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An Empirical Methodology for the Prediction of the Boiling Limits of EGR Coolers
Abstract:
Recent emissions standards have become more restrictive in terms of CO2 and NOx reduction. This has been translated into higher EGR rates at higher exhaust gas temperatures with lower coolant flow rates for much longer lifetimes. In consequence, thermal load for EGR coolers has been increasing and the interaction of boiling with thermal fatigue is now a critical issue during development.
It is almost impossible to avoid localized boiling inside an EGR cooler and, in fact, it would not be strictly necessary when it is below the Critical Heat Flux (CHF). However when CHF is exceeded, film boiling occurs leading to the sudden drop of the heat transfer rate and metal temperature rise. In consequence, thermal stress increases even when film boiling is reached only in a small area inside the part. It is very difficult to accurately predict under which conditions CHF is reached and to establish the margins to avoid it. In this work, a new empirical method has been developed that covers an innovative prototype instrumentation with thermocouples that allows the direct measurement of the metal temperature in any location inside the prototype; the boiling test methodology and the postprocess to check when CHF is reached.
An analytical correlation of a boiling parameter called Cooler Boiling Ratio enables the prediction of safety margin against CHF for any working condition. Several tests were performed to check the robustness of the method with successful results so this research represents an important progress in the empirical approach for boiling analysis in EGR coolers.