Second Law Aspects of Simplified Models for Sensible Thermal Storage

2000 ◽  
Author(s):  
K. O. Homan
Keyword(s):  
Numerical Model ◽  
Entropy Generation ◽  
Thermal Storage ◽  
Heat Fluxes ◽  
Analytical Models ◽  
Storage Devices ◽  
Thermal Mixing ◽  
Thermal Layer ◽  
Temperature Liquid ◽  
The One

Abstract This paper presents results for entropy generation during the inflow of a low temperature stream into a sensible thermal storage vessel initially filled with a uniformly high temperature liquid. The level of internal entropy generation due to thermal mixing between the cold and hot liquid corresponds to losses in the usable fraction of the stored volume and therefore decreased efficiency. Empirically, the observed behavior of sensible storage devices spans the range of nearly mixed to well stratified. In this investigation, analytical models for these two limits, the fully mixed and ideally stratified conditions, are used to bound the entropy generation levels of the observed behaviors. A numerical model for stratified storage systems based on the one-dimensional convective energy equation which accounts for aspects of the observed thermal mixing is then examined in relation to the afore-mentioned limits. The results show that even at moderate throughflow rates, the fully mixed and ideally stratified limits are separated by orders of magnitude in terms of entropy generated. The empirically-based numerical model exhibits mixing levels midway between these two limits and thereby underscores the potential for significant improvements in efficiency. Examination of the numerical model shows the crucial importance of resolving the evolution of the interior thermal layer and the boundary heat fluxes in computing the entropy generation.

Internal Entropy Generation Limits for Direct Sensible Thermal Storage

2003 ◽  
Vol 125 (2) ◽  
pp. 85-93 ◽  
Author(s):  
K. O. Homan
Keyword(s):  
Entropy Generation ◽  
Analytical Models ◽  
Liquid Volume ◽  
Liquid Stream ◽  
Storage Devices ◽  
Thermal Mixing ◽  
Energy Store ◽  
Charging And Discharging Processes ◽  
Temperature Liquid ◽  
High Or Low Temperature

This paper presents results for the entropy generated internally during the charging and discharging processes of a direct, sensible thermal energy store. The two processes correspond to the inflow of either a low or high temperature liquid stream into an enclosure initially filled with a uniformly high or low temperature liquid, respectively. The level of internal entropy generation due to thermal mixing between the inflow and the initial liquid volume corresponds to losses in the usable fraction of the stored volume and therefore decreased efficiency. Empirically, the observed behavior of direct sensible storage devices spans the range of nearly mixed to highly stratified. In the present work, analytical models for the fully-mixed and ideally-stratified limits are used to bound these behaviors and to analytically determine the corresponding entropy generation levels. The ratio of total entropy generation for the ideally-stratified limit relative to that of the fully-mixed limit is shown to vary as 8/πPe. The limiting behaviors therefore define a continuum of entropy generation levels separated by up to several orders of magnitude for typical Peclet numbers. A published numerical model which accounts for aspects of the observed thermal mixing is then examined in relation to these limits. The model predicts entropy generation levels midway between the limiting behaviors which suggests significant potential for improvements in the efficiency of direct sensible storage devices.

On Simplified Models for the Rate- and Time-Dependent Performance of Stratified Thermal Storage

2007 ◽  
Vol 129 (3) ◽  
pp. 214-222 ◽  
Author(s):  
Ying Ji ◽  
K. O. Homan
Keyword(s):  
Entropy Generation ◽  
Peclet Number ◽  
Volume Fraction ◽  
Thermal Storage ◽  
Time Dependent ◽  
Simplified Models ◽  
Péclet Number ◽  
Thermal Mixing ◽  
Discharge Volume ◽  
Cumulative Entropy

In direct sensible thermal storage systems, both the energy discharging and charging processes are inherently time-dependent as well as rate-dependent. Simplified models which depict the characteristics of this transient process are therefore crucial to the sizing and rating of the storage devices. In this paper, existing models which represent three distinct classes of models for thermal storage behavior are recast into a common formulation and used to predict the variations of discharge volume fraction, thermal mixing factor, and entropy generation. For each of the models considered, the parametric dependence of key performance measures is shown to be expressible in terms of a Peclet number and a Froude number or temperature difference ratio. The thermal mixing factor for each of the models is reasonably well described by a power law fit with Fr2Pe for the convection-dominated portion of the operating range. For the uniform and nonuniform diffusivity models examined, there is shown to be a Peclet number which maximizes the discharge volume fraction. In addition, the cumulative entropy generation from the simplified models is compared with the ideally-stratified and the fully-mixed limits. Of the models considered, only the nonuniform diffusivity model exhibits an optimal Peclet number at which the cumulative entropy generation is minimized. For each of the other models examined, the cumulative entropy generation varies monotonically with Peclet number.

scholarly journals Analytical Model of Induction Machines with Multiple Cage Faults Using the Winding Tensor Approach

Sensors ◽  
2021 ◽  
Vol 21 (15) ◽  
pp. 5076
Author(s):  
Javier Martinez-Roman ◽  
Ruben Puche-Panadero ◽  
Angel Sapena-Bano ◽  
Carla Terron-Santiago ◽  
Jordi Burriel-Valencia ◽  
...  
Keyword(s):  
Computational Cost ◽  
Induction Machines ◽  
Diagnostic Techniques ◽  
Analytical Models ◽  
Tensor Algebra ◽  
Diagnostic Systems ◽  
Technical Literature ◽  
Twin Model ◽  
Ring Segment ◽  
The One

Induction machines (IMs) are one of the main sources of mechanical power in many industrial processes, especially squirrel cage IMs (SCIMs), due to their robustness and reliability. Their sudden stoppage due to undetected faults may cause costly production breakdowns. One of the most frequent types of faults are cage faults (bar and end ring segment breakages), especially in motors that directly drive high-inertia loads (such as fans), in motors with frequent starts and stops, and in case of poorly manufactured cage windings. A continuous monitoring of IMs is needed to reduce this risk, integrated in plant-wide condition based maintenance (CBM) systems. Diverse diagnostic techniques have been proposed in the technical literature, either data-based, detecting fault-characteristic perturbations in the data collected from the IM, and model-based, observing the differences between the data collected from the actual IM and from its digital twin model. In both cases, fast and accurate IM models are needed to develop and optimize the fault diagnosis techniques. On the one hand, the finite elements approach can provide highly accurate models, but its computational cost and processing requirements are very high to be used in on-line fault diagnostic systems. On the other hand, analytical models can be much faster, but they can be very complex in case of highly asymmetrical machines, such as IMs with multiple cage faults. In this work, a new method is proposed for the analytical modelling of IMs with asymmetrical cage windings using a tensor based approach, which greatly reduces this complexity by applying routine tensor algebra to obtain the parameters of the faulty IM model from the healthy one. This winding tensor approach is explained theoretically and validated with the diagnosis of a commercial IM with multiple cage faults.

Exergy Analysis of Thermal Energy Storage With Specific Remarks on the Variation of the Environmental Temperature

1996 ◽  
Vol 118 (2) ◽  
pp. 81-88 ◽  
Author(s):  
G. Bisio
Keyword(s):  
Energy Storage ◽  
Constant Temperature ◽  
Exergy Analysis ◽  
Environmental Temperature ◽  
Variable Temperature ◽  
Thermal Storage ◽  
Exergy Efficiency ◽  
Basic Question ◽  
Storage Devices ◽  
Temperature Environment

Energy storage is a key technology for many purposes and in particular for air conditioning plants and a successful exploitation of solar energy. Thermal storage devices are usually classified as either variable temperature (“sensible heat”) or constant temperature (“latent heat”) devices. For both models a basic question is to determine the efficiency suitably: Only exergy efficiency appears a proper way. The aim of this paper is to examine exergy efficiency in both variable and constant temperature systems. From a general statement of exergy efficiency by the present author, two types of actual definitions are proposed, depending on the fact that the exergy of the fluid leaving the thermal storage during the charge phase can be either totally lost or utilized elsewhere. In addition, specific remarks are made about the exergy of a system in a periodically varying temperature environment.

The Mechanism of and Stability Criterion for Nucleate Pool Boiling of Sodium

1969 ◽  
Vol 91 (3) ◽  
pp. 315-328 ◽  
Author(s):  
I. Shai ◽  
W. M. Rohsenow
Keyword(s):  
Heat Flux ◽  
Liquid Metals ◽  
Bubble Formation ◽  
Pool Boiling ◽  
Heat Fluxes ◽  
Nucleate Pool Boiling ◽  
Thermal Layer ◽  
Minimum Heat ◽  
Bubble Growth And Departure ◽  
Recorded Temperature

Experimental data for sodium boiling on horizontal surfaces containing artificial cavities at heat fluxes of 20,000 to 300,000 Btu/ft2 hr and pressures between 40 to 106 mm Hg were obtained. Observations are made for stable boiling, unstable boiling and “bumping.” Some recorded temperature variations in the solid close to the nucleating cavity are presented. It is suggested that for liquid metals the time for bubble growth and departure is a very small fraction of the total bubble cycle, hence the delay time during which a thermal layer grows is the most significant part of the process. On this basis the transient conduction heat transfer is solved for a periodic process, and the period time is found to be a function of the degree of superheat, the heat flux and the liquid thermal properties. A simplified model for stability of nucleate pool boiling of liquid metals is postulated from which the minimum heat flux for stable boiling can be found as a function of liquid-solid properties, liquid pressure, the degree of superheat, and the cavity radius and depth. At relatively low heat fluxes, convection currents have significant effects on the period time of bubble formation. An empirical correlation is proposed, which takes into account the convection effects, to match the experimental results.

A Numerical Model of In-Channel Pebble Bed Thermal Storage for a Solar Chimney

2012 ◽  
Author(s):  
Brandon Schulte ◽  
O. A. Plumb
Keyword(s):  
Numerical Model ◽  
Present Model ◽  
Thermal Storage ◽  
Implicit Time ◽  
Heat Transfer Characteristics ◽  
Solar Chimney ◽  
Pebble Bed ◽  
One Dimensional ◽  
Time Stepping ◽  
Daily Energy

In this study, solar chimney performance is numerically modeled. Previously published models have considered water bags and natural earth as means to store daytime thermal energy for nighttime operation of the system. The present model considers in-channel pebble bed thermal storage. A one-dimensional, implicit time stepping numerical model is developed to predict solar chimney performance throughout a 24 hour period. The model is partially verified with available experimental data. The daily energy, daily efficiency and heat transfer characteristics of the solar chimney with pebble bed thermal storage are summarized. The numerical simulation showed that by introducing a pebble bed, nightly exit velocities reach 40% of the peak daytime velocity. However, the daily kinetic energy delivered by a solar chimney with pebble bed thermal storage is much less than a traditional solar chimney, suggesting pebble bed thermal storage is more practicable in building heating applications as opposed to power generation.

An approach to simulation of dual drainage

1999 ◽  
Vol 39 (9) ◽  
pp. 95-103 ◽  
Author(s):  
S. Djordjević ◽  
D. Prodanović ◽  
Č. Maksimović
Keyword(s):  
Numerical Model ◽  
Model Development ◽  
Ground Level ◽  
Surface Flow ◽  
Surface Excess ◽  
Runoff Simulation ◽  
Sewer System ◽  
Excess Water ◽  
Flow Through ◽  
The One

The paper presents the development of the field of urban drainage modelling known as dual drainage - an approach to rainfaill runoff simulation in which the numerical model takes into account not only the flow through the sewer system, but also the flow on the surface. The steps in model development are described, and necessary data, assumptions used and operations to be performed using GIS are discussed. The numerical model simultaneously handles the full dynamic equations of flow through the sewer system and simplified equations of the surface flow. The surface excess water (due to the limited capacity of inlets or to the hydraulic head in the sewer system reaching the ground level) is routed to the neighbour subcatchment (not necessarily the one attached to the downstream network node), using surface retentions, if any.

Experimental and Numerical Study of Mixing in a Hot-Water Storage Tank

2009 ◽  
Vol 131 (1) ◽  
Author(s):  
A. Aviv ◽  
Y. Blyakhman ◽  
O. Beeri ◽  
G. Ziskind ◽  
R. Letan
Keyword(s):  
Numerical Model ◽  
Storage Tank ◽  
Numerical Study ◽  
Tap Water ◽  
Three Dimensional ◽  
Hot Water ◽  
Software Validation ◽  
Water Storage Tank ◽  
Thermal Mixing ◽  
Numerical Investigations

Thermal mixing and stratification are explored numerically and experimentally in a cylindrical tank, which simulates a storage of water heated by a solar collector. The tank is 70cm in height and 24cm in diameter. The inlet and outlet are vertical and located off the centerline of the tank. The study is conducted in a transient mode, namely, the tank is filled with hot water, and as the hot water is being withdrawn, the tap water replaces it in a stratified way or by mixing. The flowrates of 2l∕min, 3l∕min, 5l∕min and 7l∕min, which correspond to superficial velocities of 4.35cm∕min, 6.52cm∕min, 10.87cm∕min, and 15.2cm∕min, are explored. Temperature of hot water ranges within 40–50°C, while the tap water is about 25–27°C. Installation of one and two horizontal baffles above the inlet is examined. Simultaneous experimental and numerical investigations are performed. In the experiment, both flow visualization and temperature measurements are used. Three-dimensional transient numerical simulations are done using the FLUENT 6 software. Validation of the numerical model is achieved by comparison with the experimental results. Then, the numerical model is applied to a study of various possible changes in the system. The results show that at low flowrates, up to a superficial velocity of about 11cm∕min through the tank, the baffles have no effect on tap water mixing with the stored hot water. At higher flowrates, a single horizontal baffle prevents the mixing and preserves the desired stratified temperature distribution in the storage tank.

scholarly journals Performance of Movable Head Disk Storage Devices with Various Disk Scheduling Algorithms

2019 ◽  
Vol 9 (2S) ◽  
pp. 486-490
Keyword(s):  
Scheduling Algorithms ◽  
Access Time ◽  
Processing Unit ◽  
Disk Scheduling ◽  
Efficient Manner ◽  
Hard Drives ◽  
Storage Devices ◽  
Disk Storage ◽  
Central Processing ◽  
The One

Hard drives are the one which needs to be accessed in an efficient manner so that it is feasible to get better recital of the central processing unit. Now a day’s magnetic disks are capable of providing more input output bandwidth yet a huge amount of this bandwidth is lost due to the access time of the hard disk. This paper discusses an analysis of performance of various disk scheduling algorithms with their merits and demerits

Sign in / Sign up

Export Citation Format

Share Document