Boiling, flow crisis

Physical Mechanisms of Flow Boiling Crisis in Visual Observations 304... 

This is an engineering textbook, and it aims to improve the performance of boiling equipment. Hence, it emphasizes the boiling crisis and flow instability. The first five chapters, besides being important in their own right, serve as preparation for understanding boiling crisis and flow instability. 

Ruddick (1953) and Lowdermilk et al. (1958) found that flow oscillation can induce a premature boiling crisis. Moreover, in a boiling water reactor the flow oscillation may induce a nuclear instability. Thus, in designing a boiling system, it is imperative to predict and prevent those operational conditions that might create flow oscillation. 

Phase 1 To understand the mechanisms of flow boiling crisis by means of... 

Visual study of boiling crisis in various flow patterns Microscopic analysis of boiling crisis in each known flow pattern... 

PHYSICAL MECHANISMS OF FLOW BOILING CRISIS IN VISUAL OBSERVATIONS... 

To understand the physical mechanisms of flow boiling crisis, simulated tests have been conducted to observe the hydraulic behavior of the coolant and to measure the thermal response of the heating surface. To do this, the simulation approaches of the entire CHF testing program are considered as follows. 

Because of the damagingly high temperature of the heater surface at DNB in a water flow, most studies of bubble behavior near the boiling crisis have been conducted on a Freon flow, where the surface temperature is much lower than in a water flow. The validity of the simulations of boiling crisis has been established in many studies, such as those of Stevens Kirby (1964), Cumo et al. (1969), Tong et al. (1970), Mayinger (1981), and Celata et al. (1985). 

Two visual studies of Freon boiling crisis were conducted at the University of Pittsburgh (Lippert, 1971) and at Michigan University (Mattson et al., 1973). Both programs succeeded in identifying the DNB under the saw-shaped bubble layer of subcooled Freon flows as shown in Figures 5.8 and 5.9, respectively... 

The bubble behavior near the boiling crisis is three-dimensional. It is hard to show a three-dimensional view in side-view photography, because the camera is focused only on a lamination of the bubbly flow. Any bubbles behind this lamination will be fussy or even invisible on the photograph, but they can be seen by the naked eye and recorded in sketches as shown in Section 5.2.3. For further visual studies, the details inside bubble layers (such as the bubble layer in the vicinity of the CHF) would be required. Therefore, close-up photography normal and parallel to the heated surf ace is highly recommended. 

Figure 5.6 Boiling crisis in subcooled Freon flow mass flux 1.5 x 106 lbm hr ft2 (2,030 kg/m2 s), pressure 40 psia (0.27 MPa), A7"sc = 30°F (16.7°C) at boiling crisis. (From Tong 1972. Reprinted with permission of U.S. Department of Energy, subject to the disclaimer of liability for inaccuracy and lack of usefulness printed in the cited reference.)...

When the boiling crisis occurs, the surface temperature rises. Because of the fairly good transfer coefficient of a fast-moving vapor core in an annular flow, the wall temperature rise after a dryout in the high-quality region is usually smaller than that in a subcooled boiling crisis. It is even possible to establish steady-state conditions at moderate wall temperatures, so that physical burnout may not occur immediately. Thus dryout is also described as slow burnout. 

Figure 5.11 Comparison of boiling-crisis mechanisms in various flow patterns. (From Tong and Hewitt, 1972. Copyright 1972 by American Society of Mechanical Engineers, New York. Reprinted with permission.)...

Liquid core temperature and velocity distribution analysis. BankofT (1961) analyzed the convective heat transfer capability of a subcooled liquid core in local boiling by using the turbulent liquid flow equations. He found that boiling crisis occurs when the core is unable to remove the heat as fast as it can be transmitted by the wall. The temperature and velocity distributions were analyzed in the singlephase turbulent core of a boiling annular flow in a circular pipe of radius r. For fully developed steady flow, the momentum equation is given as... 

Boundary-layer separation and Reynolds flux. Kutateladze and Leont ev (1964, 1966) suggested that the flow boiling crisis can be analyzed using the concept of boundary-layer separation (blowoff) from a permeable flat plate with gas injection (without condensation), as shown in Figure 5.14. Kutateladze and Leont ev (1966) also give the critical condition of boundary layer separation from a flat plate with isothermal injection of the same fluid as... 

By comparing Eq. (5-11) with existing flow boiling crisis data obtained in water at 1,000-2,000 psia (6.9-13.8 MPa) inside a single tube test section with uniform heat flux, Tong (1968) reported that C, is a function of the bulk quality and Eq. (5-11) becomes... 

In order to evaluate Eq. (5-23) in the neighborhood of the boiling crisis for given local bulk conditions (i.e., fluid pressure, bulk mass flow rate, quality, and equivalent diameter), the following simplifications are made. 

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