We examine the rotor–oscillator flow, a slow viscous flow between long parallel plates driven by the rotation of a slender cylinder (the rotor) and the longitudinal oscillation of one of the plates (the oscillator). For rotor locations of interest to us, this flow exhibits a hyperbolic mixing region, characterized by homoclinic tangling associated with a hyperbolic fixed point, and a degenerate mixing region, characterized by heteroclinic tangling associated with two degenerate fixed points on one of the boundary plates (normally the oscillator). These mixing regions are investigated both theoretically, by applying various dynamical tools to a mathematical model of the flow, and experimentally, by observing the advection of a passive tracer in a specially constructed apparatus. Although degenerate mixing regions have been largely ignored or undervalued in previous research on chaotic mixing, our results demonstrate that more mixing is associated with the degenerate mixing region than the hyperbolic one in many cases. We have also discovered a peculiar phenomenon, which we call Melnikov resonance, involving a rapid fluctuation in the size of the hyperbolic mixing region as the frequency of the oscillator is varied.

An analysis is presented that explains the variation of superheat with subcooling that has been observed by a number of researchers investigating nucleate boiling heat transfer at constant heat flux. It is shown that superheat initially increases with increasing subcooling near saturated conditions because of the way in which changes in active site density and average bubble frequency with increasing subcooling affect the rate of heat removal from the heater surface by enthalpy transport and microlayer evaporation. As subcooling increases further, natural convection begins to play an increasingly important role in the heat transfer process. Ultimately, natural convection is able to accommodate the entire imposed heat flux, after which superheat decreases as subcooling increases. The success of the analysis in explaining the variation of superheat with subcooling suggests that the rate of the heat removal from the heater surface is completely determined by the mechanisms of enthalpy transport, natural convection, and microlayer evaporation.

Radiofrequency (RF) ablation of hepatic tumors is investigated. Modeling efforts show that it is possible to determine the temperature variation in the tissue depending on the level of RF energy input. Surgeons and radiologists who are performing such procedures may find it useful to view the temperature-time history of the tissue before the actual ablation process.

Heat transfer in radiofrequency ablation therapy of liver tumors is discussed. Temporal and spatial temperature changes around a single needle and multi-prong ablation probes in monopolar and bipolar configurations based on a two-dimensional finite elements method are presented. The temperature changes and related heat transfer in the tissue model help to visualize the shape and size of the ablated region. The visualization of the tissue temperatures and their progression could be useful in clinical applications of ablation therapy. Finite-element based numerical simulation, while providing useful visualizations of the temperature changes in and around the tumor, underestimates the lesion size. The perfusion in the tissue and the possible presence of large blood vessels in or near the ablated domain, and the temperature dependency of the thermal and electrical properties of the tissue are significant complicating factors in modeling and clinical applications.

Radiofrequency ablation is a medical procedure used to treat a variety of illnesses including liver carcinomas that cannot be treated by resection or radiological procedures. A multi-physics code was used to solve the Bio-heat Equation by a finite element methodology. The volume of coagulation necrosis caused by the heating of the tumor tissue as a function of perfusion rate was determined. A multi-needle probe deposited the RF energy into the tissue. Once the tissue was heated to greater than 50°C it was considered irreversibly damaged. Increasing perfusion rates reduced the size of the lesion. In order to heat a volume to a required temperature the time elapsed did increase.

Radiofrequency ablation could be described as a thermal strategy to destroy a tissue by increasing its temperature and causing anirreversible cellular injury. Radiofrequency ablation is a relatively new modality which has found use in a wide range of medical applications and gained acceptance. RF ablation has been used in destroying tumors in liver, prostate, breast, lung, kidney, bones, and the eye. One of the early applications in clinical setting was its use in treating supraventricular arrhythmias by selectively destroying cardiac tissue. Radiofrequency ablation has become established as the primary modality of transcatheter therapy for the treatment of symptomatic arrhythmias. Radiofrequency catheter ablation of cardiac arrhythmias were investigated using a finite-element based solution of bioheat transfer equation. Spatial and temporal temperature profiles in the cardiac tissue were visualized.

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The simulation of radiofrequency thermal ablation of liver tissue using finite-element analysis is presented. Temporal and spatial temperature changes around a single needle with and without a small blood vessel were obtained.