Thermal Acceleration Research Articles

Topology optimization of regenerative cooling structures under high Reynolds number flow with variable thermo-physical properties

In the design of thermal protection system for hypersonic vehicles, the efficient convective heat transfer within regenerative cooling channels plays a critical role in maintaining their thermal performance. This study focuses on the topology optimization (TO) of the cooling channels that operate at high Reynolds number and have variable thermo-physical properties. A novel approach is introduced to enhance the density-based method by incorporating an effective artificial force correction and tabulating the temperature-dependent thermal properties. Several topology-optimized cooling layouts were generated to minimize the maximum temperature within the design domain while considering different power dissipation constraints. This confirms the validity of the proposed lumped correction term in momentum equation accounting both viscous and convective body forces. Regarding the thermal performance of the optimized layouts, the staggered arrangement of cellular ribs in the TO channel was found to induce multiple flow separations and promote turbulent mixing, thereby improving heat transfer efficiency and mitigating the thermal acceleration effect. Conjugate heat transfer calculations revealed that the optimal topology-optimized channel achieved a 24.3% improvement in overall thermal performance while being 15.3% lighter than the straight channel design. Furthermore, the optimal topology optimized layout consistently outperformed the straight channel design by 6.6% to 24.3% in terms of the overall thermal performance across a wide range of heat flux distribution and mass flow rate conditions. This investigation highlights the effectiveness of topology optimization in designing regenerative cooling structures featuring high Reynolds number hydrocarbon thermal fluid flow.

Experimental investigation on heat transfer enhancement of supercritical pressure aviation kerosene in tubular laminar flow by vibration

In advanced aero-engine thermal management systems, aviation kerosene serving as a coolant unavoidably works in a vibration environment. In this article, the laminar heat transfer performance of Chinese aviation kerosene RP-3 flowing through a horizontal micro-tube under various vibration conditions at supercritical pressures was investigated experimentally. The effects of several impact factors such as system pressure, heat flux, mass flux, inlet temperature, vibration acceleration, and vibration frequency on the heat transfer enhancement were explored in a systematic manner. Experimental results indicate that: (i) the vibration could lead to intense thermal and momentum mixing among different boundary layers of tubular laminar flow and significantly strengthens the heat transfer, and the higher Re can lead to a stronger enhancement effect. The maximum observed HTER across all experimental data is 2.8, occurring at x/d = 224.1 with the inlet temperature of 373 K; (ii) HTER hardly changes with system pressures, exhibiting a maximum relative deviation of 3.9 % at different pressures. Heat transfer enhancement has a strong dependency on heat flux, as the heat flux increases from 36 kW/m2 to 108 kW/m2, the average HTC increased by up to 36.4 %; (iii) the HTC and HTER monotonically rise with increasing vibration acceleration. Peak values in HTC and HTER are observed at vibration frequencies of 625 Hz, 191 Hz, and 242 Hz; (iv) vibration has little impact on the thermal acceleration but noticeably weakens the buoyancy close to the outlet area at high heat flux. Two well-predicted correlations for the Nu in tubular laminar flow, one with vibration and one without, are proposed.

Experimental and numerical study on the heat transfer deterioration of supercritical nitrogen in a vertical tube

Supercritical cryogenic fluids exhibit significant potential for diverse applications across various industries, including liquid air energy storage, high-temperature superconducting cables, and hypersonic vehicle engine cooling. Heat transfer deterioration (HTD) poses a substantial risk to the system safety. In this study, we constructed an experimental system and performed numerical simulations to illustrate buoyancy (Bu) and thermal acceleration (Ac) effects on HTD of supercritical nitrogen (SCN2). The newly established thresholds for buoyancy and thermal acceleration (Buth=1.8×10−4 and Acth=4.4×10−5), considering pseudo two-phase characteristics, can effectively capture buoyancy and thermal acceleration in the first and second HTD regions. The first region is influenced by the combined effect of buoyancy and thermal acceleration, while the second region is mainly influenced by thermal acceleration. The new correlations and thresholds accurately predict the occurrence of HTD and the peak position. The experimental and simulation results contribute to understanding the impact of buoyancy and thermal acceleration on SCN2 HTD.

Rotation and Dufour Impact on Unsteady Flow Past a Parabolic Accelerated Vertical Plate with Uniform Temperature and Mass Diffusion

Objective : Fluid dynamics and heat transfer theory examine the Dufour phenomenon and the rotational influence of unstable parabolic flow across an accelerating infinite vertical plate. This scenario involves several parameters, including the "mass Grashof number (Gr), thermal Grashof number (Gc), Prandtl number (Pr), Hartmann number (Ha), Schmidt number (Sc), Dufour number (Dc), and acceleration parameter". Method : We have applied the Laplace transform method to the resulting PDE. This method converts the equations to algebraic form, making it easier to solve for velocity, temperature, and concentration in terms of time and space. Graphs can show the relationship between the acceleration parameter and numerous parameters such as mass and thermal Grashof, Hartmann (Ha), and Dufour (Df), as well as the consequent velocity profile. If the velocity increases when these variables change, we'll look at the physical reasons that generate this behaviour. We will examine the conclusions using experimental data or existing theoretical models, considering the model's assumptions and limits. Finally, highlight the significant facts and insights gained from the analysis. We will discuss potential future research possibilities, such as exploring more complex geometries or accounting for new physical effects. Findings : The increase in Dufour parameter value causes an increase in temperature and level trends, demonstrating the significant influence of these factors on the researched phenomena. Novelty: This research advances our understanding of heat and mass transfer phenomena by isolating the Dufour effect in a novel scenario involving unsteady flow around a rotating vertical plate, filling a gap in the existing body of knowledge that is dominated by MHD-inclusive investigations. Suggestions: This approach is fairly general and may be applied to find analysis of heat and mass transfer on Dufour effect for other effects such as Soret effect, Hall current of heat and mass transfer. Keywords: Uniform Temperature, Thermo diffusion (Dufour), Inverse Laplace, Rotational, Constant Mass

Comprehensive analysis of the corrosion behavior of UG1 phosphate-based ultraviolet glass in water environments

We herein delve into the corrosion dynamics of UG1 in high-temperature water, employing thermal acceleration to expedite reactions. Corrosion-induced changes in spectral properties, optical constants, and morphology are analyzed. The results reveal a phosphorus-poor layer near the surface that affects transmittance, refractive index, and extinction coefficient, with corrosion rates following a parabolic model. This study provides crucial insights for protecting UG1 in humid environments and emphasizes the importance of implementing protective measures against phosphorus diffusion.

Mineralogical and chemical characterization of an oolitic iron ore, and sustainable phosphorus removal

An oolitic iron ore from Gara Djebilet, Algeria was characterized by X-ray diffraction, scanning electron microscopy, and energy dispersive spectrosopy and the phosphorus distribution between various phases studied. Two ore types were identified - a haematite-dominated, oolitic type and a more massive, magnetite-dominated type. The haematite-dominated ore had a higher phosphorus content, up to 0.6% in the iron oxide minerals, than the magnetite-dominated type (0.3% P). Owing to the very fine-grained texture of the gangue minerals, reducing the phosphorus content to levels suitable for steelmaking (below 0.05% P) by conventiobal means would entail an energy-intensive process with a high carbon footprint. Preliminary leaching test work using a sulphurous geothermal water as lixiviant, assisted by solar thermal acceleration, gave very promising results, removing 0.13% of the residual P (or 21.7% removal of total P) from the high-phosphorus oolitic ore.

Experimental and numerical investigation on the mechanisms of novel heat transfer deterioration of supercritical CO2 in vertical tubes

Understanding the mechanisms of supercritical CO2 heat transfer deterioration (HTD) has significant importance for ensuring the safety and reliability of supercritical Brayton cycles. Therefore, the effects of mass flux, wall heat flux, and tube diameters on the heat transfer characteristics of supercritical CO2 inside vertical tubes are studied using experimental and numerical methods. Results indicate that the heat transfer performance is positively influenced by an increase in mass flux and heat flux, as well as a decrease in tube diameter. When HTD occurs, the overall heat transfer performance is indeed significantly compromised. When the bulk fluid temperature exceeds Tpc, the thermal acceleration effect amplifies the mainstream velocity, stabilizing the Kelvin-Helmholtz instability between the pseudo-two-phases, thereby causing a new type of HTD that is named Type II HTD in this paper. The so-called post-dryout HTD occurs when the bulk temperature surpasses T+, causing a complete transition from pseudo-liquid to pseudo-gas state and resulting in a pseudo-single-phase flow. The heat transfer capability of the low-density and low-thermal-conductivity pseudo-gas diminishes as the temperature rises, leading to post-dryout HTD. The present work explains the mechanisms of two types of HTD and highlights the similarities between subcritical boiling and supercritical pseudo-boiling flow structures. This study also establishes a theoretical foundation for understanding pseudo-two-phase flow patterns during pseudo-boiling.

Magnetic and thermal acceleration in extragalactic jets

Aims. Relativistic jets launched from active galactic nuclei accelerate up to highly relativistic velocities within a length scale of between a few parsecs and tens of parsecs. The precise way in which this process takes place is still unclear. While magnetic acceleration is known to be able to accelerate relativistic outflows, little attention has been paid to the role of thermal acceleration. The latter has been assumed to act only on compact regions very close to the central engine, and to become negligible on parsec scales. However, this holds under the assumption of small internal energies relative to the magnetic ones, and whether or not this assumption is valid and what happens when we drop this assumption are open questions. Methods. We used a 2D relativistic magnetohydrodynamical code to explore jet acceleration from subparsec to parsec scales. As initial conditions for our models, we used observational constraints on jet properties derived by means of very long-baseline interferometry observations for a Fanaroff Riley I radio galaxy, NGC 315. We investigated the parameter space established for this source and performed a number of simulations of magnetically, thermally, or kinetically dominated jets at injection, and compared our results with the observations. Furthermore, we employed different models to characterize our jets, involving different magnetic field configurations (i.e., force-free vs. nonforce-free) and varying shear layer thicknesses. Results. Our simulated jets show that when thermal energy is comparable to or exceeds magnetic energy, thermal acceleration becomes significant at parsec scales. This result has important consequences, potentially extending the acceleration region far beyond the collimation scales, as thermal acceleration can effectively operate within a conically expanding jet. In all the models, we find acceleration to be driven by expansion, as expected. A number of our models allow us to reproduce the acceleration and opening angles observed in NGC 315. Finally, our results indicate that disk-launched winds might play an important role in jet propagation. Namely, when the jet has an initial force-free magnetic field configuration, thicker shear layers are needed to shield the internal spine from the action of the external medium and thus delay the growth of instabilities.

Numerical investigation on flow and heat transfer characteristics of a PCHE with liquid lead–bismuth eutectic and sCO2 as working fluids

The Printed Circuit Heat Exchanger (PCHE) plays a critical role in the supercritical carbon dioxide (sCO2) power generation systems, directly impacting the efficiency of the Brayton cycle. In this study,A comparative analysis was conducted to assess the flow and heat transfer characteristics of liquid lead–bismuth eutectic and sCO2 in the combined channel PCHE, as well as in the straight channel and zigzag channel PCHEs. The results indicate that the combined channel configuration of the PCHE offers improvements in heat transfer performance without significantly increasing the resistance of liquid LBE. Additionally, this study analyzes the influence of fluid velocity, pressure, temperature, buoyancy forces and thermal acceleration on the heat transfer characteristics of the combined channel.Finally, the numerical calculation results are evaluated using typical heat transfer correlation equations from the literature to assess their applicability.

Investigation of water phase change rotating cooling for high temperature turbine

Thermal protection of high temperature turbine restricts the performance of gas turbine. In order to increase turbine inlet temperature, water phase change rotating cooling scheme for turbine was proposed for using the gasification latent heat to improve cooling capacity. Numerical model of phase change rotating cooling was established to study the flow and heat transfer characteristics with Ansys CFX software using SST k-ω turbulence model. Results showed that phase change rotating cooling had good performance for cooling the rotating blades. The temperature drops of blades were more than 400 K with the average heat flux of 2.42 MW/m2. Due to the influence of flow thermal acceleration effect, the outlet pressure of cooling channel reached a maximum value of 7.23 MPa with a mass flow rate of 0.17 g/s. The outlet quality distribution under rotating condition was relatively uniform than nonrotating condition. The temperature increased firstly and then decreased in centripetal channel along phase change process because of pressure variation. Meanwhile, gas-liquid interface disappeared due to the Coriolis force, resulting in higher heat transfer coefficient than static condition. The normalized Nusselt number (Nus) of phase change rotating cooling reached 3.915, much higher than static liquid cooling.

Experimental study on the thermal runaway acceleration mechanism and characteristics of NCM811 lithium-ion battery with critical thermal load induced by nail penetration

NCM811 (Li(Ni0.8Co0.1Mn0.1)O2) lithium-ion battery (LIB) at 100 °C is in a critical state of internal chemical reaction and external thermal runaway (TR), and the coupled stimulations of nail penetration under such thermal load will accelerate TR, and coupled stimulations have hindered the development of LIBS. In this paper, an experimental platform for coupled stimulations of heat-penetration on LIBs was built, and revealed the thermal runaway acceleration mechanism, explored the influence of the SOC on the TR behavior of the cells when penetration under the critical thermal load condition. The results show that critical thermal load condition reduces the critical SOC for TR to occur, and 25% SOC NCM811 cell still produces a jet flame, elevating the fire risk of thermal runaway when compared to room temperature conditions. And the maximum temperature of 25% SOC cell was elevated by 76.1 °C, which was 18% higher than that of penetration at room temperature. Meanwhile, at critical 100 °C, as the SOC increases from 0% to 100%, the average temperature rise rate of the cell sharply increases from 1.992 °C/s to 93.033 °C/s, and the maximum temperature of cell increases from 125.9 °C to 652.9 °C, and the mass loss increases from 3.332 g to 31.180 g. The 0%SOC cell undergoes slighter TR, generating a lot of smoke but no flame. However, with the SOC increase of 25%–100%, the flame temperature increases from 437.8 °C to 918.5 °C, the flame area rise ratio reaching 281.77%. Combined with the microscopic performance characterization experiments, the dynamics behavior of particle eruption is mainly dominated by the anode graphite. The results of this study provide scientific guidance for the safety prevention of LIBs.

Thermal and Structural Performance Analysis of Piezoelectric Acceleration Sensor

Thermal and structural piezoelectric acceleration sensors are complicated and affected by many factors, especially when the sensor works in different temperature environments, its performance and structure will be affected to some extent. In this paper, the interference contact model of piezoelectric sensor is established with the method of bolt preloading, and the finite element analysis is carried out under different interference quantities. Moreover, based on the bolt preload model, the loading method of temperature load is studied, and the structural thermal adaptation characteristics of the sensor under the conditions of -70℃, -50℃, 0℃, 120℃, 160℃ and 200℃ are analyzed. Based on these thermal deformation and thermal stress analysis at different temperatures, the thermal adaptation characteristics of the sensor are analyzed. The normal operating temperature range of the sensor is -50℃ -120 ℃, and the structural parameter of the sensor (the thickness of the base) is optimized, so that the operating temperature of the sensor can reach 200℃. Ability to analyze.

MHD blood flow effects of Casson fluid with Caputo-Fabrizio fractional derivatives through an inclined blood vessels with thermal radiation

This study investigates a fractional-order time derivative model of non-Newtonian magnetic blood flow in the presence of thermal radiation and body acceleration through an inclined artery. The blood flow is formulated using the Casson fluid model under the control of a uniformly distributed magnetic field and an oscillating pressure gradient. Caputo-Fabrizio's fractional derivative mathematical model was used, along with Laplace transform and the finite Hankel transform technique. Analytical expressions were obtained for the velocity of blood flow, magnetic particle distribution, and temperature profile. These distributions are presented graphically using Mathcad software. The results show that the velocity increases with the time, Reynolds number and Casson fluid parameters, and diminishes when Hartmann number increases. Moreover, fractional parameters, radiation values, and metabolic heat source play an essential role in controlling the blood temperature. More precisely, these results are beneficial for the diagnosis and treatment of certain medical issues.

Study on the convective heat transfer characteristics of supercritical CO2 in mini-channels under unilateral heating conditions for application in a compact solar receiver

The mini-channel in compact solar receiver deliver the high performance in both heat transfer (HT) and pressure resistance. However, the severe variation of thermophysical properties of supercritical CO2 (S-CO2) coupling with the complex nonuniform distribution of solar flux leads to the abnormal flow and heat transfer (FHT) performance of S-CO2 in mini-channels. The present work numerically investigated the detailed FHT of S-CO2 in the horizontal mini-channel with one-side heating. The impacts of buoyancy and flow acceleration on FHT of S-CO2 are demonstrated in detail. The results show that the buoyancy effects can lead to the enhancement on circumferentially average heat transfer performance of S-CO2 while the local deterioration at the top. In contrast, the thermal acceleration effects can worsen the S-CO2 HT. The buoyancy and thermal acceleration effects can be ignored when Ri ≤ 2.7 × 10−3 and Ac ≤ 7.8 × 10−8, respectively. The empirical formula for S-CO2 HT in the mini-channel under single-side heating conditions is developed, with a maximum error of 4.5%.

The role of buoyancy and thermal acceleration in heat transfer deterioration and thermal oscillation for the CO2 around pseudo-critical region

Trans-critical CO2 Rankine cycle has great potential in converting low-grade heat into electricity, because of its good temperature glide matching between working medium and heat source. However, the drastic variation in thermo-physical properties of CO2 around the pseudo-critical region causes the abnormal flow and heat transfer behaviors. In this paper, the heat transfer deterioration and thermal oscillation in the heat addition of trans-critical CO2 cycle are investigated by experiments. The test section is heated uniformly by a DC power (0∼6 kW) and the minimum Reynolds number in the test section inlet is about 8000 (it is much larger than 2300) for the all cases. A shear stress reconstruction model is corrected by introducing the velocity profile of turbulence in the boundary layer to attempt to provide a quantitative criterion for deterioration and oscillation. The role of buoyancy and thermal acceleration in heat transfer deterioration and thermal oscillation is elucidated by connecting the shearing-stress reconstruction model and experimental results. It is found that, the thermal acceleration in the near-wall zone provides the key motivation to drive the heat transfer deterioration. The thermal oscillation is driven by the transition between partial and full re-laminarization flows in the local zone of heat transfer deterioration. When (Δτa/τw)<1&(Δτ/τw)tot>1 or 0.1<(Δτ/τw)tot<1, the turbulence is partially re-laminarized. When(Δτa/τw)>1&(Δτa/τw)>>(Δτbo/τw), the intense thermal acceleration causes the turbulence to be fully re-laminarized in the near-wall zone and it maintains the new laminar boundary . The turbulence is partially re-laminarized in the lower boundary and it is almost full re-laminarization in the upper boundary of the oscillation wall temperatures. The results confirm that the emergence of heat transfer deterioration and oscillation instability in mixed turbulent convective relate with the change of flow states caused by acceleration and buoyancy effects.

Accelerating turbulence in heated micron tubes at supercritical pressure

The purpose of this research was to provide further understanding of turbulent dynamics and heat transfer mechanisms in accelerating flows with thermophysical variations and pressure drops in micron tubes. Direct numerical simulations were conducted to investigate the turbulence to supercritical pressure ${\rm CO}_2$ in heated micron tubes with inner diameter $99.2~\mathrm {\mu }$ m. In general, the turbulent heat transfer enhancement/deterioration at supercritical pressure is dominated by variations in thermophysical properties, buoyancy and thermal acceleration; however, the mechanism differs in micron tubes ( $d^* &lt; 100\ \mathrm {\mu }$ m). The results showed that the pressure drop and scale effect made significant contributions to the development of turbulence flows heated at supercritical pressure in micron tubes, leading to the prominent property change and flow acceleration in the inlet fully developed turbulent flow. The deviation on temperature distribution because of pressure changes was non-negligible. The primary contribution of the acceleration was the decay of a boundary layer, which significantly suppressed the production of turbulence and decreased heat transfer. The acceleration had stabilizing effects on the ejection and sweep motions of the turbulent flow. The high-speed fluid contributed to a new disturbance scenario of the flow with a larger spanwise wavenumber superimposed on existing perturbations. The high-speed streak width in the quasilaminar region was approximately $150$ – $160\nu /u_\tau$ in accelerating flow. In the micron tubes, the Reynolds stress events of quadrant Q4 contributed 60 % of the Reynolds stress, greater than those of quadrant Q2.

EFFECT OF HEATING AND COOLING ON 6CB LIQUID CRYSTAL USING DSC TECHNIQUE

In this research, details of the heating and cooling of the liquid crystal (LC) 4-cyano-4'-hexylbiphenyl (6CB) were collected through the use of Differential Scanning Calorimetry (DSC), and then analyzed with Logger Pro. The sample of 6CB was heated from -40 °C to 80 °C and then cooled to -40 °C again at a constant rate of 20 °C/min. DSC provided data for the time, temperature, and heat flow, which was used to calculate thermal speed, acceleration, and jerk, as well as the specific heat capacity at each point. Graphs of this data contain endothermic and exothermic peaks that indicate phase transitions, revealing information such as the enthalpy of the peak and the nematic range. One focus of this study was to compare 6CB to other liquid crystals that are part of the n-alkyl-cyanobiphenyl (nCB) family to see if the difference in tail length affects the behavior of the liquid crystal. Results show that 6CB may be of interest as an alternative to other liquid crystals for liquid crystal displays (LCD).

Comparative study of quantum Otto and Carnot engines powered by a spin working substance.

Quantum Otto and Carnot engines have recently been receiving attention due to their ability to achieve high efficiencies and powers based on the laws of quantum mechanics. This paper discusses the theory, progress, and possible applications of quantum Otto and Carnot engines, such as energy production, cooling, and nanoscale technologies. In particular, we investigate a two-spin Heisenberg system that works as a substance in quantum Otto and Carnot cycles while exposed to an external magnetic field with both Dzyaloshinsky-Moriya and dipole-dipole interactions. The four stages of engine cycles are subject to analysis with respect to the heat exchanges that occur between the hot and cold reservoirs, alongside the work done during each stage. The operating conditions of the heat engine, refrigerator, thermal accelerator, and heater are all achieved. Moreover, our results demonstrate that the laws of thermodynamics are strictly upheld and the Carnot cycle produces more useful work than that of the Otto cycle.

Decomposition and reaction characteristics of C6F12O/N2 with rubber materials based on ReaxFF

C6F12O is considered to have potential for application in medium and low voltage electrical switchgear due to its excellent insulating properties. In order to ensure the stable operation of the equipment, it is necessary to study the compatibility of C6F12O/N2 gas with the rubber sealing materials used in insulating equipment. In this paper, EPDM, NBR and FKM are selected to conduct thermal acceleration experiments under 96 % C6F12O/4 % N2 gas at 40 ℃, 90 ℃, 110 ℃ and the changes of gas mixture before and after the experiments are analyzed by GC-MS. At the same time, the molecular system of gas mixture and rubber is constructed, and the reaction process of the system is simulated and the main decomposition path of rubber and the generation path of C2F are obtained at 1000–4000 K by using ReaxFF force field. The experimental results show that C6F12O/N2 gas mixture reacts with three rubbers at 40 ℃, 90 ℃, 110 ℃ and generate gas C3F6, C3F7H and by-products CS2. The theoretical results indicate that the decomposition products of mixed gas and rubber materials at high temperature are mainly H, F, CF2, CF3, and the reaction products are mainly C2F. Through pathway tracking, it is found that the decomposition products of C6F12O and rubber may react in multiple ways to generate C3F6 and C3F7H, which originate from the complex reaction of rubber and gas mixture. The compatibility between C6F12O/N2 and FKM is greater than that of NBR and EPDM. The simulation results are consistent with the experimental results. The results can provide a theoretical reference in the selection of sealing materials for equipment using C6F12O/N2 gas mixture as the insulating medium.

Numerical simulation study on the slow cook-off response characteristics of PBX explosive

The zero order reaction kinetic model was used to simulate the heating of energetic binder PBX charge at four heating rates of 3.3K/h, 6K/h, 9K/h and 30K/h, respectively. The key parameters such as the evolution of temperature field, ignition time, ignition temperature and ignition area were obtained. The results show that during the slow Cook-off process of energetic binder PBX explosive charge, before the thermal decomposition and acceleration reaction, it is a heat conduction process, with large internal temperature gradient, low charge center temperature and high charge edge temperature; When the temperature rises to 396K, the thermal decomposition rate of the explosive increases, and heat accumulation occurs in the charge; The ignition position of charges with different heating rates changes. With the increase of heating rates, the ignition position changes from a point in the center to two points on the axis that gradually move away from the center to both ends, and then to four points that gradually move away from the axis to four end angles. Finally, the ignition reaction occurs.