Thermal Energy Release Rate Research Articles

Thermal performance of phase change materials with anisotropic carbon fiber inserts

In thermal energy storage systems, phase change materials (PCMs) are widely used for thermal energy management. Most PCMs have low thermal conductivities, which limits the heat transfer rate within PCM and thus makes the phase-changing process very slow. However, thermal conductivities of PCMs can be altered by inserting targeted additives. We hypothesize that the size and shape of these additive inserts play a key role in thermal management efficiency. To this end, the impacts of carbon fiber (CF) inserts on the phase change behavior and consequent heat transfer efficiencies of inorganic and organic PCMs were investigated using experiments and simulations. Long, anisotropic CFs with high thermal conductivities formed continuous fast heat flux tunnels inside PCMs to enhance the heat transfer. Such CFs could extend the phase change fronts from the limited container-shaped interface to the larger surface of numerous CF inserts inside the PCM. These special CF inserts work with a new heat transfer mechanism, different from conventional small additives or long isotropic CF inserts. The thermal energy release rate increased by 2.5 times with 1 wt.% anisotropic CF inserts in inorganic PCM. However, CF inserts in liquid organic PCM hindered the natural convection and compromised the improved heat conduction. The lab-scale multiphysics simulations support these experimental observations and indicate that CF inserts have potential to enhance heat transfer in inorganic PCMs, but they are less effective in organic PCMs.

Parameter optimization of unevenly spiderweb-shaped fin for enhanced solidification performance of shell and tube latent-heat thermal energy storage units

This paper investigates the solidification process of fin-strengthened shell and tube latent-heat thermal energy storage unit (LTESU). An unevenly spiderweb-shaped fin is proposed to fit better with shell and tube structures. This new type of fin is connected to the radial fins by annular fins. The uneven arrangement is achieved by varying the distance between the annular fins and the width ratio of the annular fin to the radial fin. Numerical simulation is used to investigate the strengthening effect of the novel fin on the solidification performance of LTESU. Moreover, to achieve better solidification properties, three parameters of the spiderweb-shaped fin are optimized for an equal share of metal fins (the position of the outermost annular fin γ, decreasing common difference of distance between annular fins ψ, and width ratio δ). The results show that there are optimum values for all three parameters to enable LTESU to achieve the shortest complete solidification time and the highest thermal energy release rate (TERR). The optimized spiderweb-shaped fin has a structure with larger annular fin spacing close to the inner tube, smaller annular fin spacing away from the inner tube, and thicker radial fins, thinner ring fins. It is recommended that the configuration parameters for the spiderweb-shaped fin are γ = 0.94, ψ = 0.05, and δ = 0.10. Compared to conventional rectangular fins, it ultimately reduces the complete solidification time by 88.9 % and increases the TERR by 7.8 times.

Evaluation of discharging performance of molten salt/ceramic foam composite phase change material in a shell-and-tube latent heat thermal energy storage unit

Molten salts are widely used energy storage media in integrated solar power systems, however, due to their high corrosivity and the extreme high-temperature environment, many current methods of heat transfer enhancement do not apply. Herein, we proposed to use ceramic foam to enhance the discharging performance of molten salt to efficiently supply heat for power generation. The ceramic foam was prepared and its corrosion resistance was confirmed experimentally. The discharging performance in a shell-and-tube latent heat thermal energy storage unit was numerically studied. It is found that compared to the configuration without enhancement, the solidification time of the ceramic foam-enhanced unit is shortened by up to 52.0%. The unit with the upper foam insert shows better discharging performance than the one with the lower foam insert. Different foam filling height is also considered and results indicate that the thermal energy release rate always increases with the foam filling height and that of the fully foam-inserted unit is 118.1% higher than that of the none foam-inserted unit. For the first time, the discharging performance of ceramic foam-enhanced molten salt is quantitatively evaluated. This study provides guidance on designing thermal energy storage sub-system with excellent heat supply performance for solar power generation.

Thermal fracture characteristics of an interface crack subjected to temperature variations

Abstract Thermal fracture characteristics – the thermal energy release rate and thermal stress intensity factor of a semi-infinite crack at an interface between the two elastic isotropic materials, subjected to the temperature variations, are considered in this paper. Those characteristics are determined based on application of the linear elastic fracture mechanics (LEFM) concept. Expressions for obtained theoretical solutions are compared to solutions from literature and they are found to be more concise. Influence of the materials change on these two thermal fracture properties were observed, as well as the influence of the thickness ratio of the two layers constituting the interface.

Energetic Relations between the Disappearing Solar Filaments and the Associated Flare Arcades

Abstract We present the temporal and statistical relations between the mechanical energies of disappearing solar filaments and the thermal energies of the associated flare arcades in soft X-rays. Measuring the 3-D velocity fields of 10 eruptive filaments, we calculated their mechanical energy gain rate, $\epsilon_{\rm mc}$, per unit volume and compared it to the thermal energy release rate per unit volume, $\epsilon_{\rm th}$, derived with Yohkoh/SXT data. For the statistical relation, we found a relation that can be approximated as $\epsilon_{\rm th} \propto \epsilon_{\rm mc}^{1.9}$. This relation can be explained by interpreting the energy input to an arcade via the Poynting flux in the magnetic reconnection process and the acceleration of a filament by the Lorentz force. This explanation is also supported by the strong dependence of the observed increase rates of both the thermal and mechanical energy densities on the mean magnetic field strength of the source region. We also investigated their temporal variations, and found that the start time of increase in the mechanical energy of a filament preceded that of the thermal energy of the coronal arcade in some cases. These relations imply that the basic mechanisms that accelerate a filament and create a hot plasma are different, and both energy increase rates are determined primary by the magnetic field strengths.

The Relationship between Flux Rope Acceleration and Thermal Energy Release in a Model of Eruptive Solar Phenomena

In this paper, we examine the relationship between the thermal energy release rate and the flux rope acceleration in a loss-of-equilibrium CME model. The model consists of a flux rope in equilibrium in the corona and two point sources at the chromosphere. As the point sources are brought together quasi-statically, the equilibrium state of the flux rope is disturbed, and an eruption occurs. After the eruption begins, a current sheet forms under the flux rope. The thermal energy release rate is assumed to be equal to the Poynting flux swept into the current sheet as the flux rope accelerates. We find that the flux rope acceleration and the thermal energy release rate are well correlated when the background magnetic field is high and the reconnection rate is fast. They are not well correlated for events with low magnetic fields and slow reconnection rates.

Predicted Light Curves for a Model of Solar Eruptions

We determine the thermal radiation generated by a loss-of-equilibrium model for CMEs and eruptive solar flares. The magnetic configuration of the model consists of an outward-moving flux rope with a vertical current sheet below it. Reconnection at the sheet releases magnetic energy, some of which is converted into thermal energy that drives chromospheric evaporation along the newly connected field lines exiting the current sheet. The thermal energy release is calculated by assuming that all of the Poynting flux flowing into the reconnection region is eventually thermalized. We find that the fraction of the released magnetic energy that goes into thermal energy depends on the inflow Alfven Mach number. The evolution of the temperatures and densities resulting from chromospheric evaporation is calculated using a simple evaporative cooling model. Using these temperatures and densities, we calculate simulated flare light curves for TRACE, the SXT on Yohkoh, and GOES. We find that when the background magnetic field strength is weak, the radiation emitted by the reconnected X-ray loops beneath a CME is faint. Additionally, it is possible to have two CMEs with nearly the same trajectories and speeds that have a significant difference in the peak intensities of their light curves. We also examine the relationship between the thermal energy release rate and the derivative of the soft X-ray light curve and discuss the implications for the Neupert effect.

A Method to Predict the Effect of Thermal Residual Stresses on the Free-Edge Delamination Behavior of Fibre Reinforced Composite Laminates

A method is presented which accounts for the effect of process-induced thermal residual stresses on the free-edge delamination behavior of fibre reinforced laminates. A strain energy release rate fracture mechanics formulation is derived based upon the Irwin crack closure integral. The finite element method is then used to calculate Mode I and Mode II crack opening displacements and stiffnesses for free-edge delamination cracks originating at a specified ply interface from which strain energy release rates due to thermal and mechanical loading are then obtained. The effect of processing temperature on strain energy development is included in the calculation of thermal energy release rates by using experimentally measured material property data. To verify the model, (+352/-352/02/902)s graphite-reinforced PEEK tensile test specimens were manufactured and tested to failure. Measured delamination strains agree well with predictions when the effect of thermal residual stresses is included. Neglecting residual stresses over-estimates the tensile delamination strength by approximately 32%.

The influence of the variation of the coefficient of thermal expansion with temperature on the microcracking behavior of K3B/IM7 and X5260/G40-800 composites

Abstract The effects of the coefficients of thermal expansion (CTE) of K3B/IM7 and X5260/G40-800 cross-ply laminates on matrix cracking are investigated. The composite CTEs are measured in the temperature range -50°C to 175°C. Based on this measured CTE as a function of temperature, the thermal residual stresses and energy release rates at the crack tip are calculated using the finite element method and compared to those results computed under the assumption of a constant CTE value. The residual stresses in the laminates will have the potential to reduce the mechanical properties of the composites. This paper clarifies the thermal energy release rate change in a temperature range and predicts the critical applied matrix cracking load of the laminates.

53B. Thermal energy release rate of the volcano Merapi, Indonesia.(Abstracts of Papers Presented at the Fall Meeting of the Society)

53B. Thermal energy release rate of the volcano Merapi, Indonesia.(Abstracts of Papers Presented at the Fall Meeting of the Society)

Non-emptive thermal energy release rate from major active volcanoes in Japan.

Non-emptive thermal energy release rate from major active volcanoes in Japan.

6. Thermal energy release rate estimated from the volcanic cloud of Minami-dake, Sakura-jima and some applicational considerations(Abstracts of Papers Submitted for the Meeting of the Society)

6. Thermal energy release rate estimated from the volcanic cloud of Minami-dake, Sakura-jima and some applicational considerations(Abstracts of Papers Submitted for the Meeting of the Society)