항공기용 U 튜브 열교환기의 구조해석에 관한 연구

Abstract

The objective of structural analysis to develop the capability to predict the thermo-mechanical performance of the heat exchanger at engine operating conditions using validated multi-physics modelling technology. Structural analysis estimates the life of the U tube heat exchanger by thermo-mechanical and dynamic analysis.

To conduct the tensile test in high temperature (700 K, 800 K, 900 K, 1000 K) using five specimens to obtain the mechanical properties of Inconel 625 tubes.

Based on the test result, the yield strength is 308 MPa and tensile strength is 640 MPa at 1000 K.

Dynamic and thermo-mechanical analysis were carry out for the sub-model of U tube heat exchanger with baffle. The results with baffle were compared with those without baffle in order to investigate the effect of baffle on the vibration and stress level of U tube heat exchanger. The shape of baffle considered was a plate type and this plate type of baffle was assumed to be welded to the tube bundles so that the plate type of baffle moved together with tubes when the tubes moved because of the thermal deformation at the high temperature environment.

To consider different cases without baffle, with one baffle at the center and with two baffles at the lower and upper parts of U tube heat exchanger. The baffle was effective on the increasing 1st natural frequency. The maximum thermal stress of tube bundle with baffle was much larger than that without baffle, because the present plate type of baffle restrained the thermal deformation of tube bundle.

The nonlinear contact analysis with thermo-mechanical load was carried out to predict the stress level for U tube segment model with baffle. A uniform pressure of 5.5 MPa (55 bar) was applied as the mechanical surface load. A uniform metal temperature of 900 K was applied to the tubesheet and tube bundle as the thermal load. A uniform metal temperature of 350 K was applied to the baffle as the thermal load, and a reference temperature is assumed as 350 K. The tubesheet was used under the high temperature environment. The gap between tube and baffle is 0.05 mm which is considered in order to allow the sliding of tube. The maximum stress was increased with increasing the length of tubesheet at the junction part between tube and tubesheet. Through the analysis results, conduced that tubesheet width increases when stress increased. Furthermore, if baffle spacing decreases, stress would increase. The allowable width of tubesheet was suggested as 100 ~ 150 mm in the worst condition. The yield strength at the room temperature and high temperature (1000 K) of Inconel 625 thin tube (Outer dia :1.5 mm, Thickness :0.12 mm) was 440 MPa and 320 MPa, respectively. From the analysis result, it was seen that the stress level at the tube showed strong dependence on the tubesheet length.

The modular U tube heat exchanger was composed tubesheet, baffle, manifold, case and flange. Bulk temperature and heat transfer coefficients were applied to a stepwise way discontinuously. The internal driving gas temperature distribution drops from 994 K at inlet to 696 K at exit. The stress level was suggested 350 MPa in the worst condition.