열변형을 고려한 냉장고 내벽의 구조 최적화 연구

Abstract

In refrigerator constructions where in-situ foamed polyurethane thermal insulation material is employed, the structural rigidity of refrigerator cabinet is generated by foaming process which the polyurethane is solidified and hardened after injection of the foam reactant. However, the outer case may be made of steel and the inner liner of plastics. The three materials which possess different thermal properties will produce different thermal deformation and great stresses during the foaming process and subsequent operating process.

In this paper, the Finite Element Method (FEM) is applied to calculate thermal deformation of specified refrigerator model. The sequential thermo-mechanical coupling analysis in ANSYS software was also used in this study. Firstly, the constant temperature field was obtained by applying heat transfer coefficients on the refrigerator FE model, then, in the thermo-mechanical deformation analysis process, thermal analysis units were changed into structural analysis units and the mechanical field was analyzed proceeding from an already known temperature field. The finite element model was comprised of solid elements and shell elements. The foam part was meshed with solid-70 element in the thermal analysis and solid-185 element in the mechanical analysis respectively. The plastic liners and all sheet metal components were meshed with the shell-63 element.

In order to make a series of analysis of refrigerator models, we developed a method of numerical analysis based on a combination of VB 6.0 and the ANSYS Parametric Design Language (APDL). A convenient interface for parameter input was developed and thermo-mechanical analysis procedure could be carried out using a VB interface by realizing seamless integration of the two pieces of software and increasing analysis efficiency.

However, we wanted to figure out the effect of plaque parameters, which have obvious effects on decreasing thermally induced cabinet bowing deformation, including plaque depth, plaque width, interval distance between plaques, and plaque numbers using a designated distance of inner case sidewall. In this study, the plaques were designed within a designated distance of 740mm. We firstly analyzed the effect of variables of plaque depth, width and numbers on refrigerator thermal deformation. Then, with designated numbers, we analyzed the effect of plaque depth, width and interval distance.

In chapter 3, we also used a method called the BP-GA method to obtain the optimal plaque parameters for minimum refrigerator inner case deformation. The training data were obtained by FEM model utilizing ANSYS because of lack of the experimental data. A total of forty-five FEM models were analyzed among which twenty-five models were arranged by the orthogonal test method according to the plaque’s depth (D), width (W) and interval distance (S). The FEM analysis results of thirty-eight models were inputted as training data and the other seven were taken as test data after the BP network training process in order to know how perfect the predicting model obtained by ANN training would match with the actual FEM analysis results.

In chapter 4, the inner case was designed into a diamond shape so that bad appearances were anticipated to be avoided after foaming. We designed several models with a diamond shape in a hexagonal pyramid style on the inner case wall in order to analyze the cooling effect and thermal deformation. And the diamond pattern was designed separately in convex style and concave style. We took thermo-mechanical analysis of the diamond shape inner liner.

Based on the above mentioned analysis results, we came up with the concept of pre-forming. According to the inward bowing characteristic of thermal deformation, we pre-formed the sidewall by a reverse shape deformation to some suitable extent. The pre-formed area was designed to be in the central part of the inner liner area. The normal thermal deformation was found to be about five millimeter for the refrigerator model in this study. We designed convex, concave and convex-concave pre-formed models at three millimeter.