The thermophoretic deposition of aerosol particles in mixed-convection,laminar tube flow with a cooled wall is considered.The presence of free convection alters the velocity and temperature profiles for laminar tube flow relative to the constant-properties case.Previous modeling results (Walsh,Weimer,and Hrenya,in press),show that the modified flow fields lead to changes in the cumulative deposition-efficiency profiles (i.e.,deposition as a function of axial distance).In particular,deposition in vertical,downward flow is higher at short axial distances as compared to the upward counterpart,while overall deposition efficiencies (i.e.,total deposition in the tube) for both cases are closer in magnitude.In this study,experimental data is obtained in an attempt to qualitatively and quantitatively validate the model predictions for the effects of free convection on cumulative deposition efficiencies.To explore the effects of free convection,two cases are considered:(1) comparison of systems with a high and low Richardson number for vertical,downward flow,and (2) comparison of upward and downward flow for the case of a high Richardson number.Experimental results for the two cases qualitatively agree with model predictions.In particular,there is higher deposition at short axial distances in downward flow than upward flow.In Walsh et al.(in press),modeling predictions showed that the initial increased deposition in downward flow is due to increased residence time.In addition,further evidence is shown of the validity of using the local Richardson number (in addition to the bulk Richardson number) as a predictor of the effects of free convection on a system.
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