미세 U 튜브 열교환기의 형상에 따른 구조 건전성 평가에 관한 연구

Abstract

The objective of structural analysis is to predict the thermo-mechanical performance of the heat exchanger at engine operating conditions using validated multi-physics modelling technology. The plastic fracture and life on the heat exchanger are estimated by thermo-mechanical analysis.

Tensile tests are conducted 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 shows 308 MPa and tensile strength shows 640 MPa at 1000 K.

Baseline model and three kinds of improved models are proposed. The purpose of the “separated model” is to reduce tubesheet thermal stress. Unlike the baseline model, the “separated model” is divided in half so that the hot side outlet and inlets are separated. So it is allows for each part to move independently even under thermal deformation. “Flange moved model” is similarly “baseline model” However, flange position has moved to manifold side. “Solid flange model”is that tubesheet side shell flange was changed to a solid flange and modified the shape of the case wall.

Thermo-mechanical analysis was performed to estimate the structural characteristics and stress level of baseline model and 3 improved models.

Comparing with baseline model, bending deformation of tubesheet in separated model decreased. So, the maximum stress of the tubesheet reduced 15%. And the temperature difference between the tubesheet and flange decreased. Thus, the maximum stress of the flange was reduced by 4%. Temperature difference between the tubesheet and case wall was decreased. So the maximum stress of the case wall was reduced by 8%.

Comparing with baseline model, bending deformation of tubesheet in flange moved model decreased. So, the maximum stress of the tubesheet reduced by 30%. So the maximum stress of the tubesheet reduced 5%. Temperature difference between the tubesheet and case wall was decreased. So the maximum stress of the tubesheet was reduced by 7%.

Comparing with baseline model, bending deformation of tubesheet in solid flange model decreased. So, the maximum stress of the tubesheet reduced by 35%. And the temperature difference between the tubesheet and flange was decreased. Thus, the maximum stress of the flange and tubesheet reduced by 30%. Temperature difference between the tubesheet and case wall was decreased. So the maximum stress of the tubesheet was reduced by 10%.

Therefore, solid flange model is the most improved model in terms of stress. Maximum stress were reduced by 30% in flange and tubesheet. Case of the wall stress was reduced by 10%. This value exceeded the yield strength of Inconel at 1,000K. (Ref. the yield strength of inconel 625 tube is 308MPa at 1,000K.). But this value was satisfied with the fatigue strength in 1000 cycle.