Thermal storage has been considered as an important measure to extend the operation of a eoneentrated solar power plant by providing more eleetricity and meeting the peak demand of power in the time period from dusk to late night everyday, or even providing power on eloudy days. Diseussed in this paper is thermal energy storage in a thermoeline tank having a solid filler material. To provide more knowledge for designing and operating of sueh a thermoeline storage system, this paper firstly presents the application of method of characteristics for numerieally predicting the heat charging and discharging process in a paeked bed thermocline storage tank. Nondimensional analysis of governing equations and numerical solution schemes using the method of characteristics were presented. The numerical method proved to be very efficient, accurate; required minimal computations; and proved versatile in simulating various operational conditions for which analytical methods cannot always provide solutions. Available analytical solutions under .simple boundary and initial conditions were used to validate the numerical modeling and computation. A validation of the modeling by eomparing the simulation results to experimental test data from literature also confirmed the effectiveness of the model and the related numerieal solution method. Finally, design procedures using the numerical modeling tool were discussed and other issues related to operation of a thermoeline storage system were also studied.
Radio-frequency (RF) heating of soft biological tissues during electrosurgical procedures is a fast process that involves phase change through evaporation and transport of intra- and extra-cellular water, and where variations in physical properties with temperature and water content play significant role. Accurately predicting and capturing these effects would improve the modeling of temperature change in the tissue allowing the development of improved instrument design and better understanding of tissue damage and necrosis. Previous models based on the Pennes’ bioheat model neglect both evaporation and transport or consider evaporation through numerical correlations, however, do not account for changes in physical properties due to mass transport or phase change, nor capture the pressure increase due to evaporation within the tissue. While a porous media approach can capture the effects of evaporation, transport, pressure and changes in physical properties, the model assumes free diffusion of liquid and gas without a careful examination of assumptions on transport parameters in intact tissue resulting in significant under prediction of temperature. These different approaches have therefore been associated with errors in temperature prediction exceeding 20% when compared to experiments due to inaccuracies in capturing the effects of evaporation losses and transport. Here, we present a model of RF heating of hydrated soft tissue based on mixture theory where the multiphase nature of tissue is captured within a continuum thermomechanics framework, simultaneously considering the transport, deformation and phase change losses due to evaporation that occur during electrosurgical heating. The model predictions are validated against data obtained for in vivo ablation of porcine liver tissue at various power settings of the electrosurgical unit. The model is able to match the mean experimental temperature data with sharp gradients in the vicinity of the electrode during rapid low and high power ablation procedures with errors less than 7.9%. Additionally, the model is able to capture fast vaporization losses and the corresponding increase in pressure due to vapor buildup which have a significant effect on temperature prediction beyond 100 °C.
This paper explores energy dissipation in ex-vivo liver tissue during radiofrequency current excitation with application in electrosurgery. Tissue surface temperature for monopolar electrode configuration is measured using infrared thermometry. The experimental results are fitted to a finite element model for transient heat transfer taking into account energy storage and conduction in order to extract information about “apparent” specific heat, which encompasses storage and phase change. The average apparent specific heat determined for low temperatures is in agreement with published data. However, at temperatures approaching the boiling point of water, apparent specific heat increases by a factor of five, indicating that vaporization plays an important role in the energy dissipation through latent heat loss.
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