Constant Heat Input Research Articles

A study on the quality of two-wire tandem submerged arc welds under iso-heat input conditions

The influence of the leading and trailing arc welding current on two-wire tandem submerged (SAW-T) arc weld quality was studied at a iso-heat input of 2.5 kJ/mm. The weld quality includes weld bead dimensions, cooling time from 800 to 500 °C in the weld pool, weld bead ferrite phase fraction, and micro-hardness. The cooling time was calculated from the temperature distribution predicted using the numerical model. For a constant heat input, the weld penetration significantly reduced with decreasing leading arc current at a low level of trailing arc current while the final weld penetration was less sensitive to the leading arc current at higher trailing arc current. Significant variation of the average weld pool cooling time from 800 to 500 °C was not observed with increasing leading arc current from 700 to 1000 A at a constant trailing arc current and heat input of 1000 A and 2.5 kJ/mm. However, minimum weld pool cooling time was observed at higher currents. An increase of the leading or trailing arc current at a constant heat input yielded a minor rise of the weld pool acicular ferrite phase volume fraction and decreased the amount of allotriomorphic ferrite phase, leading to a small improvement in the weld bead micro-hardness. In the present study, the welds produced at a leading and trailing arc current of 1000 A and welding speed of 26.80 mm/s displayed better quality weld beads with high productivity.

Thermoacoustic sound generation under the influence of resonator curvature

This work discusses the effect of resonator curvature on the thermoacoustic effect. The introduction of curvature to the design of thermoacoustic devices has important implications to the packaging of future devices. Although some previous CFD studies have attempted to quantify the effect of this curvature, there are several mechanisms that occur in thermoacoustic engines that are difficult to model using CFD, so further work is needed. A standing wave engine design is utilized in the study with a square channel ceramic stack. The temperatures on both sides of the stack and the pressure at the antinode are examined under the influence of curvature. Increased curvature is shown to adversely effect sound pressure level and increase temperature difference across the stack at constant heat input. A close relationship is shown between the temperature variation across the stack and sound pressure level.

Experimental investigation on heat transfer enhancement from a heated circular cylinder with constant heat input power in turbulent pulsating crossflows

Experiments were carried out to study the heat transfer performance of a heated circular cylinder in turbulent pulsating crossflows. For a single heated circular cylinder, heat transfer enhancement factors up to 1.26 were observed in the studied parameter range. Two empirical correlations with different deviations were developed. The heat transfer enhancement factor was found to decrease with Strouhal number and Reynolds number, but increase in trend with the ratio of pulsating to steady Reynolds number.

Thermal response of a closed loop pulsating heat pipe under a varying gravity force

A Closed Loop Pulsating Heat Pipe made of a copper tube bent into a 2-D serpentine of 32 parallel channels and filled with FC-72, has been tested on ground and in micro/hyper gravity conditions during the 58th ESA Parabolic Flight Campaign. The device has been investigated both in horizontal and in vertical positions and at different heat loads (from 40 to 100 W). Beyond the standard thermal characterization, dynamic investigations have been performed on ground by changing the device orientation at constant heat input levels. Results show that in the vertical position the PHP thermal behavior is strongly affected by the variation of gravity field both on ground and on flight tests. In particular, during a parabolic flight, the first hypergravity period slightly assists the flow motion, while, during microgravity, the device undergoes a sudden temperature increase in the evaporator zone; the following hypergravity phase is then able to bring the PHP back to the previous thermal regime. The PHP in the horizontal position does not show any sensitive thermal variation during the parabola. A further analysis with a tilting bench in the ground lab proves that microgravity thermal behavior is comparable to the horizontal operation on ground: therefore for capillary closed loop pulsating heat pipes microgravity tests are not strictly necessary for the space application assessment.

Theoretical analysis of wall thermal inertial effects on heat transfer of pulsating laminar flow in a channel

The effects of wall heat capacity on heat transfer of pulsating laminar flow with constant heat input between two parallel plates are theoretically investigated. The analytical solution of the fully developed thermal profile under constant wall heat flux is obtained. The results show that both the fluid temperature and Nusselt number fluctuate periodically as the driven pressure pulsates. The effects of the wall thermal inertia and Prandtl number, as well as pulsation amplitude and frequency on the fluctuations are investigated. Compared with pulsating flow without considering wall thermal inertia, the fluctuation amplitude of the fluid temperature and Nusselt number is smaller while the mean temperature difference is larger. The average Nusselt number, for both considering and neglecting wall thermal inertia, is reduced by pulsating flow. The reduction is up to 10% and cannot be neglected.

Influence of welding speed and power on residual stress during gas tungsten arc welding (GTAW) of thin sections with constant heat input: A study using numerical simulation and experimental validation

The temperature distribution and residual stresses for a GTAW circumferential butt joint of AISI 304 stainless steel using numerical simulation have been evaluated. For evaluation of weld induced residual stresses, the analysis of heat source fitting was carried out with heat inputs ranging from 200 to 500J/mm to arrive at optimal heat input for obtaining proper weld penetration and heat affected zone (HAZ). For this chosen heat input, the influence of different weld speeds and powers on the temperature distribution and the residual stresses is studied. The heat source analysis revealed the best choice of heat input as 300J/mm. The residual stresses on the inner and outer surfaces, and along the radial direction were computed. Increase in temperature distribution as well as longitudinal and circumferential residual stresses was observed with the increase in weld speed and power. The validity of the results obtained from numerical simulation is demonstrated with full scale shop floor welding experiments.

Analytical modelling of boiling phase change phenomenon in high-temperature proton exchange membrane fuel cells during warm-up process

This paper investigates the thermal and water balance as well as the electro-kinetics during the warm-up process of a Hydrogen/Oxygen high-temperature proton exchange membrane fuel cell (HT-PEMFC) from room temperature up to the desired temperature of 180 °C. The heating strategy involves the extraction of constant current from the fuel cell, while an external heating source with a constant heat input rate is applied at the end plates of the cell simultaneously. A simple analytical unsteady model is derived addressing the boiling phase changing phenomenon in the cathode catalyst layer (CCL) and cathode gas diffusion layer (CGDL) of the cathode that occurs when the temperature of the fuel cell reaches the boiling temperature of water. Parameters such as the heat input rate, extracted current, cathode pressure and cathode stoichiometric flow ratio are varied and their effects on the temperature, liquid water fraction and most importantly, the voltage profiles with respect to time, are explored. A comparison between other existing heating strategies using the model suggests that there is insignificant improvement in warm-up time when current is extracted from room temperature considering a single cell. However, considering the solution for a typical 1-kW stack suggests that reductions in warm-up time and energy consumption can be expected. In addition, the results show that boiling phase change is found to be a key factor that affects the level of water saturation in the porous media such as the CCL and CGDL during the warm-up process, when current is extracted from the start of the process i.e. room temperature. However, the energy consumption due to boiling phase change is found to be negligible as compared to external heating input rate. The parametric studies show that the variation of heat input rate, extracted current and cathode pressure have significant effect on the cell voltage that is strongly dominated by the liquid water fraction in the porous media. On the other hand, the variation of cathode stoichiometric flow ratio is found to have minimal effect on the output cell voltage. The parametric studies also indicate that boiling phase change is present for a significant period of time under typical operating conditions.

Modern fiber laser beam welding of the newly-designed precipitation-strengthened nickel-base superalloys

In the present research, the modern fiber laser beam welding of newly-designed precipitation-strengthened nickel-base superalloys using various welding parameters in constant heat input has been investigated. Five nickel-base superalloys with various Ti and Nb contents were designed and produced by Vacuum Induction Melting furnace. The fiber laser beam welding operations were performed in constant heat input (100 J mm−2) and different welding powers (400 and 1000 W) and velocities (40 and 100 mm s−1) using 6-axis anthropomorphic robot. The macro- and micro-structural features, weld defects, chemical composition and mechanical property of 3.2 mm weldments were assessed utilizing optical and scanning electron microscopes equipped with EDS analysis and microhardness tester. The results showed that welding with higher powers can create higher penetration-to-width ratios. The porosity formation was increased when the welding powers and velocities were increased. None of the welds displayed hot solidification and liquation cracks in 400 and 1000 W welding powers, but liquation phenomenon was observed in all the heat-affected zones. With increasing the Nb content of the superalloys the liquation length was increased. The changing of the welding power and velocity did not alter the hardness property of the welds. The hardness of welds decreased when the Ti content declined in the composition of superalloys. Finally, the 400 and 1000 W fiber laser powers with velocity of 40 and 100 m ms−1 have been offered for hot crack-free welding of the thin sheet of newly-designed precipitation-strengthened nickel-base superalloys.

Experimental investigation on heat transfer enhancement from an inclined heated cylinder with constant heat input power in infrasonic pulsating flows

Experiments were carried out to study the heat transfer performance of an inclined heated cylinder in pulsating flows. An empirical equation was developed that correlates the heat transfer enhancement factor to the infrasonic pulsating frequency (f=6–20Hz), Reynolds number (Re=71–282), pressure amplitude (prms=9–22Pa), and inclination angle (θ=0–90°). The heat transfer enhancement factor was found to decrease with the Strouhal number, and increase in trend with the inclination angle. In addition, a critical frequency (fcr≈14–18Hz) was observed, below which the heat transfer enhancement factor decreases in trend with the Reynolds number, but it increases in trend with the Reynolds number when the frequency is above the critical frequency.

Residual stresses in laser welded ASTM A387 Grade 91 steel plates

Residual stresses in 9mm thick ASTM A387 Grade 91 steel plates, joined using constant power (8kW) low and high heat input laser welding processes, are characterised using neutron diffraction. The measured longitudinal and normal components of residual stress show a bimodal distribution across the welded joint with a low tensile or compressive trough at the weld centre flanked by high magnitude tensile peaks in parent metal adjacent to the heat affected zone boundaries. The width of the central trough and spread of the outboard tensile zones are significantly greater for the high heat input weld. In both cases, the stress distributions can be explained by the strains associated with the austenite to martensite solid-state transformation as the joint cools after welding.

Calculation of the cross-sectional area of the welded joint in automatic welding and surfacing

The results are presented of the investigation of the effect of the surfacing parameters on the penetration area and the thermal efficiency in the conditions of constant heat input in automatic deposition of a bead on a thick component. Correction coefficients used to determine by calculations the penetration area using the well-known and corrected Rykalin equation for a spot constantly acting rapidly moving heat source on the surface of a thick component are derived.

Thermal and hydrodynamic characteristics of supercritical CO2 natural circulation in closed loops

Natural convective flow of supercritical fluids has become a hot topic in engineering applications. Natural circulation thermosyphon using supercritical/trans-critical CO2 can be a potential choice for effectively transportation of heat and mass without pumping devices. This paper presents a series of numerical investigations into the fundamental features in a supercritical/trans-critical CO2 based natural circulation loop. New heat transport model aiming at trans-critical thermosyphon heat transfer and stability is proposed with supercritical/trans-critical turbulence model incorporated. In this study, the fundamentals include the basic flow and heat transfer behavior of the above loop, the effect of heat source temperature on system stability, the effect of loop diameter on natural convection supercritical CO2 loop and its coupling effect with heat source temperature and the effect of constant changing heat input condition and system behavior evolution during unsteady input or failure conditions. The fundamental potentials of supercritical/trans-critical CO2 based natural convection system are confirmed. Basic supercritical CO2 closed loop flow and heat transfer behaviors are clarified. During this study, the CO2 loop stability map are also put forward and introduced as an important feature of supercritical CO2 system. Stability factors of natural convective trans-critical CO2 flow and its implications on real system control are also discussed in this paper.

COMPARISON OF THE EFFECTS OF DIFFERENT TYPES OF TUBE INSERTS ON TWO-PHASE FLOW INSTABILITIES

Turbulence promoting inner elements such as rings, wings or winglets, twisted tapes, etc., is widely used in single- and two-phase flows. In this experimental work, the two-phase flow instabilities of forced convection boiling in a horizontal tube with ring- and twisted-tape-type inner elements inside are investigated. In order to analyze the two-phase flow of water, the experiments were conducted under a constant system pressure of 7.5 bar, and a constant heat input of 24 kW was applied to the outer surface of the circular test pipe with constant exit restriction. Three different inlet temperatures (Ti = 15, 25, and 35°C) were used to see the effect of inlet sub-cooling, and the mass flow rate was decreased from 110 to 20 g/s to obtain the characteristic curve. Two different pitch ratios for both the ring and twisted tape were evaluated in terms of stability. The results were evaluated in terms of appropriate variables for both the tube with insertions and the smooth tube. The analysis shows that the pressure drop increases with decreasing mass flow rate for all cases. The presence of the inner elements in the test pipe causes the boundaries of the oscillations to narrow in comparison with smooth tube. On the other hand, with the increase in the effective diameter of the test pipe the boundaries of the oscillations broaden. For each type of insertion, the increase of the pitch ratio makes the system less stable.

C04 スターリングエンジン用急速混合型管状火炎バーナの開発(熱交換器及び燃焼器等,構成要素機器の基本特性並びに機器性能との関連)

As a power source for emergency power supply, a Stirling engine has received keen interest due to its high thermal efficiency as well as its fuel diversity. In this study, a tubular flame burner has been developed for heating the head of a Stirling engine. To avoid flame flashback rapidly-mixed type tubular flame burner, in which fuel and oxidizer are separately injected, has been adopted. The fuel used is a biogas, which consists of 60% methane and 40% carbon dioxide by volume. Results show that stable combustion can be obtained for a wide range in equivalence ratio and in heat input. With a decrease of equivalence ratio, the head temperature, electric power generation and thermal efficiency are increased for a constant heat input, and a maximum thermal efficiency of about 8% has been achieved.

Effect of channel and plenum aspect ratios on the performance of microchannel heat sink under different flow arrangements

Microchannels based heat sinks are considered as potential thermal management solution for electronic devices. The overall thermal performance of a microchannel heat sink depends on the flow characteristics within microchannels as well as within the inlet and outlet plenum and these flow phenomena are influenced by channel aspect ratio, plenum aspect ratio and flow arrangements at the inlet and outlet plenums. In the present research work an experimental investigation has been carried out to understand how the heat transfer and pressure drop attributes vary with different plenum aspect ratio and channel aspect ratio under different flow arrangements. For this purpose microchannel test pieces with two channel aspect ratios, 4.72 and 7.57 and three plenum aspect ratios, 2.5, 3.0 and 3.75 have been tested under three flow arrangements, namely U-, S- and P-types. Test runs were performed by maintaining three constant heat inputs, 125 W, 225 W and 375 W in the range 224.3 ≤ Re ≤ 1121.7. Reduction in channel width (increase in aspect ratio, defined as depth to width of channel) in the present case has shown about 126 to 165% increase in Nusselt number, whereas increase in plenum length (reduction in plenum aspect ratio defined as width to length of plenum) has resulted in 18 to 26% increase in Nusselt number.

Calculating the cross-sectional area of the welded joint in automatic welding and surfacing

The results are presented of the investigations of the effect of the surfacing parameters on the penetration area and thermal efficiency in the conditions of constant heat input in automatic deposition of a bead on a thick component. The correction coefficients for calculating the penetration area using the well-known and corrected Rykalin equation for a constantly acting and rapidly moving point heat source on the surface of the thick component are determined.

Estimation of Milling Tool Temperature Considering Coolant and Wear

This paper presents a novel technique to estimate the temperature distribution of a milling tool during machining. In this study, heat generation during the machining process is estimated using cutting forces. We consider the heat to be time-dependent heat flux into the tool. In the proposed model, we discretize each rake face on a mill into several elements; each experiences time-dependent heat flux. Second, we calculate the time-dependent heat flux as several constant heat input starts at different time. Finally, we sum the temperature rise from each heat flux to obtain the overall temperature change. A similar concept is applied on the flank surface, where the flank wear area is modeled as an additional heat generation zone. Experimental results are presented to validate the developed model.

Synchronised Welding with Constant Heat Input

Synchronised Welding with Constant Heat Input

Effect of anodizing on pulsed Nd:YAG laser joining of polyethylene terephthalate (PET) and aluminium alloy (A5052)

A series of laser joining experiments between polyethylene terephthalate (PET) and aluminium alloy (A5052) were conducted to investigate the effect of anodizing on A5052 surface on dissimilar materials used in joining. In this study, PET/A5052 joints with anodized A5052 surface exhibited greater shear strength compared to PET/A5052 joints without anodizing. The shear strength of the joints was increased with increasing of heat input and pulse duration. This indicates that the anodizing process could improve shear strength of the laser joining specimens. Significant molten pools were formed in both PET/A5052 (anodized) and PET/A5052 (as-received) joints except for PET/A5052 (as-received) sample joined at the lowest heat input and pulse duration. For the test results from laser joining under different pulse duration at the constant heat input, it was shown that joining behaviour was dominantly controlled by heat input and not by pulse duration.

저온열원의 특성에 따른 ORC 성능해석 최적화 연구

Optimization procedures of performance analysis for ORC(Organic Rankine Cycle) system are established to the characteristics of low temperature heat sources such as open-type and closed-type. Effective heat recovery and heat extraction related to maximum power of the cycle as well as heat quality and thermal efficiency must be considered in the case of the open-type low temperature heat source. On the other hand, in the case of the closed-type low temperature heat source, only thermal efficiency is important due to constant heat input. In this study, thermal efficiency and exergy efficiency representing a level of close to Carnot cycle are studied, as useful index for the optimization of the ORC system. To validate the results of cycle analysis, those are compared with appropriate experimental data of ORC system as a thermal efficiency point of view.