Thermal Behavior of Phase Change Material During Charging Inside a Finned Cylindrical Latent Heat Energy Storage System: Effects of the Arrangement and Number of Fins

One way of storing thermal energy is through the use of latent heat energy storage systems. One such system, composed of a cylindrical container filled with paraffin wax, through which a copper pipe carrying hot water is inserted, is presented in this paper. It is shown that the physical processes encountered in the flow of water, the heat transfer by conduction and convection, and the phase change behavior of the phase change material can be modeled numerically using the finite element method. Only charging (melting) is treated in this paper. The appearance and the behavior of the melting front can be simulated by modifying the specific heat of the PCM to account for the increased amount of energy, in the form of latent heat of fusion, needed to melt the PCM over its melting temperature range. The effects of adding fins to the system are also studied, as well as the effects of the water inlet velocity.

Similarity Theory Including Variable Property Effects: A Complex Benchmark Problem

An asymptotic computational method (ACFD) to account for variable property effects is applied to a complex benchmark geometry. The method is based on the Taylor series expansion of all properties with respect to temperature and provides general results, applicable to specific problems with different fluids and heating rates. It can take into account the temperature dependence of all physical properties involved in momentum and heat transfer problems. The benchmark is a room which is ventilated by forced convection through inlet and outlet slit nozzles at the top and bottom of the side walls. Four heating elements standing on the ground floor add heat with constant heat flux density of variable strength. CFD solutions with the full coverage of all property temperature dependencies of air and SF6 are compared with the general ACFD results, applied to these fluids.

A Correlation for Nusselt Number Under Turbulent Mixed Convection Using Transient Heat Transfer Experiments

This paper reports the results of an experimental investigation of transient, turbulent mixed convection in a vertical channel in which one of the walls is heated and the other is adiabatic. The goal is to simultaneously estimate the constants in a Nusselt number correlation whose form is assumed a priori by synergistically marrying the experimental results with repeated numerical calculations that assume guess values of the constants. The convective heat transfer coefficient “h” is replaced by the Nusselt number (Nu) which is then assumed to have a form Nu = a (1+RiD) b ReDc where a, b and c are the constants to be evaluated. From the experimentally obtained temperature time history and the simulated temperature time history, based on some guess values of a, b, and c, one can define the objective function or the residue as the sum of the square of the difference between experimentally obtained and simulated temperatures. Using Bayesian inference driven by the Markov chain Monte Carlo method, one, more or all of the constants a, b and c are retrieved together with the uncertainty involved in these estimates. Additionally, the estimated parameters are compared with experimental benchmarks.

Heat Transfer Enhancement of Phase Change Materials (PCMs) in Low and High Temperature Thermal Storage by Using Porous Materials

In this paper the solid/liquid phase change heat transfer in porous materials (metal foams and expanded graphite) at low and high temperatures is experimentally investigated, in an attempt to examine the feasibility of using metal foams to enhance the heat transfer capability of phase change materials for use with both the low and high temperature thermal energy storage systems. In this research, the organic commercial paraffin wax and inorganic hydrate calcium chloride hydrate salts were employed as the low-temperature materials, while the sodium nitrate is used as the high-temperature PCM in the experiment. The heat transfer characteristics of these PCMs embedded with open-cell metal foams were studied experimentally. The composites of paraffin and expanded graphite with different graphite mass ratios, namely, 3%, 6% and 9%, were also made and the heat transfer performances of these composites were tested and compared with metal foams. Overall metal foams can provide better heat transfer performance than expanded graphite due to their continuous inter-connected structures. But the porous materials can suppress the natural convection effect in liquid zone, particularly for the PCMs with low viscosities, thereby leading to the different heat transfer performance at different regimes (solid, solid/liquid and liquid regions). This implies that the porous materials don’t necessarily mean they can always enhance heat transfer in every regime.

Turbulence Characteristics in a Natural Convection Boundary Layer Above a Heated Round Plate

A turbulent natural-convection boundary layer in air near the upper surface of a heated round plate is experimentally investigated. The heated round plate, the diameter of which is 300 mm, is horizontally placed in a test section of chamber. The upper-surface temperature is 473 K. Instantaneous temperature and two-dimensional velocity vectors near the upper surface are measured by using a cold wire and a particle image velocimetry, respectively. The measurements reveal a multi-layer structure near the upper surface. In the lower layer, velocity fluctuation of horizontal component is active compared with that of vertical component. This intermittently causes the incursion of low temperature fluids. On the other hand, in the upper layer, the intensity of vertical velocity fluctuation with high temperature fluid is much larger than that of horizontal velocity fluctuation. This multi-layer structure is mainly generated by the large-scale fluid motions, such as winding of high temperature fluids and entrainment of low temperature fluids.

Effects of Thermal Boundary Conditions on Entropy Generation During Natural Convection in a Differentially Heated Square Cavity

Natural convection is a widely occurring phenomena which has important applications in material processing, energy storage devices, electronic cooling, building ventilation etc. The concept of ‘entropy generation minimization’, which is a thermodynamic approach for optimization, may be very useful in designing efficient thermal systems. In the current study, entropy generation in steady laminar natural convection flow in a square cavity is studied with following isothermal boundary conditions: (1) Bottom wall is uniformly heated (2) Bottom wall is sinusoidally heated. The side walls are maintained cold and the top wall is maintained adiabatic. The thermal boundary condition in non-uniform heating case (case 2) is such that the dimensionless average temperature of the bottom wall is equal to that of uniform heating case (case 1). The prime objective of this work is to investigate the influence of uniform and non-uniform heating on entropy generation. The governing mass, momentum and energy equations are solved using Galerkin finite element method. Streamlines, isotherms, contour maps of entropy generation due to heat transfer and fluid friction are studied for Pr = 0.01 (molten metals) and 7 (water) in range of Ra = 103–105. Detailed analysis on the effect of uniform and non-uniform thermal boundary conditions on entropy generation due to heat transfer and fluid friction has been presented. Also, the average Bejan’s number which indicates the relative dominance of entropy generation due to heat transfer or fluid friction and the total entropy generation are studied for each case.

Air Ingress Analysis: Computational Fluid Dynamics Models

Idaho National Laboratory (INL), under the auspices of the U.S. Department of Energy (DOE), is performing research and development that focuses on key phenomena important during potential scenarios that may occur in very high temperature reactors (VHTRs). Phenomena identification and ranking studies to date have ranked an air ingress event, following on the heels of a VHTR depressurization, as important with regard to core safety. Consequently, the development of advanced air-ingress-related models and verification and validation data are a very high priority. Following a loss of coolant and system depressurization incident, air will enter the core of the High Temperature Gas Cooled Reactor through the break, possibly causing oxidation of the core and reflector graphite structure. Simple core and plant models indicate that, under certain circumstances, the oxidation may proceed at an elevated rate with additional heat generated from the oxidation reaction itself. Under postulated conditions of fluid flow and temperature, excessive degradation of lower plenum graphite can lead to a loss of structural support. Excessive oxidation of core graphite can also lead to a release of fission products into the confinement, which could be detrimental to reactor safety. Computational fluid dynamics models developed in this study will improve our understanding of this phenomenon. This paper presents two-dimensional (2-D) and three-dimensional (3-D) computational fluid dynamic (CFD) results for the quantitative assessment of the air ingress phenomena. A portion of the results from density-driven stratified flow in the inlet pipe will be compared with the experimental results.

Formation of a Counter-Rotating Vortex Pair in a Plume in a Cross-Flow

Numerical simulations are performed to study the formation of a counter-rotating vortex pair (CVP), a dominant flow feature in plumes inclined in a cross-flow. The unsteady three-dimensional flow fields are calculated by a finite difference method using the Boussinesq approximation. A plume rises from an isothermally heated square surface facing upward in air. Calculations show that the CVP originates not from horizontal spanwise vorticity in the velocity boundary layer on the bottom wall around the heated area, but from horizontal streamwise vorticity just above each side of the heated area. When the cross-flow begins after a plume forms a vortex ring in the cap above the heated area in a still environment, the vortex ring does not form a CVP. However, as the cap and the stem of the plume move downwind, a rotation about the streamwise axis appears just above each side edge of the heated area and grows into the CVP. We discuss the effect of entrainment into the stem and cap on the formation of the streamwise rotation that causes the CVP.

Air-Heating Solar Collectors for Humidification-Dehumidification Desalination Systems

Relative to solar water heaters, solar air heaters have received relatively little investigation and have resulted in few commercial products. However, in the context of a Humidification-Dehumidification (HDH) Desalination cycle, air heating accounts for advantages in cycle performance. Solar collectors can be over 40% of an air-heated HDH system’s cost, thus design optimization is crucial. Best design practices and sensitivity to material properties for solar air heaters are investigated, and absorber solar absorptivity and glazing transmissivity are found to have the strongest effect on performance. Wind speed is also found to have an impact on performance. Additionally a well designed, and likely low cost, collector includes a double glazing and roughened absorber plates for superior heat transfer to the airstream. A collector in this configuration performs better than current collectors with an efficiency of 58% at a normalized gain of 0.06 K m2/W.

The Effect of Surface Radiation and Conjugate Conduction on Laminar Free Convection From a Vertical Flat Plate

The role of conduction and surface radiation on laminar free convection heat transfer from a heated vertical flat plate has been studied. Steady state experiments have been conducted on vertical flat plates, of different thermal conductivities and surface emissivities, with an embedded heater and the results have been reported in [1]. The plate dimensions were held fixed in all the experiments. An effort is made here to identify important parameters that are involved in wall conduction - free convection - radiation interaction phenomena. The convective heat transfer from a vertical surface is affected by the surface temperature of the plate and its variations which is influenced by two other modes of heat transfer, conduction within the plate and surface radiation. Hence, the present paper attempts to understand the interaction phenomenon between the three modes of heat transfer and explain the temperature distributions within the plate, observed both experimentally and in numerical simulations. It is found that radiation is very important as it significantly affects the temperature distribution along the plate.