Heat Recuperator - AFT and Combustion efficiency
In this assignment, the effects of preheating of intake charge temperature on the adiabatic flame temperature (AFT) and combustion efficiency (CE) have been performed. In this case, methane is taken as a fuel element and air taken as an oxidizer. This work is performed using Cantera library. Heat recuperator is a device which exchanges the thermal energy between the fluids; simply we can say it is a gas to gas heat exchanger.
- Effect of preheating charge temperature in the AFT
A simple program has been written in the Cantera using the Quantity objects for mixing species. Figs. 1-3 show the effects of preheating on the AFT for different equivalence ratio (Phi). Here, methane considered as a fuel component, and oxygen and nitrogen are taken as oxidizer components. The preheating temperature has been taken from 298K to 600K and the Phi ranges between 0.8 to 1.2. Maximum AFT is observed in unity value of equivalence ratio and reduced either rich or lean mixture modes. Increasing of preheating charge temperature increases the AFT linearly. Hence, the preheating method reduces the amount of fuel requirements to do the equivalent output work without preheating unit.
Fig.1. AFT Vs preheating charge temperature (Phi=1)
Fig.2. AFT Vs preheating charge temperature (Phi=0.8)
Fig.3. AFT Vs preheating charge temperature (Phi=1.2)
- Effect of preheating charge temperature on combustion efficiency
Combustion efficiency may be defined as amount of fuel chemical energy successfully converted into thermal energy during the oxidation process. Lower heating value is calculated from the enthalpy changes of reactants and products at environment temperature and pressure (Assume). The reactants temperature is varied from 298 to 600K and combustion temperature is assumed to be 1700K. Q represents the total enthalpy changes in the combustion environment condition. Qnet denotes the specific energy liberation during the combustion process. It is useful for finding the combustion efficiency and the system assumed an adiabatic condition. The combustion efficiency is around 57% at 300K preheated charge mode and it becomes nearly 65% at 500K preheated charge condition. This may be due to improved air-fuel mixing and faster chemical reactions. Fig.4. shows the combustion efficiency vs. preheating charge temperature while the Phi=1.
CE=Qnet/LHV #LHV 50MJ/kg not using in this problem.
Results:
Increasing of intake charge preheating method improves the system combustion efficiency and the AFT in a linear manner. In this study, the exit temperature is assumed as a fixed value irrespective of intake temperature. AFT is increased with preheating of charge temperature and hence, the furnace exit temperature probably increased. Dissociation of combustion products also not taken to accounts and it becomes higher at higher combustion temperature. Therefore, the intake charge preheating value is limited to certain limits.
Refer the Dropbox link given below.
https://www.dropbox.com/sh/sfsa56xb5jz3qtq/AAAD6TgLr91MA8IcRnrRzIfNa?dl=0