Temperature Drop Process Research Articles

Aggregation behaviour of paraffin molecules with different branched distributions during crude oil gelling: A molecular dynamics study

Revealing the microdynamic mechanism and thermodynamic properties of the gelling process of waxy crude oil at the microscale plays an important role in ensuring the safe production and transport of waxy crude oil. However, few studies have dealt with the influence of paraffin molecular branching position on the gelling process. The focus of this study is to clarify the influence of branching distribution on molecular aggregation through molecular simulation, so as to further improve the mechanistic study of the gelling process of waxy crude oil. A variety of molecular models of crude oil systems containing paraffin molecules with different distributions of branched chains, asphaltenes, and light oils are developed to analyse the molecular aggregation and wax precipitation characteristics during the gelling process.The results show that the paraffin molecular chain morphology gradually from stretching to curling and the molecules gradually from dispersing to aggregating during the temperature drop process. From the van der Waals force analysis, it is concluded that isomeric alkanes with a close branching distribution form the weakest gelling structures. In addition, the phase transition temperature difference between n-alkanes and isoalkanes is large about 9 K, but the phase transition temperature difference between the two isoalkanes with different distributions of branched chains is small only about 3 K. This indicates that for alkanes with the same molecular formula, the length of the main carbon chain is a dominant factor in influencing the aggregation of the molecules and the gelling properties of the crude oil system. Under high temperature conditions, isomeric alkanes have the best mobility and are most sensitive to temperature. The results of the study can provide help for the mechanism of the gelling process of waxy crude oil to ensure the safety of the pipeline transmission process, and provide theoretical guidance for the production of depressants and catalysts.

Analysis of heat transfer characteristics of paraffin oil in oil tank based on the three-phase partition method

The temperature drop process of waxy crude-oil in an oil tank is an unsteady natural-convection process involving multi-substance coupling, multi-heat transfer coupling, phase transformation, solid-liquid interaction, rheological change, fluid-solid coupling and turbulence, and the heat transfer process is highly complicated. According to the influence of the phase change process of waxy crude-oil on the heat transfer process, based on the new three-phase partition method, the phase change heat transfer model of waxy crude-oil in an oil tank was established, and the finite volume method was used to carry out the equation discretization and solution. Through numerical simulation, the evolution laws of the temperature field, flow field, and growth of the condensate layer during the cooling process of oil in the tank were analyzed. The results had theoretical guidance and significance for the scientific design of the oil tank?s thermal insulation structure.

Influence of Water Saturation on Curtain Formation of Frozen Wall

In the process of in situ mining of underground oil shale, it is necessary to build an underground frozen wall around the mining area to prevent the inflow of groundwater and prevent oil and gas leakage. However, the water saturation of the frozen soil affects the formation of the freezing circle. In practical engineering, when building a frozen wall in soil under different water saturation conditions, the curtain is often not closed. It also affects the construction and operating costs of the entire project. In this paper, the methods of laboratory experiments and numerical simulation analysis are supplemented by the analysis of ordinary differential equations and partial differential equations. Aiming at the lowest cost, the temperature drop process and freezing time around the frozen well under different water saturation conditions are calculated and analyzed in detail. The formation law of the frozen wall curtain under different water saturation conditions is obtained. It provides a reference for the setting of the low-temperature loading time and the oil shale extraction range of frozen wells for different soils in practical engineering.

Application of magnesium oxide expansive agent in hydraulic concrete in China

This paper reviews and summarises the use of magnesium oxide (MgO) in hydraulic concrete in the following projects in China: Baishan hydropower station unexpectedly discovered the beneficial effects of MgO; Dongfeng hydropower station actively used MgO as an expansive agent for the first time in the main project; Changshaba hydropower station used MgO expansive agent for the first time in a full arch dam; Huangjiazhai hydropower station broke through 6% of MgO content for the first time in normal state concrete; and Dahe reservoir broke through 6% of MgO content for the first time in roller-compacted concrete. Findings indicate that the production process, transportation equipment, pouring process, burying of observation instrument, and so on, for concrete mixed with MgO are exactly the same as concrete without MgO. The long-term autogenous volume deformation of MgO concrete presents a good delayed micro-expansion discipline, which matches the long temperature drop process of dam concrete. The use of MgO expansive agent in dam concrete improves concrete's anti-cracking ability, simplifies temperature control measures of dam concrete, speeds up construction progress and saves project investment. At the same time, this paper points out the appropriate quality standards for the MgO expansive agent used in dam concrete and its soundness content that shall not be exceeded.

Research on Temperature Drop and Cold Release Process of Cold Wall for Backfill Body with Embedded Heat Transfer Tube

Research on Temperature Drop and Cold Release Process of Cold Wall for Backfill Body with Embedded Heat Transfer Tube

Analysis of Low-Temperature Damage of Reinforced Concrete Beams Using Acoustic Emission Parameters

Reinforced concrete (RC) beams will be affected by various external environmental in the service process, of which low-temperature is a representative factor. This paper presents a study on using the low-temperature damage test on damage and fracture of RC beams, and an acoustic emission (AE) system is used to monitor the whole process of testing. In the low-temperature damage test, AE hits are used to characterize the damage of each cooling step, and the low-temperature damage of the beam is divided into three processes: initial damage, extended damage and attenuated damage. A damage parameter (DTEM ) based on the accumulated energy of AE is used to analyze the whole process of low-temperature damage of the beam, and the percentage of low-temperature damage in each cooling step is obtained. The environment chamber is used to simulate the extreme low-temperature environment, and the damage of RC beam after the sudden temperature drop (process I) and the end of cooling (process II) is obtained. These two processes have important reference value for the damage degree analysis and damage warning of RC beams in the actual extreme low-temperature environment.

Numerical study on two-phase expansion performance and quantitative analysis of wetness loss in cryogenic turbo-expander

Cryogenic two-phase turbo-expander becomes a prevalent option in process design of high efficiency cryogenic liquefaction system, especially in modern air separation system and helium system. Computational code of turbine off-design performance prediction in overheated expansion and non-equilibrium condensation module are well established in our former works, and the mechanism of nucleation and droplet growth in spontaneous condensation is revealed. However, the analysis between single and two-phase expansion as well as wetness influence on turbine performance is not conducted yet. In this study, we investigate a turbo-expander performance analysis in a wide temperature range (Tin = 118 K ~ 98 K), and present the wetness loss assessment and quantitative calculation in two-phase expansion process. By referring to the previous off-design performance of overheated expansion, the two-phase expansion performance under different pressure ratio and rotational speed is obtained and compared. According to the continuous inlet temperature drop process, the expansion is categorized into three types and their zone boundaries are divided. The transitional feature of unexpected change is frequently figured out on the expansion zone boundary, and the reaction ratio and pressure ratio in diffuser during the continuous inlet temperature drop process are analyzed detailed, which shows the unique and significant role of wetness effect in two-phase expansion. Based on the experimental wetness loss assessment method established by Baumann in wet steam turbine, two numerical assessments by using overheated expansion and equilibrium condensation model as reference expansion are analyzed. To quantitatively analyze the wetness loss influence, the efficiency drop caused by wetness and the relative wetness loss are calculated by eliminating other component losses in turbine, and the relative wetness loss is fitted with outlet liquid mass fraction mathematically to assist future design of cryogenic two-phase turbo-expanders.

Auto-ignition performance of an electrified microreactor with constructal geometries

Common techniques employed to minimize ignition energy of gaseous fuels in micro reactors such as the use of catalyst are being plagued with setbacks. This report demonstrates the use of electric field to sustain combustion, minimize heat loss and enhance reactant mixing in microchannels of varying geometries. We set two defining constructal parameters for serpentine and straight microchannels of 0.05 to 0.25 to investigate geometric effect on the mixing of the reactants. Inlet concentration of propane and oxygen was set at 0.15 mol/dm3 and 0.7 mol/dm3 respectively. Reynolds numbers 400, 470, 530, 600 and 670 corresponding to inlet velocities of 0.06 m/s, 0.07 m/s, 0.08 m/s, 0.09 m/s and 0.1 m/s were used in the straight channel to study the dependence of reaction rate, temperature drop and reactants diffusion on the combustion process. We report that at an inlet temperature of 500 K which was below the propane’s auto ignition temperature of 743 K, the reaction could occur in the presence of a 50 V applied voltage. Increasing constructual parameter (β) yielded an increase in reaction rate and a decrease in temperature drop. At the same constructual parameter, the serpentine geometry displayed a better result with the peak reaction rate of 894 mol/m3s. More so, increase in the Reynolds number and shape factor for the two geometries led to an increase in reaction rate and propane consumption. These findings could be suitably beneficial to provide minimal fuel requirement for miniaturized vehicles and micro-heat engines.

Temperature Control Measures and Temperature Stress of Mass Concrete during Construction Period in High-Altitude Regions

The focus on the development of China’s vast hydropower resources has shifted to Tibet and other plateau regions. These areas are high-altitude regions whose basic climatic characteristics are as follows: dry climate, significant differences in daily temperature, and strong solar radiation. If a dam is built under such special climate conditions, specific and strict temperature control and crack prevention measures should be taken. Therefore, this study explores the temperature control standards, as well as temperature control and crack prevention measures, for concrete in high-altitude regions using three-dimensional finite element methods and based on the concrete gravity dam in Tibet in combination with the characteristics of material properties that are disadvantageous to temperature control and crack prevention. The temperature drop process can be optimized in time, and the temperature drop rate can be controlled to prevent excessive scale and temperature drop rates. Moreover, the temperature gradient can be spatially optimized, and thus, the differences in foundation temperatures, upper- and lower-layer temperatures, and internal and external temperatures can also be reduced. The research shows that the recommended temperature control and crack prevention measures can effectively reduce temperature stress. This study has a significant value as a reference for similar projects in high-altitude regions.

Damage effects and mechanism of the silicon NPN monolithic composite transistor induced by high-power microwaves

A two-dimensional model of the silicon NPN monolithic composite transistor is established for the first time by utilizing the semiconductor device simulator, Sentaurus-TCAD. By analyzing the internal distributions of electric field, current density, and temperature of the device, a detailed investigation on the damage process and mechanism induced by high-power microwaves (HPM) is performed. The results indicate that the temperature elevation occurs in the negative half-period and the temperature drop process is in the positive half-period under the HPM injection from the output port. The damage point is located near the edge of the base–emitter junction of T2, while with the input injection it exists between the base and the emitter of T2. Comparing these two kinds of injection, the input injection is more likely to damage the device than the output injection. The dependences of the damage energy threshold and the damage power threshold causing the device failure on the pulse-width are obtained, and the formulas obtained have the same form as the experimental equations, which demonstrates that more power is required to destroy the device if the pulse-width is shorter. Furthermore, the simulation result in this paper has a good coincidence with the experimental result.

Numerical Study on the Characteristic of Temperature Drop of Crude Oil in a Model Oil Tanker Subjected to Oscillating Motion

During tanker transportation, crude oil is heated occasionally to ensure its good flowability. Whether the heating scheme is scientific or not directly influences the safety and economy of the tanker transportation. The determination of a scientific heating scheme requires fully understanding of the characteristic of oil temperature drop during tanker transportation. However, the oscillation caused by the marine environment leads to totally different thermal and hydraulic characteristic from that of the static cases. Therefore, a systematic investigation of thermal and hydraulic process of the motion system is more than necessary. Since the marine is subjected to rotational and/or translational motion, the essence of the temperature drop process is an unsteady mixed convection process accompanied with free liquid surface movement. In this study, the movement of the free liquid surface and the characteristic of the temperature drop of the crude oil in the cargo when the tanker is subjected to rotational motion were investigated using ANSYS FLUENT (15.0, Ansys, Inc., Canonsburg, PA, USA) with user defined functions. The research result shows that the oscillating motion leads to the motion of the free surface, converting the natural convection for the static case to forced convection, and thus significantly enhancing the temperature drop rate. It is found that the temperature drop rate is positively related to the rotational angular velocity.

Application of MgO Compound Expansion Agent in Concrete Filled Steel Tube

An experimental investigation on the restrained expansion rate of motar, and a component test on the temperature and strain history of concrete filled steel tube (CFST) were carried out to evaluate the effect of calcium oxide (CaO) and magnesium oxide (MgO) compound expansion agent (EA) on the volme stability of CFST. The results indicated that CFST has experienced drastic temperature rise and drop process at its early age, but the hydration reaction activity of CaO expansion agent generally used at present was so high that they couldn’t compensate CFST thrinkage at temperature drop stage effectively. Light burned MgO clinker with active reaction time about 100 s and light burned CaO clinker were compounded in a certain ratio as a new kind of concrete expansion agent was applied in CFST and it matched the shrinkage history of CFST well, therefore, the expansion prepressing stress from core concrete to steel tube wall has been established and maintained effectively.

Investigation of temperature drop in domestic hot water circuit

The results of experimental investigation of determining the time of temperature drop in domestic hot water circuit were presented in this paper on the example of single-family house. The test were carried out on vertical sections of installations made of PE-X/Al/PE pipes (cross-linked polyethylene and aluminum) with and without insulation. The temperature drop process was investigated in temperature range typical for domestic hot water installation from 55ºC to 40ºC. On the basis of the obtained results with appropriate adjustment of the working time of circulation pump, energy savings of 90% were achieved.

Numerical study on the temperature drop characteristics of waxy crude oil in a double-plate floating roof oil tank

Physical and mathematical models for the temperature drop process of the waxy crude oil in a double-plate floating roof oil tank are established with the consideration of non-Newtonian behavior and wax precipitation accompanying the phase change of waxy crude oil. Accordingly, an integrative numerical procedure is developed for the calculation. In this study, non-Newtonian behavior is described by power-law equation. Wax precipitation is described by the enthalpy-porous media theory, in which the gel oil interface is tracked. The computer code is validated by the results in literatures. Based on the verified computer code, characteristics of temperature drop in the tank are studied and the influences of wax precipitation amount and non-Newtonian behavior on the temperature drop process are analyzed in detail. The results show that when oil is not gelled, the oil tank can be divided into two parts: (1) the top and middle part of approximate uniform temperature; (2) the bottom part of stratified distribution of temperature. It is also found that increasing of the wax precipitation amount will slow down the temperature drop process and extend the duration of the temporal stable period. Furthermore, when oil temperature is between abnormal point and thixotropy appearance point, whether considering the non-Newtonian behavior or not covers certain influence on temperature drop process, and different non-Newtonian behaviors play less effect. When oil temperature is below thixotropy appearance point, strengthening of the non-Newtonian behavior will slow down the temperature drop process slightly.

Mechanism and preparation of liquid alkali-free liquid setting accelerator for shotcrete

A new alkali-free liquid accelerator for shotcrete was prepared through normal temperature drop process by using the nano activated alumina and the modified alcohol amine as the main raw materials. The effect of alkali-free liquid accelerator on the cement setting time and the mechanical properties of mortar, the effect of the penetration strength on the shotcrete rebound were investigated. And the accelerating mechanism of the as-prepared alkali-free liquid accelerator was also analyzed via XRD and SEM characterization methods. The experimental results indicated that the hydration of C3A was accelerated by the polyamine complexation of accelerator, resulting in forming a large number of acicular ettringite and reducing the amount of Ca(OH)2 crystal, which would not affect the later hydration of cement. When the content of alkali-free liquid accelerator was 6%, the initial setting time and final setting time were less than 3min and 8min respectively, and 1d and 28d compressive strength ratios reached 207.6% and 114.2% respectively; beside that, the shotcrete rebound was very low because of the high penetration strength within 30min.

Fracture Research on Preheating Installation of Directly Buried Heating Return Water Pipe with Large Diameter

Pipe fracture and compensator tear may happen when directly buried pipes with large diameter, high temperature and pressure adopt preheated installation. Combined with engineering, this paper analyzes theoretically the fracture accident on DN1200 return water pipe during the period of cold operation and analyzes various factors which cause the pipeline fracture, when adopt open ditch electric preheating. By the numerical simulation, the size of influence factors on fracture was obtained, and the conclusion that the tensile stress is greater than the compressive stress was also obtained, where the preheating temperature was defined by means of “intercycle temperature”. It is worth nothing, the buried pipeline will emerge vertical bending under the main impact of soil and transportation loads. The vertical bending is related to pipe base, and the value of curving stress will be very big if the pipe base is mishandled. The coupling effect of curving stress and the temperature tensile stress can lead the pipeline to fracture, such as the disposable compensator place, the pipeline welding place, corners, the pipe wall flaw place and so on. This paper proposed the effective preventive measure which may reduce the occurrence probability of pipeline fracture during temperature drop process.

Interface damage and its effect on vibrations of slab track under temperature and vehicle dynamic loads

This paper presents a three-dimensional finite element model to investigate the interface damage occurred between prefabricated slab and CA (cement asphalt) mortar layer in the China Railway Track System (CRTS-II) slab track system. In the finite element model, a cohesive zone model with a non-linear constitutive law is introduced and utilized to model the damage, cracking and delamination at the interface. Combining with the temperature field database obtained from the three-dimensional transient heat transfer analysis, the interface damage evolution as a result of temperature change is analyzed. A three-dimensional coupled dynamic model of a vehicle and the slab track is then established to calculate the varying rail-supporting forces which are utilized as the inputs to the finite element model. The non-linearities of the wheel–rail contact geometry, the wheel–rail normal contact force and the wheel–rail tangential creep force are taken into account in the model. Setting the maximum interface damaged state calculated under temperature change as the initial condition, the interface damage evolution and its influence on the dynamic response of the slab track are investigated under the joint action of the temperature change and vehicle dynamic load. The analysis indicates that the proposed model is capable of predicting the initiation and propagation of cracks at the interface. The prefabricated slab presents lateral warping, resulting in severe interface damage on both the sides of the slab track along the longitudinal direction during temperature drop process, while the interface damage level does not change significantly under vehicle dynamic loads. The interface damage has great effects on the dynamic responses of the slab track.

복사조도의 번동에 따른 온도 및 적외선복사량의 변화

적외선복사에 의한 열적 반응은 물질의 온도상승이나 온도하강 등의 과정을 통하여 물질의 물리적 손상을 초래한다. 본 연구에서는 광원의 적외선복사에 의한 시료의 온도상승과 적외선복사량을 측정할 수 있는 측정시스템을 구축하고, 전시조명용으로 많이 사용하는 광원들을 대상으로 각 광원의 복사조도를 변화시키면서 온도변화와 적외선 복사량의 변화를 각각 측정하였다. 측정의 결과를 토대로 복사조도와 온도 및 적외선복사량간의 함수관계를 파악하고, 복사조도의 변동에 따른 온도변화 및 적외선복사량의 변화를 비교, 분석하였다.