Studies in Furnace Analysis: Prediction of the Variation of the Specific Exhaust Enthalpy with Output

Abstract

Prediction of the overall performance of furnaces and engines in terms of the operational and intrinsic thermal efficiencies (η and α00) has been limited in the past by need to assume empirically a linear dependence of the specific exhaust enthalpy, hg, on the useful output (heat or work), Hs. In this paper, a theoretically-based relation between hg and Hs is developed using an integral approach, following Hottel and Thring, by treatment of furnace enclosures as heat exchangers. The expression developed is shown to be supported by comparison with experiment, using measurements on a boiler obtained by Prengle at the University of Houston. The new, theoretically-based and experimentally-supported relation shows that the variation between hg and Hs is nonlinear: it also depends on a new parameter, R0, that is the ratio, at idle, of the exhaust gas temperature above ambient to the average flame temperature above ambient. The hg−Hs curve is near-linear for values of R0 near unity, and the nonlinearity increas...