J. Cent. South Univ. Technol. (2011) 18: 1278-1284

DOI: 10.1007/s11771-011-0833-0

Application of high temperature heat pipe in hypersonic vehicles thermal protection

BAI Tong(白穜), ZHANG Hong(张红), XU Hui(许辉)

College of Energy, Nanjing University of Technology, Nanjing 210009, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2011

Abstract: In order to develop further the application of high temperature heat pipe in hypersonic vehicles thermal protection, the principles and characteristics of high temperature heat pipe used in hypersonic vehicles thermal protection were introduced. The methods of numerical simulation, theory analysis and experiment research were utilized to analyze the frozen start-up and steady state characteristic of the heat pipe as well as the machining improvement for fabricating irregularly shaped heat pipe which is suitable for leading edge of hypersonic vehicles. The results indicate that the frozen start-up time of heat pipe is long (10 min) and there exists large temperature difference along the heat pipe (47 °C/cm), but the heat pipe can reduce the temperature in stagnation area of hypersonic vehicles from 1 926 to 982 °C and work normally during 1 000-1 200°C. How to improve the maximum heat transfer capability and reduce the time needed for start-up from frozen state of the heat pipe by optimizing thermostructure such as designing of a novel wick with high performance is the key point in hypersonic vehicles thermal protection of heat pipe.

Key words: thermal protection; high temperature heat pipe; heat transfer limit; start-up time

1 Introduction

Hypersonic vehicles are the aircrafts which travel at 5Ma or greater. In order to ensure the safe flight of hypersonic vehicles, it is necessary to design appropriate thermal protection structure and effective thermal control system because of the intense heat flux that hypersonic vehicles have to endure at the stagnation region such as wing leading edge and nose cap [1-2]. At present, the thermal control technology can be divided into passive and active thermal control technologies [3-6].

Though heat pipe thermal protection system is a passive thermal control technology, it can largely reduce the temperature of hypersonic vehicles at the stagnation region, and there are still some detailed problems in the research contents and methods such as the fabrication of heat pipe, theory analysis and numerical calculation of the distribution of flow flying outside the heat pipe and temperature field inside the heat pipe as well as heat pipe performance test, which are needed to be improved. The research of the application of high temperature heat pipe in hypersonic vehicles thermal protection has been carried out since 1970s [7]. GLASS [8] has made a comprehensive summary of the research of heat pipe in thermal protection system, which included the numerical simulation of the frozen start-up process and the thermal stress distribution, fabrication and performance test process of heat pipe. The results indicated that the heat pipe made of superalloy and refractory metal can meet the needs of thermal protection for hypersonic vehicles, but the conclusions did not include the influence of intense and variable input heat flux on the performance of heat pipe. CRAIG et al [9-12] provided a systematic method to study the novel leading edge of structural heat pipe. The conclusions demonstrated that the heat pipe showed great ductility and reduced the temperature of the stagnation region effectively. The research predicted that this new structure of heat pipe can endure higher heat flux with metal material, but the heat pipe used for thermal protection has not yet been produced and tested on the realistic thermal environment. LIU [13] established a numerical model of heat pipe thermal protection structure to predict the temperature field. The results showed that the steady-state analysis was more conservative than the transient analysis for safety evaluation and the heat pipe not only increased the system effectiveness but also improved the structure reliability. The most important parameter affecting the system was the shortest distance between the heat pipe and the stagnation point. But the thermal contact resistance and the stress between the heat pipe and C/C compound material were not considered.

There are some research contents and methods of heat pipe thermal protection system such as the measures to reduce the frozen start-up time, the influence of intense and variable input heat flux outside during the frozen start-up process and so on needed to be further studied. This work summarizes and analyzes the research of heat pipe thermal protection system, points out the existed problems and proposes possible solutions in order to provide the idea and method for the further research of heat pipe thermal protection system.

2 Principles of heat pipe thermal protection

In order to satisfy the needs of thermal protection at the region of leading edge and nose cap of hypersonic vehicles, high temperature heat pipes have usually been fabricated in the form of elbow, as shown in Fig.1.

Fig.1 Schematic diagram of high temperature heat pipe and heat distribution

The elbow site of the heat pipe is the evaporation section, which is usually located at the stagnation region of hypersonic vehicles and needs to endure serious aerodynamic heating. The maximum heat flux exceeds 100 W/cm² [14] and the temperature can reach 1 600 °C [15] or even higher. The working fluid absorbs the heat and evaporates at the evaporation section, and then the vapor flows to the condensation section due to the pressure difference and releases latent heat. The condensed liquid flows back to the evaporation section with the help of capillary force provided by wicks. The condensation section of the heat pipe transfers the heat absorbed by the evaporation section to the sheltered side with low temperature by radiation or convection, and it can largely reduce the temperature of the stagnation region. Therefore, the working temperature and the area of condensation section of the heat pipe should be considered while designing heat pipes. The efficiency of radiation heat transfer at the condensation section is proportional to the working temperature of the heat pipe because of the isothermal characteristic of the heat pipe. Otherwise, the heat generated by aerodynamic heating at the evaporation section has to be transferred by increasing the area of condensation section of the heat pipe or adding active radiating equipment (aircraft fuel as coolant). These measures increase the weight of thermal protection system, which means that hypersonic vehicles need more fuel, and it weakens the competitiveness of heat pipe thermal protection.

3 Research status of heat pipe thermal protection

3.1 Numerical simulation of high temperature heat pipe in thermal protection

The performance of heat transfer and start-up of high temperature heat pipe directly affect the thermal protection system. If the heat pipe cannot start to work normally in the limited time, aerodynamic heating can make hypersonic vehicles endure extreme high temperature; if the steady operation temperature of the heat pipe cannot reach the equilibrium temperature, the heat would accumulate at the evaporation section of the heat pipe, which could threaten the safe flight of hypersonic vehicles. Therefore, the simulation research of heat pipe is mainly focused on the start-up and temperature distribution of the heat pipe. The start-up progress of high temperature heat pipe can be divided into three stages [16]: 1) The heat is transferred into working fluid through wall and wicks, and the vapor density in the heat pipe is low that the vapor flow is in the molecular flow condition; 2) Sodium vapor density increases and continuum flow occurs, but in the condensation section there is still molecular flow; 3) Heat pipe starts up and works normally, and is full of continuum flow. COLWELL and HARTLEY [17] established a mathematic model to describe the physical phenomena in these three stages, and the coupled calculation was conducted to get the temperature distribution of the heat pipe. The results indicated that at the beginning of start-up progress, large temperature gradient and high temperature exist along the heat pipe due to inhomogeneous distribution of sodium vapor. Though differences exist between the simulation results and experiment results due to the structure setting of heat pipe, and material property and boundary conditions in simulation calculation are not the same with the real conditions, the established model could predict the start-up time and temperature distribution approximately. BOMAN and ELIAS [18] set a three-dimensional model and finite difference method was used to calculate the start-up progress of the heat pipe. The model assumed a continuum “front” and the start-up temperature T^* was applied to decide whether the vapor in heat pipe was in continuum flow condition. Start-up temperature T^* can be obtained iteratively by

                            (1)

where M is the relative molecular mass of gas; R is the universal gas constant; ρ is the gas density; and D is the vapor chamber diameter.

The start-up model of heat pipe is shown in Fig.2. The influence of working fluid flowing in the wick on heat transfer was not considered in the model, the heat flux in the boundary condition was assumed to be constant and there was no coupled calculation between the boundary conditions outside the heat pipe and the phase change heat transfer inside the heat pipe. This calculation method belongs to an engineering algorithm. The simulated results show that heat pipe would work normally in 10 min, and large temperature gradient   (47 °C/cm) appears along the heat pipe. The reason is that in the vapor continuum flow region, the heat is transferred by the evaporation and condensation of working fluid, and in the molecular flow region, the heat is transferred by conduction through the wall of the heat pipe, which causes low temperature at condensation section and large thermal stress along the heat pipe. These could restrain the safety operation and lifetime of heat pipe thermal protection system, and some measures like enlarging the diameter of heat pipe, reasonably arranging the length of evaporation and condensation of heat pipe, and adding non-condensable gas, are necessary to reduce the start-up time of the heat pipe.

3.2 Fabrication of high temperature heat pipe

In 1973, MDAC (McDonnell Douglas Astronautics Co.) fabricated heat pipe thermal protection structure for cooling wing leading edge of a shuttle-type vehicle with the help of NASA (National Aeronautics and Space Administration) Langley Research Center. The structure consists of twelve heat pipes, which are brazed to the inner surface of a thin hastelloy-X skin in order to fix and enlarge the heat transfer area, and the heat pipes are hastelloy-X circular cylinders with a wall thickness of 0.125 cm. Twelve heat pipes are bent to form the leading-edge shape and the diameter of the elbow is  1.25 cm. The wick consists of seven alternate layers of 100-mesh and 200-mesh stainless-steel screen. The skin of heat-resistant alloy steel is coated with a high-emissivity ceramic paint to facilitate heat rejection by radiation. This kind of structure can ensure the normal operation of the whole thermal protection system even if one heat pipe does not work and provide good stability. The details of the cross-section of the structure is shown in Fig.3.

In 1979, MDAC continued to design and improve the heat-pipe-cooled leading edge for single-stage-to- orbit vehicles in order to reduce the weight of the thermal protection structure to make it competitive with the passive thermal protection structure like carbon- carbon composite material and refractory metal. The improved design is shown in Fig.4.

The improved design used a hastelloy-X tube with “D” cross section, which was seam welded to a thin Hastelloy-X facesheet. This configuration eliminated the need for the braze fillets of the cylindrical tube design and reduced the mass by approximately 44% compared with the design that used circular cylindrical tubes brazed to a thin facesheet [19].

A sodium/hastelloy-X heat pipe was designed for cooling the wing leading edge of an advanced space transportation system and a single full-scale heat pipe was fabricated in 1985. The heat pipe was designed to reduce the maximum wing leading edge temperature from 1 927 to 982 °C. The heat pipe was in the form of “J” shape with length of 172.5 cm, and the stainless steel screen was chosen as the wick, which was diffusion welded to the inner surface of the heat pipe for good thermal contact. In the evaporation section of the heat pipe, two layers of 50-mesh stainless-steel screen were placed between two layers of 200-mesh screen on the heated surface (upwind side), while two layers of 200-mesh screen were used on the side walls. No screen was placed on the back wall (downwind side) in the evaporation section. In the condensation section, eight layers of 50-mesh screen were spot welded between two layers of 200-mesh screen on the heated surface and two layers of 200-mesh screen were used on the other three walls. The purpose of the composite wick was to achieve high capillary pressure provided by the 200-mesh screen and high permeability of the 50-mesh screen. The composite wick usually has characteristics of high capillary pressure and low resistance for fluid flow which normally decide the heat transfer performance of heat pipe [20], so the design of the wick should be the key point in fabrication of heat pipe.

Fig.2 Heat pipe start-up model: (a) τ=0, sodium frozen in wick, T<98 °C; (b) τ=τ₁, heat pipe partially operational; (c) τ=τ₂, heat pipe partially operational (T*: Transition temperature); (d) τ=τ₃, heat pipe partially operational

Fig.3 Cross-section of heat pipe thermal protection structure for cooling wing leading edge

Fig.4 Improved heat pipe design

Heat pipe technology was used to cool the wing leading edge of aerospace hypersonic vehicles with the “National Aerospace Plane” in America started in 1986. The designed heat pipe thermal protection structure had refractory-metal heat pipes embedded in a refractory- composite structure. The material of heat pipe was Mo-RE alloying steel, which is lighter than W-RE and pure RE alloying steel and had features of high strength, high temperature resistance as well as good ductility and weldability. The working fluid of the heat pipe was lithium with the saturated vapor pressure of 117 kPa at   1 370 °C and was compatible with the wall. The structure of heat-pipe-cooled wing leading edge is shown in Fig.5.

Because the diameter of the elbow at the curved heat pipe is small, how to fabricate high temperature heat pipe without fold on the curved surface should be heeded. First, the “D-shaped” molybdenum tube was bent to the wanted diameter at elevated temperature, but crimping of the tube occurred on the curved surface. A Mo-11RE tube was successfully bent at room temperature after it was heated for 4 h at 1 649 °C. Then the tube was fractured when a subsequent attempt was made to bend it at room temperature. Finally, the researchers decided to machine the components out of a solid piece of Mo-41RE and weld them together to form the final structure. Mo-41RE was chosen to be the material of heat pipe and the wick was four layers of 400-mesh Mo-5RE alloy screen, as shown in Fig.6.

Fig.5 Heat pipe thermal protection structure for cooling wing leading edge of hypersonic vehicles

Fig.6 Individual machined components in curved heat pipe

3.3 Experiment study of high temperature heat pipe in thermal protection

NASA and AFRL tested the start-up and steady-state operation performance of high temperature heat pipe with different cross-sections and shapes, and the researches were mainly centered on 1990s while there were little reports about this after 21 century. CHEN [21] did the experimental test and heated a principle thermal protection model with a high temperature heat pipe in it. The surface temperature distribution was measured by an infrared temperature instrument, and the test results demonstrated that the high temperature heat pipe transfer the heat effectively from the high temperature field to the lower field, and the temperature of stagnation point of the model decreased obviously while the thermal protection performance was improved. Other two typical heat pipe tests were selected to validate the thermal protection effect.

3.3.1 Experiment study in NASA Langley Research Center

1) Experiment object: “D” cross section high temperature heat pipe with lithium as working fluid, Mo-41RE as wall material.

2) Heating method: induction heating in vacuum chamber.

3) Experiment process: The start-up and steady state experiments of heat pipe were conducted, respectively, in order to verify whether the heat pipe could work normally at the temperature between 1 204 °C and     1 260 °C. The induction heating was stopped because of the leakage of working fluid after 1 000 s from the beginning in the start-up experiment, and due to the deformation of heat pipe, the steady state experiment was terminated when the wall temperature of the heat pipe reached 1 500 °C. It was found that the heat pipe could work normally at the expected temperature range, and the leakage of working fluid and deformation of heat pipe were caused by experiment method, which could be avoided in practical application.

4) Experiment results: The specific results are shown in Fig.7 and Fig.8. The heat pipe starts to work after 400 s and the temperature rising rate is fast while the temperature at the evaporation section has already reached 800 °C stably. There is little temperature gradient along the heat pipe when the wall temperature is 1 320 °C and the heat transfer quantity is 3.27 kW. But two problems are aroused in the experiment: the working fluid of heat pipe leaks at the place where the thermocouples are welded, and the deformation of heat pipe is due to high vapor pressure. In practical application, the heat pipe is embedded into a refractory- composite structure and this could avoid the deformation. In order to make the experiment successful, non-contact temperature measure and accurate determination of heat input are applied.

Fig.7 Steady state temperature distributions for different heat transfer quantity

Fig.8 Heat pipe start-up process

3.3.2 Experiment study in Los Alamos National Laboratory

1) Experiment object: “J-shaped” and rectangular cross section high temperature heat pipe with lithium as working fluid, Mo-41RE as wall material.

2) Heating method: induction heating in vacuum chamber.

3) Experiment process: four start-up tests were conducted for this kind of heat pipe. At the beginning of the test, heat pipe was heated up to 920 °C under the heat flux of 412 W/cm², and the hot spot was then found on the inside surface of the curved region. This is because the inside surface is heated by the induction heating but not adequately cooled by the liquid lithium due to the discontinuous wick. In the actual application, inadequate cooling of the inside surface would not be a problem since it would not be subjected to the aerodynamic heating. The approach taken to solve the hot spot problem in the experiment was to insulate the inside curved surface of the heat pipe. The heating process of the heat pipe, distribution of thermocouples and start-up process of the heat pipe are shown in Figs.9-11, respectively.

Fig.9 Heat pipe heating process

Fig.10 Thermocouple distribution along heat pipe

4) Experiment results: the first start-up experiment is representative in the four tests while the test curves of other experiments are unrepresentative and the law of some curves is still unknown. The results of the first start-up test indicate that the heat pipe starts from the  frozen state slowly and requires more time to work normally. It takes 5 h for the heat pipe before the temperature of thermocouple TC8 placed at the condensation section of heat pipe begins to rise. The highest temperature in the four tests is about 980 °C. The heat pipe is burn through in the fourth test because of the electric discharge between induction coil and heat pipe. There is light flashed from the vacuum chamber and the pressure in the vacuum chamber is increased by 90 kPa after the heating is shut down. The reason for burning through the heat pipe is that water vapor in refractory insulation is released during the heating process, which forms conducting medium and ionization between induction coil and heat pipe.

Fig.11 Heat pipe start-up process

3.4 Systematic study on leading edge structural heat pipe

CRAIG et al [9] provided a systematic method to study a novel leading edge structural heat pipe which is shown in Fig.12.

Fig.12 A leading edge structure heat pipe

The one-piece heat pipe had 100×100 metal screens as wick and truss cores for structural support to the leading edge and pathway for easy vapor flow as well as pushing the capillary limit upwards. First, the temperature and heat flux at stagnation area of leading edge of hypersonic vehicles were calculated with the Fay-Riddell equation and Sutton-Graves correlation. This could provide accurate thermal boundary conditions of heat pipe which was essential for evaluating the performance of heat pipe.

Secondly, the temperature distribution, maximum temperature and thermal stress of different heat pipes with different materials were calculated with finite element method, and heat transfer limits of heat pipes were also analyzed to evaluate the feasibility of leading edge structural heat pipe for hypersonic vehicles. The results indicate that heat pipe can largely reduce the temperature of stagnation area and provide uniform temperature distribution which could diminish temperature gradient and thermal stress, and the lithium/niobium alloy Cb-75 heat pipe has better performance than sodium/nickel alloy Inconel 625 heat pipe.

Finally, a low-temperature copper-water heat pipe was fabricated and tested to demonstrate the heat spreading effectiveness of the leading edge concept and its potential for the hypersonic thermal environment which is also the limitation of the research. The heat pipe used for thermal protection had not yet produced and tested in realistic thermal environment, and some research methods are rough like the truss cores without calculation of vapor pressure loss along pathway and how much the capillary limit could be improved by this kind of structural, the capillary limit of heat pipe without specific wick properties test and some simulating calculations like temperature and pressure distribution along the wick. Therefore, the true performance of this leading edge structural heat pipe under realistic thermal environment is still unknown.

4 Conclusions and prospect

1) High temperature heat pipe can reduce the temperature at the stagnation region like wing leading edge and nose cap of hypersonic vehicles, and provide effective thermal protection.

2) In order to increase the limited high flux capacity, heat-resistant alloying steel can be the wall material of heat pipe, or the heat pipe made of refractory metal can be embedded into a refractory-composite structure. The wick with high capillary pressure and low flow resistance as well as enhanced heat transfer in heat pipe should be designed carefully, and the hydraulic test of the wick is also needed.

3) Because of the particularity of high temperature heat pipe experiment (high heat flux, high temperature), heating method (combustion, high-speed jet fuel burner, tungsten quartz lamp, arc welding), temperature measurement and determination of heat input should be considered together to match the normal start-up and steady operation of heat pipe.

4) The influence of boundary conditions on phase change heat transfer of working fluid in heat pipe should be taken into consideration in the numerical simulation of high temperature heat pipe, which means the flow of working fluid, solid-liquid-gas phase change heat transfer and flow field outside the heat pipe should be couple-calculated.

5) The start-up characteristic and heat transfer limit capacity of high temperature heat pipe are the key points deciding whether heat pipe thermal protection works successfully. Decreasing the start-up time and increasing limited heat transfer capacity of heat pipe should be the research emphases in future.

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(Edited by HE Yun-bin)

Foundation item: Project(51076062) supported by the National Natural Science Foundation of China

Received date: 2011-01-07; Accepted date: 2011-04-02

Corresponding author: ZHANG Hong, Professor, PhD; Tel: +86-25-83587308; E-mail: hzhang@njut.edu.cn