Visualization study of film boiling onset and transition to noisy film boiling in He II

Cryogenics 35 (1995) 63 1435 0 1995 Elsevier Science Limited Printed in Great Bntain. All rights reserved 001 l-2275/95/$10.00

Visualization study of film boiling onset and transition to noisy film boiling in He II Y. Katsuki, M. Murakami,

T. lida and T. Shimazaki

Institute of Engineering Tsukuba 305, Japan

Mechanics,

Received

1994; revised

27 December

University

of Tsukuba, Tennodai

20 February

l-l-l,

Ibaraki,

1995

The transient boiling phenomena occurring on a planar heater in He II are visually studied to investigate the onset condition and the bifurcation criterion. The whole flow field is visualized with the aid of Schlieren and shadowgraph optics. After step-wise heating, local boiling sites appear and develop, eventually covering the heater surface, followed by the onset of film boiling. The state may further proceed to noisy boiling if the hydrostatic pressure is sufficiently large. A distinctive feature between the noisy and silent boiling states is visually confirmed in the present experiment. The onset time of film boiling as a function of heat flux is obtained via a transparent heater. It is found that there is some distinction between cases with large and small 9 values in the functional relationship for the onset time on 9, presumably originating from different states of the precursor nucleate boiling before film boiling.

Keywords: transient boiling phenomena; He II; visualization methods

It is known that there are two film boiling modes in He II; noisy and silent boiling. The occurrence of noisy or silent film boiling depends on the magnitude of the hydrostatic pressure and the bath temperature. For larger hydrostatic pressures and at lower temperatures, exclusively noisy boiling appears’. The transition from the silent to noisy state may be regarded as a kind of noise-induced transition to instability. In fact, it is found that a mechanical disturbance imposed on a cryostat by knocking it or a small sinusoidal thermal disturbance superposed on a constant heat flux from a heater readily gives rise to the transition from the silent boiling to noisy boiling state. The loud acoustic noise caused during noisy boiling results from large scale unstable motion of the vapour-liquid interface*. Loud noise or mechanical vibration induced in the noisy boiling mode may be harmful to cryogenic systems cooled by He II. Furthermore, it is a well known experimental fact’*3*5that the heat transfer coefficient for noisy boiling is smaller than that for silent boiling. In view of these facts, a study of the fundamental mechanism involved in noisy boiling and its control is of importance, even for such practical applications of He II as in superconducting magnet cooling. Most experiments on noisy boiling have been conducted using a thin wire heater’,“, including a visualization study with the aid of high-speed motion pictures4. Only a few have been reported on noisy boiling on a planar heater2.5.6. We have carried out such a visualization study using Schlieren and shadowgraph

methods to investigate the mechanism of noisy boiling on a planar heater in He II in an open space. In addition, the onset times of boiling phenomena in He II are investigated using the visualization method, because the existing data do not seem to be conclusive’.

Experimental Cryostat

set-up and procedure

and heater

In th present experiment, a specially designed cryostat with optical windows and an effective diameter of 50 mm is utilized for the visualization study, as illustrated in Figure I. Boiling phenomena caused in the test section are optically observed through these windows. Two kinds of planar heaters are used to cause boiling in He II. One is a transparent heater consisting of an evaporated indium oxide thin film (= 1000 8, thick) on an optical glass substrate, with dimensions of 25 mm x 25 mm, mounted vertically on the test section. Boiling phenomena can be observed through this heater, with particular detail available in the vicinity of the heater. The other heater is also planar, but is opaque, consisting of an evaporated Ni-Cr thin film on a Pyrex substrate, mounted horizontally. Several heaters of this kind of various sizes are used. The side view of boiling phenomena is observed with these heaters. He II is Joule-heated by a step-wise electrical current through a heater. The resulting flow field is visualized with the aid of Schlieren and shadowgraph methods.

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1995 Volume

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Transient

boiling

phenomena

in He II: Y. Katsuki

et al.

Mercury Lamp Mg-Spark Power Supply

Controller

Figure 2 Arrangement of Schlieren optics. In the shadowgraph visualization, the knife edge is removed

Figure 1 Schematic optical windows

Schlieren

illustration

of experimental

and shadowgraph

cryostat with

methods

Experimental

The basic idea of the Schlieren method is that some portion of the light refracted in an observation field in the test section is interrupted on its way to a screen or a photographic film and therefore the part of the field through which this light passes is locally dark. A parallel beam from a pinhole light source passes through the test section and focuses at a knife edge to interrupt some portion of the light source image. The knife edge would interrupt more or less the refracted light portion due to inhomogeneity in the refractive index or in the density of the test section. Thus the density change in the test section is converted to light intensity variation on the screen and a Schlieren image with enhanced light and shade is constructed. Thus, the darkness appearing in the photographic picture is in proportion to the first derivative of the density in the field of view. The sensitivity depends on the focal length of a concave mirror used to make a parallel beam, the size of the pin-hole and the ratio of interruption by the knife edge. The shadowgraph method is another visualization technique where the shadow of a beam caused by a density change in the observation field is directly projected on a screen. In shadowgraph optics, a point-shaped light source may be used but a parallel light beam is not necessarily required, and the knife edge is removed. Thus, the optical system and procedure are rather more simple in this technique. The darkness in a shadowgraph is in proportion to the second derivative of the density distribution, and the sensitivity is of medium grade, being less sensitive than the Schlieren method. Consequently, for visualization of the He II-vapour interface region the shadowgraph method is usualy more relevant than the Schlieren method, because the variation in density is large in this region. However, the Schlieren method is employed for visualizing the variation in density in bulk He II caused as a result of noisy boiling. These visualization images are recorded with a still camera and a video recorder. The arrangement for the Schlieren optics is shown in Figure 2. A magnesium-spark light source with a spark duration of 3 ps is utilized for the high speed still photography. A continuous light source from a high pressure mercury

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1995 Volume

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lamp is also used for video recording. Two concave mirrors, the Schlieren heads, with an effective aperture diameter of 150 mm and a focal length of 15 10 mm are used to make a parallel beam. The knife edge, which is rotatable around a horizontal axis to adjust its direction easily with respect to the direction of the maximum density gradient in the observation fields, is integrated with the still camera on a common stand.

IO

procedure

Experiments are conducted under both the saturated vapour pressure condition and a weakly pressurized condition with a hydrodynamic head of up to 40 cm. During the experiment, heat is generated step-wise from a planar heater fixed vertically or horizontally in quiescent He II. Following the onset of strong step-wise heating, a compression (first sound) wave and a second sound wave, which soon develops into a thermal shock wave, are emitteds. Then high density quantized vortices are generated and accumulate near the heater, which causes the formation of a thermal boundary layer adjacent to the heater. Finally, boiling starts on the heater surface. In the very initial precursor stage a number of nucleate boiling spots appear on the heater, and then it develops into film boiling. Noisy film boiling accompanied by loud audible noise and the more usual silent film boiling appear, depending on the magnitude of the hydrostatic pressure and the bath temperature. In this study, the time development of a series of transient boiling phenomena is visualized. After a specified delay time within the range of several tens of microseconds to several seconds from the onset of heating (according to the phenomena in question) the magnesiumspark light source is fired for a photographic record. The duration of step-wise heating is usually several milliseconds to several seconds.

Results and discussion Onset of boiling

in He /I

The rapid temperature rise of He II adjacent to the heater due to the diffusive action of high density vortices causes boiling. Here the results of our recent visualization study on the onset of boiling are only briefly described, as details have been described elsewhere’. Before the appearance of film boiling, a precursor boiling phenomenon, presumably transient nucleate boiling, takes place as seen in Figure 3. Shadowgraph visualization pictures observed through a transparent thin film heater mounted vertically are shown for two heating conditions. It is seen that the number of

Transient

boiling

phenomena

(a> 1 s

(b) 3 ms

(c)

5 ms

(d) 100 MS

(e)

(0

300 PS

200 ps

in He II: Y. Katsuki

et al.

Figure 3 Shadowgraph visualization pictures observed through transparent heater in precursor nucleate boiling stage. The heater with dimensions of 2.5 cm x 2.5 cm is verticallv mounted. (a)-(c) TB= 1.80 K, q= 10 W cm-2, r,,= l-5 ms, respectively; (d)-(f) rB = 1.80 K, q = 30 W cm-*, tH = 100-300 ps, respectively

spot-wise nucleation sites drastically increases and the onset time becomes shorter with the increase in heat flux. In cases of small q, each site expands to overlap adjacent sites with the lapse of time, eventually leading to film boiling. On the other hand, for high values of q, the transition process to film boiling seems to be different. The initial number density of boiling sites is far larger, and this turns into film boiling after a further increase in the number of sites. The critical value of q between these two transient nucleate boiling states is = 1O-20 W cm-*. This transient behaviour may be related to the two different functional dependences of the onset time of film boiling on q described later. Noisy

and silent

film boiling

It is known that immediately after the onset of regular (silent) film boiling for large q, the state may transfer to noisy boiling. For higher hydrostatic pressures, only noisy boiling appears. The temporal evolutions of the boiling patterns for the three states, i.e. the noisy, silent and transition states, are shown in Figure 4; these are shadowgraph pictures taken through the transparent heater. Here, pictures in the precursor nucleate boiling state are omitted for all cases. In the film boiling state, the noisy boiling pattern can be clearly distinguished from silent boiling. In the former case a large-scale single vapour bubble with a size comparable to the heater is seen to repeat expansion, eruption and crushing quasi-regularly on a vapour layer above the heater [pictures (a) and (b)]. The background vapour layer under the vapour bubble is fundamentally similar in nature to that in silent boiling, as seen from the minute wavy shadow patterns in the vapour layers in both cases. Loud audible noise and irregular density fluctuation are generated in connection with the unsteady expansion, eruption and crushing process. On the other hand, during silent boiling the global shadowgraph pattern, which exhibits the minute structure

of the vapour-He II interface on the heater does not change so much with time, though the detailed pattern varies unsteadily [pictures (c) and (d)]. In the silent boiling state, noise may of course be generated, but the intensity must be far smaller and the frequency range is different from those in the noisy boiling state. The distinction between the two types of boiling is quite obvious in these respects. In the transition state shown in pictures (e) and (f), a silent boiling pattern (f) is intermittently interrupted by a noisy pattern without any intentional disturbance, as seen in picture (e). It is found that mechanical disturbance or vibration caused by knocking the cryostat can readily give rise to a transition from silent to noisy boiling, or even from the non-boiling state to noisy boiling, as long as the helium hydrostatic pressure head is sufficiently high. The transition to noisy boiling may be considered as a kind of noise-induced instability. The characteristic difference between noisy and silent boiling patterns can be seen in Figure 5. These pictures are side views of boiling patterns on the horizontal heater (surface area S = 2 cm x 3 cm) visualized by the Schlieren method for higher sensitivity. It is seen from pictures (a) and (b) that a large vapour bubble full of wrinkles on the vapour-He II surface of quasi-regularly formed in the case of noisy boiling. In the case of silent boiling a vapour layer with small-scale unstable irregular fluttering of the vapourHe II interface is formed on the heater, as seen in picture (c), where the light and shade pattern seen within the heater results from the fine variations in vapour layer thickness. Another distinction between the two boiling states is the density fluctuation in the bulk He II in the noisy case, as seen in pictures (a) and (b), which results from acoustic disturbance radiation in relation to noise generation. This density fluctuation is not seen in the silent boiling stage, as shown in picture (c).

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Transient boiling phenomena in He II: Y. Katsuki et al. Onset times of boiling phenomena

(a)

500 ms

(b) 1 s

Cd) 1 s

The three onset times for initial precursor nucleate boiling, film boiling and noisy film boiling are plotted against heat flux 4 at 1.80 K in Figure 6, which all data are obtained from visualization pictures with the transparent heater mounted vertically. A series of pictures taken while changing the heating time ti..,at a specified set of conditions are examined. Here tH is the time between the onset of heating and the firing of the light source for photographing. In Figure 6, closed symbols indicate where boiling phenomena are visually confirmed and open ones indicate that boiling phenomena are not seen. The onset time occurs between an open and a closed symbol in the figure. First, the precursor transient nucleate boiling phenomenon apears before film boiling, by at least one order of magnitude. The least squares correlation for the onset time is found to be bq”, where for example a = -4.5 5 0.3 and b = 5.0 2 2.0 (with the time in s and q in W cme2) for the case of temperature lower than 2.0 K and q smaller than 10 W cm-*. As mentioned before, the data plot for precursor boiling seems to indicate that there is some distinction in the functional relationship between the onset time and heat flux bounded by =lO-20 W cm-*. The absolute value of the exponent to q, a, is larger for smaller q. This boiling begins a little earlier when the hydrostatic pressure is small. It is also seen in this case that there is a similar kind of

100,

(e)

500 ms

-7

(0 1s

Figure 4 Temporal evolution of the three boiling patterns visualized in the form of shadowgraph images through a transparent heater mounted vertically. TB= 1.80 K, q= 5 W cm-2, tH = 200 ms to 1 s. (a) and (b) noisy boiling; (c) and (d) silent boiling; (e) and (f) transition state when noisy boiling occurs intermittently

Heat Flux q (W/cm*) Figure6 Onset times of precursor boiling to, l), film boiling (A, A) and noisy boiling (0, ~1, in which all data are obtained from visualization pictures with the transparent heater mounted vertically ( Ts = 1.80 K)

Figure 5 Typical noisy and silent boiling patterns on horizontal heater (surface area 2 cm x 3 cm) visualized by Schlieren method. Ts= 1.80 K, o= 5 W cm-2, t,-500 ms. (a) and (b) noisy boiling; (c) silent boiling. The horizontal line seen in the upper half of picture (c) is a free surface

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Transient boiling phenomena

in He II: Y. Katsuki et al.

and 1,88 K (solid line in Figure 7). Agreement of the present data with the correlation by Tsoi and Lutset is fairly satisfactory in the temperature range below 2.0 K, but some deviation is seen at higher temperatures. Their experimental data at 2.11 K are found to agree more with the onset time of precursor nucleate boiling in the present experiment.

Conclusions Heat Flux q

(W/cm*)

Figure 7 Onset time of film boiling at 1.80 K (A, A), 2.00 K (0, +I and 2.10 K (V, W. The dashed lines denoted by S1.8, S2.0 and S2.1 are the correlations of onset time of film boiling proposed by Van Sciver for small 9: S1.8; tH= 1109” at 1.8 K; S2.0, f,.,= 17q4 at 2.0 K; S2.1, tH=0.5@ at 2.1 K (each line is extraoolated). The solid line labelled TL shows the results of

Transient boiling phenomena on a planar heater in He II have been studied visually and the following conclusions have been drawn. The precursor nucleate boiling appears far earlier than film boiling, by at least one order of magnitude. In the case of small q, the spot-wise nucleation sites expand, resulting in overlapping with the lapse of time. For high values of q, the number density of boiling sites increases with time. A distinctive feature between the noisy and silent boiling states is confirmed visually. In the case of noisy boiling, vapour bubbles with a size comparable to the heater repeat expansion, eruption and crushing quasiregularly on a vapour layer above the heater, generating loud audible noise. Density fluctuation in bulk He II is observed only in the case of noisy boiling. In the silent boiling state, only an irregular vapour layer appears and its global pattern does not change so much with time. The three onset times of precursor nucleate boiling, film boiling and noisy boiling are obtained as a function of heat llux from the visualization pictures with the transparent heater, and are compared with the existing correlation. Agreement of the present data is fairly satisfactory. There is some distinction in the functional relation of the dependence of the film boiling onset time on q bounded by = lo-20 W cm-*. It seems that this is due to the difference in the development process of the precursor nucleate boiling.

Tsoi &d Lutset for large 9: TL, fH= 0.25~~ at 1.79 and 1.88 K

in the onset time with heat flux bounded by = lo20 W cm-’ for the case of precursor nucleate boiling. If the hydrostatic pressure is sufficiently large, noisy boiling finally appears after the establishment of film boiling. The onset time of noisy boiling is not exactly reproducible for each heating. In the case of silent boiling for small hydrostatic pressures (silent) film boiling still continues. It should be noted here that when the heat flux q is too large (larger than ~50 W cm-‘) the boiling state does not turn into noisy boiling in spite of high hydrostatic pressures. The upper critical heat flux for the disappearance of noisy boiling seems to depend on the area of the heater and the hydrostatic pressure. The details of this phenomenon will be reported in a later paper. No boiling phenomena are observed even for 10 s of heating for q smaller than 4 W cmm2at 1.80 K under a large hydrostatic pressure condition (immersion depth >20 cm) for the transparent heater. It is also observed that this lower critical heat flux slightly depends on the area of the heater and the bath temperature. It becomes smaller for a larger heater mounted in an open space and for higher temperatures. The data measured at 1.80, 2.00 and 2.10 K are plotted in Figure 7 along with some existing data from Van Sciver’ and Tsoi and Lutset lo. Van Sciver used a solid copper rod (heater surface area S = 62.1 mm’) wrapped with manganin wire as the heater, mounted in a long tube filled with He II in his experiment. He proposed the correlation tH = Kq4 for the onset time of film boiling, where K is given as a function of temperature only. The correlation has since been justified theoretically by Dresner’ ‘. As his measurements were limited to relatively small q, his correlation is extrapolated towards higher q in Figure 7 (dashed lines). The data9 were obtained in a confined space, in a long tube, where vortices cannot diffuse away, except along the tube. In this respect, the data from Van Sciver are considered to be one-dimensional results, while ours are three-dimensional. It is seen that the present data exhibit a qualitatively similar tendency to the extrapolated correlation. A clear distinction between the data at 2.0 and 1.8 K cannot be found in the present case. Tsoi and Lutset used a planar heater with dimensions of 3 cm x 3 cm (equivalent to our transparent heater in size) and registered boiling by detecting the accompanying pressure waves with the aid of a piezoelectric transducer. They proposed another correlation tH = 0.25qw2, for cases of heat flux q >lO W cme2 at 1.79 transition

References I

2

3

4 5

6

Leonard, A.C. Helium II noisy film boiling and silent film boiling heat transfer coefficient values Proc ZCEC 3 Iliffe Science and Technology Publications, Guildford, UK (1970) 109-l 14 Katsukt, Y., Murakand, M., IIda, T., ShimazakI, T. and Sato, T. Visualization study of noisy and silent film boiling phenomena on a plane heater in He II Proc ICEC 15 Butterworths, Guildford (1994) Steed, R.C. and hey, R.K. Correlation of the depth effect on filmboiling heat transfer in liquid Helium II Adv Cryog Eng (1970) 15 299-307 Ebright, F.L. and hey, R.K. High-speed motion-picture studies of film boiling in liquid Helium II Adv Cryog Eng (1971) 16 386-392 Bet@ K.R. and Leonard, A.C. Free convection film boiling from a flat, horizontal surface in saturated He II Adv Cryog Eng (1975) 21 282-292 Kobayashi, II. and Yasukocbi, K. Maximum and minimum heat flux and temperature fluctuation in film-boiling states in superkid Helium Adv Cryog Eng (1980) 25 372-377 Nemirovskii, S.K. and Tsoi, A.N. Transient thermal and hydrodynamic processes in super&rid helium Cryogenics (1989) 29 985-994 Murakami, M., Iida, T., SbImazakl, T., Katsuki, Y. and Sato, T. The initial thermo-fluid dynamic processes of boiling phenomena in He II Cryogenics in press Van Sciver, S.W. Transient heat transport in He II Cryogenics (1979) 19 385-392 Tsoi, A.N. and Lutset, M.O. Inzhenerno-Fizicheskii ZhurnuZ(l986) 51 (Nl) 5-9 Dresner, L. Transient heat transfer in superfluid Helium Adv Ctyog .Eng(1984)29 323-333

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