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Hannah Goldstein

A Computational Study of the Effects of a Baffle and Shroud System on a Two-Cylinder Heat Exchanger During the Discharge of a Solar Thermal Storage Tank

Department: Engineering Studies
Advisor: Dr. Julia Nicodemus and Dr. Joshua Smith

Abstract:
In recent years, the concentrations of greenhouse gases released in the Earth's atmosphere continue to reach new levels due to human activity. Therefore, there is a need to lower natural gas, propane and coal use. An alternative to these environmentally dangerous energy sources is solar thermal energy, which can be used for domestic hot water and space heating systems. Improving the efficiency of these systems would make them a stronger alternative energy source for domestic and commercial use.

Storing the captured thermal energy from a solar thermal system is often done in a large hot water tank, from which the charging and discharging of the energy must be ecient. The focus of this research is to increase the heat transfer from the storage fluid to a coiled copper tube heat exchanger located at the top of the tank. Cool water is circulated through the heat exchanger during the discharge case. Heat transfer on the storage side of the heat exchanger occurs via natural convection. As the discharge process proceeds, negatively buoyant plumes form in the boundary layer at the external surface of the heat exchanger and descend through the tank [2].

There are two approaches of passively increasing heat transfer to the tank by controlling the fluid motion. The first approach is to maintain a large temperature difference between the heat exchanger and the ambient fluid for as long as possible. The second approach is to increase the velocity of the fluid around the outside of the heat exchanger. Several studies have investigated the use of baffles or baffles and shrouds to passively increase heat transfer to an immersed heat exchanger [3, 4, 5]. A baffle is a cylindrical shell that creates an open-ended annulus below the heat exchanger. Baffles isolate the falling plume from the storage fluid in the center of the tank [4]. A shroud can be used to isolate the heat exchanger itself from the bulk storage fluid and direct the plume into the baffle, causing the speed of the flow to increase locally and thereby increasing the heat transfer [6]. The shroud and baffle together maximize the velocity across the outer surface of the heat exchanger.

Previous experimental studies have concluded that the use of a shroud and baffle can passively increase heat transfer in a thermal storage tank by increasing the velocity over the heat exchanger [3, 4, 5]. Several computational studies have identified optimal dimensions of the shroud and baffle to improve heat transfer [1,2, 6]. The usefulness of the conclusions of the previous computational studies is limited by the simplications and assumptions used to model this complex flow phenomenon. These studies use a two-dimensional model with an infinitely long single cylinder, which is quite different from an actual tank. Therefore, this numerical study aims to reduce the discrepancy between a real storage tank and a numerical model. The research uses an axi-symmetric model instead of a two-dimensional planar section of the tank, in order to reflect the three-dimensional cylindrical tank used in experimental studies. Further, this research models the heat exchanger as two looped cylinders as opposed to a single infinitely long cylinder to capture the interactions between the plumes. The performance of the model was tested against a single cylinder model of the heat exchanger for both a straight baffle/shroud and a complex baffle/shroud. The straight baffle/shroud creates a long vertical cylindrical annulus of width 2D around and below the heat exchanger. The complex baffle/shroud also creates a long vertical cylindrical annulus. However, the shroud annulus around the heat exhanger has a width of 2D while the baffle annulus below the heat exhanger has a width of 0.75D. The shroud and baffle are connected by a fillet that has a radius of curvature of 1D.

It is expected that the presence of two loops of the heat exchanger will cause an increase in the heat transfer performance in comparison to a single loop heat exchanger, independent of the baffle/shroud configuration. This result is predicted because the plume from the upper cylinder will force an additional amount of fluid flow over the second cylinder, which will speed up the flow locally, thereby increasing the heat transfer performance. It is expected that heat transfer performance of the complex baffle/shroud will out perform the straight baffle/shroud, independent of the number of cylinders representing the heat exchanger, due to the constriction on the fluid flow, which causes an increase in velocity over the heat exchanger.

References:

  1. Sandra K.S. Boetcher, F.A. Kulacki, and Jane H. Davidson. Negatively buoyant plume ow in a baed heat exchanger. Journal of Solar Energy Engineering, 132:1-7, 2010.
  2. Sandra K.S. Boetcher, F.A. Kulacki, and Jane H. Davidson. Use of a shroud and bae to improve natural convection to immersed heat exchanger. Journal of Solar Energy Engineering, 134:1-7, 2012.
  3. Julia F. Haltiwanger and Jane H. Davidson. Discharge of a thermal storage tank using an immersed heat exchanger with an annual bae. Solar Energy, 83:193-01, 2009.
  4. Jackson Jerey. An investigation of the eects of a bae and shroud on an immersed heat exchanger during the discharge of a solar thermal storage tank. Honors thesis, Lafayette College, 2016.
  5. Yan Su and Jane H. Davidson. Natural convective ow and heat transfer in a collector storage with an immersed heat exchanger: Numerical study. Journal of Solar Energy Engineering, 127:324-332, 2005.
  6. Matthew K. Zemler and Sandra K.S. Boetcher. Investigation of shroud geometry to passively improve heat transfer in a solar thermal storage tank. Journal of Solar Energy Engineering, 136:011017, 2014.

About Hannah Goldstein:
Hannah Goldstein is a senior Engineering Studies and Mechanical Engineering student at Lafayette College. Her main academic interests are in energy systems and energy policy. Next fall she will be attending graduate school for an M.S. in energy systems. In addition to her academic pursuits, Hannah is a member of ASB, Lafayette’s pep band and other music ensembles, and is a Lafayette Ambassador.