PSI - Issue 42

Ricardo Pires et al. / Procedia Structural Integrity 42 (2022) 639–646 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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From Fig. 4b, it can be inferred that the exhaust gas temperature decreases with the increasing length of the fins. Note that the absolute outlet temperature is maximum [ 288.4 °C ] for their shortest length and minimum [ 275.8 °C ] for the longest. Moreover, the drop outlet temperature is thus greater than 50% of the value of the gas inlet temperature [  650 ºC], and the temperature of the coolant appears to be independent of the fins ’ length. In fact, these fluctuations are small and are within the interval [ 4.8 - 5.2 °C ] (Fig. 4b). Concerning the induced stresses due to thermal gradients, and to gas and coolant pressure, there are two critical regions in the component (Fig. 5a), where the stress values are higher than the one delimited in the caption (150 MPa - which is the Yield Strength value for the AISI 304 material at 600ºC), namely at the entrance of the EGR and at the exit section. Nevertheless, the stresses induced at the exchanger’s outlet can be disregarded since they resu lt from a fixed boundary condition defined to avoid rigid-body motion. Hence, the focus of attention should be directed to the inlet of the EGR. Fig.5b presents some critical stress values calculated for specific points in this region, namely for the curvilinear region of the fin’s geometry, where those were higher than the Yield Strength of the material. Looking in detail, one can easily find values of 200/250 MPa at the fin’s entrance and even of 400 MPa local stresses at the bent radius regions of the fin in areas under stress concentration. Additionally, the critical stress values increased with increasing values of the fins’ length . This reflects the proximity to the baffle, where high temperature values were noticed. a b

Fig. 5 (a) Section view of the EGR with the stress distribution obtained for a fin’s length of 120 mm and critical regions (outlined in red). (b) Critical stress values obtained at the fins ‘ entrance. Therefore, for the various lengths considered for the fin, the amplitude and the mean stresses were calculated considering the maximum values numerically determined. Then, based on the Morrow equation (Eq. 1), the fatigue lives for all the geometries understudy were estimated assuming a load ratio, R, equal to zero and including one hot stage and one cold stage at the beginning of the functioning of the motor (Table 2); moreover, depending on the type of conduction – for instance in a city, or on a highway, or a mixed condition – the temperature values in the EGR vary a lot along the day. Therefore, to choose the appropriate design, the target was defined as 3 cycles/day, 365 days/year and 10 years. This represents a total of about 11000 cycles, which is closest to the fin’s length of 120 mm. (1)

Table 2. Fatigue lives calculated for the EGR understudy in function of the fins ’ length. Load ratio, R=0 (  amp =  med ).