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Evaporation, Boiling, and Condensation

Two-Phase Heat Transfer and Bubble Characteristics in a Microchannel Array

[+] Author and Article Information
T. P. Lagus1

Department of Mechanical Engineering,  University of Minnesota, Minneapolis, MN 55455todd.p.lagus@Vanderbilt.edu

F. A. Kulacki2

Department of Mechanical Engineering,  University of Minnesota, Minneapolis, MN 55455todd.p.lagus@Vanderbilt.edu

1

Present address: Department of Mechanical Engineering, Vanderbilt University, Nashville TN.

2

Corresponding author.

J. Heat Transfer 134(7), 071502 (May 22, 2012) (11 pages) doi:10.1115/1.4006097 History: Received June 03, 2011; Revised November 22, 2011; Published May 22, 2012; Online May 22, 2012

Heat transfer coefficients and bubble motion characteristics are reported for two-phase water flow in an array of 13 equally spaced microchannels over an area of 1 cm2 . Each channel has Dh  = 451 ± 38 μm, W/H = 0.8, and L/Dh  = 22.2. Uniform heat flux is applied through the base, and wall temperatures are determined from the thermocouple readings corrected for heat conduction effects. The upper surface is insulated and transparent. Single-phase heat transfer coefficients are in a good agreement with comparable trends of existing correlations for developing flow and heat transfer, although a difference is seen due to the insulated upper surface. Two-phase heat transfer coefficients and flow characteristics are determined for 221 < G < 466 kg/m2 s and 250 < q < 1780 kW/m2 . Heat transfer coefficients normalized with mass flux exhibit a trend comparable to that of available studies that use similar thermal boundary conditions. Flow visualization shows expanding vapor slug flow as the primary flow regime with nucleation and bubbly flow as the precursors. Analysis of bubble dynamics reveals ∼t1/3 dependence for bubble growth. Flow reversal is observed and quantified, and different speeds of the vapor phase fronts are quantified at the leading and trailing edges of vapor slugs once the bubble diameter equals the channel width. Bubble formation, growth, coalescence, and detachment at the outlet of the array are best characterized by the Weber number.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Microchannel array and polycarbonate top. The fluid reservoir to the left provides smooth parallel flow into the channel. The 3 cm × 3 cm × 3 cm heater block fits into the recess at the bottom of the plenum.

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Figure 2

Single-phase Nusselt numbers. Harms [4] (– –), Lee and Garimella [5] (––)

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Figure 3

Transient temperatures at two axial locations. Boiling onset occurs at t ∼ 750 s. G = 342 kg/m2 s, q = 601 kW/m2 , and Bo = 7.6 × 10−04

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Figure 4

Steady-state wall temperature distributions for q = 418 kW/m2 (▴) and 366 kW/m2 (♦). G = 225 kg/m2 s

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Figure 5

Average two-phase heat transfer coefficients. (Tw  − Tsat ) = 7 K (▴) and 14 K (▪). Tsat  = 1 atm

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Figure 6

Average two-phase and single-phase (•) heat transfer flux. (Tw  − Tsat ) =  − 5 K (▪) and − 38 K (♦)

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Figure 7

Reduced heat transfer coefficients (♦). Peters and Kulacki (▴) [15]

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Figure 8

Formation and expansion of vapor slugs. q = 418 kW/m2 , qeff  = 77 W/cm2 , G = 221 kg/m2 s, and We = 0.39

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Figure 9

Front and rear interface axial velocities and rate of change in length for expanding vapor slug of Fig. 8. Front interface (♦), length rate of change (▴), rear interface (▪)

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Figure 10

Bubble radius (left axis, ▴) and volume (right axis, ▪) growth versus time during a typical nucleation sequence

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Figure 11

Interface velocities and rates of change in sudden vapor expansion shown in Fig. 1. Front interface (♦), length rate of change (▴), rear interface (▪)

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Figure 12

Bubble detachment and slug formation. q = 641 kW/m2 , G = 356 kg/m2 s, and We  = 1.02

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Figure 13

Length of bubble (solid line, primary axis) and rate of growth (dashed line, secondary axis) from Fig. 1

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Figure 14

Flow reversal. q = 366 kW/m2 , qeff  = 680 kW/m2 , G = 229 kg/m2 s, and We = 0.42

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Figure 15

Computed velocities from the flow reversal sequence in Fig. 1. Front interface (▪), rear interface (▴)

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Figure 16

Bubble formation and release to outlet plenum. q = 366 kW/m2 , G = 229 kg/m2 , and We = 0.42

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Figure 17

Bubble capture and release to outlet plenum. q = 641 kW/m2 , G = 356 kg/m2 s, and We = 1.02

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