Thermal Characterization of Protypical ICS Systems with Immersed Heat Exchangers
W. Liu, J. H. Davidson, and F. A. Kulacki
Natural convection is measured in an enclosure that represents an integral collector storage system (ICS) with an immersed tube-bundle heat exchanger. Heat transfer coefficients for bundles of 240 tubes contained in a thin enclosure of aspect ratio of 9.3:1 and inclined at 30 degrees to the horizontal are obtained for a range of transient operating modes and pitch-to-diameter ratios of 1.5, 2.4, and 3.3. Results for isothermal and stratified enclosures yield a correlation for the overall Nusselt number,
NuD = (2.45±0.03)RaD0.188, 230 < RaD < 9800.
The characteristic temperature difference in the Rayleigh number is that between the average water temperature within the bundle and the tube wall temperature. Nusselt numbers are three times larger than those for a similarly configured single-tube and an eight-tube bundle. This increase is attributed to stronger fluid motion within the bundle and higher overall large scale circulation rates in the enclosure.
Natural Convection from a Tube Bundle in a Thin Inclined Enclosure
W. Liu, J. H. Davidson, and F. A. Kulacki
Natural convection heat transfer coefficients for a rectangular array of eight tubes contained in a thin enclosure of aspect ratio 9.3:1 and inclined at 30 degrees to the horizontal are measured for a range of transient operating modes typical of a load side heat exchanger in unpressurized integral collector-storage systems. Water is the working fluid, and thermal charging is accomplished via a constant heat flux on the upper boundary. All other boundaries are well insulated. Results for isothermal and stratified enclosures yield the following correlation for the overall Nusselt number:
NuD = (0.728 ± 0.002)RaD0.25, 4.0 x 105 < RaD < 1.4 x 107.
The flow field in the enclosure is inferred from measured temperature distributions. The temperature difference that drives natural convection is also determined. The results extend earlier work for the case of a single tube and provide limiting case heat transfer data for a tube bundle that occupies the upper portion of the collector storage.
Natural Convection of a Horizontal Tube Heat Exchanger Immersed in an Integral Collector Storage
Wei Liu, Jane Davidson, and Francis Kulacki
The natural convection heat transfer coefficient from a single horizontal tube immersed in a tilted water-filled enclosure was measured under a variety of operating conditions. Heat transfer measurements are reduced to a correlation of the form Nu = CRa0.25. Aggregated results produce estimates of Nusselt numbers that are 30 percent higher than those for a heated tube in an infinite fluid. The results provide a base case for assessment of heat transfer rates in a solar water heating system comprised of an unpressurized integral collector storage (ICS) with an immersed tube bundle.
Polymeric Absorbers for Flat Plate Collectors: Can Venting Provide Adequate Overheat Protection?
Meghan Kearney, Jane Davidson and Susan Mantell
Venting and evaporative cooling are modeled as possible techniques for protecting polymer absorbers in single-glazed, flat-plate solar collectors from exceeding the material temperature limit during dry stagnation. Four venting options are considered: I) venting above the absorber plate, II) venting below, III) dual venting, and IV) venting with evaporative cooling. Results indicate that in hot, sunny conditions, venting may not provide adequate cooling to lower the absorber temperature to the relative thermal index of the polymeric materials currently in use or under consideration for this application. Venting combined with evaporative cooling from a wetted pad directly beneath the absorber plate is identified as a potential method of overheat protection in hot, dry climates.
Scaling in Polymer Tubes and Interpretation for Use in Solar Water Heating Systems
Yana Wang, Jane Davidson, and Lorraine Francis
An experimental study of the growth of scale on copper, nylon 6,6, semiaromatic high temperature nylon, polypropylene, polybutylene, and Teflon tubes exposed to hard water is presented. Results provide qualitative information on the scaling of polymer tubes in nonisothermal, flowing conditions expected in heat exchangers and solar absorbers. The 89-cm long tubes were placed in tube-in-shell heat exchangers. The tubes were exposed to flowing water for 1660 hours, a 1120-hour pretreatment phase using tap water adjusted to supersaturation of about 2 and pH of 8, followed by a 540-hour acceleration phase using tap water with an adjusted total calcium concentration of 4x10-3 M, and a pH of 9. Flow rate was 4 cm/s. A 50 percent propylene glycol solution at 60 °C was maintained on the shell-side of the heat exchanger. Sections of the tubes were removed periodically to determine the extent of scaling. Results include scanning electron microscope images of the tube surfaces before and after exposure to the flowing water, x-ray diffraction to determine the crystalline phase content of the observed deposits, and chemical analysis to estimate the mass of calcium carbonate per unit surface area. A model of the scaling process is presented to help interpret the data. The data show conclusively that polymer tubes are prone to scaling. With the exception of nylon 6,6, the scaling rate on the polymers is about the same as that on copper. The nylon 6,6 substrate appears to enhance scaling. The enhancement is attributed to hydrolysis of the substrate.
Mechanical Performance of Polysulfone, Polybutylene, and Nylon 6,6 in Hot Chlorinated Water
Andrew Freeman, Susan C. Mantell, Jane H. Davidson
Polymer materials considered for use in domestic hot water heating systems must maintain mechanical properties in the working fluid over their target lifetimes. In potable water, chlorine and pH combine to create an oxidative environment, commonly characterized by the oxidative reduction potential (ORP), that can chemically attack a polymer, resulting in permanent loss of mechanical strength and stiffness. Water absorption and hydrolysis can also impact polymer properties. In the present study, experiments were conducted to evaluate the mechanical performance of polysulfone, polybutylene, and nylon 6,6 immersed for up to 1,100 hours in water at ORP levels of 550 and 825 mV at 60 and 80°C. Mechanical performance was evaluated by measuring creep compliance and the change in tensile strength and molecular weight after exposure. Surface morphology of the exposed materials was examined using scanning electron microscopy. Nylon 6,6 showed significant degradation in strength and creep compliance in all environments. Despite some variability in measured properties, the blend of polybutylene, which has additives to prolong life, did not degrade. Polysulfone performed the best of the three materials with no discernable change in properties over the duration of the experiments.
The Benefit of Dividing an Indirect Thermal Storage into Two Compartments: Discharge Experiments
Vishard Ragoonanan, Jane H. Davidson, Kelly O. Homan, and Susan C. Mantell
Experiments are presented to demonstrate the benefits of dividing an indirect thermal storage into two compartments. The transient discharge experiments were conducted in an undivided and equally divided 126 liter rectangular storage vessel, which has a height to depth aspect ratio of 9.3:1 and is inclined at 30 degrees to the horizontal. A 240-tube copper heat exchanger with a total surface area of 2.38 m2 was immersed in the storage fluid. For the divided storage, the heat exchanger flow path was in series through the two compartments. Water flow rate through the heat exchanger was varied from 0.05 to 0.15 kg/s to demonstrate the effect of varying the number of transfer units (NTU) from 2.2 to 7 on the relative performance of undivided and divided storage vessels. Reported measurements include transient storage temperature distribution, heat exchanger outlet temperature, delivered energy, and exergy of the divided and undivided storage. The divided storage provides higher energy delivery rates and higher heat exchanger outlet temperatures during most of the discharge. The magnitude of these benefits depends on NTU and the extent of discharge. For a flow rate of 0.05 kg/s, corresponding to a nominal NTU of 7, the divided storage delivers a maximum of 11% more energy than the undivided storage when 100 liters of hot water or 55% of the stored energy has been delivered. For a flow rate of 0.15 kg/s, corresponding to a nominal NTU of 2.5, the divided storage delivers a maximum of 5% more energy at the same level of discharge. Data agree with First and Second Law analyses of a storage system comprised of two tanks in series.
Natural Convective Flow and Heat Transfer in a Collector Storage with an Immersed Heat Exchanger: Numerical Study
Yan Su and Jane H. Davidson
A three-dimensional model and dimensionless scale analysis of the transient fluid dynamics and heat transfer in an inclined adiabatic water-filled enclosure with an immersed cylindrical cold sink is presented. The geometry represents an integral collector storage system with an immersed heat exchanger. The modeled enclosure has an aspect ratio of 6:1 and is inclined at 30 degrees to the horizontal. The heat exchanger is represented by a constant surface temperature horizontal cylinder positioned near the top of the enclosure. A scale analysis of the transient heat transfer process identifies four temporal periods: conduction, quasi-steady, fluctuating and decay. It also provides general formulations for the transient Nusselt number, and volume averaged water temperature in the enclosure. Insight to the transient fluid and thermal processes is provided by presentation of instantaneous flow streamlines and isotherm contours during each transient period. The flow field consists of two distinct zones. The zone above the cold sink is nearly stagnant. The larger zone below the sink is one of strong mixing and recirculation initiated by the cold plume formed in the boundary layer of the cylindrical sink. Correlations for the transient Nusselt number and the dimensionless volume averaged tank temperature predicted from the model compare favorably to prior measured data. Fluid motion in the enclosure enhances heat transfer compared to that of a cylinder in an unbounded fluid.
Calcium Carbonate Formation on Cross-linked Polyethylene (PEX) and Polypropylene Random Copolymer (PP-R)
Patricia Sanft, Lorraine Francis, Jane H. Davidson
The accumulation of calcium carbonate (referred to as scale) on the surface of cross-linked polyethylene and polypropylene random copolymer tubes is compared to that on copper. Water with total calcium and carbonate concentration of 3x10-3 M and a pH of approximately 9.1, yielding a supersaturation of 7.8, was pumped through the tubes at a velocity of 0.07 m/s for 2.5, 5, and 7.5 hours. Flow was laminar with Reynolds numbers less than 1000. Sections of the tubes were analyzed at the designated time points to determine the extent of scaling. Results include scanning electron microscope images of the tube surfaces before and after exposure to the supersaturated water and chemical analysis to determine the mass of calcium carbonate per unit surface area. Measured scaling rates of the two polymer tubes are similar to that of copper.
A Variable Effectiveness Model for Indirect Thermal Storage Devices
A. M. Boies and K. O. Homan, J. H. Davidson and Wei Liu
The performance of indirect thermal storage systems is critically
dependent on the degree of thermal contact between the
energy storage medium and the energy transfer medium. For
liquid-liquid systems, the energy transfer occurs across a heat
exchanger for which the overall effectiveness is determined by
both tube-side and storage-side convection coefficients. While
the tube-side convection is essentially constant throughout a draw
at a constant flow rate, the storage-side convection process depends
intimately on the natural convection flow driven by the
temperature difference between the two fluids. This temperature
difference is inherently transient during the discharge process. In
the present work, analytical models are developed which predict
system behavior for constant and variable heat exchanger effectiveness.
The accuracy of each model is quantified in relation to
empirical data obtained by Liu et al. [1, 2] in a physical system
motivated by application to integral collector storage (ICS) solar
water heating devices. From analysis of the empirical data,
discharge-averaged values in the constant effectiveness model and
in the variable effectiveness model are determined for a range of
empirical conditions. The results show that the initial flow transients
generated by the start of the discharge process are flow rate
dependent and have a significant impact on the observed overall
heat transfer coefficients.
Analysis of Sandwich Panels for an Energy Efficient and Self-Supporting Residential Roof
Daniel Thomas, Susan C. Mantell, and Jane H. Davidson
Louise F. Goldberg and John Carmody
The structural and thermal feasibility of a self-supporting sandwich panel for energy efficient residential roof applications is assessed. The assessment is limited to symmetric sandwich panels comprising two face sheets and an insulating core. Feasible panel designs are presented for loading conditions, corresponding to southern and northern climates in the United States. The base case panel is 5.5 m long for a nominal 4.6 m horizontal span and an 8/12 roof pitch. Face sheet materials considered are oriented strand board, steel, and fiber reinforced plastic. Core materials considered are expanded polystyrene, extruded polystyrene, polyurethane and poly(vinyl-chloride) foams. A wide range of material options meet building code limits on deflection and weight and prevent face sheet fracture and buckling, and core shear failure. Panels are identified that have structural depths similar to conventional wood rafter construction. Shortening the overall panel length provides greater choice in the use of materials and decreases the required panel thickness. Suggestions for improved panel designs address uncertainty in the ability of the plastic core to withstand long term loading over the expected life of residential buildings.
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