A numerical study of flow and heat transfer characteristics within a liquid nitrogen cryoprobe

The use of low temperature as a medical tool can be traced back to as early as 3500 B.C., when Egyptians applied ice as a local anesthetic to wounds and other ailments. However, the local application of cold in cancer therapy was not reported until 1851, when a British physician employed a container filled with ice-brine mixture in direct contact with skin cancer to inhibit its growth. The modern era of cryosurgery is believed to begin with the development of the first cryoprobe in 1961 by Dr. Irving Cooper. Many attractive medical and economical advantages, e.g. low bleeding, good esthetic results, minimal use of anesthetics, short period of recovery, and low cost of procedure, make cryosurgery an ideal treatment for the destruction of cancerous tumors. During cryosurgery, freezing is used to destroy diseased or undesired tissue, which is accomplished by insertion of one or more cryoprobes into the target tissue. As the core of a cryosurgical system, the cryoprobe has witness the development of various designs using diverse cryogens and methods of refrigeration. Among those, the liquid nitrogen (LN ₂) cryoprobe has the strongest freezing capacity, which makes it the only choice for the treatment of invasive cancer. The LN ₂ cryoprobe is composed of two concentric tubes: an outer layer of vacuum insulation and an inner tube for passage of liquid nitrogen to the freezing tip. The open end of the inner tube reaches closely to the closed end of the outer one. During cryosurgery, the flow of liquid nitrogen, going through and towards the open end of the inner tube, is constrained to return along the annular gap between the two tubes. The target tissue near the cryoprobe tip can therefore be frozen locally due to the strong heat transfer between the cryoprobe and the tissue. Therefore, one of the fundamental tasks in the design of cryosurgical instrumentation has been to control surface heat transfer from tissue to a probe as efficiently as possible. Geometrically, a cryoprobe belongs to the category of bayonet tube heat exchanger, which has been extensively studied. However, the majority of the study is focused upon the heat transfer through the outer sidewall, and the tip of the outer tube in the study is always insulated, which makes the direct application of the results in cryoprobe design impossible. The paper presents a numerical analysis of the flow and heat transfer in a liquid nitrogen cryoprobe. The numerical model is firstly validated by simulating the published experimental results. The parametric study is then performed and different geometry factors, e.g. outer tube end surface designs, ratios of inner/outer tube diameters and end clearances, are considered. The results are valuable and applicable for cryoprobe design and cryosurgery protocol optimization.