Study On Heat Transfer Mechanism And Flow Visualization For Three-dimensional Enhanced Surfaces Of Horizontal Dimpled Tubes

Since the early 1970s,nations around the world have been forced to adopt the strict resource management systems to solve the problem of low utilization efficiency and over-consumption of energy,due to the emergence of major environment issues and the ever-increasing imbalance between energy supply and demand.However,the decreased growth rate of energy efficiency and the steady growth in energy demand have become a great challenge for the global energy industry.Currently,Extended Surface is the main heat transfer enhancement technology used in the thermal design of commercial heat exchangers,which is used to reduce the footprint and improve the thermal performance of heat exchanger.As a type of newly-developed 3D enhanced surface tube,dimpled tubes show the superior economic benefit due to its low costs of manufacturing,daily operation and maintenance and the low pressure loss in pipelines.Among them,the novel dimpled tubes?1EHT tube?having the Vipertex surface base with scale inhibition effect show a broader development prospect.In the paper,a research review on heat transfer enhancement in enhanced surface tubes at this stage was carried out,and a brief summary of the research work on flow visualization in tube was conducted.The shortages of current research area in dimpled tubes were also discussed.Then,the high-precision measurement system of saturated convection heat transfer in horizontal tube and its supporting experimental equipment for flow visualization were introduced.The composite surface structures of the 1EHT-type dimpled tubes were measured by a non-contact optical profilometer.In view of the fact that all dimple tubes have a 3D two-side enhanced surface,it is necessary to use the Wilson plot to obtain the water-side single-phase heat transfer coefficient in the annular channel outside a test tube.The heat transfer coefficient of inner surface of the test tube and the uncertainty of main experimental parameters were calculated by a thermal resistance model and an error delivering algorithm described by Moffat,respectively.In addition,a heat balance analysis of the test rig and a verification of single-phase heat transfer coefficient of the test section were described successively.In this thesis,the in-tube two-phase heat transfer coefficient of a smooth copper tube and two 1EHTa-type dimpled tubes having the same outer diameter of 12.70 mm were measured by using refrigerant R410A as the working fluid.And the saturation temperatures of flow boiling and forced-convection condensation were 6?and 45?,respectively.Besides,the heat transfer coefficient curve varied with the mass quality and mass flux.The flow pattern image corresponding to each data point was recorded by a high-speed camera.Meanwhile,the flow pattern category was identified by the oscillometric measurement of threaded stud shadow and the stripe contrast method.This paper also introduces the major flow patterns for convective condensation and flow boiling heat transfer in a horizontal tube.For each test tube,the two-phase flow pattern maps were described by the X_tt-J_G plot to explain the relationships between two-phase flow pattern and experimental heat transfer coefficient for the horizontal test tubes.Meanwhile,the influence of internal surface structures of the 1EHTa-type dimpled tube on the change of saturated-convection flow pattern was discussed based on heat transfer mechanism.Finally,the mechanism of enhanced heat transfer caused by the surface structures of new dimpled tubes was revealed.In this study,in order to distill the gas-liquid interface profile in the model of gas-liquid two-phase stratified flow in a circular tube,computer scripts were built by using the MATLAB software to carry on the binary processing for flow pattern image,thereby obtaining the in-tube void fraction under the given conditions.Moreover,the experimental values were compared with the predicted values given by three classical models to propose the prediction model of void fraction applied to two 1EHTa-type dimpled tubes under all operating conditions.Then,new correlations for flow pattern transition in the previously-mentioned new dimpled tubes were established based on the corresponding two-phase flow pattern maps.Finally,the prediction models of heat transfer coefficient were built for the convective condensation and flow boiling in two1EHTa-type dimpled tubes.For the conditions of a large vapor quality change occurring in test tube with an inlet quality of 0.2/0.8 and an outlet quality of 0.8/0.2,the effects of working fluid,tube wall material,operation parameters and geometric parameters on two-phase heat transfer coefficient and frictional pressure drop in the 1EHT-type dimpled tubes were investigated experimentally in detail.To provide reference for researchers in this field when they study the heat transfer in other enhanced surface tubes,the influence rules of experimental variables were summarized by referring to the flow pattern maps.