Liquid Phase Region Research Articles

Blade Designs For Improved Multi-Phase Performance In sCO2 Compressors; Part II - Optical Diagnostics In sCO2 And Experimental Evaluation With Particle Image Velocimetry

Abstract This paper presents the second part of a study in which the leading-edge and suction surface of a compressor blade was modified to delay onset of phase change for sCO2 compressors operating near the critical point. Using a first-of-its-kind apparatus for the measurement of sCO2 flow fields, Particle Image Velocimetry (PIV) is used for local flow field measurements of two compressor blade geometries: the modified “biased-wedge,” and a conventional constant thickness blade. Utilizing the developed hardware, the feasibility of a simple, laser-based diagnostic for qualitatively measuring liquid phase regions, is also presented. The design of the optical diagnostics rig, a discussion of numerous challenges, and necessary considerations involved in performing optical-based measurements like PIV, in sCO2, are discussed. Velocity field measurements for the modified compressor profile show a much lower suction peak compared to a conventional blade. These results validate numerical results at the tested conditions, where the suction side profile of the biased wedge works to minimize the local pressure gradient.

A molecular dynamics study on the supercritical evaporation of liquid ammonia under forced convection conditions

Although the behavior of the supercritical transition during the evaporation of liquid fuels in engine combustion chambers has been studied by many researchers, the effects of convection on the phase transition of liquid fuel in supercritical environments have been little studied. Previous studies mainly focused on hydrocarbon fuels for supercritical evaporation, with fewer on the zero-carbon liquid ammonia fuel. Aiming at the above deficiencies, this study used molecular dynamics simulations (MD) to explore the phase transition of liquid ammonia in a nitrogen supercritical environment under forced convection conditions. The Peng-Robinson equation of state (PR-EoS) was used to calculate the vapor–liquid equilibrium of the ammonia–nitrogen system to determine the supercritical transition time of liquid ammonia under forced convection conditions. The effects of forced convection on the heat and mass transfer process and the evolution of the liquid-phase region and the vapor–liquid interface region were analyzed. It was found that forced convection can promote the mass and heat transfer process and accelerate the phase transition of liquid ammonia to some extent. Forced convection may have a greater effect on the single-phase diffusive mixing process compared to the two-phase evaporation process. The increase in the interfacial thickness under forced convection conditions is mainly attributed to the temperature increase promoted by convection. The interfacial temperature dominates the development of its thickness and the thickness is approximately linear with temperature during the diffusive mixing stage. The shear effect induced by convection has an inhibitory effect on the diffusion of ammonia molecules within the interface region along the vertical convection direction. The cases in this study indicate that forced convection may promote the supercritical transition of liquid ammonia. In addition, a dimensionless number κ was proposed to explore the possibility of scaling up the MD results to the engineering scale.

A semi-analytical method based on the Green's function for the laser melting process of the metal materials

Thermal responses of the metal materials subjected to intense laser irradiation are complicated due to the phase change in the melting process. In order to simplify this problem, previous studies usually ignored the liquid phase or took the usage of finite element method, which limited the achievement of theoretical breakthroughs. In the present study, a one-dimensional heat transfer model with consideration of phase change was established to describe the melting process of the metal materials caused by a constant intense laser beam. The governing equations with three domains were constructed in the piecewise form: the liquid phase region (melted already), the solid phase region (unmelted yet) and the melting region. The limited discretization method was employed to describe the moving of the soli-liquid interface and the energy exchange. The piecewise governing equations in liquid and solid region were solved separately and analytically by means of the Green's function method, and the calculation result was verified by the finite element simulation. The results of the analytical model were in good agreement with those of the finite element simulation, and the maximal prediction errors of the temperature field prediction between the theoretical model and FE simulation is limited to 2.8 %. In addition, the influences of heat source intensity, material heat conductivity, latent heat on the melting process were discussed in detail. The analytical and simulation results reveal that the semi-analytical solution in piecewise for 1-D laser melting process can be successfully used to reflect the dynamic temperature response of the metal materials and the movement of the solid-liquid interface. This work is of guiding significance for the analytical solution of the multi-dimensional melting process of the metal materials under the fixed and continuous laser beams.

Carbon Macrosegregation Pattern in 30Cr15Mo1N Ingot: Impact of Pouring Rate

The effect of the pouring rate on the carbon macrosegregation in 30Cr15Mo1N ingots has been clarified by investigating changes in the content of carbon‐containing precipitated phases, cooling rate, secondary dendrite arm spacing (SDAS) and the flow characteristics of molten steel. During the solidification process, the solute‐enriched liquid steel migrates toward the center of the ingot under the action of convection, resulting in an increasing segregation ratio from the edge to the center. However, the growth of equiaxed grains partially restricts the flow of solute‐enriched liquid steel, which leads to a maximum segregation ratio not at the center but at approximately a quarter radius from the center. Furthermore, due to the upward flow of molten steel in the center, positive segregation occurs at the top. An increment in the pouring rate reduces the SDAS by enhancing the cooling rate, and thus the permeability decreases. It aggravates the carbon macrosegregation horizontally. However, increasing the pouring rate decreases the carbon macrosegregation vertically. This is due to the more intense flow of molten steel and the larger liquid phase region at higher pouring rate, which reduces the solute enrichment degree and promotes compositional homogeneity above and below the ingot.

Study on structure and viscosity evolution mechanism of titanium-containing iron liquid in full viscosity range

This study aims to investigate the relationship between the viscosity of Fe-4.5wt%C-0.2wt%Ti melt and its liquid structure. The actual experimental results of viscosity measurement reveal that the viscosity range of Fe-4.5wt%C-0.2wt%Ti melt can be segmented into three intervals. Through thermodynamically calculated equilibrium phases and high-temperature confocal experiments, it is observed that the abrupt change in viscosity in the lower temperature range is attributed to the precipitation of the graphite phase. Moreover, differential scanning calorimetry experiments exhibit a distinct phase transition in the liquid phase region. To delve into the atomic structure of the liquid phase region, molecular dynamics simulations are employed. The high-temperature liquid structure of Fe-4.5wt%C-0.2wt%Ti melt predominantly consists of vacancies, atomic clusters with Ti as the core, and atomic clusters with C as the core. Within the high-temperature liquid phase region, the conversion of Free-Fe and C1-Fe to Cx-Fe occurs, resulting in a reduction of free volume and enhanced stability of the cluster. Consequently, the inhomogeneity of the Fe-4.5wt%C-0.2wt%Ti melt increases, and the viscosity exhibits a significant increase with temperature, leading to a variation in the viscosity within the liquid phase interval.

The Impact of In-Flight Acceleration Environments on the Performance of a Phase-Change Heat Exchanger Unit with Layered Porous Media

The Phase-Change Heat Exchanger Unit in Layered Porous Media (PCEU-LPM) is obtained through frozen pouring processing, and exhibits characteristics such as high thermal conductivity, high latent heat, and high permeability, making it suitable for dissipating heat in airborne electronic devices. This study numerically investigates the impact of aircraft speed acceleration conditions, which lead to weightlessness or overload, on the performance of the PCHEU-LPM, with a particular focus on the influence of natural convection in the liquid-phase region. Initially, a microscale thermal analysis model is established based on the Navier–Stokes equation scanning electron micrograph to calculate the effective thermal conductivity and permeability of the PCHEU-LPM under different porosities. Subsequently, these parameters are incorporated into a macroscale thermal analysis model based on Darcy’s law, employing an average parameter approach. Using the macroscale thermal analysis model, temperature and velocity fields are computed under various porosities, acceleration magnitudes, and directions. The calculation results indicate that as the acceleration increases from α = 0 to α = 10 g, the interface temperature of the PCHTU-LPM decreases by approximately 5.2 K, and the temperature fluctuation decreases by 2.4 K. If the porosity of the PCHTU-LPM is increased from ε = 70% to ε = 85%, the influence of acceleration change on natural convection will be further amplified, resulting in a decrease in the interface temperature of the PCHTU-LPM by approximately 10.2 K and a decrease in temperature fluctuation by 5.8 K. When the acceleration direction is +z, the interface temperature of the PCHTU-LPM is at its lowest, while it is highest when the acceleration direction is −z, with a maximum difference of 15.4 K between the two. When the acceleration direction is ±x and ±y, the interface temperature lies between the former two cases, with the interface temperature slightly higher for ±y compared to ±x, with a maximum difference of 3.9 K between them.

Dynamic Response of Phase Change Heat Exchange Unit with Layered Porous Media for Pulsed Electronic Equipment

Effective heat dissipation challenges transient high-power electronic devices in hypersonic vehicle cabins. This study introduces a Phase Change Heat Exchange Unit with Layered Porous Media (PCHEU–LPM) employing pulsed heat flow at the top and forced convection at the bottom. The primary aim was a comparative parametric study analyzing the thermal response of the heating surface under pulsed heat flow conditions. The geometric model was generated using electron microscopy images of manufactured objects and the numerical model was established based on the enthalpy–porosity method. Numerical simulations explored amplitude and frequency effects on pulsed thermal excitation, evaluating temperature and phase fields. A comprehensive time-frequency transformation assessed the temperature response. The results indicated an initial decrease and subsequent increase in interface temperature fluctuation with pulse heat flux amplitude growth. Temperature field uniformity correlated with natural convection strength in two-phase and liquid-phase regions. At mid and low frequencies, the phase change process increasingly suppressed interface temperature fluctuations. Optimal pulse thermal excitation selection was crucial for minimizing temperature fluctuations while maintaining the interface temperature within the expected phase transition range. In conclusion, a novel design concept is posited herein, aiming to enhance surface temperature uniformity and broaden the applicability of electronic devices through the manipulation of porosity rates.

Phase diagram of ternary Co–Fe–Ge system (I): Experimental

Co–Fe–Ge is an important material system for magnetic and catalyst applications. However, phase equilibria information of the Co–Fe–Ge ternary system is limited. Ternary Co–Fe–Ge alloys equilibrated at 950 °C were prepared. The microstructures, chemical compositions and crystal structure of each phase were determined. The phase equilibria isothermal section at 950 °C of the Co–Fe–Ge ternary system was then proposed. A continuous solid solution phase was formed between the Co5Ge3 and Fe5Ge3 phases and was labeled the β(Co,Fe)5Ge3 phase. There are five three-phase regions in the system at 950 °C. A wide α2 single-phase, and liquid phase regions with large composition ranges being confirmed at the Ge-poor and Ge-rich sides, respectively.

Effect of adaptive nanocrystalline behaviors on the cavitation erosion performance of Cu47.5Zr45.1Al7.4 bulk metallic glass

Amorphous alloys have attracted much attention in the field of cavitation erosion (CE) due to their good comprehensive properties. However, due to the special amorphous structures that are difficult to characterize, their micro-response behaviors and damage mechanisms during the CE process have not been well elucidated. In this paper, advanced techniques including focused ion beam (FIB) and transmission electron microscope (TEM) were used to comparatively study the microstructure evolution and volume loss rules of Cu47.5Zr45.1Al7.4 and Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glasses (BMG) under the action of cavitation load and cavitation heat to reveal their CE mechanisms. The results showed that the absence of small atoms in Cu47.5Zr45.1Al7.4 led to a large free volume, thereby compromising its hardness, modulus, and yield strength. However, this also made it more susceptible to shear banding, which in turn enhanced its energy absorption capabilities. Although Cu47.5Zr45.1Al7.4 had a higher glass transition temperature (Tg) and onset temperature of crystallization (Tx), as well as a wider supercooled liquid phase region (ΔTx), its exothermic peak was obviously pronounced and the transformation temperature range was significantly narrower. Therefore, it is more likely to adaptively form Cu-rich nanocrystals under the action of cavitation heat. Additionally, the larger free volume in the shear band was conducive to the diffusion of atoms and the occurrence of adaptive nanocrystallization behaviors in Cu47.5Zr45.1Al7.4. These nanocrystals were able to effectively force crack deflection, thereby dissipating impact energy and delaying the fatigue spallation of the material. Consequently, Cu47.5Zr45.1Al7.4 exhibited a far longer incubation period and a noticeably lower cumulative volume loss.

Effect of TiO2 addition on microstructure and mechanical properties of Al2O3 ceramic joints brazed with Ag‐CuO fillers

AbstractIn our work, 0.5 mol.% TiO2 was added to Ag‐xCuO (x = 2, 4, 8 mol.%) fillers whose compositions at 1050°C were in the L2 single liquid phase region, the boundary of the miscible phase region, and the L1+L2 miscible phase region, respectively. The role of TiO2 was derived by comparing the quenching microstructure of the fillers on the Al2O3 ceramics and the effect on the interfacial microstructure and shear strength of the joints before and after the addition of TiO2. The results showed that the TiO2 would dissolve into the CuO‐rich L1 liquid phase of the filler, leading it to spread outside the molten droplet. When brazing with Ag‐4CuO‐0.5TiO2 filler, the CuO at the joint interface almost completely disappeared, and the shear strength of the joint was significantly reduced. In contrast, the joint strength showed a small increase when using Ag‐8CuO‐0.5TiO2 filler, as the excess CuO at the interface of the brazed joint was reduced. When the filler composition was in the L2 single liquid phase region, CuO was diffusely distributed throughout the molten droplet. TiO2 was hardly contacted with CuO and its addition had no significant effect on the interfacial microstructure and the shear strength of the joints.

Statistical characteristics of convective and stratiform precipitation during the rainy season over South China based on GPM-DPR observations

South China is one of the wettest regions in the world, and is prone to precipitation-related disasters. The GPM-DPR product is utilized in this study to investigate the vertical structure and drop size distribution (DSD) characteristics of different precipitation types, i.e., stratiform precipitation (SP) and convective precipitation (CP) in South China. The precipitation frequency and accumulation in South China are dominated by SP and deep CP, respectively. Shallow CP has comparable occurrence ratio to deep CP, but the mean intensity is much lower. As such, it contributes <5% to the total rainfall. Vertical features differ among precipitation types and intensities. Higher storm top altitude (STA) is roughly related to more intense precipitation, especially for CPs. As surface PR increases in deep CP, the contribution of increase of PR (ΔPR) in liquid-phase region to total PR also increases, from approximately 10% for 4 mm/h to 55% for 100 mm/h. The intensity of shallow CP is generally low, but ΔPR in the liquid-phase region is significant. For SP and shallow CP, higher precipitation intensity is often caused by higher hydrometer concentration. Differently, for deep CP with PR >15 mm/h, both the increases in hydrometer size and concentration contribute to the increase in PR. The collision-coalescence process is dominant in the low-level layers in SP without bright band and CP, and approximately four fifth of shallow CP samples are influenced by collision-coalescence process. Compared with inland mountainous region, collision-coalescence process plays more important role in intense precipitation in coastal region. The results of this study indicate that it is important to consider the vertical evolution and DSD characteristics of different precipitation types for improving the accuracy of near surface quantitative precipitation estimate.

Interaction between growing dendrite and rising bubble under convection

Prediction and control of gas porosity defect during solidification is challenging because of multiphase and multiphysics characteristics. In this work, a multiphase-field lattice Boltzmann method is employed to study the interaction between solidifying dendrite and rising bubble. The influencing factors are divided into two categories including external and internal factors, and the effects of the multiple factors are discussed and compared. The bubble changes the dendrite skeleton by hindering the solid growth, including entrapped in the dendrite root, tilting growth towards the neighboring bubble, forming the knot structure, and growing along the radial direction of the bubble. The bubble in the columnar dendrite array with random orientation distribution has larger effects on the average primary dendrite arm spacing. Based on the summary of different interaction results, a phase diagram is proposed to distinguish different interaction regimes. Further, the size of the closed liquid phase region is measured to evaluate the porosities tendency and guide how to reduce the porosities.

Comparative molecular dynamics study of liquid hydrogen annular jets in subcritical and supercritical environments

In this paper, the injection characteristics of a liquid hydrogen propellant jet under subcritical and supercritical operating conditions are comparatively studied at the molecular scale using molecular dynamics methods in the context of spacecraft propulsion systems. To control the temperature and pressure within the environment and nozzle respectively, two sub-computational domains are set up for separate modeling. The sub-computational domain merging process eliminates the broken molecular bonds by using the computational domain reduction in the x-direction. The jet velocity, density and other physical property distributions at different temperatures and pressures were comparatively studied. It is found that the rate of change of the radial density of the jet under supercritical conditions is small. Based on this, the surface area and volume of the liquid phase region of the jet were extracted and calculated. Compared to result of subcritical case, the volume of the liquid hydrogen jet in the supercritical environment is reduced by 1.9%, while the surface area is increase by 6.7%. To explain the variation in the surface area due to molecular diffusion, the molecular stress results revealed that the stress in the hydrogen jet in supercritical environment is only 88.9% of that in subcritical environment.

Analysis of the structure and viscosity of iron melts containing titanium at various concentration

This study aims to investigate the correlation between viscosity and liquid structure in Fe-4.5 wt%C-xTi melts. As the temperature decreases, Free-Fe and A-Fe species undergo a transformation into B-Fe, resulting in a significant reduction in free volume. This leads to the aggregation and growth of existing clusters as well as the formation of new clusters, thereby increasing both the number and average volume of atomic clusters. Consequently, the viscosity of the melt experiences a substantial increase as the temperature decreases. In the high-temperature liquid phase region, an elevated titanium content enhances the abundance of titanium-centered clusters within the melt, promoting the stable growth of atomic clusters. Additionally, the increased presence of titanium facilitates the binding of a greater number of carbon atoms and enhances the occupation of free space by iron atoms. As a combined effect, the viscosity of the molten iron rises with an increase in titanium content. At temperatures below the liquidus temperature, a higher titanium content decreases the precipitation temperature of graphite. Consequently, melts with a greater titanium content exhibit a pronounced increase in viscosity at a lower temperature threshold. The primary factor influencing viscosity is the mass fraction of graphite precipitation, and the impact of titanium content diminishes as the titanium content increases.

A Revisit of Superconductivity in 4Hb-TaS2−2xSe2x Single Crystals

Previous investigations of 4$H_b$-TaS$_{2-2x}$Se$_{2x}$ mainly focused on the direct competition between superconductivity and charge density wave (CDW). However, the superconductivity itself, although has been prominently enhanced by isovalent Se substitution, has not been adequately investigated. Here, we performed a detailed electrical transport measurement down to 0.1 K on a series of 4$H_b$-TaS$_{2-2x}$Se$_{2x}$ single crystals. A systematic fitting of the temperature-dependent resistance demonstrates that the decreased Debye temperatures ($\Theta_{D}$) and higher electron-phonon coupling constants ($\lambda_{e-p}$) at the optimal Se doping content raise the superconducting transition temperature ($T_c$). Additionally, we discovered that the incorporation of Se diminishes the degree of anisotropy of the superconductivity in the highly layered structure. More prominently, a comprehensive analysis of the vortex liquid phase region reveals that the optimally doped sample deviates from the canonical 2D Tinkham prediction but favors a linear trend with the variation of the external magnetic field. These findings emphasize the importance of interlayer interaction in this segregated superconducting-Mott-insulating system.

Purifying Fe-based amorphous alloys to enhance glass-forming ability by designing a novel refining slag

• A novel refining slag design strategy was proposed for Fe-based amorphous alloys. • The designed SiO 2 -CaO-Al 2 O 3 -B 2 O 3 refining slag was effective in enhancing the GFA. • Inclusions with small lattice disregistry can promote the heterogeneous nucleation. • The enhanced GFA is due to the reduced impurities and inclusions after refining. Designing low melting point and low basicity refining slag suitable for Fe-based amorphous alloys and understanding the inclusions’ formation, removal, influencing mechanisms are quite vital in the fields of metallurgy and materials. In this study, a novel 13%SiO 2 -32%CaO-30%Al 2 O 3 -25%B 2 O 3 (wt.%) refining slag was designed after careful calculations of the liquid phase region, slag-metal equilibrium, surface tension, viscosity, deoxidation capability and sulfur distribution ratio. After refining with our designed slag, the content of impurities and the number density of inclusions in a representative Fe 83 Si 2 B 15 (at.%) amorphous alloy were significantly reduced. Moreover, the glass-forming ability (GFA) of the alloy was also enhanced, enabling the preparation of amorphous ribbons with a lower cooling rate. Based on the impurities in Fe-based amorphous alloys as well as the calculated oxide and sulfide free energy diagrams, CaO, SiO 2 , Al 2 O 3 oxides and CaS, TiS, MnS sulfides will form in the master alloy. The high melting point inclusions in the melt are generally removed via a floatation-separation-absorption process and the Mn, Ti, S impurities are removed via slag-metal interface reactions during refining. As for the detrimental effect of inclusions on glass formation, the small lattice disregistry between Ti, Mn-containing inclusions and primary α-Fe gains reveal that these inclusions are effective in promoting the heterogeneous nucleation, and therefore greatly deteriorate the GFA. These findings are important and provide an ideal solution to purifying the Fe-based amorphous alloys by refining and enhancing the GFA for industrial production.

Characterization of Two-Phase Wakes in an Upward Adiabatic Liquid-Gas Flow Around a Cylinder

AbstractTwo-phase wakes generated from a cylinder in a crossflow were experimentally studied. A water–air mixture traveled through a vertical water channel with a rectangular cross section, in which a cylinder was installed horizontally. Liquid Reynolds numbers, based on a cylinder diameter of 9.5 mm, were varied from Re = 100 to 3,000; the air superficial velocities were varied from jg = 0.06 m/s to 0.60 m/s; and mean bubble diameters were varied from 0.48 mm to 3.5 mm. Void fraction distribution in the wake of the cylinder was determined from high-speed visualizations, where a correlation was applied to the shadow fraction measurements to account for overlapping bubble images. It divided the wakes into a liquid-phase region with a low void fraction relative to its freestream condition (α/α∞&amp;lt;1/2) and a bubble-trapping region with a relatively high void fraction (α/α∞&amp;gt;2). The liquid-phase region occurred in all flow conditions, but its length decreased with increasing Reynolds number. In contrast, the bubble-trapping region occurred only at relatively high Reynolds numbers depending on the bubble size and air superficial velocity. Transitional bubble-trapping behavior was identified at Re = 1,200 for the 3.5 mm bubbles, where bubble trapping only occurred at low air superficial velocities. Once the bubble-trapping region developed sufficiently, the location of the maximum void fraction was consistently located at y/D = 1.3–1.5 downstream from the center of the cylinder.

Experimental investigation on collision characteristics of dual-spray with different physical-chemical fuel properties: A case study of butanol-biodiesel fuel combination

The dual direct injection strategy can flexibly control the reactivity distributions and equivalence ratio, which shows a great potential in realizing clean and efficient combustion. However, the sprays’ collision is inevitable when the fuel sprays are injected into the combustion chamber simultaneously owing to limited in-cylinder space, which is rarely investigated. The investigation aims to explore the atomization and combustion characteristics of collision between biodiesel and butanol sprays at different injection delay times of biodiesel (0, 0.5, 1, 1.5, 2 ms). Experiments are conducted in a constant volume combustion chamber. The collision spray and combustion images are captured by optical diagnosis techniques. Several macroscopic parameters are obtained, including spray behavior, spray area, ignition delay, ignition location, flame behavior, flame lift-off length, and natural luminosity intensity. Results show that the variation of injection delay time can effectively change the spray distribution and realize the concentration stratification. The liquid-phase diffusion region increases with an increased premixed charge of butanol spray, but the total evaporation time has a tiny difference. The sprays’ collision can promote the ignition process, but the ignition location is slightly affected by the injection delay time of biodiesel. A trade-off relationship between the vapor-phase diffusion area and combustion characteristics is found. When the injection delay time exceeds 0.5 ms, a longer injection delay time can lead to a larger vapor-phase area, which causes smaller equivalent ratio, longer flame-off length, and lower luminosity intensity. However, a longer injection delay would lead to an uncomplete combustion of the n-butanol spray. At the experimental cases, the injection delay time of biodiesel should be 1/2–2/3 injection duration to realize complete combustion and small emissions.

Dynamic tuning of phase change interfacial thermal energy transport in hybrid nano-enhanced phase change materials by a non-uniform magnetic field

The present study aims to explore the synergistic enhanced phase change heat transfer behavior between magnetic field and hybrid nanoparticles under the consideration of non-Newtonian effects. The visualized experiments are conducted using thermochromic liquid crystal technology to extract the interconnection between dynamic phase change interface evolution and temperature change of hybrid nano-enhanced phase change material (HNEPCM), and thermocouples to measure temperature data inside the latent heat thermal energy storage unit. The magnetic field and hybrid nanoparticles synergistic mechanism is then further revealed based on the analysis of liquid phase fraction, Nusselt number, energy storage and energy storage efficiency. The results show that under magnetic field, the magnetic nanoparticles form a “thermal conductivity layer” at the solid-liquid interface, while the non-magnetic nanoparticles in the liquid phase region drive the phase change heat transfer to proceed. The non-Newtonian effect induced by high concentration of HNEPCM can weaken the convection intensity. The competition between natural convection and magnetic viscous effect under magnetic field is induced at different hybrid ratios of magnetic and non-magnetic nanoparticles. High hybrid ratios (R = 1.0, 1.5 and 2.0) will be more favorable to satisfy the high thermal conductivity requirement in the liquid phase region and the dynamic adjustment condition at the phase change interface. Under a non-uniform magnetic field, 1 wt.% HNEPCM with R = 1.0 is considered as the optimum strategy with better energy storage performance and higher magnetic field effect.

Study of fuel spray characteristics using a high-pressure common rail diesel injection system by the method of Schlieren

The supercritical atmosphere created by the increase in temperature and pressure in the diesel engine cylinder is the key factor that causes changes in the fuel atomization process. In this study, the differences in spray morphology of n-heptane under different ambient temperature and pressure conditions were investigated using a constant volume combustion bomb combined with the Schlieren method. Based on the Peng-Robinson equation of state (PR equation), several transport characteristics models are constructed to predict the transport characteristics of n-heptane under different operating conditions. During the experiment, the injection pressure and fuel temperature were kept constant (80 MPa, 300 K), the ambient pressure was varied from 2 to 5 MPa, and the fuel temperature was changed from 400 to 800 K, spanning from subcritical to supercritical state. Three different spray regions have been identified based on the fuel temperature and spray geometry: liquid phase region, gas-liquid interface mixing layer, gas phase region. The results show that in a subcritical environment (400 K, 2 MPa), the fuel injection and atomization process is dominated by evaporation. The degree of heat exchange between the fuel and the medium gas is very low, while a small amount of phase change occurs. When the fuel is in low (600 K, 3 MPa) and medium supercritical (600 K, 4 MPa) ambient atmosphere, turbulence becomes the dominant factor in the fuel atomization process. The mixing layer at the gas-liquid interface at the edge of the liquid jet increases significantly. When the fuel is in a high supercritical (700 K, 4 MPa; 800 K, 5 MPa) atmosphere, the jet takes on a shape similar to that of a gas jet, and the mixing layer at the gas-liquid interface becomes the main part of the spray projection area.