1. Introduction
In space remote sensing, CCD(Charge-Coupled Device) and CMOS(Complementary Metal Oxide Semiconductor) are the most widely applied core photosensitive devices on space optical payloads due to their high definition and high quantum efficiency imaging, real-time data transmission and wide spectrum range [
1]. These photosensitive components are quite sensitive to temperature, and excessive temperature fluctuation may worsen imaging quality by giving rise to dark current and thermal noise. For CCD, according to the research by Ahmad [
2], a dark current would double itself when the temperature increases from 6 to 9 °C, for CMOS, the range is from 8 to 10 °C by Chen [
3]. Generally, for CCD in space infrared remote sensors, the working temperature range shall be kept from −70 °C to −10 °C, and for the visible spectrum, the maximal upper limit can be 35 °C [
4].
While in orbit, the thermal environment for remote sensors is serious: solar radiation, earth infrared radiation, earth albedo and space environment heat and cool the sensor alternatively. Considerable heat is generated when these power consuming assemblies are working. Since CCD and CMOS are small in size and of low heat capacity, excessive temperature fluctuations may easily occur if no reliable thermal control measures are taken.
Unlike ground devices which could dissipate heat through conduction or convection [
5,
6], in space, inner heat generation of CCD and CMOS would eventually be diffused into space through radiation. As a key part of the thermal control system of focal plane assemblies, the design of the space radiators is a vital design task. To solve the design problem, an equivalent thermal resistance method [
7,
8,
9,
10] is widely used. With the method, the temperature of CCD assemblies would be determined in advance, then equivalent heat resistance for each part is evaluated to define the temperature of each part along the heat transfer path. Based on such estimation, the area of the radiator is evaluated. Finally, the validation of the design is done through CAE (Computer Aided Engineering) tools [
2,
7,
8,
9,
10]. Although the heat resistance method has proven itself as a reliable tool, the engineering experience and thermodynamic calculation based classical method is time-consuming and the performance of the design relies heavily on the designer’s experience. Furthermore, with complicated constraints, the improvement of heat dissipation efficiency on radiator design is hard to achieve. As in space observation, CCD and CMOS have strict requirements on working temperature, and the design problem is calling for new methods to improve heat dissipation efficiency.
In recent research concerning the optimization design of space radiators, most works are focused on large-scale radiators with a honeycomb structure [
11,
12,
13]. In honeycomb radiator optimization designs, parameters such as the gap between every two adjacent heat pipes [
13], thickness, or the materials and coating [
12] are usually used as the optimization variations. The topology optimization method is also applied to determine the layout of heat tubes [
11] in some cases. For radiators that work on miniaturized optical devices, due to strict installation and mass limits, a common solution is to use the outer cover of the optical device as the radiator [
14]. In this case, it is feasible to improve the heat dissipation efficiency by optimizing material distribution of the outer cover based on the topology optimization method.
As a promising optimization design method, topology optimization, which is capable of systematically distributing materials in a certain domain under prescribed constraints, in recent years has become a popular optimization design method in structural engineering since its proposal in late 1980s by Bendsøe and Kikuchi [
15]. Topology optimization is also effective in solving heat transferring problems, and many of the topology optimization techniques and methods are introduced and promoted in solving heat transfer optimization problems. Previous research is mainly focused on conduction and convection problems: J. Haslinger and A. Hillebrand [
16] applied a homogenization method to optimize a heat conducting structure by controlling the variables represented by coefficients of elliptic equations; Bendsøe [
17] and C.Seonho [
18] introduced the SIMP [
19] (Solid Isotropic Material with Penalization) method into heat conduction problems; A.Iga [
20] summarized the topology optimization process of the problems concerning thermal conduction and convection with design-dependent effects; Alexandersen [
21] proposed an optimization with a large scale three-dimensional sink under natural convection; Li [
22,
23] and Xie [
24] dealt with heat conduction problems with the ESO (Evolutionary Structural Optimization) method [
23].
Concerning optimization design problems with radiation boundary, the research is quite minimal. For the design of a radiative enclosure, D.A. Castro [
25] calculated a design-dependent view factor between the radiating surfaces and discussed two objectives, aiming at maximizing the net flux and minimizing the temperature summation. Through topology optimization, Castro changed the maximal temperature from 845.97 to 800.98 °C. The radiative enclosure is simplified into a two-dimensional plane model, and only radiation is considered in the optimization. In Fan’s research [
11], a two step optimization is done to strengthen the heat dissipation efficiency of a large-scale butterfly wing radiator. The optimization decreases 73.4 °C on maximal temperature under a uniform heat source. In the first step, the shape of the radiator is selected from many Pareto solutions, the objective represents a weighted combination of heat dissipation per mass and temperature differences at boundary points. Then in the second step, the heat tube layout is decided by a topology optimization only based on mean compliance concerning heat conduction. For the radiator discussed in this paper, since the material is homogeneous and the heat conduction and radiation of the radiator are coupled, the objective of the topology optimization should be able to describe the influences of both conduction and radiation, which is seldom discussed in previous research.
In this paper, to improve the heat dissipation efficiency of a radiator that works for focal plane assemblies on a miniaturized space optical device, topology optimization is introduced into the design process. For realization of topology optimization, an objective based on maximal thermal stiffness concerning radiators is formulated. After reconstructing the radiator based on the topology optimization result, the final design could be validated through transient simulation. Simulation results indicate that after topology optimization, the maximal temperature of CCD assemblies is decreased by 1.167 °C. The reconstructed radiator design optimized the material distribution under prescribed constraints, which is meaningful in future applications.