Heat transfer characteristics of boiling nitrogen and neon in narrow annuli

Abstract

The boiling characteristics of liquid neon and nitrogen in vertical annuli ranging between 0.006 and 0.080 in. wide were studied. The center tube wall, submerged to various depths, formed the boiling surface. Observations suggested a convective heat transfer mechanism, and correlation by a Dittus-Boelter equation form proved successful.Interest in cryogenically cooled magnets is rapidly increasing in view of the substantial energy reduction achieved at low temperatures. The compactness and low resistivity obtainable with cryomagnets are necessary to obtain very high field strengths in the order of lo" oersteds or higher.Magnets may be maintained at their desired temperatures by removing the required energy input as sensible heat in a fluid or as latent heat. Air, water, or kerosene are used for magnets operating at ambient temperatures. Removal of the heat generated in cryogenic magnets by means of a boiling liquid offers two major advantages over the use of a fluid utilizing sensible heat transfer: a lower temperature difference can be maintained between the boiling liquid and the magnet surface, hence, a lower resistivity, and a much smaller volume of coolant is required.At 30°K. the onIy fluid refrigerants are helium, hydrogen, and neon. Hydrogen and neon are more suitable from the heat transfer point of view, but safety considerations preclude the use of liquid hydrogen until more operating experience with cryomagnets is obtained. Other refrigerants are available for operation at higher cryogenic tempeatures. Some cryogenically cooled magnets have been operated at 78°K. by using liquid nitrogen as a coolant.The investigation was divided into two parts: 1. Study of pool boiling on a short vertical pipe with liquid nitrogen and neon. 2. Study of nitrogen and neon boiling in narrow annuli.The first part of this study was presented at the A.1.Ch.E. National Meeting, Pittsburgh, Pennsylvania (1 ) . This paper will present the second part of the study covering the various factors affecting the boiling performance in narrow annuli.The technical literature contains many studies and analyses of heat transfer to boiling liquids. These studies take many forms such as surveys (2, 3 ) , theoretical analyses ( 4 , 5, 6, 7, 8), experimental work (9, 10, 11, 1 2 ) , and statistical analyses ( 1 3 , 1 4 ) . Heat transfer to boiling cryogenic liquids has been thoroughly reviewed by Class et al. ( 2 ) and Richards et al. ( 3 ) . Both conclude that there are large variations in the magnitude of heat flux and in the shape of the heat flux VS. temperature difference curves, obtained by different investigators. These differences may be due to uncontrolled parameters such as surface roughness and contamination.The study of heat transfer to boiling liquids flowing in narrow channels is limited. The following is a brief review of the pertinent literature in this area. L.A. Wenzel is at Lehigh University, Allentown, Pennsylvania. (1) when short round tubes and rectangular channels having an equivalent diameter of 0.332 and 0.091 in., r...