Research On Nanoparticle-laden Two-phase In Curved Pipes

The system of particles in the order of nanometer suspending in fluid can be described as nanoparticle two-phase system. In this system, the Saffman force and Magnus force which were more concerned in the conventional multiphase flow researches are relatively not important, but the intermolecular van der Waals force and the Brownian motion caused by collisions, etc., which were often neglected before, need to gain more consideration. Thus, there are more difficulties for the theoretical studies of nanoparticles two-phase system. And because of the tiny quality and size of nanoparticles, even below the diffraction limit of (visible) light, it would be impossible to monitor the nanoparticle processes such as deposition directly in experiment.After an analytical overview of current domestic and international researches on the nanoparticle two-phase flow, this doctoral theses focuses on the nanoparticles suspending in the curved pipes, which are widely used in industrial applications. The nanoparticles in a rotating (or not) curved pipe with a circular and rectangular cross-section have been firstly researched by employing numerical simulation (mainly) and theoretical analysis (aided). We discussed in detail the influence of a number of non-dimensional parameters such as non-dimensional curvature, Reynolds number, Schmidt number and F number on the nanoparticles mass distribution on the pipe section, deposition enhanced factor onto the pipe wall, deposition efficiency, and the evolution of the particle size and the geometric standard deviation in space and time scales. A number of important results have been achieved.The numerical and analysis results for nanoparticles in a circular curved pipe are as follows: (1) When the control parameters are relative small, the nanoparticle distribution pattern is similar to the axial velocity; and when the parameters are median, the pattern is symmetrical with respect to the top and the bottom sides of the bend, and there are a series of concentric close line placed in both the top and bottom semicircles. (2) As Schmidt number (both small and median value) increases, the nanoparticle mass fraction decreases, especially in the maximum region. But it has little influence on the distribution pattern. (3) For both small and median parameters, the particle deposition at the outside edge is the most intensive, while the deposition at the inside edge is weakest.The research on the nanoparticle Brownian coagulation in circular curved pipes shows: (1) The region of high mass concentration tends to be elongated and shifts to be outside of bend tube due to the centrifugal force and the presence of secondary flow. (2) The distributions of other investigated particle dynamics including number concentration, second moment and particle size have the nearly same characteristics with the mass concentration. (3) The increase of Reynolds number leads to the decrease of particle coagulation probability over the entire cross-section, and so the particle growth. (4) The geometric mean deviation for particle size distribution increases with decreasing Reynolds number and initial particle size.The influences of the rotating are as follows: (1) When the pipe co-rotates, the distributions of nanoparticle mass fraction are similar to that for the stationary case. When the pipe counter -rotates, the Coriolis force pushes the region with high value of nanoparticle mass fraction towards inside edge of the bend. (2) When the pipe co-rotates or keeps stationary, there is a "hot spot" deposition region near the outside edge. When the pipe counter -rotates, the "hot spot" point appears at the inside edge. (3) The co-rotation of the pipe makes the particle deposition efficiency a reduction, while high counter-rotation of the pipe only slightly affects the deposition efficiency. When two kinds of secondary flows are coexisting, the relative deposition efficiency is larger than that for the stationary case.We used Perturbation method to obtain the analytical expression of the deposition enhancement factor on the pipe wall and the curvature relative deposition efficiency, and found that the curvatures of tube, the Reynolds number and the Schmidt number have effects of second order, forth order, and first order on the relative deposition efficiency, respectively. We firstly employed a new moment method called Taylor-Expansion Moment Method (TEMOM), which ensures the accuracy and has a good balance between efficiency and computational cost, to study the coagulation of nanoparticles associated with Brownian motion in circular curved pipes. The results provide a numerical basis for a better understanding of the dynamic behavior of particles, and also provide a feasible direction of the follow-up studies on particle collisions or the growth of the particle surface.