Study Of The Natual Circulation Flow Boiling Characteristics And Its Regulation Mechanisms In Horizontally Arranged Evaporators

Due to its simple structure,flexible layout and driven by the density difference,natural circulation thermosyphon loop was widely applied in nuclear power plants,electronic devices cooling,parabolic trough solar collector systems etc.In parabolic trough collector natural circulation system,because of the horizontally-arranged evaporator,the low heat flux,the countercurrent flow and flow pattern transition,the heat transfer coefficient was far from expected.In order to solve this problem,a natural circulation thermosyphon loop with horizontal arranged evaporator was set up and visualization experiments of gas-liquid stratified flow boiling was carried out.Surfaces with various wettability degrees were prepared.The contact angles of the fabricated surfaces and original copper surface were around 5°,30°,60° and 80°,respectively.By applying these fabricated surfaces to the different regions of evaporator,the flow boiling characteristics were improved.The working fluid was deionized water and the system pressure was maintained at 0.15MPa.Flow pattern in the evaporator was gradually developed from the slug/stratified-wavy flow to the stratified-wavy/stratified-mist flow along the flow direction.The flow pattern transition process at each section included six stages:start of flow cycle,speed up of gas flow,start of flow pattern transition,flow pattern transition,fluctuation and backflow.With the change of the heating power,the time ratio of the stratified flow and the time of the whole cycle would be changed because of the different flow patterns occurred in the evaporator.The countercurrent flow at the exit prevented the development of the stratified-wavy flow to the mist flow which subsequently resulted in a drop of superheat.The countercurrent flow also suppressed the decrease of the height of the gas-liquid interface and prolonged the time ratio of the stratified flow.Along the flow direction,the average temperature increased at first and then decreased at the cross section C as a turning point.Though the average temperature at cross section C was the highest,the temperature difference was the smallest.With the increasing of the heating power,the average temperatures in the evaporator increased and the heat transfer coefficient decreased.Accounting for the effects of the subcool and flow pattern transition,a modified correlation based on the flow pattern was developed for predicting the local heat transfer coefficient.When the main flow pattern in the evaporator was slug flow,Cooper's pool boiling model can obtain better prediction accuracy.When the main flow pattern was gas-liquid stratified flow,a modified model like htp= tstrat/tcyclehtp-strat+(1-tstrat/tcycle)htp-nonstrat was applied.The deviation between the prediction and the experiment was acceptable.When the heating power was 500W,the fluctuation of the flow parameters was dominated by the low dryness fraction density wave oscillation.As increasing of the heating power,there were three kinds of flow oscillation.The flow pattern transition oscillation which was directly related to the heat transfer had the lowest period and amplitude.The period and amplitude of pressure drop type oscillation were highest.The oscillation of wall temperature experienced the process of stabilization to instability and finally to stabilization with the increasing of heating power.When the heating power increased to 1500W,the pressure drop type oscillation had been replaced by density wave oscillation.After applying the super-hydrophilic surface,the intensity of pressure drop type oscillation increased and the trend of instability to stability was delayed.The adoption of fabricated surface also increased the intensity of density wave oscillation,especially in the lower heating power.However,there was no obvious difference on flow pattern transition oscillation no matter what the surface conditions liked.Under the same heating power,the wettability of the evaporator with the super-hydrophilic fabrication was better and the average height of gas-liquid interface increased.The occurrence frequency of the mist flow was lower and so did the lasting time.Different kinds of surface wettability brought different degrees of superheat reduction.At the same time,the heat transfer coefficient was also enhanced.The enhancement ratio of the overall heat transfer in the evaporator was about 2.0 when the adoption of the super-hydrophilic surface.The enhancement ratio on the surface which had a contact angle of 30° reached the maximum.Among these three kinds of surfaces,the enhancement of the heat transfer coefficient was the least significant.The enhancement ratio fluctuated in a small range of 1.19?1.39 without changing with the heating power.The mechanism of how the different fabrication regions affected the flow boiling characteristics along the whole evaporator was also discussed.The results showed that local modification not only enhances the heat transfer where the boiling surface was fabricated,but also the original boiling surface.The longer of the fabrication area,the better enhancement of heat transfer it would get.Moreover,it was better to fabricate at the early status of flow boiling in the condition of the same fabrication length.The gap between these two optimizations gradually decreased as the heating power increased.Through the research of this topic,a scientific basis for the optimization design and application of the natural circulation parabolic trough solar collector system was provided,which had a positive academic and engineering significance.