Cooling-down Phase Research Articles

High-efficiency control strategies of a hydrogen turbo-expander for a 5 t/d hydrogen liquefier

Liquid hydrogen plays a crucial role in the large-scale storage and long-distance transportation of hydrogen energy. Effectively controlling hydrogen turbo-expanders is an important strategy for reducing energy consumption during liquefaction process. This study conducts both experimental and numerical investigations on a hydrogen turbo-expander within a 5 t/d hydrogen liquefaction system. A CFD model is developed to predict the performance of hydrogen turbo-expanders, which is then validated against experimental data. The numerical predictions of isotropic efficiency closely match experimental results, with a maximum deviation of 10 %. Throughout the cooling-down process, the efficiency of the hydrogen turbo-expander varies significantly, indicating a strong dependence on the characteristic ratio. Consequently, an optimal characteristic ratio method is proposed to maintain the hydrogen turbo-expander at peak efficiency. Compared to traditional control methods based on fixed brake pressure, the proposed approach achieves a maximum efficiency enhancement of 23.8 %. At the optimal characteristic ratio, the rotational speed can be maintained at the design level by adjusting brake pressure, which remains below the design value during the cooling-down phase. This investigation presents an effective method for optimizing the control of turbo-expanders in hydrogen liquefaction systems.

Discrete element method to simulate interface delamination and fracture of plasma-sprayed thermal barrier coatings

This contribution deals with a discrete element method (DEM) framework to simulate and investigate the mechanisms leading to the failure of plasma-sprayed thermal barrier coating (TBC) systems. A hybrid lattice-particle approach is proposed to determine residual stress fields induced by the coefficient of thermal expansion mismatch during a cooling-down phase. Besides, this is combined with a mixed-mode cohesive zone model to simulate interface delamination, and the removed discrete element failure criterion to model crack initiation and propagation in TBC system. The context of a unit cell model with a perfectly sinusoidal interface profile is first investigated to highlight the suitability of the proposed DEM-based approach in terms of stress fields and failure process. The case of a real microstructure reproduced by the image processing is then discussed. This underlines the effect of porosity and surface asperities on the failure mechanisms. Results exhibit the potential of the proposed DEM approach to model complex cracks phenomena occurring in TBC systems under thermal loading.

Reaction Stages of Feather Hydrolysis: Factors That Influence Availability for Enzymatic Hydrolysis and Cystine Conservation during Thermal Pressure Hydrolysis

The vast amount of feathers generated (>1 Mtons/a in Europe) in the poultry industry is an opportunity of upcycling by-product materials and improving sustainable practices. Feathers are potentially interesting materials as feed protein ingredients due to their high protein (>85 wt%) and cystine content (>7 wt%). However, due to their challenging recalcitrant nature, they have to be processed to make feather protein suitably digestible. The objective was to investigate the effects of temperature (120oC–160oC) and time (10, 30, and 60 min) in thermal pressure hydrolysis of feathers on availability for enzymatic hydrolysis (AEH) and cystine conservation. AEH is defined as degree of degradation of processed feather protein by two digestive enzymes pepsin and pancreatin (Boisen). The present study identified and assessed four temperature stages that take place during feather processing. The four temperature stages are 120oC–135oC, 140oC–155oC, > 160oC, and the cooling-down phase. The second stage has the greatest influence on AEH. As well as temperature, hydrolysis time is also an essential parameter that had a major impact in the second stage (140oC–155oC). Both temperature and time influence negatively cystine content and stability. The present study demonstrates for the first time the importance of four reaction stages during feather hydrolysis and the impact of four stages on AEH of the obtained products.

Numerical study of residual stress and crack nucleation in thermal barrier coating system with plane model

The residual stresses could cause extensive damage to thermal barrier coatings and even failure. A finite element model of thermal barrier coating system had been designed to simulate the residual stresses and then to analyze the crack nucleation behavior. The distribution of normal and tangential stress components along top coat (TC) / thermally grown oxide (TGO) and TGO / bond coat (BC) interfaces are shown in this work. It is found that the maximum tensile stress along TC/TGO interface occurs in the peak region during heating-up, and that along TGO/BC interface is also located in the peak region, but during the process of cooling-down. A parameter correlating the normal stress component with corresponding tangential one was used to evaluate the interfacial cracks, indicating that cracks will initiate at the peak-off region of TC/TGO interface in the heating-up phase, but for TGO/BC interface, cracks will initiate at the peak position in the cooling-down phase.

Kinetic path towards the passivation of threading dislocations in GaN by oxygen impurities

Defect tolerance can be critically important for optoelectronics. GaN, specifically, tolerates a relatively large concentration of threading dislocations, but the physical origin of this tolerance remains a mystery. First-principles calculations reveal the removal of deep-level states from edge dislocations by oxygen passivation. This removal is, however, not a thermodynamic ground state but kinetically driven. Oxygen incorporation during growth can be harmful; it becomes beneficial if introduced in the cooling-down phase or post-growth thermal treatment at a significantly lower temperature. Our findings extend first-principles defect study to the nonequilibrium regime where low-diffusion-barrier defects affect electronic behavior of semiconductors in unexpected fashion.

Step-heating infrared thermographic inspection of steel structures by applying least-squares regression

This paper reports the application of the least-squares regression method in the step-heating thermographic inspection of steel structures. The surface temperature variation of a slab with finite thickness during both the step-heating phase and the cooling-down phase is presented. A mild steel slab with holes of various depths and diameters is chosen as the specimen. The step-heating thermographic inspection experiments are carried out on the specimen with different heating times. The heating as well as the cooling-down phases are recorded with an infrared camera and are analyzed separately by linear regression of the double logarithmic temperature increase versus time plots. Three statistics of the linear regression, the slope, the coefficient of determination, and the F-test value, are used to create image maps according to the processing results. The signal-to-noise ratio of each map is calculated to evaluate the performance of the three imaging methods with different durations of heating time and cooling time. The results prove that the F-test value maps present a good performance for the sequences of the step-heating phase, while the slope maps present a good performance for the sequences of the cooling-down phase. The optimal heating time and cooling time for a steel structure are also concluded. The comparison with the results of the thermographic signal reconstruction (TSR) method proves that the least-squares regression method has better detectability and a higher inspection efficiency.

Effect of perspiration on skin temperature measurements by infrared thermography and contact thermometry during aerobic cycling

The aim of the present study was to compare infrared thermography and thermal contact sensors for measuring skin temperature during cycling in a moderate environment. Fourteen cyclists performed a 45-min cycling test at 50% of peak power output. Skin temperatures were simultaneously recorded by infrared thermography and thermal contact sensors before and immediately after cycling activity as well as after 10min cooling-down, representing different skin wetness and blood perfusion states. Additionally, surface temperature during well controlled dry and wet heat exchange (avoiding thermoregulatory responses) using a hot plate system was assessed by infrared thermography and thermal contact sensors. In human trials, the inter-method correlation coefficient was high when measured before cycling (r=0.92) whereas it was reduced immediately after the cycling (r=0.82) and after the cooling-down phase (r=0.59). Immediately after cycling, infrared thermography provided lower temperature values than thermal contact sensors whereas it presented higher temperatures after the cooling-down phase. Comparable results as in human trials were observed for hot plate tests in dry and wet states. Results support the application of infrared thermography for measuring skin temperature in exercise scenarios where perspiration does not form a water film.

Ethylene oxide and acetaldehyde in hot cores

[Abridged] Ethylene oxide and its isomer acetaldehyde are important complex organic molecules because of their potential role in the formation of amino acids. Despite the fact that acetaldehyde is ubiquitous in the interstellar medium, ethylene oxide has not yet been detected in cold sources. We aim to understand the chemistry of the formation and loss of ethylene oxide in hot and cold interstellar objects (i) by including in a revised gas-grain network some recent experimental results on grain surfaces and (ii) by comparison with the chemical behaviour of its isomer, acetaldehyde. We test the code for the case of a hot core. The model allows us to predict the gaseous and solid ethylene oxide abundances during a cooling-down phase prior to star formation and during the subsequent warm-up phase. We can therefore predict at what temperatures ethylene oxide forms on grain surfaces and at what temperature it starts to desorb into the gas phase. The model reproduces the observed gaseous abundances of ethylene oxide and acetaldehyde towards high-mass star-forming regions. In addition, our results show that ethylene oxide may be present in outer and cooler regions of hot cores where its isomer has already been detected. Despite their different chemical structures, the chemistry of ethylene oxide is coupled to that of acetaldehyde, suggesting that acetaldehyde may be used as a tracer for ethylene oxide towards cold cores.

The Progamic Phase in High-Mountain Plants: From Pollination to Fertilization in the Cold.

In high-mountains, cold spells can occur at any time during the growing season and plants may be covered with snow for several days. This raises the question to what extent sexual processes are impaired by low temperatures. We tested pollen performance and fertilization capacity of high-mountain species with different elevational distribution in the European Alps (Cerastium uniflorum, Gentianella germanica, Ranunculus glacialis, R. alpestris, Saxifraga bryoides, S. caesia, S. moschata) during simulated cold snaps in the laboratory. Plants were exposed to 0 °C (the temperature below the snow) for 12, 36, 60 and 84 h. In S. caesia, the experiment was verified in situ during a cold snap. Sexual processes coped well with large temperature differences and remained functional at near-freezing temperatures for a few days. During the cooling-down phase a high percentage (67–97%) of pollen grains germinated and grew tubes into the style. At zero degrees, tube growth continued slowly both in the laboratory and in situ below the snow. Fertilization occurred in up to 100% of flowers in the nival species and in G. germanica, but was strongly delayed or absent in the alpine species. During rewarming, fertilization continued. Overall, progamic processes in high-mountain plants appear fairly robust toward weather extremes increasing the probability of successful reproduction.

Formation of Glaciation Epochs

The effect of Earth precession angle on a climate is presented here. It is shown that the glaciation epochs occurred only when the precession angle was low. After the continental glaciation formed in the Northern hemisphere, Earth’s spherecal symmetry was disrupted and its precession angle increased drastically. As a result, a drastic and rapid climate warm-up occurred, the glaciers melted down and an interglacial stadial1 began. Subsequently, affected by the Lunar-Solar gravity pull on the Earth’s equatorial swelling, the precession angle gradually decreased and a new cooling-down phase occurred. As a result, there was nonlinear oscillation of Earth’s climate with periods on the order of 100 - 120 MY.

Thermal and structural response of two-storey two-bay composite steel frames under furnace loading

The collapse of the World Trade Center Towers and other recent fires in tall buildings has motivated this study to understand the performance of structural frames under fire loading. Three two-storey, two-bay composite steel frames were constructed and subjected to dead loads by applying weight blocks, and to thermal load by placing the frame in a furnace. The furnace was specially designed to allow for controlled heating of the structural elements that formed the four compartments of the test frame. This paper describes the experimental results of furnace test conducted on the three full-scale composite frames. The three tests differed from each other in the number and location of compartments that were heated by the furnace. For each test, the structural elements were subjected to a heating-up phase followed by a cooling-down phase. The furnace temperatures and the steel and concrete temperatures recorded during the test are presented. The thermally induced horizontal displacements of the columns and vertical deflections of the composite beams are discussed. Observations on local buckling of the steel beam, cracking of the concrete slab and failure of the beam-to-column connections are tabulated. Experimental results of the three tests are compared with each other by studying the complete deformation process of the test frames over time. Results indicate that the deformation process of the test frames was highly dependent on the number and location of compartments that were subjected to thermal loading.

Vergleichende Untersuchungen zum Brandverhalten der Borken von Föhre, Lärche und Robinie | Comparative studies on the firing behaviour of the bark of Scots pine, larch and black locust

During firing experiments on the bark of Scots pine (Pinus sylvestris), larch (Larix decidua) and black locust (Robinia pseudoacacia) the temperature pattern was determined for the duration of the firing phase and the cooling-down phase in two measuring depths (8 mm and 12 mm). Additionally several wood physical (density, coefficient of thermal conductivity for varying temperatures) and anatomical (structure, cell wall / lumen ratio) properties were ascertained and the burning progress as well as the carbonisation documented. The temperature pattern showed a strong correlation between the burn-through-time and the density of the bark. Strong distinctions were apparent among the tree species. Black locust, with its very homogeneous and low density scale-fibre bark,showed a much steeper temperature pattern with a considerably higher temperature maximum than the scale bark of the conifers. The almost linear and slightly delayed temperature pattern in the pine bark might indicate a good opportunity for a possible industrial utilisation.

Consequences of intra-cascade clustering on defect accumulation and materials performance

Considerations of intracascade clustering of self-interstitial atoms (SIAs) and vacancies during the cooling-down phase of a cascade and of the difference between thermal stabilities of SIA and vacancy clusters give rise to the concept of production bias. In recent years, various aspects of this concept including the effects of cluster annihilation by one-dimensional glide have been investigated. The salient features of these investigations and their results are summarized. The most important conclusion emerging from these investigations is that a realistic modelling of accumulation of surviving defects under cascade damage conditions must explicitly consider the role of formation, thermal stability and mobility (glide and climb) of clusters produced in the cascades. Examples are shown where appropriate considerations of these features have led to predictions regarding dose, temperature and recoil energy dependence of void swelling which are consistent with experimental results. The special feature such as enhanced damage accumulation near grain boundaries has been successfully explained in terms of production bias and one-dimensional glide of small loops produced in the cascades. Finally, it is shown that the decoration of grown-in dislocations by gliding SIA loops produced in the cascades may also explain the irradiation-induced increase in the upper yield stress.

Effects of Nonsteady Hot-water Greenhouse Heating on Heat Transfer and Microclimate

Hot-water tubes are used for heating in many agricultural buildings. Owing to the thermostatic control of the heating system, the tubes usually operate under non-steady-state conditions. Experiments were carried out to determine the effects of non-steady heating on the heat transfer coefficients of the heating tubes, on the amount of energy supplied in the different phases of the heating cycle and on the temperature variation of the air and the crop. The experimental data show that the heat transfer coefficients decreased during the cooling-down phase of the heating cycle. In addition, there was a continuous change in the ratio between the heat lost by convection and that lost by radiation from the outer surface of the tubes. At the beginning of the cooling-down phase, convection contributed more than radiation to the total heat loss, while towards the end of this phase radiation made a slightly larger contribution than convection. From energy calculations, it appears that approximately 60–70% of the energy used was supplied to the greenhouse during the heating-up and quasi-steady heating phases; the remainder was supplied during the cooling down of the tubes. Owing to the non-steady-state heating, the temperatures of the greenhouse air and crop varied in a cyclic manner with respect to time. These temperatures were closely correlated and the difference between them also changed in a cyclic manner.

Recent advances in the understanding of damage production and its consequences on void swelling, irradiation creep and growth

The consequences of displacement damage produced by energetic particles on physical and mechanical properties of metals and alloys have been investigated both experimentally and theoretically for several decades. Over the years, a number of theoretical models have been proposed to rationalize the rate and magnitude of defect accumulation under different irradiation conditions. In recent years, significant advances have been made in understanding the nature of the damage produced in this form of multi-displacement cascades. The new knowledge regarding the intra-cascade recombination and clustering of self-interstitial atoms and vacancies during the cooling-down phase of cascades makes it necessary to re-examine the appropriateness of the available models for describing the accumulation of damage under cascade damage conditions. In this paper, recent advances in the understanding of damage production and its consequences are reviewed. A historic perspective is adopted. A comprehensive analysis of the effects of temperature, dose rate and particle type on multi-phenomena (swelling, creep, growth, microstructure evolution, RED, RIS) is presented to discuss the strength and weakness of various models, as they have evolved with the understanding of the damage processes. It has been shown that the irradiation damage modeling has progressed from the standard rate theory model to the BEK model to the production bias model with an increasing degree of sophistication as increasingly more realistic features of the irradiation damage production process were incorporated. It is shown that the newly proposed production bias model uniquely includes the necessary features of cascade damage production in its treatment of the damage accumulation.

Daily variation in energy expenditure during weight-training versus continuous arm cranking and cycling.

To quantify day-to-day variability in energy expenditure (EE) during weight-training, 10 male subjects completed 6 training sessions that consisted of a warming-up and cooling-down phase (10 min; arm cranking or cycling) and a computer-paced weight-training phase using standard weights (42 min; 10 exercises, 3 x 15 repetitions, work to rest ratio = 1:1). The first 2 sessions were used to accommodate. During sessions 3-6 (labelled session A, B, C and D) cardiorespiratory response was continuously measured. Gas-exchange was converted to EE using the Weir formula. In the last two sessions training weights were raised by +/- 50% and the initial warming-up and cooling-down exercise being arm-cranking was replaced by cycling. Mean EE was calculated for last five minutes of the warming-up (WU5) and cooling-down (CD5) and the complete weight-training phase (WT). Mean EE during WT for session A to D was 20.0 +/- 2.7, 19.1 +/- 2.5, 22.2 +/- 2.2 and 21.9 +/- 2.4 kj.min-1, respectively. Daily variation was tested by comparing test-retest phases (A vs B and C vs D). Intra-individual variability in EE during respectively the lower (session A vs B) and higher (session C vs D) weight-training intensity was expressed as: absolute test-retest difference (0.9 +/- 0.5 and 0.7 +/- 0.6 kj.min-1), difference relative to the mean of the test-retest measurements (2.3 +/- 1.4 and 1.6 +/- 1.4%) and as the mean coefficient of variation (3.3 and 2.2%). There were no differences in variability between both WT-intensities nor exercise modes, irrespective of the way variability was expressed. In conclusion, intra-individual variability in EE during weight-training does not differ from variability during arm cranking or cycling.

Production bias driven defect accumulation in the transient regime

Abstract Consequences of the nature of the damage production in multidisplacement cascades are analyzed. Considerations of intracascade clustering of self-interstitial atoms (SIAs) and vacancies during the cooling-down phase of a cascade and of the difference between thermal stabilities of SIA and vacancy clusters give rise to the concept of production bias. Within this concept, the fraction of vacancies and SIAs escaping the cascade region is taken to be the effective rates of defect production. The fraction of vacancies in excess of SIAs has been calculated and used for determining the temporal dependence of void swelling purely in terms of production bias. It is proposed that small SIA clusters/loops may perform one-dimensional glide motion and may get transported to dislocations and grain boundaries. This is considered to be a viable mechanism for the removal of a fraction of SIA population. The resulting vacancy supersaturation would drive void nucleation, growth and void swelling. The swelling behaviour in the transient regime predicted by the production bias is found to be in reasonable agreement with the experimental results.

Dependence of the UO 2-PuO 2 pellet structure on fabrication parameters

The fabrication of fuel pellets with a given structure and certain properties requires detailed knowledge of the effects of the individual fabrication parameters. The correlation between some of these parameters and the pellet structure has been investigated with UO 2-PuO 2 pellets which were prepared by the so-called OCOM process. The most important results are: The fraction of open porosity decreases with increasing duration of the milling process due to enhancement of the sintering activity. The addition of an appropriate pore former gives the possibility for a well-defined reduction of the sintering density generally achieved as a result of the milling process. An increase of maximum pore size and an inhomogeneous distribution of pore volume has to be accepted. The O/M ratio can be adjusted to a certain value by moistening of the sintering gas Ar/H 2. The occurrence of cracks with this procedure can be avoided, if dry gas is fed during the cooling-down phase.

ＪＴ‐６０真空容器加熱冷却装置の設計製作と性能試験

This report describes design, fabrication and performance test of heating and cooling system of JT-60 vacuum vessel. The vacuum vessel was designed to be bakable up to 500°C by electric heaters as a means of ultra-high vacuum treatments, and was required to be cooled down by water or nitrogen gas. Results of the on-site baking test after assembly of the machine showed that the design requirements for the maximum temperature, temperature difference, heating time, ultimate vacuum and outgassing rate were satisfied, and no leaks were detected with helium leak testing. It was also found that the slide mechanism for thermal expansion during the heatingup and cooling-down phase functioned satisfactorily.