing approaches, based on inverse heat conduction methods (IHCM), were developed and employed during the course of this work.

The wind tunnel, specifically designed for the aerodynamic research, was able to replicate the air flow conditions on the by-pass flow of a turbofan. For the thermal research the test section walls were adapted to support direct IR optical access to the model. However due to the complexity of the geometry, full optical access on one complete fin was not possible. Therefore the 3D heat transfer process over one full fin was retrieved using the developed IHCM. The test was performed on a model with continuous fins along the axial direction of the engine. The experiments were started by imposing an initial wall temperature (Twi) to maximize the temperature difference between the wall and the flow temperature (Tf ) resulting in higher heat flux values. After thermal steady state was reached (1) the blow down was started. When steady flow conditions were attained the heaters were turned off (2). The wall temperature was monitored during the entire process by the infrared camera. Based on a linear fitting between the measured wall temperature and the computed heat flux, the adiabatic condition was obtained by extrapolating the fitted curve to null heat flux. The correspondent temperature is the adiabatic wall temperature (Tad) while the slope represents the adiabatic heat transfer coefficient (had).

The adiabatic heat transfer coefficient presents a maximum at the leading edge, followed by an exponential decrease along the axial direction (for X>50 the results were neglected due to the high uncertainty at these locations), as observed frim the results obtained for the lateral wall of the fin. Such evolution in the streamwise direction is in accordance with the one expected for a turbulent boundary layer. The values of the adiabatic wall temperature proved to be similar to the total inlet flow temperature. The estimated values of had and Tad on the bottom surface revealed to be higher when compared to the lateral wall of the fin. The values of the had in the bottom surface decreased in fins flow passage.

The developed data processing technique allowed the determination of the adiabatic heat transfer coefficient and the respective adiabatic wall temperature along the walls of the heat exchanger. An estimation of the heat dissipation capacity of the studied surface cooler is now possible by imposing the expected boundary condition, for the air flow and the oil, at engine operating conditions.

Thermal performance of the ACOC (oil side)

Using different SACOC geometries on the air side, different breadboards representative of the oil-side of the SACOC have been tested by ULB. In total 6 different oil-side configurations have been tested. The baseline prototype was defined as using rectangular straight fins separated by 1 mm. One parameter to explore has been the fin spacing. Therefore, a breadboard has its oil side manufactured with fins separated by 2 mm. Two breadboards are made with fins following a zigzag structure (“offset” configuration), therefore increasing the heat exchange surface. They differ by having their main network parameter, the lance length.

Finally, two breadboards consist of a waved fine plate with or without brazing junctions, with the aim to compare their respective heat exchange characteristics.

Interesting and useful results on the oil side have been obtained after the ACOC-TH test campaign. Because of a high dissipation of oil in the pipes connecting the oil and the air circuits (more than 10 m in total, mainly for safety and human health reasons) and the ambient cold air in the test bench room, the maximum oil temperature that could be reached at the inlet of the breadboards is limited to 130°C in order to preserve the integrity of the test facility. The heat dissipated by the heat exchanger is measured the most accurately on the oil side, because of the lack of adequate instruments to measure the heat exchanger fin temperature during the tests. The global trend is the dissipated heat increases with the cooling air velocity.

Another thermal vs. pressure drop comparison for different oil-side breadboards has been made with the air velocity limited to 120 m/s. To observe the highest pressure drop differences between the different breadboards, the comparison was performed for an oil flow of 1000 l/h.

As for the air side, one can observe a high correlation between the dissipated heat and the pressure drop. The comparison between the waved metal sheet configurations with and without brazed junctions shows that the brazing junctions improve the thermal performance, while slightly diminishing the pressure drop, naturally thanks to the connection made between the waved sheet and the air fins. The breadboard with the 2 mm distance between fins shows naturally a lower pressure drop with a lower dissipated power, because of the lower surface exchange compared to the baseline configuration. On the contrary, both offset matrix breadboards show tremendous thermal performance, but along with high pressure drops. Nevertheless, the offset matrix n°2, with the highest lance length exchanges even more heat with pretty much the same pressure drop.

Purely based on the heat transfer point of view, the offset matrix configuration for the oil-side of the (S)ACOC seems to be the most efficient technology to be chosen (together with the straight fins on the air side). However, a high pressure drop must be taken into account as those heat exchangers are placed before scavenge pumps in the oil lubrication circuit.

The risk with too low oil pressure is to observe cavitation in the oil pump, which could be very harmful for the pumps and the whole circuit or sensors. Also, their complex form implies higher production costs, in time and money.

A good alternative would be to stick with simple straight fins as well on the oil side. The distance between fins would be then subjected to a compromise as a shorter distance increases the pressure drop as well as the thermal performance. Unfortunately, there were not enough breadboards and not enough budget to quantify this influence on the ACOC performance even if an extrapolation of the tests results has been done.

With this ACOC-TH test rig, very useful results have been obtained for the oil side of the (S)ACOC. Test results show that the offset matrix configuration has clearly the best heat transfer performance, but also the worst pressure drop. An alternative would be to use straight fins, but with a careful choice of the distance between fins to make a compromise between pressure drop and heat transfer performance.

These results also provide data to tune the CFD and CHT design tools, developed by lubrication system partners as Cenaero, Techspace Aero, Nottingham University or others.

Highlights

Detailed characterization of the ACOC both on air and oil side was performed through experimental and numerical analysis.


Reported by

VON KARMAN INSTITUTE FOR FLUID DYNAMICS
Belgium
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