Research On Convective Heat Transfer Of Super-critical Carbon Dioxide In Vertical Tube

Due to its unique advantages,super-critical carbon dioxide(scCO2)cycle power generation is a research hotpots in the fields of high efficient and flexible thermal power generation,high temperature Solar-thermal power generation,new generation nuclear power generation,waste heat power generation and advanced ship power system.ScCO2 flow and heat transfer is very important for the safe operation of scCO2 power cycle system.At present,the convective heat transfer of scCO2 in tubes mainly focuses on the?8 MPa,tube diameter less than 10 mm and uniform heating.There are no super-critical heat transfer correlations suitable for a wide range of experimental data.Super-critical heat transfer has been treated based on the hypothesis of single-phase fluid,the buoyancy and the flow acceleration effect is emphasized for a long time,which are difficult to explain the unique phenomenon.Therefore,it is of great academic significance and value to conduct experiments,numerical simulation studies and theoretical analysis on the heat transfer characteristics of scCO2 with a wide range of parameters,reveal the heat transfer mechanism,and develop the high-precision criterion to predict the onset of heat transfer deterioration and heat transfer and resistance coefficient correlation.The results provide the theoretical guideline for the design and optimization of scCO2 power cycle.The large experimental apparatus of scCO2 heat transfer under the condition of uniform heating and non-uniform heating was designed and established.Compared with other experimental apparatus in the world,the power consumption of this experimental apparatus is 120 kW,The testing temperature is allowed to range from 0 to 500? and experiments of different pressures(1 to 25 MPa),tube diameter(2 to 14 mm),tube length(1 to 6 m)and heat flux(0 to 400 kW/m2)can be conducted,which makes up for the shortage of existing experimental facilities.The establishment of this experimental facility can meet the needs of data for high temperature and high pressure CO2,mechanism research and engineering design.The experiment of scCO2 heat transfer is performed in a 10.0 mm inner diameter tube under uniform heating,covering ranges of P=7.5-21.1 MPa,G=488-1600 kg/m2·s,qw=74-413 kW/m2.The experimental results show that the system operating parameters have a great impact on the heat transfer of scCO2,and increasing the mass flux and pressure can effectively delay or inhibit the occurrence of heat transfer deterioration(HTD).The concept of pseudo-boiling is introduced.Analogy analysis between sub-critical boiling and super-critical heat transfer creates two new dimensionless parameters,the super-critical boiling number(SBO)and super-critical K number,SBO=qw/G·ipc,K=(qw/G·iw)2?b/?w.K number represents evaporation induced momentum force relative to inertia force to govern the growth of wall attached vapor-like fluid layer thickness.Small SBO and K number correspond to thin film thickness and good heat transfer.In contrast,large SBO and K numbers correspond to thick film thickness and poor heat transfer.The experiment found that the onset of HTD is found to occur at a critical SBO=5.126×10-4,and our new criterion is also suitable for other experiments reported in literature(din=2?10 mm).For a specific experimental case,when SBO is less than the critical value,heat transfer is normal.On the contrary,when SBO is greater than the critical value,the heat transfer deteriorates.SBO reflects the comprehensive effects of heat flux,mass flux and pressure on heat transfer.The existing research on scCO2 heat transfer is carried out under the condition of uniform heating.But in actual engineering applications,For example,in the scCO2 power generation system driven by solar energy,the vacuum tube is the key element of the parabolic trough solar collector.The sunny side of tube supplies higher heat flux but the night side has weak heat absorption.In a scCO2 coal fired power plant,for each tube,the side facing combustion zone of furnace receives radiation heat flux,but the back side is thermally insulated.In the above two kinds of applications,the heating tubes show strong non-uniform heating characteristics.In this paper,the circumferential non-uniform heating is simulated by electroplating silver on the half circumference of the tube.The inner diameter of the test section is 10 mm.The experimental parameters range is as follows:P=7.67-20.7 MPa,G=498.3-1229.2 kg/m2·s,qw=43.7-488 kW/m2.The experimental results show that the inner wall heat flux and temperature are strongly correlated with the circumferential angle.Compared with uniform heating,non-uniform heating can delay or eliminate the deterioration of heat transfer ahead of the pseudo-critical point and improve the heat transfer obviously.However,beyond the pseudo-critical temperature,non-uniform heating deteriorates the heat transfer.For non-uniform heating,the critical value of normal heat transfer and heat transfer deterioration is SBO=8.908x10-4Totally,5560 data points,including our newly obtained 2028 data points for scCO2 with pressures up to 21 MPa,and other 3532 data points cited from 18 articles for carbon dioxide,water and R134a,are used to determine the heat transfer correlation,yielding the expression of Nu=0.0012Reb0.9484Prb0.718K-0.0313.In addition to Re and Pr,only one K term is added to the correlation.The prediction accuracy is better than the widely cited heat transfer correlation in the literature,and it can be applied to different working fluid,tube diameter,normal heat transfer and heat transfer deterioration.The negative exponent-0.0313 for the K number indicate that K number has an negative effect on super-critical heat transfer,which conforms to the physical mechanism.For Vertical upward flow,the variation trend of super-critical K number is consistent with that of friction coefficient.Based on this,a new K number resistance correlation is proposed,and its prediction accuracy is greatly improved compared with other correlations.