Classification of boiling

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10.1 Introduction

When a solid surface is immersed in liquid and the solid surface temperature, Tw, exceeds the saturation temperature, {T_{sat}}({p_\ell }), for the liquid at that pressure, vapor bubbles can form, grow, and detach from the solid surface. This phase change process is called boiling, and the energy transport involved is classified as convective heat transfer with phase change. Two types of boiling are commonly distinguished: pool boiling and flow or forced convective boiling. In the former case, the bulk liquid is quiescent while the liquid near the heating surface moves due to free convection and the mixing induced by bubble growth and detachment. In the latter case, bulk liquid motion driven by some external means is superimposed on the motion that also occurs in pool boiling. Vapor bubbles within the liquid phase are the primary visual characteristics that distinguish boiling from evaporation. The influence of bubbles is also the primary cause of differences in the thermodynamic and hydrodynamic analyses of these two phase change modes. Fig. 10.1 compares, in schematic fashion, the temperature field of a quiescent volume of fluid experiencing evaporation with that of one experiencing boiling. As the figure indicates, boiling requires a much larger temperature difference between the bulk liquid and the heating surface than evaporation does. A second distinction that arises from the figure is that the thermal boundary layer is less precise in the case of boiling; this may be attributed to the mixing action caused by the release of bubbles from the heating surface. In addition to the pool and forced convection classifications, boiling can also be categorized according to the initial temperature of the liquid. Fig. 10.2 shows pool boiling for different initial liquid temperatures. Subcooled boiling occurs if the liquid temperature starts below the saturation temperature. In this case, bubbles formed at the heater surface experience condensation as they rise through the cooler bulk liquid and may collapse before reaching the free surface. Saturated boiling, on the other hand, occurs if the temperature of the liquid equals the saturation temperature. In this case, bubbles form at the heating surface, travel intact through the liquid, and escape at the free surface.

Schematic comparison of evaporation and nucleate boiling temperature profiles
Figure 10.1 Schematic comparison of evaporation and nucleate boiling temperature profiles.

Figure 10.2 Effects of liquid temperature
(a) {T_\ell } < {T_{sat}} (b) {T_\ell } = {T_{sat}} Figure 10.2 Effects of liquid temperature: (a) subcooled boiling, (b) saturated boiling ( vapor, liquid).

Section 10.2 introduces the pool boiling curve and the various regimes (free convection, nucleate boiling, transition boiling, film boiling) of which it is composed. After this introduction, the four regimes and the characteristic points of the curve are then discussed in detail. Section 10.3 discusses nucleate boiling, including nucleation, bubble dynamics and detachment, nucleation side density, numerical simulation of bubble growth and merger, and heat transfer analysis. The critical heat flux is presented in Section 10.4, followed by discussions on transition boiling and minimum heat flux in Section 10.5. Section 10.6 discusses film boiling, including film boiling laminar boundary layer analysis and correlations, direct numerical simulations, and Leidenfrost phenomena. This chapter is closed by a discussion of boiling in porous media (Section 10.7), including boiling on a wicked surface, boiling in porous media heated from below, and an analysis of film boiling in porous media. Forced convection boiling in both macro and micro tubes, which involves liquid-vapor two-phase flow with transitions between several characteristic flow patterns, is addressed in Chapter 11 under the heading of Two-Phase Flow.


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