OCR Text |
Show Efficiency", if -- at zero output; and (2) that the operational efficiency is an asymmetric inverted V-shaped curve going to zero at both zero output and at the maximum output, and going through a maximum at some point in between. (these simple equations work as a first approximation but in a second approximation the linearity of wall loss and/or exhaust enthalpy must be modified) These curves, provide a clear and simple general pattern of behavior that applies to all furnaces and engines and that, potentially, can be used for making design and operational decisions such as, for example, selecting the design output that will be closest to maximum efficiency, if this is a design requirement. This integral approach has both advantages and limitations. The principal advantage is the generality of the results obtained, with applicability to all thermal devices, whether First or Second Law. The principal disadvantage is the inability to predict from first principles the relevant "constants" in the Perfonnance equations: H;; H;; and aO. In principle, these can be predicted by mechanistic analysis, based on detailed examination of the internal characteristics of the heat transfer, fluid dynamics, and reaction kinetics behavior. For analytical and design purposes it would, of course, be desirable to have computer codes that would complete the required internal analysis of the detailed thennal interactions, and a number of codes, far too numerous to list here, have been written to do this. However, they all have the present disadvantage that the results of the computations generally depend critically on the boundary conditions, and particularly on the shape, temperatures, and related properties of the combustion container. To handle this problem, so far, has required the use of simplified boundary conditions, notably the common use of symmetry in the combustion chamber (typically cylindrical). Consequently, general codes that can successfully accommodate a wide variety of boundary conditions, and particularly the asymmetric geometrical designs that most accurately represent reality are not widely accessible to practicing engineers, largely due to the influence of turbulence. There is the further problem that the full scope of the fluids and heat transfer equations also requires computational facilities such as supercomputers and parallel processing machines that are not readily available and may still not be adequate for the computation needs of the problems defined. One solution to this has been to use very simple geometric shapes and/or to use simplified equations systems but with the result that such codes may then be too limited for use in the design process. 6 |