Numerical Study On Radiative And Evaporative Characteristics Of Liquid Droplet Radiator

As the spacecraft scale increases, the power needed and waste heat generated increase rapidly. The thermal control technique currently used may not meet the requirements of the waste heat dissipation. The dissipation of waste heat requires a light mass and high efficiency heat radiator. In this background, Liquid Droplet Radiator (LDR) is a very promising solution.The LDR system is mainly composed of the droplet generator, collector, pump, heat exchanger and pipes. First, the working fluid is heated in the heat exchanger by the waste heat of spacecraft, and then turned into billions or trillions of droplets in a droplet generator before ejected into space. The droplets radiate energy during flight and are captured and reformed into continuous liquid in a droplet collector, then repressurized and recirculated back to the heat exchanger.The droplet layer between the generator and collector is the effective radiating area of a LDR system, and its radiative characteristic determines the cooling capacity of the LDR, while the evaporative characteristic has a crucial impact on the LDR system lifetime.In the study of radiative and evaporative characteristics, the very sparse layer is discussed firstly. For the layer with the ratio of space to diameter larger than 10, it can be treated as isolating droplets. The model of the radiation and evaporation process of a single droplet is developed, and the influence of the initial temperature and radius on the final temperature and evaporation ratio (the ratio of evaporation mass to initial mass of droplet) is discussed. Then the influence on the radiation energy of per mass of droplet and lifetime of system is obtained. The results show that the higher initial temperature and smaller initial radius of droplet are, the lager radiation energy of per mass of droplet and shorter lifetime of system are. For a given mass of droplets layer and system lifetime, the permitted range of initial temperature reduces as the initial radius of droplet reduces, and the radiation energy is greater when the system adopts smaller initial radius and corresponding maximum initial temperature.Second, the layer of optical thickness smaller than one is discussed. One dimensional evaporative mass flux expression was obtained and combined with the radiation heat transfer model. The combined radiation-evaporation model was used to analyze the influences of the exit temperature and optical thickness of the droplet layer on the temperature distribution, evaporation loss rate and system lifetime. In the case of a certain droplet diameter, the numerical results show that, the evaporation loss rate increases as the exit temperature and optical thickness increase, and the main contribution to the evaporation loss rate comes from the high temperature portion of the liquid layer near the exit of the liquid generator, i.e., the evaporation loss rate increases rapidly in a short length of liquid droplet layer, and approaches a stable value as the length reaches a certain value. With a same working fluid mass overloading proportion of the droplet layer, the system lifetime is mainly determined by the exit temperature of the liquid droplet layer. For example, if the exit temperature decreases from 320 to 310 K the system lifetime increases around 3 times. On the contrary, the system lifetime has a weak relation with optical thickness, and the system lifetime of different optical thickness tends to the same value as the layer length increases.Third, the layer of arbitrary optical thickness is discussed. The combined radiation-evaporation model was used to analyze the influences of the optical thickness on the temperature distribution, average temperature, and evaporation loss rate as well as system lifetime. In the case of a certain droplet diameter and overloading 10% (mass) working fluid of the droplet layer, the numerical results show that, the temperature of droplet layer increases with the optical thickness, and the difference of temperature between the center and boundary increases too. WhenκD≥8, the temperatures distribution across the layer at the entrance of the collector approaches a same one. The numerical results of average temperature is very close to the results obtained by the analysis formulae deduced in the optically thin limit, as a result, in the acceptable error range, the average temperature can be obtained by the analysis formulae. The evaporation loss rate and system lifetime increase with the optical thickness. When the optical thickness is small (κD≤1), the evaporation loss rate is approximately proportional to the optical thickness, and it increases rapidly withκD, while the system lifetime increases slowly. On the contrary, when the optical thickness is large (κD> 4), the evaporation loss rate increases a little with the optical thickness, while the system lifetime increases rapidly.In the end, taking non-gray body and non-constant properties into account, the radiative performance of the layer is discussed, and the effects of the droplet diameter, layers and jet frequency on the sheet temperature, radiation power and radiation power per mass were discussed. The results show that, the larger the droplet diameter and jet frequency are, the less the corresponding critical layers (the layers corresponding to 95% of the equilibrium radiation power of droplet sheet) is; it is better to take the small droplet diameter, and the radiation power can be increased by increasing the width of the radiator generator and the jet frequency. Also, it can be increased by increasing the layers when it is below critical layers.