THE STRESS ANALYSIS OF A BUFFER AIR HEAT EXCHANGER

 

YONGSHENG GEa, IGOR DOKLESTICb & STEVE HUGHESa

 

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:Traditional heat exchangers: Introduction

4

Newly developed heat exchangers:Newly developed heat exchangers: Introduction

5

Newly developed heat exchangers:Newly developed heat exchangers: Introduction

Hot air inlet Hot air outlet

Cold air inlet

Cold air outlet

6

Newly developed heat exchangers:Newly developed heat exchangers: Introduction

baffles

Tubes in matrix

7

Flight Certification Requirement

• The flight certification requirement for this new product would The flight certification requirement for this new product would have been meet by Pressure, Temperature and Flow (PTF) testing have been meet by Pressure, Temperature and Flow (PTF) testing at a representative flight cycle for 80,000 cycles. at a representative flight cycle for 80,000 cycles.

• The purpose of the PTF test is to demonstrate the ability of the The purpose of the PTF test is to demonstrate the ability of the design and construction to meet the fatigue life requirements. An design and construction to meet the fatigue life requirements. An alternative to the full scale PTF test was agreed that a fully alternative to the full scale PTF test was agreed that a fully calibrated FEA model can be used for the fatigue life predictions.calibrated FEA model can be used for the fatigue life predictions.

• With the flight cycle compressed into 600 seconds, the test With the flight cycle compressed into 600 seconds, the test duration would have been approximately two years, which was duration would have been approximately two years, which was beyond the service introduction date and would have been very beyond the service introduction date and would have been very expensive.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)

596.86 0.318 228.29 130.01 0.515 32.08

TC node Metal temp. deg.F FE prediction Deg F Diff in Deg F Difference by %

2 20576 284 275 9 3.2

3 28815 330.8 323.6 7.2 2.2

4 26187 480.2 485.6 -5.4 -1.1

5 20096 233.6 239 -5.4 -2.3

6 28713 314.6 305.6 9 2.9

7 13961 581 590 -9 -1.5

8 29055 174.2 172.4 1.8 1.0

9 18213 392 375.8 16.2 4.1

11 23143 406.4 411.8 -5.4 -1.3

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.

0 2 6 8 300 315 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)

200

600

50

225

14

The Validation of the FE Model

Temperature data-match of a simple transient cycle

Thermal couples Test data FE results Difference in Deg F difference in %

TC2 285 273 12 4.2%

TC3 322 312 10 3.1%

TC4 471 482 -11 -2.3%

TC5 233 238 -5 -2.1%

TC6 309 299 10 3.2%

TC7 560 583 -23 -4.1%

TC8 166 160 6 3.6%

TC9 389 369 20 5.1%

TC11 389 410 -21 -5.4%

TC13 574 565 9 -1.6% 

15

The Validation of the FE Model

Temperature data-match of a simple transient cycle

0.00

100.00

200.00

300.00

400.00

500.00

600.00

700.00

0.0 50.0 100.0 150.0 200.0 250.0 300.0 350.0 400.0 450.0 500.0

Time [s]

Te

mp

era

ture

[d

eg

F]

TC13_test

TC13_FEA

16

The Validation of the FE Model

Strain data-match of a simple transient cycle

-1.50E-03

-1.00E-03

-5.00E-04

0.00E+00

5.00E-04

1.00E-03

1.50E-03

2.00E-03

0 50 100 150 200 250 300 350 400 450 500

Time [s]

Str

ain

FE predictions on "D"

test data on "D"

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 Good temperature data match has been obtained for both steady-state and transient conditions. Reasonably both steady-state and transient conditions. Reasonably good strain match is observed to give further confidence good strain match is observed to give further confidence

in the FE model.in the FE model.

Conclusions

• The model is recognised by Serck Aviation’s The model is recognised by Serck Aviation’s customer for further fatigue life prediction. It is time customer for further fatigue life prediction. It is time and cost efficient to certificate the product by using and cost efficient to certificate the product by using FE model than by carrying out full test.FE model than by carrying out full test.