A coupled wicking and evaporation model for prediction of pool boiling critical heat flux on structured surfaces

Abstract

Boiling is an effective heat transfer mechanism that is central to a variety of industrial processes including electronic systems, power plants, and nuclear reactors. Micro-/nanostructured surfaces have been demonstrated to significantly enhance the critical heat flux (CHF) during pool boiling, but there is no consensus on how to predict the structure-induced CHF enhancement. In this study, we develop an analytical model that takes into consideration key mechanisms that govern CHF during pool boiling on structured surfaces, namely, capillary wicking and evaporation of the liquid layer underneath the bubble. The model extends existing wicking-based CHF theories by introducing the competing evaporation mechanism. The model reveals a wicking-limited regime where CHF increases monotonically with the wicking flux, and an evaporation-limited regime where additional increases in the wicking flux do not significantly affect CHF. The model predictions are shown to agree with experimental CHF data from the literature for boiling of water on surfaces structured with square micropillar arrays. A parametric study is performed for such micropillared surfaces and has identified the optimal structure based on the competition between wicking and evaporation, capillary pressure and viscous resistance, and conduction and liquid-vapor interfacial resistance.

Nanoparticle-Assisted Pool Boiling Heat Transfer on Micro-Pin-Fin Surfaces

“…Hu et al 77 proposed two parameters to define the wicking flux and the evaporation flux, which are characteristic wicking flux (ϕ wi,ch ) and characteristic evaporation flux (ϕ ev,ch ). Then, the critical heat flux gain is expressed as The characteristic wicking flux was analytically obtained by solving the Brinkman equation as 77 where K and ε are the surface permeability and porosity, respectively, h is the height of a micro pin fin, P c is the capillary pressure, and ρ l , ρ v , μ l , σ lv , and g are the liquid density, vapor density, liquid dynamic viscosity, liquid–vapor surface tension, and gravitational acceleration, respectively. In the present study, the permeability of CPF-1 and SPF-1 was calculated by the model proposed by Sangani and Acrivos, 78 while the permeability of SPF-2 and the micropost surface 40 was calculated by the model proposed by Drummond and Tahir, 79 considering the configuration of micro pin fins.…”

“…The characteristic evaporation flux was derived based on an equilibrium meniscus formed between micro pin fins beneath bubbles where a thermal network model was applied that has balanced thermal conduction across the liquid film and evaporation across the liquid–vapor interface. 77 where R is the radius of the equilibrium meniscus, δ( y ) is the meniscus profile (the liquid film thickness) with respect to the position along the pin fin height, k l is the liquid thermal conductivity, θ rec is the receding contact angle, d and p are the diameter of a micro pin fin and the pitch between neighboring pin fins, respectively, and h lv is the evaporation heat transfer coefficient. In the present study, the receding contact angle is assumed to be zero and the meniscus profile is assumed to be a circular profile.…”

“…Liu et al , further considered the regulation of micro/nanostructures on bubble dynamics at critical heat flux and proposed a model that incorporates wicking fluxes and bubble departure frequencies. Recently, Hu et al proposed a coupled wicking and evaporation model to predict the critical heat flux on structured surfaces, which was great inspiration for the present study. This model further considers the evaporation beneath bubbles.…”

“…Hu et al 77 proposed two parameters to define the wicking flux and the evaporation flux, which are characteristic wicking flux (ϕ wi,ch ) and characteristic evaporation flux (ϕ ev,ch ). Then, the critical heat flux gain is expressed as…”

Research of Thermal Hydraulic Conditions Effect on PWR CIPS Risk

“…Some researchers believe that the CIPS is a phenomenon caused by the local increase of boron precipitation mass of fuel assemblies. It is necessary to simulate the local CRUD behavior of fuel assemblies in more detail (Haq et al, 2011;Hu et al, 2019) and calculate the power offset degree of fuel assemblies by coupling nuclear physics calculation codes to accurately predict the CIPS risk of the PWR. However, Table 1 is still the most widely accepted CIPS risk criteria.…”