tailieunhanh - Engineering Tribology Episode 1 Part 9

Tham khảo tài liệu 'engineering tribology episode 1 part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Hydrodynamic Lubrication 175 Conducted Convected Heat Ratio From the above equations the ratio of conducted to convected heat can be calculated to determine which of the two mechanisms of heat removal from the bearing is the more significant. Combining and the ratio is given by xcond H xconv K 2B pỡ Uh2 where K pơ X is the thermal diffusivity of the fluid m2 s . Typical values of density specific heat thermal conductivity and thermal diffusivity are shown in Table Chapter 2. EXAMPLE Find the ratio of conducted to convected heat in a journal bearing of diameter D m and length L m which operates at 3000 rpm . The hydrodynamic film thickness is h m . The bearing is lubricated by mineral oil of thermal diffusivity X X 10-6 m2 s . Since D m B nD 2 m and H . . 2 X X 10-6 - _Ĩ 3000 nx 60 it is clear from the above example that for journal bearings the amount of convected heat is quite significant. When the film thickness of lubricant is much smaller or a highly conductive fluid such as mercury is used as a lubricant then the ratio of conducted to convected heat will be different. Hydrodynamic lubrication at much thinner film thicknesses is discussed in the chapter on Elastohydrodynamic Lubrication . The significance of the ratio of conducted to convected heat for hydrodynamic bearings is that convection must be included in the equations of heat transfer in a hydrodynamic film. This condition renders the numerical analysis of the heat transfer equations much more complicated than would otherwise be the case as is demonstrated in the next chapter. Another ramification of the above result is that conductive heat transfer is still significant although it is often the smaller component of overall heat transfer. Most hydrodynamic bearings operate under a condition between adiabatic and isothermal heat transfer. Adiabatic heat transfer can be modelled by a perfectly insulating shaft and .