Reduced mechanisms

Mechanical. Premature wearout or loss of contact metal during engagement and separation can result in loss of tolerances, reduced spring forces, formation of loose metallic wear debris, which may short-circuit contacts, and development of porosity in noble metal contacts. Underplatings, contact lubricants, and hard materials reduce mechanical wear. 

For instance, power can be transmitted from one machine to the other through shafts, flexible couplings, and gear reducers (mechanical equipment). Power can be transmitted through a torque converter (hydraulic equipment) or by a combination of mechanical and hydraulic equipment. 

A direct relationship is found between the hardness of a material and its ability to withstand mechanical wear, and as a rule the harder the material, the greater is its wear resistance. A related factor is that harder materials usually take a better polish and, as a result, the wear due to surface roughness mentioned above can be minimized. Choosing material combinations with a low coefficient of friction is also an important factor in reducing mechanical wear. 

Titanium Carbide. TiC, with its great hardness and wear resistance, is particularly suitable to reducing mechanical and abrasive wear. However, it is susceptible to chemical attack and is not a good diffusion barrier. 

Chen, J. Y., R. W. Dibble, and R. W. Bilger (1990). PDF modeling of turbulent nonpremixed CO/H2/N2 jet flames with reduced mechanisms. In 23rd International Symposium on Combustion, pp. 775-780. Pittsburgh, PA The Combustion Institute. 

Swaminathan, N. and R. W. Bilger (1997). Direct numerical simulation of turbulent nonpremixed hydrocarbon reaction zones using a two-step reduced mechanism. Combustion Science and Technology 127, 167-196. 

Bilger, R.W. On reduced mechanisms for methane-air combustion in non-premixed flames, Combustion Flame, 80, 135-149, (1990). 

Mechanism reduction, nevertheless, may be necessary in some applications — for example, to model multidimensional reactive flows. Even the fastest computers today cannot handle such problems using detailed mechanisms in a reasonable time frame. It must be recognized, however, that models that utilize reduced mechanisms would have a far narrower range of applicability than the ones that use comprehensive reaction mechanisms. Furthermore, models that are based on reduced mechanisms cannot be expected to be valid outside the limits set in the mechanism reduction step. 

Steady-state approximations are introduced into the skeletal mechanism to obtain the following six-step reduced mechanism ... 

Hewson, J. C., and M. Bollig. 1996. Reduced mechanisms for NOj, emissions from hydrocarbon diffusion flames. 26th Symposium (International) on Combustion Proceedings. Pittsburgh, PA The Combustion Institute. 2171-79. 

Chronic constriction injury (CCI) models (Bennett and Xie, 1988 Kim and Chung, 1992 Mosconi and Kruger, 1996) have been used to examine chronic pain states in experimental animals. In these models, intrathecally administered a2-agonists reduced mechanical allodynia and thermal hyperalgesia that developed following nerve constriction (Levy et al., 1994 Yaksh et al., 1995). This... 

A.L. Forchielli H.A. Aaronson, Desensitized Explosives - RDX Composition B of Reduced Mechanical Sensitivity , PATR 1766 (1950) 47) H.A. Aaronson, Desensitization... 

Analytically Reduced Mechanisms Some problems can be described by models that involve a full reaction mechanism in combination with simplied fluid dynamics. Other applications may involve laminar or turbulent multidimensional reactive flows. For problems that require a complex mathematical flow description (possibly CFD), the computational cost of using a full mechanism may be prohibitive. An alternative is to describe... 

A number of different approaches have been suggested for systematic reduction of detailed reaction mechanisms [160,313], The most common approach involves a two-stage procedure. First, a skeletal mechanism is established by removing all redundant species and reactions. Second, the skeletal mechanism is further reduced by order-of-magnitude approximations, resulting in the analytically reduced mechanism. 

Once the skeletal mechanism is established, a reduced mechanism is developed by applying steady-state and partial-equilibrium assumptions. The criteria for assuming steady-state or partial-equilibrium are discussed in Section 13.2.5. The concentration of species, typically radicals, that can be assumed in steady state can be estimated based on concentrations of other species and rate constants for relevant reactions. Thereby the steady-state species can be eliminated from the reaction mechanism. After elimination of steady-state species, the required number of multi-step reactions is determined. The reaction rate for these multi-step reactions can be calculated from the reaction rates of the original mechanism. The multi-step reaction rates depend on the concentration of the eliminated steady-state species. Partial equilibrium assumptions are often applied to the fastest elementary reactions to simplify the estimation of the steady-state concentrations. 

Reduced mechanisms are used increasingly to describe chemical reactions in computational fluid mechanics. However, the development of a reduced mechanism often requires a thorough knowledge of the chemical kinetics of the system of interest, and the results obtained with the reduced mechanism are only valid in a limited domain of initial and operating conditions. Methods to automate the reduction procedure are currently being developed to facilitate the use of this modeling approach, for example, as discussed in Ref. [314],... 

There are a number of alternatives and variations to the reduced mechanism method. The intrinsic low dimensional manifold (ILDM) approach [253] and similar methods [399] seek to decouple the fastest time scales in the chemistry. There is a wide range of time scales for chemical reactions in most high-temperature processes, from 10-9 second to seconds. Fast reactions, or reactions with small time scales, quickly bring composition points down to attracting manifolds in the composition space. Then composition points move along on manifolds. In the ILDM approach it is assumed that any movement of the... 

N. Peters and B. Rogg, editors. Reduced Mechanisms for Applications in Combustion Systems. Lecture Notes in Physics, Springer-Verlag, New York, second edition, 1993. 

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