Flow boiling pattern

FLOW BOILING PATTERN AND HEAT TRANSFER RATE... 

Knowledge of dominant two-phase flow patterns in micro-channels is a key factor in developing accurate and physically sound predictive tools for heat sink design. Unfortunately, interfacial interactions between the vapor and liquid phases during flow boiling in a micro-channel are often far too complex to permit accurate measurement or quantitative assessment of flow patterns. 

Flow boiling is distinguished f rom pool boiling by the presence of fluid flow caused by natural circulation in a loop or forced by an external pump. In both systems, when operating at steady state, the flow appears to be forced no distinction will be made between them, since only the flow pattern and the heat transfer are of interest in this section. 

Typical flow pattern for flow boiling are shown in Fig. 5. For subcooled boiling and high liquid flow rate, the observed bubble detachment size is smaller than the gap size. The treatment of the data showed that correlation [26] obtained for a large tube flow boiling can be applied to predict subcooled flow boiling heat transfer in a confined space. For saturated flow, the vapor bubbles have a tendency to merge and produce the... 

Kuznetsov, V.V., Shamirzaev, A.S., (1999), Two-phase flow pattern and flow boiling heat transfer in non- circular channel with a small gap, Two-Phase Elow Modeling and Experimentation, Pisa, Italy, v 1, pp. 249-253. 

Current experimentation on micro-channel two-phase flows has provided some evidence of the heat transfer mechanisms that govern the micro-scale flow boiling process (i) at low vapor qualities, when bubbly flow is the dominant flow pattern, thermal transport is primarily associated to nucleate boiling, (ii) at intermediate vapor qualities, with the intermittent passage of elongated bubbles and slugs of liquid, heat is transferred by single phase... 

N. Kattan, J. R. Thome, and D. Favrat, Flow Boiling in Horizontal Tubes Part 1—Development of a Diabatic Two-Phase Flow Pattern Map, ASME J. Heat Transfer, Vol. 120, pp. 140-147,1998. 

Whereas the heat transfer coefficient a for convective pool boiling is calculated according to Sect. 4.3.2.3, the prediction of the heat transfer coefficient a for convective flow boiling is more complicated. Here the vapor fraction plays a significant role, because the flowing gas influences the flow pattern of the liquid. 

Pressure Drop and Visualization of Flow Patterns To analyze flow boiling instabilities in a minichannel or a microchannel, pressure measurements recorded at a high frequency (e.g., 200 Hz) are usually performed. The analysis of the microchannel pressure drop is then related to flow patterns to understand destabilization mechanisms. 

Scaling laws help to understand the flow patterns and heat transfer phenomenon without any constraint of geometry, fluid type, or operation conditions. The terms used to determine these scaling laws are dimensicmless and usually balance the physical processes involved. In the present case of flow boiling, the pressure drop is considered without dimensicHi usually using the dynamic pressure. 

Destabilization Mechanism To use or predict flow boiling instabilities, it is essential to understand the mechanisms which lead to the instabilities. These mechanisms can usually be found by analyzing the flow patterns. For flow boiling in a microchannel, the previous section evidenced that flow boiling instabilities appear and lead to periodical pressure oscillations. The flow even returns to the entrance (Fig. 7). To determine the destabilization mechanism which occurs in such a situation, we quantified the phenomena involved in the instability, such as vapor generation rate, total channel pressure drop, etc. 

As observed and characterized by Thome and co-workers [13,14], the flow patterns and their transitions encountered during flow boiling of R-134a in a 0.5 mm tube are as follows ... 

Qu et al. [2] found evidence of two kinds of unsteady flow boiling for 21 parallel microchannels measuring 231 x 713 xm. They observed in their parallel microchannel array either a global fluctuation of the whole two-phase zone for all the microchannels (Fig. 1) or chaotic fluctuations of the two-phase zone (Fig. 2) over-pressure in one microchannel and under-pressure in another. The individual microchannel mass flow rate was not controlled. Hetsroni et al. [3] created an experimental setup to study liquid-gas and liquid-vapor flow in parallel triangular microchannels with diameters of 103 to 161 p.m. They used a fast video camera coupled with a microscope through a Pyrex plate to record the flow patterns. They showed the influence of the injection method (plenum shape) and found evidence for the same inlet conditions. 

In high heat flux (heat transfer rate per unit area) boilers, such as power water tube (WT) boilers, the continued and more rapid convection of a steam bubble-water mixture away from the source of heat (bubbly flow), results in a gradual thinning of the water film at the heat-transfer surface. A point is eventually reached at which most of the flow is principally steam (but still contains entrained water droplets) and surface evaporation occurs. Flow patterns include intermediate flow (churn flow), annular flow, and mist flow (droplet flow). These various steam flow patterns are forms of convective boiling. 

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