Applied Heat Load Research Articles

Damage and deuterium retention of re-solidified tungsten following vertical displacement event-like heat load

Surface morphology and hydrogen isotope retention of W specimen melted with vertical displacement event-like heat load and subsequent deuterium (D) plasma exposure were studied. Applied heat loads using electron beam without raster scanning were about 190 and 230 MW/m2 in heat flux and 0.08, 0.12 and 0.16s in duration. After the heat load application, specimens showed apparent melting spots with grain growth or dense micrometer scale convex structure. Cracks were observed only in the part with the convex structure. D retention in the melted part of specimens was not significantly larger than in the reference specimen despite large changes of surface characteristics.

Vapor pocket behavior inside the porous wick of a capillary pumped loop for terrestrial application

This paper presents a numerical study of a Capillary Pumped Loop with an unsaturated porous wick. The CPL model is divided into subsystems, for which transient mass, energy, and momentum balances have been developed. The evaporator is described by a two-dimensional unsteady mathematical model, model which takes into account the existence of a vapor pocket inside the porous wick. The comparison of the numerical results and data experience which makes it possible to validate the proposed model in a transient regime. Simulations show that the vapor pocket size is almost independent of the applied heat load at a steady state. The evaporator behavior is then studied for rapid transient application of heat power, when used in terrestrial applications.

Steady-state modeling and analysis of a loop heat pipe under gravity-assisted operation

Loop heat pipes (LHPs) are efficient two-phase heat transfer devices that have found many space and terrestrial applications. This work addresses our insufficient understanding of LHP operation under gravity-assisted attitude, i.e. the condenser is located higher than the evaporator. A steady-state mathematical model of a LHP under gravity-assisted operation was established based on two driving modes: gravity driven mode and capillarity-gravity co-driven mode, determined by a defined transition heat load. The model was validated by the experimental results, and was employed to predict the operating characteristics of a LHP under the gravity-assisted attitude. Comparing to LHPs operating under horizontal or antigravity attitudes, some distinctive features have been identified, which include: i) the total mass flowrate in the loop shows a unique V-shape with the increase of applied heat load; ii) the steady-state operating temperature is much lower under the gravity driven mode, and is in similar values under capillarity-gravity co-driven mode and iii) the thermal conductance of the LHP increases with increasing positive elevation especially in the variable conductance zone. Such results contribute greatly to the understanding of the complicated operating principle and characteristics of LHPs especially for terrestrial applications.

A novel ultra-high performance liquid chromatography method for the rapid determination of β-lactoglobulin as heat load indicator in commercial milk samples

The level of undenatured acid-soluble β-lactoglobulin can be used as an indicator to assess the heat load applied to liquid milk, thus further allowing the discrimination between milk originating from different thermal production processes. In this work, a new UHPLC method for the rapid determination of bovine β-lactoglobulin in 1.8min only (total runtime 3min) is presented using simple UV detection at 205nm. Separation selectivity for possibly co-eluting other major whey proteins (bovine serum albumin, lactoferrin, α-lactalbumin, immunoglobulin G) was verified, and the method validated for the analysis of liquid milk samples regarding linearity (20–560μg/mL, R2>0.99), instrumentation precision (RSDs<2.8%), limits of detection and quantification (7 and 23mg/L milk), repeatability of sample work-up (RSDs≤2.6%) and method recovery (103%). In total, 71 commercial liquid milk samples produced using different preservation techniques (e.g., thermal or mechanical treatment), hence featuring different applied heat loads, were profiled for their intrinsic undenatured acid-soluble β-lactoglobulin levels. As expected, pasteurized milk showed the highest concentrations clearly above 3000mg/L due to pasteurization being the mildest thermal treatment, while in contrast, ultra-high temperature heated milk featured the lowest amounts (<200mg/L). For extended shelf life (ESL) milk, quite diverse levels were determined ranging from ∼100 up to 4000mg/L, thus clearly illustrating variable applied heat loads and impacts on the “nativeness” of milk essentially due to the fact that the production technologies used for ESL milk may differ significantly, and are currently not regulated in the EU.

Three dimensional liquid and vapour distribution in the wick of capillary evaporators

Heat and mass transfer with liquid–vapour phase change in a representative unit cell of a capillary evaporator is studied using a mixed pore network model. The model combines the computation of temperature and pressure fields in vapour and liquid pores according to mean field approaches with pore scale invasion rules depending on the capillary pressure thresholds associated with each local constriction between two pores. The metallic body through which heat is transferred to the porous wick is also taken into account in the simulations. After comparisons with a visualisation experiment, numerical simulations performed in three dimensional pore networks lead to the identification of three main regimes depending on the applied heat load. Compared with previous works using the so-called vapour pocket assumption, the 3D simulations reveal a regime where the phase distribution within the wick is fundamentally different. This regime is characterised by the coexistence of both the liquid and vapour phases underneath the casing within the wick. This regime is shown to correspond to the best evaporator performance.

Experimental and Theoretical Evaluation of On-Chip Micro Heat Pipe

The performance of a specially fabricated micro heat pipe etched on a silicon wafer is analyzed to evaluate its cooling potential. Triangular microgrooves are etched on a silicon wafer via a lithographic process with two reservoirs at the two ends. Anodic bonding and electrochemical spark erosion techniques are used to seal the microgrooves with a thin Pyrex glass and to create holes for vacuum connections and coolant insertion, respectively. The cooling potential of the micro heat pipe is evaluated by accurately measuring the temperature distributions along the channel length at different values of the applied heat load. A mathematical model is proposed and numerically solved to evaluate the axial variation of the radius of curvature of the coolant film to examine the cooling potential of the on-chip micro heat pipe system.

Analytical model of cryo-target in thermo-vacuum chamber

Nitrogen and Flelium based Cryo-Target (CT) systems are used for various experimental, industrial and testing applications in high vacuum chambers. Thermal performance of such systems depends upon a number of factors such as thermal exchange coefficient of coolant, various forms of heat losses, coolant properties, CT plate characteristics, externally applied heat loads etc. Therefore an analytical formulation is necessary in order to quantify the effect of such factors over thermal performance of CT system. In this paper analytical model is developed for flat plate CT system based on the positive root of heat balance quartic equation. Important analytical formulas related to characterization of thermal performance of CT systems are presented. This paper doesn't consider mixed phase flow in CT.

Analysis of overloaded micro heat pipes: Effects of solid thermal conductivity

Starting from the principles of mass, momentum, and energy conservations as well as the Young–Laplace capillary equation, a mathematical model of triangular MHPs has been developed, primarily to investigate the effects of the thermal conductivity of the solid wall on their performance under overloaded conditions. For this purpose, two solids of significantly different thermal conductivities, copper and nickel, have been selected for the wall material. Using the model, a map encompassing the various possible operation zones of an MHP was constructed, to provide insight into the modes of operation under a given operating temperature but with varying heat loads and charge levels of the working fluid. The model predicts that in the overloaded zones, the dryout and flooded lengths increase with the applied heat load, resulting in a decrease in the effective length of the MHP. Moreover, it is also observed that the existence of dryout is accompanied by a large temperature rise over the dry region and, thus, a large total axial temperature drop, which increases rapidly with the applied heat load. Finally, comparison between copper and nickel MHPs on how the key performance indicators, such as the dryout and flooded lengths, the effective length, and the total axial temperature drop, response to the applied heat load shows that an MHP of solid wall of higher thermal conductivity out-performs one with lower solid thermal conductivity.

Comparison of quasi-steady-state and unsteady-state formulations in a freeze dryer modeling

An unsteady-state model is developed for primary and secondary stages of freeze drying process of skim milk. The results are compared with those obtained from a quasi-steady-state (QSS) formulation. The QSS formulation is not valid where the applied heat load is high. The applied heat load affects on the drying time the most compared to other parameters like chamber pressure and the radiation surface temperature.

Modeling, development, and experimental validation of a Joule–Thompson superfluid refrigerator using a pulse tube cryocooler

An economical method of refrigeration to temperatures near 1K is introduced in this work. Unlike traditional methods, our refrigeration system does not rely on the use of a helium bath; helium enters a Dewar at room temperature and pre-cools, condenses, and sub-cools to temperatures slightly lower than the normal boiling point of helium during its heat exchange with various components that are connected to a Cryomech pulse tube cryocooler (PT-410). A fixed Joule–Thompson restriction valve is used to create a large pressure difference between the incoming high-pressure helium line and the vacuum pumped space of the 1K pot causing the helium temperature to drop to its corresponding saturation condition in the pot upon an isenthalpic expansion across the valve. This is a comprehensive report on the performance of a Joule–Thompson superfluid refrigerator; we will discuss our model, techniques in the experimental setup, and present our data for this type refrigerator. This refrigerator reached an ultimate low temperature of 1.39K with no applied heat load and provided 150mW of cooling power at 1.65K.

Evaporative heat transfer analysis of a micro loop heat pipe with rectangular grooves

This study focuses on the thermal and capillary analysis of a micro loop heat pipe for the thermal management of electronic devices and systems. A model is developed using the principles of thin film evaporation to predict the evaporative heat transfer coefficient in grooved capillary structures. In addition, a micro-flow submodel is developed to compute the dry-out distance in rectangular capillary grooves. These submodels are incorporated into our previously developed system-level loop solver model to simulate the performance of a flat micro loop heat pipe. The integrated model predicts the thermal performance, evaporator surface temperature, and local and average heat transfer coefficients as a function of the applied heat load. The modeling results are verified by comparison with the experimental data for a similar device and a good agreement is obtained.

Interface engineering to enhance thermal contact conductance of evaporators in miniature loop heat pipe systems

While miniature loop heat pipes (mLHP) have significant potential for electronic cooling, they are only used in a narrow niche of applications, such as space or military. Complicated fabrication and system integration leading to high cost devices are the main culprit. To this end, this paper explores a low-cost sintering method for fabricating evaporators for mLHP that have increased heat transfer performance. Through this method, the porous wick of the evaporator is fabricated to partially fill the vapor collection channels embedded in the base plate of the evaporator. The sintering method employs an organic material used to define the vapor collection channels, which is sublimated at the end of the sintering process. Interpenetrating these two, otherwise distinctive, parts of the evaporator results in an increased contact area and thermal conductance. The heat transfer performance of an mLHP employing the new evaporator is compared to that of a system using a standard evaporator configuration, where the porous wick is rested against a flat base plate. It is found that the thermal contact conductance increases about 25%, depending on the applied heat load, while the total thermal resistance of the mLHP with the new evaporator decreases approximately by a factor of two.

Validation of an active magnetic regenerator test apparatus model

This work focuses on the development and validation of a transient one-dimensional numerical model of an active magnetic regenerator (AMR) test apparatus. Simulation results are validated by comparison to room temperature experiments for varying hot heat sink temperature, system pressure, and applied heat load. Three different second-order magnetocaloric materials are used. In addition to external heat leaks, parameters such as thermal conductivity, Curie temperature, and peak magnetocaloric effect are adjusted to obtain better fits to experimental results. In the case of gadolinium, where material properties are well-characterized, the inclusion of parasitic heat leaks as well as an increase in diffusivity resulted in good fits across a broad range of operating conditions. Adjustments to Curie temperature and peak magnetocaloric effect produced good matches with experimental data for Gd0.85Er0.15. Predictive simulations of a Gd – Gd0.85Er0.15 two-layer regenerator are briefly discussed.

Thermal performance of screen mesh wick heat pipes using water-based copper nanofluids

Fairly stable surfactant free copper–distilled water nanofluids are prepared using prolonged sonication and homogenization. Thermal conductivity of the prepared nanofluid displays a maximum enhancement of ∼15% for 0.5 wt% of Cu loading in distilled water at 30 °C. The wall temperature distributions and the thermal resistances between the evaporator and the condenser sections of a commercial screen mesh wick heat pipe containing nanofluids are investigated for three different angular position of the heat pipe. The results are compared with those for the same heat pipe with water as the working fluid. The wall temperatures of the heat pipes decrease along the test section from the evaporator section to the condenser section and increase with input power. The average evaporator wall temperatures of the heat pipe with nanofluids are much lower than those of the heat pipe with distilled water. The thermal resistance of the heat pipe using both distilled water and nanofluids is high at low heat loads and reduces rapidly to a minimum value as the applied heat load is increased. The thermal resistance of the vertically mounted heat pipe with 0.5 wt% of Cu–distilled water nanofluid is reduced by ∼27%. The observed enhanced thermal performance is explained in light of the deposited Cu layer on the screen mesh wick in the evaporator section of the heat pipe.

Evaporative heat transfer model of a loop heat pipe with bidisperse wick structure

A mathematical model of evaporative heat transfer in a loop heat pipe was developed and compared with experiments. The steady-state thermal performance was predicted for different sintered nickel wicks, including monoporous and bidisperse structures. The effect of wick pore size distribution on heat transfer was taken into consideration. The wick in the evaporator was assumed to possess three regions during vaporization from an applied heat load: a vapor blanket, a two-phase region, and a saturated liquid region. The evaporator wall temperature and the total thermal resistance at different heat loads were predicted using ammonia as the working fluid. The predictions showed distinct heat transfer characteristics and higher performance for the bidisperse wick in contrast with monoporous wick. A bidisperse wick was able to decrease the thickness of the vapor blanket region, which presents a thermal resistance and causes lower heat transfer capacity of the evaporator. Additionally, a validation test presented good agreement with the experiments.

Evaluation of permeability of ceramic wick structures for two phase heat transfer devices

This work reports on different approaches to predict the pressure drop in capillary structures for two phase heat transfer devices (TPHTD). For this purpose, several porous wick structures were produced by slip casting and characterized concerning their porosity, critical pore diameter, capillary pressure and permeability. The wick structures exhibited porosity in the range of 48–58%, pore diameter ranging from 1 μm to 4 μm, with capillary pressure of 80 and 22 kPa, respectively. Two expressions were developed to simulate the pressure drop permeabilities as a function of the heat load applied to the TPHTD evaporator. The prime target of this work is to evaluate the differences in predicting pressure drop just by using the Darcyan constant, k 1 , and both by using Darcyan and non-Darcyan constant, k 1 and k 2 . When k 1 was used only it was considered as Darcyan approach and when both constants were taken into account it was considered as Forchheimer approach. Additionally, ceramic wick structures have been tested as capillary evaporator in a TPHTD. The startup was successful after 20 min the inlet and outlet evaporator temperatures remain constant. It confirms that all applied heat load was transported to the condenser. ► We manufactured and characterized ceramic wick structures for CPL applications. ► We evaluated their permeability regarding Darcyan and non-Darcyan constants. ► The pressure drop in the CPL could be predicted by Darcyan and Forchheimer laws. ► We demonstrated a successful performance of a CPL at different operations conditions.

Advanced Conductive Composite Materials for Spacecraft Application

In this study, thermal responses of advanced fiber/epoxy matrix composite materials are considered for spacecraft thermal design. These thermal responses are important, because the localized thermal behavior from applied heat loads can induce thermal stresses, which can lead to functional failure of the spacecraft system. Since most of polymer matrices exhibit relatively poor thermal conductivity, the composite materials have been widely considered only for structural application and little for thermal application. However, recently pitch-based high performance carbon fiber becomes available and this fiber shows high thermal conductivity. Because of this combination of low CTE and high thermal conductivity, continuous carbon fiber composites make them suitable for thermal management of spacecraft. The advanced composite material is composed of a continuous high modulus pitch based fiber (YS90A) and DGEBA epoxy resin(RS3232). It was demonstrated that advanced composite material satisfied thermal requirement for a lightweight thermal radiator for heat rejection of communication satellite.

Evaluation of novel Ti-doped 3D carbon–carbon composites under transient thermal loads

3D Ti-doped and undoped carbon–carbon composites (CFCs) were exposed to transient thermal loads to simulate plasma disruptions, in the electron beam test facility JUDITH at different power densities and multiple shots in order to study the evolution in the behavior of the material. The thermal shock response of the undoped and Ti-doped materials was compared in order to study the influence of titanium carbide as dopant. The erosion itself is driven during the first shots by macroscopic erosion (brittle destruction), which is a result of thermally induced stresses. With increasing number of shots, no more brittle destruction is observed and the main erosion mechanism is sublimation due to local overheating. This is also confirmed by the decrease of the erosion rate with increasing the number of shots. The pitch fibers are hardly affected by the applied heat loads and they show almost no erosion, especially in the Ti-doped composite.

Improved performance of an engineering model cryogen free double adiabatic demagnetization refrigerator

This paper describes the design, development and performance of the engineering model double adiabatic demagnetization refrigerator (dADR) built and tested under contract to the European Space Agency for its former mission XEUS (now IXO). The dADR operates from a 4 K bath and has a measured recycle and hold time (with a parasitic load of 2.34 μW) at 50 mK of 15 h and 10 h, respectively. It is shown that the performance can be significantly improved by operating from a lower bath temperature and replacing the current heat switches with tungsten magnetoresistive (MR) heat switches, which significantly reduce the parasitic heat load. Performing the latter gives an anticipated recycle and hold time of 2 and 29 h (with a 1 μW applied heat load in addition to the parasitic load), respectively. Such improved performance allows for a reduction in mass of the dADR from 32 kg to 10 kg by operating from a 2.5 K bath (which could be reduced further by optimising the magnet design). Ultimately, continuous operation could be achieved by linking two dADRs to a common detector stage and operating them alternately. Based on this design the mass of the continuous ADR is estimated to be about 4.5 kg.

PORE NETWORK MODELS FOR THE STUDY OF TRANSFERS IN THE POROUS WICK OF LOOP HEAT PIPES

This paper presents an overview of pore network models for the study of vaporization phenomena occurring within a porous wick in relation with loop heat pipes (LHPs). Pore network models are a powerful tool to study two-phase flows in a porous structure. These models are based on a representation of the pore space in terms of a system of sites (pores) connected by throats (constrictions). First developed for studying steady-state regimes, pore network models are now used for the study of transient regimes in relation with the start-up phase of LHP and changes in the applied heat load during normal operation. Some open problems are also discussed.