Critical Heat Research Articles

Effect of thermal cycling induced phosphorus grain boundary segregation on embrittlement of welding heat affected zones in 2·25Cr–1Mo steel

Phosphorus induced embrittlement of welding heat affected zones (HAZs) in a 2·25Cr–1Mo steel was examined by measuring the ductile to brittle transition temperature (DBTT) of the HAZs simulated with a peak temperature of 1320°C at different welding heat inputs. At the same heat input, the DBTTs of the HAZs in the P doped samples were apparently higher than those in the undoped samples and phosphorus grain boundary segregation was mainly responsible for the DBTT increase. A critical welding heat input was found to be between 36 and 100 kJ cm−1, being close to 60 kJ cm−1, at which the maximum segregation of phosphorus could be produced during the corresponding thermal cycling, leading to the maximum DBTT difference between the P doped and undoped samples.

Effect of underfill defects on distortion and tensile properties of Ti-2Al-1.5Mn welded joint by pulsed laser beam welding

Owing to its more precise control of heat input, pulsed laser beam welding is expected to be a choice for welding thin sheets. In this paper, the Ti-2Al-1.5Mn titanium alloy sheets with a thickness of 0.8 mm were welded by a pulsed Nd:YAG laser beam welding system. The underfill defects, welding distortions, microstructures, and tensile properties of the joints were investigated. The results show that the welding defects are strongly influenced by the welding parameters. The residual distortion is related to the specific heat input. Particularly, the maximum angular distortion increases remarkably for the specific heat input higher than 328.5 J/cm. Further investigations reveal that this is mainly due to the formation of underfill defects, while it is not affecting the maximum residual longitudinal bending distortion. In forming underfill defects and changing acicular α′ phase morphology in the fusion zone (FZ) of the welded joint, 328.5 J/cm is a critical specific heat input. Furthermore, tensile test results reveal that the fracture positions are closely related to the underfill rate.

Analysis of the Zero-Pressure-Difference Point and Determination of the Pump Head in Distributed Power System

Using pressure diagram to analyze the pressure distribution of distributed power system when the zero-pressure-difference point is in different positions. Conclusions can be drawn as follows: the zero-pressure-difference point should be between the critical point and heat source to eliminate the energy consumption of regulators. And as a result the transmission energy consumption of the system becomes lowest. When the zero-pressure-difference point is in different positions between the critical point and heat source, arrangements of pumps of the system are different. But the total output power of the pumps is the same. The zero-pressure-difference point and the schemes for pump head can be conveniently determined by graphical method like pressure diagram or numerical method.

지중송전관로 되메움재의 종류에 따른 열 소산 효과의 비교에 관한 연구

지중송전관로를 통한 송전용량 증대를 위해서는 송전 시 발생하는 열을 빠르게 소산시킬 수 있도록 습윤 시 <TEX>$50^{\circ}C$</TEX>-cm/Watt, 건조 시 <TEX>$100^{\circ}C$</TEX>-cm/Watt 이하의 열 저항률을 갖는 재료를 되메움재로 사용하는 것이 요구되며, 이를 위해 선행연구에서는 요구되는 조건을 만족시키는 열 소산 되메움재를 개발하였다. 이 연구에서는 개발된 열 소산 되메움재의 열 소산효과를 알아보기 위하여 1회선의 단면을 현장에 설치하고 개발된 열 소산 되메움재와 강모래, 현장토에 대하여 열을 직접 가하고 일정한 지점에서 온도를 측정함으로서 현장시험을 수행하였고, 시험결과와 유한요소해석과의 비교를 통해 열 소산효과를 비교하였다. 비교결과, 열원과 가까운 지점에서는 개발된 열 소산 되메움재가 강모래와 현장토에 비해 열평형상태에 빨리 도달하는 것으로 나타났으며, 열평형에 도달하는 온도도 높은 것으로 나타나 열 소산효과가 우수한 것으로 나타났다. 또한 다른 지점에서도 개발된 되메움재가 다른 재료보다 온도가 높아 열 소산효과가 우수한 것으로 나타났다. 또한 함수비의 변화에도 열 소산효과가 크게 변화하지 않는 것으로 나타났다. Backfill material with thermal resistivity which has <TEX>$50^{\circ}C$</TEX>-cm/Watt in wet and <TEX>$100^{\circ}C$</TEX>-cm/Watt in dry is requested to improve the power transfer capability for dissipation of heat production in underground power cables. In the field test performed by buried cable backfills, the backfill material developed from this study is compared with river sand and weathered soil (native soil) to investigate the effect of heat transfer in various seasons and locations of thermal sensors. As a result, the developed backfill material is faster approaching yielding temperature (critical heat) than that of river sand and weathered soil, and it has good dissipation capacity rather than other materials by keeping moisture content at dry season.

Evaluating the non-isothermal crystallization behavior of organic molecules from the undercooled melt state using rapid heat/cool calorimetry

Understanding the inherent crystallization tendency of organic molecules from the undercooled melt state is an important criterion to assess the glass-forming ability of a compound, and hence the ability to produce and maintain amorphous materials, an important area for drug delivery of poorly water-soluble therapeutics. The main objective of this study was to determine the critical cooling (Rcrit,cool) and/or critical heating (Rcrit,heat) rates to inhibit crystallization for a group of compounds with diverse chemistries and different crystallization tendencies. Compounds were cooled at various cooling/heating rates using a prototype rapid heat/cool (RHC) calorimeter, and crystallization (or lack thereof) from the undercooled melt was recorded. Results showed some compounds have very high crystallization tendencies [class (I–A)] from the undercooled melt state, with Rcrit,cool rates above 1000 °C min−1, while some compounds with low crystallization tendencies upon cooling [class (II) compounds] exhibited very high crystallization tendencies (Rcrit,heat > 1000 °C min−1) upon subsequent reheating from the amorphous state. The pronounced asymmetry observed between Rcrit,cool and Rcrit,heat rates for class (II) compounds was greatly reduced by rapidly quenching the compound from the melt state, preventing the formation of quenched-in nuclei during cooling and initiating crystallization upon reheating. The ability to apply controlled rapid heating and cooling rates to organic compounds thus provides useful insight into the crystallization tendency of organic compounds.

ANALYSIS OF TENDENCIES TOWARDS CHANGES IN TEMPERATURES OF RAILWAY VEHICLE AXLE BOXES / RIEDMENŲ AŠIDĖŽIŲ KAITIMO TEMPERATŪRŲ KITIMO TENDENCIJŲ ANALIZĖ

The paper analyzes variations in the temperature of axle-box heating of railroad rolling stock considering two modes of the train - driving and parking position. Additionally, the article describes the influence of the axle load and season on the heat of the axle-box, forecasts intolerable residual resources and explains a possibility of detecting likely breakage at the very beginning of defects. The paper also deals with scientific literature related to axle-box heating and focuses on the conducted practical and theoretical experiments. The carried out research has established the rates of operating heating and critical heating. The application of numerical values for the algorithm of automatic control devices could help with detecting the breakdowns of the axle-box before they reach a critical state and assist in forecasting residual operating resources. Santrauka Nagrinėjama geležinkelio riedmenų ašidėžių kaitimo kitimas traukinio važiavimo ir stovėjimo metu, riedmenų krovumo ir metų laiko įtaka ašidėžei kaisti, neleistinai kaistančios ašidėžės liekamojo resurso prognozė, ašidėžės gedimo nustatymo galimybė pačioje gedimo atsiradimo pradžioje. Išanalizuota mokslinė literatūra, susijusi su riedmenų ašidėžių kaitimu, taip pat išanalizuoti atlikti eksperimentiniai ir teoriniai darbai. Tyrimo metu nustatytos ašidėžių darbinės ir kritinės riedmenų ašidėžių kaitimo temperatūros. Šių temperatūrų skaitines reikšmes pritaikius riedmenų automatinės kontrolės prietaisų algoritmui būtų galima aptikti ašidėžių gedimus, kol jie nepasiekė kritinės būklės, ir prognozuoti liekamąjį darbo resursą.

Effect of Thermal Treatment on Physical Properties of CdTe Thin Films Deposited by a Solution-Based Method

The influence of substrate and post-annealing temperature steps on the unique physical, structural, morphology and optical properties of CdTe thin films deposited by a non-vacuum spray process were investigated for solar cells. Scanning electron microscopy (SEM) and X-ray diffraction spectrometer (XRD) were employed to study surface morphologies and structural changes, respectively. It was observed that CdTe films produced at substrate temperature of 130°C had substantially larger grain size and showed denser morphology. Annealing at 130°C–330°C in air caused small grains throughout the surface at random. Annealing at 430°C, however, initiated fusing between grains and produced uniform morphology with an average grain size of about 60 nm. XRD analyses showed that CdTe films deposited at 130°C and then post-annealed at 430°C behaved in the originally strong (111) direction without the formation of secondary CdTeO3 phase. Optical band gap measurements indicated that the values of CdTe films grown at 130°C and post-annealed at 430°C in ambient condition were about 1.45 eV. All results exhibit 130°C and 400°C to be critical deposition temperature and heat treatment temperature at which physical, optical and structural properties of CdTe thin films start to change.

Experimental Evaluation of Natural Circulation Pressure Drop in a Boiling Channel

Using natural circulation as the primary core cooling mechanism in next generation nuclear reactors provides advantages such as improved safety, less operation and maintenance costs (because of elimination of pumps), and simplicity of system. Large scale deployment of natural circulation based reactors and safety systems depend on the successful resolution of the challenges specific to natural circulation such as driving force, system pressure drops, instability effects, and critical heat flux.In this work, natural circulation two-phase flow pressure drops in a single channel are studied experimentally. For this purpose, natural circulation hydrodynamic loop was designed. The overall pressure drop was measured by use of pressure transducer sensors and the void fraction in visible boxes which located at the end of heated tube is measured by use of high speed camera. The frictional and acceleration pressure drop are evaluated in different conditions from experimental data and corresponding theoretical formulas. The results could be useful in natural circulation based reactor design and computer codes validation.

Effect of Heat Input on HAZ Properties and Microstructure of SMA490BW Corrosion Resistance Steel

Welding thermal cycle of SMA490BW corrosion resistance steel with different heat inputs were simulated by using Gleeble-3500 thermo-mechanical simulator to investigate the microstructure and impact toughness of the heat-affected zone (HAZ). The results shows that the microstructure of the coarse-grain heat affected zone(CGHAZ) is mainly composed of lath bainites and the surpass critical coarse-grain heat affected zone(SCCGHAZ) consists mainly of granular bainites with a small content of ferrites. The grain size increased with the heat input increasing, Besides, the impact energy of the surpass critical coarse-grain heat affected zone(SCCGHAZ) is higher than that of the coarse-grain heat affected zone(CGHAZ), when the heat input energy is 15kJ/cm after twice thermal cycle, the impact energy is highest.

Specific Heat and Magnetization Studies of the Superconducting Mixed State of UBe13

Low-temperature specific heat and static dc magnetizetion measurements have been performed for the superconducting state of the heavy-fermion compound UBe 13 . The superconducting phase diagram B c2 – T ( B ∥[110]) has been obtained by our specific heat and magnetization measurements. From the slope of the upper critical field and specific heat jumps at superconducting transition temperatures in magnetic fields, we have also obtained a temperature dependence of Ginzburg–Landau parameter κ 2 along [110]. It has been revealed that κ 2 decreases with decreasing temperature down to 0.5 K, which indicates that a paramagnetic effect may be present at least down to 0.5 K for [110].

Liquid exploding emission during boiling nucleation in capillary tubes

A series of experiments was conducted to visually observe boiling nucleation phenomena in micro capillary tubes employing a high speed CCD. At a higher heat flux the liquid in capillary tubes was emitted instantaneously rather than generating nucleate boiling, which is referred as liquid exploding emission in this paper. Both critical emission heat flux and temperature corresponding to this phenomenon were measured for tubes with different heating lengths and arrangements of two end sections. Boiling nucleation and liquid film evaporation played critical roles in different stages during exploding emission. Dimensionless critical emission volumetric heat was introduced to describe the conditions of the emission instead of heat flux, and theoretical discussions were conducted to explore the influences of thermal properties and geometrical configuration, particularly heating length and asymmetry of two end sections of tubes.

A two-dimensional study on natural convection and heat transfer in the enclosure with heat transfer and radiation coupled in natural convection

Numerical investigation using SIMPLE algorithm with QUICK scheme for natural convection and heat transfer in the enclosure bounded by a solid wall and with heat transfer and radiation coupled in natural convection has been conducted. The various parameters are: Rayleigh number (from 103 to 105), dimensionless conductivity of bounding wall (from 0 to 100), dimensionless wall thickness (from 0 to 0.6) and radiation emissivity of all surfaces (from 0 to 1). The results suggest that flow and heat transfer are influenced by radiation. Radiation is a dominant action on flow and heat transfer. With increase of the thermal conductivity of wall, flow and heat transfer turn stronger. The temperature distribution changes obviously. When the thermal conductivity of wall is over a certain critical number, the increasing trend of flow and heat transfer may disappear. With increase of enclosure wall thickness, flow and heat transfer turn slighter. When the enclosure wall thickness is over a certain critical number, the flow and heat transfer will turn slow.

Surfactant Effects on Critical and Minimum Heat Flux

Surfactant Effects on Critical and Minimum Heat Flux

An Efficient Welding Method to Control the HAZ Softening of Ultra Fine Grained Steel Weldment

The HAZ softening of the ultra fine grained steel weldment occurs when welding heat input is higher than the critical value. The critical welding heat input was found to be about 10kJ/cm for the steel studied. The lowest hardness value and the width of the softened HAZ are dependant on the applied welding heat input. For obtaining high productivity with the low heat input, less than 10kJ/cm, a high speed FCA welding process with 2 electrodes was developed and verified to be an efficient method for the ultra fine grained steel. The weldment made by this process did not have the softened HAZ; and thus, satisfied the tensile strength required.

Critical Environmental Heat Stress Limits for Exercising Heat-Acclimated Lean and Obese Boys

Critical Environmental Heat Stress Limits for Exercising Heat-Acclimated Lean and Obese Boys

Rewetting analysis of hot surfaces with internal heat source by the heat balance integral method

A two region conduction-controlled rewetting model of hot vertical surfaces with internal heat generation and boundary heat flux subjected to constant but different heat transfer coefficient in both wet and dry region is solved by the Heat Balance Integral Method (HBIM). The HBIM yields the temperature field and quench front temperature as a function of various model parameters such as Peclet number, Biot number and internal heat source parameter of the hot surface. Further, the critical (dry out) internal heat source parameter is obtained by setting Peclet number equal to zero, which yields the minimum internal heat source parameter to prevent the hot surface from being rewetted. Using this method, it has been possible to derive a unified relationship for a two-dimensional slab and tube with both internal heat generation and boundary heat flux. The solutions are found to be in good agreement with other analytical results reported in literature.

Phytoplankton from two sub-Arctic fjords in northern Norway 2002–2004: I. Seasonal variations in chlorophyll a and bloom dynamics

Chlorophyll a, meteorology and hydrography were sampled during a 3 year period in Altafjord and Porsangerfjord. The analysis demonstrated a decoupling between spring bloom progression and the classical ‘Sverdrup’ regulators, i.e. there was no correlation between spring bloom biomass and critical depth, mixed or station depth and irradiance. The only significant correlation was linear between bloom magnitude and day length. We also observed significant correlations between log chlorophyll a and critical depth, irradiance and heat flux. From early March to mid-May, day length increases from 7 to 24 h in the area, whereas daily integrated irradiance increases ca. 14 times. Then, if blooms have not yet triggered early in April, water turbidity will remain low and critical depth will deepen rapidly, causing blooms to start after this date, i.e. irradiance will become the main forcing function if initial stocks are present. Another characteristic feature in the area is that spring bloom concentrations are low and that production is more spread out throughout the growth season, contrary to what is common further south (spring bloom, summer recession and autumn bloom).

Protein Denaturation, Rheology, and Gelation Characteristics of Radio-Frequency Heated Egg White Dispersions

Gel properties of radio frequency (RF) heated egg white dispersions at 27.12 MHz were studied as function of concentration (2.5–2.5 kg/100 kg sample), pH (3–11) and heating time (60–180 s). Egg white dispersions demonstrated a gradual liquid-solid transformation as they denatured and gelled during RF treatment. The critical concentration and heating period for egg white protein denaturation and gelation were found to be 7.5% (w/w) and 150 seconds. The elastic modulus (G′) of RF-heated samples increased with concentration and heating period (temperature), whereas complex viscosity (η*) increased exponentially with concentration. In an alkaline condition, the egg white dispersion did not produce a gel; however, in acidic condition it resulted in a strong gel with significantly (P < 0.05) higher G′. This could be attributed to the high dielectric constant (ε′) and loss factor (ε″) values of acidified samples as compared to the alkaline and control egg white dispersion. Effect of heating rate (1, 5, 10, and 20°C/min) in situ on rheometer plate significantly affected gel rigidity; the RF treated sample rigidity was comparable to samples heated at the rate of 5 and 10°C/min. Differential scanning calorimetry, dielectric measurement, and sodium dodecyl sulfate (SDS) PAGE electrophoresis results were used to confirm gelation behavior during both conventional and RF heating conditions.

Three-Dimensional Mixed Convection in Plane Symmetric-Sudden Expansion: Bifurcated Flow Regime

Simulations of three-dimensional laminar mixed convection in a vertical duct with plane symmetric sudden expansion are presented to illustrate the effects of the buoyancy-assisting force and the duct’s aspect ratio on flow bifurcation and heat transfer. The stable laminar bifurcated flow regime that develops in this geometry at low buoyancy levels leads to nonsymmetric temperature and heat transfer distributions in the transverse direction, but symmetric distributions with respect to the center width of the duct in the spanwise direction. As the buoyancy force increases, due to increases in wall heat flux, flow bifurcation diminishes and both the flow and the thermal fields become symmetric at a critical wall heat flux. The size of the primary recirculation flow region adjacent to the sudden expansion increases on one of the stepped walls and decreases on the other stepped wall as the wall heat flux increases. The maximum Nusselt number that develops on one of the stepped walls in the bifurcated flow regime is significantly larger than the one that develops on the other stepped wall. The critical wall heat flux increases as the duct’s aspect ratio increases for fixed Reynolds number. The maximum Nusselt number that develops in the bifurcated flow regime increases as the duct’s aspect ratio increases for fixed wall heat flux and Reynolds number.

Bose–Einstein condensation of a relativistic Bose gas trapped in a general external potential

Bose–Einstein condensation of an ideal relativistic Bose gas trapped in a generic power-law potential is investigated. The analytical expressions for some important parameters such as the critical temperature, ground-state fraction and heat capacity are derived. The general criteria on the occurrence of Bose–Einstein condensation and the discontinuity of heat capacity at the critical temperature are obtained. The results obtained here present a unified description for the Bose–Einstein condensation of a class of ideal Bose systems so that many important conclusions in the literature are included in this paper.