2013 IEEE/ASME International Conference on Advanced Intelligent Mechatronics (AIM) Wollongong, Australia, July 9-12, 2013
Transient Performance Investigation of a Self-Driven Adaptive Thermostatic Valve for Single-phase Space Thermal Control Loop* Jin Wang, Yun-Ze Li, Member, IEEE, and Jun Wang Abstract—A novel thermal control system which adopt a self-driven adaptive thermostatic valve for single-phase space thermal control Loop (SSTCL-SATV) has been proposed to improve the thermal control adaptability and energy-saving requirement of spacecraft. This self-driven adaptive thermostatic valve can realize the automatic proportional control of flow and temperature by applying the linear expansion or contraction characteristic of sensitive wax. The mathematic model of SSTCL-SATV such as SATV, radiator, and heat source are established for predicting temperature dynamic performance of the thermal control system by lumped parameter method. The transient response of temperature under different sensitive wax parameters such as layout, phase-change range, time constant and temperature delay are presented by numerical simulation to analyze the control effect of SATV by comparing the control overshoot, settling time and finial offset values. These results will supply a design reference of SSTCL-SATV to adapt the complicated space environment.
I. INTRODUCTION With the development of space technology, there is an urgent need to improve the thermal management of spacecraft. Not only is it desirable to enable higher device heat flux, but also to do so with high reliability. Single-phase fluid loop system as an active thermal control technology, have been widely applied to effectively improve the adaptability of spacecraft thermal control system such as the space shuttle, International Space Station and other manned spacecraft thermal control system [1-3]. Single-phase fluid loop system as an active thermal control technology has been studied by many researchers. In general, these researchers always focus on that whether the thermal control can keep the controlled objects in safe work range while ignoring to consider the energy consumption and reliability of this thermal control system. However, with the limitations of the operating costs of the spacecraft, as well as the complexity of the task requirement, the problem is proposed that how to solve the higher requirements of reliability and adaptability for unexpected situations [4-5]. 1 As we all know, the performance of a single-phase fluid loop thermal control system largely depends on the components such as pumps, heat exchangers and valves [6]. Therefore, it is essential to improve the system performance and reliability by applying suitable component. In this paper, we propose a self-driven adaptive thermostatic valve (SATV) instead of the traditional electric thermostatic valve in *Research supported by the National Natural Science Foundation of China under Grant 50506003 Jin Wang, Yun-Ze Li, and Jun Wang are with the School of Aeronautic Science and Engineering ,Beihang University, Beijing 100191, China (e-mail:
[email protected];
[email protected];
[email protected]).
978-1-4673-5320-5/13/$31.00 ©2013 IEEE
single-phase fluid loop to overcome the disadvantage such as unstable and high energy consumption under electric control [7]. SATV is a self-driven adaptive thermostatic valve which can realize the automatic proportional control of flow and temperature by applying the linear expansion or contraction characteristic of sensitive wax.. As an appealing choice for spacecraft thermal control system, SATV has been studied by many researchers [8-12]. In addition to analyze the dynamic behavior of applications such as cooling system of automotive engine and air-conditioning system with SATV by establishing the mathematical models and numerical simulation [8-9], the characteristic of sensitive wax which impact the thermal control effect of SATV are also studied [10-12]. The experiment about the thermal characteristic of sensitive wax has been studied by B. Z. Yang to analyze the influence of temperature and pressure on the wax thermal expansion characteristics [10]. It is essential to design the thermal characteristic parameters of SATV which impact the thermal control effect in SSTCL to reach highly effective thermal control requirement of spacecraft. From the Ref. [13], it has been concluded that the thermal characteristics of SATV include three important parameters which are phase-change range, time constant and temperature delay respectively. Besides, the working condition of SATV also plays an important role on the thermal control effect of the SSTCL-SATV. Therefore, the purpose of this paper is to investigate the thermal control effect of SSTCL-SATV under different layout and characteristic parameters such as phase-change range, time constant and temperature delay respectively to achieve the parameters selection rules which will supply the theoretical basis for the optimal design of SSTCL-SATV. The rest of the paper is organized as follows. Section Ⅱ introduces the construction and working principle of a self-driven adaptive thermostatic valve for single-phase space thermal control Loop (SSTCL-SATV), and also describes the thermal characteristics of sensitive wax. Then the mathematic model of SSTCL-SATV such as SATV, radiator, heat source are established for predicting temperature dynamic performance of the thermal control system; Section Ⅲ simulates and analyzes the control effect of SATV under different sensitive wax parameters such as layout, phase-change range, time constant and temperature delay. In the last section, we draw the conclusion of the paper. II. BRIEF INTRODUCTION AND DYNAMIC MODELING A．Brief Introduction of SSTCL-SATV
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A self-driven adaptive thermostatic valve for single-phase space thermal control Loop (SSTCL-SATV) is proposed as shown in Fig 1. Fig. 1 (a) illustrates that SATV is located in the inlet of MCHE-s, which is at the combine condition; Fig. 1 (b) illustrates that SATV is located at the outlet of MCHE-s, which is at the diversion condition. As we know, the SSTCL -SATV typically includes a hot circuit, a cold circuit, a pump and a three-way SATV.
The hot circuit is composed of MHEs-c which is tightly attached on electronic components (heat source), where heat is transferred into the fluid loop as cold fluids flow through the MHEs-c to the cold circuit.
In the cold circuit, heat is removed from the heated fluid to the outer space through radiator located outside of the spacecraft.
The pump provides the cycle power for the single-phase fluid flowing from hot circuit to cold circuit.
The SATV realize the control of fluid flowrate through MHEs-r by comparing the sensitive wax reference temperature with the mixed fluid temperature. The objective of SATV is to control the temperature at the hot circuit inlet in the set value which can maintain the electronics in a safe working range considering the disturbance from heat load or heat flux from radiator.
The detailed working principle of SATV under combine condition is described as follows. Firstly, the MCHE-s channel is closed, all fluids flow into SATV through bypass channel. The thermo-bulb in the SATV measure and compare the temperature with the reference value. If the bypass fluid temperature is higher, sensitive wax in the thermo-bulb linearly expands and pushes the actuator to open the MCHE-r channel and partially close the bypass channel. Then, the fluid from bypass and MCHE-r channel is mixed in valve body flowing into the hot circuit through the mixed fluid channel. When there is a temperature difference between mixed fluid and reference temperature in the thermo-bulb, sensitive wax is achieved linear expansion or contraction to continually adjust the actuator to realize the flowrate control through MCHE-r and bypass channel until the temperature of mixed fluid reaches the reference value.
Figure 2. The schematic of SATV in combine condition
Thermal characteristics [13] of SATV include three important performance parameters which are phase-change range, time constant and temperature delay respectively. Phase-change range represents the temperature range from starting melting to completely melt into liquid state; The time constant means the time delay of temperature measure in thermo-bulb which is related with heat capacity and surface heat exchanger of the thermo-bulb; The temperature delay is illustrated as that temperature difference from continuous expansion to contraction of sensitive wax .Figure 3 shows a typical curve of thermal characteristic of sensitive wax[14], where represent the volume expansion rate of sensitive wax, Tsen is the sensitive wax temperature.
(a) Combine condition
(b) Separate condition Figure 1. The schematic of SSTCL-SATV
The traditional construction in SATV is consisted of a thermo-bulb, an actuator and a valve body. The thermo-bulb and actuator realize the implement of temperature measurement, comparison and flowrate control respectively according to the compared temperature error between the sensitive wax and reference temperature. 394
Figure 3. The thermal characteristic of sensitive wax
0，Tsen Tset m (T T )， sen set mr Tp Tset Tsen Tset Tp m，Tsen Tset Tp
B. Heat Transfer and Temperature Dynamic Model In order to simplify the mathematical modeling process, some assumptions are adopted as follows: The thermal resistance in thermo-bulb, MCHE-r, MCHE-s and radiator are neglected;
The mechanical and the flow characteristics of SATV are ignored;
The flow field is steady state, one-dimensional and laminar;
The whole system is adiabatic without heat leak expects the outer surface of MCHE-r which is attached by the radiator;
The friction of fluid flow in line and pump do not generate temperature rise;
resistance
The temperature dynamics of the SSTCL-SATV are contributed by three sources: the radiator and MCHE-r in the cold circuit, the heat source and MCHE-c in the hot circuit, and the SATV in the mixed point of hot and cold circuit. This network models the MCHE-r and radiator, MCHE-c and heat source, and SATV as three lumped thermal nodes. The lumped parameter method is used to establish the mathematical model of SATV under combine condition. The average temperature of the lumped unit Tsen can be calculated by
ksen
dTsen (t ) Tsen (t ) Tm (t ) dt
0，Tsen Tset Ths m (T T T )， sen set hs mr Tp Tset Ths Tsen Tset pT hsT m，Tsen Tset Tp Ths
Csen hsen Asen
Where mr is the fluid mass flowrate through MCHE-r channel, m is the total mass flow, Tp and Ths are phase-change range and temperature delay of sensitive wax respectively. The mixed point of bypass and MCHE-r channel is typical dynamic temperature control point, according to the law of conservation of mass, the relationship of m, mb and mr can be expressed as
m mb mr
Where mb is the bypass fluid mass flow.
The differential form energy conservation equation of mixed point can be concluded by
Where ksen is the time constant of SATV, is expressed as
ksen
mc p
Where Csen is the heat capacity of SATV, hsen and Asen are the convective heat transfer coefficient and heat transfer area between thermo-bulb and mixed fluid in the valve body, Tm is the mixed fluid temperature. From Fig.3 we can see that temperature delay exists between the expansion and contraction process of sensitive wax. Therefore, it is essential to determine the temperature change trend to choose control equation. When dTsen/dt>0, opening valve flow control is started to reflect the expansion process; When dTsen/dt
