Liquid ring pumps are used in aircraft fuel systems in conjunction with main impeller pumps and serve the function of removing fuel vapor and air from the fuel. Thus, their reliable functioning plays a critical role in the safe operation of aircraft during flight. As cavitation has the potential to severely limit the operability of these liquid ring pumps and in severe cases may lead to their structural failure, accurate prediction of cavitation in the pumps is extremely important to design them for safe operation.
The cavitation phenomena occur in regions where large pressure drops results in the local pressure falling below the vapor pressure, resulting in formation of vapor bubbles. Typically for pumps, cavitation occurs in the suction side of the pump blades, which in turn results in a reduction of effective area of the blade, thereby diminishing the efficiency of the pumps.
The formation of vapor bubbles and their subsequent bursting creates pressure impulses on the blade surfaces, which leads to vibration and fatigue-induced structural damage leading to pump failures.
The liquid ring pump's main function is to remove air and fuel vapor during the priming process of the main fuel pump. This becomes necessary for situations where a large quantity of fuel vapor is formed in the ullage area. In those situations, the main centrifugal pumps require the liquid ring pumps to generate a positive suction head and to separate the air and fuel vapor from the liquid fuel through compression.
A typical liquid ring pump consists of a static casing that forms the outer surface of main pump and has an eccentrically mounted rotor assembly consisting of multiple rotating blade elements that are also referred to as impellers. The rotor assembly is mounted on the shaft of the main pump and is driven by the main fuel pump itself. As the impeller rotates, the centrifugal force causes the liquid fuel present in the pump to form a ring around the pump periphery. This draws in the air and fuel vapor mixture in to the inlet port with the liquid fuel being discharged to the fuel tank and supplied to the engine units through the outlet port.
In a recent study, researchers used a steady state multiple reference frame (MRF) methodology and a transient sliding mesh methodology to compare their ability to predict cavitation and pump performance in liquid ring pumps. As the assessment for the two computational approaches was relative to each other, no special emphasis was placed to determine grid dependency individually for each of the approaches.
The liquid ring pump configuration that was studied consisted of a single stage impeller with 14 blades arranged in an equispaced manner circumferentially. The impeller blades were of constant thickness and were mounted eccentrically relative to the pump casing. The cross section of the inlet and the outlet ports were airfoil shaped to minimize pressure losses.
The CFD simulations carried out for both methodologies employed a fully structured hexahedral mesh, which was generated in commercially available software. The conformal mesh consisting of around 110,000 cells had an equivolume skewness and aspect ratio of the individual cells below 0.9 and 58, respectively. The same mesh was used for both the MRF and the sliding mesh calculations to achieve mesh consistency between the two models for relative comparison and assessment.
Results indicated that though the computation efforts were cheaper for the steady state MRF model, the results obtained were physically not possible. The computationally expensive transient sliding mesh approach resulted in realistic predictions.
Due to unavailability of experimental data, a quantitative validation of the sliding mesh approach for cavitation prediction could be not be performed, but the trends observed in the results showed promise in this approach as compared to the MRF approach. Further investigation along with experimental validation would be required to refine the prediction fidelity of the transient sliding mesh based cavitation model for liquid ring pump applications.
This article is based on SAE International technical paper 2013-01-2238 by Manoj Radle and Biswadip Shome, Tata Technologies Ltd.