Experimental evidence of a new regime for boiling of water at subatmospheric pressure

https://doi.org/10.1016/j.expthermflusci.2014.07.011Get rights and content

Highlights

  • Experimental set-up for subatmospheric pool boiling characterization was built.

  • The non-homogeneity of the boiling environment is highlighted.

  • Bubbles with “mushroom” shape were observed and analyzed.

  • Instantaneous boiling curves were plotted for pressure range of 0.8–100 kPa.

  • A new regime specific to subatmospheric pool boiling was observed.

Abstract

The objective of this paper is to underline the specific characteristics of water pool boiling at subatmospheric pressure compared to atmospheric pressure in order to optimize the design of evaporators dedicated to sorption systems. An experimental test set up was built to characterize the pool boiling of water at pressure down to 0.85 kPa. The major specificity of the pool boiling at low pressure lies in the non homogeneity of the boiling environment, both in terms of pressure and subcooling degree. This non-homogeneity makes really different boiling phenomena than those usually observed. That is why a particular shape and size of bubbles at low pressure is observed. Bubbles have centimeter size (up to 15 cm at 1.2 kPa) and are often followed by a liquid jet at the departure that confer them a “mushroom” shape. Boiling curves from 100 kPa to 0.85 kPa were plotted and related to specific subatmospheric pressure pool boiling regime. This regime is characterized by significant wall temperature fluctuations (up to 20 K for our experimental conditions) affecting the heat transfer coefficient and causing material fatigue.

Introduction

Refrigeration by means of sorption systems is desirable to answer new environmental regulations, as sorption systems substitute a part of their needs in mechanical energy by thermal energy. Moreover, for air-conditioning applications the refrigerant classically used is water (LiBr/H2O absorption, or silica gel/H2O, zeolite/H2O adsorption systems) which is fully environment-friendly. However, to develop small cooling power sorption systems, the current size of these machines must be reduced. Evaporator is one of the components which needs to be improved. The studies on these heat exchangers for sorption systems are still scarce so far [1], [2], [3] and thus, the design of these evaporators still remains mainly empirical. Fundamentals work on the driving phenomena in actual operating conditions, i.e. at subatmospheric pressure near 1 kPa, are scarce as well.

The mains parts of the investigations done at subatmospheric pressure are experimental and focus on bubble growth rate or on heat transfer enhancement by changing the surface treatment. That is why general features of boiling at low pressure are the high bubbles growth rates, the large bubble volumes at detachment, the long waiting time between bubbles [4], [5], [6], [7], [8], and the deterioration of the boiling performance [2], [6], [9], [10]. Few studies focus on the fundamental phenomena of water pool boiling on a plain plate surface whereas many phenomena not explained yet to the authors’ knowledge were observed.

The analysis of the entire bubble life cycle remains to be done despite its special fundamental interest because of the particular bubble shape at such low pressure [11], [12]. Some results report bubble growth curves as the bubble equivalent radius vs. time, which is insufficient information in that sense that the bubbles are far from being spherical. Some authors also observed instabilities during the boiling at low heat fluxes [5], [6], [7], [13], [14] but, to the authors’ knowledge, no further investigations were done. Schnabel et al. [2] noticed the importance of the filling levels on the bubble growth at such low pressure (in their experiments the filling water levels was lower than the bubble size) but no other authors mentioned the non-homogeneity characteristic of the boiling environment at such low pressure and the influence of the vapor saturation pressure as well as the influence on the liquid height on the experimental conditions. Only the influence of the size of the vessel or of the heat transfer area relative to the bubble size was eventually mentioned [5], [13], [14].

The aim of the present work is to go further in characterizing the behavior of pool boiling at subatmospheric pressure. From Bonjour et al. [15] to Siedel et al. [16], [17], works on boiling phenomena on an isolated artificial nucleation site permitted large contribution to the fundamental analysis of the pool boiling phenomenon under different running conditions. Thus, an experimental test bench inspired from these works was built to answer low pressure specifications. It allows to study the bubble dynamics of water on a copper horizontal plane surface. In the present article, one discusses on the non-homogeneity of the boiling environment and on its influence on the bubbles size. Bubbles growth dynamics was investigated by means of a high speed camera. As some authors mentioned significant fluctuations of the wall superheated in certain conditions of imposed heat flux [5], [6], [12], boiling curves for pressures ranging from 0.8 kPa to 100 kPa were plotted as the instantaneous wall superheat vs. the instantaneous heat fluxes for a given imposed heat flux at the heater. The existence of a particular boiling regime was thus detected when reaching low pressures.

Section snippets

Experimental test set up

The experimental test facility is constituted by a 200 mm inner diameter and 420 mm high stainless steel cylinder (Fig. 1). Four circular viewports (diameter 100 mm) arranged on the side face of the chamber allow observing the sample. A fifth viewport of the same diameter is located at the base of the vessel and is used to insert the test sample into the vessel.

For the study, experiments are performed using distilled water as working fluid. A valve connected to a vacuum pump at the top of the

Boiling environment in subatmospheric pressure

At subatmospheric pressure, as the pressure is very low, the static head imposed by the liquid height can be of the same order of magnitude as the fluid saturation pressure. As a consequence, the pressure and saturation temperature (or subcooling degree) fields of the bulk fluid in the vessel are highly non-homogeneous, which affects the bubble growth dynamics and the bubble shape. As the liquid weight is non negligible, a variation of the liquid height also means a variation of pressure. The

Bubbles dynamics at different pressure

As explained above, there must be a pressure (for a given liquid height) for which the influence of the subcooling degree is more significant than the influence of other parameters (surface tension, vapor density, diffusivity, etc.). Fig. 6 shows bubbles, just before their detachment, obtained for vapor saturation pressures of 100 kPa, 15 kPa, 4.2 kPa, 1.8 kPa, 0.85 kPa and a given imposed heat flux of 10.2 W cm−2. The liquid height is 200 mm.

As shown in Fig. 6, bubbles and regimes obtained at

Thermal analysis of the boiling phenomenon

As large subatmospheric bubbles grow and detach, some authors report large fluctuations of the wall temperatures [5], [6]. Van Stralen et al. [5] observed a local temperature drop up to 11 K during the bubble growth. The temperature then increased again during the succeeding waiting time, i.e. during the amount of time required to superheat the liquid in the vicinity of the wall. For Van Stralen et al. [5], these periodic temperature fluctuations inside the copper (a highly conducting heating

Conclusions

An experimental test set-up was designed and built to study pool boiling of water over a large range of pressure (between 100 kPa and 0.85 kPa) on a horizontal heated copper disk. It was shown that a major specificity of boiling at subatmospheric pressure lies in the non homogeneity of both saturation temperature and pressure of the boiling environment. Especially the bubble dynamics at subatmospheric pressure was described and briefly compared to the dynamics of bubbles obtained at atmospheric

Acknowledgements

The authors wish to thank the ANR (Agence National de la Recherche, French funding organization) for funding this study and all the partners of the ANR Project ECOSS (Contract No: ANR-11-SEED-0007-001). This project focuses on compact evaporator in sorption system using water as refrigerant in order to give guidelines to design them properly.

References (33)

  • R. McGillis, V.P. Carey, J.S. Fitch, W.R. Hamburgen, Pool boiling on a small heat dissipating element in water at low...
  • V.V. Yagov

    Bubble growth rate at pool boiling in wide range of reduced pressures

    Exp. Heat Transfer, Fluid Mech. Thermodyn.

    (2001)
  • D.S. Cryder et al.

    Heat transmission from metal surface to boiling liquids: effects of temperature of the liquid on the liquid film coefficient

    Am. Inst. Chem. Eng.

    (1937)
  • M.A. Chan et al.

    Pool boiling heat transfer of water on finned surfaces at near vacuum pressures

    ASME J. Heat Transfer

    (2010)
  • R. Rullière, B. Siedel, P. Haberschill, Experimental evaluation of bubble growth of water at very low pressure, in: ECI...
  • F. Giraud, R. Rulliere, C. Toublanc, M. Clausse, J. Bonjour, Experimental study of water pool boiling at very low...
  • Cited by (61)

    View all citing articles on Scopus
    View full text