PW1200G: LPC Clearance Check

Engine efficiency is a big deal in the commercial airline business.  Less fuel burn means more money in the airlines pocket. Which, ideally would mean lower airfare, but who am I kidding? It’s also important to make sure parts won’t smash into each other when you turn the engine on.

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A typical turbine setup.  The red line shows blade tip clearance, which is usually only a few thousandths of an inch.

A jet engine is essentially a series of ducted fans, and a ducted fan’s efficiency increases drastically as the gap between the fan tip and the shroud decreases.  So, part of my job on this project was to provide blade tip deflections to our performance department so that they could measure the gap.

Ducted fans being put to good use.

Doing the Math

Using Finite Element Analysis, we are able to mathematically calculate just how much the blade tips of our “fan” deflect.  An FEA model built to Pratt & Whitney standards is accurate enough to determine these deflections to within a few thousandths of an inch.  For this particular assessment, a 3D sector model of the compressor blisk was necessary to fully capture the deflection of the blade tip.  There are a handful of missions that the performance department are interested in, so I ran the model through those.  Once I had results back, all I had to do was tabulate the deflection values over the course of the mission and violà, we can verify our design will meet performance metrics.

Processing the data

Tabulating the deflection data would’ve been tedious if I hadn’t written a script to do so.  There were 3 separate models to run through 5 combinations of mission/boundary conditions, each with over 40 mission time points to collect data from.  So, using ANSYS Parametric Design Language, I was able to have the computer do it for me.  The script simply looped through the mission time points and queried the deflection values at the blade tips.  From there, it nice and neatly formats the data in a text file readable by the Performance Department’s software.

I sent those text files over the fence to the performance folks.  They were able to do their thing, the clearances met requirements, smiles all around.  Maybe some of that fuel cost savings will make its way to people buying airfare some day.

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