Convective Flow Boiling in Coiled Tubes

Convective flow boiling of liquid nitrogen has been investigated using coiled-tube heat exchangers in order to determine the heat transfer characteristics. This project focused on the identification of the primary variables involved in establishing the flow boiling coefficient for nitrogen using a coiled-tube flow configuration. The need for an accurate predictive correlation for this coefficient was recognized after an extensive testing program yielded results that did not correspond well with published information. A test method was developed which allowed for the accurate measurement of the boiling coefficient over a wide range of nitrogen flow rates, excess temperatures (Twall Tsat), and tube coil geometries. In evaluating nitrogen boiling rate data using a large number of f l ow conditions and coil geometries, it was found that va p o r i z a t i o n rates are a function of three main variables. These variables are : (1) Mass velocity of the liquid nitrogen inside the tubes; (2) Excess temperature involved in the boiling process; (3) Coiled-tube geometry, specifically the ratio of the inside tube diameter to the average coil diameter. A correlation has been developed which accounts for these va r iables. This correlation has been shown to be accurate in pre d i c t i n g the flow boiling coefficient within 10% for all cases tested.