Contents lists available at ScienceDirect Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs Effect of subatmospheric pressures on heat transfer, vapor bubbles and dry spots evolution during water boiling Anton Surtaev a,b, ⁎ , Vladimir Serdyukov a,b,c , Ivan Malakhov a,b a Novosibirsk State University, Pirogov str. 1, Novosibirsk, Russia b Kutateladze Institute of Thermophysics SB RAS, Lavrentiev ave. 1, Novosibirsk, Russia c Chinakal Institute of Mining SB RAS, Krasny ave. 54, Novosibirsk, Russia ARTICLE INFO Keywords: Boiling Subatmospheric pressure Dry spot evolution Bubble dynamics Multiscale heat transfer ABSTRACT The present paper reports the results of the comprehensive experimental investigation of an influence of sub- atmospheric pressures on multiscale heat transfer characteristics during liquid pool boiling. Experiments were carried out in the pressure range of 8.8–103 kPa at saturated water boiling using high-speed IR thermography, high-speed visualization from different sides and the specially designed transparent ITO heater. This made it possible to obtain simultaneously extensive data set on the effect of reduced pressure on main characteristics of boiling, including heat transfer coefficients, nucleation site density, growth rate and departure diameter of vapor bubbles. High-speed visualization from a bottom side of transparent heater allowed to investigate an evolution of dry spots bounded by triple contact line depending on pressure for the first time. It was demonstrated that the growth rate of dry spots is constant in time and has a non-monotonic dependence on pressure. 1. Introduction Being one of the most effective heat transfer regimes boiling is quite often used in practice. But despite numerous studies there are still questions related to the description of dynamics of two-phase flows, the theory of heat transfer and crisis phenomena development during nu- cleate boiling [1,2]. Commonly, dimensionless correlations presented in the literature were obtained for certain fluids and are valid only in certain pressure range. For example, at pressures range of p/ p cr < 0.002 the well-known hydrodynamic theory of pool boiling crisis [3,4] shows significantly overestimated results than the experiments [5,6]. The complexity of the theoretical description of the boiling process is primarily due to the fact that this is conjugate task, which requires taking into account the influence of the physical and chemical surface properties, including its geometry, morphology, wetting prop- erties, etc. Secondly, boiling is a multiscale non-stationary process and for its description it is necessary to consider the effects that occur on different spatial and temporal scales. These features of the boiling also create additional complexity for the experimental study of this process. It is well known that the system pressure is one of the most im- portant parameters which has the complex effect on the nucleation, the heat transfer rate and critical heat fluxes at nucleate boiling. In the second half of the last century various authors [7–15] showed that with pressure reduction, the sharp decrease in the density of nucleation sites and the emission frequency of vapor bubbles, as well as the increase in the growth rate and departure diameters of bubbles are observed. This reflects the fact, that with pressure reduction vapor density and surface tension dramatically change, which leads to the increase in the critical radius of the vapor bubble and wall superheating corresponding to boiling incipience, and as a result to the increase in the Jakob number. The change in the nucleation site density and the emission frequency of vapor bubbles leads to significant surface temperature fluctuations. A significant change in the local boiling characteristics and in the dy- namics of two-phase flows near a heated wall at subatmospheric pres- sures has a negative effect on the intensity of heat transfer and the value of the critical heat flux. In the literature, a lot of attention is paid to an investigation of the dynamics of vapor bubbles during boiling of various liquids at sub- atmospheric pressures. In particular, authors of [7,8,13,16–20] ana- lyzed a growth rate of vapor bubbles at pool boiling down to p = 1 kPa with the use of high-speed video recording from the side of heating surface. It was shown that a growth rate of vapor bubbles at sub- atmospheric pressures boiling cannot be described in frame of heat diffusion-controlled scheme of bubble growth, at which the interfacial heat transfer is the only limiting factor. Bubble growth curves obtained for different reduced pressures are characterized by different exponents n in power law R eq (t) ~ t n , which demonstrates the manifestation of different mechanisms of bubble growth with pressure change [21]. The https://doi.org/10.1016/j.expthermflusci.2019.109974 Received 5 July 2019; Received in revised form 29 September 2019; Accepted 27 October 2019 ⁎ Corresponding author. E-mail address: surtaev@itp.nsc.ru (A. Surtaev). Experimental Thermal and Fluid Science 112 (2020) 109974 Available online 30 October 2019 0894-1777/ © 2019 Elsevier Inc. All rights reserved. T