Comparing Velocity Profiles Along the Rod Length of a Helical Coil Steam Generator Model
A model of a specific type of tube and shell heat exchanger, the helical coil steam generator (HCSG), was designed and constructed to study multiple fluidic properties that attribute to the efficiency of the heat transfer it provides. Tube and shell heat exchangers are currently used throughout industry in chemical, power and petroleum engineering. Most commonly, a primary flow is circulated through a shell, while the secondary flow is run through several small tubes that are bundled together. Methods to increase the heat transfer between the two flows include adding baffles to force shell side flow over more tubes, fins added in-between layers of tubes and passing tubes multiple times through the shell to increase heat exchanger effectiveness. The HCSG incorporates many of the proposed methods but also relies on a complex geometry to increase the heat transfer between tube and shell side flow. In an area where cost effectiveness is favorable, such as inside a nuclear reactor, helically banked steam generators makes extensive use of the room with no sharp tube bends. This increases the pressure loss on the secondary flow and can solve potential temperature manufacturing issues at the welds of junctions. Additionally, they have exhibited pure cross flow behaviors with high heat transfer coefficients and axial temperature profiles that correspond to pure counter-flow with high mean temperature differences. Early work showed that helical banks can have a significant increase of 16 to 43% higher heat transfer coefficient than straight pipe heat exchangers. While many previous studies have looked at the heat transfer, pressure drop and flow fields of a tube and shell heat exchanger, they all focus on tube bundles that are either in-line or staggered. The early flow visualization studies on these types of bundle arrangements reported that attached eddies were formed alternatively behind each of the rods and were occasionally dragged into the main stream of shell side flow. Later studies suggested that observable periodic vortex shedding was dependent on geometry. Experimental data taken using techniques such as laser Doppler anemometry (LDV) by Simonin and Barcouda (1986) as mentioned in Hassan (2002), hot-wire anemometry with Tsuchida (1982) and particle image velocimetry (PIV) conducted by researchers as late as 2007 have been used to characterize turbulent flow and distinguish flow field structures within in-line and staggered tube bundles. By studying these flow field structures, their contribution to fluidic properties such as heat transfer efficiency and pressure drop can be characterized effectively. Literature review lead to a design that incorporated five layers of coils wrapped around one another. The outermost and middle layers of tubes coiled at a specific pitch in a clockwise direction while the other two layers coiled counterclockwise. The proposed design incorporates a constantly changing lateral pitch ratio, a, between adjacent rods. The constructed test section isolates a single interface between two adjacent coiling rod bundles of twelve rods. Three distinct cross sectional planes along the rod bundles were looked at to study the effect of the local geometry to flow fields, Section A, B and C. Particle Image Velocimetry (PIV) frames of 896 x 848 pixels were captured at each section across a specific height. Profiles of mean velocity fields for each section are compared and discussed as possible features of the changing lateral pitch ratio.