TECHNICAL REPORT NO. CORRELATION OF MAXIMUM HEAT FLUX DATA FOR BOILING OF SATURATED LIQUIDS BY WARREN M. ROHSENOW and PETER GRIFFITH FOR THE OFFICE OF NAVAL RESEARCH CONTRACT N5ori-07827 NR-035-267 D.I.C. PROJECT NUMBER 6627 MARCH 1, 1955 -NINO 7-0 MASSACHUSETTS INSTITUTE OF TECHNOLOGY DIVISION OF INDUSTRIAL COOPERATION CAMBRIDGE 39, MASSACHUSETTS MITLibraries Document Services Room 14-0551 77 Massachusetts Avenue Cambridge, MA 02139 Ph: 617.253.5668 Fax: 617.253.1690 Email: docs@mit.edu http://Iibraries.mit.edu/docs DISCLAIMER OF QUALITY Due to the condition of the original material, there are unavoidable flaws in this reproduction. We have made every effort possible to provide you with the best copy available. If you are dissatisfied with this product and find it unusable, please contact Document Services as soon as possible. Thank you. Due to the poor quality of the original document, there is some spotting or background shading in this document. CORRELATION OF MAXILW IEAT FLUX DATA FOR BOILING OF SATURATED LIQUIDS by arren M. Rohsenow * and Peter Griffith Characteristic data for the boiling of single oamponent liquids show in the region of transition fran nucleate boil ing to vapor film boiling the existence of a maximum heat flux as the wall temperature, henoe heat flux, is raised. and If electric heating is employed, the maximum heat flux point usually coincides with a burnout point since the metal surface temperature usually rises wiell above the melting point in attempt.ing to transfer this maximum heat flux after nucleate boiling changes to vapor film type boiling. These two terms - maximum heat flux and burnout -- will be used interchangeably. A correlation has been attempted by Addams(l)(2) in which a plot is made of (q/A)max V v h a g 1/5vs. i The quantity (q/A)mai'/ yhfg is an average velocity of vapor leaving the heating surface and the quantity (a g) was introduced merely to produce a dimensionless group, which follows logically from dimensional analysis if a and g are included as significant quantities. Since it was observed(3) that bubbles "shoot" away frem a surface perpendicularly fran vertical surfaces it appears that gravity should perhaps not have a significant effect on the peak heat flux values. Associate Professor of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts. Instructor of Mechanical Engineering, Massachusetts Institute of Teohnology, Cambridge., Massachusetts. The correYation methrl proposad here is based on reasoning similar to that used in developing the boilin, heat transfer correlation tmploying the concept of a bubble Reynolds number(4)O. Proposed Correlation At present, no satisfactory peak heat flux correlation exists which gives insight into the mechanism of vapor bindingo However, it is possible to eliminate some variables and predict a qualitative effect for the others o The mechanism of transition to vapor binding may be visualized, in qualitative terms, as follows. bubbles form on certain spots At the onset of nucleate boiling, Under certain oonditiqns these bubbles .e.ave due to the action of gravity and, under other conditions, to the inertia of the surrounding liquid. nearly independent of q/Ao The size and frequency, however, are As one goes to high q/A, the number of spots at which bubbles form increases in direo't proportion to the q/A (5)a At some point the spacing of the bubbles becomes so small that neighboring bubbles join and the surface becomes partially covered with vapor, thus suddenly decreasing q/A at that point if the wall temperature is held constant. The maximum heat flux exists just before the transition fran nucleate to film boiling occurs,, With this physical picture in mind, it becomes possible to postulate a "burnout" criterion. Imagine an idealized condition on the surface such that the vapor bubbles touch each other as sketched in Fig. 1 .Considering this to be the vapor binding conditions, the number, "n", places on the surface at which bubbles form is equal to /Db per unit of length or 1/%2 per unit area. We can say then that the criterion for q/A) or burnout is n= Cvb b Db2 (1) where Cyb is -unity for the condition of touching bubbles shown in Fig. I but is probably les than unity when burnout actually occurs. The heat transfer to the bubbles was shown(4) to be proportional to q/A)ttgo 4 a q hf where "f" is the frequency. n Db3 f Then (2) Jakob(6) found that the time interval during which a bubble grows on a surface and becomes detached approximately equals the time interval required to form a new bubble at the particular point on the surface. He also observed that the distance of the center of gravity of the bubble above the heater strip increased almost linearly with time. The total time interval elapsed is 1/f and the distance traversed is Db; hence the bubble velocity at detaohment is Db ~ tb Yb Thus, ve oan put Equation (1) into Equation (2) q/A)., I I (f D) (4C where f Db a the bubble velocity. The diameter of the bubble at departure is dependent on the mechanism of departure. Since our knowledge of this mechanism is quite inognplete, we are unable to ocmplete this correlation. Jakob(6) found M 4 - that f Db was nearly the same for boiling CC1 mately to 920 ft/hro sa 20 and equal approxi- If this is universally true, a burnout criterion might be (q/A) - vf~g (Db where ( vs0 is a bouyancy term which varies inversely with pressure and hence is essentially a pressure effect. Actually, since f Db is taken as a constant, it is omitted in the plot shown in Fig0 2 which includes data of Cichelli and Bonilla(7), Addcs(4), and Braunlich(8). abscissa as plotted has units of ft/hr. like the dimensionless quantity. Then the Nevertheless, the quantity behaves It could actually have been made dimen- sionless by dividing the plotted nunber by 920 ft/hr. 'This seemed like an unnecessary stepl thus it was not taken. The deviation of the data from the suggested correlation line in Fig. 2 is only slightly less than in Addams' correlation. This is due to the fact that (aJg)l/3 for the range of data considered varied only by a factor of approximately 2. It should be noted that in both the present correlation and the Addoms correlation (q/A)a varies by a factor of about 30 hf (f- ) by about 3 end 3 V by about 1500a Since by about 4, 'V is by far the strong- est variable and it appears in both abscissa and ordinate, the data tend toward a slope of unity. (q/A) Further. since the quantity of interest is which appears only in the ordinate, the percentage deviation fram the correlation curve represents the degree of correlation of (q/A) A pl ot of max 4 - 5 (q/A) shows the same deviation as the plot in Fig0 2, The equation of the straight line drawn through the data points on Fig 4 2 is (q/A) ma-x 0,6 ? 143 () ft/hr () with an approximation deviation range of about + 11%. NGaENCLATURE a - thermal diffusivity of liquid, k/jo o - specific heat f - frequency with which bubbles form and depart at a point on the heating surface - acceleration of gravity h - latent enthalpy change during evaporation k - thermal conductivity n - muber of places on surface at which bubbles form per unit area g/A- heat transfer rate per unit area 0q coefficient in Equation (2) Cvb Db coefficient in Equation (1) bubble diameter at deparbure fran surface density of vapor v - density of liquid REFEMSUCES 1. Addans J. N., SeD thesis, Chemioal Engineering Departent, Llass. Inst. of Tech., 1948. 2. *Heat Tranuaission," by W. H. MoAdamso UoOrawHIll, 1954, Ord Edition, Fig. 1448 B. "Study of MNohni of Boiling Beat Transfer," by W. U. RohsenOw and J. A. Clark, ASME Trans., vol. 73, 1961, pp. 609*620. 4. "A Method of Correlating Heat Transfer Data for Surfam Boiling of Liquids," by W. M. Rohsenow, ASI Trans., vol. 74, No. 8, Aug. 1962. 5. "Reet Transfer," by M. Jakob, 6. "Heat Transfer," by M. Jakob, %,107, 7. Ciohelli, M. T., and C. F. Bonilla, Trans. AIMhE, 1946, vol. 41, p. 766. 8. Branlich, R. H., Thesis, Chenical Engineering Department, Mass. Inst. of Teoh., 1941. eliey, 1049, p. 627. Fs1I 1049, p. 6838. -L~ DISTRIBUTION LIST Chief of Naval Research Department of the Navy Washington 25, D. C. Attnt Code 458 Library of Congress ashington 25, D. C. Attn: Navy Research Section 1 Director, Naval Research Laboratory Washington 25, D. C. Attnt Tech. Info. Officer (9) Technical Library (1) Mechanics Division (2) Comanding Officer Office of Naval Research Branch Office 150 Causeway Street Boston 14, Massachusetts Comanding Officer Office of Naval Research Branch Office 346 Broadway New York 13, N. Y. (2) Chief of Bureau of Ordnance Navy Department Washington 25, D. C. Attns Ad-3, Technical Library (1) 844 N. Rush Street Cammanding Officer Office of Naval Research Branch Office 1000 Geary Street San Franc isco 24, Calif. 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