# Reduction of the Cavitation Noise in an Automotive Heater Core
## 3 Identification of the Cavitation Noise Generation Mechanism  
To determine whether cavitation occurs in the heater core or the water pump, transparent tubes were installed in the inlet and outlet pipes of the compact passenger car, as shown in Figure 12, to observe the presence of cavitation-induced bubbles. If the bubbles are observed in the transparent tube of the inlet pipe, it strongly suggests that cavitation originates in the water pump. Conversely, if bubbles are absent in the inlet pipe but appear in the transparent tube of the outlet pipe, it confirms that cavitation occurs in the heater core. Vibration signals were measured at an initial coolant temperature of 1 \({}^{\circ}\)C and at two engine speeds: 4000 and 5000 RPM. As shown in Figure 13, accele  
Figure 11: Frequency spectra for the noise and vibration signals generated from the heater core in the compact passenger vehicle: (**a**) noise and (**b**) vibration. Figure 12: Transparent tubes installed on the inlet and outlet pipes to observe bubbles generated by cavitation. five points on the inlet and outlet pipes of the heater core and six points on the upper and lower tanks. These sensors were used to measure vibration signals to identify the locations where bubbles collapse due to cavitation. To attach the accelerometers to the heater core tank surfaces, some sections of the HVAC system were removed to expose the heater core. In addition to measuring the pressures in the inlet and outlet pipes, pressure gauges were also installed at the ends of these pipes, and a flow meter was also installed to measure the flow rate. Experimental results indicate that at an engine speed of 4000 RPM, no cavitation-induced bubbles were observed in either the inlet or outlet pipes. However, at 5000 RPM, bubbles were absent in the inlet pipe but were detected in the outlet pipe, confirming that cavitation occurs in the heater core. The vibration signals measured at point F on the upper tank of the heater core are shown in Figure 14. Figure 14a presents the vibration signal at 4000 RPM without cavitation, while Figure 14b shows the signal at 5000 RPM with cavitation. As seen in these figures, the vibration magnitude during cavitation is higher than that in non-cavitation conditions. Furthermore, in the presence of cavitation, the vibration signals exhibit impacts caused by bubble collapse against the inner walls of the tank. Figure 14: Vibration signals measured by the accelerometer attached to point F on the upper tank: (**a**) at an engine speed of 4000 RPM (without cavitation) and (**b**) at an engine speed of 5000 RPM (with cavitation). Figure 13: Attachment points of the accelerometers for measuring vibration signals: (**a**) points on the inlet and outlet pipes and (**b**) points on the upper and lower tanks. The labels 1, 2, 3, 4, 5 and A, B, C, D, E, F indicate the designated sensor positions used during the measurements. To further identify cavitation bubble collapse locations in the heater core, vibration magnitudes at different measurement points, as shown in Figure 13, were compared. As indicated in Figure 11b, frequency spectrum differences due to cavitation are observed above 2 kHz. Therefore, the root mean square (RMS) values of vibration magnitudes within the 2-10 kHz frequency range were chosen for comparison. The vibration magnitudes measured at five points on the inlet and outlet pipes (Figure 13a) and six points on the upper and lower tanks (Figure 13b) are compared in Figure 15. Figure 15a shows the vibration magnitudes at the five pipe measurement points, while Figure 15b shows those at the six tank measurement points. The blue bars represent vibration magnitudes at 4000 RPM without cavitation, whereas the pink bars indicate those at 5000 RPM with cavitation. Figure 15 reveals that vibration magnitudes at all points are higher when cavitation occurs than when it does not. This indicates that collapsing cavitation bubbles induce vibrations in the heater core pipes and tanks.