Development and characterization of a laminar aerosol flow tube

Abstract

We have developed a new laminar aerosol flow tube (AFT) to study transformations such as ice nucleation, deliquescence, and efflorescence in model atmospheric aerosols. The apparatus consists of four sections which can be independently cooled to reproduce temperature profiles relevant to the troposphere and stratosphere. An automatic control system maintains the average axial temperature along each section between 100 and 300K, within ±0.1K. Changes in aerosol composition, phase, and size distribution are monitored at the tube exit using infrared spectroscopy (AFT-IR). We used computational fluid dynamics simulations to investigate flow velocity and temperature distributions within the flow tube. Based on these computations, the final design was formulated to eliminate turbulent mixing zones and buoyancy-driven convection cells. The latter can occur even under conditions where the Reynolds number indicates laminar flow. In either case, recirculation causes aerosol residence times and temperature histories to be poorly defined, leading to erroneous interpretation of experimental measurements. The resulting AFT design used copper fins to reduce temperature gradients and axial mixing of aerosol and carrier gas flows in the inlet section to reduce turbulence. The performance of the new AFT is significantly better than for previous designs.

A New Aerosol Flow System for Photochemical and Thermal Studies of Tropospheric Aerosols

“…Thus, natural convection dominates radial movement as the aerosol/air mixture moves down the reaction/photolysis section of the flow tube (Khalizov et al 2006a(Khalizov et al , 2006bIncropera 2007). As a result, convective cells are expected which, while promoting good mixing of reactants, will also increase the residence time.…”

“…There is also a short elbow constructed of 3.2 mm stainless steel tubing that passes through the side of the 1.90 cm tube at a location exterior to the endplate of the flow tube. This elbow is directed co-axially downstream (see Figure 1b) in order to minimize recirculation within the inlet (Khalizov et al 2006b), and it is used for the addition of aerosol particles. A significant portion of the carrier gas flow is added through a 6.35 mm Swagelok fitting at the head of the 1.90 cm tube, and additional reagents could also be included in this flow.…”

“…The system also exhibits forced convection in which motion of air is caused by external sources such as the pump at the end of the flow tube. In order to determine the dominant mechanism, the dimensionless Richardson number is introduced, Ri = (gβ TD)/U 2 where g is the acceleration due to gravity, β the thermal expansion coefficient of air, T the temperature difference between the lamp housing and room temperature, and D the diameter of the flow tube (Khalizov et al 2006a(Khalizov et al , 2006b). The Richardson number quantifies the ratio of natural convection to forced convection.…”

“…Flow systems are readily disassembled for cleaning and provide access to different reaction times in one experiment via sampling from ports at different locations along the length of the flow tube. Experimental conditions can be rapidly modified, and temperature and total pressure control more readily provided than is the case for the large chambers (Khalizov et al 2006a;2006b). However, sampling volumes are limited and the short total reaction times are not sufficient for studying slower processes that occur over time periods of hours or more.…”

The Caltech Photooxidation Flow Tube reactor: design, fluid dynamics and characterization

“…Such reactors comprise both chambers and flow reactors. The flow tube reactor has emerged as a widely used platform (Bruns et al, 2015;Chen et al, 2013;Ezell et al, 2010;Kang et al, 2007Kang et al, , 2011Karjalainen et al, 2016;Keller and Burtscher, 2012;Khalizov et al, 2006;Lambe et al, 2011aLambe et al, , b, 2012Lambe et al, , 2015Li et al, 2015;Ortega et al, 2013Ortega et al, , 2016Palm et al, 2016;Peng et al, 2015Peng et al, , 2016Simonen et al, 2016;Tkacik et al, 2014). The flow tube reactor is generally operated under steadystate conditions.…”