Research On Fundamental Characteristics Of Marangoni-ther Mocapillary Convection And Buoyancy-marangoni-thermocapillary Convection In A Shallow Annular Pool With Bidirectional Temperature Gradients

The uneven surface tension and density induced by temperature gradient can drive the fluid. The surface tension driven convection can take two different forms: one is the thermocapillary convection caused by the temperature gradient parallel to the free surface, and the other is the Marangoni convection driven by the temperature gradient perpendicular to the free surface. The flow driven by density gradient is called buoyancy convection. Under gravity condition, Marangoni or thermocapillary convection and buoyancy convection coexist, however, the buoyancy convection disappears while the Marangoni or thermocapillary convection still exists under microgravity. Up to now, most researchers have only studied these convections due to a unidirectional temperature difference. However, the vertical and horizontal temperature differences often coexist in nature and many actual production processes, these two thermodynamic forces drive the fluid system away from equilibrium state together, which makes the result different from the situation with unidirectional temperature difference. In this thesis, the Marangoni-thermocapillary and BuoyancyMarangoni-thermocapillary convections of 0.65 cSt silicon oil with moderate Prandtl number in a shallow annular layer under the combined action of horizontal and vertical temperature gradients were investigated by numerical simulation and experiment, and numerical simulation is adopted as the main method. The study focuses on the respective roles of horizontal temperature difference and vertical bottom heat flux in these convections, the basic pattern and instability characteristic of Marangoni-thermocapillary convection are obtained, the effect of free surface heat transfer on the basic pattern and instability characteristic is discussed, and the action mechanism of gravity for Buoyancy-Marangoni-thermocapillary convection is analyzed. The obtained results are helpful to not only expand the theory on the thermocapillary convection but lay a certain theoretical foundation for many actual production processes. The main works and results are as follows:Firstly, the Marangoni-thermocapillary convection of 0.65 cSt silicon oil in a shallow annular layer under bidirectional temperature gradients is investigated by numerical simulation, the respective roles of horizontal temperature difference and vertical bottom heat flux in convection are identified: horizontal temperature difference makes surface temperature increase from inner wall to outer wall, accordingly the surface fluid flows from the outer wall to inner wall, while bottom heat flux makes surface temperature decrease from the middle region to the inner and outer walls, accordingly the surface fluid flows from the middle region to the two walls. The fluid is driven by these two thermodynamic forces together and the convection pattern depends on their strength relationship. When the horizontal temperature difference plays a leading role in convection, the surface temperature increases from inner wall to outer wall; When the vertical bottom heat flux dominates the convection, the surface temperature decreases from the middle region to the two walls; When both of them act effectively, the highest surface temperature locates at the region between the middle and outer wall.Secondly, horizontal temperature difference and vertical bottom heat flux can destabilize Marangoni-thermocapillary convection, which is proved by studying the effects of horizontal temperature difference and vertical bottom heat flux on the convection. Surface temperature radial fluctuation can characterize the flow strength while the circumferential fluctuation can characterize the instability strength because that the circumferential fluctuation doesn’t exist for steady convection, and the stronger the instability, the stronger the circumferential fluctuation, while the radial flow induced by the radial fluctuation dominates the convection. The instability of Marangoni-thermocapillary convection can be categorized according to the instability strength: the circumferential fluctuation with punctate wave represents the weak instability; For the strong instability, it is a double hydrothermal wave when the vertical bottom heat flux dominates the flow, it is a hydrothermal wave when the horizontal temperature difference plays a leading role, and it is a hydrothermal wave with shorter wavelength when both of horizontal temperature difference and vertical bottom heat flux act effectively; It is a chaos wave for the too strong instability. Here, the punctate, hydrothermal and double hydrothermal waves are a regular instability. Punctate wave moves along the positive radial direction over time and the wave gradually abates during the movement while the new wave appears near the inner wall, this process cycles and the basic wave pattern doesn’t change over the whole period; Hydrothermal and double hydrothermal waves rotate over time and the basic patterns are also consistent.Thirdly, the effects of surface heat exchange characterized by Bi number and gravity characterized by Gr number on three typical unsteady Marangonithermocapillary convections with strong instability are investigated, the results show that the surface heat loss and gravity stabilize the Marangoni-thermocapillary convection with bidirectional temperature differences. The mechanism that gravity suppresses the convection is as following: surface fluid flows from the hot end to the cold end while the bottom fluid flows along the opposite direction, which makes the temperature of upper fluid higher than the temperature of lower fluid somewhere, fluid flows along the negative direction of Z axis in this region, which is against the buoyancy, therefore, gravity suppresses the convection.Finally, the effects of vertical bottom heat flux and horizontal temperature difference on Buoyancy-Marangoni-thermocapillary convection are studied, the results of numerical simulation and experiment show that the roles of vertical bottom heat flux and horizontal temperature difference in Buoyancy-Marangoni-thermocapillary convection are similar to the ones in Marangoni-thermocapillary convection; When both of vertical bottom heat flux and horizontal temperature difference act effectively or horizontal temperature difference dominates the flow, gravity succeeds in suppressing the flow cell near inner wall but fails to weaken the one near outer wall; when the highest surface temperature is away from the outer wall, the surface temperature circumferential fluctuation mainly concentrates in the region where the highest surface temperature locates, with the increase of bottom heat flux, the highest surface temperature moves towards the middle region, accordingly, the fluctuation moves towards the middle region; However, the evolution of surface temperature circumferential fluctuation with the non-equilibrium thermodynamic forces is complicated when the surface temperature increases along the positive radial direction.