Study On The Effect Of Gravity On The Liquid Flow And Heat Transfer Characteristics In Capillary Tubes

With the rapid development of microelectronics technology,the integration of components is increasing,as well as the size is decreasing,so the power density of these devices is also increasing rapidly with the increase of chip integration.Microchannel twophase flow is an effective way to solve the heat dissipation problem.The size of the capillary tube is generally within 1 mm and the flow state of the internal fluid can be clearly observed.Therefore,it is an ideal geometric channel for studying heat dissipation in microchannels.Evaporative cooling at the interface of the meniscus inside the capillary tube is widely used in the field of micro-scale cooling.So it is important to study the evaporative characteristics of the fluid inside the capillary tube for the optimization of the heat transfer system.This paper focuses on the differences of ethanol evaporation characteristics for two different cross-sectional shapes(circular and triangular)of capillary tubes at four different placement angles(horizontal 0°,30°,60° and vertical 90°).We used 3D digital video microscope,infrared thermal imaging camera and microscopic particle imager PIV to record the morphological changes of the curved moon surface,temperature distribution and flow pattern during the natural evaporation of ethanol in the capillary tube,respectively.We analyzed the force on the liquid surface,the temperature distribution,and the distribution of velocity and vortex during the flow on the curved moon surface.The following conclusions were obtained:(1)Both round and triangular capillaries have a spherical shape at the base of the meniscus.The triangular tube forms a liquid film at the wall and the film does not disappear as evaporation proceeds.For circular capillaries,the combined evaporation driving force on the liquid surface at an inclination of 60° is the largest and the evaporation rate is the fastest.For triangular capillaries,the presence of the liquid film affects evaporation.The larger the angle of inclination,the smaller the thickness of the liquid film,the smaller the heat transfer thermal resistance,so the faster the evaporation rate.(2)The circular capillary tube has the lowest overall temperature at an inclination angle of 60° because of the fast evaporation rate.And the temperature difference between the wedge-shaped area and the central area is the largest,and the temperature gradient is the largest.We analyzed the temperature distribution on one side of the triangular capillary and found that the larger the inclination angle of placement,the lower the temperature.At inclination angles of horizontal,30° and 60°,the temperature difference between the edge and the center is small and there is a small temperature gradient only at the edge and almost no temperature difference at the center.The temperature difference between the tube wall and the center of the tube wall is the largest at about 1.5 ℃ when the inclination angle is placed vertically at 90°.(3)The flow during the evaporation of ethanol in a circular capillary tube shows a conical roller motion in three dimensions.The Marangoni flow seen mainly in the vertical cross section is a single large vortex.There is no significant vortex flow when placed vertically.Only a small vortex is seen at the wedge-shaped pinch.When placed at an inclination of horizontal,30° and 60°,the center of the vortex moves toward the bottom of the wall as the inclination angle increases.Comparing the vortex volume and velocity distribution at different inclination meniscus,it is found that the velocity value and vortex volume values are maximum at 60° placement,which is larger than the horizontal placement case.The inclination angle of 30 ° and vertical placement are both smaller than the horizontal case,indicating that the velocity and intensity of flow over the meniscus is maximum at 60°placement,which in turn causes good heat dissipation.(4)Different angles of placement of the triangular tube will see completely different flow patterns.When placed horizontally,a semi-circular arc is seen.At inclination angles of30° and 60°,the flow line shows a circular,closed curve.At an inclination angle of 90° the flow line is similar to that of a circular tube both have no distinct shape and the flow is more chaotic.The flow when placed vertically shows an opposite pattern to the other angles,and the velocity values and vortex values are greater than the case of horizontal placement,indicating that the flow in the capillary tube is strong when placed vertically.