Numerical and experimental investigation of thermal louvers for space applications

Numerical and Experimental Investigation of Thermal Louvers for Space Applications

Issamu Muraoka

[e-mail]

Fabiano Luis de Sousa

[e-mail]

Fernando Manuel Ramos

[e-mail]

Instituto Nacional de Pesquisas Espaciais - INPE

Caixa Postal 515

12201-970 São José dos Campos, SP. Brazil

Wilson Roberto Parisotto

Electrolux do Brasil

Rua Ministro Gabriel Passos, 360

81520-500 Curitiba, PR. Brazil

[e-mail]

Thermal louvers, using movable or rotating shutters over a radiating surface, have gained a wide acceptance as highly efficient devices for controlling the temperature of a spacecraft. This paper presents a detailed analysis of the performance of a rectangular thermal louver with movable blades. The radiative capacity of the louver, determined by its effective emittance, is calculated for different values of the blades opening angle. Experimental results obtained with a prototype of a spacecraft thermal louver show good agreement with the theoretical values.

Keywords: Thermal louver, spacecraft thermal control, radiation heat exchange

Introduction

Louvers are thermally activated shutters that regulate the thermal environment of structural and electronic equipment during spaceflight. They provide a radiative surface with an adjustable effective emittance that compensates for coating degradation, internal power fluctuations or seasonal flux changes. A thermal louver generally consists of an array of low-emissivity, pivoting blades, whose movement is controlled by bi-metallic actuators set for a predetermined temperature range of a base plate or space radiator. As the temperature of the base plate decreases, the bi-metallic actuators close the blades creating a shield that reduces the heat losses from the high-emissivity radiator surface to space. As the temperature increases, the bi-metallic sensor rotates the shutters towards an open position, allowing the heat to escape.

The heat dissipation capability of thermal louvers has been the subject of several analytical and experimental studies (Furukawa, 1979; Hwangbo and Kelly, 1980; Karam, 1979; Ollendorf, 1966; Plamadon, 1964). In the present paper we describe the development of a computational model, based on the lumped-parameter method, for predicting the thermal behavior of a louver system. The louver performance is characterized by its effective emittance, defined as the ratio of the net heat transfer from the louvered area to the radiation from an equivalent unlouvered black area, at the same temperature. We also describe the test in a cryogenic vacuum chamber of a real-sized louver prototype. The experimental results were used to validate our computational model.

Nomenclature

A = louvered area

B = conduction exchange factor

Cp = specific heat

m = mass

p = pressure

Q = internal heat dissipation or net heat transfer

R = radiation exchange factor

T = temperature

t = time

Greeks

e = emitance

e _eff = effective emittance

j = infrared specularity

q = blande opening angle

s = Stefan-Boltzmann constant

Subscripts

b = louver base plate

i, j = node number

w = chamber internal wall

Mathematical Model

The louver system is modeled as a network of isothermal nodes, conductively and radiatively interconnected (Bastos, Muraoka and Cardoso, 1990). In the present analysis, the thermal louver was divided into n=208 two-dimensional, planar nodes. The temperature T_i of the i-th node is given by the heat balance equation:

with i=1,......,n, where m_i is the mass of the i-th node; Cp_i is the specific heat; R_jiand B_ji are, respectively, the radiation and conduction exchange factors between the nodes j and i; Q_i is the internal heat dissipation at the i-th node; s is the Stefan-Boltzmann constant; and t is the time.

The temperature field, at a given time, is approximately determined by finite differencing the time derivative in Eq. (1), using a Crank-Nicolson scheme, which averages the energy exchanges through conduction and radiation between the current and the previous time step. The resulting set of equations is solved iteratively by the SOR method with an optimized relaxation factor. Steady-state solutions are obtained setting the rate of change of enthalpy to zero in Eq. (1). Thermophysical properties are assumed to be constant and independent of the temperature of the nodal points. Radiation exchange factors are calculated by the Monte Carlo method, taking into account the infrared emittance (e) and infrared specularity (j) of the surfaces. The conductive couplings are determined as the inverse of the total thermal resistance in the path of the heat flow between two adjacent nodes, assuming one-dimensional conduction.

The behavior of the thermal louver was numerically simulated according to a two-step approach. Firstly, considering that our computational model cannot simulate dynamic changes in the geometry of the system, five configurations were analyzed, each one representing a different blade opening angle q (0° , 22.5° , 45° , 67.5° and 90° ). A mesh of 208 nodes was used in our simulations. For each angle, a constant and uniform heat dissipation was imposed to the louver base and the resulting average base temperature was determined. The results were condensed in a curve showing the effective emittance as a function of q. The effective emittance, defined as the ratio of the net heat transfer Q from the louvered area A to the radiation from an equivalent unlouvered black area, at an uniform temperature equal to the average base temperature T_b, is given by:

Secondly, a one-node model was developed to simulate the dynamic behavior of the thermal louver. In this model the louver is represented just by its base plate as a planar radiator with temperature dependent emittance. The impact of the blades and their movement on the radiative exchanges between the base plate and the environment is simulated by the effective emittance curve, obtained previously. A linear relation between the blade opening angle and the base temperature was assumed.

Louver and Test Set-up Description

The louver prototype consisted of a square aluminum frame (30 cm x 30 cm x 7.5 cm), with 8 pairs of parallel pivoting blades (13.6 cm x 4 cm, 0.5 mm thick), a base plate radiator (30 cm x 30 cm, 3 mm thick) and an actuator housing cover. Figure 1 shows a schematic view of its major components. No bi-metallic actuators were used in the prototype and, thus, all the settings of the blades angular positions were performed manually during the validation tests. The main structural elements (blades, frame, base plate and housing) were manufactured in Al5052, and chemically polished to enhance their reflectivities. The base plate radiating surface was painted with a high-emissivity black coating (e = 0.90).

The validation tests were conducted in a cryogenic vacuum chamber (p @ 10^-6 Torr, T_wall < -145 ^oC), 1m in diameter and 1m long. The thermal louver was instrumented with 36 copper-constantan thermocouples (AWG 36). In order to simulate the internal heat loads, film heaters were bonded to the rear surface of the base plate and covered with a multi-layer insulation (MLI) blanket, made up of 11 sheets of aluminized Mylar with nylon interspacers, for minimizing heat losses. The assembly was suspended in the chamber by polyester wires as shown in the Fig. 2. Steady-state conditions were achieved for two base plate temperature levels (0° and 30° C), and five blade angular positions (0° , 22.5° , 45° , 67.5° and 90°).

Results

The louver effective emittance as a function of the blades opening angle is depicted in Figs. 3 and 4, for two temperatures of the base plate. The theoretical results were computed considering four different values of blade infrared specularity j (0.75, 0.50, 0.25 and 0). We found a good agreement between experimental data and theory, for j =0.50. When the blades are assumed to be perfectly diffuse surfaces (j =0), the calculated values are always smaller than the measured ones because the model tends to underestimate the net heat transfer from the louvered area. This discrepancy grows with the opening angle, attaining a maximum (approximately 14%) at 90° . The same behavior has already been reported in the literature (Ollendorf, 1966).

The automatic opening or closing of louvers can accommodate large internal power fluctuations, maintaining the base plate temperature within a relatively narrow band. This is well illustrated in Fig. 5 which compares the calculated equilibrium temperature of movable and fixed louvers (opening angles of 0° , 22.5° , 45° , 67.5° and 90°), for a given heat dissipation level. A linear relation between the blades opening angle and the temperature of the bi-metallic actuators, within the range of 10° (fully closed) to 20°C (fully open), was assumed in this simulation.

To illustrate its transient behavior, a cyclic dissipation is imposed to the thermal louver. In this simulation, the heat load oscillates from 4 W to 26 W with a period of 2000 seconds. The results, presented in Fig. 6, are compared with a similar curve obtained with an ordinary radiator of emittance 0.46. While the thermal louver maintains the base plate temperature between 10 and 20 ° C, the ordinary radiator temperature varies in a wider range, from -1 to 32 ° C . However, this advantage is not general and depends on the dynamics of the applied heat load. Repeating the previous simulation using a cyclic dissipation with a period four times smaller (500 seconds), the louver displays the same performance of the ordinary radiator, as shown in Fig. 7. Therefore, the thermal louvers are specially recommended in cases where the satellite is submitted to large seasonal flux variations.

Finally, in order to show how a louver could improve the thermal design of real satellite, we considered the case of the SCD1 satellite, launched in February, 1993. SCD1 is a small spin-stabilized data collecting satellite, with a total mass of 115 Kg, and a maximum electrical power of 110 W. It was placed in a circular low-earth orbit, with an altitude of 700 km and an inclination of 25^o. One of the major constraints when designing SCD1 was the thermal control of the battery (Muraoka and Leite, 1994). The design solution adopted for the flight model was the use of an ordinary radiator, installed at the shadow panel of the satellite, in order to keep the battery temperature within its operational limits (-5 to 25 °C). To illustrate the impact of a louver system in SCD1 thermal performance in orbit, we replaced the ordinary radiator by a thermal louver of same area, keeping all other design parameters unchanged. Louver and radiator were assumed to have the same spectral absortivities. The results, presented in Table 1, show that the use of a louver allowed the reduction of the predicted temperature range for the battery from -3 ~ 23 ° C to -2 ~ 16 ° C. Although the lower limit was little affected, the decrease in the range amplitude was considerable.

Conclusion

In this paper we analyzed the thermal behavior of a louver system using a lumped-parameter numerical method. The radiative capacity of the louver, defined in terms of its effective emittance, was calculated for different blades opening angles. For validating our computational model, we constructed a real-size louver prototype and tested it in a cryogenic vacuum chamber. The experimental data were found to be in good agreement with the theoretical results. Particularly, we demonstrated through simulations with the SCD1 satellite, how a louver system can improve the thermal design of a real spacecraft.

Acknowledgement

This work was partially supported by CNPq-Brazil.

Manuscript received: March 2000, Technical Editor: Angela O. Nieckele.

• Bastos, J.L.F., Muraoka, I., Cardoso, H.P., 1990, "Pacote de Análise Térmica PCTER", I Simpósio Brasileiro de Tecnologia Aeroespacial, São José dos Campos, SP, Brazil, pp.256-257.
• Furukawa, M., 1979, "Analytical Studies on Design Optimization of Movable Louvers for Space Use", Journal of Spacecraft and Rockets, Vol. 16, pp. 412-425.
• Hwangbo, H. and Kelly, W.H., 1980 , "Transient Response of Thermal Louvers with Bimetallic Actuators", 15th AIAA Thermophysics Conference, Snowmass, CO, AIAA paper no. 80-1539.
• Karam, R. D., 1979, "Temperature Distribution on Louvered Panels", Journal of Spacecraft and Rockets, Vol. 16, pp. 92-97.
• Muraoka, I. and Leite, R.M.G., 1994, "Thermal Design of the SCD1 Satellite", 19th International Symposium on Space Technology and Science, Yokohama, Japan, Paper ISTS 94-e-18.
• Ollendorf, S., 1966, "Effective Emittance of an Insulated Louver System", Journal of Spacecraft and Rockets, Vol. 3, pp. 950-952.
• Plamadon, J. A., 1964, "Analysis of Movable Louvers for Temperature Control", Journal of Spacecraft and Rockets, Vol. 1, pp. 492-497.

Publication Dates

• Publication in this collection
  24 Oct 2001
• Date of issue
  2001

History

• Received
  Mar 2000