Fluid motion and heat transfer of boiling with impinging flow in a mini-tube with nonlinear thermocapillary solutions

Boiling heat transfer with impinging flow can be an effective way for cooling a small heated area such as CPUs and laser emitting devices. In the phenomena the movement of liquid layer on the heated surface strongly affects the detachment of boiling bubbles and the heat flux. In this study, nonlinear thermocapillary solutions such as button aqueous solutions were applied to this type of boiling with impinging flow aiming to promote heat transfer. These solutions have special characteristics that the surface tension increases as the temperature is raised over some temperature. It is expected that this tendency about the surface tension will promote the wetting of the heated surface and the detachment of boiling bubbles. In the experiment, T-shaped mini tubes were built with quartz tubes and used for flow boiling. The inner diameter of the tube was 2 mm and the outer diameter was 4 mm. The liquid flow impinged at the junction point where small area was heated by using a conducting thin film coated at the outer surface of the tube. The test fluids were butanol aqueous solution and pure water. The flow rate of the liquid was the order of 1 ml/min, the concentration of the butanol aqueous solution was 7.15 wt %. The liquid motion was observed by CCD video camera system. It was found from the experiment that the motion of the liquid layer of the butanol solution at the impinging area was very different from that of pure water. The layer of the butanol solution tended to extend to the hotter part of the heated area. In another experiment for precisely fixing the imposed heat flux value, T-shaped mini channel with small copper surface installed for heating the fluid was prepared. The cross section of the channel was rectangular shape of 3 mm x 3 mm, and the entire channel was made of insulating polymer material. It was found that the heat transfer of the boiling with impinging flow in using butanol solution was more promoted than that in using pure water.