Analysis Of CHF Flow Behavior In A Restricted Channel

As one of the three thermal design criteria of the reactor,the determination of the critical heat flux not only guarantees the safety of the reactor,but also has an important impact on the economics of the reactor.Therefore,accurate prediction of the critical heat flux is of great significance to the reactor.With the trend towards smaller and integral reactors,plate-type fuel elements with the characters of narrow rectangular channels have entered the field of vision of a large number of researchers.Compared with conventional channels,narrow rectangular channels have the advantages of higher power density and greater heat exchange area,lower temperature,and safer.However,the narrow rectangular channels have huge differences due to its special geometric structure to round tubes and rod bundle channels.Therefore,the CHF correlation based on the above channels and the influence law of thermal parameters on CHF cannot be directly applied to rectangular channels.The experiment of critical heat flux in rectangular flow channel is to verify and improve the conclusions above based on other channels.In this paper,deionized water is used as the working fluid,and the critical heat flux visualization experiment under the condition of one-sided heating upflow in a rectangular channel is carried out.The results can be divided into two categories based on the visualized image.The cutoff point is about G=400 kg/m~2s.When G<400 kg/m~2s,during the entire heating process,bubble flow,slug flow,and churn flow appear in turn in the rectangular flow channel.During the duration of the slug flow,the inlet and outlet pressure drops will have unusual pressure fluctuations.Before the boiling crisis,the heating plate will have a large area of dryness,and the flow will fluctuate greatly.When the boiling crisis happens,a part of the dryout area of the heating plate will have a heat transfer deterioration zone.When dryout appears in the heat transfer deterioration area,the area cannot be wetted and get effectively cooling and cause boiling cirsis.When G>400 kg/m~2s,in the entire heating process,diffuse bubbly flow,coalesced bubble flow,churn flow and vapor core slug flow appears in the rectangular flow,because there is no slug flow,during the entire heating process,the pressure drop between the inlet and outlet increases continuously with the increase of the heating power,and there is no abnormal pressure drop fluctuation.When boiling crisis occurs,the thin liquid film layer on the heating plate is evaporated to dryness.When dry-out occurs,the heating power is high at this time.Thus the temperature rises at the same time as the dry-out occurs,and the boiling crisis occurs.The high-flow condition is less affected by the flow instability.Through the analysis of experimental data,it is found that as the mass flux increases,the critical heat flux increases almost linearly;at all boiling crisis conditions in this experiment,the thermodynamic equilibrium quality of the outlet is greater than 0,so the thermodynamic equilibrium quality is the quality.The relationship between the quality at the exit and the mass flow rate is non-linear.When G>.350 kg/m~2s and G<350 kg/m~2s,the thermodynamic gas content decreases as the mass flow rate increases respectively.When G near 350 kg/m~2s,it does not conform to this law,which is because the two types of boiling thresholds are caused by different mechanisms;the increase of pressure makes the bubble detachment diameter smaller and the quality higher when boiling crisis happens,then the critical heat flux increases accordingly;the increase of the inlet subcooling will also increase the critical heat flux,but the increase is not large.The effect of the inlet subcooling on the critical quality is not obvious.Comparing the experimental results with the predicted values of the Sudo correlation、the predicted values of the Katto correlation and the predicted values of the Mishima correlation,it is found that the predicted value of the Sudo correlation is generally small,but all the experimental values are within the tolerance range of its correlation;The overall predicted value of the Katto correlation is large;the predicted value of the Mishima correlation agrees best with the experimental results.The mean average error of the prediction value is 19.4%,and mean average error of the modified Mishima correlation prediction is11.2%.