EXPERIMENTAL STUDY OF WATER POOL BOILING AT VERY LOW PRESSURE ON A VERTICAL OR HORIZONTAL HEATED SURFACE

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1 EXPERIMENTAL STUDY OF WATER POOL BOILING AT VERY LOW PRESSURE ON A VERTICAL OR HORIZONTAL HEATED SURFACE Florine Giraud (a), Romuald Rulliere (a), Cyril Toublanc (b), Marc Clausse (b)(c), Jocelyn Bonjour (a) (a) Université de Lyon, CNRS, INSA-Lyon, CETHIL, UMR5008, F-69621, Villeurbanne, France Université Lyon 1, F-69622, France (b) Laboratoire de Génie des Procédés pour l Environnement, l Energie et la Santé (LGP2ES - EA 21), CNAM, ICENER, case 2D3P20, 292 rue Saint-Martin, Paris Cedex 03, France (c) ESIEE Paris, 2 boulevard Blaise Pascal, Cité Descartes, BP 99, Noisy le Grand Cedex romuald.rulliere@insa-lyon.fr ABSTRACT The objective of this study is to describe properly the boiling heat transfer phenomena at very low pressure, a fundamental knowledge acquired to optimize the design of evaporators in absorption or adsorption systems using water as the refrigerant (H 2 O-LiBr absorption or, silicagel/h 2 O, zeolite/h 2 O adsorption systems). We will first describe the experimental set up which permits the characterization of water pool boiling at pressure below 1.8 kpa. Then, results obtained on a vertical or horizontal flat copper surface of 19.6 cm² will be presented. We will show boiling curves at decreasing flux in order to study influences of the liquid height and of the wall orientation. As the static head imposed by the water level has an important influence on the absolute pressure, dynamics bubbles in this inhomogeneous environment was also studied. Bubbles with particular shape and with a diameter up to 10 cm were observed. 1. INTRODUCTION Sorption refrigeration systems permit to substitute a part of their needs in mechanical energy by thermal energy. Furthermore, to answer the new environmental regulations concerning the refrigerant nature and the global warming, natural working fluids are desirables. For air-conditioning applications, the refrigerant classically used is water (absorption: water/lithium Bromide, adsorption: silicagel/h 2 O, zeolite/h 2 O). In these applications, the temperature of the water in the evaporator is about 10 C so that the saturation pressure is about 10 mbar. Knowledge on boiling heat transfer phenomena at such low pressure is insufficient and thus, the design of evaporators still mainly remains empirical despites non-conventional behavior of these heat exchangers (Clausse et al., 2011). The aim of the present work is to characterize the boiling heat transfer processes at low pressure levels. The study will especially aim at showing the influence of the pressure on the heat transfer coefficients and bubble dynamics. An experimental test bench was thus built. It allows characterizing the pool boiling of water on a heated copper vertical or horizontal plane surface at very low pressure. 2. EXPERIMENTAL TEST SET UP 2.1 Experimental test facility The experimental test facility (fig. 1a) is constituted by a stainless steel cylindrical pool-boiling vessel of a 200 mm inner diameter and a height of 420 mm (fig. 1b). To facilitate observation and photography of boiling phenomena at the test sample level, the vertical face of the vessel is equipped with three circular viewports of diameter 100 mm. Using two other openings of the same diameter, the test sample can be inserted in the vessel either from the side (vertical configuration) or from the base face of the vessel (horizontal configuration). 1

2 Figure 1. (a) experimental test facility (b) vacuum vessel The working fluid in the present study is water in saturated conditions. A heat exchanger inside the wall of the vessel in which circulates a coolant allows maintaining the desired saturation and pressure conditions of the working fluid during the experiments. Initially, the tank is evacuated then filled with water. The height of the free surface between the liquid and the vapour phases in the vessel is estimated thanks to a transparent hose connected with the chamber. A valve situated at the bottom of the vacuum chamber allows emptying or filling the vessel with water. Another valve situated at the top of the chamber allows the connecting of a vacuum pump. All the components are made in vacuum technology (ISO-K) to ensure a high gas-tightness. Very low pressures in the order of 0.5 kpa can be maintained. To record temperatures and pressures inside the vessel, two K type thermocouples and two vacuum pressure transducers are used. The two vacuum traducers measure the pressure of vapour at the top of the vessel and have an operational range of 0-1 bar and bar, respectively. The two thermocouples measure the vapour and the liquid temperature. The latter is situated at a distance of 100 mm from the bottom of the vessel. The test heater (fig. 2) is a 40 mm diameter and 78 mm high cylindrical copper block. A fin of 3 mm in thickness and 50 mm in diameter has been made at the end of the cylinder to promote boiling on an artificial nucleation site. Two cartridge heaters of 600 W each are inserted in the bottom of the copper cylinder. Eight K type thermocouples are located along the cylinder by rows of four and with an angle of 45 from each other. These thermocouples are used to determine the wall temperature by means of an inverse heat conduction method. The copper block is isolated on its perimeter with PTFE (fig. 3). A toric seal is placed between the copper block and the PTFE in order to have an efficient gas-tightness. The boiling surface has an area equal to 19.6 cm². This copper plate surface is carefully prepared to avoid any parasite boiling. The surface is polished until reaching a roughness lower than 0.4 µm (measured with a confocal microscope). A conical artificial nucleation site (110 µm in diameter and 73 µm in depth) is made at the center of the plate. Figure 2. Detail of the test heater 4th IIR Conference on Thermophysical Properties and Transfer Processes of Refrigerants, Delft, The Netherlands, 2

The acquisition frequency of the camera is set to 1000 images per second to obtain the best trade-off between acquisition speed and image size.

3 Boiling surface PTFE (a) (b) Figure 3. Test sample (a) profile view (b) front view 2.2 Visualization facility To study the dynamics of bubble growth, a high-speed video camera is used. Boiling is recorded through a lateral viewport while the boiling area is illuminated through the opposite viewport (fig. 4). The acquisition frequency of the camera is set to 1000 images per second to obtain the best trade-off between acquisition speed and image size. The numerical photos are analyzed with the software Octave in order to determine the bubble geometrical characteristics. Bubbles grow in a very inhomogeneous environment in temperature and pressure. The shape and dimensions of bubbles are thus not the same from the front view as seen in profile, especially in vertical orientation. As photos describe bubbles only in two dimensions, it matters to specify from which view they are taken. In the present work, only photos from profile view are introduced. Figure 4. Visualization facility 3. EXPERIMENTAL RESULTS The presented experimental results were realized with water, for vapour pressures between 1.2 kpa and 1.8 kpa. Because of the very low pressure work, the static head imposed by the liquid can be of the same order of magnitude as the fluid saturation pressure (for instance, the hydrostatic pressure is equal to 2 kpa if the liquid height is h = 0.2 m). The influence of the liquid height is therefore significant and makes bubbles grow in a very inhomogeneous environment in temperature and pressure. The absolute pressure P at the level of the artificial nucleation site can be calculated from the relation: P = Psat + ρgh (1) With ρ assumed to be constant in this pressure range. As a thermocouple placed close to the heated surface would disturb liquid movements, it was preferred to determine the saturated temperature at the level of the artificial nucleation site from a calculation: The formula comes from the properties of water in this pressure range. T * = ln( P + gh) sat ρ (2) sat 3

3.1 Experimental boiling curves Figure 5 shows the influence of the liquid height on the boiling curves on a vertical and on a horizontal surface.

The liquid height is defined as the distance between the nucleation site and the liquid-vapor free interface in the vessel. Theses curves were obtained at decreasing flux.

4 3.1 Experimental boiling curves Figure 5 shows the influence of the liquid height on the boiling curves on a vertical and on a horizontal surface. For each case, the pressure of vapour is constant and, as explained before, liquid height has an influence on the absolute pressure at the level of the nucleation site. The liquid height is defined as the distance between the nucleation site and the liquid-vapor free interface in the vessel. Theses curves were obtained at decreasing flux. Each point represents the average of sixty data collected during ten minutes after steady state was obtained. Heat Flux (W.cm-²) hv = 74 mm hv = 115 mm hv = 200 mm Heat Flux (W.cm-²) hh = 74 mm hh=108 mm hh=200 mm Wall superheat (K) Wall superheat (K) Figure 5. Boiling curves for different water heights (a) on vertical heated surface (b) on horizontal heated surface (P = 1.8 kpa) The figure 5b shows that, in horizontal orientation, the heat transfer improves with increasing the pressure. This confirms the trend observed by Rulliere et al. (2012). However, the present work reveals an opposite trend in vertical (fig 5a). In vertical orientation (fig 5a), the minimum wall superheat required to maintain nucleation increases with the liquid height. In this specific orientation, three mains phenomena improve heat exchanges: convection phenomena, bubbles shape and pump effect. Convection phenomena are due to inhomogeneous density of the fluid along the heat surface. The specific bubble shape in vertical orientation is due to buoyancy forces (Katto et al., 1970). This shape fosters the microlayer evaporation (Bonjour et al., 1997) and allows bubble sliding. As the bubble slides, cool liquid is carried up by the bubble s wake. This is what we call in the present work the pump effect. Pressure has a significant influence on the bubble shape. As the pressure decreases, bubble diameter increases (Van Stralen et al., 1975, Zuber 1959, Cole and Rohsenow, 1968). Bubbles are bigger at low pressure and thus at low liquid height. Due to the bubble size, the length of the microlayer increases with the decreasing pressure as well as the velocity of the bubble slide on the wall (fig. 6). These phenomena improve the pump effect and so liquid motion along the heat surface. microlayer microlayer 2 cm 2 cm t = 73 ms, h v = 50 mm t = 45 ms, h v = 186 mm Figure 6. Visualization of the microlayer (P = 1.3 kpa, q = 2.9 W.cm -2 ) 4

5 3.2 Bubble dynamics at low pressure levels Figure 7 presents the visualization with a high speed camera of the bubble growth on a vertical heat surface at P = 1.8 kpa and q = 7.0 W.cm -2. At the first moments of the growth, the bubble is hemispherical. The radius of the dry heated zone increases. As the bubble keeps growing a flattening of the base is observed. The dry heated zone remains at a fixed location until the flattened base reached that specific location as depicted schematically on figure 8. Then, the bubble starts sliding upward along the heated surface before collapsing as usual behavior in subcooled liquids. Figure 7. Visualization of bubble growth (P = 1.8 kpa, T l = 15 C, T* sat = 21 K, q = 7.0 W.cm -2 ) Figure 8. Schema of the mains moments of the bubble growth At this range of pressure, bubbles have centimeter sizes. These results are in agreement with Van Stralen et al. (1975) or more recently with Rulliere et al. (2012). Only the phase of inertial growth (also called isotherm growth) exists. The phase of isobaric growth, which is controlled by the heat diffusion, is never reached. The bubble has thus no spherical shape. According to Katto (1970), the flattening of the base is due to the 5

buoyancy force. The bubble has not reached its departure diameter but keeps growing. As the mass of vapour tends to keep on increasing, bubble shape changes to minimize the surface tension.

6 buoyancy force. The bubble has not reached its departure diameter but keeps growing. As the mass of vapour tends to keep on increasing, bubble shape changes to minimize the surface tension. The bubble sliding velocity depends on the size of the bubble and so decrease when the pressure increases. For example the velocity at q = 2.65 W.cm -2 and P = 3.0 kpa is around 240 mm.s -1 and around 470 mm.s -1 at P = 1.7 kpa. However, bubbles have a maximal horizontal diameter of 15 mm in the first case whereas the maximal horizontal diameter is 36 mm in the second case. The same trend is observed for the maximal vertical diameter (22 mm in the first case, 41 mm in the other). This could be explained by the fact that the vapour density falls by a factor 2 between 3 kpa and 1.7 kpa. Advancing and receding angles are however the same, around 20 and around 48, respectively. Different boiling flows patterns were observed and characterized mostly by bubble size and bubble departure frequency. For a specific area of the boiling curve, the flow pattern is characterized by a long waiting time (up to 120 s) then by the departure of a large bubble and immediately after by a lot of bubbles which pop up on the entire heated surface (fig. 9). Theses bubbles have different sizes and collapse relatively quickly. Figure 9. Visualization of bubbles crisis (P = 1.8 kpa, T l = 15 C, T* sat = 21 K, q = 12.7 W.cm -2 ) As Eddington and Kenning (1978) observed, this is probably due to the site seeding. During the growth of a large bubble, a microlayer is formed and then evaporated. The dry spot completely covers sites free of vapour which are still inactive. The bubble departure leaves a residue of vapour in these inactive sites, which are then able to promote bubbles nucleation. Judd and Lavdas (1980), who have studied the formation of a large bubble like in this present study, agreed with this explanation. As mentioned by Shi et al. (1993), when vapour is trapped in inactive sites, a decrease in the surface heat flux and a decrease in the wall superheat for the next generation are observed in our case. Figure 10 shows the variation of these parameters corresponding to this specific flow pattern for P = 1.8 kpa and q = 9.6 W.cm -2. Such trends were also observed by Shi et al. (1993). 6

7 Figure 10. Wall temperature and heat flux during boiling crisis (P = 1.8 kpa and q = 9.6 W.cm -2 ) 3. CONCLUSIONS An experimental test set-up has been designed and built to study boiling at very low pressure (between 1.2 kpa and 1.8 kpa) on a vertical or horizontal heated copper disk. As the static head imposed by the liquid has a significant influence on the absolute pressure, boiling curves at decreasing flux were first obtained to evaluate the effect of the liquid height in vertical orientation. Increasing the liquid height leads to increase the minimum wall superheat required to maintain nucleation and to decrease the heat transfer. The effect of the wall orientation on the boiling curve was also addressed. According to the literature, opposite trends were observed in horizontal configuration compared to those observed on the vertical one. As bubbles grow in a very inhomogeneous environment in temperature and pressure due to the importance of the static heads, bubbles dynamics were also observed. Bubbles filmed by means of a high-speed camera have centimeter size and present a particular shape with a flattening on their base. Finally the paper focuses on a special boiling flow pattern observed: large bubble departure followed by the generation of many bubbles of different sizes which collapses before leaving the wall. Waiting time up to 120 s can be reached between two cycles. ACKNOWLEDGEMENTS The authors wish to thank the ANR (National Agency for Research, French funding organization) for funding this study and all the partners of the ANR Project ECOSS (contract n : ANR-11-SEED ). This project aims to focus on compact evaporator in sorption system using water as refrigerant in order to give guidelines to design them properly. NOMENCLATURE g gravitational acceleration (m.s - ²) h height (m) P pressure (kpa) q heat flux (W.cm - ²) ρ density of water (kg m 3 ) T temperature (K) Subscripts h horizontal l liquid sat saturation v vertical * local 7

8 REFERENCES Bonjour J., Boulanger F., Gentile D. and Lallemand M., Etude phénoménologique de l ébullition en espace confine à partir d un site de nucléation isolé. Rev. Gén. Therm. 36, Calka A. and Judd R.L., Some aspects of the interaction among nucleation sites during saturated nucleate boiling. Int. J. Heat Mass Transfer. 28, 12: Chekanov V. V., Interaction of centers in nucleate boiling. Teplofiz. Vysok. Temp. 15, Clausse, M., Leprieur, J., Meunier, F., Experimental test of plate evaporator for sorption refrigeration systems, ISHPC 11, Padova Italie, mai 2011, paper I-86 Cole R. and Roshenow W. M., Correlation of bubble departure diameters for boiling of saturated liquids. Chem. Engineering Prog. Symposium Series. 92, 65: Judd R.L. and Lavdas C. H., The nature of nucleation site interaction. J. Heat Transfer. 102, Katto Y., Yokoya S. and Yasumaka M., Mechanism of boiling crisis and transition boiling in pool boiling. 4 th Int. Heat Transfer Conf. 4, Rulliere R., Siedel B. and Haberschill P.,2012. Experimental evaluation of bubble growth of water at very low pressure. ECI 8 th Int. Conf. On Boiling and Condensation Heat Transfer. Lausane. Switzerland. Shi M. H., Ji M. and Wang B. X, Analysis on hysteresis in nucleate pool boiling heat transfer. Int. J. Heat Mass Transfer. 36, 18: Van Stralen S. J. D., Cole R., Sluyter W. M. and Sohal M. S., Bubble growth rates in nucleate boiling of water at subatmospherique pressure. Int. J. Heat Mass Transfer. 18, Zuber N, Hydrodynamic aspects of boiling heat transfer. Report AECU-4439, U.S. Atomic Energy Commission. 8

Visual Observation of Nucleate Boiling and Sliding Phenomena of Boiling Bubbles on a Horizontal Tube Heater

Proceedings of the 2 nd World Congress on Mechanical, Chemical, and Material Engineering (MCM'16) Budapest, Hungary August 22 23, 216 Paper No. HTFF 146 DOI:.11159/htff16.146 Visual Observation of Nucleate