THE STRESS ANALYSIS OF A BUFFER AIR HEAT EXCHANGER YONGSHENG GE a, IGOR DOKLESTIC b & STEVE HUGHES a a Serck Aviation, Oscar House, Wharfdale Road, Tyseley, Birmingham, B11 2DG b Stirling Dynamics Ltd, 2 Tyndall’s Park Road, Clifton, Bristol, BS8 1PG

2 Outlines Introduction to Serck Aviation products Certification requirements for the new product FE modelling by Stirling Dynamics Ltd The validation of the FE model The Fatigue Life Prediction Conclusions

3 Traditional heat exchangers: Introduction

4 Newly developed heat exchangers: Introduction

5 Newly developed heat exchangers: Introduction Hot air inletHot air outlet Cold air inlet Cold air outlet

6 Newly developed heat exchangers: Introduction baffles Tubes in matrix

7 Flight Certification Requirement The flight certification requirement for this new product would have been meet by Pressure, Temperature and Flow (PTF) testing at a representative flight cycle for 80,000 cycles. The flight certification requirement for this new product would have been meet by Pressure, Temperature and Flow (PTF) testing at a representative flight cycle for 80,000 cycles. The purpose of the PTF test is to demonstrate the ability of the design and construction to meet the fatigue life requirements. An alternative to the full scale PTF test was agreed that a fully calibrated FEA model can be used for the fatigue life predictions. The purpose of the PTF test is to demonstrate the ability of the design and construction to meet the fatigue life requirements. An alternative to the full scale PTF test was agreed that a fully calibrated FEA model can be used for the fatigue life predictions. With the flight cycle compressed into 600 seconds, the test duration would have been approximately two years, which was beyond the service introduction date and would have been very expensive. With the flight cycle compressed into 600 seconds, the test duration would have been approximately two years, which was beyond the service introduction date and would have been very expensive.

8 The Finite Element Model Tube plate Baffles

9 The Finite Element Model It was assumed that geometry and loading of the heat exchanger are symmetrical; therefore, only one half of the cooler was modelled. Basic assumptions: Tubes and areas where the tubes are brazed into the tube sheet were not modelled. The tube matrix (tube plate) was modelled as a continuous plate. For this area effective material properties were defined to account for the reduction in conductivity, density and Young’s modulus. Calculation of the stress concentration around the tube holes was not part of this analysis. Baffles were additionally modelled to stiffer the casing.

10 The Finite Element Model Loading conditions: Thermal Thermal transient loading was applied to the surfaces of the model via a film heat transfer coefficient and fluid temperatures. This loading was implemented with the user subroutines FILM and DFLUX, specified in the ABAQUS input file. Formulas and constants for the heat transfer coefficients and temperatures were supplied by SERCK Aviation. The FILM subroutine was applied to all surfaces except the top of the tube plate for which the equivalent material properties were used. The DFLUX subroutine was applied to the volume of elements in the tube plate area with equivalent material properties.

11 The Finite Element Model Loading conditions: Pressure Pressure transient loading was applied to the internal surfaces of the model via the DLOAD subroutine specified in the ABAQUS input file. Pressure distribution was defined by SERCK Aviation.

12 The Validation of the FE Model Temperature data-match at steady-state points Buffer air in temperature (Deg F) Buffer air flow (pps) Buffer air in pressure (Psi) Coolant air in temperature (Deg F) Coolant air flow (pps) Coolant air in pressure (Psi) TCnodeMetal temp. deg.FFE prediction Deg FDiff in Deg FDifference by %

13 The Validation of the FE Model Temperature data-match of a simple transient cycle Constant coolant air inlet temperature of 100 deg.F & Wc=0.52pps with 32psia inlet Time (seconds) 294 sec pressure dwell, Wh=0.32pps max Wh=0.09pps min 292 second temperature dwell Buffer Pressure (psi) Buffer Temperature (deg.F)

14 The Validation of the FE Model Temperature data-match of a simple transient cycle Thermal couplesTest dataFE resultsDifference in Deg Fdifference in % TC % TC % TC % TC % TC % TC % TC % TC % TC % TC %

15 The Validation of the FE Model Temperature data-match of a simple transient cycle

16 The Validation of the FE Model Strain data-match of a simple transient cycle

17 The Fatigue Life Prediction Fatigue life prediction using FE Safe, based on the results from the validated FE model

18 Good temperature data match has been obtained for both steady-state and transient conditions. Reasonably good strain match is observed to give further confidence in the FE model. Good temperature data match has been obtained for both steady-state and transient conditions. Reasonably good strain match is observed to give further confidence in the FE model. Conclusions The model is recognised by Serck Aviation’s customer for further fatigue life prediction. It is time and cost efficient to certificate the product by using FE model than by carrying out full test. The model is recognised by Serck Aviation’s customer for further fatigue life prediction. It is time and cost efficient to certificate the product by using FE model than by carrying out full test.