Enthalpy Change of Combustion of Hydrogen in CFD Simulations
Enthalpy of reaction defines the energy consumed or released during a chemical reaction.
Reacting fluids will have some enthalpy of reaction that modifies ideal fluid flow behavior.
By including heat and temperature, system designers can better model fluid behavior in combustion reactions.
Reactions between fluids are normally taught in simple ways without getting into the finer details of fluid flow and mixing. Students who learn about reacting fluids focus on things like ion exchange, stoichiometry, and enthalpy without worrying about the dynamics of a fluid itself. In real systems, such as in engines or fluid reactors, the motion of the mixing and reacting fluids matters greatly for controlling reaction progress. Combustion is one area where the goal is to produce heat and mechanical energy, so the flow characteristics of the reacting fluids involved need to be understood to ensure maximum system effectiveness.
As a new and developing area of technology, hydrogen combustion promises an alternative to fossil fuels in consumer vehicles and in aerospace propulsion systems. Like any other combustion engine, these systems use a chemical reaction to produce heat that will drive a cylinder, producing the desired mechanical motion. To control the flow characteristics of these combustion systems, the enthalpy change of combustion for hydrogen reactions must be included in CFD simulations for flow behavior.
Knowing the enthalpy change of combustion for hydrogen reactions is important, but what simulation model should be used in CFD simulations? This article will provide some paths forward for selecting the best turbulence model to understand and model combustion reactions, including hydrogen combustion.
Enthalpy Change of Combustion of Hydrogen
Enthalpy is a thermodynamic quantity that is equivalent to the total heat content of a system as measured by a system’s pressure and volume. Enthalpy changes in chemical reactions measure the input heat required to drive the reaction or the heat released during the reaction. The products of the reaction may end up hotter or colder, depending on the sign of the change in enthalpy (energy consumption or loss in the reaction). In the case of compressible fluids, the products in the reaction may expand or contract due to their change in temperature during the reaction. This is the entire basis for internal combustion engines, including hydrogen combustion engines.
The reaction equation for hydrogen combustion is shown below. The reaction is exothermic as indicated by the sign of the enthalpy value.
Hydrogen combustion reaction
The reaction produces liquid water, which is carried away with the leftover unreacted gasses and air as a multiphase flow. During the reaction, gaseous reactants and liquid products can coexist, thus we have a multiphase compressible flow at high temperature and pressure.
The above reaction equation defines the energy released in the reaction on a per molecule (or equivalently per mole) basis. However, this does not tell you the available thermal energy that is released during the reaction, which will then tell you how much mechanical energy you can access from the combustion reaction. What is needed is a CFD model that accounts for heat released during the reaction so that pressure changes in a reaction vessel can be determined.
Selecting a Combustion Model for CFD Simulations
While in principle, it is possible to build a numerical model for a combustion system directly from the full Navier-Stokes equations, the resulting model will likely be very complex and require a lot of computational resources to execute with high accuracy. Therefore, reduced models have been developed to target specific types of flow situations, structures, and asymptotic limits on material or flow behaviors. A basic understanding of some system and material parameters will aid the selection of an appropriate model for use in CFD simulations.
In combustion reactions, there are two parameters that can aid the selection of a reduced set of CFD equations for modeling the turbulent flow field in combustion reactions.
- Reaction rate compared to the fluid flow rate
- The turbulent length scale compared to the reaction kinetics length scale
The former is taken to be much larger than the latter for both points in combustion reactions. These two points are normally applied with the CFD-based definition of enthalpy to develop reduced fluid flow models that account for combustion reactions.
Some Reduced Combustion Models
Some of the best-known models for simulating turbulent combustion in CFD simulations are:
- Progress variable (C) equation (RANS based)
- Extended coherent flame model (RANS based)
- Level set (G) equation (Not RANS based)
- Zimont flame speed model
- Multi-zone extended coherent flame model
These pre-mixed flow models apply to hydrogen engines, natural gas combustion, and other fuel combustion situations where mixing is enforced before or during the combustion reaction.
Some Combustion Is Laminar
Not all flows involving combustion reactions are turbulent. For example, sheet flows involving combustion like bunsen flows and radiant burners are known to operate in the laminar regime. However, for hydrogen-based internal combustion engines and propulsion systems, the turbulent flow characteristics are known to influence exhaust and thermal efficiency. Systems designers working in these areas will need the best applications to accurately model these flows and design the most efficient systems.
Whenever you need to model enthalpy change of combustion of hydrogen in an engine or jet propulsion system design, use the complete set of CFD simulation features in Fidelity from Cadence. With Fidelity, you can examine all aspects of flow behavior using modern numerical approaches. Simulation models and algorithms used in aerodynamics simulations, turbulent and laminar flow simulations, reduced fluid flow models, and much more can be implemented in Cadence’s simulation tools.
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