Study on dynamics and structure of evaporation front in ethanol depending on pressure and subcooling

This paper is dedicated to study of self-sustained evaporation front in subcooled ethanol at stepwise heat generation. The data on evaporation front velocity and microstructure at different subcoolings and pressure are presented. Experiments show that development of the evaporation front drastically depends on studied parameters. At low pressures initial vapour bubbles grow without losing interface stability and initiation of evaporation fronts doesn’t occur. At higher pressures the fronts propagate along the heater, and its structure depends on temperature.

dynamics at non-stationary heat release the setup, shown schematically in Fig. 1, was developed.
The frame 2 with refill tank 3 and working chamber 4, housing the working section 10, is hinged on bed 1.The working chamber is a sealed cylindrical stainless steel vessel.The inner diameter of the chamber is 250 mm, and the height of the workspace is 250 mm.The working chamber is equipped with four windows 5.The bellows 6 with an adjusting screw 7 allows creating the required pressure in the working chamber at the closed valve 8. Heat exchanger 9 located in the bottom of the working chamber allowed setting the desired temperature of liquid.

Mechanisms of front initiation
The data obtained using macro videoshooting allows describing the front initiation mechanism as follows.Vapor bubble, beginning to grow at the heater surface, has smooth interface while it is within the metastable termal layer near the heater wall (Fig. 3a).At the lowest pressure 0.0075 MPa vapor bubbles grow up to big size without losing interface stability, until they merge, and vapor layer covers the whole heater.At higher pressures, when a bubble grows out of the thermal layer into the subcooled area, condesation processes begin to compete with evaporation, and the bubble interface loses stability (Fig. 3b).After that evaporation front starts to propagate with its headmost point within the thermal layer (Fig. 3c).Size of the interface perturbations is comparable with the thermal layer thickness.

Fig. 1 .Fig. 2 .
Fig. 1.Scheme of experimental setup.The working section was made of the stainless steel tube with outer diameter of 8 mm, wall thickness of 0.2 mm and length of 50 mm.The rectangular current impulse of 420 A was supplied to the working section.Temperature of the heater was determined by the current impulse duration.After current was turned off, the wall temperature remained almost constant at the times of the evaporation front propagation.Numerical solution of heat equation has shown that for the time of front passage (much lesser than the time of convection development) the wall temperature before the front dropped no more than by 0.2 K. Thus in each experiment, the propagation of self-sustained evaporation front was investigated at the pre-set constant wall superheat.The working section temperature until the moment of vapor phase emergence was measured using temperature dependence of the heater resistance.For this purpose, two conductors of 0.05-mm diameter were welded in the middle of the tube to measure the voltage drop in the region of 30-mm length.In each measurement, calibration was performed by the temperature of undisturbed fluid.This technique allowed measuring the average temperature of the working section with an error less than ±1.5 K.After the vapor phase formation, the wall temperature was determined numerically based on the nonstationary heat equation.This approach is valid under conditions of our experiments since the times of the evaporation front propagation (about 50 ms) were much less than the time of convection development (about 200 ms).Visual observations of vapor phase formation and propagation on the heating surface were performed using a high-speed digital video camera Phantom v7.0.The shooting rate was up to 45 000 frames per second with the exposure of 19 microseconds.When analyzing the results of high-speed digital video-shooting, the line of the interfacial boundary was determined on each frame using specially developed software.The obtained data was used to study dynamic properties of evaporation front boundary.

Fig 3 .DOI
Fig 3. Evolution of a vapor bubble and emergence of the evaporation front.P = 0.101 MPa.ΔTsub = 40.3K. Heating time was 40 ms (T-Tsat = 56 K).(a) -60.15 ms after beginning of heat release; (b) -62.62 ms; (c) -66.88 ms.Experiments in reflected light show that development and structure of the front significantly depend on pressure and wall superheat.As mentioned above, initiation doesn't