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7-9 October 2009, Leuven, Belgium captured at higher frame rates. B. Flow Instabilities Chen and Garimella [10] investigated the effect of dissolved air on flow boiling of FC-77 in microchannels using a carefully designed degassing scheme. Their study showed that dissolved air significantly affects both heat transfer and pressure drop in microchannels. They also observed larger flow instabilities in terms of pressure drop fluctuations for the undegassed liquid. In all the rest of the experiments conducted in the authors’ group, the liquid in the test loop is fully degassed before initiating each test to help minimize flow instabilities. Also, a throttling valve positioned upstream of the test section serves to suppress instabilities in the microchannel heat sink. Mild flow reversals were still observed at the inlet of the microchannels at the highest heat fluxes studied, for microchannels of cross-sectional area 0.144 mm 2 and smaller. However, these instabilities did not affect the inlet fluid temperature, which is held constant throughout each test. C. Data Reduction The local heat transfer coefficient, h , is calculated from h = q′′ w / ( Tw − Tref ) (1) where T is the local mean fluid temperature in the singlephase region and the liquid saturation temperature in the ref two-phase region, and T w is the local microchannel wall temperature. The heat flux used in (1) is the wall heat flux and is defined as ( ) q′′ = q / A /25 (2) w net t where q net is the net heat transfer rate to the fluid and A t is the total heated area of the microchannels, with L being the microchannel length: t ( 2 ) A = N w+ d L (3) The calculated local heat transfer coefficients presented here are based on measurements from the temperature sensor located along the centerline of the test piece near the flow exit. Flow visualizations are also reported from this location. Important nondimensional parameters often used in flow boiling include Reynolds number, Re, Bond number, Bo, and Boiling number, Bl. Reynolds number is calculated using the liquid phase mass flux as: Re = GD / μ (4) where G is the liquid mass flux and μ is the dynamic viscosity of the liquid. Bond number represents the ratio of buoyancy force to surface tension force and assumes importance in microscale boiling: ( ) 2 f g / Bo = g ρ − ρ D σ (5) here, ρ f and ρ g are the density of the liquid and vapor phases, respectively, and σ is the liquid surface tension. As demonstrated in Harirchian and Garimella [11], the channel cross-sectional area plays a critical role in determining microchannel boiling mechanisms and heat transfer; therefore, the length scale used in (4) and (5) is the square root of the cross-sectional area of one channel rather than its hydraulic diameter. Boiling number is the nondimensional form of the heat flux and is calculated using the liquid mass flux and latent heat, , as follows: h Bl q′′ / Gh w fg fg = (6) Following a standard uncertainty analysis [19], the uncertainties associated with the wall heat flux and the heat transfer coefficient are estimated to be 2 to 4% and 2.2 to 4.8%, respectively, for the cases considered. Details of the measurement uncertainties are discussed in [11]. IV. RESULTS AND DISCUSSION A. Flow Visualizations In Harirchian and Garimella [12], flow visualizations were performed with simultaneous heat transfer measurements during flow boiling in microchannels of different sizes for different flow rates. Five major flow regimes of bubbly, slug, churn, wispy-annular, and annular flow, and a postdryout regime of inverted-annular flow were identified in these microchannels, for which representative visualization images are provided in Fig. 2. Detailed descriptions of these regimes and the changes in flow regimes with microchannel size and mass flux are discussed in detail in [12]. Fig. 3 shows a summary of the existing flow regimes at different microchannel sizes and mass fluxes. It is seen that in the smaller microchannels and at lower mass fluxes, bubbly flow is not established; instead, slug flow is observed for low heat fluxes. In slug flow, elongated vapor bubbles are confined within the channel cross-section and are separated from the walls by a thin liquid layer. As the heat flux is increased, an alternating churn and confined annular flow appears in these microchannels. In confined annular flow, the vapor core occupies the whole cross-section of the microchannels and is separated from the walls by a thin liquid film. As the channel cross-sectional area or the mass flux increases, bubbly flow is observed at low heat fluxes. In the bubbly flow regime, bubbles are smaller relative to the cross section of the channels and confinement is not observed. At higher heat fluxes, alternating churn and wispy-annular or annular flow occurs. In wispy-annular or annular flow, the vapor core does not necessarily occupy the entire crosssection and can instead exist on only one side of the channel; in other words, the flow is not confined by the channel walls. For example, as can be seen in Fig. 4(b) for the 2200 μm × 400 μm microchannels the wispy-annular flow (or annular flow in Fig. 4(c)) and churn flow patterns are distributed side by side across the width of the channel due to the large channel aspect ratio, as also explained in [12]. ©EDA Publishing/THERMINIC 2009 103 ISBN: 978-2-35500-010-2
7-9 October 2009, Leuven, Belgium Flow direction Bubbly flow Vapor bubbles (a) Bulk liquid Slug flow Elongated vapor bubbles (b) Liquid slug Vapor bubbles Churn flow Bulk liquid (c) Vapor chunks Wispy-annular flow Vapor core Thin liquid film (d) Liquid droplets Vapor bubble Annular flow Vapor core Thin liquid film (e) Liquid droplets Inverted annular flow Liquid core Thick vapor blanket (f) Fig. 2. Description of observed flow boiling regimes [12]. 1420 G(kg/m 2 s) 225 630 1050 B C/W B C/W B C/W B C B B/S C/A S C/A B B B/S B B B/S B C/W B/S S B B/S C/A S C/A S C/A S C/A C/A C/W C/A C/W C/A C/W C/A C/W C/A C/A C/A 5850 x 400 (2.201) 2200 x 400 (0.815) 1000 x 400 (0.366) 1000 x 220 (0.231) 400 x 400 (0.144) 250 x 400 (0.089) 400 x 220 (0.079) 100 x 400 (0.037) 400 x 100 (0.026) 100 x 220 (0.021) 100 x 100 (0.009) B C/W C/A 5850 x 400 (2.201) B/S C/W C/A 2200 x 400 (0.815) B/S C/W C/A 1000 x 400 (0.366) B/S C/A 400 x 400 (0.144) S C/A 250 x 400 (0.089) S C/A 100 x 400 (0.037) 0 100 200 300 400 q" (kW/m 2 ) Width x Depth (Area) (µm) x (µm) (mm 2 ) 5850 x 400 (2.201) 2200 x 400 (0.815) 1000 x 400 (0.366) C/W 400 x 400 (0.144) 250 x 400 (0.089) 100 x 400 (0.037) B C/W 5850 x 400 (2.201) B B/S C/W 2200 x 400 (0.815) B C/W 1000 x 400 (0.366) B C/W 400 x 400 (0.144) B B/S C/A 250 x 400 (0.089) S C/A 100 x 400 (0.037) B: Bubbly S: Slug C: Churn W: Wispy-annular A: Annular B/S: Alternating bubbly/slug flow C/W: Alternating churn/wispy-annular flow C/A: Alternating churn/annular flow : Single-phase flow Fig. 3. Summary of boiling flow patterns in the microchannel test pieces; the microchannel dimensions are presented as width (μm) × depth (μm) with a single-channel cross-sectional area (mm 2 ) in parentheses [14]. ©EDA Publishing/THERMINIC 2009 104 ISBN: 978-2-35500-010-2
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International Workshop on THERMal I
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Poster session: Introduction* 7-9 O
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