Steam penetration in long, narrow channels during steam sterilization: A combined study using wavelength modulation spectroscopy and CFD

• Measurement of the absolute water vapor mole fraction during a dynamic sterilization cycle with high temporal resolution at the end of a long, narrow channel. • Results indicate a nonlinear pressure-dependence of the water vapor self broadening parameter outside the isolated lines assumption. • By adding a plateau phase at low pressure, the water vapor mole fraction at the end of a 1 m long pipe was increased by 0.3 compared to the original autoclave cycle. • Measurement setup can be mounted on any autoclave by using the required test connection (DIN EN 13060:2014 and DIN EN 285:2015) Effective steam sterilization requires the complete removal of non-condensable gases, which is especially challenging in long, narrow channels commonly found in medical devices. To better understand the water vapor flow in such passages, both experimental measurements using wavelength modulation spectroscopy (WMS) at 1364 nm and numerical simulations with computational fluid dynamics (CFD) were conducted. The measured WMS signals at water vapor partial pressures below 90 kPa could be well reproduced with a recently published spectral database using quadratic speed-dependent Voigt line profiles. However, at higher partial pressures the database values failed to predict observed line shapes. Therefore, line-profile coefficients of two transitions were updated with a nonlinear pressure-dependence of the self broadening and by including first-order collisional line mixing, which reduced the residuals between the two signals significantly. CFD simulations reproduced the measured water vapor mole fraction with good accuracy. Additionally, modifying the sterilization cycle by introducing a low-pressure holding phase enhanced steam penetration at the pipe end by 0.3 in mole fraction without requiring additional water or energy. This work demonstrates a validated approach for characterizing and enhancing steam penetration in long, narrow channels, with potential to improve the sterility of medical devices and make their reuse safer for patients.