Input impedance measurement is commonly used in the design of internal combustion engines to improve their performance. The water-cooled charge-air cooler used in supercharged engines imposes a strong longitudinal thermal gradient on the air contained in its narrow channels. To isolate the resonances caused by the thermal gradient, the input impedance of a semi-infinite narrow pipe with multiple longitudinal temperature profiles is studied experimentally using the Two-Measurement Three-Calibration method. Three known non-resonant loads are used to calibrate the test bench. The importance of using a semi-infinite pipe as a calibration load in narrow pipes is further demonstrated. In the case of this paper, the acoustic propagation is highly influenced by the presence of microphones. The calibration process helps to take into account this influence. Measurements show that the presence of a non-uniform longitudinal temperature profile in the air inside a pipe modifies its input impedance.

In this paper, the normalized acoustic input impedance of a narrow pipe with a strong axially non-homogeneous thermal profile subjected to low Mach number flow of up to 0.03 M is studied. The analysis is done experimentally, using the two-microphone three-calibration technique and the results are compared with numerical simulations, using an industrial finite element package. The main application of this study is in the acoustic design of water-cooled charge-air coolers in internal combustion engines. The measurements are done on an effectively semi-infinite narrow pipe. It is shown that neglecting the flow effect in the calibration process of narrow pipes can significantly increase the measurement error. The results show that although the direct modification of the normalized acoustic input impedance due to low Mach number flow is negligible, the flow exerts an indirect but significant effect on this parameter by modifying the thermal gradient profile. Therefore, taking this effect into account can help to achieve an improved acoustical design of engine intake lines.