Abstract
Abstract. This work describes the latest design, calibration and application of a near-infrared laser diode-based photoacoustic (PA) hygrometer developed for total water content measurement in simulated atmospheric freezing precipitation and high ice water content conditions with relevance in fundamental icing research, aviation testing, and certification. The single-wavelength and single-pass PA absorption cell is calibrated for molar water vapor fractions with a two-pressure humidity generator integrated into the instrument. Laboratory calibration showed an estimated measurement accuracy better than 3.3 % in the water vapor mole fraction range of 510–12 360 ppm (5 % from 250–21 200 ppm) with a theoretical limit of detection (3σ) of 3.2 ppm. The hygrometer is examined in combination with a basic isokinetic evaporator probe (IKP) and sampling system designed for icing wind tunnel applications, for which a general description of total condensed water content (CWC) measurements and uncertainties are presented. Despite the current limitation of the IKP to a hydrometeor mass flux below 90 gm-2s-1, a CWC measurement accuracy better than 20 % is achieved by the instrument above a CWC of 0.14 g m−3 in cold air (−30 ∘C) with suitable background humidity measurement. Results of a comparison to the Cranfield University IKP instrument in freezing drizzle and rain show a CWC agreement of the two instruments within 20 %, which demonstrates the potential of PA hygrometers for water content measurement in atmospheric icing conditions.
Highlights
In this work we describe the latest design, preliminary calibration, and basic properties of a new PA hygrometer and two-pressure humidity generator, developed with the goal of providing the total water measurement and calibration ranges typical for simulated atmospheric icing conditions applied in aviation testing and certification
Only a single hygrometer used for total water (TW) measurement was available, the isokinetic evaporator probe (IKP) was used without the rearward-facing probe and background water vapor (BWV) was estimated from icing wind tunnels (IWTs) humidity sensors
A hygrometer based on intensity-modulated photoacoustic spectroscopy with a near-infrared laser diode was developed and combined with a two-pressure humidity generator and an isokinetic evaporator probe to provide a new instrument capable of measuring total or background water content in simulated atmospheric icing conditions
Summary
Atmospheric water in the form of clouds and precipitation is of particular concern to aviation at temperatures below freezing, as supercooled liquid water and ice crystal environments present potentially hazardous conditions to aircraft, leading to airframe and air data probe icing (Vukits, 2002; Gent et al, 2000) or in-flight engine power loss (Mason et al, 2006). Both methods are either known or suspected to suffer from size- and water-content-dependent inaccuracies in large drop or ice crystal icing environments due to uncertainties in collection efficiency and mass losses before accretion or evaporation (Cober et al, 2001a; Strapp et al, 2003; Emery et al, 2004; Isaac et al, 2006; Korolev et al, 2013; Steen et al, 2016) This situation has led to the development of new benchmark isokinetic evaporator probe (IKP) instruments for CWC measurement (Davison et al, 2008; Strapp et al, 2016), regarded as closest to a first principles measurement and primarily designed for and deployed in the characterization of high IWC mixed-phase and glaciated conditions (e.g., Ratvasky et al, 2019).
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