Thermal Effect Of Gas Film On Spiral Groove Dry Gas Seal

Spiral groove dry gas seal is widely used in rotating machines which require shaft seals because of its advantages of strong hydrodynamic effects, high opening force and film stiffness. During the operation of dry gas seal, the gas temperature varies along with the gas flowing through the seal face due to viscous shear, compression and expansion of interfacial gas film. Excessive gas temperature and temperature gradient can cause thermal deformation of the seal rings, the instability of operation, increase of leakage, decrease of film stiffness and so on. The seal failure may occur in severe cases. The research on thermal effect of interfacial gas film in spiral groove dry gas seal has been done in this paper.To calculate the heat caused by viscous shear, a simplified calculation method of the end face frictional torque of spiral groove dry gas seal, which was influenced by the depth of spiral groove, was proposed. And it was analyzed comparing with other calculation methods. In the common range of 3 μm to 5μm operating clearance of spiral groove dry gas seal, the presented calculation method has sufficient accuracy.In order to analyze the variation of gas temperature along with the gas flowing through the seal face, which is simply caused by thermodynamic process, the Muijderman’s governing equations of fluid film pressure with spiral grooves was modified by the Sutherland’s viscosity-temperature equation and the process equation of ideal gas. Then the variation of the gas temperature along the radial direction of the end face, which is simply caused by compression and expansion, has been obtained by solving the modified equations just obtained. The results show that the film pressure in the same position decreases slightly with the increasing of polytropic exponent (m). For the process of gas flowing from the inlet to the outlet, if the film pressure is higher than the ambient pressure (p0) caused by compression, the film temperature is higher than the ambient temperature (To), and the film temperature rises with increasing of m. If the film pressure is less than p0 caused by expansion, the film temperature decreases gradually; and the greater of m, the more rapidly the temperature drops.When the heat generated due to viscous shear is equal to the heat loss due to expansion, the gas film basically reaches heat equilibrium and is approximatively isothermal under the condition of ignoring the heat transfer between the gas film and the seal rings. The expressions of the two kinds of heat varying with the film thickness were obtained by using least square fitting method, respectively. And the film thickness of heat equilibrium (hp) was got by making the two equations equal. The results show that hp varies generally between 2.5μm and 4.5 μm. And hp increases gradually with the increasing of rotating speed (ω) and heat transfer coefficient (σ), and decreases gradually with the increasing of groove depth (hg) and the width ratio of groove and dam (φ).The numerical simulations for the fluid flow and the heat transfer of the interfacial gas film in two cases (i.e., ideal gas and real gas) by Fluent were obtained, respectively. The results show that the gas film temperature varies both in radial and circumferential direction, and lower gas film temperature appears near the inlet of the groove. The higher temperature area of gas film gradually shifts from the land to the dam with the increasing of the film thickness (h). The thicker h is, the lower the average film temperature would be. The film temperature is significantly affected by the rotational speed (n). As the rotational speed (n) increases, the gas film temperature increases rapidly. However, increasing sealed gas pressure (po) reduces the temperature of the gas film. The main reason is that as the sealed gas pressure (po) increases, the gas leakage (St) increases gradually, which will take more heat away. In the range of pressure no more than 4.6 MPa, the sealing performance of air and N2 real gas is basically the same as those of ideal-gas. The opening force and the leakage of CO2 real gas in spiral groove dry gas seal are greater than those of ideal-gas, while the opening force and the leakage of H2 real gas are slightly less than those of ideal-gas. The real gas behaviour has obvious effects on the leakage of the dry gas seal, but little effects on the opening force.