Design Optimization of Evaporator Clip by FEA (ANSYS)

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1 Design Optimization of Evaporator Clip by FEA (ANSYS) Giridhar Kumar Abhijit Kulkarni Global Technology and Engineering Centre,Whirlpool of India Ltd. Pune, India Abstract Structural rigidity of refrigerator components is one of the important aspects of refrigerator s structural design for its safe and efficient operation. Generally, the loading conditions are severe during the transportation of the product. To ensure safe transportation, design has to withstand transportation test. We were referred to a component, evaporator clip, which failed during a standard transportation test. Evaporator clip is an aluminum alloy component which holds the evaporator tube assembly in place. The two evaporator clips, shaped like J hooks, were getting straightened during the transportation test. A dimple in the hook was indenting the very tube where it was supported. The obvious solutions were (a) To add one more clip to reduce load per clip or (b) Use high strength material or (c) Increase the thickness of clip material. But, all these options were increasing the manufacturing and product cost, which was not acceptable. This paper covers the success story of how we arrived on two safe design solutions without incurring substantial extra cost using ANSYS 7.1. Introduction In a typical refrigerator, evaporator assembly was a cooling unit in which refrigerant evaporated by extracting heat from inside, through surrounding fins mounted on evaporator tube. Evaporator assembly consists of parallel bent tubing, fins mounted on tubes and a defrost heater (Figure 1). The evaporator assembly was supported on two evaporator clips hanging on the heat shield. Tube was press fitted in the bend part ( J shape) of the clip. Other end of clip was engaged in a groove made on heat shield (aluminum body covering evaporator assembly). Three dimples on the bent part of clip held the tube. Two dimples on the side ensured the locking of the tube and another dimple at the bottom supported the tube (Figure 2). Both clips were getting straightened in transportation test due to cyclic loading. Also, lower dimple of clip was tending to penetrate the tube (Figure 3). Initially, original clip design was analyzed using ANSYS 7.1 by applying load in increments so that the stress induced in the clip crosses the yield stress. This load was used as a reference to analyze the different new design using ANSYS 7.1. These FEA analysis results were main guiding tools to evaluate the new ideas and find the optimum clip design. Analysis Model simplification Evaporator assembly which rests on evaporator clips is complex and asymmetric (Figure 1). The tubes are loosely held in holes on the fins. The defrost heater and evaporator tube were held together by clamps. Hence the whole assembly has lot of curvatures and loose joints which make FEA model complex with poor quality mesh. Hence model simplification was required.

Hence for the sake of simplicity, one clip and a part of tube with equivalent weight was considered

2 For clip design, evaporator assembly load is key parameter. Total load of evaporator assembly was held by two clips through tube. Hence for the sake of simplicity, one clip and a part of tube with equivalent weight was considered in FEA model. Heat shield Evaporator tube Fins Defrost heater Evaporator Clips Evaporator Tube Figure 1. Evaporator assembly (Evaporator clip, Tubes, Fins & Heater assembly)

Mid plane model of tube and clip was constructed in Pro/E and an IGES file was created. In ANSYS 7.1 IGES file was imported.

3 Dimples Figure 2. Original design of evaporator clip Clip before transportation test Clip after transportation test Figure 3. Existing design of clip before & after the test FEA modeling The 3D models of all the components were available in Pro/Engineer. Mid plane model of tube and clip was constructed in Pro/E and an IGES file was created. In ANSYS 7.1 IGES file was imported. Mid plane geometry of tube and clip was meshed by Shell 93 element. Surface to surface contact elements were created between tube surfaces touching the clip/dimples for transmitting the load from tube to clip. Clip contact surface was created as Target 170 elements and tube contact surface was created as Contact 174 elements.

Figure 4. FEA model of tube and clip original design with forces and BCs Loading and Boundary conditions Incremental load was applied as nodal force on the tube top surface nodes.

4 Figure 4. FEA model of tube and clip original design with forces and BCs Loading and Boundary conditions Incremental load was applied as nodal force on the tube top surface nodes. Nodes of upper part of the clip which were engaged with heat shield were constrained in all DOF (Figure 4). Analysis Results & Discussion Analysis of original design Original design of clip was analyzed as per above assumptions and boundary conditions with incremental loads. The load was found out by several trials of FEA analysis which was crossing yield stress at failure point. By applying load 13.2 N (double of equivalent evaporator assembly weight); stress at bend part was exceeding the yield limit of 195 N/mm 2 (Figure 5a &5b). Hence this load was considered as a reference load to check the new design. Also high stress was observed at tube, near dimple at tube contact point (Figure 6).

5 Figure 5a. Stress plot in original design of clip Figure 5b. Stress plot in original design of clip

6 Stresses in Tube 5 Figure 6. Stress plot in original design of tube Development and analysis of new design supported by formed feature (Design 1) FEA result was showing high stress at bend part of clip and also straightening was occurring at this location, hence various ideas to support at this point were generated. The various ideas were evaluated by FEA simulation. Final shape and size of most suitable formed feature to support the hanging part of clip, was decided based on FEA results (Figure 7a & 7b). Also lower dimple was removed to create a line contact between tube and clip, instead of point contact in original design. Clip s J shape hook was adjusted to retain tube center at original position. Stress and deflection of clip (Figure 8) and tube (Figure 9) have drastically gone down compared to original design and also below yield stress limit of the material.

7 Figure 7a. Formed feature to support hanging part of clip Figure 7b. Formed feature to support hanging part of clip

8 Figure 8. Stress plot with support at hanging part Figure 9. Stress plot of tube with line contact

Comparison of maximum stress developed in original design and new design with formed feature (Design 1) Yield Original New Design - 1 Percentage Result Stress Design (with support feature ) stress

9 Comparison of maximum stress developed in original design and new design with formed feature (Design 1) Yield Original New Design - 1 Percentage Result Stress Design (with support feature ) stress reduction Max. Stress in Clip (MPa) % Max. Stress in Tube (MPa) % Development and analysis of new design with rib (Design -2) Another design was developed by providing vertical rib at bend. Different shape and size of rib was modeled and analyzed by using ANSYS 7.1. On initial trial, stress concentration was observed at the termination point of ribs (Figure 10). By increasing the rib length & tangentially merging the both ends of rib have resolved the stress concentration problem (Figure 11). Most suitable shape and size of rib was accepted base on FEA results. By providing rib, stress has come down below yield stress limit of the material (Figure 12). Figure 10. Stress plot of initial clip design with rib

10 Figure 11. New design 2 with rib at bending Figure 12. Stress plot with rib

11 Comparison of maximum stress developed in original design and new design with rib (Design 2) Result Yield stress Original Design New Design - 2 (with rib feature) Percentage stress reduction Max. Stress in Clip (MPa) % Max. Stress in Tube(MPa) % Conclusion Both the new designs have shown significant reduction in stress levels, on the clip as well as on the tube. Although design 1 has higher safety in comparison to design 2, the two options have been provided for manufacturing to choose from. With the help of these options manufacturing can choose the easier of the two changes based on their current tooling configuration. Both solutions are capable of solving the problems observed during transportation test. It can be noted that neither solution adds to the existing manufacturing / material costs. This is a recent analysis and implementation work is in progress. References 1. ANSYS User Manual 2. Concepts and Applications of Finite Element Analysis by R. D. Cook & David S. Malkus. 3. Finite Element Method by J N Reddy. 4. Machinery's Hand books, 26th Edition, Industrial Press Inc, New York.

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