Interfacial Heat and Mass Transfer

Interfacial transport of scalar quantities is important to many industrial processes involving multi-fluids flows. In natural systems, the transport mechanism governs the pollutant transport, heat transfer, and reaeration processes at water surfaces. It affects the exchange of greenhouse gases between the atmosphere and oceans, which is essential to long term climate change, as the oceans have an enormous capacity for gas absorption. Despite the substantial efforts made in previous experimental, numerical, and theoretical studies, the knowledge of the underlying fundamental physics and the prediction capability of free-surface scalar transport is far from satisfactory. Taking the transfer of carbon dioxide at sea surfaces as an example, there still exist large uncertainties in the estimation of its oceanic uptake rate using even the state-of-the-art hydrodynamic parameterizations. 

Our study aims at investigating the effect of nonlinear wave dynamics on the advection and diffusion of scalar quantities near the free surface. The importance of surface wave effects in interfacial transport has long been recognized, but its mechanism remains elusive, with the understanding that wave nonlinearity and wave-turbulence interaction should play an essential role. Our progress in the study of free-surface turbulence will make possible the simulation of turbulence scalar transport in the presence of dynamically-evolving capillary-gravity waves to reveal the detailed structure of the flow and scalar fields. We are also interested in the surface fluxes under violent surface conditions that involve high winds, wave breaking, bubbles, and sprays. Such processes are important to the study of hurricanes and storms. Our long term research goal in this area is to establish a physical basis for the development of next-generation models for interfacial mass and heat transfer.

Interfacial mass and heat transfer
Interfacial mass and heat transfer

 

Evaporation is a uniquely important process in the Earth System linking water, energy, and carbon cycles. Monitoring and modeling evaporation over water surfaces such as lakes and oceans remains challenging. Better quantification and modeling of water evaporation require an improved understanding of the physical processes across the water-atmosphere interface. An outstanding scientific question is the role of the top water layer where temperature increases with depth, known as the inverse-temperature layer (ITL), in evaporation. Our present research is to reveal and understand physical mechanisms underlying the dynamics of the ITL and its effect on ET at diurnal and seasonal scale through large-eddy simulations (LES) and theoretical and modeling analysis.

Scalar_Concentration.png
LES of near-surface turbulence and scalar transport. Vortices in water, scalar concentration on vertical planes, scalar flux at wave surface, and velocity vectors on a vertical plane are plotted. The surface is lifted up for better visualization.

 

 

Selected Publications:

  • Xie, H., Zong, Y., Shen, L. & Dai, G. (2021), “Interfacial mass transfer intensification with highly viscous mixture,” Chemical Engineering Science, Vol. 236, 116531.
  • Yang, D. & Shen, L. (2017), “Numerical simulation of scalar transport in turbulent flows over progressive surface waves,” Journal of Fluid Mechanics, Vol. 819, pp.58-103.
  • Khakpour, H.R., Igusa T. & Shen, L. (2012), “Coherent vortical structures responsible for strong flux of scalar at free surface,” International Journal of Heat and Mass Transfer, Vol. 55, pp.5157-5170.
  • Kermani, A., Khakpour, H.R., Shen, L. & Igusa T. (2011), “Statistics of surface renewal of passive scalars in free-surface turbulence,” Journal of Fluid Mechanics, Vol. 678, pp.379-416.
  • Khakpour, H.R., Shen, L. & Yue, D.K.P. (2011), “Transport of passive scalar in turbulent shear flow under a clean or surfactant-contaminated free surface,” Journal of Fluid Mechanics, Vol. 670, pp.527-557. 
  • Kermani, A. & Shen, L. (2009), “Surface age of surface renewal in turbulent interfacial transport,” Geophysical Research Letters, Vol. 36, L10605.