Application of Fractal Contact Model in Dynamic Performance Analysis of Gas Face Seals

Chinese Journal of Mechanical Engineering, Apr 2018

Fractal theory provides scale-independent asperity contact loads and assumes variable curvature radii in the contact analyses of rough surfaces, the current research for which mainly focuses on the mechanism study. The present study introduces the fractal theory into the dynamic research of gas face seals under face-contacting conditions. Structure-Function method is adopted to handle the surface profiles of typical carbon-graphite rings, proving the fractal contact model can be used in the field of gas face seals. Using a numerical model established for the dynamic analyses of a spiral groove gas face seal with a flexibly mounted stator, a comparison of dynamic performance between the Majumdar-Bhushan (MB) fractal model and the Chang-Etsion-Bogy (CEB) statistical model is performed. The result shows that the two approaches induce differences in terms of the occurrence and the level of face contact. Although the approach distinctions in film thickness and leakage rate can be tiny, the distinctions in contact mechanism and end face damage are obvious. An investigation of fractal parameters D and G shows that a proper D (nearly 1.5) and a small G are helpful in raising the proportion of elastic deformation to weaken the adhesive wear in the sealing dynamic performance. The proposed research provides a fractal approach to design gas face seals.

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Application of Fractal Contact Model in Dynamic Performance Analysis of Gas Face Seals

Hu et al. Chin. J. Mech. Eng. Application of Fractal Contact Model in Dynamic Performance Analysis of Gas Face Seals SongT‑ao Hu 0 Wei‑Feng Huang 0 Xiang‑Feng Liu 0 Yu‑Ming Wang 0 0 State Key Laboratory of Tribology, Tsinghua University , Beijing 100084 , China Fractal theory provides scale‑ independent asperity contact loads and assumes variable curvature radii in the contact analyses of rough surfaces, the current research for which mainly focuses on the mechanism study. The present study introduces the fractal theory into the dynamic research of gas face seals under face‑ contacting conditions. StructureFunction method is adopted to handle the surface profiles of typical carbon‑ graphite rings, proving the fractal contact model can be used in the field of gas face seals. Using a numerical model established for the dynamic analyses of a spiral groove gas face seal with a flexibly mounted stator, a comparison of dynamic performance between the Majumdar‑ Bhushan (MB) fractal model and the Chang‑ Etsion‑ Bogy (CEB) statistical model is performed. The result shows that the two approaches induce differences in terms of the occurrence and the level of face contact. Although the approach distinctions in film thickness and leakage rate can be tiny, the distinctions in contact mechanism and end face damage are obvious. An investigation of fractal parameters D and G shows that a proper D (nearly 1.5) and a small G are helpful in raising the proportion of elastic deformation to weaken the adhesive wear in the sealing dynamic performance. The proposed research provides a fractal approach to design gas face seals. Fractal theory; Asperity contact; Gas face seal; Dynamic performance 1 Introduction Face contact is an important physical reality in a number of research fields [ 1–3 ]. In the field of face seals, for contacting face seals, face contact is inevitable during the opened operation. For non-contacting face seals such as spiral groove gas face seals as shown in Figure 1, they should possess a proper gas film thickness to avert face contact during the opened operation. Even so, face contact does occur during the startup and shutdown operations [ 4 ], and is also a risk from disturbances during the opened operation [ 5 ]. Therefore, it is imperative to choose an adequate asperity contact model in the analyses of face seals. With respect to asperity contact, Greenwood and Williamson (GW model) [ 6 ] have done a pioneering work, developing an elastic contact model between rough surfaces. McCool [ 7 ] and Bhushan [ 8 ] added asperity slope and curvature to capture rough surfaces. Chang et  al. [ 9 ] proposed the CEB elastic-plastic contact model based on volume conservation during plastic deformation to improve the GW model. Kogut and Etsion (KE model) [ 10, 11 ] developed a finite element method to investigate the contact between a deformable spherical asperity and a rigid flat, showing dimensionless contact load and contact area over the increase in the interference range from purely elastic through elasticplastic to fully plastic contact. However, Sayles and Thomas [ 12 ] revealed that many engineered surfaces have the multi-scale characteristic. Bhushan et  al. [ 13 ] found that statistical parameters depend strongly on the resolution of measuring instruments, and are not unique for a surface because of the multi-scale characteristic. It leads to the result that measurements with different resolutions and scanning lengths wouldn’t yield unique statistical parameters for a surface. Moreover, statistical contact models overlook the fact that the curvature radius of an asperity is a function of asperity size, and surely assume a constant curvature radius for all asperities. Majumdar and Bhushan [ 14 ] used the Weierstrass-Mandelbrot (WM) function to develop the first fractal contact model for real rough surfaces where the assumption of variable curvature radius was adopted. This fractal contact model has been of interest to many researchers, and has been applied to various applications. Wang and Komvopoulos [ 15, 16 ] researched the interfacial temperature factor in the fractal contact. Komvopoulos and Yan [17] generated a three-dimensional fractal surface by the WM function and introduced it into the contact model. Sahoo and Chowdhury [ 18, 19 ] analyzed the friction and the wear of fractal surfaces. Ciavarella et al. [20] investigated the elastic contact stiffness and the contact resistance of fractal surfaces. Kogut and Jackson [ 21 ] used both statistical and fractal approaches to characterize simulated surfaces, and obtained substantial differences between the two. Morag and Etsion (ME model) [ 22 ] argued that a single asperity transferring from plastic to elastic when the load increases and the contact area becomes larger in the MB model is in contrast with classical contact mechanics. They suggested the real deformation is an independent parameter ranging from zero to ful (...truncated)


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Song-Tao Hu, Wei-Feng Huang, Xiang-Feng Liu, Yu-Ming Wang. Application of Fractal Contact Model in Dynamic Performance Analysis of Gas Face Seals, Chinese Journal of Mechanical Engineering, 2018, pp. 27, Volume 31, Issue 1, DOI: 10.1186/s10033-018-0224-7