Determining the mixing length for hydrogen injection into natural gas pipelines

Injecting hydrogen into natural gas pipelines offers promising opportunities for storage, transportation, and
decarbonisation. However, gas mixing does not occur instantaneously, which may pose challenges for pipeline
safety, custody transfer, and end-use applications. This study performs numerical simulations to model the
minimum pipeline length required to attain homogeneous gas mixing. The analysis considers direct injection
within a straight pipeline across a variety of injection and flow conditions, including the diameter ratio between
natural gas and hydrogen pipelines, the hydrogen blending ratio, the natural gas mass flow rate, injection
orientation, and pipeline temperature and pressure. Mixing is quantified by the coefficient of variation of
hydrogen molar concentration, plotted against flow distance for various dimensionless parameters, such as the
momentum flux ratio (M), Reynolds number (Re), Froude number (Fr), and Grashof number (Gr). A simplified
correlation incorporating these dimensionless parameters has been developed using linear regression, and is
calibrated for the ranges 0.01 < M < 20, 1.1 × 104 < Re < 8.5 × 104, 0 < Fr < 0.84, and 3 × 108 < Gr < 1.6 ×
109. These ranges correspond to fully turbulent, moderate-pressure flow conditions, which are highly relevant to
practical hydrogen–natural gas distribution networks.