Proceedings of the 9th World Congress on Momentum, Heat and Mass Transfer (MHMT 2024)
London, United Kingdom – April 11 – 13, 2024
Paper No. ICMFHT 164
DOI: 10.11159/icmfht22.164

The Role of Surface Roughness on the Increased Heat Transfer Rate in
Pool Film Boiling of a Flat, Upward Facing Surface
Zhuo Li, Vladan Prodanovic, Matthias Militzer

The Centre for Metallurgical Process Engineering
The University of British Columbia
2355 East Mall, Vancouver, BC, Canada, V6T 1Z4
zhuo.li@ubc.ca; vladan.prodanovic@ubc.ca; matthias.militzer@ubc.ca

Extended AbstractBoiling heat transfer is commonly utilised in the steel making process to achieve high heat transfer rates during castingor hot rolling of steel products. Various process parameters, such as water flow rates, water temperature, or time of exposureof steel surface to water jet, can be adjusted to achieve the desired heat transfer rates and uniform cooling. This uniform
cooling across the surface of steel is particularly important to avoid residual thermal stresses and achieve the flatness of theproduct.
When water is deposited on the hot metal surface, which is typically in the range of 850 - 900 oC, film boiling quickly
becomes the dominat heat transfer mode. As the surface is cooled beyond the Leidenfrost point, the heat transfer modechanges to transition boiling and eventually to nucleate boiling. Water cooling for most steel products ends when metal
surface temperatures are still sufficiently high, such that nucleate boiling is seldom achieved.
Surface roughness has been identified as an important factor affecting the cooling rate of steel products in the hot rolling
process [1-2]. The quality of the product is often reduced by uneven cooling of steel, which is contributed by variable
roughness of the surface. While most cooling systems employ jets or sprays to deposit water on the cooled surface,understanding the effect of surface topography on cooling characteristics in pool boiling represents an important benchmark
from a phenomenological point of view. Previous experimental work on pool film boiling of hot metal plates with various
surface roughness [3-5] revealed earlier onset of transition boiling with an increase in surface roughness, resulting in fastercooling and non-uniform surface temperature distribution. It is equally important to develop models which can be used as
predictive tools to adjust process parameters in cooling equipment, and optimise the process for better quality control.
Several hydrodynamic models based on Rayleigh-Taylor instability, which addresses the structure of the vapour-liquidinterface, have been developed in the past to predict the onset of transition boiling at the Leidenfrost temperature. Whilethese models make accurate predictions for smooth surfaces, incorporating the effect of surface roughness is still achallenging task. This paper presents a simplified model to describe the role of surface roughness in pool film boiling and at
the Leidenfrost point, for an upward facing flat surface. The model is based on quantifying liquid intrusion due to instabilitiesof the liquid-vapour interface, and assumes that enhanced heat transfer from a rough surface results from frequent transientcontact between the liquid and rough elements. Based on modelling of the vapour-liquid interface with respect to the mostdangerous wavelength of Rayleigh-Taylor instability, it addresses the area fraction of liquid-solid contact with respect ofsurface roughness, and the duration of liquid-solid contact.
The model has been validated with pool boiling experiments on samples with surface roughness between 1-40 µm. The
experimentat data indicated an increase of cooling rate of about 50% with the increase of surface roughness. Model
predictions, which align well with experimental data, suggested intermittent liquid-solid contact area occuring at a frequency
of about 10 ms, and the fraction of the total contact area increasing with surface roughness up to 20% of the contact area inthe vicinity of the Leidenfrost point.

References[1] C.F. Gomez, C.W.M. Van Der Geld, J.G.M. Kuerten, M. Bsibsi, B.P.M. Van Esch, “Film Boiling in Quench Cooling with Hightemperature Jets”, Int. J. Heat Mass Transf. Vol. 164, 2021, https://doi.org/10.1016/j.ijheatmasstransfer.2020.120578

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[2] M. Chabicovsky, M. Hnidzil, A.A. Tseng, M. Raudensky, “Effects of oxide layer on Leidenfrost temperature during spray coolingof steel at high temperatures”, Int. J. Heat Mass Transf. Vol. 88, 2015, pp. 236-246,
https://doi.org/10.1016/j.ijheatmasstransfer.2015.04.067[3] V. Prodanovic, M. Militzer, H.J. Kirsch, R. Schorr and V. Schwinn, “ The Effect of Surface Roughness on Accelerated Cooling,”
5th International Conference on Thermomechanical Processing of Steel, Milan, Italy, 2016.
[4] M. Militzer, V. Prodanovic, H. Kirsch, R. Schorr, V. Schwinn, Accelerated Cooling Studies of Steel Plates on a Pilot-scale RunoutTable, Proc. 2nd Int. Symp. Recent Dev. Plate Steels, AIST, Warrendale, PA, pp. 103–109, 2018
[5] S. Yao, R.E. Henry, “An Investigation of the Minimum Film Boiling Temperature on Horizontal Surfaces”, ASME J. Heat Transf.
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