Hosei Nagano

Development of An Experimental Small Loop Heat Pipe with Polytetrafluoroethylene Wicks

This paper reports the development of an experimental small loop heat pipe with polytetrafluoroethylene (PTFE) wicks. Three kinds of PTFE porous materials were fabricated, and their wick properties were evaluated. It was clarified that the peak pore radii of the PTFE porous materials ranged from 0.8 to 2:2 � m, and their porosities ranged from 27 to 50%. A small loop heat pipe with PTFE wicks was designed using these properties, and an experimental small loop heat pipe, for which the wick in an evaporator is exchangeable, was fabricated. The experimental loop was tested with 1.2 and 2:2 � m PTFE wicks under atmospheric conditions. The test result with a 2:2 � mPTFEwickshowedahighoperatingtemperature,possiblyduetovaporleakfromtheevaporatorgroovesto the compensation chamber. The test results with the 1:2 � m PTFE wick showed a lower operating temperature, lower thermal resistance, and stable operation of the loop heat pipe.

Capillary Limit of a Multiple-Evaporator and Multiple-Condenser Miniature Loop Heat Pipe

An experimental investigation was conducted on the capillary limit of a miniature loop heat pipe with multiple evaporators and multiple condensers. Tests were conducted under various operating conditions: 1) heat load to one evaporator only; 2) even heat loads to both evaporators; 3) no temperature control of either compensation chamber; 4) controlling the temperature of one or both compensation chambers using thermoelectric devices; 5) placing the loop in a horizontal position with evaporators and compensation chambers on the same plane; and 6) placing the loop in a vertical position with evaporators above the compensation chambers. The physical processes that lead to evaporator deprime and the recovery from the deprime were examined. The effect of the gravity on the capillary limit and the compensation chamber temperature was also investigated.

Loop Heat Pipe Operation with Thermoelectric Converters and Coupling Blocks

This paper presents theoretical and experimental studies on using thermoelectric converters (TECs) and coupling blocks to control the operating temperature of a miniature loop heat pipes (MLHP). The MLHP has two parallel evaporators and two parallel condensers, and each evaporator has its own integral compensation chamber (CC). A TEC is attached to each CC, and connected to the evaporator via a copper thermal strap. The TEC can provide both heating and cooling to the CC, therefore extending the LHP operating temperature over a larger range of the evaporator heat load. A bi-polar power supply is used for the TEC operation. The bipolar power supply automatically changes the direction of the current to the TEC, depending on whether the CC requires heating or cooling, to maintain the CC temperature at the desired set point. The TEC can also enhance the startup success by maintaining a constant CC temperature during the start-up transient. Several aluminum coupling blocks are installed between the vapor line and liquid line. The coupling blocks serve as a heat exchanger which preheats the cold returning liquid so as to reduce the amount of liquid subcooling, and hence the power required to maintain the CC at the desired set point temperature. This paper focuses on the savings of the CC control heater power afforded by the TECs when compared to traditional electric heaters. Tests were conducted by varying the evaporator power, the condenser sink temperature, the CC set point temperature, the number of coupling blocks, and the thermal conductance of the thermal strap. Test results show that the TECs are able to control the CC temperature within k0.5K under all test conditions, and the required TEC heater power is only a fraction of the required electric heater power.

Measurement of the Thermal Diffusivity of an Anisotropic Graphite Sheet Using a Laser-Heating AC Calorimetric Method

The thermal diffusivity of a graphite sheet having an extremely high anisotropy has been measured by a laser heating AC calorimetric method in the temperature range from 30 to 350 K. This graphite sheet has characteristics of high thermal diffusivity and high anisotropy, and it is only 100 μm thick. Thus, it is difficult to apply the conventional AC technique. Therefore, we propose a simultaneous measurement method for the in-plane and out-of-plane thermal diffusivities, by analyzing the three-dimensional heat conduction process, which contains the effects of anisotropy and thermal wave reflections. This method was verified by checking with thermal diffusivity measurements of isotropic materials such as stainless steel and pure copper and was then applied to the anisotropic thermal diffusivity measurement of the graphite sheet.

Thermophysical Properties of High-Thermal-Conductivity Graphite Sheets for Spacecraft Thermal Design

Thermophysical properties of a new material-a graphite sheet, which has characteristics of high thermal conductivity, anisotropy, lightweight and flexibility-have been measured in order to apply this sheet to a spacecraft thermal control material. The following measurements were performed: 1) The thermal diffusivities in the in-plane and out-of-plane directions were measured over the temperature range from 100 to 350 K using a laser heating ac calorimetric method. 2) The specific heat and the total hemispherical emittance were measured over the temperature range from 173 to 375 K using a transient calorimetric method. 3) The solar absorptance was measured using a spectroscopic method for angles of incidence ranging from 5 to 60 deg. Additionally, the thermal conductivities and the specific thermal conductivities were calculated using the measured results, and the high potential of this graphite sheet as a material for spacecraft thermal control was confirmed.

Transient Thermo-Fluid Modeling of Loop Heat Pipes and Experimental Validation

A transient mathematical model is developed to study the transient response and analyze the distribution of heat load in a loop heat pipe. The model is based on the one-dimensional and time-dependent conservation equations for heat and fluid flow. The momentum and energy conservation equations for each of the loop heat pipe components are solved. The model results are compared against the data obtained from two miniature loop heat pipes using polytetrafluoroethylene wicks, ethanol, and acetone as working fluids. The mathematical model satisfactorily predicts the dynamic behavior of the loop heat pipe unit. It is shown that the percentage of heat leak across the wick decreases and the ratio of latent heat increases with increasing heat load. Some temperature overshoots observed in the calculation results are not observed in the experimental data. When a new power is applied, no time lag is observed in the loop heat pipe response between the simulation and experimental results.

Current status of ASTRO-H Hard X-ray Telescopes (HXTs)

ASTRO-H is an international X-ray mission of ISAS/JAXA, which will be launched in 2014. One of the main characteristics of ASTRO-H is imaging spectroscopy in the hard X-ray band above 10 keV. ASTRO-H will carry two identical Hard X-ray telescopes (HXTs), whose mirror surfaces are coated with Pt/C depth-graded multilayers to enhance hard X-ray effective area up to 80 keV. HXT was designed based on the telescope on board the SUMIT balloon borne experiment. After feasibility study of the HXT design, the FM design has been deteremined. Mass production of the mirror shells at Nagoya University has been going on since August 2010, and production of mirror shells for HXT-1 was completed in March 2012. After the integation of X-ray mirrors for HXT-1, we measured hard X-ray performance of selected mirror shells for HXT-1 at a synchrotron radiation facility, SPring-8 beamline BL20B2. We will perform environment tests and ground calibarations at SPring-8 for HXT-1. In HXT-2, foil production is going on.

Capillary Limit of a Miniature Loop Heat Pipe with Multiple Evaporators and Multiple Condensers

An experimental investigation of a miniature loop heat pipe with multiple evaporators and multiple condensers were conducted in order to evaluate its capillary limit. The experimental tests were conducted by varying heat load to one or both evaporators, with and without active temperature control of compensation chamber (CC) using the thermoelectric devices, and variable tilts between the evaporators and the CCs. The physical process of the loop and thermal conductance of the heat leak from evaporator to (CC) were discussed based on the test results. The difference of the temperature profiles between with and without active control of CC temperature was evaluated. The effect of the gravity on capillary limit and CC temperature was also evaluated by comparing the test result in horizontal position with that in vertical position. The loop recovery after capillary limit was exceeded was also described.

Simple deployable radiator with autonomous thermal control function

The concept, detailed design, fabrication, and test results of a reversible thermal panel, which is a new, passive, and lightweight 100 W-class deployable radiator with an environment-adaptive function, is described. The reversible thermal panel changes its function reversibly from a radiator to a solar absorber by deploying/stowing the radiator/ absorber reversible fin upon changes in the heat dissipation and thermal environment, and is effective for the thermal control of high power density small satellites, landers, and interplanetary spacecrafts. Parametric studies were conducted with detailed simulation models, and the reversible thermal panel configuration that satisfies thermal requirements with the restraints of weight and fin efficiency variation was determined. Based on the analytical results, a reversible thermal panel engineering model was fabricated using high thermal conductivity graphite sheets in the reversible fin and a shape-memory alloy in the passive deployment/stowing actuator. Test results indicated excellent performance of the reversible thermal panel as a passive radiator from the standpoints of heat-rejection capability, specific heat rejection, and variation of the fin efficiency. The effectiveness of the reversible thermal panel as a solar absorber that can be substituted for a survival heater was also demonstrated.

Small Loop Heat Pipe with Plastic Wick for Electronics Cooling

In this paper, fabrication and test results of a small loop heat pipe (LHP) for electric cooling over a distance of 1100 mm are discussed. Poly(tetrafluoroethylene) porous material with 1.2 µm pore radius was used as a wick material for a primary pump of the loop to satisfy in both high thermal performance and low cost. High-grade ethanol was selected as a working fluid inside the loop. The small LHP with an outer diameter of 12 mm and length of 77 mm of an evaporator was made of stainless steel. The vapor and liquid transport lines are 1.8 mm in inner diameter and 1100 mm in length respectively in order to demonstrate the long-distance heat transport in a small tube, against the frictional resistance in the loop. The start-up process was monitored with an infrared camera, and quick start-up and its stable operation after the start-up were visually demonstrated. The loop could operate up to 50 W heat load with the operating temperature of around 70 °C in the evaporator. The thermal resistance between the evaporator and the condenser of the loop was 0.58 K/W at 50 W.