Transient Natural Convection in Enclosures With Application to Solar Thermal Storage Tanks

1992 ◽  
Vol 114 (3) ◽  
pp. 175-181 ◽  
Author(s):  
D. T. Reindl ◽  
W. A. Beckman ◽  
J. W. Mitchell
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Heat Exchangers ◽  
Thermal Storage ◽  
Solar Thermal ◽  
Storage Tanks ◽  
Time Duration ◽  
Source Temperature ◽  
Fluid Field ◽  
Isothermal Fluid

Many previously studied natural convection enclosure problems in the literature have the bounding walls of the enclosure responsible for driving the flow. A number of relevant applications contain sources within the enclosure which drive the fluid flow and heat transfer. The motivation for this work is found in solar thermal storage tanks with immersed coil heat exchangers. The heat exchangers provide a means to charge and discharge the thermal energy in the tank. The enclosure is cylindrical and well insulated. Initially the interior fluid is isothermal and quiescent. At time zero, a step change in the source temperature begins to influence the flow. The final condition is a quiescent isothermal fluid field at the source temperature. The governing time-dependent Navier-Stokes and energy equations for this configuration are solved by a finite element method. Solutions are obtained for 103≤RaD≤106. Scale analysis is used to obtain time duration estimates of three distinct heat transfer regimes. The transient heat transfer during these regimes are compared with limiting cases. Correlations are presented for the three regimes.

Effect of Shroud and Baffle on Heat Transfer in a Solar Thermal Storage Tank

2009 ◽  
Author(s):  
S. K. S. Boetcher ◽  
F. A. Kulacki
Keyword(s):  
Heat Transfer ◽  
Heat Exchanger ◽  
Transfer Rate ◽  
Thermal Storage ◽  
Solar Thermal ◽  
Discharge Process ◽  
Storage Tanks ◽  
Nusselt Numbers ◽  
Enhancing Heat Transfer ◽  
Rayleigh Numbers

Enhancing heat transfer during the charge and discharge of solar thermal storage tanks is an ongoing technical challenge. The types of thermal storage systems considered in the present study comprise an immersed heat exchanger at the top of a solar thermal storage fluid. The discharge process of a thermal store with specified dimensions is numerically simulated over a range of Rayleigh numbers, 105 < RaD <107. The immersed heat exchanger is modeled as a two-dimensional isothermal cylinder which is situated near the top of a water-filled tank with adiabatic walls. An adiabatic shroud whose shape is parametrically varied is placed around the cylinder. In addition, the shroud is connected to an adiabatic baffle situated beneath the cylinder. Nusselt numbers are calculated for different shroud shapes at different Rayleigh numbers. Results show that the shroud is effective in increasing the heat transfer rate. Optimal shroud and baffle geometries are presented as well as qualitative flow results.

Novel Ionic Liquid Thermal Storage for Solar Thermal Electric Power Systems

2001 ◽  
Author(s):  
Banqiu Wu ◽  
Ramana G. Reddy ◽  
Robin D. Rogers
Keyword(s):  
Heat Transfer ◽  
Ionic Liquids ◽  
Thermal Power ◽  
Thermal Storage ◽  
Solar Thermal ◽  
Liquid Temperature ◽  
Storage Density ◽  
Heat Transfer Fluids ◽  
Solar Thermal Power ◽  
Storage Media

Abstract Feasibility of ionic liquids as liquid thermal storage media and heat transfer fluids in a solar thermal power plant was investigated. Many ionic liquids such as [C4min][PF6], [C8mim][PF6], [C4min][bistrifluromethane sulflonimide], [C4min][BF4], [C8mim][BF4], and [C4min][bistrifluromethane sulflonimide] were synthesized and characterized using thermogravimetric analysis (TGA), differential scanning calorimeter (DSC), nuclear magnetic resonance (NMR), viscometry, and some other methods. Properties such as decomposition temperature, melting point, viscosity, density, heat capacity, and thermal expansion coefficient were measured. The calculated storage density for [C8mim][PF6] is 378 MJ/m3 when the inlet and outlet field temperatures are 210°C and 390°C. For a single ionic liquid, [C4mim][BF4], the liquid temperature range is from −75°C to 459°C. It is found that ionic liquids have advantages of high density, wide liquid temperature range, low viscosity, high chemical stability, non-volatility, high heat capacity, and high storage density. Based on our experimental results, it is concluded that ionic liquids could be excellent liquid thermal storage media and heat transfer fluids in solar thermal power plant.

Numerical analysis of a hybrid tubular and cavity air receiver for solar thermal applications

2020 ◽  
Vol ahead-of-print (ahead-of-print) ◽  
Author(s):  
Sayuj Sasidharan ◽  
Pradip Dutta
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Numerical Analysis ◽  
Heat Loss ◽  
Solar Thermal ◽  
Heat Transfer Analysis ◽  
Receiver Design ◽  
Radiation Emission ◽  
Content Type ◽  
Thermal Receiver

Purpose This paper aims to deal with characterisation of the thermal performance of a hybrid tubular and cavity solar thermal receiver. Design/methodology/approach The coupled optical-flow-thermal analysis is carried out on the proposed receiver design. Modelling is performed in two and three dimensions for estimating heat loss by natural convection for an upward-facing cavity. Heat loss obtained in two dimensions by solving coupled continuity, momentum and energy equation inside the cavity domain is compared with the loss obtained using an established Nusselt number correlation for realistic receiver performance prediction. Findings It is found that radiation emission from a heated cavity wall to the ambient is the dominant mode of heat loss from the receiver. The findings recommend that fluid flow path must be designed adjacent to the surface exposed to irradiation of concentrated flux to limit conduction heat loss. Research limitations/implications On-sun experimental tests need to be performed to validate the numerical study. Practical implications Numerical analysis of receivers provides guidelines for effective and efficient solar thermal receiver design. Social implications Pressurised air receivers designed from this method can be integrated with Brayton cycles using air or supercritical carbon-dioxide to run a turbine generating electricity using a solar heat source. Originality/value The present paper proposes a novel method for coupling the flux map from ray-tracing analysis and using it as a heat flux boundary condition for performing coupled flow and heat transfer analysis. This is achieved using affine transformation implemented using extrusion coupling tool from COMSOL Multiphysics software package. Cavity surface natural convection heat transfer coefficient is obtained locally based on the surface temperature distribution.

Effect of Natural Convection on Thermal Energy Storage in Supercritical Fluids

2013 ◽  
Author(s):  
Reza Baghaei Lakeh ◽  
Adrienne S. Lavine ◽  
H. Pirouz Kavehpour ◽  
Gani B. Ganapathi ◽  
Richard E. Wirz
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Natural Convection ◽  
Energy Storage ◽  
Supercritical Fluid ◽  
Supercritical Fluids ◽  
Thermal Energy ◽  
Thermal Energy Storage ◽  
Storage System ◽  
Thermal Storage

Heat transfer to the storage fluid is a critical subject in thermal energy storage systems. The storage fluids that are proposed for supercritical thermal storage system are organic fluids that have poor thermal conductivity; therefore, pure conduction will not be an efficient heat transfer mechanism for the system. The current study concerns a supercritical thermal energy storage system consisting of horizontal tubes filled with a supercritical fluid. The results of this study show that the heat transfer to the supercritical fluid is highly dominated by natural convection. The buoyancy-driven flow inside the storage tubes dominates the flow field and enhances the heat transfer dramatically. Depending on the diameter of the storage tube, the buoyancy-driven flow may be laminar or turbulent. The natural convection has a significant effect on reducing the charge time compared to pure conduction. It was concluded that although the thermal conductivity of the organic supercritical fluids are relatively low, the effective laminar or turbulent natural convection compensates for this deficiency and enables the supercritical thermal storage to charge effectively.

The Correlation of Heat Transfer Coefficients for the Laminar Natural Convection in a Circular Finned-Tube Heat Exchanger

2017 ◽  
Vol 140 (3) ◽  
Author(s):  
Hie Chan Kang ◽  
Se-Myong Chang
Keyword(s):  
Heat Transfer ◽  
Experimental Data ◽  
Nusselt Number ◽  
Natural Convection ◽  
Heat Exchanger ◽  
Heat Exchangers ◽  
Finned Tube ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Laminar Natural Convection

This study proposes an empirical correlation for laminar natural convection applicable to external circular finned-tube heat exchangers with wide range of configuration parameters. The transient temperature response of the heat exchangers was used to obtain the heat transfer coefficient, and the experimental data with their characteristic lengths are discussed. The data lie in the range from 1 to 1000 for Rayleigh numbers based on the fin spacing: the ratio of fin height to tube diameter ranges from 0.1 to 0.9, and the ratio of fin pitch to height ranges from 0.13 to 2.6. Sixteen sets of finned-tube electroplated with nickel–chrome were tested. The convective heat transfer coefficients on the heat exchangers were measured by elimination of the thermal radiation effect from the heat exchanger surfaces. The Nusselt number was correlated with a newly suggested composite curve formula, which converges to the quarter power of the Rayleigh number for a single cylinder case. The proposed characteristic length for the Rayleigh number is the fin pitch while that for the Nusselt number is mean flow length, defined as half the perimeter of the mean radial position inside the flow region bounded by the tube surface and two adjacent fins. The flow is regarded as laminar, which covers heat exchangers from a single horizontal cylinder to infinite parallel disks. Consequently, the result of curve fitting for the experimental data shows the reasonable physical interpretation as well as the good quantitative agreement with the correction factors.

A Numerical Study Concerning the Influence of the Grashof Number in the Indirect Ice Storage Tanks Performance

2006 ◽  
Author(s):  
Cleyton S. Stampa ◽  
Angela O. Nieckele
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Phase Change ◽  
Flow Pattern ◽  
Storage Tank ◽  
Vertical Wall ◽  
Heat Transfer Analysis ◽  
Ice Storage ◽  
Storage Tanks ◽  
Grashof Number

The present paper deals with typical chiller-based ice storage tanks. Natural convection of water (Phase Change Material-PCM) near its density maximum leads to a peculiar nature of the flow pattern in the liquid phase of the PCM, giving rise to a multi-cellular regime that affects drastically the heat transfers within the tank. So, this work intends to examine numerically how the flow pattern affects qualitatively the performance of such thermal storage devices. This is done by investigating the influence of the non-dimensional parameter Grashof number during the charging operation step of such devices that corresponds to the ice making process occurring within the storage tank. Besides, the tank is assumed to be vertically positioned, as well as their internal tubes through which the secondary fluid flows. In order to analyze the heat transfer between the PCM and one internal tube during the growth of an ice layer around it, one selected a vertical annulus as the physical model. The inner vertical wall represents one of the tubes packed into a typical storage tank, while the outer vertical wall represents the thickness of formed ice around the tube. Regarding the annulus, the top and bottom walls, as well as the outer vertical wall were considered thermally insulated. The thermal analysis is focused in the heat transfer at the inner wall for different values of Grashof, keeping unchanged all other parameters that govern the natural convection problem with phase change. An overview of the cooling process is analyzed through streamlines and isotherms, for specific instants of the physical process. Further, a heat transfer analysis for the total charging stage is presented.

The Cooling Potential of Sky Radiation With Variations in System Parameters

2019 ◽  
Vol 141 (4) ◽  
Author(s):  
M. Adrienne Parsons ◽  
M. Keith Sharp
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Heat Pipe ◽  
Storage Capacity ◽  
Thermal Storage ◽  
Ambient Air ◽  
View Factor ◽  
Water Tank ◽  
Cooling Load ◽  
Tank Wall

This study evaluated the building cooling capacity of sky radiation, which was previously identified to have the greatest cooling potential among common ambient sources for climates across the U.S. A heat pipe augmented sky radiator system was simulated by a thermal network with nine nodes, including a thin polyethylene cover with and without condensation, white (zinc oxide) painted radiator plate, condenser and evaporator ends of the heat pipe, thermal storage fluid (water), tank wall, room, sky and ambient air. Heat transfer between nodes included solar flux and sky radiation to cover and plate, wind convection and radiation from cover to ambient, radiation from plate to ambient, natural convection and radiation from plate to cover, conduction from plate to condenser, two-phase heat transfer from evaporator to condenser, natural convection from evaporator to water and from water to tank wall, natural convection and radiation from tank wall to room, and overall heat loss from room to ambient. A thin layer of water was applied to simulate condensation on the cover. Nodal temperatures were simultaneously solved as functions of time using typical meteorological year (TMY3) weather data. Auxiliary cooling was added as needed to limit room temperature to a maximum of 23.9 °C. For this initial investigation, a moderate climate (Louisville, KY) was used to evaluate the effects of radiator orientation, thermal storage capacity, and cooling load to radiator area ratio (LRR). Results were compared to a Louisville baseline with LRR = 10 W/m2 K, horizontal radiator and one cover, which provided an annual sky fraction (fraction of cooling load provided by sky radiation) of 0.855. A decrease to 0.852 was found for an increase in radiator slope to 20 deg, and a drop to 0.832 for 53 deg slope (latitude + 15 deg, a typical slope for solar heating). These drops were associated with increases in average radiator temperature by 0.73 °C for 20 deg and 1.99 °C for 53 deg. A 30% decrease in storage capacity caused a decrease in sky fraction to 0.843. Sky fractions were 0.720 and 0.959 for LRR of 20 and 5, respectively. LRR and thermal storage capacity had strong effects on performance. Radiator slope had a surprisingly small impact, considering that the view factor to the sky at 53 deg tilt is less than 0.5.

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