A Study of Alternative Working Fluids and Corresponding Effects for Condensation Nuclei Counters Applied on Automotive Exhaust

Introduction: An established measurement device for particle number (PN) concentrations in automotive exhaust gases in scientific as well as for industrial applications are condensation nuclei counters (CNC), which typically use butanol as a working fluid. With the inauguration of partly biogenic fuels some new effects like strong system drifts and shortened lifetimes of wicks occurred. These effects can partly be related to carboxylic acid butanol ester chains resulting from the imperfect burning of biogenic components of the fuel and can be avoided by the use of a suitable working fluid instead of butanol.

Methods: A pre-selection of 88 working fluids was done considering critical parameters as e.g. diffusivity, kelvin diameter, dielectric constant, vapour pressure, etc. and supported by CFD simulations and practical tests under laboratory conditions n-decane and n-dodecane were selected as most suitable candidates. Based on literature research regarding capillary condensation, a theoretical model was set up to describe the processes during the nucleation event in a CNC for vapours of different fluids.

Results: Tests with a CNC using n-decane in comparison to one using butanol showed at least comparable, or even superior counting efficiency for the decane CNC using soot as condensation nuclei. Theoretical considerations reveal until now a strong dependence of the kelvin diameter to the morphology of the particle but on the other hand suggest no or just a weak dependency of the working fluid.