Publisher Summary The chapter presents the experimental data with no particular theory, to search for significant effects of physical parameters. The present method gains power, scope, and speed by avoiding redundancies among the fluid properties, which otherwise lead to intolerable and unhelpful complexity of analysis and to a confusing multiplicity of correlations that look different but are often numerically rather similar. The chapter examines existing correlations, finding a reason for the fact that they are often numerically similar, despite using very different properties that leads to the improved method of analysis. It has always required a major effort of data reduction to compare experimental data with correlations that involve many properties. Comparisons between competing claims of different correlations are confused by uncertainties and indeed errors in tabulated property values, especially for uncommon fluids or old tabulations. These problems are greatly reduced by using a simpler correlation for example, in terms of reduced pressure, where the only tabulated property required for each fluid is critical pressure. The chapter concludes that taking advantage of that simplicity, correlations of a more subtle type can be considered without undue labor in data reduction.

Heat Flow Rates in Saturated Nucleate Pool Boiling-A Wide-Ranging Examination Using Reduced Properties

▪ Abstract This review examines recent advances made in predicting boiling heat fluxes, including some key results from the past. The topics covered are nucleate boiling, maximum heat flux, transition boiling, and film boiling. The review focuses on pool boiling of pure liquids, but flow boiling is also discussed briefly.

Boiling heat transfer

Small circular- and rectangular-channel boiling with two refrigerants

In flow boiling, the nucleate and convective components are superimposed by a very complex mechanism, which so far is not well understood Two models exist in present literature, one by Chen [3] (1963), using addition of the two components with a suppression factor; and one by Shah [8] (1976), using the “greater of” the two components with a Bo-number simplified correlation Neither model presents a satisfactory solution, as attested by the numerous methods published since then, mostly based only on regression analysis-derived correction factors In this article a new model, based on asymptotic addition of the two boiling components, is introduced It follows the established principles of flow boiling and converges correctly to the extremes of all parameters Tested on the University of Karlsruhe data bank containing over 13,000 data points in vertical flow boiling, results superior to previous correlations are demonstrated

Flow Boiling Heat Transfer in Vertical Tubes Correlated by an Asymptotic Model

The forces due to surface tension and momentum change during evaporation, in conjunction with the forces due to viscous shear and inertia, govern the two-phase flow patterns and the heat transfer characteristics during flow boiling in microchannels. These forces are analyzed in this paper, and two new nondimensional groups, K1 and K2 , relevant to flow boiling phenomenon are derived. These groups are able to represent some of the key flow boiling characteristics, including the CHF. In addition, a mechanistic description of the flow boiling phenomenon is presented. The small hydraulic dimensions of microchannel flow passages present a large frictional pressure drop in single-phase and two-phase flows. The small hydraulic diameter also leads to low Reynolds numbers, in the range 100‐1000, or even lower for smaller diameter channels. Such low Reynolds numbers are rarely employed during flow boiling in conventional channels. In these low Reynolds number flows, nucleate boiling systematically emerges as the dominant mode of heat transfer. The high degree of wall superheat required to initiate nucleation in microchannels leads to rapid evaporation and flow instabilities, often resulting in flow reversal in multiple parallel channel configuration. Aided by strong evaporation rates, the bubbles nucleating on the wall grow rapidly and fill the entire channel. The contact line between the bubble base and the channel wall surface now becomes the entire perimeter at both ends of the vapor slug. Evaporation occurs at the moving contact line of the expanding vapor slug as well as over the channel wall covered with a thin evaporating film surrounding the vapor core. The usual nucleate boiling heat transfer mechanisms, including liquid film evaporation and transient heat conduction in the liquid adjacent to the contact line region, play an important role. The liquid film under the large vapor slug evaporates completely at downstream locations thus presenting a dryout condition periodically with the passage of each large vapor slug. The experimental data and high speed visual observations confirm some of the key features presented in this paper. @DOI: 10.1115/1.1643090#

M. Abdelsalam

Papers

Heat-transfer correlations for natural convection boiling