Investigation of Coupling Guard Temperature Rise Using Advanced CFD Analysis

Increasingly, the turbomachinery industry is facing issues related to high temperatures and oil misting inside coupling guards. This leads to machinery down-times and loss of revenue. Many in the turbomachinery industry are investing significant time and effort to reduce heat generation within coupling guards, particularly at elevated temperatures and high peripheral speeds. The API 671 recommends the maximum allowable guard temperature limit of 160⁰F. The present computational fluid dynamics (CFD) investigation of Air+Oil fluid flow and heat transfer in the coupling guard was conducted to investigate heat generation issues. In prior investigations, the emphasis has been mostly placed on predicting coupling guard surface temperature by modeling pure air as fluid flow in the coupling guard. This paper presents CFD simulations completed under two conditions:  1) with pure AIR as fluid – in a Single-phase simulation and 2) a multi-phase simulation using AIR+OIL mixture as fluid. Multiphase CFD simulation approach provides higher coupling guard surface temperatures compared to single phase simulation.

The coupling guard geometry is cylindrical in nature. Given the symmetry in the geometry, a 90-degree sector was modeled, allowing significantly smaller meshes and lower run times without compromising quality of results. In this paper, the modeled geometry includes both Fluid + Solid domains that do not contain an inlet or outlet. This work explains the CFD methodology for multiphase and conjugate heat transfer analysis.

The objective of this paper is to predict the temperature rise on coupling guard considering Air + Oil mixture as fluid and conjugate heat transfer. The domain evaluated consists of several stationary and rotating components. The CFD domain used for the conjugate heat transfer analysis consists of fluid between the rotating shaft and the stationary cover. The solid domain includes the coupling guard cover. CFD analysis was carried out using commercially available software ANSYS CFX 18.1.  The mesh was generated using Tetrahedral/Prism elements.  Both steady state and unsteady CFD analyses were completed. As noted, multiphase CFD analyses were carried out using Air and Oil VG68 as a homogenous type of mixture.

Detailed flow field characteristics (total and static temperature, pressure, streamlines colored by Mach number, etc.) and CFD-predicted temperatures are compared between the AIR and AIR+OIL mixture simulations. In addition, available literature measurements or test data were used to validate the CFD results. Temperature results obtained from the CFD simulations were found to be in good agreement with the experimental data. This developed CFD modeling approach can guide the design of new coupling guards with lower surface temperatures.