bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamic Forces From Single Gland Labyrinth Seals: Part I—Ideal and Viscous Decomposition
Authors: Millsaps K. T. , Martinez-Sanchez M.
Published: June 2008

A theoretical and experimental investigation on the aerodynamic forces generated by a single gland labyrinth seal executing a spinning/whirling motion has been conducted. A lumped parameter model, which includes the kinetic energy carryover effect, is presented along with a linear perturbation solution technique. The resulting system is nondimensionalized and the physical significance of the reduced parameters is discussed. Closed-form algebraic formulas are given for some simple limiting cases. It is shown that the total cross force predicted by this model can be represented as the sum of an ideal component due to an inviscid flow with entry swirl and a viscous part due to the change in swirl created by friction inside the gland. The frequency-dependent ideal part is solely responsible for the rotordynamic direct damping. The facility designed and built to measure these frequency dependent forces is described. Experimental data confirm the validity and usefulness of this ideal/viscous decomposition. A method for calculating the damping coefficients based on the force decomposition using the static measurements only is presented.
Dynamic Forces From Single Gland Labyrinth Seals: Part II—Upstream Coupling
Authors: Millsaps K. T. , Martinez-Sanchez M.
Published: June 2008

The standard lumped parameter model for flow in an eccentrically offset labyrinth seal, which assumes constant upstream and downstream boundary conditions, has been extended to include the effects of a nonuniform upstream cavity flow due to coupling. This new model predicts that the upstream perturbations in pressure and azimuthal velocity caused by this coupling can have a very strong impact on the pressure distribution in the seal gland itself. Augmentation by a factor of four, over the uniform inlet model, is predicted under some circumstances. Although no precise comparison to the experimental data with this new model was possible, due to the lack of control over the face seal venting the upstream cavity to the center hub plenum, the calculated effect of this coupling was shown to be approximately what was required to restore quantitative agreement between the data and theory. The new theory can explain the anomalously large pressure nonuniformity previously found by other authors in short seals as well as the first few glands of multicavity seals.
Application of Computational Fluid Dynamics to Turbine Disk Cavities
Authors: Virr G. P. , Chew J. W. , Coupland J.
Published: June 2008

A CFD code for the prediction of flow and heat transfer in rotating turbine disk cavities is described and its capabilities demonstrated through comparison with available experimental data. Application of the method to configurations typically found in aeroengine gas turbines is illustrated and discussed. The code employs boundary-fitted coordinates and uses the k–ε turbulence model with alternative near-wall treatments. The wall function approach and a one-equation near-wall model are compared and it is shown that there are particular limitations in the use of wall functions at low rotational Reynolds number. Validation of the code includes comparison with earlier CFD calculations and measurements of heat transfer, disk moment, and fluid velocities. It is concluded that, for this application CFD is a valuable design tool capable of predicting the flow at engine operating conditions, thereby offering the potential for reduced engine testing through enhanced understanding of the physical processes.
Effects of Embedded Vortices on Injectant From Film Cooling Holes With Large Spanwise Spacing and Compound Angle Orientations in a Turbulent Boundary Layer
Authors: Ligrani P. M. , Mitchell S. W.
Published: June 2008

Experimental results are presented that describe the effects of embedded, longitudinal vortices on heat transfer and film injectant downstream of a single row of film cooling holes with compound angle orientations. Holes are spaced 7.8 hole diameters apart in the spanwise direction so that information is obtained on the interactions between the vortices and the injectant from a single hole. The compound angle holes are oriented so that their angles with respect to the test surface are 30 deg in a spanwise/normal plane projection, and 35 deg in a streamwise/normal plane projection. A blowing ratio of 0.5 is employed and the ratio of vortex core diameter to hole diameter is 1.6–1.67 just downstream of the injection holes (x/d=10.2). At the same location, vortex circulation magnitudes range from 0.15 m2/s to 0.18 m2/s. The most important conclusion is that local heat transfer and injectant distributions are strongly affected by the longitudinal embedded vortices, including their directions of rotation and their spanwise positions with respect to film injection holes. To obtain information on the latter, clockwise rotating vortices R0–R4 and counterclockwise rotating vortices L0–L4 are placed at different spanwise locations with respect to the central injection hole located on the spanwise centerline. With vortices R0–R4, the greatest disruption to the film is produced by the vortex whose downwash passes over the central hole (R0). With vortices L0–L4, the greatest disruption is produced by the vortices whose cores pass over the central hole (L1 and L2). To minimize such disruptions, vortex centers must pass at least 1.5 vortex core diameters away from an injection hole on the upwash sides of the vortices and 2.9 vortex core diameters away on the downwash sides of the vortices. Differences resulting from vortex rotation are due to secondary flow vectors, especially beneath vortex cores, which are in different directions with respect to the spanwise velocity components of injectant after it exits the holes. When secondary flow vectors near the wall are in the same direction as the spanwise components of the injectant velocity (vortices R0–R4), the film injectant is more readily swept beneath vortex cores and into vortex upwash regions than for the opposite situation in which near-wall secondary flow vectors are opposite to the spanwise components of the injectant velocity (vortices L0–L4). Consequently, higher Stanton numbers are generally present over larger portions of the test surface with vortices R0–R4 than with vortices L0–L4.
Unsteady Wake Over a Linear Turbine Blade Cascade With Air and CO2 Film Injection: Part II—Effect on Film Effectiveness and Heat Transfer Distributions
Authors: Mehendale A. B. , Han J.-C. , Ou S. , Lee C. P.
Published: June 2008

The effect of unsteady wake flow and air (D.R. = 0.97) or CO2 (D.R. = 1.48) film injection on blade film effectiveness and heat transfer distributions was experimentally determined. A spoked wheel type wake generator produced the unsteady wake. Experiments were performed on a five-airfoil linear cascade in a low-speed wind tunnel at the chord Reynolds number of 3 × 105 for the no wake case and at the wake Strouhal numbers of 0.1 and 0.3. A model turbine blade with several rows of film holes on its leading edge, and pressure and suction surfaces ( −0.2<X/C< 0.4) was used. Results show that the blowing ratios of 1.2 and 0.8 provide the best film effectiveness over most of the blade surface for CO2 and air injections, respectively. An increase in the wake Strouhal number causes a decrease in film effectiveness over most of the blade surface for both density ratio injectants and at all blowing ratios. On the pressure surface, CO2 injection provides higher film effectiveness than air injection at the blowing ratio of 1.2; however, this trend is reversed at the blowing ratio of 0.8. On the suction surface, CO2 injection provides higher film effectiveness than air injection at the blowing ratio of 1.2; however, this trend is reversed at the blowing ratio of 0.4. Co2 injection provides lower heat loads than air injection at the blowing ratio of 1.2; however, this trend is reversed at the blowing ratio of 0.4. Heat load ratios under unsteady wake conditions are lower than the no wake case. For an actual gas turbine blade, since the blowing ratios can be greater than 1.2 and the density ratios can be up to 2.0, a higher density ratio coolant may provide lower heat load ratios under unsteady wake conditions.
Prediction of Surface Roughness and Incidence Effects on Turbine Performance
Authors: Boyle R. J.
Published: June 2008

The use of a Navier–Stokes analysis to predict the change in turbine efficiency resulting from changes in blade surface roughness or incidence flow angles is discussed. The results of a midspan Navier–Stokes analysis are combined with those from a quasi-three-dimensional flow analysis code to predict turbine performance. A quasi-three-dimensional flow analysis code was used to determine turbine performance over a range of incidence flow angles. This analysis was done for a number of incidence loss models. The change in loss due to changes in incidence flow computed from the Navier–Stokes analysis is compared with the results obtained using the empirical loss models. The Navier–Stokes analysis was also used to determine the effects of surface roughness using a mixing length turbulence model, which incorporated the roughness height. The validity of the approach used was verified by comparisons with experimental data for a turbine with both smooth and rough blades tested over a wide range of blade incidence flow angles.
Temperature Field and Reaction Zones in Biomass Gasification
Authors: Wang Y. , Kinoshita C. M.
Published: June 2008

A method used to measure and validate the temperature field in an experimental downdraft biomass gasifier and reformulate the discrete test data into continuous mathematical functions is described. The reformulated temperature field is analyzed to identify different reaction zones within the gasifier. Distinct endothermic and exothermic regions, separated by rather complicated boundaries, are found.
Direct Contact Condensation in the Presence of Noncondensables in OC-OTEC Condensers
Authors: Ghiaasiaan S. M. , Wassel A. T. , Lin C. S.
Published: June 2008

Analyses are presented for the performance of direct contact jet and spray condensers in the presence of noncondensables. A solution procedure is developed which obtains analytically the liquid-side transient temperature profile, including the thermal entrance effect. The approach avoids iteration at the liquid-vapor interface, which usually is required to determine the interface temperature, by locally linearizing the energy and mass transfer relationship at the gas-liquid interface. Comparisons are carried out with some available data and correlations and show good agreement. The analyses are shown to provide a rational basis for correlating the experimental data. Unlike previous correlations, which are basically curve fits to the applicable data, the present analysis shows clearly the effects of noncondensables, flow, and geometric parameters on the condenser performance. Results are presented for conditions typical of open-cycle ocean thermal energy conversion (OC-OTEC).
Simulations of Airborne Pollutant Levels and Infiltration Flows in an Enclosure
Authors: Rieder W. , Delfanian F.
Published: June 2008

A study of infiltration-type air flows through a room-size enclosure with an obstructing wall was conducted. A finite volume (cell finite difference) method was used, and a k-ε turbulence model was retained in the simulations. Detailed monitoring of how a simulated gaseous pollutant was diluted and dispersed over time was accomplished, and showed that nearly stagnant zones exist. The simple air change (ACH) method of rating the ventilation capabilities of an enclosure was judged to be marginally acceptable, with a local air age scheme a suggested alternative for multizone cases. Based on the simulations, it was argued that the tracer-gas empirical method of determining ventilation capability may be prone to large uncertainties if flow obstructions or multiple zones exist and only a few sampling locations are used.
Determination of Sorption Rate and Apparent Solid-Side Diffusivity of Pure H2O in Silica Gel Using a Constant Volume/Variable Pressure Apparatus
Authors: Lu L. T. , Charoensupaya D. , Lavan Z.
Published: June 2008

Silica gel-water vapor is commonly suggested as a working media for solar-powered desiccant cooling systems since the system can be regenerated at relatively low temperatures. In the present study the sorption rates of water vapor by regular density silica gel particles were measured in the pressure range of 1–25 torr using a constant volume/variable pressure apparatus. The sorption rate was determined from recorded time variation of water vapor pressure in a test unit of known volume during the sorption process. The mass transfer film resistance was eliminated by evacuating the system and by introducing pure water vapor into the test unit. The apparent solid-side diffusivity was obtained by matching the analytical solution of the simultaneous heat and mass transfer governing equations to the experimental data. The uptake measurements had been performed for three particle sizes of silica gel (150 μm, 1 mm, and 3 mm). The tests were performed sequentially in small steps over a range of initial silica gel moisture content ranging from near zero up to 0.25 kg H2O per kg dry silica gel. The effect of moisture content and particle size on the sorption rate and apparent diffusivity were determined. The effect of charging on time variation of pressure was evaluated and used for correction on all sorption data.
Optimal Building Energy Management: Part I—Modeling
Authors: Be´nard C. , Guerrier B. , Rosset-Loue¨rat M.-M.
Published: June 2008

We analyze the advantages of solving building energy management problems with the techniques of optimal control. Our approach consists of describing the dynamic behavior of a heated building with a simple model and controlling the whole system by minimizing a criterion defined for a time horizon of a few days. The two control components are the heat delivered to the building, and the variable heat exchange through the building envelope. In Part I, input (control and meteorological data) and output (indoor temperature) are related through a simplified state-space representation of the building. Part II is devoted to the actual computation of the control input. Results are given for two categories of buildings: The first is characterized by important direct solar gains. The inside structure is of low thermal inertia and so is the heating system. The second type of building is well insulated, with less glazing and less solar gain. The heavy internal structure of the building and the distribution of heat give a large thermal inertia to the system.
Topics in Gradient Maintenance and Salt Recycling in an Operational Solar Pond
Authors: Swift A. H. P. , Golding Peter
Published: June 2008

Since 1986, the 3355 m2 salt gradient solar pond facility in El Paso, Texas, has operated with a temperature difference between the upper and lower zones of 55 to 75° C while delivering industrial process heat, grid-connected electrical power, and thermal energy for the experimental production of desalted water. Because the El Paso solar pond is an inland facility, it is necessary to recycle the salt in a sustainable salt management system. A method that uses the main pond surface for initial brine concentration and short-term storage was developed after it became evident that the original evaporation pond system was undersized. This paper examines the method for brine concentration and storage, the effects of a brine storage zone on pond operation, and the installation of an enhanced evaporation net system and an automatic scanning injection system. A short description of the performance history and current status of the project is also included.
Comparison of Wind Tunnel Airfoil Performance Data With Wind Turbine Blade Data
Authors: Butterfield C. P. , Scott George , Musial Walt
Published: June 2008

Horizontal axis wind turbine (HAWT) performance is usually predicted by using wind tunnel airfoil performance data in a blade element momentum theory analysis. This analysis assumes that the rotating blade airfoils will perform as they do in the wind tunnel. However, when stall-regulated HAWT performance is measured in full-scale operation, it is common to find that peak power levels are significantly greater than those predicted. Pitch-controlled rotors experience predictable peak power levels because they do not rely on stall to regulate peak power. This has led to empirical corrections to the stall predictions. Viterna and Corrigan (1981) proposed the most popular version of this correction. But very little insight has been gained into the basic cause of this discrepancy. The National Renewable Energy Laboratory (NREL), funded by the DOE, has conducted the first phase of an experiment which is focused on understanding the basic fluid mechanics of HAWT aerodynamics. Results to date have shown that unsteady aerodynamics exist during all operating conditions and dynamic stall can exist for high yaw angle operation. Stall hysteresis occurs for even small yaw angles and delayed stall is a very persistent reality in all operating conditions. Delayed stall is indicated by a leading edge suction peak which remains attached through angles of attack (AOA) up to 30 degrees. Wind tunnel results show this peak separating from the leading edge at 18 deg AOA. The effect of this anomaly is to raise normal force coefficients and tangent force coefficients for high AOA. Increased tangent forces will directly affect HAWT performance in high wind speed operation. This report describes pressure distribution data resulting from both wind tunnel and HAWT tests. A method of bins is used to average the HAWT data which is compared to the wind tunnel data. The analysis technique and the test set-up for each test are described.
On Protection of Freedom’s Solar Dynamic Radiator From the Orbital Debris Environment: Part I—Preliminary Analysis and Testing
Authors: Rhatigan Jennifer L. , Christiansen Eric L. , Fleming Michael L.
Published: June 2008

A great deal of experimentation and analysis has been performed to quantify penetration thresholds of components which will experience orbital debris impacts. Penetration has been found to depend upon mission-specific parameters such as orbital altitude, inclination, and orientation of the component; and upon component specific parameters such as material, density, and the geometry particular to its shielding. Experimental results are highly dependent upon shield configuration and cannot be extrapolated with confidence to alternate shield configurations. Also, current experimental capabilities are limited to velocities which only approach the lower limit of predicted orbital debris velocities. Therefore, prediction of the penetrating particle size for a particular component having a complex geometry remains highly uncertain. This paper describes the approach developed to assess on-orbit survivability of the solar dynamic radiator due to micrometeroid and space debris impacts. Preliminary analyses are presented to quantify the solar dynamic radiator survivability, and include the type of particle and particle population expected to defeat the radiator bumpering (i.e., penetrate a fluid flow tube). Results of preliminary hypervelocity impact testing performed on radiator panel samples (in the 6 to 7 km/sec velocity range) are also presented. Plans for further analyses and testing are discussed. These efforts are expected to lead to a radiator design which will perform to Space Station Freedom requirements over the expected lifetime.
On Protection of Freedom’s Solar Dynamic Radiator From the Orbital Debris Environment: Part II—Further Testing and Analysis
Authors: Rhatigan Jennifer L. , Christiansen Eric L. , Fleming Michael L.
Published: June 2008

Recent progress to better understand the environmental threat of micrometeoroid and space debris to the solar dynamic radiator for the Space Station Freedom power system is reported. The objective was to define a design which would perform to survivability requirements over the expected lifetime of the radiator. A previous paper described the approach developed to assess on-orbit survivability of the solar dynamic radiator due to micrometeoroid and space debris impacts. Preliminary analyses were presented to quantify the solar dynamic radiator survivability. These included the type of particle and particle population expected to defeat the radiator bumpering. Results of preliminary hypervelocity impact (HVI) testing performed on radiator panel samples were also presented. This paper presents results of a more extensive test program undertaken to further define the response of the solar dynamic radiator to HVI. Tests were conducted on representative radiator panels (under ambient, nonoperating conditions) over a range of particle size, particle density, impact angle, and impact velocity. Target parameters were also varied. Data indicate that analytical penetration predictions are conservative (i.e., pessimistic) for the specific configuration of the solar dynamic radiator. Test results are used to define more rigorously the solar dynamic radiator reliability with respect to HVI. Test data, analyses, and survivability results are presented.
Parametric Studies of a Spectrally Selective, Two-Layered, Porous, Volumetric Solar Collector
Authors: Kaminski D. A. , Kar S.
Published: June 2008

A porous, packed bed, volumetric solar collector consisting of two dissimilar layers of spherical beads is numerically modeled. The bed is irradiated on the top surface by concentrated solar flux isotropic within a known cone angle. A gas stream perfusing the bed is heated by convection with the solid particles. The equation of radiative transfer, which accounts for absorption, emission, and linearly anisotropic scattering in the bed, is simplified by employing the P1 differential approximation. The bed materials are spectrally selective in the solar and infrared wavelengths. Sensitivity studies are used to identify the critical input parameters of the system, and a baseline configuration, which incorporates the key requirements of an efficient solar collector, is adopted. Parametric studies are conducted on the mass flow rate, incident solar flux, top layer porosity, solar absorptivity, particle diameter, and degree of back scatter. Tailoring of the particle and fluid temperature profiles and enhancing the efficiency of the collector by an appropriate selection of these critical parameters is demonstrated. Various high-temperature ceramics with suitable radiative properties are identified and their relative performance in the collector is assessed.
Effect of Shielding Surfaces on the Irradiance Received by Tilted Collectors
Authors: Al-Beirutty Mohammad H. , Elsayed Moustafa M.
Published: June 2008

A model is developed to predict the total solar irradiance received by a tilted collector in the presence of a large shielding tilted surface. The model is suggested to replace the commonly accepted isotropic model which does not account for a shielding surface. The present model is found to depend mainly on the configuration factors between the collector and the sky and the ground in the presence of a shielding surface. Expressions for these configuration factors are derived for a general geometry of the collector and a large shielding surface. These expressions are found to depend on the tilt angles of the collector and the surface, the orientation angle and the location of the collector relative to the surface, and the height and width of the shielding surface. The expressions are verified numerically and also, by comparing them with the results obtained from the literature for special geometries. The expressions are simplified to facilitate their use for the case of a vertical collector and a vertical shielding surface. This special case is needed to accurately estimate the solar irradiance received by glass windows of buildings.
Field Test Evaluation of Solar-Heated Evaporators
Authors: Guinn Gerald R.
Published: June 2008

The cost of disposal of liquid industrial waste is of intense concern to all industrial waste generators, but especially so to smaller generators which are penalized by the economy of scale. As a last resort to on-site treatment or recycling, the smaller generator is forced to depend upon commercial waste disposal at costs approaching $6.00 per gallon. The use of solar energy to evaporate water from aqueous wastes is a potentially viable alternative if the contaminants are salts having low volatility. This paper describes a field test conducted with a small solar evaporator installed in a machine shop. A naturally vented, basin-type evaporator was used to evaporate water from a spent plating solution used to anodize aluminum. Measured evaporation data validated performance predictions made earlier using TRNSYS and TMY weather data. The data also indicated that accepted correlation models for predicting performance of solar stills underpredict evaporator performance by about 20 percent.
On the Determination of the Average Temperature in a Horizontal Storage Tank and Its Use in Monitoring the Performance of a Solar Thermosyphon System
Authors: Bannerot Richard B.
Published: June 2008

The mass weighted average storage temperature is a useful parameter for evaluating the performance of thermal storage systems. It is commonly used in system simulations since detailed temperature data in the storage medium are not known (or even considered). However, it is rarely used in the evaluation of operating systems because of the difficulty in determining it, especially in potentially multidimensional storage configurations, like a horizontal tank. This paper examines the issue of the evaluation of the average storage temperature, specifically from the point of view of reproducibility and how much data is required for a confident calculation. Unfortunately, this issue was not completely resolved and was shown to be strongly dependent on the locations of the measurements. Two examples are presented that demonstrate the usefulness of knowing the average storage temperature. The examples also give a quantitative demonstration of the effect of withdrawal flow rate on the amount of thermal energy that can be withdrawn from a given system. A reduction in flow rate by a factor of four increased the delivered energy by 70 percent.
Second-Law Analysis of a Two-Phase Self-Pumping Solar Water Heater
Authors: Walker H. A. , Davidson J. H.
Published: June 2008

Entropy generated by operation of a two-phase self-pumping solar water heater under Solar Rating and Certification Corporation rating conditions is computed numerically in a methodology based on an exergy cascade. An order of magnitude analysis shows that entropy generation is dominated by heat transfer across temperature differences. Conversion of radiant solar energy incident on the collector to thermal energy within the collector accounts for 87.1 percent of total entropy generation. Thermal losses are responsible for 9.9 percent of total entropy generation, and heat transfer across the condenser accounts for 2.4 percent of the total entropy generation. Mixing in the tempering valve is responsible for 0.7 percent of the total entropy generation. Approximately one half of the entropy generated by thermal losses is attributable to the self-pumping process. The procedure to determine total entropy generation can be used in a parametric study to evaluate the performance of two-phase hot water heating systems relative to other solar water heating options.
Unglazed Solar Collector Performance Characteristics
Authors: Morrison G. L. , Gilliaert D.
Published: June 2008

A modified form of the Hottel-Whillier-Bliss solar collector efficiency function is investigated for unglazed solar collector characterisation. The modified function includes allowance for long-wave sky radiation and wind speed on collector performance. The effect of uncertainties in long-wave radiation and wind speed measurement on the evaluation of the collector efficiency is discussed. The use of four coefficient and three coefficient forms of the modified Hottel-Whillier-Bliss function are considered from the point of view of test data reliability.
A General Analysis of Phase-Change Energy Storage for Solar Energy Applications
Authors: Hsieh C. K. , Choi Chang-Yong
Published: June 2008

A unified approach is developed for the analysis of one, two, or three-phase melting or solidification of a semi-infinite medium with or without subcooling or superheating and imposed with constant, monotonic, or cyclic temperature or flux conditions. A source and sink method is presented in which a sink front is used to characterize a melt front while a source front is used to characterize a freeze front. An integrodifferential equation is then derived for the interface position which is linearized locally for numerical solution. This position is, in turn, used as input for the determination of the temperature distribution and energy storage and release in different phases of the medium. The numerical solution presented in this paper has shown to be unique, convergent, stable, and accurate. The analysis has been applied to the study of phase change in a subcooled paraffin wax imposed with a cyclic temperature condition. Test results yield some interesting phenomena related to the merging of phase-change fronts and hysteresis of energy storage and release, among others, which have not previously been reported in the literature. Their relations to the energy storage and release are particularly stressed in the paper.
Effect of Attic Ventilation on the Performance of Radiant Barriers
Authors: Medina Mario A. , O’Neal Dennis L. , Turner W. Dan
Published: June 2008

The objective of the experiments was to quantify how attic ventilation would affect the performance of a radiant barrier. Ceiling heat flux and space cooling load were both measured. Results of side-by-side radiant barrier experiments using two identical 13.38 m2 (nominal) test houses are presented. The test houses responded similarly to weather variations. Indoor temperatures of the test houses were controlled to within 0.2°C. Ceiling heat fluxes and space cooling load were within a 2.5 percent difference between both test houses. The results showed that a critical attic ventilation flow rate of 1.3 (l/sec)/m2 of the attic floor existed after which the percentage reduction in ceiling heat fluxes produced by the radiant barriers did not change with increasing attic airflow rates. The ceiling heat flux reductions produced by the radiant barriers were between 25 and 35 percent, with 28 percent being the percent reduction observed most often in the presence of attic ventilation. The space-cooling load reductions observed were between two to four percent. All results compiled in this paper were for attics with unfaced fiberglass insulation with a resistance level of 3.35 m2 K/W (nominal) and for a perforated radiant barrier with low emissivities (less than 0.05) on both sides.
Conceptual Design of a Downward-Convecting Solar Pond Filled With a Water-Saturated, Porous Medium
Authors: Hadim A. , Burmeister L. C.
Published: June 2008

Previous results from a stability analysis of natural convection in a water-saturated porous medium with externally imposed downward flow of the fluid and distributed internal heat generation are used in this study for a preliminary design of a saltless solar pond. The pond is filled with a water-saturated porous medium to inhibit natural convection. Downward flow of water is steadily imposed to allow the absorbed solar radiation, dependent upon the extinction coefficient of the water-saturated porous medium, to be carried down to the bottom of the solar pond from whence warm water is conveyed to the point of use. The downward velocities used have small magnitudes to allow useful bottom temperatures to be obtained. A steady-state analytical study was performed to determine the maximum temperature that can be obtained at the bottom of the saltless solar pond which resulted in a relationship involving the internal and external Rayleigh numbers, extinction coefficient, and Peclet number of the imposed downward flow. Results from this analytical study are combined with results from the stability analysis in a design procedure. This procedure is used to design a saltless solar pond of the concept described above for several cases.
Performance and Cost Implications of the Fixed Mirror, Distributed Focus (FMDF) Collector
Authors: Dirks J. A. , Williams T. A. , Brown D. R.
Published: June 2008

The Fixed Mirror, Distributed Focus (FMDF) hemispherical bowl is a solar thermal power technology which has been studied for several years as a potentially attractive means of producing electricity. This paper discusses projections of the collector (concentrator and receiver) performance and cost that could be achieved by five variations of the FMDF technology, based on the assumption of continued technology development and high volume production. The major loss mechanisms were determined for the concentrators and receivers and then design point losses and efficiencies were developed. The Pacific Northwest Laboratory (PNL) computer code SOLSTEP was then used to determine the annual performance. The cost analysis was based on a “commercial” solar thermal industry that could exist by the late 1990s. The industry is assumed to be installing several hundred megawatts per year of electric capacity, and is assumed to have improved the maturity of solar thermal components so that technical and economic risks are similar to other energy systems.
Effects of Irreversibility and Economics on the Performance of a Heat Engine
Authors: Ibrahim O. M. , Klein S. A. , Mitchell J. W.
Published: June 2008

Previous investigators have shown that an internally reversible Carnot cycle, operating with heat transfer limitations between the heat source and heat sink at temperatures TH and TL, achieves maximum power at an efficiency equal to 1−TL/TH independent of the heat exchanger transfer coefficients. In this paper, optimization of the power output of an internally irreversible heat engine is considered for finite capacitance rates of the external fluid streams. The method of Lagrange multipliers is used to solve for working fluid temperatures which yield maximum power. Analytical expressions for the maximum power and the cycle efficiency at maximum power are obtained. The effects of irreversibility and economics on the performance of a heat engine are investigated. A relationship between the maximum power point and economically optimum design is identified. It is demonstrated that, with certain reasonable economic assumptions, the maximum power point of a heat engine corresponds to a point of minimum life-cycle costs.
Analysis of Thermal Energy Storage Material With Change-of-Phase Volumetric Effects
Authors: Kerslake Thomas W. , Ibrahim Mounir B.
Published: June 2008

NASA’s Space Station Freedom proposed hybrid power system includes photovoltaic arrays with nickel hydrogen batteries for energy storage and solar dynamic collectors driving Brayton heat engines with change-of-phase thermal energy storage (TES) devices. A TES device is comprised of multiple metallic, annular canisters which contain a eutectic composition LiF-CaF2 phase change material (PCM) that melts at 1040 K. A moderately sophisticated LiF-CaF2 PCM computer model is being developed in two stages considering first one-dimensional and then two-dimensional canister geometries. One-dimensional model results indicate that the void has a marked effect on the phase change process due to PCM displacement and dynamic void heat transfer resistance. Equally influential are the effects of different boundary conditions and liquid PCM free convection. For the second stage, successful numerical techniques used in the one-dimensional phase change model are extended to a two-dimensional (r,z) PCM containment canister model. A prototypical PCM containment canister is analyzed and the results are discussed.
The Equilibrium Solar Pond: A Laboratory Model for the Gradient Layer
Authors: Harel Z. , Tanny J. , Tsinober A.
Published: June 2008

Salts, whose solubility in water increases strongly with temperature, can be utilized as a solute in the equilibrium solar pond. In solutions of such salts, a temperature gradient usually gives rise to thermal diffusion of salt from lower to higher temperature zones in the fluid (negative Soret effect). If the direction of this molecular cross flux is opposite to that of the regular molecular salt flux (down its own gradient), the two fluxes can balance so that the net salt flux is zero and the system is in equilibrium. In the equilibrium solar pond suggested here, the insulating non-convective fluid layer is stratified by a stable concentration graident and a destabilizing temperature profile, distributed in such a way that the regular and cross-diffusion fluxes of salt are exactly balanced and the net salt flux is zero. An experimental study was carried out to investigate such a double-diffusive system. A simple method is suggested to build up the concentration and temperature gradients: a two-layer stratified fluid consisting of KNO3 solution and contained within a large tank is heated at its bottom up to a prescribed temperature above the ambient, while the upper free surface is exposed to steady room conditions. The resulting double diffusive system is found to be highly stable; observations and measurements indicate that the net salt flux is zero, and as long as the temperature gradient across the layer is maintained, the system is at equilibrium and basically remains unchanged (for a period of at least one week). The concept of the equilibrium solar pond is considered as a generalization of the saturated solar pond. Futher laboratory studies on practical aspects associated with the equilibrium solar pond are needed before the above ideas can be applied to a real solar pond.
Minimum-Weight Analysis of Anisotropic Plane-Fin Heat-Pipe Space Radiators
Authors: Lund Kurt O. , Baker Karl W.
Published: June 2008

Equations are formulated for the two-dimensional, anisotropic conduction of heat in space radiator fins. The transverse temperature field is obtained by the integral method, and the axial field by numerical integration. A shape factor, defined for the heat-pipe interface boundary condition, simplifies the analysis and renders the results applicable to general heat-pipe/conduction-fin designs. The thermal results are summarized in terms of the fin efficiency, a fin length parameter, and a radiation/axial-conductance number. These relations, together with those for mass distribution between fins, heat pipes, and headers are used in formulating a radiator mass/heat-rate criterion function. Minimization of the criterion function results in asymptotic solutions for the optimum radiator geometry and conditions. The effect of physical properties on the optimum design is determined; in particular, performance is found to vary with fin conductivity to the 1/3 power for large conductivity values.
Numerical Modeling and Experimental Testing of a Solar Grill
Authors: Olwi Ibrahim , Khalifa Adel
Published: June 2008

A detailed study of a solar cooker used for meat grilling was performed. Experiments were undertaken to test the effects of several parameters on the cooker performance. A mathematical model for the solar grill was developed. Heat balance equations were solved using the fourth-order Runge-Kutta technique. It was concluded that an air-tight oven with double glazing and maximum meat charge will give the best performance and highest efficiency for the solar grill.
Comparison of Experimental and Simulated Thermal Ratings of Drain-Back Solar Water Heaters
Authors: Davidson J. H. , Carlson W. T. , Duff W. S. , Schaefer P. J. , Beckman W. A. , Klein S. A.
Published: June 2008

Short-term experimental tests of drain-back solar water heaters are compared to ratings obtained using TRNSYS to determine if computer simulations can effectively replace laboratory thermal ratings of solar domestic hot water heating systems. The effectiveness of TRNSYS in predicting changes in rating due to limited changes in collector area, collector flow rate, recirculation flow rate, storage tank volume, and storage tank design is validated to within ±10 percent. Storage tank design is varied by using a stratification manifold in place of the standard drop tube. Variations in other component sizes and operating factors are based on current industry standards.
Temperature Distribution Inside a Solar Collector Storage Tank of Finite Wall Thickness
Authors: Al-Nimr M. A.
Published: June 2008

A mathematical model for the transient conjugated behavior of a hot-water storage tank having finite wall thickness is presented. An analytical closed-form solution for the temperature field within the tank is obtained. This solution takes into consideration the axial conduction of heat in both fluid and solid wall and the heat capacity of the solid wall. Plots of the solution show that finite wall thickness tends, as expected, to decrease the thermal stratification within the tank. This effect becomes less apparent at high Peclet numbers.
Use of Simplified System Models to Measure Retrofit Energy Savings
Authors: Katipamula S. , Claridge D. E.
Published: June 2008

The retrofit of dual-duct constant volume systems (DDCV) with energy-efficient variable air volume systems (VAV) has become common in recent years. In general, the energy savings from such retrofits are estimated by developing a temperature-dependent regression model using whole building preretrofit energy consumption data. Model predictions are then compared with measured post retrofit consumption, to determine the savings. In cases where the preretrofit energy consumption is not available such a method cannot be implemented. This paper describes a method that can be used to calculate savings in such cases. The method is based on use of simplified calibrated system models. A VAV model was developed based on the ASHRAE TC 4.7 Simplified Energy Analysis Procedure (SEAP) (Knebel, 1983) and calibrated with the postretrofit energy consumption of a large engineering center in Central Texas. The loads from the calibrated VAV model were then used with the DDCV model to estimate the preretrofit energy use, also based on TC 4.7 SEAP, and apparent savings were determined as the difference between the DDCV predicted consumption and measured energy consumption for the postretrofit VAV system. The simulated hourly cooling energy consumption from the VAV model was within ±1GJ (±20 percent) of the measured consumption. The simulated daily consumption (the sum of 24 hours of consumption) compared better with the measured daily consumption (within ±7 percent). The apparent saving from the retrofit of the DDCV system with VAV was about 684 GJ in cooling energy and 324 GJ in heating energy for a three-week period June–July 1991.
Cyclic Methylsiloxanes as Working Fluids for Space Power Cycles
Authors: Angelino G. , Invernizzi C.
Published: June 2008

The potential merits of cyclic polymethylsiloxanes, particularly those conventionally denominated D4 and D5, as working fluids for space power cycles are discussed. The attractive technical characteristics of these substances which are fully nontoxic, moderately flammable, and stable at high temperature are presented. Some experimental results on vapor pressure and on thermal stability are reported. A maximum operating temperature of about 400°C appears achievable. A comprehensive thermodynamic analysis comparing siloxanes with other classes of high temperature fluids is performed. The peculiar characters of siloxane cycles are found to be: a good overall efficiency achieved through a massive regeneration, a moderate expansion work, and an abundant volume flow at turbine exhaust. A number of two-stage turbines for two power levels (i.e., 30 and 5 kW) were designed using an appropriate optimization program. The resulting main features of such expanders were a satisfactory efficiency, a low rotating and peripheral speed, and a comparatively large wheel diameter. These characteristics seem of particular interest for low capacity systems where, with other fluids, turbines tend to be impractically small and fast rotating and where a high level of regeneration becomes more acceptable. In considering for the sake of comparison the thermodynamic performance of many classes of organic fluids, it becomes apparent that the full potential of organic power cycles in view of the variety of future needs has not yet been thoroughly investigated.
Considerations of Gravity Effects on VAWT Rotor Configurations Which Minimize Flatwise Moments and Stresses
Authors: Eggers A. J. , Ashley H. , Digumarthi R. V.
Published: June 2008

Anticipating that composite materials will permit greater freedom when future VAWT rotor blades are configured, this paper further examines the benefits of adopting true Troposkien shapes for blades which do not use horizontal struts. Two parameters are identified which control blade geometry under combined rotational and gravitational loads. Representative zero-bending-moment shapes are calculated, both for constant mass distribution and for several new variable-mass families. The latter include one case whose mathematical solution proves remarkably simple. Steady design stresses are found to be severely affected by deviations from the ideal shape. The concept of “aerodynamic Troposkien,” which preserves zero bending moments in the presence of 1-P cyclic normal forces, is extended to account for gravity as well as inertial effects on blade shape. This concept is limited to unstalled flow and it is found to require unrealistic blade chord distributions near the tower when gravity effects are included. It is noted that both steady gravitational loads and nonsteady aerodynamic normal force loads will become increasingly important in achieving high-cycle fatigue life with larger-scale rotor blades. It is indicated that this achievement will depend primarily on configuring the rotor blades to minimize the cyclic flatwise bending moments and stresses due to the nonsteady normal forces.
The Fractal Dimension of the Wind
Authors: Syu C. Y. , Kirchhoff R. H.
Published: June 2008

In this paper the concept of a fractal and its dimension is presented. The fractal dimension of the horizontal component of several wind speed time series is determined by the variation method. It is concluded that the fractal dimension D=1.60 can be used to describe the wind both at Windsor, MA, and Altamont, CA. The fractal dimension is related to the universal estimate of the decay rate of turbulent kinetic energy in the wind.
Alternative Designs for Passive Cooling of Homes for Venezuela
Authors: Almao de Herrera Nastia , Rincon Jose
Published: June 2008

A two-dimensional model has been applied to predict the indoor temperature fluctuations of a building using a finite difference technique based on the control volume approach and the “SIMPLE” algorithm. The model simulates thermal performance of a rectangular section where inside air motion by free convection and unsteady heat conduction through walls and roof, are allowed. Since it is a two-dimensional model and only the thermal load through external surfaces is considered, numerical results allow one first to study qualitative performance of a rectangular building section and, secondly, to make quantitative comparisons among alternative designs. In order to show the usefulness of the model, eight design cases were simulated under the transient climatic conditions of Maracaibo (latitude = 10° 30′; longitude = 71° 36′), a hot and humid region in Venezuela. Numerical results show how thermal load through external surfaces can be reduced 60 percent and over in relation to the highest thermal load case.