Envelope radiation characteristics of stratospheric airship

Abstract

To investigate the envelope radiation characteristic of stratospheric airship, three kinds of airship models with different dimensions (fineness ratio of 3, 4, 5) are selected. After establishing the thermal equilibrium equations of the selected models, the influence of the envelope radiation characteristic on the envelope and airship gas is first addressed to determine the optimal fineness ratio by considering the flight dynamics characteristics. Furthermore, the effects of envelope emissivity, envelope absorptivity, solar cells radiation characteristics, and typical envelope materials on the envelope radiation characteristics of airship are discussed in depth. The results show that: (1) With the assumption of same absorptivity and emissivity, the larger the fineness ratio of the airship, the smaller the diurnal temperature difference of the main helium gasbag, and the better the thermal characteristics of airship as well. When the envelope radiation characteristics vary, the temperature difference of main helium gasbag with a fineness ratio of 4 in one day is always less than 1 K, compared with the fineness ratio of 5. However, the temperature difference of main helium gasbag with a fineness ratio of 3 exceeds that with the fineness ratio of 5, and reaches its maximum of 6 K. Therefore, the fineness ratio of 4 is regarded as optimal by considering the floating constraints of low drag and high buoyancy. (2) In the case of higher long-wave emissivity and lower short-wave absorptivity of the envelope material and solar cells, the diurnal helium temperature difference is the smallest, and the thermal performance of the aerostat is better. (3) Among the five typical envelope materials, i.e., white Tedlar film, white PU film, silver-plated Teflon film, aluminized PET film, and white PVF film, the aluminized PET film is more conducive to controlling the helium temperature difference and reducing the volume change of gasbags. The aforementioned results can provide a theoretical basis for improving the thermal performance of stratospheric airships.

Introduction

Stratospheric airships as representative vehicles for long endurance in military and civil fields have attracted researchers’ concern in recent years (Yang, 2019, Zhao et al., 2016). Their lift mainly depends on density difference between buoyancy gas and ambient air, which makes airships large and vulnerable to environmental disturbance (Wang and Shan, 2011). In the design and operation of the stratospheric airships, thermal performance is the primary issue to be tackled (Qin, 2015, Wang and Xie, 2020, Li et al., 2016). In the past decades, great efforts have been made to improve thermal performance. It has been proved that the envelope radiation characteristic is one of the most important factors that affect the thermal performance of a stratospheric airship. Zhang et al. (2016) found that the envelope material with lower short-wave absorptivity and higher long-wave emissivity can benefit the thermal performance of airships. Li et al. (2012) provided the temperature distribution of the airship hull under diverse radiation properties. Liu et al. (2010) investigated the effects of different envelope materials on thermal performance. The above investigations on envelope radiation characteristics of airship are confined to the method of a single factor without applying orthogonal analysis, thereby the obtaining results that lack of universality. Although the single factor study is easy to perform, the coupling effect increases its uncertainty. The stratospheric airship has a complex structure, which is strongly coupled with the radiation characteristics due to the variation of radiation area and angle. Considering the airship shape, the existing research mainly focuses on its influence on the lifting resistance characteristics. Khoury (2012) reported that the fineness ratio of airships should be between 4 and 5.5 for reducing the drag. Li et al. (2019) found that minimum drag coefficient of the stratospheric airship was concentrated in the fineness ratio of 4.25–4.75. Zhou et al. (2017) concluded that the fineness ratio more than 3 was the best choice to reduce envelope tension. However, not the slender the better, because the increase in fineness ratio led to a decrease in free buoyancy. Therefore, the contribution of different fineness ratios to the thermal characteristics of airships is a problem worth of investigation.

This paper established the thermal and mathematical model based on an airship with additional helium gasbag. The coupling effects of fineness ratio and the envelope radiation characteristics on the thermal performance of stratospheric airship were analyzed. Then, a preferred fineness ratio was determined based on the thermal behaviors. Finally, the effects of the envelope emissivity, the envelope absorptivity, the solar cells radiation characteristics and the different envelope materials were discussed in detail. The results of this study would provide a useful reference to improve the thermal performance of stratospheric airships.