Session: ASME Undergraduate Student Design Expo

Paper Number: 172904

Chemical Equilibrium-Based Combustion Modeling of Low-Gwp Refrigerants 

Global climate regulations necessitate the phase-in of lower global warming potential (GWP) hydrofluoroolefin (HFO) refrigerants. However, the formulation of these refrigerants results in a greater flammability as a trade-off to lowering their GWP. To accelerate an understanding of the combustion behavior of newly developed fluorinated refrigerants and refrigerant blends, a chemical equilibrium-based thermodynamic model for adiabatic flame temperature and pressure that operates under constant pressure or constant volume conditions was developed. The model accounts for initial temperature, pressure, air-fuel ratio, and relative humidity, and is refrigerant- and mass-composition general. These parameters make it applicable to a significant number of environmental conditions, including real-world experiments and laboratory testing scenarios. In addition, this model calculates combustion product concentrations as a supporting mechanism, which can be helpful in evaluating the safety of novel refrigerants and refrigerant blends.

The model was developed using both the MATLAB and GNU Octave computational programs and relied on iterative numerical methods due to a high degree of nonlinearity. The fmincon optimization function in addition to exterior convergence techniques were utilized to increase the repeatability, robustness, and accuracy of the model. To quantify the model’s accuracy, adiabatic flame temperature values were compared to experimental literature values for several refrigerants and refrigerant blends, equivalence ratios, and initial conditions. Even though the model’s complexity is significantly lower than a full chemical kinetics-based model, predicted adiabatic flame temperatures are still accurate to within 100 K of experimental data, and can be used as a screening tool for the consideration of new refrigerants and refrigerant blends without any experimental testing or advanced computation. 

A second outcome of this project is its educational impact. Because it is a chemical equilibrium-based model, the equations it is based upon can be derived from fundamental thermodynamic principles rather than the higher-level calculations seen in homogeneous chemical kinetics-based combustion models. The thermodynamic equations used for the development of the model were conservation of mass, conservation of energy, minimization of Gibbs or Helmholtz free energy, and the ideal gas law. The use of these fundamental principles lowers the barrier of entry for otherwise complex topics in the field of thermodynamics, such as combustion and flammability. An open source version of the model is additionally available through the GNU Octave platform, further lowering the barrier of entry and allowing students to explore thermodynamically nuanced case studies for refrigerant combustion. 

One outcome of this model’s development was the accumulation of a considerable amount of thermodynamic data for novel pure refrigerants. Heat of formation values, NASA Chemkin Polynomial coefficients, and GWP values were gathered for 22 modern-age refrigerants. These data were compiled to act as a thermodynamic database for newer-age HFOs to expedite more in-depth research into refrigerant flammability behavior, especially as newer HFO-based refrigerants and refrigerant blends are developed.

Presenting Author: Adam Smith University of Kansas

Presenting Author Biography: Adam Smith is a rising senior studying Mechanical Engineering at Arizona State University. He worked with Dr. Christopher Depcik at the University of Kansas this summer within the EARTH NSF ERC through the REU program on refrigerant combustion modeling. He also has research experience at Arizona State University under Dr. Sui Yang in metasurface development and characterization for optical applications in the renewable energy field.

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