bluuebunny/crossref_metadata_2025 huggingface dataset:
A Stability Theorem for Hill’s Equation for Engineering Applications
Authors: Bhattacharyya R.
Published: February 2008

A criterion for determining the stability of solutions of Hill’s equation containing even periodic potential is developed based on Hill’s method of solution. In the special case of the Mathieu equation, the theorem is applied to generate stability maps that compares well with those obtained by existing methods. Finally, stability of Poynting oscillations of a rigid disk attached to a neo-Hookean rubber rod is briefly discussed.
Experimental Investigation of Isolated and Simultaneous Internal Resonances in Suspended Cables
Authors: Lee C. L. , Perkins N. C.
Published: February 2008

The near resonant response of suspended elastic cables driven by harmonic, planar excitation is investigated experimentally. Measurements of large amplitude cable motions confirm previous theoretical predictions of fundamental classes of internally-resonant responses. For particular magnitudes of equilibrium curvature, strong modal interactions arise through isolated (two-mode) or simultaneous (three-mode) internal resonances. Four qualitatively different periodic responses are observed: (1) pure planar response, (2) 2:1 internally resonant nonplanar response, (3) 1:1 internally resonant nonplanar response, and (4) simultaneous, 2:2:1 internally resonant nonplanar response. Quasiperiodic responses are also observed.
Modal Parameter Estimation for Piezostructures
Authors: Cole D. G. , Saunders W. R. , Robertshaw H. H.
Published: February 2008

This paper is motivated by the need for consistency between piezostructure measurements and existing modal analysis approaches. Fundamental relationships are developed which reveal that the existing framework of traditional modal analysis approaches can be used to estimate modal parameters which describe the piezostructure dynamics. The modal analysis technique is a frequency domain method where the relationship between pole-residue models for conventional structures and piezostructures is developed. Since typical arrangements of piezoelectric sensors and actuators for modal testing lead to ambiguous mode shape estimates, the use of sensoriactuator transducers provides critical drive-point response information. Also, the existence of a transformation between the structure’s modal matrix and the piezostructure’s electromechanical coupling matrix is shown. It is shown that combining the results of a traditional modal test and a piezostructure modal test enables a modal filtering operation which produces experimental measurements of the electromechanical coupling matrix. This method of modal analysis of a piezostructure is demonstrated numerically for a cantilevered beam.
Nonlinear Normal Modes of a Cantilever Beam
Authors: Nayfeh A. H. , Chin C. , Nayfeh S. A.
Published: February 2008

Two approaches for determination of the nonlinear planar modes of a cantilever beam are compared. In the first approach, the governing partial-differential system is discretized using the linear mode shapes and then the nonlinear mode shapes are determined from the discretized system. In the second approach, the boundary-value problem is treated directly by using the method of multiple scales. The results show that both approaches yield the same nonlinear modes because the discretization is performed using a complete set of basis functions, namely, the linear mode shapes.
On Vibrations of a System With an Eigenfrequency Identical to That of One of Its Subsystems
Authors: Pesterev A. V. , Bergman L. A.
Published: February 2008

The free vibrations of a complex system with a natural frequency identical to that of one of its subsystems is discussed. Such eigenvibrations need special consideration in many modal synthesis methods as the characteristic matrix (equation) does not exist. Particular emphasis has been placed on the case where a resonating subsystem is subjected to forces from other subsystems, which has not received sufficient attention in the literature on substructure synthesis methods. Examples of such eigenvibrations for two simple systems are given. A general equation for system eigenfunctions corresponding to eigenfrequencies of the isolated subsystems is presented and discussed.
Direct Time Integration Methods for Structural Acoustics and Far Field Scattering Computations
Authors: Wright J. P.
Published: February 2008

Procedures are described for doing: (a) transient three dimensional structural acoustics computations, using a combination of modal and finite element techniques in space and explicit integration in time; (b) far field scattering computations, using orthogonal function expansions and the method of characteristics. These methods are discussed and preliminary validation of the approach is accomplished by comparing numerical solutions with available analytic results for some problems in spherical geometry.
Linearization Techniques for Seismic Analysis of Piping System on Friction Support
Authors: Reddy G. R. , Suzuki K. , Watanabe T. , Mahajan S. C.
Published: February 2008

Generally, industrial piping systems are supported on hangers, snubbers, friction supports, etc. Friction supports are used for free thermal expansion of the piping systems. They also have the property to absorb energy from earthquake excitation. In this paper, equivalent linearization techniques such as the Caughey method for bilinear system and the energy method are used to calculate equivalent damping of typical industrial piping system on friction support. These methods are compared in terms of the equivalent damping. An iterative response spectrum method is tried for evaluating response of the piping system using equivalent damping obtained by linearization techniques. Maximum response displacement obtained at friction support is compared with the experimental values. At the end it is concluded that the Caughey method and the energy method evaluate similar damping for the piping on friction support and also concluded that the iterative response spectrum method is easy and reasonable for use in design.
Low-Cycle Fatigue Behavior of a Nickel-Based Alloy Under Combined Bending/Tension Loading
Authors: Julien D. , Bui-Quoc T. , Bernard M. , Saad N. R. , Nguyen H. Luan
Published: February 2008

In this paper, the effect of a combined bending/tension loading on the fatigue resistance and on the fatigue crack growth characteristics of a nickel-based alloy at room temperature is studied. For this purpose, a device was specifically designed so that it can be mounted onto a servohydraulic push-pull testing machine. With the device, a simultaneous displacement and rotation of the specimen extremities generate a combined bending/axial stress; the ratio between the bending stress and the axial stress may be specified by adjusting the eccentricity between the specimen axis and the load axis. Stress-controlled fatigue tests were carried out on plate specimens under bending/tension loading with a surface stress ratio of −0.52 (ratio between the maximum cyclic stress on the back face and that on the front face of the specimen). During each test, the fatigue crack length was monitored using two traveling video-cameras. The experimental results obtained under bending/tension loading have been analyzed in connection with the data obtained under pure tension loading. In particular, the fatigue crack propagation rate expressed in terms of the stress intensity factor of a crack under combined loading was examined.
Compressive Thermal Yielding Leading to Hydrogen Cracking in a Fired Cannon
Authors: Underwood J. H. , Parker A. P. , Cote P. J. , Sopok S.
Published: February 2008

Investigation of environmental cracking of a 1100-MPa yield strength A723 steel cannon tube subjected to prototype firings is described. Metallographic results show cracking of the steel beneath a 0.12-mm protective layer of chromium. Cracks undermine and remove sections of chromium and lead to localized erosion that ruins the cannon. Key features of the firing thermal damage and cracking are: (i) recrystalization of the chromium to a depth of up to 0.08 mm; (ii) steel transformation to 0.19 mm below the chrome surface; (iii) two different periodic arrays of cracks normal to the hoop and axial directions, with mean depths of 0.23 and 0.46 mm, respectively. Time-temperature-depth profiles for the firing cycle were derived via bi-material finite difference analysis of a semi-infinite solid which incorporated cannon combustion gas temperatures and material properties that vary as a function of temperature. The temperature and depth associated with the steel transformation were used to solve iteratively for the convective heat transfer coefficient. This value was further confirmed by the depths of chromium recrystalization and of the crack arrays in the two orientations. A profile of maximum temperature versus depth is used to determine the near-bore applied and residual stress distributions within the tube. The measured volume change of steel transformation is used to determine an upper limit on applied and residual stresses. These stresses are used to determine crack-tip stress intensity factors for the observed crack arrays, and hence provide some explanation for the differential depths of cracking. The near-bore temperature and residual stress distributions are used to help determine the cause of hydrogen cracking and measures to prevent cracking. Compressive yielding due to thermal loading produces near-bore tensile residual stresses, and thereby causes hydrogen cracking. Prevention of cracking is discussed in relationship to hydrogen crack growth rate tests of alternative alloys and coatings.
Remote Identification of Impact Forces on Loosely Supported Tubes: Analysis of Multi-Supported Systems
Authors: Paulino M. , Antunes J. , Izquierdo P.
Published: February 2008

Impact forces are useful information in field monitoring of many industrial components, such as heat exchangers, condensers, etc. In two previous papers we presented techniques—based on vibratory measurements remote from the actual impact locations—for the experimental identification of isolated impacts (Arau´jo et al., 1996) and complex rattling forces (Antunes et al., 1997). In both papers a single gap support was assumed. Those results concern systems which are simpler than the actual multi-supported tube bundles found in heat exchangers. Impact force identification is a difficult problem for such systems, because 1) when sensed by the remote motion transducers, the traveling waves generated at several impact supports are mixed, and there is no obvious way to isolate the contribution of each support; 2) multi-supported tubes may be quite long, with significant dissipative effects (by interacting flows or by frictional phenomena at the clearance supports), leading to some loss of the information carried by the traveling waves; 3) in multi-supported systems, some of the supports are often in permanent contact, leading to nonimpulsive forces which are difficult to identify. In this paper, we move closer towards force identification under realistic conditions. Only the first problem of wave isolation is addressed, assuming that damping effects are small and also that all clearance supports are impacting. An iterative multiple-identification method is introduced, which operates in an alternate fashion between the time and frequency domains. This technique proved to be effective in isolating the impact forces generated at each gap support. Experiments were performed on a long beam with three clearance supports, excited by random forces. Beam motions were planar, with complex rattling at the supports. Experimental results are quite satisfactory, as the identified impact forces compare favorably with the direct measurements.
On the Choice of Initial Geometric Imperfections in Externally Pressurized Shells
Authors: Błachut J. , Jaiswal O. R.
Published: February 2008

Localized and global, of eigenmode type, initial geometric imperfections were superimposed on perfect torispherical, ellipsoidal, and toroidal shells of circular and elliptical cross section. Reduction of the load-carrying capacity was then calculated numerically for various geometries and the yield point of material which was assumed to be mild steel. Results show that the buckling strength of torispheres and ellipsoids could be strongly affected by imperfections, but reduction of its magnitude was dependent on the choice of imperfection shape and, more importantly, on the imperfection’s location. Calculations carried out for closed toroids of circular cross section show that these shells are not sensitive to eigenmode-type imperfections, while toroids with elliptical cross sections are sensitive to eigen-imperfections.
Effect of Shear Deformation and Relaxation of Support Conditions on Elastic Buckling of Pressurized Expansion Bellows
Authors: Skoczen B.
Published: February 2008

Mechanical stability of pressurized expansion bellows and tube-bellows-tube interconnects is considered. Effect of shear deformation on buckling pressure of bellows is shown by using Engesser’s model. A generalized equivalent column concept is developed in order to study effect of relaxation of support conditions on stability of interconnects containing expansion bellows. Two general modes of buckling are discussed: column instability of well-supported bellows (I) and buckling of tube-bellows-tube interconnect if the support-bellows distances are large enough (II). A dramatic drop of critical pressure in the transition region from mode I to mode II is shown.
An Investigation of Nonlinear Models for a Cylinder Row in a Cross Flow
Authors: Thothadri M. , Moon F. C.
Published: February 2008

An array of elastically supported cylinders placed in a uniform fluid flow perpendicular to their long axis has been known to perform large-amplitude oscillations when the flow velocity is increased past a critical value. Experimental investigations have shown that the linear stability of the cylinder row is lost through a subcritical Hopf bifurcation resulting in the now well-known hysteresis regime. In this study, we investigate the nonlinearities in the dynamics of the fluid-elastic system, with particular emphasis on capturing the global bifurcation behavior of the cylinders by proposing two nonlinear models. Although the proposed nonlinear models are mostly arbitrary, when appropriate choices are made for the unknown coefficients in the models, based on the theory of center manifolds and normal forms, the predictions of the models, based on the theory of center manifolds and normal forms, the predictions of the models are reasonable. While one of the models captures the experimental bifurcation diagram qualitatively, the other nonlinear model exhibits secondary bifurcation, resulting in coexisting periodic and quasi-periodic solutions.
Thermomechanical Fracture on a Pressurized Cylindrical Vessel Induced by a Localized Energy Source
Authors: Beraun J. , Chen J. K. , Tzou D. Y.
Published: February 2008

Thermally induced elastic fracture around a localized energy source on a pressurized cylindrical vessel is studied in this work. Analytical solutions are obtained via the method of dual integral transform, with emphasis on the identification of the dominating parameters in thermal cracking. Directions for crack extension from the heat source are examined by the stress and strain-energy-based criteria, including the effects of internal pressure and axial stress. Special features include the intrinsic transition between the thermally driven and the mechanically driven fracture patterns. The physical parameters governing such transition are determined.
Effects of Crack Starter Bead Preparation in Drop-Weight Test on Nil-Ductility Transition Temperature
Authors: Higuchi M. , Yamauchi T. , Tanaka Y. , Iida K.
Published: February 2008

The procedure for making crack starter weld deposit on drop-weight test (DWT) specimens was altered from two passes to one pass in about 1990. The effects of some parameters of crack starter weld process on drop-weight test results were studied. Results of this study indicated that length of overlap of the second pass and height of crack starter beads were most effective on nil-ductility temperature (TNDT). When overlap length and bead height of two-pass deposit were small enough, TNDT obtained by two-pass deposit became lower than one-pass TNDT, the discrepancy being by as much as 25°C. TNDT values for 24 Japanese steels were determined using two different DWT methods, one-pass deposit and two-pass deposit having small overlap length and bead height. The difference of TNDT depending on DWT method could be seen only for high-toughness low-alloy steel base metals. For other materials (i.e., low-to-medium-toughness low-alloy steel base metals, weld metals, and high-toughness carbon steels), TNDTS by two-pass and one-pass deposits were essentially the same. For lower-toughness steels, TNDT was frequently lower than the temperature of vTcv − 33°C), and thus, the reference nil-ductility temperature RTNDT was determined from Charpy impact test results. These results can be taken as a way of interpreting the past toughness evaluations made for operating plants using the two-pass TNDT.
Three-Dimensional Analysis of a Semi-Elliptical Crack Emanating From an Erosion at the Bore of an Autofrettaged Pressurized Cylinder
Authors: Perl M. , Levy C. , Bu J.
Published: February 2008

The effect of various erosion configurations on the mode I stress intensity factor (SIF) distribution along the front of a semi-elliptical crack, emanating from the deepest line of the erosion surface (DLES) at the bore of an autofrettaged, pressurized thick-walled cylinder of outer to inner radius ratio, Ro/Ri = 2, is investigated. The three-dimensional (3-D) linear elastic problem is solved via the finite element (FE) method using the ANSYS 5.2 standard code. Hill’s autofrettage residual stress field is simulated by an equivalent thermal load and the SIFs are determined by the nodal displacement method. SIF distribution along the front of semi-elliptical cracks of various crack depths to wall thickness ratios, a/t = 0.05 to 0.25, and ellipticities, a/c, ranging from 0.5 to 1.5, emanating from the DLES, are determined. Three groups of erosion geometries are considered: (a) arc erosions of constant relative depth, d/t, equal to 5 percent and with relative radii of curvature, r′/t, between 5 and 30 percent; (b) semi-elliptic erosions of constant relative depth, d/t, of 5 percent with erosion ellipticity, d/h, varying from 0.3 to 2.0; and (c) semi-circular erosions of relative depth, d/t, between 1 and 10 percent of the wall thickness. The effective SIF along the crack front results from the superposition of KIP—the SIF due to pressurization, and KIA—the negative SIF due to the autofrettage residual stress field. KIP is highly dependent on the stress concentration ahead of the DLES which directly relates to the erosion geometry. The absolute value of KIA is just slightly reduced by the presence of the erosion. Its change solely depends on, and is directly proportional to, the erosion depth. Thus, while deep cracks are almost unaffected by the erosion, the effective SIF for relatively short cracks is found to be significantly enhanced by its presence and might result in a shortening of the vessel’s fatigue life by up to an order of magnitude. Also, it is shown that 2-D analysis may lead to a nonconservative estimate of the vessel’s fatigue life.
Added Fluid Mass for Bellows Expansion Joints in Axial Vibrations
Authors: Jakubauskas V. F.
Published: February 2008

This paper presents the results of investigation of the fluid added mass in axial vibrations of bellows expansion joints. The added mass is shown to consist of three parts, one due to convolution translation in axial direction, another due to distortion of the convolution during vibration, and the third one due to the return flow in central section of bellows. The latter component was ignored due to its negligibility in comparison with the first two parts. The distortion component for a half-convolution has been determined using finite element analysis and the results are presented in graphical form for a typical range of bellows geometry. The total added mass is given in a form suitable for hand calculations.
Effect of Gap Between Pad and Vessel for Moment Loading on Nozzle
Authors: Sang Z. F. , Li L. , Zhou Y. J. , Widera G. E. O.
Published: February 2008

The purpose of this paper is to present a study of the effect of a geometric gap between the cylindrical shell and reinforcement pad on the local stresses in the area of the intersection when the nozzle is subjected to moment loading. Experimental and finite element analyses were performed on two test vessels (four nozzles). A comparative study of stresses in the intersection region for different geometric gaps was also carried out.
ISO Test Method to Determine Sustained-Load-Cracking Resistance of Aluminium Cylinders
Authors: Bhuyan G. S. , Rana M. D.
Published: February 2008

Leak as well as rupture types of failures related to sustained-load-cracking (SLC) have been observed in high-pressure gas cylinders fabricated from certain aluminium alloy. The stable crack growth mechanism observed primarily in the cylinder neck and shoulder area have been identified as the SLC mechanism occurring at room temperature without any environmental effect. The International Organization for Standardization (ISO) Sub-Committee 3, Working Group 16 has developed a test method to measure the SLC resistance using fracture mechanics specimens along with an acceptance criterion for aluminium cylinders. The technical rationale for the proposed test method and the physical significance of the acceptance criterion to the cylinder performance in terms of critical stress-crack size relationship is presented. Application of the developed test method for characterizing new aluminium alloy for manufacturing cylinders is demonstrated. SLC characteristics of several aluminium cylinders as well as on-board cylinders for natural gas vehicles assessed by the authors are discussed.
Modeling of Tuned Liquid Damper With Submerged Nets
Authors: Kaneko S. , Ishikawa M.
Published: February 2008

An analytical model for describing the effectiveness of tuned liquid damper (TLD) with submerged nets for suppressing horizontal vibration of structures is proposed. Dissipation energy due to the liquid motion under sinusoidal excitation is calculated based on nonlinear shallow water wave theory. In particular, the effects of hydraulic resistance produced by nets installed in a tank on the performance of TLDs are examined. The results of dissipation energy theoretically obtained are confirmed by experiments. To show the effectiveness of the proposed analytical model for TLD-structure interaction problems, the case in which TLD with nets is installed on top of a pylon of a cable-stayed bridge is described and the calculated results are compared with the actual experimental data. In the calculation, it is shown that the optimal damping factor, as is the case for tuned mass dampers (TMD), can be produced by the nets, and the effectiveness of TLD with nets are demonstrated.
Weld Repair of 2-1/4Cr-1Mo Service-Aged Header Welds
Authors: Viswanathan R. , Gandy D. , Findlan S.
Published: February 2008

The objective of this investigation was to evaluate the efficacy of different weld repair techniques as applied to service-aged 2-1/4Cr-1Mo steel weldments. A header which had been in service for 244,000 h at 1050°F (565°C) was utilized for the study. Three girth welds were partially excavated and subjected to repairs using gas tungsten arc welding (GTAW), shielded metal arc welding (SMAW) with postweld heat treatment (PWHT), and without postweld heat treatment using a temperbead technique. Results show that all the weld repairs improved the creep rupture lives of the ex-service weldments and that remaining lives of several decades could be achieved in the repaired condition. The SMAW-temperbead repairs resulted in increase of rupture life, tensile strength, and impact toughness compared to the SMAW-PWHT repairs. The GTAW-PWHT repairs also produced a superior combination of mechanical properties. Remaining creep rupture lives were a function of the extrapolation procedure and specimen size. These results are described here and discussed in comparison with results previously reported for a less severely degraded condition of the steel in order to delineate the effect of prior degradation on weld repair performance.
Residual Stress Analysis by Simplified Inherent Strain at Welded Pipe Junctures in a Pressure Vessel
Authors: Mochizuki M. , Hayashi M. , Hattori T.
Published: February 2008

We present a new and simplified method of estimating residual stress in welded structures by using inherent strain. The method makes use of elastic analysis by means of the finite element method and is used to calculate the residual stress in complicated three-dimensional structures efficiently. The inherent strain distribution in a welded joint of a small-diameter pipe penetrating a pressure vessel was assumed to be a simple distribution, and the residual stress was calculated. Inherent strain distributions were inferred from those of welded joints with simple shapes. The estimated residual stress using these inferred inherent strains agrees well with the measurements of a mock-up specimen. The proposed method is a simple way to estimate welding residual stress in three-dimensional structures of complicated shapes.
Residual Stress Estimation in Crossbores With Bauschinger Effect Inclusion Using FEM and Strain Energy Density
Authors: Badr E. A. , Sorem J. R. , Tipton S. M.
Published: February 2008

Crossbore intersections in liquid ends of positive displacement pumps (PDPs) have regions with high stress concentration. Due to the cyclic loading that occurs in most PDPs, these stress concentration points are susceptible to fatigue cracking. In order to prolong their life, the liquid ends are often overpressurized (autofrettaged), thus inducing beneficial compressive hoop stresses in these critical regions upon removal of the autofrettage pressure. This autofrettage process drives the region of high stress concentration beyond the elastic limit and well into the elastic-plastic region. Elastic-plastic stresses and strains due to loading and unloading were analyzed in crossbore geometries, with Bauschinger effect included, using 3-D finite element analysis of the liquid end. For comparison, an analytical approach was developed, based on the strain energy density criterion first proposed by Glinka. The approach was modified to include the Bauschinger effect for precise estimation of such stresses and strains. Good correlation was observed between elastic-plastic crossbore stresses and strains predicted by the analytical approach and the finite element analysis.
High-Temperature Buckling Analysis of Titanium Cans Under External Pressure
Authors: Koundy V. , Thiebaut C.
Published: February 2008

Powder metallurgy techniques are often used in the fabrication of finished and semi-finished articles. At the CEA, a cold followed by a hot isostatic pressing method is used to produce solid forms from various types of powder. Occasionally, during the hot isostatic pressing part of the process, buckling of the titanium envelope near to the sealed end leads to fracture of the can and incomplete powder consolidation. The aim of this paper is to investigate by numerical finite element simulation the fracture process. A 2-D shell element model with Fourier series taking into account plastic deformation of the can material has been considered and used to determine the buckling critical pressure and the corresponding buckling mode. The simulation has been used to eliminate failure of the can by modifying the temperature-pressure schedule or by changing the can design. The calculations show that reducing the sharp angles of the initial titanium can near the rupture area can resolve the buckling problem; however, this solution is not totally satisfactory due to the development of a zone of constriction (breaks and irregularities) in the compacted powder just behind the modified can wall. This geometrical defect leads to difficulties in machining the final product. A better solution to the problem is to increase the initial can temperature prior to application of the pressure. This leads to a can of enhanced ductility with a better ability to deform. This latter solution, which can be employed with or without can modifications, eliminates both the can’s buckling and the zone of constriction. These numerical results have been validated by recent tests performed in our laboratories.
Performance of Surface and Volumetric Solar Thermal Absorbers
Authors: Spirkl W. , Ries H. , Kribus A.
Published: February 2008

Thermal surface absorbers convert all incident radiation to heat at a single local temperature. The fluid flows perpendicular to the radiation’s propagation direction. In contrast, in volumetric absorbers the fluid flows parallel to the radiation’s propagation direction, and the absorber may exhibit temperature stratification along the radiation’s direction. This raises the question whether reabsorption of parts of the thermal emission coming from the hotter absorber sections renders the volumetric absorber superior to the surface absorber. For the case of isotropic radiation, we compare the efficiency of the volumetric and the surface absorber with each other and with an isothermal absorber. We find that the nonselective volumetric absorber is less efficient than the nonselective surface absorber for the assumption of perfect heat transfer between absorber and fluid. Thus we conclude that in practical high-flux applications, the superiority of volumetric absorbers stems from the enhanced heat transfer area. If the fluid flows against the direction of radiation propagation the volumetric absorber is slightly more efficient than if it flows with this direction. We also discuss the effect of the two-flux approximation on simulation of isotropic volumetric absorbers.
Optimal Parallel Flow in Solar Collectors for Nonuniform Irradiance
Authors: Spirkl W. , Ries H. , Kribus A.
Published: February 2008

We consider volumetric thermal absorbers exposed to nonuniform irradiance. The fluid flows perpendicular to the absorber surface. The fluid transport from front to rear surface elements is connected in a parallel configuration. We investigate the optimum adjustment of the local mass flux or, equivalently, the local absorber temperature. Three different strategies are compared: (1) strategy of uniform mass flux, (2) strategy of uniform exit temperature, and (3) a strategy with a locally optimized temperature-flow profile. Significant improvement of strategy (2) compared to (1) and of strategy (3) compared to (2) is observed for the case of a black-body absorber which is isothermal in the direction normal to the absorber surface. We find it to be advantageous to include even regions where the stagnation temperature is below the desired exit temperature.
Compensation of Self-Heating in the Measurement of the Dynamic Response of a Thermoresistive Sensor
Authors: Neto J. S. R. , Deep G. S. , Freire R. C. S. , Lima A. M. N. , Lobo P. C.
Published: February 2008

In the experimental determination of the response time of a thermoresistive sensor to a radiation step, one needs to monitor the variation of the ohmic resistance of the sensor with time. This requires passing electrical current through the sensor, which introduces error in the measured response time due to the additional self-heating of the sensor by the electrical current. A theoretical method to eliminate this error is formulated and experimental results are presented. The proposed method also permits experimental determination of the heat capacity (mc) and heat transfer coefficient (UA). The sensitivity analysis of τ = mc/UA permits the optimal choice for the resistance measurement currents.
Field Performance of Photovoltaic Solar Water Heating Systems
Authors: Fanney A. H. , Dougherty B. P. , Kramp K. P.
Published: February 2008

Energy consumed for water heating accounts for approximately 17.9 EJ of the energy consumed by residential and commercial buildings. Although there are over 90 million water heaters currently in use within the United States (Zogg and Barbour, 1996), durability and installation issues as well as initial cost have limited the sales of solar water heaters to less than 1 million units. Durability issues have included freeze and fluid leakage problems, failure of pumps and their associated controllers, the loss of heat transfer fluids under stagnation conditions, and heat exchanger fouling. The installation of solar water heating systems has often proved difficult, requiring roof penetrations for the piping that transports fluid to and from the solar collectors. Fanney and Dougherty have recently proposed and patented a solar water heating system that eliminates the durability and installation problems associated with current solar water heating systems. The system employs photovoltaic modules to generate electrical energy which is dissipated in multiple electric heating elements. A microprocessor controller is used to match the electrical resistance of the load to the operating characteristics of the photovoltaic modules. Although currently more expensive than existing solar hot water systems, photovoltaic solar water heaters offer the promise of being less expensive than solar thermal systems within the next decade. To date, photovoltaic solar water heating systems have been installed at the National Institute of Standards and Technology in Gaithersburg, MD and the Florida Solar Energy Center in Cocoa, FL. This paper will review the technology employed, describe the two photovoltaic solar water heating systems, and present measured performance data.
Experimental Investigation of Convective Cells Generated by Side Walls in Solar Ponds Using Various Salts
Authors: Jubran B. A. , Iweida J. A. , Haimour N. M.
Published: February 2008

This paper reports an experimental investigation to study the effect of various parameters that might affect the generation of convective cells in solar ponds with sloping walls. The parameters studied include the variation of the angles of the walls, types of salt used, and the surface roughness of the sloping wall. Four angles were used: 90, 60, 45 and 30, as well as four different types of salts: sodium chloride, potassium chloride, ammonium Sulphate, and carnalite. It was found that increasing the sloping angle of the heated wall of the pond tends to decrease the activity of the convective cells. Furthermore, the growth rate of the convective cells is very much dependent on the type of salt. The results indicate that carnalite and ammonium sulphate tend to have low growth rates of convective cells compared with sodium chloride and potassium chloride. Finally, the surface roughness of the heated wall tends to reduce the activity of the convective cells.
Thermal Storage in Ammonium Alum/Ammonium Nitrate Eutectic for Solar Space Heating Applications
Authors: Jotshi C. K. , Hsieh C. K. , Goswami D. Y. , Klausner J. F. , Srinivasan N.
Published: February 2008

Ammonium alum and ammonium nitrate in the weight ratio of 1:1 form a eutectic that melts at 53°C and solidifies at 48°C. The thermophysical properties of this eutectic were measured in detail and the eutectic was found to have properties desirable for energy storage for solar space heating applications. The eutectic was encapsulated in 0.0254-m diameter high-density polyethylene (HDPE) balls and packed into a cylindrical bed in a scale model for testing its heat transfer characteristics when exposed to an air flow. Test results indicate that the thermal extraction efficiency of the model was 89 percent with an uncertainty of ±8.0 percent. The packed bed had a Stanton number value in close agreement with that predicted with an empirical equation for sensible heat extraction from the eutectic in the solid phase. This Stanton number was increased by about 74 percent for sensible heat extraction from the eutectic in the liquid phase, a phenomenon not previously reported in the literature.
The Determination of Stochastic Loads on Horizontal Axis Wind Turbine Blades
Authors: Freeman L. N. , Wilson R. E.
Published: February 2008

The FAST Code which is capable of determining structural loads of a flexible, teetering, horizontal axis wind turbine is described and comparisons of calculated loads with test data are given at two wind speeds for the ESI-80. The FAST Code models a two-bladed HAWT with degrees-of-freedom for blade bending, teeter, drive train flexibility, yaw, and windwise and crosswind tower motion. The code allows blade dimensions, stiffnesses, and weights to differ and the code models tower shadow, wind shear, and turbulence. Additionally, dynamic stall is included as are delta-3 and an underslung rotor. Load comparisons are made with ESI-80 test data in the form of power spectral density, rainflow counting, occurrence histograms, and azimuth averaged bin plots. It is concluded that agreement between the FAST Code and test results is good.
An Analysis of Flat-Plate Photovoltaic Output in Southern Nevada
Authors: Boehm R. , Chen Y.-T. , Thomas J.
Published: February 2008

A study is reported of predicted flat-plate photovoltaic output in Southern Nevada, and this output is compared to the time variation of the electrical utility load for this area. Included in the comparisons are typical summer time variations of predicted outputs and loads on an hourly basis. Also shown are the time differentials between the utility peak load and the peak photovoltaic output. Plots are also given of the fractions of the output of the photovoltaic systems relative to their peaks that occur at the time of the utility load peak. Finally, the results of a study are reported that investigates the influence of time-of-day purchase prices on the orientation of the modules to maximize revenues. A simple price model based on one used by the local utility for independent power producer purchases is incorporated to show the influence of pricing on optimal orientation.
The “Porcupine”: A Novel High-Flux Absorber for Volumetric Solar Receivers
Authors: Karni J. , Kribus A. , Rubin R. , Doron P.
Published: February 2008

A new volumetric (directly irradiated) solar absorber, nicknamed Porcupine, is presented. It was tested over several hundreds of hours at the Weizmann Institute’s Solar Furnace, using several flow and geometric configurations, at various irradiation conditions. The experiments, which were conducted at a power level of about 10 kW, showed that the new absorber can accommodate different working conditions and provide a convective cooling pattern to match various irradiation flux distributions. The capability of the Porcupine to endure a concentrated solar flux of up to about 4 MW/m2, while producing working gas exit temperatures of up to 940°C, was demonstrated. In comparative tests, the Porcupine sustained an irradiation solar flux level about four times higher than that sustained by other volumetric absorbers (foam and honeycomb matrices). Due to its ability to sustain and transport a much higher energy fluxes, the Porcupine yielded twice the power output of the other absorbers while its exit gas temperature was 300–350°C higher. The Porcupine design is highly resistant to thermal stresses development; none of the Porcupine absorbers tested showed any sign of deterioration after hundreds of operating hours, although temperature gradients of several hundreds °C/cm developed in some experiments. The basic Porcupine structure provides convective and radiative energy transport between the matrix elements, therefore alleviating the development of flow instabilities; this phenomenon causes local overheating and restricts the operation of other volumetric matrices. A Porcupine absorber was subsequently incorporated into the directly irradiated annular pressurized receiver (DIAPR), where it has been operating flawlessly at an incident flux of several MW/m2 and temperatures of up to 1,700°C.
Development of Graphical Indices for Viewing Building Energy Data: Part I
Authors: Haberl J. S. , Abbas M.
Published: February 2008

Graphs are an effective way of displaying quantitative and statistical information. Unfortunately, only a few studies have systematically looked at presenting building energy data in forms which are both informative, visually efficient, and easy to comprehend. Recent advancements in computing, including the development of sophisticated graphing packages and powerful desktop computers have made the concept of graphical displays or graphical indices more timely than ever before. In this two part paper, graphical displays or indices are reviewed and discussed that can be used to analyze several years of hourly data (20,000 to 30,000 data points). The purpose of these papers is to show how graphical indices can be used to help a building energy analyst view large amounts of hourly building energy consumption data in order to quickly and efficiently analyze the data, check for errors, or establish time and temperature related trends over a large period of time. Data from case study buildings in Texas’ are used to demonstrate the indices. Part I reviews the literature on graphical presentations of building energy use data and graphical perception and presents two graphical indices. Part II provides four additional graphical indices in use in the data review process for the Texas LoanSTAR program2.
Ambient-Temperature Regression Analysis for Estimating Retrofit Savings in Commercial Buildings
Authors: Kissock J. K. , Reddy T. A. , Claridge D. E.
Published: February 2008

This paper describes a procedure for estimating weather-adjusted retrofit savings in commercial buildings using ambient-temperature regression models. The selection of ambient temperature as the sole independent regression variable is discussed. An approximate method for determining the uncertainty of savings and a method for identifying the data time scale which minimizes the uncertainty of savings are developed. The appropriate uses of both linear and change-point models for estimating savings based on expected heating and cooling relationships for common HVAC systems are described. A case study example illustrates the procedure.
Uncertainty in Baseline Regression Modeling and in Determination of Retrofit Savings
Authors: Reddy T. A. , Kissock J. K. , Ruch D. K.
Published: February 2008

The objective of this paper is to discuss the various sources of uncertainty inherent in the estimation of actual measured energy savings from baseline regression models, and to present pertinent statistical concepts and formulae to determine this uncertainty. Regression models of energy use in commercial buildings are not of the “standard” type addressed in textbooks because of the changepoint behavior of the models and the effect of patterned and non-constant variance residuals (largely as a result of changes in operating modes of the building and the HVAC system). This paper also addresses such issues as how model prediction is impacted by both improper model residuals and models identified from data periods which do not encompass the entire range of variation of both climatic conditions and the different building operating modes.
A Robust Controller for a Novel Variable Speed Wind Turbine Transmission
Authors: Idan M. , Lior D. , Shaviv G.
Published: February 2008

A hybrid mechanical-electrical variable speed transmission for wind turbines is presented. The concept consists of a planetary differential transmission in which one input is driven by the variable speed rotor and the second by the variable speed control system, both driving the main generator at a constant speed. The increase in the energy output is estimated to be of about 15 percent–20 percent using the new transmission with the same rotor. The control system, designed to operate efficiently at the entire anticipated wind speed range, varies the rotor speed to provide optimal output power for slow wind speed variations and to attenuate high frequency shaft torque fatigue damage in gusty wind operation. The net result is a cost effective system compared to contemporary variable speed solutions. Dynamic simulations are presented to demonstrate the improved system performance.
Experimental Study for a Double-Glazed Forced-Flow Solar Collector/Regenerator
Authors: Yang R. , Wang P.-L.
Published: February 2008

Experimental study of a double-glazed forced-convection solar collector/regenerator for absorption solar cooling is presented. The south facing experimental solar collector/ regenerator with 10 deg slope is located at Kaohsiung, Taiwan at 120°18′ E longitude and 22°34′ N latitude. The size of the collector is 1 m wide and 7 m long with an effective regeneration area of 0.9 m by 6 m. Previous study for single-glazed forced-convection solar collector/regenerator operated at the same location has shown to have a best day-average efficiency of 17 percent. In order to raise the system performance, a double-glazed collector/regenerator is constructed such that air can be preheated in the upper channel flow. The preheated air is then conducted into the lower channel where it contacts with the film flow of solar heated lithium-chloride solution and regenerates the solution by carrying out the evaporated water vapor. The preheated air has lower relative humidity but the same humidity ratio since it is sensibly heated. Therefore, the regeneration driving potential is increased. The present study shows that the best day-average efficiency can reach 20 percent which increases the feasibility of the open-cycle absorption solar cooling system. Effects of controlling parameters on the collector/regenerator performance are studied, and heat and mass transfer correlations are also presented for design purposes.
Surface Precision of Inflatable Membrane Reflectors
Authors: Jenkins C. H. , Marker D. K.
Published: February 2008

Space-based inflatable technology is of current interest to NASA and DOD, and in particular to the Air Force and Phillips Laboratory. Potentially large gains in lowering launch costs, through reductions in structure mass and volume, are driving this activity. Diverse groups are researching and developing this technology for radio and radar antennae, optical telescopes, and solar power and propulsion applications. Regardless of the use, one common requirement for successful application is the accuracy of the inflated surface shape. The work reported here concerns the shape control of an inflated thin circular disk through use of a nonlinear finite element analysis. First, a review of the important associated Hencky problem is given. Then we discuss a shape modification, achieved through enforced boundary displacements, which resulted in moving the inflated shape towards a desired parabolic profile. Minimization of the figure error is discussed and conclusions are drawn.
A General Model for Analyzing the Thermal Characteristics of a Class of Latent Heat Thermal Energy Storage Systems
Authors: Yanbing Kang , Yinping Zhang , Yi Jiang , Yingxin Zhu
Published: February 2008

The present study describes and classifies latent heat thermal energy storage (LHTES) systems according to their structural characteristics. A general model is developed for analyzing the thermal characteristics of the various typical LHTES systems to simulate thermal characteristics such as instantaneous heat transfer rate, instantaneous thermal storage capacity, etc. of the various typical LHTES systems. The model can calculate some important but difficult to measure system parameters for monitoring the charging or discharging processes of the systems. The model is verified using experimental data in the literature. Results from the model can be used to discuss the influence of the characteristic geometric parameters of LHTES units, the physical properties of the phase change material (PCM), the flow type and the velocity of heat transfer fluid (HTF) on the system thermal performance and to identify the key factors influencing the system thermal performance. The general model can be used to select and optimize the system structure and to simulate the thermal behavior of various typical LHTES systems.