Numerical Simulation Of Flow And Heat Transfer Characteristics In Annular Closed Interlayer Of Low-pressure Slip Regime With Nonuniform Solar Flux Conditions

Due to heat transfer oil decomposition,air leakage,getter failure,component deflation,etc.,the accumulation of residual gas in the annular interlayer is the main cause of evacuated collector tube vacuum failure.The change of pressure in the interlayer is accompanied by the change of heat transfer mechanism.By studying the influence of pressure on the flow and heat transfer characteristics in the interlayer under different working conditions,it not only provides theoretical support for the design of evacuated collector tube with low heat loss and good quality,but also enriches the flow and heat transfer theory of rarefied gas in the annular closed interlayer under complex thermal boundary conditions.In this paper,the radiation heat transfer between the inner and outer walls of the interlayer,the heat transfer of residual gas in the interlayer,the forced turbulence in the absorber tube and the heat transfer outside the envelope are considered.Based on the first-order velocity slip and temperature jump boundary conditions,Monte Carlo ray tracing method,finite volume method,UDF programming and other technical means are adopted.When the DNI?direct normal intensity?=1000W/m²,the effects of pressure p,gas types,heat transfer fluid temperature T_f and solar azimuth on the heat loss characteristics and temperature distribution characteristics of evacuated collector tube are studied.The following conclusions can be drawn from numerical results.When the rays is normal incident,the nonuniform solar flux distribution of the outer wall of the absorber tube is approximately symmetrical,meanwhile the bottom region has the highest solar flux value but the top region'value is the smallest.In winter,the solar flux is extremely uneven in both circumferential and axial distribution,and fluctuates greatly at different times.In summer,the solar flux is evenly distributed in the axial direction,and only the phase position changes at different times.The change in the phase position of the nonuniform solar flux causes a special circumferential circulation in the interlayer,but has little effect on the heat transfer behavior of gas in the interlayer.In the low pressure region of 1.83<p<36.33 Pa,significant temperature jump and velocity slip can be observed near the gas-solid interface of the annular interlayer.The“additional temperature difference”caused by temperature jump directly affects the heat transfer loss and temperature distribution of the collector tube.The larger the p or the lower the T_f,the weaker the temperature jump near the gas-solid interface,and the temperature jump has basically disappeared at p=130Pa.The temperature jump is most obvious when the residual gas is H₂ under the same working conditions.Higher T_f,higher thermal conductivity k of residual gas,higher p,will cause a large increase in heat loss of the collector tube.In the low pressure region,the radiant heat loss Q_r accounts for the majority of the total heat loss.It decreases slightly with the increase of p,and increases greatly with the increase of T_f.The larger the p,the greater the heat exchange loss Q_conon of residual gas in the interlayer,and the rapid increase of Q_conon in the high pressure region is the main cause of the decrease in thermal efficiency.The higher the T_f is,the more Q_conon increases in the same pressure interval.Both Q_con?H₂?and Q_con?He?are much larger than Q_con?N₂?.It is noted that Q_con?N₂?increases substantially at p<70Pa and p>20000Pa,which is related to the transition between different heat transfer mechanisms.When p=1-130Pa,the temperature distribution characteristics of the collector tube under different working conditions are summarized as follows:The outer wall temperature of the envelope T_11w1w and the inner wall temperature of envelope T_12w2w increase with the increase of p,and the higher the T_f,the greater the temperature rise in the same pressure interval.k and thermal adaptation coefficient?_T have a great impact on T_11w,while kinematic viscosity?has a weak influence on T_11w.The temperature of the outer wall of the absorber T_21w1w is directly related to T_f,and is minimally affected by gas types and p.The distribution of circumferential temperature difference?T is related to p,gas types and T_f.The higher the T_f is,the larger the circumferential temperature difference?T is,but the smaller the fluctuation in the same pressure interval.k and?_T affect the circumferential temperature difference of the outer wall of the envelope?T_11w1w and its variation.?T_11w?He?is always the largest and?T_11w?N₂?is the smallest under the same conditions.The circumferential temperature difference of the outer wall of the absorber?T_21w1w decreases slightly with the increase of p.However the circumferential temperature difference of the residual gas near the absorber outer wall?T_21g1g increases with the increase of p,and gradually overlaps with?T_21w1w in the high pressure region.Under the same working conditions,the increase of?T_21g?H₂?is always the highest in the same pressure interval.