Experiments on thermal characteristics of a natural circulation loop with latent heat energy storage under cyclic pulsed heat load

 An experimental investigation has been made of thermal characteristics of a rectangular, annular single-phase natural circulation loop with the inner tube filled with a solid–liquid phase change material (PCM) under cyclic pulsating heat load. A rectangular, annular loop of 150 cm in height and 75 cm in width was constructed with an annular gap of 0.6 cm, within which water was filled. The inner tube of the annular loop was filled with a PCM (n-Eicosene) or air. Under the cyclic pulsating heat load, temperature field within the water-filled annular loop with PCM- or air-filled inner tube was found to evolve into a steady periodic variation for the range of parameters considered. The water temperature and/or its fluctuating amplitude along the heated or cooled sections of the loop with the PCM-filled inner tube were found to be markedly lower than those measured in the loop with the air-filled inner tube under the identical conditions. On the other hand, along the insulated sections of the loop a somewhat minute difference in temporal variations of the water temperatures exists between the loops with PCM- and air-filled inner tube. In addition, at the outer wall along the cooled section, a time-periodic variation of temperature was detected in synchronizing with the pulsating heat load. Parametric effects of varying amplitude and time-period of the pulsating heat input, as well as of varying the inlet coolant temperature of the cooling jacket were investigated. Received on 30 June 2000 The authors are grateful for the support for this study from National Science Council of Republic of China in Taiwan under the Project Nos. NSC87-2212-E006-054 and NSC88-2212-E006-022.     

Effects of the heat transfer fluid velocity on the storage characteristics of a cylindrical latent heat energy storage system: a numerical study

A numerical study of the effects of the thermal fluid velocity on the storage characteristics of a cylindrical latent heat energy storage system (LHESS) was conducted. Due to the low thermal conductivity of phase change materials (PCMs) used in LHESS, fins were added to the system to increase the rate of heat transfer and charging. Finite elements were used to implement the developed numerical method needed to study and solve for the phase change heat transfer (melting of PCM) encountered in a LHESS during charging. The effective heat capacity method was applied in order to account for the large amount of latent energy stored during melting of the PCM and the moving interface between the solid and liquid phases. The effects of the heat transfer fluid (HTF) velocity on the melting rate of the PCM were studied for configurations having between 0 and 18 fins. Results show that the overall heat transfer rate to the PCM increases with an increase in the HTF velocity. However, the effect of the HTF velocity was observed to be small in configurations having very few fins, owing to the large residual thermal resistance offered by the PCM. However, the effect of the HTF velocity becomes more pronounced with addition of fins; since the thermal resistance on the PCM side of the LHESS is significantly reduce by the large number of fins in the system.     

Effects of the number and distribution of fins on the storage characteristics of a cylindrical latent heat energy storage system: a numerical study

A numerical study of the effects of the number and distribution of fins on the storage characteristics of a cylindrical latent heat energy storage system (LHESS) was conducted. Due to the low thermal conductivity of phase change materials (PCMs) used in LHESS, fins were added to the system to increase the rate of heat transfer and charging. Finite elements were used to implement the developed numerical method needed to study and solve for the phase change heat transfer (melting of PCM) encountered in a LHESS during charging. The effective heat capacity method was applied in order to account for the large amount of latent energy stored during melting of the PCM and the moving interface between the solid and liquid phases. The effects of increasing the number and distribution of fins on the melting rate of the PCM were studied for configurations having between 0 and 27 fins for heat transfer fluid (HTF) velocities of 0.05 and 0.5?m/s. Results show that the overall heat transfer rate to the PCM increases with an increase in the number of fins irrespective of the HTF velocity. It was also observed that the total amount of energy stored after 12?h increases nearly linearly with the addition of fins up to 12 fins; further addition of fins increasing the total energy stored by ever smaller amounts.     

Fins and turbulence promoters for heat transfer enhancement in latent heat storage systems

One of the serious problems associated with the operation of PCM storage system is the heat transfer in and out of the element containing the PCM. This paper presents the results of an experimental investigation of the effects of radial fins and turbulence promoters on the enhancement of phase change heat transfer external to a horizontal tube submersed in the PCM with the working fluid flowing through it. The experimental measurements were realized on a bare cupper tube and an identical cupper tube fitted with radial fins. The fins investigated are 40, 60, 120 and 180 mm diameters. A turbulence promoter made of stainless steel wire of 1.0 mm diameter coiled in a helical form with a pitch of 25.0 mm was inserted into the cupper tubes. The tests were realized on bare tubes, finned tubes and finned tubes with the turbulence promoter inserted into the finned tubes. The measurements were realized for the working fluid temperatures in the range of −10 °C, to −25 °C and six values of the mass flow rate ranging from 0.013 to 0.031 kg/s. The position of the phase interface was photographed by a high resolution digital camera and scanned to determine the real interface position by comparison with a precision measuring scale. The results of the phase interface position, velocity of the interface, solidified mass fraction and the time for complete solidification are presented in function of the working fluid temperature, the working fluid mass and the tube arrangements. The results are presented and discussed.     

Modeling and experiments of energy storage in a packed bed with PCM

This work presents a numerical and experimental study of the transient response of a packed bed filled with a granular phase change material (PCM). The proposed numerical model accounts for the progressive evolution of the enthalpy with temperature during the phase change rather than using a constant phase change temperature. This temperature-dependent enthalpy is included in the model as an apparent specific heat that is dependent on temperature according to the measurements obtained by differential scanning calorimetry (DSC). The model also includes the energy stored in the wall, which has been shown to have a non-negligible effect in several experimental facilities.The equations presented are non-dimensionalized, which results in the same differential equation system regardless of whether a granular PCM or sensible heat storage material is used. In this manner, the same numerical method can be used in cases with or without a granular PCM. Numerical and experimental results are obtained for a conventional granular material (sand) and two commercial granular PCMs with different phase change temperatures. The numerical and experimental heating results exhibit good agreement, and the energy stored in the wall of the bed represents between 8 and 16% of the energy stored in the granular material.     

Performance limit for base-excited energy harvesting,and comparison with experiments

Nonlinear Dynamics - We consider the theoretical maximum extractable average power from an energy harvesting device attached to a vibrating table which provides a unidirectional displacement...     

Experimental and numerical investigations on the performance of dehumidifying desiccant beds composed of silica-gel and thermal energy storage particles

Enhanced efficiency of the adsorption process in the dehumidifier is a key element for improved performance of desiccant cooling systems. Due to the exothermic nature of the adsorption process, the dehumidification and cooling capacity are limited by significant temperature changes in the adsorption column. In the present study, the effects of integration of sensible and latent heat storage particles in the desiccant bed for in situ management of released adsorption heat are investigated. For this purpose, column experiments are performed using an initially dry granular bed made of silica-gel particles or a homogeneous mixture of silica gel and inert sensible or latent heat storage particles. The packed bed is subject to a sudden uniform air flow at selected values of temperature and humidity. Also, a packed bed numerical model is developed that includes the coupled non-equilibrium heat and moisture transfer in the solid and gas phases. Investigations of the heat and mass transfer characteristics are reported using the composite structure and the results are compared with the base case of simple silica gel bed. Improved desiccant cooling system performance can be obtained by appropriate adjustment of desiccant cycle operation and proper choice of the volume ratio of thermal energy storage particles.     

Fully developed laminar flow of non-Newtonian liquids through annuli: comparison of numerical calculations with experiments

Experimental data are reported for fully developed laminar flow of a shear-thinning liquid through both a concentric and an 80% eccentric annulus with and without centrebody rotation. The working fluid was an aqueous solution of 0.1% xanthan gum and 0.1% carboxymethylcellulose for which the flow curve is well represented by the Cross model. Comparisons are reported between numerical calculations and the flow data, as well as with other laminar annular-flow data for a variety of shear-thinning liquids previously reported in the literature. In general, the calculations are in good quantitative agreement with the experimental data, even in situations where viscoelastic effects, neglected in the calculations, would be expected to play a role.     

Carbonate-salt-based composite materials for medium- and high-temperature thermal energy storage

This paper discusses composite materials based on inorganic salts for medium- and high-temperature thermal energy storage application. The composites consist of a phase change material （PCM）, a ceramic material, and a high thermal conductivity material. The ceramic material forms a microstructural skeleton for encapsulation of the PCM and structural stability of the composites; the high thermal conductivity material enhances the overall thermal conductivity of the composites. Using a eutectic salt of lithium and sodium carbonates as the PCM, magnesium oxide as the ceramic skeleton, and either graphite flakes or carbon nanotubes as the thermal conductivity enhancer, we produced composites with good physical and chemical stability and high thermal conductivity. We found that the wettability of the molten salt on the ceramic and carbon materials significantly affects the microstructure of the composites.     

Numerical simulation for fin effect of a rectangular latent heat storage vessel packed with molten salt under heat release process

The present numerical study has dealt with the enhancement of latent heat Release by using plate type fins mounted on the vertical cooling surface in the rectangular vessel packed with molten salt as a latent heat storage material. It was found that the fin thickness and pitch exerted an influence on solidification heat transfer in a liquid layer of a nitric molten salt. The numerical results elucidated the flow pattern, velocity profile and heat transfer rate in the melted liquid layer.     

Three-dimensional numerical analysis of heat and mass transfer in heat pipes

A three-dimensional finite-element numerical model is presented for simulation of the steady-state performance characteristics of heat pipes. The mass, momentum and energy conservation equations are solved for the liquid and vapor flow in the entire heat pipe domain. The calculated outer wall temperature profiles are in good agreement with the experimental data. The estimations of the liquid and vapor pressure distributions and velocity profiles are also presented and discussed. It is shown that the vapor flow field remains nearly symmetrical about the heat pipe centerline, even under a non-uniform heat load. The analytical method used to predict the heat pipe capillary limit is found to be conservative.     

Effect of porosity and the inlet heat transfer fluid temperature variation on the performance of cool thermal energy storage system

This paper discusses the results of numerical and experimental study of an encapsulated cool thermal energy storage system. The storage system is a cylindrical storage tank filled with phase change material encapsulated in spherical container, placed in a refrigeration loop. A simulation program was developed to evaluate the temperature histories of the heat transfer fluid and the phase change material at any axial location during the charging period. The present analysis aims at studying the influence of the inlet heat transfer fluid temperature and porosity on system performance. An experimental setup was designed and constructed to conduct the experiments. The results of the model were validated by comparison with experimental results of temperature profiles for different inlet heat transfer fluid temperatures and porosity. The results are in good agreement with the experimental results. The results reported are much useful for designing cool thermal energy storage systems.     

Beading instability in soft cylindrical gels with capillary energy: Weakly non-linear analysis and numerical simulations

Soft cylindrical gels can develop a long-wavelength peristaltic pattern driven by a competition between surface tension and bulk elastic energy. In contrast to the Rayleigh–Plateau instability for viscous fluids, the macroscopic shape in soft solids evolves toward a stable beading, which strongly differs from the buckling arising in compressed elastic cylinders.This work proposes a novel theoretical and numerical approach for studying the onset and the non-linear development of the elasto-capillary beading in soft cylinders, made of neo-Hookean hyperelastic material with capillary energy at the free surface, subjected to axial stretch. Both a theoretical study, deriving the linear and the weakly non-linear stability analyses for the problem, and numerical simulations, investigating the fully non-linear evolution of the beaded morphology, are performed. The theoretical results prove that an axial elongation can not only favour the onset of beading, but also determine the nature of the elastic bifurcation. The fully non-linear phase diagrams of the beading are also derived from finite element numerical simulations, showing two peculiar morphological transitions when varying either the axial stretch or the material properties of the gel. Since the bifurcation is found to be subcritical for very slender cylinders, an imperfection sensitivity analysis is finally performed. In this case, it is shown that a surface sinusoidal imperfection can resonate with the corresponding marginally stable solution, thus selecting the emerging beading wavelength.In conclusion, the results of this study provide novel guidelines for controlling the beaded morphology in different experimental conditions, with important applications in micro-fabrication techniques, such as electrospun fibres.     

Targeted energy transfer with parallel nonlinear energy sinks,part II: theory and experiments

In this paper governing equations of general multi degrees of freedom (dof) systems with trees of parallel Nonlinear Energy Sink (NES) devices at each dof are derived. Then these equations are summarized for a 4 dof structure with two parallel NES at the 4^th dof in order to control the first mode of the system. A prototype four storey structure with two parallel NES at the top floor is studied experimentally. The NES of the mentioned system is designed by endowing the suggested method in the Part I of this paper. A couple of experimental tests are carried out on the structure at the DGCB laboratory of the ENTPE, France. The aim is to control the first mode of the compound nonlinear system by demonstrating the efficiency of the parallel NES systems on the intended task.     

The concrete columns as a sensible thermal energy storage medium and a heater

This study investigated storage possibility of sensible thermal energy in the concrete columns of multi-storey buildings and the heating performance of the indoors with the stored energy. In the suggested system, the dry air heated in an energy center will be circulated in stainless steel pipes through columns. The sensible thermal energy would firstly be stored by means of forced convection in column medium. Then, the stored thermal energy will transfer by natural convection and radiation from the column surfaces to indoor spaces. The transient thermal calculations are realized for a flat of the 11-storey building in Kayseri city of Turkey. The thermal energy requirement of the flat is nearby 5.3 kW as an average of a winter season. The simplified transient calculations were carried out over a concrete hollow cylindrical column having outer radius of 0.31 m and inner radius of 0.05 m corresponding an averaged column section in the sample flat. The flow temperature was selected between T = 350 and 500 K, which are considerably lower than the temperature of 573 K assumed as a limit for thermal strength of the concrete in the literature. The flow velocity ranges were selected between V = 1.0 and 5.0 m/s. The initial temperature was assumed as 293 K. After the first energy charging process of 23 h, for T = 350 K and V = 1.0 m/s, the total heat flux from the column surfaces into indoors are nearby 5.5 kW. The first charging time required to reach the energy requirement of 5.3 kW is decreased by increasing the flow velocity and temperature. Also for 5.0 m/s–350 K and 5.0 m/s–450 K, this time can decrease to 10 and 4.5 h, respectively. In addition, with 4.0 m/s–360 K or 2.0 m/s–400 K, after the energy charging of 8 h, the energy requirement of 5.3 kW can be provided by the energy discharging of 16 h and the energy charging of 8 h during 7 days. The results are very attractive in terms of the building heating systems of the future.     

Wave induced thermal boundary layers in a compressible fluid: analysis and numerical simulations

The response of a semi-infinite compressible fluid to a step-wise change in temperature of its boundary is investigated analytically and numerically. Numerical results of the boundary layer structure are compared with Clarke’s analytical solution for a gas with thermal conductivity proportional to temperature. To avoid unwanted numerical dissipation in the numerical analysis, the space-time conservation element and solution element (CESE) method has been adopted to solve the unsteady 1-D Navier-Stokes equations. Good agreement between analytical and numerical results has been found for the development of the thermal boundary layer on a long time scale. Weak shock waves and expansion waves induced by the thermal boundary layer due to its compressibility, are observed in the numerical simulation. Finally, the numerical method has been applied to the reflection of a non-linear expansion wave and to a shock wave from an isothermal wall, thereby illustrating the effect of the boundary layer on the external flow field.     

Finite viscoelasticity of filled rubber: experiments and numerical simulation

Summary  Constitutive equations are derived for the viscoelastic behavior of particle-re-inforced elastomers at isothermal deformation with finite strain. A filled rubber is thought of as a composite medium where inclusions with high and low concentrations of junctions between chains are randomly distributed in the bulk material. The characteristic length of the inhomogeneities is assumed to be small compared to the size of the specimen and substantially exceed the radius of gyration for macromolecules. Inclusions with high concentration of junctions are associated with regions of suppressed mobility of chains that surround isolated clusters and/or the secondary network of filler. Regions with low concentration of junctions arise during the preparation process due to a heterogeneity in the spatial distribution of the cross-linker and the filler. With reference to the concept of transient networks, the time-dependent response of an elastomer is attribute d to thermally activated rearrangement of strands in the domains with low concentration of junctions. Stress–strain relations for particle-reinforced rubber are developed by using the laws of thermodynamics. Adjustable parameters in the constitutive equations are found by fitting experimental data in tensile relaxation tests for several grades of unfilled and carbon black-filled rubber. It is demonstrated that even at moderate finite deformations (with axial elongations up to 100%), the characteristic rate of relaxation is noticeably affected by strain. Unlike glassy polymers, where the rate of relaxation increases with longitudinal strain, the growth of the elongation ratio results in a decrease in the relaxation rate for natural rubber (unfilled or particle-reinforced). The latter may be explained by (partial) crystallization of chains in the regions with low concentration of junctions. Received 16 October 2001; accepted for publication 25 June 2002 Present address: A. D. Drozdov Department of Production, Aalborg University, Fibigerstraede 16, DK-9220 Aalborg, Denmark We would like to express our gratitude to Dr. K. Fuller (TARRC, UK) for providing us with rubber specimens and to Prof. P. Haupt and Dr. S. Hartmann (University of Kassel, Germany) for sending their experimental data. We are indebted to Mr. G. Seifritz for his assistance in performing mechanical tests. ADD acknowledges stimulating discussions with Prof. N. Aksel (University of Bayreuth, Germany).     

Non-linear dynamics of curved beams. Part 2, numerical analysis and experiments

The aim of this work is to formulate a model for the study of the dynamics of curved beams undergoing large oscillations. In Part 1, the interest was oriented to the formulation of a consistent analytical model and to obtain the equations of motion in weak form. In Part 2, a case-study is considered and the response for various initial curved configurations, obtained by varying the initial curvature, is analyzed. Both the free and the forced problems are considered: the linear free dynamics are studied to detect how the initial configuration affects the modal properties and to enlighten the typical phenomena of frequency coalescence and avoidance; the forced dynamics are then studied for different internal resonance conditions to enlighten the phenomenon of the dynamic instability under a shear periodic tip follower force and to describe the various classes of post-critical motion. The results of experimental tests conducted on a slightly imperfect straight beam prototype are eventually discussed.