For example 2h2o 2h2 o2 36 2 thermodynamics of

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: X ^ Ni;P hi;P ¼ i X ^ ^ Ni;P ½Dho;P þ hsi;P ðTP ފ ¼ HR ðTR Þ ¼ i i X ^ Ni;R hi;R i Next, we rearrange the above equation to find an expression for the sensible enthalpy of the products as X i X i X i ^ Ni;P Dho;P þ i X i ^ Ni;P hsi;P ðTP Þ ¼ ^ Ni;P hsi;P ðTP Þ ¼ X X ^ Ni;R Dho;R þ i i ^ Ni;R Dho;R À i i ^ Ni;P hsi;P ðTP Þ ¼ ÀQ0 ;p þ rxn X X X ^ Ni;R hsi;R ðTR Þ i ^ Ni;P Dho;P þ i i X ^ Ni;R hsi;R ðTR Þ (2.45) i ^ Ni;P hsi;R ðTR Þ: i With an initial guess of flame temperature, Tp1, one evaluates Hp(Tp1) from tables such as those in Appendix 3. If Hp(Tp1) < HR(TR), we guess a higher flame temperature, Tp2. One repeats this process until the two closest temperatures are found such that Hp(Tf1) < HR(TR) < Hp(Tf2). The product temperature can be estimated by linear interpolation. This method, although more accurate, still assumes complete combustion to the major products. Method 3: Equilibrium State (Free software: Cantera; Commercial software: Chemkin) Dissociation5 of products at high temperature (T > 1,500 K at ambient pressure) can take a significant portion of energy from combustion and hence the product 5 Dissociation is the separation of larger molecules into smaller molecules. For example, 2H2O ↔2H2 + O2. 36 2 Thermodynamics of Combustion temperature is lower than that calculated with only major components as products. The equilibrium state determines the species concentrations and temperature under certain constraints such as constant enthalpy, pressure, or temperature. The equilibrium flame temperature is expected to be lower than the temperatures estimated with Method 1 or Method 2. In addition, the chemical equilibrium state is often used in combustion engineering as a reference point for chemical kinetics (the subject of Chap. 3) if infinite time is available for chemical reactions. At this ideal state, forward and backward reaction rates of any chemical reaction steps are balanced. By constraining certain variables such as constant pressure and enthalpy, the chemical equilibrium state can be determined by minimizing the G...
View Full Document

Ask a homework question - tutors are online