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Evaporation, Boiling, and Condensation

Residence Time and Heat Transfer When Water Droplets Hit a Scalding Surface

[+] Author and Article Information
Ji Yong Park1

Materials Research Laboratory and Department of Materials Science and Engineering,  University of Illinois, Urbana, IL 61801park98@illinois.edu

Chang-Ki Min, Steve Granick, David G. Cahill

Materials Research Laboratory and Department of Materials Science and Engineering,  University of Illinois, Urbana, IL 61801

1

Corresponding author.

J. Heat Transfer 134(10), 101503 (Aug 07, 2012) (7 pages) doi:10.1115/1.4006802 History: Received August 21, 2011; Revised May 04, 2012; Published August 06, 2012; Online August 07, 2012

We study, using pump-probe optical methods with a time resolution of 1 ms, heat transfer when a series of water droplets impact a smooth surface whose temperature exceeds the boiling point. The volume of the individual water droplets is ≈10 nl, the time between droplets is ≈0.3 ms, and the number of water droplets in the series of droplets is 3, 20, or 100. In the temperature range 100 °C < T < 150 °C, our measurements of the heat transfer, and the residence time of water in contact with the surface, show that nearly all of the dispensed water vaporizes, but more rapidly, the higher the temperature. At higher temperatures, 150 °C < T < 220 °C, droplet shattering plays an increasingly important role in limiting heat transfer and, as a result, the volume of water evaporated and residence time decrease strongly with increasing temperature.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 3

High speed camera (300 × 600 pixels at 5 × 104 fps) images (a) at 150 °C and (b) at 190 °C. Here, water of 0.19 mm3 is dispensed, which is equivalent to 20 droplets. These images are captured immediately after all droplets in the droplet stream have arrived at the surface. In (a), a water drop has formed on the surface that evaporates without significant additional nucleation of vapor bubbles. In (b), water droplets shatter upon impact and water left on the surface boils.

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Figure 2

(a) Illustrative optical microscopy images and (b) the time-evolution of the diameter of a water drop on the surface of a sample at 110 °C. The surface was dosed with 1 μl of water at t = 0. In (a), the drop is viewed from the side and appears dark in this image; the solid surface is represented as a solid line which divide the images into the reflection of the water droplet (below the line) and the direct image of the droplet that is blocking the illumination (above the line). In (b), the symbols are the measured diameter d of the water drop on the surface and the solid line is a fit to the data assuming a functional form d ∝(t0-t) where t0 is the extrapolated time at which the water drop disappears.

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Figure 1

(a) Schematic diagram of the experimental configuration, not to scale. The Al heater block is 1 cm thick and the diameter of the hole in the Al heater block is 1.5 cm. Droplet generator, sample, and objective lens are vertically aligned. (b) The sample is a 1 mm thick double-side polished Si wafer. The bottom film of TiO2 is an antireflection coating and the top film of TiO2 thermally isolated the Ti thin film from the Si wafer. The Pt film provides a chemically inert surface, stable against boiling water. (c) Optical layout of the pump-probe system used. Sample region is described in detail in Fig. 1.

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Figure 4

Examples of (a) transient absorption data acquired at small values of the delay time between pump and probe, − 2 < t< 2 ps; and (b) time domain thermoreflectance data acquired at large delay times. 0.1 < t< 3 ns. Vin and Vout are the in-phase and out-of-phase signal from rf lock-in amplifier, respectively. The peak of the transient absorption caused by two-photon absorption at t = 0 (marked by an arrow in (a)) is used to measure the temperature of the sample. The ratio signal, -Vin /Vout , at t = 0.5 ns (marked by an arrow in (b)) is used to measure the effective thermal conductance of the Pt/water interface.

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Figure 5

Illustrative data showing transient changes of temperature and effective thermal conductance created by the impingement of dispensed water volume of 0.19 mm3 . Panels (a) and (c) are for a relatively low sample temperature of 130 °C. Panels (b) and (d) are for a relatively high sample temperature of 210 °C. Time zero is defined by the electronic trigger of the microdispenser. The series of water droplets arrives at the sample surface ≈50 ms after the trigger.

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Figure 6

(a) Heat transfer (the amount of thermal energy transferred from the sample to the water), (b) residence time, and (c) average heat flux plotted as a function of sample temperature. In (a), the thermal energy transferred is scaled by the deposited volume Ω (0.04, 0.19, and 1.0 mm3 ). Dashed line is placed at E/Ω of 2.6 J mm− 3 (volumetric enthalpy difference between liquid water at 25 °C and water vapor at 100 °C ) for reference. In (c), a filled square at 40 W cm− 2 and 120 °C from the work of Bernardin [5-6] is added for comparison. In (a) and (b), error bars describe the standard deviation of the mean for 5 sets of 64 repetitions of each experiment. In (c), error bars are calculated from the square root of the sum of the squares of the error bars in (a) and (b). Symbols are labeled by the dispensed water volume: diamonds 0.04 mm3 (filled diamonds), 0.19 mm3 (open squares), and 1.0 mm3 (filled circles).

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