טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentBeckerman Genady
SubjectNumerical Model of Solidification by an External Cryoprobe
of a Material with an Embedded Cylindrical Heat
Source
DepartmentDepartment of Mechanical Engineering
Supervisors Professor Emeritus Avraham Shitzer
Professor Emeritus David Degani


Abstract

Cryosurgery is a medical technique in which malignant or undesired tissues undergo localized controlled freezing aimed at destroying them. The device used to freeze the tissue is called a cryoprobe. Large blood vessels passing inside, or in the vicinity of the target tumor are considered to be a major thermal disturbance, as they act as an effective heat source and impede the propagation of the freezing front and distort the freezing lump’s shape.

The purpose of this study is to analyze numerically the thermal interaction between a flat circular cryoprobe, applied on the surface of a tissue-like phase-changing medium (PCM), inside which there is an embedded tube. Solution of the temperature field in the PCM is performed numerically by a commercial software (ANSYS7.0) by coupling the independent solutions of the energy equation and the equations governing the flow inside the embedded tube.

Various flow rates in the tube (30 and 100ml/min) and cooling rates of the cryoprobe (-4, -8 and -12ºC/min) were applied and their effect on the volume of the frozen medium were examined. The results include 3-D transient solutions of the temperature fields in both the PCM and the region that remains unfrozen, as well the temperature field of the fluid inside the embedded tube. A number of the numerical results were validated against the experimental results obtained in a separate study yielding good agreements. The numerical results support the observation reported previously of the overwhelming influence of the warm fluid flowing in the embedded tube on the development of the frozen lump, which is manifested in the appearance of a “hot region” under the tube representing the blood vessel.  The volume of the frozen lump is found to grow at a faster rate at the beginning of the freezing process and then slows down. The volume enclosed by the -40 °C isotherm, considered in the literature to be the threshold temperature under which cell destruction rate is maximal, is developing later, initially growing at a fast rate, until reaching a certain maximum and then it is slowing down. Under the conditions of this study, the flow inside the tube has not been blocked due to freezing, although the fluid temperature in the region closer to the cryoprobe has been reduced significantly.

The results of this study will have bearings on the design and performance of cryosurgical procedures of tumors in vicinity to thermally significant blood vessels.