The importance of surfare condition on nucleate boiling has long been recognized. It has also been known that only cavities of a narrow size range can be active nucleation sites. In order to define the size range of active cavities as a function of wall temperature or heat flux, a model is proposed. The model pictures a bubble nucleus at a site enveloped by a warm liquid. The nucleus will begin to grow into a bubble only when the surrounding liquid is sufficiently superheated. The time required for the liquid to attain this superheat is called the waiting period. The transfer of heat from the superheated liquid into the bubble is considered to be a transient conduction process. A cavity is considered effective only if the waiting period is finite. This criterion gives the limiting sizes of effective cavities. The equations show that maximum and minimum sizes of effective cavities are functions of subcooling, pressure of the system, physical properties, and the thickness of the superheated liquid layer. Comparison of theoretical prediction with experimental data from several sources was made. The fluids considered were ether, pentane, and water, with water under various degrees of subcooling. The theory did predict the incipience of boiling and size range of cavities successfully.
When heat is transferred from a fin into a nearly saturated liquid, nucleate 'boiling may set in if the temperature on the fin is sufficiently higher than the liquid's saturation temperature (1). Such a mode of heat transfer would be very efficient because of the high heat transfer coefficient of nucleate boiling over an extended surface area. Prior to this report, only complicated numerical solutions for nucleate boiling from a fin had been proposed (1, 2 ) . The purposes of this report are to present a simplified model for establishing a criterion for the incipience of nucleate boiling on a straight fin, to propose an expression for the length of the boiling section, and to determine the heat flux for a iven base temperature. The the need of resorting to a tedious step-by-step numerical calculation with difference equations.The temperature profile on a straight fin was determined, and this temperature was compared with the existing criterion for the incipience of nucleate boiling. The temperature for the incipience of nucleate boiling can be determined either experimentally or by use of the equation proposed by Hsu (3) :simplified method will provi f e a quick estimate without Consider the convective section first, and assume that the heat transfer coefficient is represented by a constant value h,.The boundary conditions are Use of Equation (2a) and the previous boundary conditions gives the classical fin equation for a fin of heightIn the present study, the temperature profile distribution along the straight fin was derived to determine the location where the criterion for the inci ience of nucleate boilknown, the heat flux at the base can be determined.A simple experiment was conducted to check the theory. Also, a comparison was made between the present theory and the experimental data on nucleate boiling from a fin reported by Haley and Westwater (1 ) .ing is satisfied. Once the length o P the boiling section is ANALYSIS If nucleate boiling does occur on a fin, it will first take place at the root of the fin. As the temperature of the fin is increased, the nucleate boiling section will extend out further and further on the fin. The one fin can thus be considered as made up of two separate fins in contact with each other. The section near the root is hot enough to have nucleate boiling taking place while the end section is being cooled by convection (which could be either forced or free convection). The model is shown in Figure 1 for a straight fin of rectangular shape. Similar analyses can be made on other geometries, such as a cylindrical fin, without much additional difficulty.The fin could be either tion of the temperature profile on a straight fin were applied herein, namely, constant roperties, flat profile in t~ tion for a straight fin is direction, and negligible end e P ect. The differential equa-The heat flux at x = Li is For the boiling section, the heat transfer coefficient varies as the temperature difference 8 to the nth power. Thus, Equation (2) should take the form where and ei are the heat ...
Heat transfer measurements were made with vertical stainless steel bayonet tubes, 3/8 to 3/4 in. O.D., with lengths from 2.6 to 6.5 in. The heat source was steam. The boiling I3.m AT ranged from 154" to 314'F. for three organic liquids and from 547" to 788°F. for nitrogen, all at 1 atm. No forced convection was used. Benzene, carbon tetrachloride, and nitrogen on the longer tubes had h values two or three times greater than predicted by the Bromley equation; however, the Reynolds numbers were found to exceed 2,000. Nitrogen on the 2.6-in. length obeyed the equation; the Reynolds numbers were less than 2,000. Methanol is an anomaly; although the Reynolds numbers were less than 2,000, the flow was proved by photography to be turbulent and the h values were much higher than predicted for viscous flow. A correlation is given which fits all the data except for methanol. It shows that a vertical orientation is superior to the horizontal for liquids boiling outside tubes.
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