Highly Transient High Speed Jet Flow From Different Orifice Geometries

Abstract

Abstract The majority of the many types of different industrial flows are not in the laminar flow regime, but rather these flows are well beyond laminar and continue to exceed the turbulent flow transition criterion for internal and free shear flows to be fully turbulent and highly unsteady involving the transfer of fluid through circular conduits or round pipes as well as other conduit geometries including the issuing of jet flows into some type of ambient environment. High speed jet flows have a wide range of applications in many areas of engineering. The understanding of jet flow theory has progressed substantially. However, there is a very little investigation into the transient nature of the high-speed jet flows and how the structure of these jet flows differs according to the geometry of the orifice out of which the jet flow emanates. The transient nature of these jet flows allows the applications into which they are installed to be optimized according to the characteristics of the jet flow and the configuration of the system. The focus of the present work is to characterize transient high speed jet flows from the differing orifice nozzle geometries and the introduction of a swirling motion into the jet flow and how this affects the characteristics of the jet flow from the reference jet flow that is free from swirling motion momentum. Another important but niche or specialised jet flow application is in reverse pulse-jet (RPJ) cleaning systems, of which is the focus application of the present work. A typical RPJ cleaning system consists of three main components: compressed air supply, valve and blowtube. The blowtube is the name given to the pipe connected to the valve and configured with a number of outlets or orifices where the flow exits into a plenum thereby entraining and inducting flow into a filter that is aligned with the orifice from which the jet flow issues. In the present work, improvements are sought from the blowtube or more specifically the exit pipe orifices for a more efficient operation of a well-designed cleaning system. The present paper will discuss and compare the flow through a number of different orifice geometries for the type flow that is typically experienced in this type of application. The operation of a single event or an actuation of the pulse-jet valve, is extremely rapid; typically approximately 300 ms. The valve is actuated and the diaphragm moves and allows the compressed air to travel from the pressure vessel or header tank through the valve past the valve seat into the blowtube and exits through plain orifices or nozzles. The extremely rapid event generates highly transient, highly turbulent free shear jet type flow from either the plain orifice or nozzle with a circular orifice geometry through which the flow exits. Advancements made by the author in subsonic flows and high-speed gas dynamic flows could provide not only improvements to the flow but further insight to the physics of high speed flows in particular around pipe exit orifices. This investigative study of the jet flow was based upon a computational analysis. It was shown that the base reference case of the jet flow that was solely a non-swirling flow although the jet flow was highly transient in nature that the centreline velocity of the jet flow had variability from the inner core to the outer extremities of the jet flow. The comparison of the base reference case with swirling jet flow will produce a longer coherent jet flow using the different orifice geometries. The stability of the jet flow was improved with the introduction of the swirling motion to the jet flow. Future developments of the transient nature of the jet flow will include experimental studies to verify the flow control methods that were used in the swirling jet flow cases.