Effect Of Surface Coating Distribution Configurations On The Falling Liquid Film Flow And Heat Transfer

Environment protection and energy conservation is a popular issue now, and it is becoming a highly valued opinion of how to recovery the low temperature waste heat occurring in industrial processes. Absorption heat transformer (AHT), as an effective energy saving device, is qualified to be employed in saving energy and enhancing energy utilization. It is known that an absorber is one of the most important components in absorption heat-transformer, which can influence the performance of AHT, the cost of investment and maintenance. Heat and mass transfer modes of absorption are mainly classified into two types: falling film mode and bubble mode. Falling film absorption has been widely used because of its high heat transfer rate, small temperature difference. In the falling film absorption process, The high efficient components, adding surfactant agent, novel configuration of absorber and nanofluids are commonly used to enhance the heat and mass transfer of absorption..Specific coating is sprayed on the external surface of bare brass tube with the designed surface distribution form. The objective of this thesis is to investigate the effect of specific coatings with the different wettability of solid surface on the hydrodynamics of falling liquid film. The disturbance of falling liquid film is strengthened due to the existence of coated region, resulting in stronger mixing occurring and driving liquid particles from internal of liquid film to the interface, which greatly contribute to the enhancement of heat transfer. The main contents of the present paper include:The photographs of the liquid film flow patterns on the various coated tubes are observed by video camera. The minimum spray densities on different coated distribution tubes are determined experimentally. To characterize the coated distribution tube surfaces, the effective contact angle is introduced as weighted average contact angle to calculate the minimum spray density. The experimental minimum spray densities were compared with those predicted by the correlations in literature.The liquid film thickness and wavy amplitudes are measured by the capacitance method. The effect of solid surface with different surface energy on the film thickness and wavy amplitudes is investigated. The liquid film velocity is measured by the thermal tracing technique. Meanwhile, further analysis is carried out to simulate the flow field in the liquid film by the Commercial Software Fluent. The results show that the film thickness on the PFA coating distribution tube is thicker than that on the bare brass tube, and the wave amplitude is lower than that on the bare brass tube. There is periodic fluctuation on the coated distribution tube, and the fluctuation periods decrease with the increase of spray dansity. The liquid film velocity on the coated distribution tube is slower than that on the bare brass tube. There is conversion from axial velocity to radial velocity on the coated distribution tube. The radial velocity on the coated distribution tube is higher than that on the bare brass tube.By using the thermal imaging technique, the heat transfer characteristics of liquid film on the coated distribution tube are experimentally investigated. The fluctuations of liquid film temperature at different liquid film spray dansities and initial temperature were measured. The surface temperatures on coated distribution tubes are higher than that on the bare brass tube at the same falling film position. There are several peak values of temperature on the coated distribution tubes, and that the positions of the peak value are just at the coated regions. The temperature fluctuation periods increase with the decrease of spray dansity and initial temperature. The temperature wave intensity and temperature increment on the coated distribution tube are higher than that on the bare brass tube.A new thermal tracing technique is developed to assess the mixing effect of falling liquid film on the coated distribution tube. By using the high speed video camera and thermal imaging technique, the flow patterns and mixing effect of liquid film on coated distribution tubes are investigated experimentally. Flow visualization experiments reveal that the liquid film forms a bifurcate flow at the coated distribution surface; the bifurcate flows converges and forms uniform liquid film again on the next bare region, which resulting in deformation of temperature distribution on the coated distribution tube. The average mixing intensity on the coated distribution tube is about 1.2 times larger than that on the bare brass tube.The experimental investigation of the vertical falling film absorption at the high temperature is conducted. The effect of solution spray dansity and inlet temperature on the heat and mass transfer are studied. The result shows that the heat transfer coefficients on the coated distribution tube are higher than that on the bare brass tube, further validate there is enhancement function in the falling film heat and mass transfer process on the coated distribution tube.