Characteristics Of Flow And Heat Transfer And Ash Deposition Of 3-d Finned Tube With Dusty Gas Crossing

The industrial flue gas is typically in the middle and low temperature.The conventional heat exchanger made by smooth tube has the disadvantages of large space required,heavy metal consumption and low heat transfer efficiency.In recent years,some studies have shown that three-dimensional extended surface can achieve better heat transfer performance.However,the waste heat recovery from industrial flue gas generally has the problem that fly ash deposit on the heat transfer surface and affects its flow and heat transfer performance.And the understanding of the fly ash deposition characteristics on the three-dimensional extended surface is still insufficient.And there is rarely research on the change law of flow and heat transfer performance of 3-D finned tube under ash deposition.Therefore,this paper systematically studies the ash deposition characteristics when dusty flue gas across the 3-D finned tube and its influence on flow and heat transfer performance.Which can provide a theoretical basis for the engineering application.Based on the orthogonal experiment of Taguchi method,this paper firstly studies the effects of the structural parameters of 3-D finned tubes on heat transfer performance and flow resistance characteristics in the air cross-flow.The effects of multi-parameters on heat transfer and flow resistance are obtained.The optimal structural parameters within the experimental range are proposed by using a comprehensive performance evaluation criterion.Then,through the visualization experiments,the location and the development process of the deposition layer are analyzed by combining the deposition mechanism.The fouling mass of fly ash is quantitatively characterized by weighing.And the intuitive analysis of Taguchi method is used to study the influence of fin structure parameters on the deposition.Finally,through the experiment of heat transfer performance of 3-D finned tubes in low temperature dusty flue gas,the effects of fin structure parameters on fly ash deposition morphology,fly ash fouling mass and particle size distribution of fouling layer are investigated.The variation of Nusselt number and fouling resistance in the ash deposition process for different fin structure is obtained.By comparing the commonalities and differences between the smooth tube and 3-D finned tube,the heat transfer characteristics of 3-D finned tubes in dusty flue gas are revealed.The main research contents and conclusions are as follows:(1)The fin structure parameters are systematically considered in the study of the heat transfer and flow resistance characteristics of 3-D finned tube.Empirical correlations are developed for the Nusselt number and friction factor by multiply linear regression.The performance evaluation criterion,which characterizes the heat transfer performance under the same pumping power,is revealed to be 2.7-2.9 times greater than that of the smooth tube.The optimal combination of fin structure parameters in the experimental range is obtained with a fin height of 3.0 mm,fin width of 1.5 mm,axial fin pitch of 2.0 mm and circular fin pitch of 2.0 mm.(2)Fly ash deposition characteristics of smooth tube and 3-D finned tubes are compared at ambient temperature condition.The development process of the deposition layer,deposition morphology,fouling mass,particle size distribution of the smooth tube and 3-D finned tube are obtained.Compared with the smooth tube,the fouling mass on the windward side of the 3-D finned tube is greatly reduced,thereby effectively reducing the total fouling mass.In addition,the effects of fin structure parameters on the particle fouling mass are studied by orthogonal experiment.It is found that the axial fin pitch has the greatest influence on the deposition of fly ash on the windward side,and the fin width has the greatest influence on the deposition of fly ash on the leeward side.The combinations of fin structure parameters which are beneficial to reduce the windward and leeward surfaces are obtained respectively.(3)The heat transfer characteristics of 3-D finned tubes in flue gas are researched.The variation of the Nusselt number and fouling resistance of 3-D finned tubes with different fin structure parameters are obtained.The results show that with the increase of fin height,the area that large particle deposited on the finned tube shifts to the windward side,and the mean diameter of the deposited ash increases.The non-deposited area of the windward surface of the finned tube increases with the increase of the fin width and the fouling mass decreases with the increase of the axial fin pitch.By comparing the ash deposition and heat transfer characteristics of the smooth tube and 3-D finned tube,it is found that the fouling resistance is gradually increased with the continuous deposition of fly ash.The fouling resistance for 3-D finned tube is smaller than that of the smooth tube,which indicates that the better heat transfer performance is achieved.After 5 h of experiment,the Nusselt numbers of the 3-D finned tubes are 1.27-2.65 times greater than that of the smooth tube.At this time,the fouling resistance of the 3-D finned tube tends to be steady,and the fouling resistance of the smooth tube still has a growing tendency.