Date of Award
Doctor of Philosophy (PhD)
P. E. Wood
Reaction injection moulding (RIM) is commercially used in industry for making polymer parts. In this process, two or more jets produce impingement mixing of low viscosity monomers or oligomers in a mixhead. In order to better understand an impingement mixer, it is essential to understand the flow field created by the opposed laminar jets in a similar configuration. In this thesis, an experimental investigation and analysis of the flow field created by the two equal and opposed cylindrical laminar jets impinging near the closed end of a confined space (mixhead) has been conducted. Three important flow and geometrical parameters which influence the flow field are considered. These are (i) jet Reynolds numbers (Re), (ii) H/D, a ratio of the position (H) (the distance in - Z direction from the impingement point to the closed end) to mixhead diameter (D), and (iii) viscosity of the fluid used. Both qualitative and quantitative studies have been performed using an experimental model of the mixhead which simulates a RIM type configuration. For qualitative analysis, two flow visualization methods, namely, dye injection and particle tracing methods have been used. For quantitative analysis, the laser Doppler anemometer (LDA) technique has been used to quantify the velocity fields (only U and V components).
It has been found that Re shows a significant effect on the flow field. For Re< 50, flow remains in a statically stable mode and rotating vortices are formed near the impingement plane. At slightly higher Re (≥50), the flow pattern changes to dynamically stable mode and the impingement plane oscillates. At Re≥90, the flow attains an unstable mode. In fact, the vortices gradually grow with increasing Re and at a critical value of Re they dynamically interact with each other causing oscillations about the impingement plane. For both static and dynamic stable zones, the length of circulating region increases as a function of Re (l/D = a - be⁻ᶜᴿᵉ) between 50 to 90. For Re > 90, the recirculating region shrinks before breaking into an unstable zone at a Re of approximately 125.
The piston position (H/D) plays an important role in the stability of the flow pattern. An unstable flow pattern transforms to a steady flow pattern when the piston is moved down from H/D = -0.5 to -0.75 at Re = 150. Furthermore, it appears that the flow patterns are always unstable at very high Re values since no stable patterns could be obtained up to HID = -1.0 at Re = 200. In stable modes, flow fields of both side of impingement plane are like a mirror image of each other. It appears that viscosity of the fluid has no significant influence on the flow patterns at constant Re. The vector and streamline plots obtained by LDA for Re = 50, and 150 agree with the flow pattern obtained with flow visualization methods. Also, the experimental measurements confirm that the flow in the vortex regions is 3-dimensional.
Vigorous fluctuations are evident up to one-half mixhead diameter above the impingement point. The axial velocity (V) along the mixhead axis reaches a velocity profile of a developed one dimensional steady flow within 1 to 1.5 mixhead diameters. However, the radial velocity (U) along the mixhead axis reduces to an insignificant value at a distance equal to approximately two jet-diameters above the impingement point. The frequency analysis of the time series of velocity components showed higher harmonics as the Re increases from Re = 50 to 150. The periodic structure completely vanishes at Re = 200. The periodic nature of time series which vanishes at Re = 200 for H/D = 0, vanishes at Re = 150 when H/D = -0.5. This shows that H/D ratio also influences the breakdown of any periodic nature in the flow field.
Tyagi, Ashok K., "Jet to jet impingement in a confined space" (1997). Open Access Dissertations and Theses. Paper 3405.