Thermodynamically, a body of combustion products in equilibrium may be classed as a system, since it is a ‘fixed quantity of matter enclosed by a boundary defining a region in space’. Conservation of energy demands that any energy transfer across the boundary incurs a corresponding change in the total stock of energy of the system itself. Energy can transfer across the boundary in the forms of work W and/or heat Q, and the sign convention is such that work flow outwards and heat flow inwards are both classed as positive. Hence the total outwards flow of energy = (W − Q), in view of the sign convention, and this corresponds to a loss of equal magnitude in the intrinsic energy stock of the system. The following expressions apply respectively to systems that are stationary, and to those that are flowing but with negligible changes in potential (height) and kinetic (velocity) energies.
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