Hermetic Reciprocating Compressor, CFD, Lubrication, Sliding Mesh Method, Moving Reference Frame
The capacity of inverter compressors is adjusted with speed. Therefore, rotational speed of the hermetic reciprocating compressor is changed in accordance with the requirements of capacity in the refrigerator. Operation at different rotational speed makes the design of oil management system more critical for those compressors. Especially, the preference of operation at lower speeds causes a higher risk for refrigeration applications. Insufficient lubrication of the hermetic compressor causes the compressor to stop working or even become inoperable. Several experimental and numerical studies which are dedicated to overcoming insufficient lubrication problems can be found in the literature. Among these studies, numerical investigations are at the forefront especially at the first design process due to some difficulties of the experimental investigations such as high cost and long-running time. On the other hand, there are various disadvantages of the numerical studies. For example, the numerical modeling approach preferred in the study determines the accuracy of the results. Because the performance of any modeling approach depends heavily on the application used. Therefore, which modeling approach gives the best results for the case under investigation should be carefully determined before the ultimate investigations. In this regard, two different numerical modeling approach, namely Sliding Mesh (SM) and Moving Reference Frame (MRF), are used to numerically investigate the lubrication system of a compact type hermetic reciprocating compressor (CIC). SM method is a special type of dynamic mesh motion which boundaries and mesh cells move together in a rigid-body motion. As a consequence, SM model is inherently unsteady due to the motion of the mesh with time. The cells in the zones are not deforming and the governing equations of the fluid motion are different than used in MRF method. SM model is defined as the most accurate method for simulating flows in multiple reference frame, but also the most computationally demanding. In MRF method, different rotational speed is defined at each cell zones and the flow in the zone is solved using moving reference frame equations. The MRF approach does not account for the relative motion of a moving zone, the mesh remains fixed for the computational time. This approach also named as "freezing rotor approach" because of the freezing motion of moving part in a fixed initial position. The MFR method is widely used in numerical studies related to this subject. This paper reports the performance of the SM and the MFR methods in modeling lubrication system of a KIK compressor. The two-phase flow is simulated using the Volume of Fluid Method (VoF) under transient, laminar and incompressible flow conditions. The simulations are carried out for a wide range of rotational speed. The modeling performance of the SM and the MFR models is evaluated by investigating the oil climbing time which is defined as the required time to reach oil to the uppermost of the crankshaft, and the flow fields obtained by the two methods.