A Combined Modeling and Experimental Investigation of Nan-Particulate Transport in Non-Isothermal Turbulent Internal Flows.

Abstract

Particles in particle-laden flows are subject to many forces including turbulent impaction, Brownian, electrostatic, thermophoretic, and gravitational. Our scaling analysis and experiments show that thermophoretic force is the dominant deposition mechanism for submicron particles. One common example of industrial devices in which thermophoretic particle deposition occurs is exhaust gas recirculation (EGR) heat exchangers used on diesel engines. They are used to reduce intake charge temperature and thus reduce emissions of nitrogen oxides. The buildup of soot particles in EGR coolers causes a significant degradation in heat transfer performance (effectiveness) generally followed by the stabilization of cooler effectiveness (no more degradation) for longer exposure times. To investigate the initial sharp reduction in cooler effectiveness, an analytical solution, computational one dimensional model and an axi-symmetric model are developed to estimate particulate deposition efficiency and consequently the overall heat transfer reduction in tube flows. Internal flows (tube/channel) are employed in this dissertation to resemble real EGR coolers. The analytical solution is employed for a parametric study and sensitivity analysis to highlight the effect of critical boundary conditions. The computational models are developed to solve the governing equations for exhaust flow and particles. Model output including predicted mass deposition along the tube and the tube effectiveness drop has been compared against experiments conducted at Oak Ridge National Laboratory with good accuracy. CFD models improve the output compared to the analytical solution while the axi-symmetric model is significantly closer to the experiments due to accurate calculations of near wall fluxes. Mechanisms responsible for the cooler effectiveness stabilization in long exposure times are not clearly understood. To address the stabilization trend, a visualization test rig is developed to track the dynamics of particulate deposition and removal in-situ, and a digital microscope records any events. Interesting results are observed for flaking/removal of the deposit layer at various boundary conditions. In contrast to conventional understanding, large particles (tens of microns) were also observed in diesel exhaust. Water condensation occurring at a low EGR cooler coolant temperature resulted in a significant removal of deposit in the form of flakes while thermal expansion alone did not remove the deposit layer.