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Critical Heat Flux and Subcooled Nucleate Boiling in Transient Region Between a Two-Dimensional Water Jet and a Heated Surface

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Critical Heat Flux and Subcooled Nucleate Boiling in Transient Region Between a Two-Dimensional Water Jet and a Heated Surface CRITICAL HEAT FLUX AND SUBCOOLED NUCLEATE BOILING IN TRANSIENT REGION BETWEEN A TWO-DIMENSIONAL WATER JET AND A HEATED SURFACE Yoshiki MIYASAKA, Shigeaki INADA AND YOSHIHIKO OWASE Department of Mechanical Engineering, GunmaUniversity, Kiryu 376 The present paper is concerned with a high subcooling water jet boiling system at the stagnation point. The purpose of the investigation is to obtain a high heat flux and to reveal the high heat flux mechanism. Most of the discussion in this paper is based on experimental results and con- cerns the effect of subcooling, jet velocity, and stagnation pressure on critical heat flux and nucleate boiling in the transient region. The entire range of the boiling curve is divided into three heat transfer regions which maybe hypothesized to exist in nucleate boiling. curves in subcooled pool boiling is discussed. Introduction In the previous study6\ the effect of forced con- Whenthe boiling phenomenonis used effectively in vection heat transfer on nucleate boiling was con- a technological application, it is convenient and im- sidered from the standpoint of superposition of the portant to removeenormousquantities of heat per heat flux. unit time and area from a heated surface and to ac- The present study was made within the same sub- complish this heat transfer at a relatively low surface cooling and the jet velocity ranges as in the previous temperature without burn-out of the boiling surface. paper. The important parameters of this study are Recently, a study of the effect of jet velocity on critical subcooling, the jet velocity, the stagnation pressure heat flux has been made by Monde and Katto7). and contamination of the boiling surface. Generally, Their experimentwascarried out with a systemcovered when the boiling surface is contaminated in some with a flowing film of saturated water at atmospheric degree, the coefficient of heat transfer increases pressure being maintained by a small circular jet of remarkably in the transient boiling region. In this water held at the center of the heated surface. study, it is considered that the boiling surface is not The present paper is concerned with a high sub- completely contaminated, but is moderately contami- cooling water jet boiling system at the stagnation nated. point. The purpose of the present investigation is Most of the discussion in this paper is based on the to obtain a high heat flux and to reveal the boiling experimental results and concernsthe effect of these mechanism. In this study, a heat flux of up to 4X parameters on critical heat flux and nucleate boiling. 107 kcal/m2-hr could be obtained in steady-state The boiling curve at high subcooling was not separated condition over the entire range of superheat 10- clearly into nucleate, transition and film boiling re- 800°C, by using a heated surface with a small diameter gimes. Heat flux was increased gradually over the of 1.5 to 2.0mm. Use of such a small diameter was entire range as superheat rose. necessary because it was very difficult experimentally The entire range is divided into three heat transfer to remain a high heat flux above 107 kcal/m2-hr in regions which may be hypothesized to exist in nu- steady-state condition by using a heated surface with cleate boiling. diameter above 2.0 mm.However, a heated surface 1. Experimental Apparatus and Procedure of 10 mmdia. was used for subcooled pool boiling. The effect of heated surface diameter on the boiling Figure 1 shows the experimental apparatus for the impinging jet, where high heat flux nucleate boiling Received May 4, 1979. Correspondence concerning this article should be addressed to Y. Miyasaka. Y. Owase is now with Kobe Steel Co., Ltd., of subcooled water was generated on a heat transfer Osaka 541. surface which was fixed on the copper heat conductor VOL. 13 NO. 1 1980 29 the top of the conical copper block and used to con- duct heat to the boiling surface. The base of the conical copper block was finished to 25 mmdia. and a carbon plate of thickness 2 mmwas placed on it in order to obtain radiant energy efficiently. The ceram- ic fiber heat insulator KAOWOOLwas packed tightly around the copper cylinder and conical cop- per block to reduce heat losses. This heat insulator possessed the function of the receiver of melting cop- per as well. Copper-constantan thermocouples of 0.1 mmdia. were used to measure temperature. The thermo- couples were calibrated over a wide temperature range of 100 to 900°C. Two constantan wires of 0.1 mm dia. were attached to the surface of the 1.5 mm-dia. copper cylinder by spot welding, one located 1.0 mm, Fig. 1 Experimental apparatus for impinging jet the other 2.0mm below the boiling surface. A copper wire of0.1 mmdia. was placed in a 0.5 mm-dia. hole drilled into the conical copper block and was stuffed with copper chips. The temperature indicated by these thermocouples was used as the average temperature in a cross section of the copper cylinder and heat flux was measured by calculations based on Fourier's equation for steady- state heat flow. Temperature of the boiling surface was measured by extrapolating the temperatures con- sidering the thickness of 0.05 mmof platinum foil. All the thermoelectric forces were recorded on a gal- vanograph through an attenuator and a changing- over switch. Fig. 2 Main part of apparatus for impinging jet The platinum foil attached on the copper cylinder was formed into a circular fin. The edge of the plati- by the diffusion-bonding method. This surface was numcircular fin was soldered to the brass plate to made of a 0.05mm thickness platinum foil. Dif- bridge the gap, leaving about 5 mmbetween copper fusion-bonding was achieved by keeping the tempera- cylinder and brass plate. Epoxy resin was used to ture of the contact face between platinum and copper prevent leakage of water at the outer edge of the brass at about 600°C for twenty-four hours in an electric plate. furnace filled with argon gas. The heat flux measured with the boiling surface The radiant energy of a molybdenumheater, being covered with Teflon rod showed that the heat losses heated by alternating current, was transferred to the through the platinum fin and the side of the copper end of the copper heat conductor. The a. c. power cylinder were negligible comparedto the rate of heat input was controlled with a 10-KVA adjustable transfer through the boiling surface. transformer. The furnace was filled with hydrogen Figure 3 shows the experimental apparatus for sub- gas in order to prevent the molybdenumheater from cooled pool boiling. Figure 4 shows the detailed oxidation. main part of the apparatus. The furnace used for A two-dimensional water jet was impinged upward subcooled pool boiling was identical to that shown in vertically on the heat transfer surface being standed Fig. 1. A 2.0mm-dia. copper cylinder, 5mm long, upside down in order to prevent the heater from adhe- was set facing upward and the platinum boiling sion of melting copper. surface was attached to it by the diffusion-bonding The test water was pumpedup to a height of about method. Furthermore, two boiling surfaces (3 mm 15 mand poured into the storage tank. The tempera- dia., 10 mmdia.) were used to investigate the effect of ture of the waterjet measured at the outlet of the noz- diameter on the boiling curves. Three 0.1 mm-dia. zle was kept constant at about 15°C. Figure 2 shows constantan wires were attached to the side of the cop- the details of the main part of the apparatus. A 1.5 per cylinder by spot welding. The copper wire of mm-dia. copper cylinder, 3 mmlong, was attached to 0.1 mmdia. was attached in muchthe same way as 30 JOURNAL OF CHEMICAL ENGINEERING OF JAPAN in Fig. 1. In the case of the 10mmdia. boiling sur- face, further, three constantan wires of 0.1 mmdia. isolated in a glass tube were fixed by spot welding in 0.5 mm-dia. holes drilled into the center of the copper cylinder with separation distance 1.0 mmbelow the heated surface. The test liquid, about ten liters of water, was placed into the boiling vessel so that the water level was about 250 mmabove the heated surface. The liquid tem- perature was measuredby time-interpolation of the temperatures at the beginning and end of the run. The maxmumvariation of liquid temperature from start to end of run was found to be about 10°C. The heated surface used in this work was cleaned with a 5/0 emery paper and acetone before each ex- perimental run. The power input to the molybdenum heater was increased very slowly. After a steady- Fig. 3 Experimental apparatus for subcooled state condition was reached, as indicated by tempera- pool boiling ture measurement, experimental data were taken. Each experimental run required about eight hours. 2. Boiling Curve 2. 1 Subcooled pool boiling curve Heat flux vs. superheat data measuredin subcool- ing 85°C at atmospheric pressure are plotted in Fig. 5. At a heat flux of about 106kcal/m2-hr, violent nucleate boiling on the heated surface was observed. The vapor generated at the active sites on the heated Fig. 4 Main part of apparatus for subcooled pool surface was in the form of discrete bubbles of 0.1 mm boiling dia., and was condensed immediately. As the tempera- ture of the heated surface was raised, heat flux increas- ed. Though the data showed scatter in some degree, these data appeared as an extension of the ordinary high heat flux saturated pool boiling curve at atmos- pheric pressure.
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