Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng Research Paper Characteristics of nucleate boiling under conditions of pulsed heat release at the heater surface Anatoliy Levin ⁎ , Polina Khan Melentiev Energy Systems Institute SB RAS, 130, Lermontov St., Irkutsk 664033, Russia HIGHLIGHTS • The initial stages of the nucleate boiling of the subcooled water flow were investigated • The unsteady stepwise heat flux ranges from 0.56 to 2 MW/m 2 . • New data of the wall overheat and the values of the nucleation density were obtained. • New data were generalized by the dependences developed for the stationary boiling. • The heat balance model was used for numerical calculation of the temperature field. • Predictions of the D m made by using superheated layer thickness are most adequate. ABSTRACT This paper presents results of the experimental study for the initial stage of the explosive boiling, as well as an attempt to simulate them in order to clarify whether the existing approaches can be extended to the case of the non-stationary heat release. We present data on the onset of nucleate boiling (ONB) and nucleation density for pulsed heat release in the range from 0.56 to 2 MW/m 2 . To determine the dynamics of the temperature field under conditions of non-stationary nucleation, a numerical algorithm was implemented, which includes the calculation of the heat fluxes associated with the interphase interaction in the bubble neighborhood. It was shown that for the non-stationary case the previously developed approaches can be applied to numerically predict nucleate boiling characteristics using the Fourier number. 1. Introduction Emergency situations in nuclear reactors are often accompanied by a rapid increase of energy release, which, in terms of the surface unit, significantly exceeds the critical heat flux (CHF). In this case, the boiling modes quickly change each other, and the characteristics of heat transfer in the bubble mode can differ significantly from similar con- ditions during steady-state heating, since the formation and collapse of bubbles occur in a non-stationary temperature field. With many papers addressing nucleate boiling at heat fluxes significantly smaller or reaching the CHF, nucleate boiling at higher heat fluxes has not been studied well. Thus, the study of boiling in a stream of subcooled liquid for the heat fluxes exceeding the CHF remains a relevant problem, the solution of which will improve predictive cooling models for atomic reactors in emergency and pre-emergency conditions. The prospective findings can also be useful in applied problems characterized by a high heat flux density. The high efficiency of boiling heat transfer is the main reason for many researchers to focus their studies on such local characteristics of nucleate boiling as onset of nucleate boiling, nuclea- tion density, bubble departure frequency and bubble detachment dia- meter. The extensive development of modeling tools, as well as the growing requirements for detailed predictive models, aroused great interest in developing new approaches [1–3] based on the spatial dis- tribution of the main parameters (temperature, pressure, flow quality). At the same time, recent advances in experimental research sig- nificantly expanded the database on the dynamics of steam structures in conditions of stationary and non-stationary heat fluxes. Some works, for example, [4–6] continued to study the experimental determination and construction of models for predicting the main characteristics of nu- cleate boiling, such as bubble diameters, nucleation rates, and the nu- cleate density. In [7], non-stationary conditions with periodic pulsa- tions of the heat flux were investigated. The relationship between the key characteristics of nucleate boiling and the level of heat flux was demonstrated. The work [2] is focused on the application of the RPI (Rensselaer Polytechnic Institute) wall model to pressures up to 15 MPa. Numerical verification showed that the approach developed in https://doi.org/10.1016/j.applthermaleng.2018.12.126 Received 8 September 2018; Received in revised form 7 December 2018; Accepted 24 December 2018 ⁎ Corresponding author. E-mail address: lirt@mail.ru (A. Levin). Applied Thermal Engineering 149 (2019) 1215–1222 Available online 27 December 2018 1359-4311/ © 2018 Elsevier Ltd. All rights reserved. T