Investigation On The Growth Characteristics And Fouling Prediction Model Of Combined Fouling On The Internal Surface Of Enhanced Tubes

Enhanced tubes have been widely used in traditional water-cooling condensers because of their superior heat transfer qualities.Fouling issue is one of the most significant problems for enhanced tubes installed in the water-cooling condensers.In current studies,there are rather big differences between the test conditions of the accelerated fouling test and the actual operating conditions,and the non-accelerated fouling data is very limited.In addition,the existing fouling models and the constant fouling factor（or fouling resistance）cannot forecast the deposition process of combined fouling accurately,which have limitation in engineering.To fill the research gap of non-accelerated fouling tests and solve the above-mentioned problems,this paper studied the growth law of waterside combined fouling of enhanced tubes through experimental studies,theory analysis,and numerical simulation.Detailed effects of water velocity,water quality,and tube geometries on the fouling deposition process for heat transfer tubes were revealed.A generalized fouling prediction model was developed and further validated.The effect of the flow field caused by the micro-rib on the behavior of fouling was clarified.The deposition characteristics of combined fouling were revealed from the macro-and micro-perspective.Firstly,the author provided a very brief introduction of the non-accelerated fouling measurement system（FMS）.Based on this system,a baseline test and analysis were conducted for test tubes in clean condition.To minimize the error of fouling resistance caused by the fluctuation of water velocity and heat flux,the correction correlations of fouling resistance were developed before the non-accelerated fouling test.Meanwhile,the differences between each set of baseline tests using DI water with a velocity of 1.6 m/s were compared and analyzed.In addition,the analysis of non-condensable gases（such as air,etc.）in refrigeration systems was carried out to ensure that the fouling tests under different experimental conditions were comparable.Secondly,five sets of continuous non-accelerated fouling tests on eight enhanced tubes and one plain tube were performed over a duration of 86 to 143 days.Cooling water with two different fouling potentials（low and medium）and three different water velocities（0.9 m/s,1.6 m/s,and 2.4 m/s）was studied in fouling tests.Results showed that the average asymptotic fouling resistance for all test tubes increased by90.72%as the average Langelier’s Saturation Index（LSI）rose by 28.97%,suggesting that the amount of fouling deposition was positively associated with the fouling potential of cooling water.At the same time,when compared with two-dimensional enhanced tubes and the plain tube,water quality had more influence on the variation of heat transfer coefficient caused by the fouling deposition for three-dimensional enhanced tubes.In addition,for one enhanced tube,the water velocity increased from0.9 m/s to 2.4 m/s not only resulting in the reductions of asymptotic fouling resistance（75.86%～91.70%）but also causing the duration of the introduction period of the fouling process to reduce from 53 days to 13 days.Thirdly,in order to further explore the deposition characteristics of combined fouling theoretically,two key elements affecting the fouling growth process（the deposit bond strength and the sticking probability）and their influencing factors were analyzed and discussed.The calculation correlations of sticking probability and deposit bond strength used to calculate the asymptotic fouling resistance directly were developed respectively,based on the non-accelerated fouling data.Results indicated that the change of water velocity had the complete opposite effect on deposit bond strength in the deposition and removal process for one enhanced tube.With the same water quality,the lower sticking probability（P）resulted in a lower average cohesive force between the foulant.Therefore,the deposit bond strength was weakened when the water velocity changed from 0.9 m/s to 2.4 m/s（considering only the deposition process）.Due to the wall shear stress increasing with the water velocity,there was an increased probability that the fouling with lower deposit bond strength was sheared off in the higher velocity.In other words,the increased water velocity also increased the“dressing”effect of cooling water on the accumulated fouling surface.Thus,the deposit bond strength enhanced with the rise of the water velocity in the removal process.Subsequently,this paper not only compared the difference between the experimentally-determined fouling resistance（factor）in the field projects and the recommendations made in the AHRI Guideline E-1997（or GB/T 151-2014 Heat Exchangers）but also compared the results of accelerated fouling tests and non-accelerated fouling tests.The shortcomings of existing standards and the accelerated experimental data were verified.To overcome these challenges,this research developed and validated two fouling prediction models based on non-accelerated tests.Both models considered the impact of water quality,water velocity,and the tube geometries on the fouling deposition with acceptable accuracy for prediction.Results indicated that the generalized fouling prediction model as a ratio of asymptotic fouling resistance had good suitability and accuracy in practical project applications,which could be used to guide the design of heat exchangers.In addition,to evaluate the performance of enhanced tubes,a lifetime comprehensive performance evaluation criteria was proposed,which could be used to guide the selection of the tube geometries for enhanced tubes under different operating conditions.Finally,to make up for the shortcomings of the insufficient number of experimental test tubes and the insignificant influence of tube geometries on the growth of combined fouling,the numerical simulation study on 25 internally helical enhanced tubes with different tube geometries was carried out.The influence laws of flow field caused by micro-rib on the fouling process were revealed.The sensitivity of fouling deposition to the geometric parameters of micro-rib on the surface was clarified.The simulation results showed that when using the medium fouling potential water（LSI=～1.2）with the water velocity of 1.6 m/s,the asymptotic fouling resistance for enhanced tubes increased with the increase of the number of starts,helix angle,rib height,and the decrease of internal diameter.And their affecting degree diminished in order within the typical range of tube geometries.The corresponding range of the average asymptotic fouling resistance was 2.44×10^-5～8.01×10^-5（m²·K）/W.This paper has carried out multiple sets of non-accelerated fouling tests,providing important data support for the fouling research field where the non-accelerated experimental data is very scarce.Meanwhile,the deposition characteristics of combined fouling were revealed.The mechanisms of tube geometries,water quality,and water velocity on the growth of combined fouling were expounded.In addition,the fouling prediction model was developed and verified in this paper.All of these results are of great importance to theoretical study and engineering design.