Variable Conductance Heat Pipe Research Articles

Experimental study on the detection of frozen diffused ammonia blockage in the inactive section of a variable conductance heat pipe

Variable Conductance Heat Pipes (VCHP) are mainly employed to cool down electronic systems in spacecraft applications, as they can handle high temperature fluctuations in their cold source, preventing thus the systems from damaging. These fluctuations, as well as ultra-low temperatures, are always present in outer space, and one of the key steps in a VCHP design is therefore to make sure that they endure these conditions. However, not much has been written about their resilience during and after a long exposition to subfreezing conditions, i.e. temperatures lower than the freezing point of the working fluid. In this paper we implement and validate a computational routine based on a modified Flat-Front Approach to predict the VCHP temperature profile and to determine the location of the gas–vapor front. Then we continuously expose an ammonia/stainless-steel VCHP to temperatures below the ammonia freezing point for 211 h, to later examine the formation and subsequent dynamics of a thin block of frozen ammonia which is diffused into the inactive part of the heat pipe condenser. We describe as well how a strong correlation between the adiabatic section and the reservoir temperatures is maintained (or broken) upon the occurrence (or absence) of the blockage of frozen ammonia.

Experimental validation and numerical assessment of a temperature-controlled thermoelectric generator concept aimed at maximizing performance under highly variable thermal load driving cycles

Although the internal combustion engine (ICE) dominance in mobility is being challenged, ICE R&D enables a non-disruptive transition towards sustainable transportation through hybridization and carbon neutral fuels. The efficiency improvement of ICE-based mobility through Waste Heat Recovery (WHR) is thus a priority, especially in the area of hybrid, plugin hybrid and long-haul transportation, where the recovery potential is high, and ICEs will still be used for a long time. Thermoelectric (TE) Generators (TEGs) have been assessed as a possible low maintenance WHR solution for long. However, viable market solutions are still not a reality. While material costs are steadily being overcome with novel affordable TE materials, there is still the issue of low average conversion efficiency under realistic driving conditions. This is caused by the fact that the efficiency of TEGs is further deprecated by the big challenge of thermally optimizing the TEGs under highly variable conditions of exhaust flow rate and exhaust temperature during real-world driving. This thermal optimization mainly means that the hot side temperature should be as close as possible to the maximum allowable temperature of the modules, but not higher.The authors have been exploring exhaust heat exchanger (HX) concepts that allow achieving this thermal optimization under highly variable thermal loads by using phase change. The strategy used incorporates the spreading of excess heat along the HX by variable conductance heat pipes (VCHPs) embedded in it, thus enabling to maximize the energy conversion without thermal dilution under low thermal loads or overheating risk under high thermal loads. This was done by pre-regulating the pressure of the non-condensable gas (NCG) present inside the VCHPs so that these devices would start performing the spreading of excess heat from over-heated to under-heated regions of the HX at a precise phase change temperature, optimized to the optimal thermal level of the thermoelectric generators.The merit of the aforementioned strategy was confirmed through modelling and illustrated in previous work. The scientific novelty of present study relatively to previous work by the authors or other researchers is that it finally provides a full experimental validation of this concept with two different prototypes attached to a light duty engine. The experimental proof-of-concept results were successfully compared against predictions. Finally, theoretical performance analysis of the impact of the application on a light duty vehicle in terms of fuel economy and GHG emissions during the WLTC class 3 b was assessed with the help of driving cycle and engine simulations performed with AVL Suite software.The unique heat spreading and temperature control features of the concept were confirmed experimentally. In addition, predictions made by the model were far beyond the current state of the art for light duty automotive TEG generators. Namely, maximum output powers exceeding 2 kW and 1.2 kW were predicted during the WLTC cycle when using commercial bismuth telluride and the silicide-Tetrahedrite modules explored by the authors, respectively. Average fuel/CO2 savings of 4% were predicted for the WLTC cycle, with these savings raising to 12% during the highway portion of the cycle. This unparalleled performance is explained by the possibility of achieving both an optimized hot face temperature and a high heat exchanger effectiveness even under highly variable thermal loads.

Thermal and hydrodynamic analysis of a self-purging hot reservoir variable conductance heat pipe

A new design of a hot reservoir variable conductance heat pipe (VCHP) is proposed and investigated in this study. While a hot reservoir VCHP can provide much tighter thermal control performance in comparison to a traditional cold reservoir VCHP, it was seen that the hot reservoir design is prone to more common operating failures due to the accumulation of the working fluid inside the reservoir. Once accumulated, this excessive working fluid in the reservoir can only be removed by diffusion which takes a long time. In this study, we propose and investigate a new design for the hot reservoir VCHP that would perform an additional function of working fluid vapor purging in addition to operating as a normal heat pipe. In the proposed design, the non-condensable gas (NCG) section of the heat pipe is connected to the reservoir by an external tube, forming a NCG flow-inducing loop configuration, hence the name self-purging hot reservoir VCHP (SPHR-VCHP). With proper placement of the NCG pipe in the VCHP a pressure-induced flow is generated through the loop from the NCG section towards the reservoir. It is shown that this loop flow can remove the excess working fluid vapor in the reservoir significantly faster than by diffusion. A Fortran code to simulate the two-phase flow in the heat pipe section combined with an ANSYS Fluent CFD model of the external loop is employed to predict the fluid flow, heat transfer, and mass transfer behaviors inside the SPHR-VCHP. An experiment is set up for the SPHR-VCHP that is made of a 291 mm long titanium tube with an inner diameter of 14.7 mm. The working fluid and the NCG are acetone and helium, respectively. The CFD predictions of wall temperature are validated by the experimental data with mean absolute error equal to 2.68%. A thorough CFD study is then performed to optimize the loop design in order to enhance purging rates by changing the size of the NCG tube to operate under microgravity conditions for space applications. Based on this study an optimum NCG pipe diameter equal to 40% of the inner diameter and pipe length equal to the length of the evaporator section is suggested. The wall temperature results show that the SPHR-VCHP design does not impact the thermal performance of the original hot reservoir heat pipe, while also having a self-purging capability that makes the design safe from working fluid accumulations issues.

Complex Fluid Flow in Microchannels and Heat Pipes with Enhanced Surfaces for Advanced Heat Conversion and Recovery Systems

This paper addresses a multiscale approach for heat recovery systems, used in two distinct applications. In both applications, a microscale approach is used (microchannel heat sinks and heat pipes) for macroscale applications (cooling of a photovoltaic—PV cell), and the thermal energy of exhaust gases of an internal combustion engine is used for thermoelectric generators with variable conductance heat pipes. Several experimental techniques are combined such as visualization, thermography with high spatial and temporal resolution, and the characterization of the flow hydrodynamics, such as the friction losses. The analysis performed evidences the relevance of looking at the physics of the observed phenomena to optimize the heat sink geometry. For instance, the results based on the dissipated heat flux and the convective heat transfer coefficients obtained in the tests of the microchannel-based heat sinks for cooling applications in PV cells show an improvement in the dissipated power at the expense of controlled pumping power, for the best performing geometries. Simple geometries based on these results were then used as inputs in a genetic algorithm to produce the optimized geometries. In both applications, the analysis performed evidences the potential of using two-phase flows. However, instabilities at the microscale must be accurately addressed to take advantage of liquid phase change. In this context, the use of enhanced interfaces may significantly contribute to the resolution of the instability issues as they are able to control bubble dynamics. Such an approach is also addressed here.

Experimental investigation and multiparameter analysis of variable conductance heat pipes

This paper presents an experimental investigation of the influence of multiple parameters on the thermal performance of variable conductance heat pipes (VCHPs) according to theoretical design and analysis. The operating characteristics of four VCHPs, such as the steady-state temperature, thermal resistance, and degree of self-control, were systematically analysed and compared. The design principles were examined considering the structural matching design, thermal-physical properties, and charging mass of the fluids. The examined types included the cold-gas reservoir VCHP (CGR-VCHP) and the hot-gas reservoir VCHP (HGR-VCHP), whose steady-state operating temperatures and thermal resistances were compared. The HGR-VCHP attained a higher degree of self-control than the CGR-VCHP for the same heat pipe (HP) structure size. The effects of the reservoir volume of the CGR-VCHP were experimentally and theoretically assessed. Compared to a small volume of 33.66 mL, a large volume of 43.27 mL increased the self-control, with dTva/dQ dropping from 0.272 K/J to 0.042 K/J. Moreover, the temperature difference decreased from 13.6 to 2.09 °C under the same heat load increase of 50 W. Both R22 and ammonia were chosen as working fluids due to their similar sensitivity factors. The self-control ability of the ammonia CGR-VCHP was higher than that of the R22 CGR-VCHP based on the obtained experimental and theoretically calculated results. This research provides valuable insights into VCHP multiparameter analysis, which are of great significance for the practical application of VCHPs in self-control thermal.

Compact automotive thermoelectric generator with embedded heat pipes for thermal control

Currently, the automotive industry faces challenges to implement solutions that provide reductions in energy consumption, pollutants and greenhouse-gas (GHG) emissions. Exhaust heat recovery employing Thermoelectric generators (TEGs) enables the direct conversion of heat into electric energy without moving parts and little to no maintenance. On-board electrical production is especially useful given the growing electrification trend of road vehicles. The present work assesses the performance of a novel temperature-controlled thermoelectric generator (TCTG) concept in a light duty vehicle and its impact on fuel economy and GHG emissions under realistic driving conditions. The novel exhaust heat exchanger (HE) concept consists of corrugated pipes embedded in a cast aluminium matrix along with variable conductance heat pipes (VCHPs) acting as spreaders of excess heat along the longitudinal direction. This concept seems to have a quite good potential for highly variable thermal load applications, as it is able to avoid overheating by spreading heat instead of wasting it through by-pass systems. Furthermore, when compared to previous concepts by the group, it does not need gravity assistance and has a form factor similar to conventional generators. It also appears to be capable of delivering a breakthrough electric output for TEG systems in such light vehicles, with as much as 572 W and 1538 W of average and maximum electric powers during a driving cycle, respectively, and showing a quite promising reduction of 5.4% in fuel consumption and CO2 emissions.

Experimental investigation into the feasibility of using a variable conductance heat pipe for controlled heat release from a phase-change material thermal store

Traditional heat storage uses large water tanks, with wide operating temperature differentials that restrict their use with heat pumps and Stirling engine DCHP systems. In contrast, the system developed under the ‘THERMAC’ project uses phase change materials (PCMs) for thermal storage to reduce system size, with a self-regulating, variable conductance heat pipe (VCHP) to control heat output. The VCHP is a heat pipe whose condenser is linked to a reservoir of non-condensable gas – in this case argon. Small temperature gradients expand or contract the argon along the condenser, so that it acts as a thermal switch by controlling the active length of the condenser.The study was successful in its main objective of demonstrating the basic technical feasibility of (i) Erythritol PCM as a means of heat storage (ii) over-coming the PCM’s low thermal conductivity by using fins on the VCHP, and, (iii) a VCHP as a means of controlling the thermal output of the PCM storage system, thereby increasing their overall efficiency.Recommendations are made for further development of the ‘THERMAC’ concept.

Compact Heat Rejection System Utilizing Integral Variable Conductance Planar Heat Pipe Radiator for Space Application

ABSTRACT In order to meet heat rejection requirements for future NASA exploration, scientific, and discovery missions, a study is being conducted for the feasibility of integral variable conductance planar heat pipe (VCPHP) technology. This represents a novel, low technology readiness level (TRL) heat rejection technology that, when developed, could operate efficiently and reliably across a wide range of thermal environments. The concept consists of a planar heat pipe whose evaporator acquires the excess thermal energy from the thermal control system and rejects it at its condenser whose outer surface acts as a radiating surface. The heat pipe is made from thermally conductive polymers in order to minimize its mass. It has a non-condensable gas that changes the active radiator surface depending on the heat load. A mathematical model of steady-state variable conductance heat pipe is developed. Two planar heat pipes are designed, fabricated, and tested to validate the theoretical model. The feasibility of the proposed VCPHP working in a space environment is discussed, based on the model.

In-Flight Performance of the OCO-2 Cryocooler

The Orbiting Carbon Observatory-2 (OCO-2) will have completed its first year in space on July 2, 2015. The OCO-2 instrument incorporates three bore-sighted, high-resolution grating spectrometers, designed to measure the near-infrared absorption of reflected sunlight by carbon dioxide and molecular oxygen. The cryocooler system design is coupled with the instrument's thermal control design to maximize the instrument's performance. A single-stage NGAS pulse tube cryocooler provides refrigeration to three focal plane arrays to ∼120 K via a high conductance flexible thermal strap. A variable conductance heat pipe (VCHP) based heat rejection system (HRS) transports waste heat from the instrument located inside the spacecraft to the space-viewing radiators. The HRS provides tight temperature control of the optics to 267 K and maintains the cryocooler at 300 K. Soon after entering the A-Train on August 3, 2014, the optics and focal planes were cooled to their operating temperatures. This paper provides a general overview of the cryogenic system design and reviews the in-flight cryogenic performance during the Observatory's first year.

Thermoelectric Exhaust Heat Recovery with Heat Pipe-Based Thermal Control

Heat pipe (HP)-based heat exchangers can be used for very low resistance heat transfer between a hot and a cold source. Their operating temperature depends solely on the boiling point of their working fluid, so it is possible to control the heat transfer temperature if the pressure of the HP can be adjusted. This is the case of the variable conductance HPs (VCHP). This solution makes VCHPs ideal for the passive control of thermoelectric generator (TEG) temperature levels. The present work assesses, both theoretically and experimentally, the merit of the aforementioned approach. A thermal and electrical model of a TEG with VCHP assist is proposed. Experimental results obtained with a proof of concept prototype attached to a small single-cylinder engine are presented and used to validate the model. It was found that the HP heat exchanger indeed enables the TEG to operate at a constant, optimal temperature in a passive and safe way, and with a minimal overall thermal resistance, under part load, it effectively reduces the active module area without deprecating the temperature level of the active modules.

Design and verification of thermal control system for Chang’E-3 probe

ChangE-3 probe is the first Chinese spacecraft landed on the lunar surface and survived for lunar day/night cycles. The ChangE-3 thermal control system developed new elements such as closed two-phase thermosyphons, variable conductance heat pipes (VCHP), high-temperature adiabatic shield and radioisotope heater units (RHU) to deal with the harsh environment. Ground test methods for simulating the effect of lunar gravity and descent engine were established as well. Thermal control system worked well in orbit and on the lunar surface, and maintained the probe components and structure within the required range of temperatures throughout the mission. First-hand data of the lunar surface probe, which is useful for the future mission, was obtained during the flight test. This paper introduces the primary requirements, chief factors and the characteristics of the thermal control system design. The final scheme, key elements and flight performance of thermal control system are also described.

Reduced Power Precision Temperature Control Using Variable Conductance Heat Pipes

This paper assesses the use of variable conductance heat pipes (VCHPs) for reduced-power precision temperature control of photonics components. When subambient cooling is not required we consider only a VCHP and where it is required we consider a VCHP-thermoelectric module (TEM) assembly. In the former case, the setpoint of the component mounted to the VCHP is 70°C and a range of heat loads (0 to 12 W) and ambient temperatures (-5°C to 65°C), representative of a photonics component in a telecommunications environment, are imposed on the system. In passive operation, the VCHP provided good temperature control for varying heat load, but not for varying ambient temperature. In active operation, i.e., when the reservoir of noncondensable gas on the VCHP is heated, good temperature control is achieved for ambient temperatures from 10°C to 65°C. The experimental measurements are compared with the theoretical predictions of the flat front model. For the TEM-VCHP assembly experiments, the component is maintained at a representative setpoint temperature, 63±1°C, for heat loads from 2 to 7 W and ambient temperatures from 0°C to 60°C, such that a TEM is essential. The TEM-VCHP assembly power consumption is quantified and compared with a TEM-constant conductance heat pipe assembly. The maximum TEM power consumption for the TEM-VCHP assembly is over 40% lower.

中性子ラジオグラフィによる可変コンダクタンスヒートパイプ(VCHP)の可視化

中性子ラジオグラフィによる可変コンダクタンスヒートパイプ(VCHP)の可視化

Influence of Heat Pipe Operating Temperature on Exhaust Heat Thermoelectric Generation

Increasingly stringent targets on energy efficiency and emissions, as well as growing vehicle electrification are making attractive the electric recovery of the energy normally wasted through the tailpipe of Internal Combustion Engines. Recent developments in thermoelectrics (TE) may soon make them a viable solution for such applications [1]. This team has been exploring the potential of using TE modules in combination with variable conductance heat pipes for transferring the exhaust heat to the generator with very low thermal resistance and at a constant, prescribed temperature. This passive temperature control eliminates the need for bypass systems in the event of temperature overshoots. The operating temperature of a generator should be as high as possible in order to maximize the Seebeck effect. However, currently available modules are temperature limited. Moreover, the higher the HP temperature the less the usable thermal power at the exhaust will be (heat can only be transferred to from a hotter to a colder body). The present work assesses both theoretically and experimentally the influence of the HP temperature in the electric output of a thermoelectric generator. A small diesel engine and a generator were tested and it was found that a high HP operating temperature is only limitative for performance in the cases where low exhaust temperature and low engine power are present. In those cases it is possible to estimate an optimal HP temperature in order to maximize power output. The combined use of Seebeck modules and heat pipes was found to be highly advantageous in various ways.

Thermoelectric Module-Variable Conductance Heat Pipe Assemblies for Reduced Power Temperature Control

Thermoelectric modules (TEMs) are used to precisely maintain the setpoint temperature of photonic components generating variable heat loads under varying ambient conditions. The non-component side of TEMs is mounted onto conventional heat sinks (CHSs). At any combination of setpoint temperature, heat load and ambient temperature, there is a unique thermal resistance between the non-component side of a TEM and the ambient corresponding to minimal TEM power consumption. Indeed, a zero thermal resistance heat sink minimizes power consumption when a TEM operates in refrigeration mode, but when it operates in heating mode a relatively high thermal resistance one is optimal. This paper considers replacing a CHS with a variable thermal resistance heat sink in the form of a finned variable conductance heat pipe (FVCHP). An FVCHP passively and (generally) favorably alters its thermal resistance in response to changes in operating conditions, thereby reducing TEM power consumption. An analytical model of a TEM-FVCHP assembly is developed in order to quantitatively estimate achievable energy savings. The model assumes that there is 1-D flow of heat and charge through a TEM and that a “flat front” divides the condenser section of an FVCHP into thermally active and inactive regions. Calculations are performed for representative TEM-FVCHP assemblies that maintain the setpoint temperature of a photonics component over prescribed ranges of heat loads and ambient temperatures. Average TEM power consumption is reduced by up to 55% compared to a comparable TEM-CHS assembly. Finally, further optimization of the TEM and FVCHP design parameters is considered.

An experimental and analytical study of a variable conductance heat pipe: Application to vehicle thermal management

Abstract This paper reports on about an analytical and experimental study of a Variable Conductance Heat Pipe (VCHP) applied to vehicle thermal management. The reduction of engine energy consumption after a cold start by controlling heating–cooling cycle of oil is the objective of the present study. A theoretical model based on a nodal method and an experimental test bench was developed to study the performance of a copper/water VCHP, using nitrogen as a non-condensable gas. VCHP operate as a thermal switch, with a start-up temperature of 80 °C. The effect of the inclination angle (i.e., adverse-gravity, horizontal position or gravity-aided) was studied. The results allowed us to identify the effect of the air mass flow rate on the condenser and the effect inclination angle on the performance of the VCHP. Our results show that the VCHP performance improves with a small inclination angle aiding liquid return to the evaporator.

Temperature-Controllable Oscillating Heat Pipe

Experiments are conducted to investigate whether an oscillating heat pipe with a liquid reservoir can serve as a thermal control device in space. The oscillating heat pipe consists of a stainless steel capillary tube (inner and outer diameters of 0.8 mm and 1 mm, respectively), which meanders between a heating section and a cooling section 15 times in each direction (30 times in total). A 50 mL reservoir is connected to the oscillating heat pipe via another capillary tube. The 1,1,1,2-tetrafluoroethane (HFC-134a) is used as the working fluid. The heat input to the heating section is increased from 0 and 70 W, in 10 W increments. When the oscillating heat pipe is set horizontally, the temperature of the heating section remains at about the reservoir temperature of 40 C for three orientations of the reservoir with respect to gravity: vertical, horizontal, and vertically inverted. In the top-heating mode, the temperature of the heating section also remains at about the reservoir temperature. The oscillating heat pipe with a liquid reservoir is confirmed to operate as a variable conductance heat pipe, and its operating temperature can be controlled to almost be the liquid reservoir temperature for each investigated orientation of the reservoir. The oscillating heat pipe with a liquid reservoir is confirmed not to lose its temperature control function in gravity; thus, the operating temperature can be controlled by regulating the liquid reservoir temperature, not only on the ground but also in space.

Analysis and application of variable conductance heat pipe air preheater

The heat transfer analysis of variable conductance heat pipe air preheater was carried out. The temperature transfer matrix was obtained for the air preheater that comprises several discrete heat transfer units with same or different heat transfer surface area in a parallel or counter flow mode. By using the temperature transfer matrix, the outlet fluid temperatures could be easily calculated for a given air preheater and inlet fluid temperatures. The active length of condenser in a variable conductance heat pipe is determined according to the flat interface model. With the same initial conditions, the comparisons between variable conductance heat-pipe air preheater and regular heat pipe air preheater has been analyzed and tested in terms of heat pipe wall temperature, heat transfer surface area and outlet fluid temperatures. Based on the real industrial applications, it has been confirmed that the variable conductance heat pipe air preheater has excellent performance of anti-corrosion and anti-ash-deposition especially at the variable working condition and the sulfur coal (5%–6% mass fraction of sulfur) condition.

Evaluation of liquid behavior in a Variable Conductance Heat Pipe by neutron radiography

Abstract A Variable Conductance Heat Pipe (VCHP) is used as a cooling device for electrical equipments. The condensation area is passively controlled by the non-condensable gas volume in the VCHP depending on the heat load. The VCHP has often a bent pipe between the evaporation and condensation area. The heat pipe performance depends much on the bent pipe shape and configuration because a liquid plug is formed in the bent pipe and disturbs the refrigerant circulation. However, the mechanism has not been clarified well. The neutron radiography system at the JRR-3 in Japan Atomic Energy Agency (JAEA) was used to visualize the refrigerant behavior in the VCHP. Effects of the thin plate inserted in the pipe, refrigerant filling ratios and heat pipe configuration were examined on the heat pipe performance. The liquid plug was formed at the bend and caused to decrease the performance. It was confirmed that the thin plate insert was effective to disturb the liquid plug formation.

可変コンダクタンスヒートパイプ（VCHP）安定化技術の開発 第１報：気液分離構造の評価

可変コンダクタンスヒートパイプ（VCHP）安定化技術の開発 第１報：気液分離構造の評価