Contents lists available at ScienceDirect International Journal of Heat and Fluid Flow journal homepage: www.elsevier.com/locate/ijhff Evaluation of the microlayer contribution to bubble growth in horizontal pool boiling with a mechanistic model that considers dynamic contact angle and base expansion Wei Ding a, ⁎ , Eckhard Krepper a , Uwe Hampel a,b a Institute of Fluid Dynamics, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany b AREVA Endowed Chair of Imaging Techniques in Energy and Process Engineering, Technische Universität Dresden, Germany ARTICLE INFO Keywords: Nucleate boiling Microlayer Force balance Dynamic contact angle Dynamic base expansion Bubble geometry ABSTRACT Recently a new mechanistic model for pool and nucleate flow boiling was developed in our group. This model is based on the balance of forces acting on a bubble and considers the evaporation of the microlayer underneath the bubble, thermal diffusion around the cap of bubble due to the super-heated liquid and condensation due to the sub-cooled liquid. Compared to other models we particularly consider the temporal evolution of the microlayer underneath the bubble during the bubble growth by consideration of the dynamic contact angle and the dynamic bubble base expansion. This enhances, in our opinion, the model accuracy and generality. In this paper we further evaluate this model with experiments and direct numerical simulation (DNS) in order to prove the importance of dynamic contact angle and bubble base expansion. 1. Introduction Nucleate boiling is an efficient heat transfer process. Its physical modeling is still not fully mature as it involves complex two-phase fluid dynamics with mass, momentum and energy transfer at the liquid- vapor interface and further heat conduction through solid walls. The bubble dynamics of nucleation boiling has been heavily investigated since the 1950s, first in pool boiling. In the 1950s Forster and Zuber (1954) as well as Plesset and Zwick (1954) modelled the bubble growth in a uniformly superheated liquid. Zuber (1961) extended this model to non-uniform temperature fields. Then Mikic et al. (1970), Prosperetti and Plesset (1978), and Labuntsov (1974), derived di- mensionless relations for inertia controlled and heat (or thermal dif- fusion) controlled growth. Cooper and Lloyd (1969) identified a thin liquid microlayer underneath the bubbles and modelled it on the basis of experimental findings. Then van Stralen et al. (1975) proposed a model based on the evaporation of the microlayer underneath the bubble and heat diffusion from a relaxation microlayer around the bubble. In 1993, Klausner et al. (1993) developed a model based on the balance of the forces acting on the bubble to predict its departure and lift-off. The authors obtained satisfactory prediction accuracy against their own data of flow boiling with refrigerant R113. They re- commended a fixed bubble base diameter (contact diameter) of 0.09 mm, an advancing contact angle of π/4 and a receding contact angle of π/5. Later, modified versions of the Klausner model have been brought up by others with other values of base diameter, advancing and receding contact angle to predict their own experimental data. Ex- amples are Yun et al. (2012), Situ et al. (2005), Sugrue (2012), Thorncroft et al. (2001) and Chen et al. (2012). Klausner applied the Mikic model to simulate the bubble growth while Situ and most of the latter authors employed the Zuber (Mikic et al., 1970) formulation. Zuber included in his formulation a parameter b to account for bubble sphericity. This parameter has been used by the latter authors with different values between 0.24 and 24 to fit the models with their ex- perimental data (Colombo and Fairweather, 2015). Yun et al. (2012) improved Klausner's model by incorporating a bubble condensation model as well as evaluating the model for a wider range of pressure, temperature, and flow rates for water. More recently, in 2015, Colombo and Fairweather (2015) developed a mechanistic model to simulate the bubble growth and departure. In the model, they con- sidered the contribution of the microlayer, the superheated thermal liquid layer and the condensation to bubble growth (Fig. 1). Based on the suggested contact angles from Klausner et al. (1993) and other empirically measured contact angles, the model gave a good agreement with data from different experiments. Later in 2017, Raj et al. (2017) tried to formulate a similar model as an analytical solution with countable validations. In 2018, Mozzocco et al. (2018) developed a model for the mechanistic prediction of bubble departure and lift off. https://doi.org/10.1016/j.ijheatfluidflow.2018.06.009 Received 9 February 2018; Received in revised form 11 May 2018; Accepted 12 June 2018 ⁎ Corresponding author. E-mail address: w.ding@hzdr.de (W. Ding). International Journal of Heat and Fluid Flow 72 (2018) 274–287 0142-727X/ © 2018 Elsevier Inc. All rights reserved. T