A Study Of Effect Of Dynamic Load On Steam-Water Two-Phase Flow Boiling Critical Heat Flux In Narrow Rectangular Channels

The cooling of electronic devices with high heat flux stimulates the researches on the flow boiling in narrow rectangular channels.The flow boiling characteristics in rectangular channel are very complex,and the critical heat flux is an important parameter to prevent tubes from drying out.The fluid in channels will be subjected to different dynamic loads during the maneuver of aircraft,which increases the intensity of disturbance in flow boiling.Based on the existing research achievements at home and abroad,the experimental research and numerical simulation of two-phase flow boiling and its critical heat flux in rectangular channels under high gravity have been conducted in this thesis,and the effects of dynamic load,mass flow velocity,heating orientation,sub-cooling and the ratio of height to width of tube section on critical heat flux were obtained,which are significant in developing airborne vapor cycle cooling system and can be added in the database in investigating the characteristics of two-phase flow boiling and critical heat flux.The rotating platform was used to simulate the dynamic load,on which a circular loop of two-phase flow pipeline was established and adjusted.Using distilled water as working fluid,one side heating rectangular channel was employed to carry out experimental investigation into the effects of dynamic load and heating orientation on the flow boiling critical heat flux by changing test section position,rotary speed,inlet temperature and mass flow velocity.After processing the original data,the variations of mass velocity,the pressure drop in test section and its wall temperature before and after reaching critical heat flux were obtained.The effects of inlet sub-cooling,mass flow velocity,dynamic load and heating orientation on critical heat flux of flow boiling were analyzed,and the flow instability was also discussed.The results show that as the critical heat transfer occur,the wall temperature rises rapidly,the effective heat flux decreases,and the pressure drop of test section increases with severe disturbance.The critical heat flux increases with the increases of inlet sub-cooling,mass velocity and dynamic load.The critical heat flux for upward heating are obviously greater than downward heating as effect of gravity,and the critical heat flux value for right heating and left heating are influenced by Coriolis force.The critical heat flux value increases with the increase of channel's aspect ratio.To sum up,the shear stress from the dynamic load can increase the critical heat flux.However the bubbles accumulating on the heating surface,which caused by buoyancy,decreases the critical value.Two newly dimensionless formulas for predicting the critical heat flux in rectangular channels under side load and inverse load were obtained respectively through fitting the experimental data for upward and downward(except for inverse load condition)heating bymodifying the Qu-Mudawar correlation,and the predicting values agree well with the experimental date.VOF model of FLUENT was employed to investigate flow boiling characteristics in rectangular channels through meshing test section and inducing user defined function(UDF)to describe the phase changing process.The method to determine critical heat flux occurring was introduced.The influence factors on critical heat flux was analyzed.The numerical simulation results were compared with the experiment data,which show that the varying tendency of predicting values agrees well with that of experimental results,but their magnitudes have some differences.The experimental method and the numerical model of the present research have academic significance and applied value in researching on critical heat flux in channels under dynamic loads,which might be expected to be of benefit to new achievement in heat and mass transfer of two-phase flow.