Flow Boiling Instability

BoiUng and Evaporation in Microchannels Bubble-Actuated Microfluidic Switch Cavitation in Microdomains Droplet Dispensing > Flow Boiling Instability... 

Between 1985 and 2006, 1,247 papers were published on ScienceDirect on the subject of boiling, of which only 49, i.e., about 4 %, deal with mini- or microchannels. Four of these 49 papers report flow instabilities. Even if flow boiling is a well-developed research subject, flow boiling in microchannels, not to mention instabilities which occur in these microchannels, have not been discussed or reported to any considerable extent in the literature. In Table 1, a few studies dealing with flow boiling instabilities in microchannels are listed. We will discuss some of these studies below. 

Flow Boiling Instability, Fig. 1 Top view of a severe pressure drop oscillation [2]... 

Flow Boiling Instability, Table 2 Classification of flow instabilities (based on [5]) ... 

As for flow boiling instabilities in classical channels, several types of instabilities have been observed and reported by different authors. Table 2 summarizes these instabilities and their physical consequences on the fluid flow. We also show in the following table which publications reported these instabilities in microchannels. This classification is based on the one proposed for channels by Bergles in 1976 [5] and is divided into two categories ... 

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. 

Flow Boiling Instability, Fig. 5 Local pressure in a minichaimel for two situations a mainly liquid flow and a mainly two-phase flow... 

Flow Boiling Instability, Fig. 7 Vapor slug formation during flow boiling example of backflow (5 ms between 2 pictures) (some bubble outlines were evidenced in order to follow their evolution)... 

Flow Boiling Instability, Fig. 8 Average pressure loss versus inlet Reynolds number when the buffer is not connected to the loop for flve heat fluxes... 

Flow Boiling Instability, Fig. 9 Pressure loss scaling law for all heat flux densities provided non-dimensioned pressure loss function of the ratio between the phase change number and the Reynolds number for only exit vapm qualities strictly between 0 and 1... 

Flow Boiling Instability, Fig. 10 Non-dimensioned oscillation frequency as a function of Afpch/R o... 

Scaling laws of flow boiling instabilities can also be applied to heat transfer. In Brutin et al. [9], the heat transfer coefficient calculated is based on the average surface and fluid temperature. A total heat flux is provided (Qw). whereas locally the heat flux is redistributed inside the aluminum rod. Thus, the local surface and fluid... 

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. 

Research over the last decade has provided a huge amount of data on flow boiling instabilities in microchannels. As far as we know today, the instabilities which occur in microchannels appear in the same way as in classical channels (see Table 2). Scaling laws have been evidenced for the stability transition criteria, such as the oscillation frequency of the instabilities, as has been shown in the previous sections. This enables us to conclude that aU flow boiling instabilities in microchannels have at least one origin, which is space confinement. However, other phenomena, such as coupling with the experimental loop or the injection device, can lead to other supplementary instabilities. 

When two-phase flow phase change occurs in a channel with a small hydraulic diameter compared with the capillary length, flow boiling instabilities may arise. As in classical-sized channels, the instabilities can be static or dynamic however their intensity is higher in microchannels due to the higher rate of volumic generation of vapor which induces considerable pressure drops. 

Flow Boiling Instability, Hgure 6 Typical outlet pressure signal during unsteady boiling shown over 2 periods, = 888.9 irm, /. = 200 mm,... 

---