Objective Optical solar reflector(OSR)exhibit a very low absorptance-emittance ratio,ensuring stability and high performance in space applications.They efficiently aid in cooling or heat dissipation when attached to the exterior of radiative cooling panels.Understanding the impact of space environment on the OSR and the degradation mechanisms of their thermal control properties is crucial for designing long-lasting thermal control systems.Through analysis of temperature data from cameras onboard four satellites operating in geostationary orbit for 7 years,4 years,3 years,and 2.5 years,coupled with thermal simulation analysis models,the degradation model of the OSR solar absorptance solar absorptance is obtained.The deterioration model of the OSR solar absorptance solar absorptance is derived by analyzing temperature data from cameras on four satellites that have been in geostationary orbit for seven,four,three,and two and a half years.This data is combined with thermal simulation analysis models.The results reveal that the solar absorptance of the OSR were 0.125 in the first year of operation,degraded to 0.134 after 2 years in orbit,and changed to 0.175 after 7 years,with the degradation curve exhibiting linear characteristics and an annual degradation rate of approximately 0.74%.Additionally,based on this degradation model,the solar absorptance is predicted to be 0.195 after 10 years.OSR has strong spatial adaptability and stability and the results of this article offering guidance for subsequent in-orbit temperature control and new thermal control system designs.

The loop heat pipe (LHP) is an advanced, efficient two-phase heat transfer unit, whose operational performance may be affected by microgravity conditions in contrast to ground-based applications. The performance of on-orbit temperature data and ground test of a copper-propylene LHP with a condenser temperature range of 243.15 K to 303.15 K were employed to compared and analyzed. The LHP has successfully started up for more than 193 times with a good heat transfer performance and a stable start-up stabilization on-orbit under a complex orbital heating environment for more than eight months. With a small heat load (10.0 W), the average start-up time is 110.0 s while the start-up temperature ranges from 5.71 K-12.78 K. The start-up time at large temperature differences in the high temperature zone will be higher than the time required for start-up at smaller temperature differences in the low one. When the condenser temperature is 250.0 K, the stable temperature difference on orbit is 3.83 K, which is generally consistent in heat transfer compared to 2.20 K in the ground test. In this paper, we can conclude that the on-orbit flight data up to now can provide a reference to the design of subsequent LHP space applications.

基于航空航天等领域对环路热管长距离传热的需求,设计制造了一套传热距离8 100.0 mm的圆柱型蒸发器环路热管,试验了不同加热功率、不同冷凝温度下该环路热管的启动和变工况运行性能,并对其热阻及最大传热能力进行了分析.研究结果表明:当其他条件一致、初始气液分布相同和不同时,加热功率由100增大至160 W后环路热管启动时间和启动温升均发生一定程度的下降;加热功率100 W时,冷凝温度由10.0降低至-10.0 ℃使得环路热管启动时间增加,加热功率160 W时,冷凝温度由10.0降至-10.0 ℃对环路热管的启动时间影响不大.在冷凝温度0.0 ℃下,该环路热管在100～500 W内均能稳定运行,且200 W时环路热管传热效率最高,传热温差最小,稳定运行温度最低;另外,由于系统传输距离较长,每个工况达到稳定所需要的时间也较长,分布于1 000～3 500 s内.随着加热功率的增大,环路热管热阻先减小后逐渐增大,该环路热管传热热阻最大不超过0.090 ℃/W,最小为0.024 ℃/W;随着传热距离的增大,管路的热损失增加,总压降和热阻也变大.当传热距离基本相同时,蒸发器容积的大小、冷凝器的冷凝能力及气液管线的布置.形状均在一定程度上影响环路热管的最大传热能力.

A loop heat pipe (LHP) is a kind of passive heat transfer device that uses the latent heat of the working fluid and the capillary forces of the capillary wicks. It demonstrates high heat transfer efficiency, long-distance heat transfer, and high pipeline flexibility. The multi-evaporator loop heat pipe (MeLHP) is a special loop heat pipe with multiple evaporators so that heat collection and emission from multiple heat sources can be achieved. In this paper, a new type of the multi-evaporator loop heat pipe prototype with a dual-layer condenser was designed, which can ensure the uniform and symmetrical layout of pipelines. The working temperature was 20°C, and propylene was used as the working fluid. The performance of the same evaporator in a single-loop LHP was considered as a reference. The experiment was conducted under two heating modes, i.e. single-evaporator heating and multi-evaporator heating, and the working stability of the prototype was verified by applying periodic heating power change and adverse elevation condition. It was observed that the prototype can be successfully started in different heating modes with a heat transfer limit of 230 W. In the test, the four loops were different in heat transfer limit due to the differences of flow resistance, and less power distribution to the loop with lowest heat transfer limit was considered to be beneficial to the prototype’s performance. Meanwhile, the prototype showed good heat sharing characteristic as the maximum temperature difference is low (smaller than 2 K in single-evaporator heating mode and 0.5 K in multi-evaporator heating mode). The prototype was of good operational reliability and found to be adaptable to the adverse elevation and cyclic variation of the heating power to a certain extent.

This study used aluminum powder as a raw material to fabricate a wick for loop heat pipes by the powder metallurgy method and took advantage of the excellent corrosion resistance, low density and low cost of aluminum. The average pore diameter, porosity and permeability of the aluminum wicks were 9 ?m, 47.65%, and 2.1 ? 10?13 m?2, respectively. Then, to verify the feasibility of the aluminum wick, it was installed into a loop heat pipe to test the heat transfer performance. The experimental results showed that the loop heat pipe could transport a heating load of 130 W with a thermal resistance of 0.04 K/W under horizontal condition. A steady-state loop heat pipe mathematical model was developed, and the numerical results were compared with the experimental data. The results show that the model data are consistent with the experimental data, which means that Al wicks are suitable for use in the case of a high heating load and light weight.

The cryogenic fluids' flow performance and condensation characteristics with different heating powers in cryogenic loop heat pipes (CLHPs) have not been studied due to the difficulty of measurement at low temperature. In this study, a test system was designed and fabricated to characterize the condensation flow patterns of propylene CLHP and its heat transfer performance with different heating powers. The results show that the two-phase region length and flow pattern in the condenser were closely related to the operation mode of the CLHP. The vapor front in the condenser oscillated at the condenser inlet at a low heating power, which resulted in an unstable operation of the CLHP. Moreover, by comparing the condensation heat transfer coefficient (HTC) with the condensation correlations, the Cavallini correlation is recommended for the design of the CLHP condenser.

均温板作为一种新型的两相流散热技术,具有导热性高、均温性好、热流方向可逆等优点,克服了传统热管接触面积小、热阻大、热流密度不均匀等问题,已经成为解决未来电子工业中高热流密度电子器件散热有效途径之一.本文总结了3种吸液芯种类:微槽道型、烧结粉末型、烧结丝网型,阐述每种毛细芯的制备方法,并比较它们的优缺点;简述了当前国内外对均温板传热传质理论的最新研究进展,学者们利用输运模型沸腾理论捕捉气液界面,确定临界热通量,分析工质在均温板内的流动和传热的规律.本文剖析了影响均温板性能的各个因素,包括流体选择、充液率、热源输入功率大小和分布位置、工作角度等.最后从背景环境角度对均温板的应用方向进行了分析和展望.

Two kinds of new refrigerant R1234ze(E) and R245fa were discussed as substitutes or supplements to traditional working fluids of loop heat pipes (LHPs) based on their favorable thermophysical properties and characteristics such as being safe and environmentally friendly. Thermal characteristics of a loop heat pipe with sintering copper wick at different charging ratios were experimentally investigated under variable heat loads. The results showed that the optimal charging ratio in the loop heat pipe range from 65% to 70%, and at this charging level, the R1234ze(E) system had better start-up response, while the R245fa system presented a stronger heat transfer capacity. The characteristic temperature of R1234ze(E) system was below 35 degrees C, and the corresponding thermal resistance was 0.08 K/W-1.62 K/W under heat loads ranging from 5 W to 40 W. The thermal resistance of the R245fa system was 0.18 K/W-0.91 K/W under heat loads of 10 W-60 W, and the operating temperature was below 60 degrees C. The loop heat pipes charged with the proposed new refrigerants exhibit superb performance in room temperature applications, making them beneficial for enhancing the performance of electronics and could provide a distinctive choice for the cooling of small-sized electronics especially.

针对环路热管内部工质相变及流动换热问题,设计了环路热管蒸发器中心通道可视化实验平台,研究了不同加热方式对热管内工质状态和传热特性的影响.结果表明:加热方式直接影响热管10 W启动过程,双面加热启动速度最快.相同热载荷时,不同加热方式下环路热管热阻及蒸发器中心通道内液面高度和成核情况存在差异.10～40 W热载荷时,随着热载荷的增大,三种加热方式的传热热阻均在减小.40～50W热载荷时,顶部加热方式下的热管性能出现恶化,底部加热传热性能出现停滞,仅双面加热性能稳定并有提高趋势.随着热负荷的增加,蒸发器中心通道内气液界面升高、气泡的产生变得更加剧烈,蒸发器通过吸液芯向储液器的漏热量增加,进而影响环路热管的性能.

The wick is the most crucial element of loop heat pipes (LHPs). At present, wick materials are limited to copper, stainless steel, nickel, and so on. Little research has been carried out on new wick materials. This paper reports the fabrication of aluminum wicks and performance tests of LHPs using aluminum wicks. Experimental data show that this LHP is capable of transferring a heating load of 130 W, while the thermal resistance is only approximately 0.04 K/W. Then, the experimental data were compared with those for LHPs using a polytetrafluoroethylene (PTFE) wick. Based on the SIMPLE algorithm, a 2-D unsteady-state heat transfer model and a 2-D steady-state flow model of the wicks are established to analyze the effects of wick material, pore size and porosity on the performance of the evaporator. The experimental data for LHPs with PTFE wicks are from the literature. The simulation results show that a PTFE wick can effectively reduce heat leakage, while an aluminum wick has better temperature uniformity and a lower casing temperature. However, wicks with large porosities have lower flow resistance.

为了探索不同加热方式对环路热管的启动及稳定运行性能的影响,对环路热管的蒸发器补偿器进行了可视化试验研究.环路热管以R245fa为工质,充液率为50％,分别采用了三种不同的热负载施加方法:蒸发器顶部加热、蒸发器上下同时加热、蒸发器底部加热.根据启动过程中蒸发器空腔内的主要相变模式不同,对应地分为3种启动模式:蒸发启动模式、蒸发沸腾混合启动模式、沸腾启动模式.结果发现:在5W启动过程中,蒸发沸腾混合模式和沸腾模式下的启动速度最快,并在蒸发器空腔气体槽道出口处伴有气泡溢出,分别历时760s、1180s,远小于蒸发启动模式的2370s.分析可知,环路热管的启动速度与蒸发器空腔内的初始液面及其平均液体消失速度密切相关.另外,为研究蒸发器空腔内液相工质的沸腾,对不同的启动模式下空腔内的气泡生长进行了探索.在稳定工况中,同热负载下不同加热方式的环路热管的热阻及补偿器液面高度都不相同,其中底部加热方式的环路热管热阻最小.经分析发现,同热负载下不同加热方式会影响蒸发器内液相工质的蒸发效率,同时也会改变补偿器液面高度和蒸发器向补偿器的漏热,进而影响环路热管性能.

The loop heat pipe is a passive high-efficiency two-phase transfer device. To study the effects of the liquid and vapor line lengths on the performance of ethane loop heat pipe (ELHP), a performance test system has been set up based on a pulse tube refrigerator (PTR). The lengths of liquid line (LL) and vapor line (VL) of the ELHP are changed in this test system. The LL length is set as 180 mm, 500 mm, 800 mm, 1100 mm and 1500 mm, respectively, when the VL length is 500 mm. In addition, the VL length is set as 800 mm, 1100 mm, and 1500 mm, respectively, as the LL length is 500 mm. The thermal performances of ELHP including startup performance, thermal resistance and temperature difference are investigated in detail under 170 ~ 240 K temperature regions. The experiment results show that the lengths of LL have an important effect on the startup performance when the heat load (HL) of the ELHP is 0 W. The shorter the LL length is, the easier the startup is at 0 W HL. When the LL length is longer, the ELHP is more stable and the temperature oscillation is smaller. The effect of the VL length on the ELHP startup performance is the same as that of the LL length. Due to the thermophysical properties of ethane, the ELHP in the 240 K temperature region has the best performance.

With the rapid development of space exploration technology, the traditional point-to-point heat transfer methods cannot fulfill the heat dissipation requirements of multi- and large-array detectors. Loop heat pipe is an effective and efficient twophase heat transfer device that has played a significant role in spacecraft for the thermal control system. In addition, it has high heat transfer efficiency, long heat transfer distance, heat switch characteristics, and pipeline flexibility. However, higher requirements are proposed for space thermal control. We need to design and handle more complicated detector structures, heat dissipation from scattered distribution, and heat dissipation of large area array. In this work, a heterotypic loop heat pipe has been investigated to solve the heat dissipation issue of a multi-point distributed heat source. A multi-evaporator loop heat pipe (MeLHP) in low temperature is designed and manufactured, via connecting three evaporators in parallel with gas coupling method. The pipelines are arranged asymmetrically as required. The working temperature is set to 170 K while ethane has been used as a working fluid. A series of experiments are conducted to study the start-up characteristics of the prototype. Different heating methods and various charging ratios are compared with a single evaporator loop heat pipe, where the MeLHP prototype is explored variously in the process of cooling and start-up. Initially, a start-up experiment of a single evaporator loop heat pipe is carried out. As we know, the loop heat pipe (LHP) research is relatively mature, so all the conditions of LHP are adopted for the startup characteristics of MeLHP's. It is found that every single evaporator of MeLHP has similar performance to that of LHP. Later, we employ different experimental conditions on the evaporators such as various charging ratios and different heating methods. The temperature fluctuation and heat sharing characteristics are compressively analyzed from the comparison of temperature curves at different working conditions. Finally, the experimental process is verified from the nice comparison of single evaporator temperature change and cooling rate of each case. In addition, we observe that evaporators in each loop of MeLHP are highly uniform compared to that of a single evaporator loop heat pipe, during the start-up process. So, MeLHP can start under the condition of single evaporator heating and multiple evaporators heating as well. When a single evaporator heating process takes place in MeLHP, the unheated evaporators share the heat via the influence of gas coupling from the heated evaporators. So, this process simultaneously starts three evaporators; the initial state of working fluid would influence the temperature of each evaporator during cooling, while the heat exchange of gas coupling effectively inhibits the effect of evaporators' temperature. The structure of gas coupling effectively shares the heating load among all evaporators. Different charging ratios affect the start-up process of MeLHP. Compared to the charging ratio of LHP (0.6), the results of MeLHP are consistent with LHP at a charging ratio of 0.7. So, the fluid charging ratio of MeLHP should be appropriately increased to obtain an equivalent performance to that of a single loop heat pipe. Our experiments have verified the start-up feasibility of a multi-evaporator loop heat pipe with parallel gas coupling, revealed the law of cooling, and the start-up process, which is highly significant for the promotion and application of MeLHP.

基于中子成像(NR)技术对铜-丙烯环路热管(LHP)在不同加热功率(0、5和10 W)下的运行进行了可视化研究.结果 表明:在加热功率为5W时蒸发器内液体工质在减少,冷凝器能够充分冷凝,液体管线充满液体工质;在加热功率10W时热管内部冷凝的液体工质量在减少,蒸发器开始出现烧干现象;环路热管能够成功启动并稳定运行,且随加热功率增加启动时间减少;LHP在5W时内部的气液分布使得传热性能最佳,在10 W时风扇的强制对流并不能达到充分冷凝的效果,导致热管热阻增大,性能变差;LHP运行过程中气体管线部分存在残留液体工质,这会减少LHP的工质实际循环量,降低其传热性能.

为了进一步提高回路热管仿真精度并丰富回路热管实验研究方法,本文对回路热管瞬态传质进行实验研究.使用高精度质量流量计分别对以丙酮、乙醇、丙烯为工质的回路热管进行不同负载功率下的质量流量测量研究.结果表明:启动阶段,热负载10W时,丙烯回路比丙酮回路热管启动快,且两者的温度稳定均滞后于质量流量;稳定阶段,随着热负载功率增大,不同工质的回路热管的平均质量流量均线性增长,而瞬态质量流量则持续波动,其质量流量波动幅度均呈现先减小后增大的趋势.质量流量波动幅度会受到气体工质的可压缩性与作用在毛细芯内部上的热量的共同影响.通过频谱分析发现,液相质量流量波动还会受到冷凝器两相区的影响.高热负载下,作用在毛细芯内部上的热量占主导地位,质量流量波动加剧,同时出现周期性大幅波动,且其波动频率随着热负载增大而增大.

环路热管是以多孔毛细芯抽吸力为动力的相变传热设备,可根据实际应用改变结构形式,能在远距离传热的同时保持良好的均温性,并且可在微重力环境下运行.环路热管工作温区较广,按照其工作温区一般可分为高温环路热管(350 K以上)、常温环路热管(200～350 K)和低温环路热管(200 K以下).为了满足深空探测的需要,低温环路热管广泛应用于航天设备温控系统中并表现出优异的性能.按照孔隙特征和结构形式将用于环路热管的毛细芯分为四种,简要阐述每种毛细芯制备和特点;综合分析了近年来低温环路热管技术主要理论和实验研究成果,将目前低温环路热管常见的工作温区分成五个部分,分析影响低温环路热管传热性能的因素,包括工质充装量、反重力高度、次蒸发器功率等.最后,提出优化措施以满足未来深空以及地面应用的需求.

This research evaluated the working fluids for the loop heat pipes (LHPs) at the working temperature of 190-260 K and verified the using of the figures of merit (FOM) to evaluate the working fluids for LHPs. Ethane, propylene and the rarely reported ethylene, were selected as the working fluids. The thermal resistance and heat transfer limits of two geometrical different LHPs under various operating temperatures and heat loads were investigated. The thermal resistances of the LHPs charged with ethylene were lower than those charged with ethane and propylene under the same heat loads, and the thermal resistances decreased as the operating temperature increased, which is consistent with the predicted results of the Joung Parameter. The experimental heat transfer limits of the two LHPs significantly deviating from the predictions using the Dunbar Parameter and the Mishkinis Criterion. This deviation was analyzed and corrected by considering the pressure loss and contact angle in the derivation of the updated FOM. The experimental heat transfer limit of the LHPs are in good agreement with the updated FOM with different contact angles at different temperatures, and the experimental data is overall closer to the theoretical estimation by the updated FOM than the Mishkinis Criterion which indicates errors up to 70%. The Joung Parameter and the updated FOM in this work may help evaluating and selecting a working fluid suitable for the LHP to meet the design goals of low thermal resistances and significant heat transfer limits, respectively.

A cryogenic loop heat pipe (CLHP) is an effective means of temperature control at low temperatures. However, the physical properties of cryogenic working fluids are quite different from those of room-temperature working fluids, and their phase change and heat transfer characteristics need to be further studied. In this paper, a test system for evaluating the loop heat pipe’s (LHP) heat transfer performance and visualizing the condensation phenomenon in the condenser was fabricated. The performance of a propylene LHP under 10 W of heating power was tested as the condenser temperature was varied from 283 to 193 K, and the flow patterns and heat transfer characteristics during condensation were obtained. The experimental results show that with decreasing condenser temperature, the length of the two-phase region decreased, and the main flow patterns of LHP with low power include stratified, wavy and slug flow. The comparison with the heat transfer correlation shows that Cavallini correlation can predict the condensation heat transfer coefficient well.

通过采用石英补偿器和高速摄像机实现了对丙烯环路热管补偿器的可视化实验研究,重点研究了补偿器内工质的状态随充装量和传热量的变化及充装量对环路热管传热性能的影响.研究发现,容积为51.4 mL的环路热管最佳充装量约为19.7 g.充装量小于最佳充装量的各工况下,能观察到对应补偿器内工质液面高度低于引流管,蒸发器和补偿器之间相变换热强烈,引流管外壁面明显有工质的冷凝及流动,且工质冷凝和流动的速度随着传热量的增加而加快;随着充装量增加,环路热管传热热阻减小,280 K工作温度以下的传热量增大.最佳充装量对应的补偿器内液面高度浸没引流管而接近蒸发器核心通道顶端,得到280 K以下最大传热量为40 W,对应的最优传热热阻为2 K/W.充装量大于最佳充装量的工况下,补偿器内液面高度超过蒸发器核心顶端,随着充装量增加,环路热管传热热阻增大,280 K以下的传热量减小.补偿器和蒸发器核心通道内的工质分布能影响蒸发器向补偿器的漏热量,这是充装量影响环路热管性能的重要原因.