# Reduction of the Cavitation Noise in an Automotive Heater Core
## 4 Reduction of the Cavitation Noise  
In the previous section, it was demonstrated that cavitation noise was generated at the junction between the upper tank and the outlet pipe. This phenomenon was attributed to the abrupt change in cross-sectional area at the junction, which induced a high coolant velocity and a rapid pressure drop, creating favorable conditions for cavitation. Related to this issue, structural modifications were implemented at the junction to reduce cavitation noise. Figure 20 illustrates the design changes. The original configuration (Figure 20a) featured a step-like transition between the upper tank and the outlet pipe, resulting in a sudden contraction in flow area. This was redesigned into a tapered connection (Figure 20b), allowing for a smoother and more gradual transition in cross-sectional area. Additionally, the cross-sectional area of the outlet pipe was increased, thereby reducing  
Figure 19: Velocity distribution of the coolant: (**a**) around the junction between the upper tank and the outlet pipe and (**b**) in the cross-section of the outlet pipe at the junction. the area ratio between the outlet pipe and the upper tank from 54% to 38%. These changes were aimed at reducing the flow velocity and pressure drop at the junction, which are primary contributors to cavitation. To evaluate the effectiveness of the modified design, a CFD simulation was performed under the same boundary conditions used in the previous analysis to ensure a fair comparison. The resulting velocity distribution near the junction is presented in Figure 21a, while Figure 21b shows the velocity distribution across a cross-section of the outlet pipe close to the junction. The simulation results reveal that the maximum velocity in the improved configuration is significantly lower than that in the original design (Figure 19). This reduction is directly attributed to the elimination of the step structure and the increase in outlet pipe diameter. The effect of these design modifications on cavitation potential was assessed using the cavitation index. Figure 22 presents the distribution of the cavitation index at the mid-plane of the heater core. In the region where cavitation was most severe in the original design (highlighted by a circle in Figure 22), the minimum cavitation index was improved from 0.876 to 5.604. A cavitation index of 5.604 indicates that the local pressure conditions are no longer conducive to cavitation, suggesting that the redesigned junction effectively suppresses cavitation onset. Figure 21: Velocity distribution of the coolant after the structural modification: (**a**) around the junction between the upper tank and the outlet pipe and (**b**) in the cross-section of the outlet pipe at the junction. Figure 20: Structural modification to the junction between the upper tank and the outlet pipe for cavitation noise reduction: (**a**) before modification and (**b**) after modification. To experimentally validate the CFD findings, a prototype heater core incorporating the improved junction geometry was fabricated. The prototype maintained identical dimensions to those used in the CFD model. Transparent tubes were installed on the inlet and outlet pipes to visually monitor bubble formation, and an accelerometer was mounted at point F on the upper tank (as shown in Figure 13b) to capture vibration signals associated with cavitation. The engine was accelerated from a stationary state to 5000 RPM and held at that speed for approximately 100 s, during which both visual observation and vibration measurements were conducted. No cavitation noise was detected from the structurally modified heater core, and no bubbles were observed in the transparent sections of the inlet and outlet pipes. Furthermore, the vibration signal shown in Figure 23 did not exhibit the impulse peaks that were present in the original configuration (Figure 14b), which were previously linked to the collapse of cavitation bubbles striking the tank wall.