Research On The Thermal Performance And Environment Control System For The Near-Space Airship

Compared to traditional aircraft,near-space airship with significant characteristics of long duration,high cost-effectiveness,and recycle has attracted increasing attention in commercial and military fields.The lift of the near-space airship is mainly from the density difference between the buoyancy gas and the ambient air,which differs from that of propulsion aircraft.The pressure and temperature characteristics of the inner air and helium gas decide that it will influence the flight performance.Inflate or deflate process of the inner air bag adjust the pressure change caused by temperature change of the inner gas of the traditional aircraft.But it does come with a significant drawback: As the valves open and close,letting in charges of air,the height of the near-space airship fluctuates greatly because of the mass change of the airbag.Therefore,a new fixed-point adjustment method of airship is proposed in this paper.Static and thermodynamic model of the fixed-point adjustment method is also developed.the temperature performance of the helium bag,vice helium bag and outer membrane of the airship are obtained,and the influences of sunshine time and seasons are conducted.Meanwhile,temperature variation of the inner bags is a big factor in the decision to flight performance and heat convection of outer membrane remarkably influence the temperature of the inner bags.so high precision calculation of convective heat transfer coefficient improve reliability of the study become very important.A lack of numerical method of convective heat transfer coefficient considering slenderness ratios,Re,angle of attack should be solved immediately.Finally,some equipment required to execute the task is installed in payload cabin.Therefore,the payload cabin needs artificial temperature control.Based on the analysis of the working performance of the near-space airship,the temperature control method of the cabin and the electronic equipment is proposed and its performance throughout the flight altitude is investigated.The thesis has the following major contents:(1)A thermal and motion model of near-space airships during ascent,floating,and descentThe thermal performance of near-space airships depends on the film radiation properties and convective heat transfer characteristics.It analyzed the atmospheric environment of the airship at first.The method of simulation of direct solar radiation,diffuse solar radiation and ground reflection is investigated.Based on the analysis of the working environment,control style and inner gas physical properties of the near-space airship,a thermal and motion model are developed to simulate the flight and thermal performance of near-space airships during ascent,floating,and descent.(2)Heat convection outside the near-space airshipthe near-space airship is an ellipsoid and is designed with various sizes by different research teams.The mathematical model of forced convection outside the near-space airship is usually based on the geometry figure of sphere and infinite horizontal cylinder,which cannot reach the required accuracy.some relevant scholars began to focus on the calculation correlation based on elliptical shape.Unfortunately,high angle of attack is ignored,leading to the narrow scope of application.The forced convective heat transfer model which is suitable for different Re number and angles of attack is developed.Using the computational fluid dynamic code of Fluent,the forced convective heat transfer of an ellipsoid is simulated.The mean forced convective heat transfer coefficient at different Re number is calculated.Based on the simulation results,a new mean forced convective heat transfer model is proposed by regression analysis.(3)Thermal analysis of the near-space airship during floating processthe outer membrane of the airship is a curved surface in space,lumped parameter method is not accurate enough.Therefore,the outer membrane is dispersed into triangle units by using finite element method.The temperature variations of the membrane and air bags are obtained.the gas flow rate of helium bag is analyzed considering the influences of the volume proportion of the helium bags,seasons,latitude,orientation of the airship and different position of air bags in the ship.(4)Deicing heat of the drain tap during the flightIce blockage phenomenon can occur inside the drain tap during the flight of the near-space airship.The electro-thermal deicing system is proposed in ascent.Based on the thermal and motion model of near-space airships during ascent,the flight and thermal parameters of the airship are obtained.On this basis,the extraction flow rate of the drain tap is obtained and the mass of ice forming is achieved.Meanwhile the effects of air humidity,temperature of the airbags and latitude on the power for deicing are investigated.(5)The parallel cooling method for the cabin using a carbon dioxide refrigeration systemBased on the analysis of the working performance of the near-space airship cabin,the temperature control method of the cabin and the electronic equipment is proposed and its performance during the flight is analyzed.The results show that variable air volume control system can solve the problem of global temperature in the cabin.The electronic equipment with high heat flux can be cooled by cold plate and also it is essential to use frequency regulating speed ventilator.Meanwhile,considering the higher surface temperature of cold plate below 3000 m in summers and higher requirements of air flux at the high altitude,a parallel cooling system is proposed.The results are helpful for the design and operation of a safe and reliable high altitude near-space airship.