Procedure for the Calculation of Sampling Line Penetration Functions and Line Loss Correction Factors

Scope:

This SAE Aerospace Recommended Practice (ARP) describes a method for assessing size dependent particle losses in a sampling and measurement system of specified geometry utilizing on the non-volatile PM (nvPM) mass and number concentrations measured at the end of the sampling system. The penetration functions of the sampling and measurement system may be determined either by measurement or by analytic computational methods. Loss mechanisms including thermophoretic (which has a very weak size dependence) and size dependent losses are considered in this method along with the uncertainties due to both measurement error and the assumptions of the method. The results of this system loss assessment allow development of estimated correction factors for nvPM mass and number concentrations to account for the system losses facilitating estimation of the nvPM mass and number at the engine exhaust nozzle exit plane. As the particle losses are size dependent, the magnitude of correction factors can vary as a function of many factors including combustor technology and engine operating condition. Implementation of the nvPM sampling and measurement system for aircraft engine testing, as per ARP6320, requires a sample line of up to 35m and includes several sampling and measurement system components, which result in significant particle loss on the order of 50% for nvPM mass and 90% for nvPM number. The system loss correction factors are estimated based on a model with the following inputs and assumptions: engine exhaust exit plane nvPM have a lognormal distribution, known size dependent values of nvPM effective density and geometric standard deviation, a minimum particle size cut-off of 10nm, and no coagulation.

Rationale:

This Aerospace Recommended Practice describes a method for calculating correction factors to account for system particle losses when performing non-volatile Particulate Matter (nvPM) measurement as specified in ARP6320. Such sampling and measurement systems have significant line length and several components that result in particle losses. The particle losses are size dependent and hence depend on many factors including combustor technology and engine operating condition resulting in a reduction in measurement of the order of 50% for nvPM mass concentration and 90% for nvPM number concentration. Estimation of engine exit plane nvPM mass and number concentrations are improved by developing a calculation method to account for these losses. The approach used in this ARP will involve separate correction factors for measured nvPM mass and number concentrations, which will be calculated using measured or calculated line and component penetration efficiencies. These calculations will be based on assumptions of a lognormal particle size distribution at the engine exit with a known associated lognormal width, and an equivalent spherical particle shape with a corresponding known effective particle density. These resulting correction factors will then be used to estimate the total particle losses in the sampling and measurement system for nvPM mass and number, and will thus be used to infer the engine exit plane concentrations of nvPM mass and number.

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