Hydrothermal waves in a liquid bridge subjected to a gas stream along the interface

In the presence of temperature gradients along the gas-liquid interface, a liquid bridge is prone to hydrothermal instabilities. In the case of a coaxial gas stream, in addition to buoyancy and thermocapillary forces, the shear stresses and interfacial heat transfer affect the development of these instabilities. By combining experimental data with three-dimensional numerical simulations, we examine the evolution of hydrothermal waves in a liquid bridge with Pr = 14 with a gas flow parallel to the interface. The gas moves from the cold to the hot side with a constant velocity of 0.5 m s(-1) and its temperature is the main control parameter of the study. When the thermal stress.T exceeds a critical value Delta T-cr, a three-dimensional oscillatory flow occurs in the system. A stability window of steady flow has been found to exist in the map of dynamical states in terms of gas temperature and applied thermal stress Delta T. The study is carried out by tracking the evolution of hydrothermal waves with increasing gas temperature along three distinct paths with constant values of Delta T: path 1 is selected to be just above the threshold of instability while path 2 traverses the stability window and path 3 lies above it. We observe a variety of dynamics including standing and travelling waves, determine their dominant and secondary azimuthal wavenumbers, and suggest the mathematical equations describing hydrothermal waves. Multimodal standing waves, coexistence of travelling waves with several wavenumbers rotating in the same or opposite directions are among the most intriguing observations.