Numerical model of a balloon-based cryogenic probe

Atrial fibrillation is a disease that alters the speed and rhythm of the heartbeat, greatly affecting the patient’s quality of life. To treat recurrent forms, the pulmonary veins are electrically isolated, freezing the cardiac tissue surrounding them. This treatment is carried out using a cardiac probe consisting of a capillary for the delivery of the coolant in a balloon expansion chamber, and a return line to discharge the exhausted fluid. The aim of this thesis is to develop a 1-D numerical model of a balloon-based cryogenic probe to analyse the phenomena involved. The behaviour of the fluid inside the injection and return line was modelled using the continuity, momentum and energy equations written under the assumption of steady-state implementing an homogeneous model for two-phase fluid. The fluid friction was evaluated using an empirical correlation. In order to select the most suitable model for the case under consideration, a comparison of the various available ones was carried out. In order to direct the fluid towards the walls of the balloon, small orifices are present at the end of the capillary, for this reason a flow model was implemented in order to preliminarily assess the mass flow rate disposed of by the probe. Single-phase and two-phase heat transfer models were also introduced in order to simulate the heat exchanged by the lines. The system of ordinary differential equations was discretized using the finite volume approach, and the resulting system of non-linear equations was solved using the iterative Newton-Raphson procedure. The developed numerical model was experimentally validated by means of a test bench specifically set up to perform cryoprobe measurements. Finally, the performance of a commercial cryoballoon was simulated and the numerical results obtained were compared with those available in the literature