Effect of Unsteady Flow on Intercooler Performance

Two compact intercoolers are designed for the Rotax 914 aircraft engine to increase engine power and avoid engine knock. A study is performed to investigate the effects of unsteady airflow on intercooler performance. Both intercoolers use air-to-liquid cross flow heat exchangers with staggered fins. The intercoolers are first tested by connecting the four air outlets of the intercooler to a common restricted exit creating a constant back pressure which allows for steady airflow. The intercoolers are then tested by connecting the four air outlets to a 2.4 liter, 4 cylinder engine head and varying the engine speed from 6000 to 1200 RPM corresponding to decreasing flow steadiness. The test is performed under average flight conditions with air entering the intercooler at 180°F and about 5 psig. Results from the experiment indicate that airflow unsteadiness has a significant effect on the intercooler's performance. Temperature spread across the intercooler's outlets varies from 30°F to 5°F as airflow unsteadiness increases. The Stagnation pressure drop across the intercooler varies from 0.8 psi to 2.7 psi as airflow unsteadiness increases. The effectiveness of the intercooler without internal baffling is independent of the level of flow steadiness with a value of about 0.55. The effectiveness of the intercooler with an internal baffling system has a value of about 0.53 when disconnected from the engine and about 0.58 when connected to the engine. These results indicate that flow unsteadiness has a strong effect on intercooler performance and should be accounted for when designing and testing an intercooler.