Heat Input Levels Research Articles

A pulsating heat pipe embedded radiator: Thermal-vacuum characterisation in the pre-cryogenic temperature range for space applications

The use of Pulsating Heat Pipes (PHPs) in the space field is still an open issue because of the lack of data obtained during actual operation in relevant environment. A considerable amount of data is available in the literature on the thermal response of PHPs to a variable gravity condition and on the operation in the cryogenic field but in both cases the heat load at the condenser is rejected by convection to a constant temperature sink. On the other hand, barely any work in the PHP’s literature deals with thermal radiation to a low temperature sink as only heat transfer mode. The present work attempts to fill the gap by testing a PHP radiator (16 turns, 1.1 mm inner diameter, 50% filled with FC-72 at 293 K) in thermo-vacuum conditions in horizontal orientation at different heat loads and different environment (chamber) temperature. Fluid pressure measurements coupled with the frequency analysis characterised the effect of the cold source temperature on the device operational limits and efficiency. Results show that the device thermal performance in the radiative configuration is mostly affected by the lower operating temperatures needed to obtain a sensible heat rejection, rather than the heat transfer mode itself. The decrease of the environment temperature shortens the operational heat load range: the start-up occurs at higher heat input levels while the thermal crisis occurs at lower heat loads. The frequency analysis reveals that the equivalent thermal resistance is positively affected by higher values of the dominant frequency for all the cases.

Microstructure, mechanical properties and pitting corrosion of TIG weld joints alternative low-cost austenitic stainless steel grade 216

This present work aims to study 200 series austenitic stainless steel as a replacement for 300 series austenitic stainless steel. This is due to 200 series austenitic stainless steel being inexpensive and also due to its popularity in Southeast Asia. This study investigated the behavior and properties of the weld metal produced from 216 grade austenitic stainless steel. The microstructure, mechanical properties and pitting corrosion resistance that were produced by the tungsten inert gas welding process (TIG) with filler metal ER316 are studied in this paper. The variable parameters of the welding process were heat inputs at high (0.78 kJ/mm), medium (0.52 kJ/mm) and low (0.39 kJ/mm) ranges. It was found that the weld metal with a high heat input level increased the width of the face weld, root weld, PMZ and HAZ. However, the opposite result was achieved when the heat input was reduced. The microstructure of all the weld metals formed delta-ferrite content in the austenite matrix. This was shown to be greater in the weld metal with a low heat input, under the solidification mode FA. The results were shown to agree with the predictions outlined in the Schaeffler diagram. In addition, it was discovered that a high heat input level produced a more discontinuous form of delta-ferrite content than for the low heat input model. This corresponded to the dendrite length and inter-dendrite spacing in the weld zone. The hardness, ultimate strength and elongation tended to decrease when the heat input increased. The fractures for all the weld metals were found to occur in the location of the weld zone. The pitting corrosion resistance increased when the heat input was reduced, which related to the volume of delta-ferrite content in the weld metal zone. This corresponds to the pitting corrosion resistance in the weld metals being higher than for the base metal material. Furthermore, it was found that pitting occurred initially at the interface between the delta-ferrite and austenite phase and the corrosion then spread into the austenite phase.

Microstructure of In-Situ Friction Stir Processed Al-Cu Transition Zone

The majority of literature sources dedicated to dissimilar Al-Cu friction stir welding testifies to the formation of intermetallic compounds (IMC) according to diffusion-controlled reactions, i.e., without liquation on the Al/Cu interfaces. Fewer sources report on revealing Al-Cu eutectics, i.e., that IMCs are formed with the presence of the liquid phase. This work is an attempt to fill the gap in the results and find out the reasons behind such a difference. Structural-phase characteristics of an in-situ friction stir processed (FSP) Al-Cu zone were studied. The single-pass FSPed stir zone (SZ) was characterized by the presence of IMCs such as Al2Cu, Al2Cu3, AlCu3, Al2MgCu, whose distribution in the SZ was extremely inhomogeneous. The advancing side SZ contained large IMC particles as well as Al(Mg,Cu) solid solution (SS) dendrites and Al-Al2Cu eutectics. The retreating side SZ was composed of Al-Cu solid solution layered structures and smaller IMCs. Such a difference may be explained by different levels of heat input with respect to the SZ sides as well as by using lap FSP instead of the butt one.

Controlling the porosity using exponential decay heat input regimes during electron beam wire-feed additive manufacturing of Al-Mg alloy

Electron beam wire-feed additive manufacturing is given less attention in research community compared with other additive manufacturing methods, despite it allows higher deposition rate and less porosity. However, both gas and shrinking porosity are still met even with this method especially if applied to light alloys. The excess heat input may be the reason for evaporation of volatile metals, and forming the gas pores especially in the top layers of the built sample where cooling rate is reduced. Therefore, exponential decay heat input was used to grow AA5356 samples. Microstructural and mechanical characterization of the samples obtained at mean low, medium, and high heat input levels was carried out. An optimal heat input gradient was determined which allowed growing a defect-free metal in the bottom part of the sample and forcing out the shrinkage cavities to the top part. Shrinkage and gas pore structure formation was analyzed as a function of the heat input gradient and along the building direction. Gas pores resulted at two higher heat input regimes as a result of evaporation of magnesium as supported by the results of EDS and XRD. Tensile test showed the ultimate strength of the defect-free part equal to that of the base AA5356.

Effect of working fluid inventory and heat input on transient and steady state behavior of a thermosyphon

The effect of working fluid inventory and heat input on the performance of thermosyphon is investigated in this paper. First, a thermosyphon with diameter of 16 mm made of steel with toluene as working fluid and the length of the evaporator, adiabatic and condenser section 10, 23 and 17 cm was considered, respectively. The working fluid inventory was considered 0.1, 0.14, 0.18 and 0.24 of total volume of thermosyphon and in each case 54–235 W of heat input applied to the evaporator area also. In order to evaluate the transient behavior of the thermosyphon, in any amount of working fluid, 28 W of heat input was considered and temperature of the evaporator area were recorded at different times. The results showed that in startup, in the values of the working fluid equal to 0.14, 0.18 and 0.24, the evaporator temperature has fluctuated behavior that is indicative of the occurrence of geyser boiling phenomenon. When the volume of the working fluid was more than 0.24, thermosyphon performance was associated with vibration, indicating a high fluidity and a lack of proper functioning of the thermosyphon. The amount of optimal working fluid with respect to the total thermal resistance of thermosyphon is equal to 0.18 that in this case, the efficiency of thermosyphon at different levels of heat input is between 73 and 78%. In addition, in any amount of working fluid, increase of heat input leads to increment in the evaporator and condenser heat transfer coefficient increases and reduction in the total thermal resistance of thermosyphon.

Investigation on the Pulsed TIG Welding of Nickel-Base Alloy for Improved Microstructure and Weld Penetration

In this study, a novel pulsed tungsten inert gas (TIG) welding technique that is capable of effectively improving the weldabilities of nickel-based alloys was developed.  The correlation between process parameters and material properties was studied. The effects of ratios of pulse to background welding currents on hardness and penetration in welds and heat-affected zones (HAZs) of Inconel 718 alloy were investigated experimentally by using the pulsed TIG welding technique. It is found that with an increasing ratio of the pulse to background welding current, the hardness in welds increased from 2% to 3.5%, compared to that in constant current TIG welding at the same level of heat input. When the ratio of pulse to background welding current is higher than 6, the penetration in weld was improved significantly by up to 61%. Research work has demonstrated that the pulsed TIG welding technique is an effective approach to control the microstructure and reduce cracking susceptibilities.

Correlation Among the Input Thermal Parameters and Thermography Measurements Data of the Resistance Seam Welding

Abstract Resistance Seam Welding (RSEW) is commonly used in continuous steel processing for joining steel coils. Experimental studies focused on the welding process quality controls are justified by their wide application and influence on the performance of steelmaking processes. The present study focuses on the continuous annealing line welding process using as sample, a low carbon steel (0.07%C) with 0.21 mm thickness. The work proposes a correlation of thermal input, electric current parameter and thermogram monitoring during RSEW procedures. A methodology was developed to determine the real thermal profiles values by means of association of the thermograms with the sample emissivity curve. Three heat input levels were used, 3.64 J.mm-1, 5.09 J.mm-1, and 6.38 J.mm-1, respectively, for typical C-Mn steel. The thermal welding profile obtained experimentally exhibited linear correlation coefficients (R) of 0.99 with the electric current welding parameter and (R) of 0.99 with the calculated thermal input, respectively. The welds metallurgical characterizations results, expressed through the ductility and welds nuggets analysis, were conducted to correlate the welds quality with the welding thermal input. The Erichsen cupping test showed that the higher the thermal input corresponds to higher ductility of the welds. Nevertheless, the heat input of 5.09 J.mm-1 indicated better-balanced properties: ductility, weld nugget discontinuity, nugget size, and no expulsion of the melt material.

Effect of Heat Input on Weld Formation and Tensile Properties in Keyhole Mode TIG Welding Process

Keyhole mode Tungsten Inert Gas (K-TIG) welding is a novel advanced deep penetration welding technology which provides an alternative to high power density welding in terms of achieving keyhole mode welding. In order to facilitate welding procedure optimisation in this newly developed welding technology, the relationship among welding parameters, weld formation and tensile properties during the K-TIG welding was investigated in detail. Results show that except for travel speed, the heat input level also plays an important role in forming undercut defect by changing the plasma jet trajectory inside keyhole channel, leading to the formation of hump in the weld centre and exacerbation of undercut formation. Both undercut defect and root side fusion boundary can act as a stress concentration point, which affects the fracture mode and tensile properties considerably. The research results provide a practical guidance of process parameter optimisation and quality assurance for the K-TIG welding process.

RESEARCH OF PROPERTIES OF THE METAL OF THE HEAT-AFFECTED ZONE FOR SURFACING BY STAPE ELECTRODE WITH THE CONTROLLED TRANSFER OF THE ELECTRODE METAL

The quality of the deposited layer is determined by the conditions of its formation, as well as the properties of local zones, due to the parameters of the thermal cycle of surfacing (mechanical, microstructural, chemical, etc.). When surfacing the austenitic layer on pearlitic steels, the properties of the heat-affected zone, along with the resistance of the surface layer to corrosion, largely determine the performance and durability of the deposited product. To reduce the amount of heat input and effectively control the structure and properties of the heat-affected zone when restoring working surfaces by submerged arc surfacing, a method for surfacing with a tape electrode with controlled mechanical transfer of electrode metal is proposed. Studies have shown that the imposition of longitudinal sinusoidal vibrations on the end of the tape electrode can reduce the level of heat input in the heat-affected zone by increasing the efficiency of fusion of the base metal. At the investigated frequency of 50 Hz, the duration of the dots on the boundary of the fusion line and below it in the range 900 ... .1100 °С is less than when surfacing without controlled mechanical transfer. Reducing heat input into the heat-affected zone in the temperature range 900 ... 1100 °С provides a finer-grained structure. The use of forced vibrations of the end of the tape when surfacing a high-alloy austenitic layer on low-carbon steel makes it possible to obtain a finer-grained structure at the fusion boundary in the heat-affected zone. This will increase the resistance of the heat-affected zone to the formation of defects associated with a decrease in its mechanical properties.

Influence of the heat input and aging treatment on microstructure and mechanical properties of AISI 317 L steel weldments using 0020 robotic–pulsed GMAW

In this work, the effect of heat input and aging treatment on the microstructural characteristics and mechanical properties of similar AISI 317L austenitic stainless steel weldments used in the petroleum industry was investigated. The filler metal used was the AWS ER317L electrode at two different heat input levels (4 and 8 kJ/cm) in order to verify the influence of this parameter on the precipitation of deleterious phases. The specimens were aged at 700 °C for 50 and 100 h. Quantification and microchemical mapping of precipitated phases after welding and aging thermal treatment (ATT) were performed. Vickers hardness and tensile tests were used to evaluate the mechanical properties. It was observed that aging promoted a refinement of the base metal region, and all delta ferrite was transformed into sigma phase. The delta ferrite present in the fusion zone was completely transformed into sigma and chi phases. In the aged specimens for 100 h a lower occurrence of the secondary austenite phase (γ2) was identified, which indicates that with the increase of ATT time the dissolution of γ2 occurred in the already precipitated sigma phase. All welding conditions showed an increase in tensile strength, yield limit and hardness with the ATT.

Developing flow pattern maps for accelerated two-phase capillary flows

The prediction of flow pattern transitions is extremely important to understand the coupling of thermal and fluid dynamic phenomena in two phase systems and it contributes to the optimum design of heat exchangers. Two phase flow regimes have been extensively studied under controlled mass flow rate and velocity. On the other hand, less effort has been spent in the literature on the cases where the flow motion is purely thermally induced and consequently the mass flow rate or the velocity of the phases are not known a priori. In the present work, flow pattern transitions and bubble break-up and coalescence events have been investigated in a passive two phase wickless capillary loop, where the mass flow rate is intrinsically not controllable. Modified Froude, Weber and Bond numbers have been introduced, considering the actual acceleration of the fluid and the length of the bubble as merit parameters for the transitions. The proposed nondimensional investigation was developed by analysing experimental data obtained with ethanol and FC-72, as working fluids, different heat input levels (from 9 to 24 W) as well as three different gravity levels (through a parabolic flight campaign). A new empirical diabatic flow pattern map for accelerated two-phase capillary flows is presented, together with quantitative criteria for the calculation of the flow regime transitions, defining the physic limits for the bubble coalescence and break-up. This kind of new regime maps will be useful to the further development of comprehensive designing tools for passive two-phase wickless heat transfer devices.

Investigations on the microstructure and mechanical properties of laser welded dissimilar galvanized steel–aluminum joints

In this study, galvanized high-strength steel and aluminum alloy sheets were laser-welded in zero-gap lap joint configuration. In order to determine the influences of the heat input levels, microstructural and mechanical properties of the joints were investigated. The weld bead geometry, microstructure, and intermetallic phases at the interface of welded joints were investigated using an optical microscope and scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS) at different heat input levels. Mechanical properties and microhardness distribution of the welded joints were examined according to the weld bead dimension. The results revealed that there is a correlation between the weld seam geometry and intermetallic phase formation. At relatively high heat input level, the penetration depth increased, and thick Al-rich intermetallic layer was observed at the interface of the weld seam, which deteriorated the tensile strength of the joint. It has been found that without the need for any additional precaution, the thickness of the IMC layer can be limited to 5–15 μm when the optimum welding parameters were conducted. The experimental results showed that with limited heat input and penetration depths, up to 108.7-N/mm tensile strength could be achieved and fracture initiated at the weld seam–steel interface.

Thermal Performance of Pulsating Heat Stripes Built With Plastic Materials

A low-cost, flexible pulsating heat pipe (PHP) was built in a composite polypropylene sheet consisting of three layers joint together by selective laser welding, to address the demand of heat transfer devices characterized by low weight, small unit thickness, low cost, and high mechanical flexibility. A thin, flexible, and lightweight heat pipe is advantageous for various aerospace, aircraft, and portable electronic applications where the device weight, and its mechanical flexibility are essential. The concept is to sandwich a serpentine channel, cut out in a polypropylene sheet and containing a self-propelled mixture of a working fluid with its vapor, between two transparent sheets of the same material; this results into a thin, flat enclosure with parallel channels hence the name “pulsating heat stripes” (PHS). The transient and steady-state thermal response of the device was characterized for different heat input levels and different configurations, either straight or bent at different angles. The equivalent thermal resistance was estimated by measuring the wall temperatures at both the evaporator and the condenser, showing a multifold increase of the equivalent thermal conductance with respect to solid polypropylene.

Effect of Welding Parameters on Junction Structure and Fracture Behavior of Friction-Stir-Welded Joints of 2024 Aluminum Alloys

In this paper, the effect of welding speed and tool rotation speed on the junction structure and fracture behavior of friction-stir-welded joints was investigated with use of 2024 aluminum alloys of 3.2 mm thick. Results show that the formation of junction structure is more affected by tool rotation speed than by welding speed due to the differences of heat input levels. When welding speed is at 100 mm/min and tool rotation speed is in the range of 400-600 r/min, the joint junction structure causes complete bonding with no welding defects. While under improper welding parameters, the junction structure causes the precipitate aggregation on the bonding line and the partial bonding with the appearance of both tunnel defects and kissing bond defects. Meanwhile, the results of tensile tests show that the tensile fracture of all FSW joints occur at the weld center. The failure mode of joints with defects-free junction structure characterized by shear fracture is ductile. While the failure mode of joints with defects-existent junction structure characterized by layered fracture surface is brittle. Furthermore, that heterogeneous microstructure, precipitate aggregation and welding defects converge inside the junction structure is the main factor affecting the tensile properties and fracture behavior of the friction-stir-welded joint.

Design, Fabrication, and Characterization of a Compact Hierarchical Manifold Microchannel Heat Sink Array for Two-Phase Cooling

High-heat-flux removal is critical for the next-generation electronic devices to reliably operate within their temperature limits. A large portion of the thermal resistance in a traditional chip package is caused by thermal resistances at interfaces between the device, heat spreaders, and the heat sink; embedding the heat sink directly into the heat-generating device can eliminate these interface resistances and drastically reduce the overall thermal resistance. Microfluidic cooling within the embedded heat sink improves the heat dissipation, with two-phase operation offering the potential for dissipation of very high heat fluxes while maintaining moderate chip temperatures. To enable multichip stacking and other heterogeneous packaging approaches, it is important to densely integrate all fluid flow paths into the device; volumetric heat dissipation emerges as a performance metric in this new heat sinking paradigm. In this paper, a compact hierarchical manifold microchannel design is presented that utilizes an integrated multilevel manifold distributor to feed coolant to an array of microchannel heat sinks. The flow features in the manifold layers and microchannels are fabricated in silicon wafers using deep reactive-ion etching. The heat source is simulated via Joule heating using thin-film platinum heaters. The on-chip spatial temperature measurements are made using four-wire resistance temperature detectors. The individual manifold layers and the microchannel-bearing wafers are diced and bonded into a sealed stack via thermocompression bonding using gold layers at the mating surfaces. Thermal and hydrodynamic testing is performed by pumping the dielectric fluid HFE-7100 through the device at a known flow rate, temperature, and pressure at different levels of chip heat input. A volumetric heat density of up to 2870 W/cm3 is dissipated at a chip temperature less than 112 °C and microchannel pressure drop less than 27 kPa. The overall pressure drop is governed by flowing through the manifold, rather than the microchannels, in this compact heat sink that occupies envelope of 5 mm $\times 5$ mm $\times2.3$ mm including all the functional flow features.

Microstructural evolution and chemical corrosion of electron beam wire-feed additively manufactured AISI 304 stainless steel

Using an electron beam wire-feed additive manufacturing three sets of samples were obtained at different heat input levels. The samples contained different amounts of vermicular δ-ferrite that depended on the heat input value used. Chemical corrosion tests were carried out to find out that the corrosion resistance of a sample is determined by the δ-ferrite amount in it. High δ-ferrite content in the sample obtained at medium used heat input value caused enhanced corrosion in the Cr-depleted austenite grains. Otherwise, when using the maximum heat input the additively manufactured sample contained the minimum amount of δ-ferrite and demonstrated the highest corrosion resistance as well as formation of a cubic crystallographic texture. No such texture was found in samples grown at lower heat input values. In addition, no inter-granular corrosion induced open cracks have been observed on all samples after 90° angle flexural test.

Metallurgical, impact and fatigue performance of electron beam welded duplex stainless steel joints

Fusion zones of electron beam welded (EBW) 2205 duplex stainless steel (DSS) joints pertaining to two different levels of heat input, exhibited variable microstructural morphology as well as ferrite-to-austenite (α/γ) ratio. Both the weld zones showed phase imbalance with ferrite being the dominating phase over austenite. However, as governed by the associated thermal cycles experienced by these welds, low heat input weld metal exhibited a relatively higher proportion of austenite as compared to the high heat input weld. Thermal aging at 850 °C/0.5 h promoted the formation of Cr and Mo-rich intermetallic σ-phase and carbides in the welds, whereas welding resulted into Cr2N precipitation, which consequentially influenced their impact toughness and fatigue crack growth behavior. The aged joints showed a severe degradation in their impact toughness, the extent of which was more in the base metal than in the weld metal. The role played by the prevalence of secondary phases, unlike impact toughness degradation, was not detrimental but rather beneficial, as aged weld metals exhibited significantly superior fatigue crack growth resistance as compared to their unaged counterparts, which probably could be attributed to dispersion strengthening that occurred due to intermetallic precipitation.

Start-up in microgravity and local thermodynamic states of a hybrid loop thermosyphon/pulsating heat pipe

A wickless passive two phase closed loop heat transfer device especially designed for a future implementation on the heat transfer host module of the International Space Station is tested in relevant environment on board a parabolic flight. The tube internal diameter (3 mm) is larger than the static capillary threshold evaluated in normal gravity for this working fluid (FC-72), leading the device to work as a loop thermosyphon on ground and in hyper-gravity conditions, and as a Pulsating Heat Pipe when micro-gravity occurs. Novel start up tests, where the heat load has been provided after the occurrence of microgravity, show that the 20 s microgravity period is enough for the device activation and, most important, that the device activation is purely thermally induced and not affected by the previous acceleration field. Two miniaturized pressure transducers and direct fluid temperature measurement via two micro-thermocouples, allow to provide a detailed insight on the fluid local thermodynamics states both in the evaporator and in the condenser zone during microgravity. It is shown that the two-phase fluid close to the evaporator and the condenser is subjected to several degrees (up to 5 K) of superheating or subcooling. The level of subcooling seems to increase with the heat input level both in terms of temperature difference and in terms of percentage time with respect to the whole microgravity period.

The effect of pulsed Nd:YAG laser welding parameters on defects of Kovar to AISI 304L dissimilar joint

Kovar, due to its close thermal expansion coefficient to glass, can be used as a mediating alloy in joint of glass to metals such as AISI 304L. In the present research, the effect of parameters of pulsed laser on dissimilar welding of Kovar to AISI 304L has been investigated. For observation and investigation of the results, metallography, Energy dispersive X-ray spectroscopy (EDS) and microhardness tests have been used. Results obtained from microstructural examinations, indicated the existence of solidification crack and lack of complete mixing in the weld metal. According to the results, the heat input must have an optimum value equaling approximately 19.1 J/mm. At higher levels such as 32.6 J/mm, the TCL (total crack length) value reached 760 μm. Increasing the heat input lead to increasing thermal stresses and formation of crack in the weld metal. On the other hand, at heat input levels less than the value of 15 J/mm, cracks are eliminated and non-mixing is clearly observable because sufficient time is not provided for mixing and, then, solidification for the weld metal. The weld metal hardness is also higher than the base metals due to the fine dendrite structure and the presence of hardening elements.

Effect of filling ratio and orientation on the performance of a multiple turns helium pulsating heat pipe

A multi-turn Pulsating Heat Pipe (PHP) made of stainless steel (I.D. 0.5 mm, O.D. 0.8 mm), filled with helium is experimentally investigated. The lengths of the evaporator, adiabatic and condenser sections are 50 mm, 100 mm and 50 mm, respectively. The study focuses on the effect of the filling ratio and the inclination angle (gravity) at different heat input levels on the thermal performance of the cryogenic PHP. Results show that the obvious temperature fluctuation occur at low filling ratios, such as 16.4%, 31.8% and 48.8%. Despite the relatively high number of turns, the gravity still plays a critical role on the PHP thermal performance, especially at medium filling ratio (i.e. 48.8%). At a filling ratio of 31.8%, 66.1% and 94.2%, the thermal performance in 60° orientation is the same as that in vertical orientation. The optimal filling ratio may be within the range between 48.8% and 66.1% and related to the heat input level.