Prediction of the Thermodynamic Properties of Carbon-Free Flue Gases from Hydrogen-Powered Engines

Bo Chen; Yimin Xuan

doi:10.1016/j.eng.2026.03.006

Engineering ›› :202603006 DOI: 10.1016/j.eng.2026.03.006

Green Aviation Propulsion—Article

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Prediction of the Thermodynamic Properties of Carbon-Free Flue Gases from Hydrogen-Powered Engines

Bo Chen
, Yimin Xuan ^*

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Abstract

Hydrogen-powered engines generate carbon-free flue gases with water vapor as their main combustion product. Under extreme temperature and pressure conditions, water vapor exhibits pronounced non-ideal gas behavior, resulting in thermophysical properties with significant pressure dependency. Concurrently, radiative heat transfer is strongly influenced by pressure-induced spectral line broadening and the high optical thickness that results from elevated water vapor concentrations. Existing thermophysical correlations are generally valid only within limited operating envelopes and lack the theoretical justification needed to reliably extend predictions to high-temperature regimes. Furthermore, conventional weighted-sum-of-gray-gases (WSGG) models rarely account for specific hydrogen-powered engine conditions. Most existing WSGG models employ coupled variables, in which pressure and concentration effects are lumped into a single pressure-path-length product, making them inadequate for capturing complex independent dependencies. In this study, sensitivity analyses of key thermophysical parameters were conducted to identify the parameters’ functional dependence on temperature and pressure, yielding predictive models for specific enthalpy, heat capacities, and viscosity that enable physically consistent extrapolation to temperatures exceeding 2000 K. For radiative characteristics, a unified hydrogen-oriented WSGG (H-WSGG) framework was developed that decouples the nonlinear influences of water vapor concentration and total pressure, treating temperature, pressure, concentration, and path length as fully independent variables. Unlike conventional methods, these nonlinear effects are explicitly integrated into the absorption coefficients and weighting factors. Based on this framework and the Brayton cycle characteristics, two practical models were derived: the constant-pressure H-WSGG-C model and the constant-concentration H-WSGG-T model. Validation against original data and line-by-line (LBL) calculations under non-isothermal and non-uniform conditions demonstrate that the proposed thermophysical models achieve high accuracy and the H-WSGG models exhibit strong agreement with LBL benchmarks.

Keywords

Carbon-free flue gases / Thermal radiation / Thermal properties / WSGG

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Bo Chen, Yimin Xuan. Prediction of the Thermodynamic Properties of Carbon-Free Flue Gases from Hydrogen-Powered Engines. Engineering 202603006 DOI:10.1016/j.eng.2026.03.006

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