Buoyancy-thermocapillary convection with simple fluids

Research project: Buoyancy-thermocapillary convection of volatile fluids in confined and sealed geometries

This work represents the first systematic description of the two-phase flow problem. A comprehensive model was developed, numerically implemented, and successfully validated against the available experimental results. The results of numerical simulations were used to determine the key physical processes that control heat and mass flow in two-phase cooling devices. Furthermore, a simplified analytical description of transport in both the liquid and the gas layer was developed and the solutions were used to describe flow stability and determine the critical Marangoni number and wavelength describing the onset of the convective pattern. As a result, the results presented in the thesis should be of interest both to engineers working in heat transfer and researchers interested in fluid dynamics and pattern formation.

Fundamental understanding of two-phase flows of volatile fluids

  • Developed a simplified analytical description of two-dimensional flow in a layer of liquid under a mixture of its vapor and noncondensable gases in confined geometries.
  • Discovered that the heat and mass transport through the gas layer have a crucial effect on the stability of the flow in the liquid layer.
  • Demonstrated that transport in the gas phase essentially controls the convection in the liquid phase and can only be neglected in certain specific and limited cases.
  • Developed a numerical code based upon an open source CFD package, OpenFOAM, which can be used for fundamental studies of thermocapillary-buoyancy convection.

Modeling of evaporative two-phase cooling devices

  • Developed a simple transport model for heat pipes with wicks and micro-heat pipes that can analytically predict the heat flux and heat transfer coefficient.
  • Predicted the optimal size for the pores of the wick (or width of the microchannels) based on the constraints imposed by the capillary forces (capillary limit) and the conduction of heat through the liquid (conduction limit).
  • Discovered that there is no “adiabatic” region in micro-heat pipes at low concentrations of noncondensables, which requires a new modeling approach for such devices.
  • Predicted the range of concentrations of noncondensables beyond which the thermal performance of evaporative cooling devices will be constrained by either the diffusion of coolant vapor through the gas (diffusion limit) or by thermocapillary forces (thermocapillary limit).