Transient Heat Transfer Behavior Research Articles

Numerical simulation for heat transfer behavior of copper slag ladle under air-cooling mechanism

Copper slag ladle cooling process is divided into two stages: air-cooling and water-cooling, is one of the important processes in copper production process from copper slag flotation recovery of valuable metals. This study investigates the transient heat transfer behavior of copper slag ladle during air-cooling mechanism based on the finite-volume method. The innovative aspect of this research lies in development of a 1:1 scale 3D model of slag ladle based on industrial-scale dimensions, obtaining the relevant thermophysical parameters of copper slag by experiment, and validating simulation results against actual industrial production data. The results of study show that: (i) The temperature distribution of copper slag within slag ladle exhibits a “concentric circle” pattern with the formation of a “liquid core” zone, indicating that the temperature is significantly higher in central region compared to periphery, revealing a notable temperature gradient during the air-cooling process; (ii) The heat flux is most concentrated in the central region of slag ladle, suggesting that the heat transfer intensity is the greatest in this area, the temperature variation of copper slag in proximity to this region is the most pronounced; (iii) The cooling path of copper slag proceeds from outer layers to inner layers, with the cooling rate decreasing from fast to slow, reflecting the temperature change trend of copper slag during air-cooling, which transitions from rapid to gradual cooling. This study provides new perspectives and data support for exploring air-cooling process of copper slag ladle and contributes to the further advancement of this field.

A novel coaxial air-R134a spray cooling for heat transfer enhancement of laser dermatology

Efficient cooling is essential in laser dermatology to prevent thermal damage, but current methods are often hindered by thermal barrier film deposition. This study presents coaxial air-R134a spray cooling for heat transfer enhancement and deposited-film mitigation. Surface temperature was measured using a TFTC thermocouple, while heat flux was estimated through the improved Duhamel theorem. High-speed camera visualization with the Mie-scattering technique captured the spray and film behavior. The results showed uniform and effective cooling via transient surface heat transfer and film behaviors. The boiling curve identified three distinct cooling phases: transition boiling, nucleate boiling, and single-phase film deposition. Airflow adjustments not only enhance the heat transfer coefficient but also control spray dynamics, droplet rebounding, and film spreading. Self-organizing maps revealed that medium-to-high nozzle spacing (y/D) reduces surface temperature and increases the heat transfer coefficient. Film resistance time is minimized by either high airflow and low y/D or low airflow and high y/D. For critical heat flux, low y/D and airflow are optimal. A y/D = 32 and 60 L/min airflow provided appropriate operating conditions. However, airflow increased frost deposition during off-duty, suggesting moisture-free air in the future.

Numerical investigation of the transient fluid flow and heat transfer behavior in self-driven natural circulation cooling loop

An innovative open natural circulation system is proposed in this work, which utilizes the difference of the heat flux loaded on different parts of the pipe to form a self-circulation flow of working fluid. Via CFD simulation established by embedding self-programmed code into commercial software, the transient performance of a three-dimensional natural circulation loop is studied. The results indicate that: applying uniform heat flux on the wall of the loop, the fluid flow state successively experiences three stages, i.e., liquid expansion, partial circulation, and phase change, and finally a vapor plunger blocks the pipe, leading to local heat transfer deterioration and temperature increase; applying non-uniform heat flux, the fluid can be driven to form self-circulation flow, which can not only cool the structure via convection, but also prevent bubbles from accumulating, stabilizing the maximum temperature of the system around the boiling point; further for the loop with a more complex configuration, the self-circulation can be also started but with different start modes. Therefore, the new natural circulation system can achieve effective and adaptive cooling according to the external thermal environment, avoid fluctuation and vapor block phenomenon caused by violent phase change, and reduce the driving equipment and control unit. This work provides a reference for the design of the natural circulation system applied in a non-uniform heating environment. Through transient numerical simulation of the start-up stage, the fluid flow, phase change, and heat transfer characteristics in the natural circulation loop are studied, and the start mode and mechanism are analyzed.

Transient behavior of liquid film deposition and surface heat transfer upon cryogen-spray cooling coupled with cold air jet

Cryogen spray cooling (CSC) has become a promising auxiliary-cooling technique for laser dermatology, which recently enhanced by cold air jet (CSC-CAJ). However, the CAJ-to-spray interactions could worsen the CSC performance and associated liquid film behavior. In this study, the transient behavior of liquid film deposition and surface heat transfer of CSC-CAJ was investigated. A high-speed camera was used to capture the film intensity through Mie-scattering technique. Fast-response TFTC thermocouple was used to measure the transient surface temperature. The large eddy simulation (LES) was used to extract the vortical structure, while the discrete-phase model (DPM) and Eulerian wall film (EWF) were coupled for the spray process and the transient film modeling. The results showed that the CAJ significantly enhanced CAJ-to-spray mixing and mitigated the film thickness (i.e., 32 % reduction). The CSC-CAJ had asymmetric film coverage due to induced counter-rotating vortex-pairs (CRVP) from the CAJ-to-spray interaction, which laterally clustered the spray. It permitted fast evaporation with dominant CAJ at the spray-periphery while permitting fast evaporation with dominant spray at the spray-core. Consequently, the CSC-CAJ showed 75 % temperature increment at the windward side and 15 % temperature reduction with 60 % heat transfer coefficient enhancement at the impingement point. The experimental film intensity and simulated film thickness were linearly correlated and significantly influenced the surface temperature. The combined results affirmed the significant impact of the CAJ on the transient film deposition, which potentially helps promote advanced cooling technology.

Simulation Study on Transient Periodic Heat Transfer Behavior of Meniscus in Continuous Casting Mold

Simulation Study on Transient Periodic Heat Transfer Behavior of Meniscus in Continuous Casting Mold

Experimental study on transient heat transfer performance of high temperature heat pipe under temperature feedback heating mode for micro nuclear reactor applications

The use of heat pipe-cooled micro nuclear-powered reactors (HPNRs) is crucial for shaping a low-carbon energy structure in the future. However, there is still a lack of experimental investigations on the transient heat transfer performance of high-temperature heat pipes (HTHPs) under the application of HPNRs. To address this issue, a hardware-in-the-loop (HIL) test platform has been developed to dynamically adjust the heating power of the heat pipe in real-time considering the temperature feedback (TF) effect. A horizontal test rig composed of a hex block, a sodium heat pipe, and six electric heaters has been built in the laboratory, and its thermal effectiveness has been evaluated at 60%. Subsequently, a transient HIL test under the TF heating mode has been conducted. The results show that the frozen start-up of the HTHP can be divided into five stages based on the power exponential growth rate and the state of the sodium vapor. During the transient start-up process, the worst axial heat transfer of the HP occurs when the front of the continuum vapor arrives at the adiabatic section, where the maximum thermal resistance is 500 K/W. Heat pipes with a high filling ratio can avoid integral overheating of the evaporator when occurring viscosity limit at the start-up period. Different time scales of neutron growth and thermal inertia cause alternating oscillation of power and temperature. This work provides valuable insights into the transient heat transfer behavior of the HTHP under the running process of the HPNR system.

Effect of Nozzle Position on Fluid Flow and Solidification in a Billet Curved Mould using Mould Electromagnetic Stirring

Owing to the smaller cross‐sectional size of the billet, the effect of the position of the submerged entry nozzle (SEN) on the fluid flow in a curved mold is more prominent. The transient fluid flow, heat transfer, and solidification behavior at different nozzle positions and mold electromagnetic stirring (M‐EMS) are investigated using a 3D transient mathematical model. The symmetry index, S, is introduced to quantitatively evaluate the symmetry of the flow field. When the SEN is offset to the outer curve, S is the highest, the flow field mixing is the best, and the liquid level fluctuation is minimal. When the SEN is offset to the inner curve, S is the lowest, and the washing of the solidified shell on the inner curve side is more significant, causing the solidified shell thickness on the inner curve to be thinner, which can easily cause steel breakout accidents. The M‐EMS can improve the uneven flow field caused by the nozzle position. In actual production, the SEN can be appropriately offset to the outer curve, and the application of M‐EMS can improve the flow field distribution of molten steel.

Chaotic Cattaneo-LTNE porous convection

The chaotic convection in a porous medium has been studied with a local thermal nonequilibrium (LTNE) model and hyperbolic-type heat transport equation in the solid arising due to Cattaneo heat flux law. The Brinkman-extended Darcy model is considered to describe the flow in a porous medium. A system of autonomous nonlinear ordinary differential equations is derived by using a truncated double Fourier series expansion such that the linear stability theory results are the same as those for the full two-dimensional problem. The stability of equilibrium points of the system is discussed in detail for the governing parameters involved therein. The transition to chaos in different phase planes and the transient behavior of heat transfer are analyzed by solving the model equations numerically using the Runge-Kutta-Fehlberg technique. The effect of increasing scaled solid thermal relaxation time parameter is to speed up the transition to chaos and to decrease the heat transfer.

Effect of Swirling Flow Nozzle on Fluid Flow and Solidification in a Round Bloom Continuous Casting Mold

The nozzle structure has an important effect on the fluid flow in the mold, which can significantly improve the solidified shell and product quality of alloy steel round bloom. The transient fluid flow, heat transfer, and solidification behavior under different nozzle structures and mold electromagnetic stirring (M-EMS) are investigated using a 3D transient mathematical model. The results show that a third small recirculation zone appears near the meniscus after the application of the swirling flow nozzle (SFN). The impact depth of SFN is shallower than that of the original submerged entry nozzle (SEN) impact, and the lower circulation zone is shifted upward. The horizontal swirling flow generated by SFN can significantly weaken the washing of the initial shell by high-temperature steel and improve the uneven growth phenomenon of the inner and outer curved solidified shell caused by mold curvature. The swirling flow produced by M-EMS in the mold can also improve the washing of the initial shell by the high-temperature jet and the uneven growth of the inner and outer curved shell. M-EMS can expand the high-temperature zone in the upper part of the mold, promote the superheat dissipation of the molten steel, and promote the growth of the solidified shell. In addition, after the application of M-EMS, the tangential velocity of -15° SFN in the meniscus is smaller, and the resulting liquid level fluctuation is lower at 5.07 mm, which is less likely to produce slag entrapment and is conducive to improving the quality of round bloom.

High-precision 2D surface phosphor thermometry at kHz-rates during flame-wall interaction in narrow passages

This work demonstrates high-speed 2D wall temperature measurements occurring during flame-wall interaction (FWI) within a narrow channel. Such measurements are essential to understand transient wall heat transfer and flame behavior occurring within micro-combustors and designated engine crevices. Wall temperature is measured using the phosphor Bismuth-doped Scandium vanadate (ScVO4:Bi3+). ScVO4:Bi3+ exhibits a short phosphorescence lifetime (∼ 2 µs at room temperature), enabling kHz measurement rates. ScVO4:Bi3+ also exhibits a high temperature sensitivity, which yields single-shot precision < 0.5 K within the temperature range of 295 – 335 K. A frequency-doubled Ti:Sapphire laser emitting light at 400 nm is used to excite ScVO4:Bi3+, and wall temperature is measured within a ∼ 22 × 22 mm2 region with 380 µm spatial resolution. Phosphor thermometry and CH* imaging are combined at 1 kHz to measure the spatiotemporal dynamics of the flame and wall temperature (Twall) within a 2 mm crevice passage in a fixed volume chamber designed for heat transfer studies. Measurements describe Twall signatures associated with transient FWI events, including unique Twall features associated with wrinkled flame fronts. For our operating conditions, Twall associated with flame cusps consistently exhibit temperatures 5 – 20 K lower than flame crest regions. The most extreme difference is seen for large cusp formation, where local wall cooling is noticeable at the trough of the flame cusp. This cooling feature may be caused by intrinsic flame/flow instabilities, which locally and temporally cool the wall.

Transient heat transfer characteristics of submerged heat source under seismic excitation

In this experimental based study, experiments are carried out to investigate the transient heat transfer behavior from the horizontal heat source submerged in the partial filled water pool including the effect of submergence depth of heat source. Further, experiments are conducted and analyses have been made to investigate the heat transfer behavior under the seismic environment along with the consideration of seismic excitation parameters: natural frequency of liquid and amplitude of displacement.The transient heat transfer coefficient for natural convection and boiling region are evaluated with and without excitation cases. Experimental heat transfer coefficients were compared with the available correlations and found inadequate. Therefore, new correlations have been proposed for transient heat transfer coefficient for each region. For the seismic condition, separate correlation has been proposed to predict the vibrational heat transfer coefficient in case when strong sloshing occurs with wave breaking at the free surface. Proposed correlations are predicted quite satisfactory results.

Numerical Simulation of the Fluid Flow, Heat Transfer, and Solidification in Ultrahigh Speed Continuous Casting Billet Mold

Ultrahigh‐speed casting is the most remarkable feature of billet high‐efficiency continuous casting. The transient fluid flow, heat transfer, and solidification behavior under different casting speeds, submerged entry nozzle (SEN) parameters, and mold electromagnetic stirring (M‐EMS) are investigated using a 3D transient mathematical model. The results show that a typical roll flow pattern is generated for all casting speeds, while a third small recirculation zone developed near the solidified shell at the casting speed of 6.0 m min−1. When the casting speed increases, the impact depth of the molten steel increases from 0.748 to 0.844 m, the velocity and level fluctuation at the steel/slag interface increases, the high‐temperature zone moves downward, and the shell thickness is reduced from 16.2 to 11.1 mm. As the SEN inner diameter and immersion depth increases, the impact depth increases, the velocity and level fluctuation at the steel/slag interface decreases, and the shell thickness at the mold exit is ≈11.0 mm. When the SEN immersion depth and inner diameter are 120 and 40 mm, respectively, the flow and temperature fields in the mold are the most appropriate. In addition, the M‐EMS have a great effect on the fluid flow and heat transfer behavior.

A coupled thermal-hydraulic-mechanical model for the kinetic dissociation of methane hydrate in a depressurizing well

Methane hydrate (MH) has significant potential to become a major resource of the future energy supply. Depressurization is one of the major methods of MH development used in MH production wells. The redistributions of pore pressure and hydrate saturation change the effective stress and mechanical properties of MH-bearing sediment, resulting in complicated thermal-hydraulic-mechanical behaviours. In this study, the transient multi-phase seepage and transient heat transfer behaviour around a wellbore were analysed. A multiannulus wellbore was obtained via the relations between the hydrate saturation and the mechanical parameters. The analytical stress and displacement solutions for a multi-annulus wellbore were deduced according to elastic mechanics. Then, a new thermal-hydraulic-mechanical coupling model was proposed to gain a better understanding of the kinetic dissociation behaviour of MH in a depressurizing well. The results showed that pore pressure diffusion transforms from a transient state to a relatively steady state in only a few minutes, while a transient state of heat diffusion exists for a relatively long time. The dissociating front moves farther at the beginning of dissociation and then shrinks back towards the wellbore wall. Thereafter, the dissociating front moves nearly linearly with the production time. Due to the flowability differences among the gas, water and MH, the area where the gas and water saturations change is larger than that where the MH saturation decreases. There exists an annulus zone surrounding the wellbore where the mechanical parameters are much lower than those of the zone far from the wellbore. The outer diameter of this annulus increases slowly with the production time. Stress transilience surrounding the wellbore is caused by the transilience of mechanical parameters after the decomposition of MH.

Transient hydrodynamics and heat transfer behaviour in a pressurized circulating fluidized bed during abrupt changes in operating pressure

Pressurized circulating fluidized beds (PCFB) are subjected to various fluctuating conditions due to fluctuations in load demands owing to its vast utilization as a combination of heat and power generating unit. As such, analysis of transient heat transfers and hydrodynamics during these fluctuations in PCFB is very important. The present work includes experimental investigation on the transient behaviour of a PCFB unit during abrupt changes in operating pressure. The experiments are performed on a laboratory scale PCFB set up consisting of a stainless-steel riser of height 2 m and an inner diameter of 54 mm. The operating pressure is regulated by a pressure regulator and is measured using a piezo resistive pressure sensor attached to the inlet of the riser. Static pressures and temperatures are also measured along the riser column at various heights. Experiments are performed with 500 g sand as bed inventory at two operating pressure of 2 and 3 bar. Drastic changes in voidage values were observed mostly in the upper region of the riser column for transient conditions in the operating parameters. The experiments showed that with a sudden drop in operating pressure, the heat transfer coefficient momentarily increase and subsequently decrease steadily. The reverse is observed for a sudden increase in operating pressure.

EXPERIMENTAL INVESTIGATION OF CYLINDRICAL FOOD PRODUCT DURING FORCED CONVECTION COOLING

The objective of this work is to study the transient heat transfer behavior during precooling of cylindrical food product at different air flow rates. Temperature measurement is carried out at the center of the specimen at different air flow rates (3.8, 4.0, 4.2 &amp; 4.5 m/s). These temperature distributions are plotted with time. The terms of Fourier equation were evaluated and thermal diffusivity was plotted against the temperature variation during cooling of the cylindrical food product.Surface film conductance values is also calculated in present work, at different air flow rates. These values are effective values, which included the additional cooling effect due to moisture evaporation from the exposed product surface. Variation of Reynolds number and Nusselt number is also plotted with different air velocities and it is found that more favorable results of thermal conductivity and thermal diffusivity is obtain at 3.8 m/s. Which ultimately saves energy and provide optimum cooling for the specimen at the given air velocity.

Transient heat transfer processes in a single rock fracture at high flow rates

Understanding the process of heat transfer in fractures and the influence of fracture surface morphology is crucial for optimized heat mining. In this work, we investigated transient heat transfer behavior at high flow rates, ranging from 80 to 350 mm/s, at various inflow temperatures. A split sandstone core was installed in an experimental setup that allowed the fluid to be circulated, cooled and reheated. Water was circulated through the fracture and a temperature step was induced with the help of a heat exchanger upstream to the core. Thermocouples before and after the core enabled the measurement and evaluation of transient cooling and heating curves. The thermal breakthrough curves reveal the capacity of the rock to maintain maximum outflow temperature for a substantial amount of time. Thermal and mass dispersion coefficients are comparable but analytical solutions for heat transport can only be used with strong limitations. Heat transfer coefficients were determined using the stationary state. The evaluation of the surface roughness identifies its substantial influence on the heat transfer coefficient. Our experiments show that flow velocities beyond 200 mm/s lead to cold outflow temperatures, causing undesired cooling and prolonged heat transfer processes in the reservoir. These effects should not be underestimated in terms of the efficiency of geothermal energy generation.

Dynamic surface heat transfer and re-attachment flow measurement using luminescent molecular sensors

Luminescent molecular sensors, which can be used as Temperature Sensitive Paint (TSP) or Pressure Sensitive Paint (PSP) in respective temperature or pressure measurements, has been proved to be one promising surface quantity measurement technology in recent years. It is advantageous in the experiments with complicated surface and is able to give flow field information such as surface flux and wall shear-stress. The current study is focused on the dynamic heat transfer measurement of a backward-facing step model, using luminescent molecular sensors as temperature probe. The effects of flow separation and re-attachment after a back-step model were experimentally discussed in wind tunnel tests. The experimental system was consisted of a molecular sensor calibration system, a dynamic data recording system and a data processing system. It is found that the reattachment process will form a low temperature region, which then gives the clear temperature field of the flow. Dynamic temperature field data show a re-attachment position around x/h = 5.7, which agrees well with oil-flow measurements as well as previous experiments. The dynamic temperature fluctuation data is discussed with the vibrations of flow and transient heat transfer behaviors after the backward step, which then is used in the analysis of surface wall shear-stress variations. It is concluded that the current luminescent molecular sensor method is capable of quantitative measurement for surface heat transfer and fluid flows.

Methodology for spatially resolved transient convection processes using infrared thermography

ABSTRACT Studies have previously looked at the transport processes in convection but there is a lack of detail on methods to characterize transient heat transfer behavior. An infrared methodology to spatially and temporally resolve transient heat transfer is presented and tested on turbulent impinging (cooling) jets (non-swirling, swirling, Re = 11,600 to 35,000, S = 0 to 1.05). The intricacies of data filtration and spatial discretization, as well as the structure of the image processing code are reported and a parametric study of factors influencing the time to reach steady-state is done. Results show that variations in jet behavior are manifest in the time to reach steady-state and can be resolved even in the first few seconds.

Graphene and CNT impact on heat transfer response of nanocomposite cylinders

AbstractReinforcing polymers with nanofillers is an advanced approach to improve and manage the thermal behaviors of polymeric nanocomposite materials. Among the proposed nanofillers, graphene and carbon nanotube (CNT) with superior thermal conductivity are two advanced nanofillers, which have extensively been utilized to enhance the heat transfer responses of host polymeric materials. In this work, the impacts of randomly oriented graphene and CNT to steady state and transient heat transfer behaviors of functionally graded (FG) nanocomposite cylinders have been investigated using an axisymmetric model. Nanocomposite cylinders have been assumed to be under heat fluxes, heat convections or temperatures as different types of thermal boundary conditions. The thermal properties of the resulted nanocomposite materials are estimated by micromechanical model. Moreover, the governing thermal equations of axisymmetric cylinders have been analyzed using a highly consistent and reliable developed mesh-free method. This numerical method predicts temperature fields via MLS shape functions and imposes essential boundary conditions with transformation approach. The effects of nanofiller content and distribution as well as thermal boundary conditions on the heat transfer responses of nanocomposite cylinders are studied. The results indicated that the use of nanofiller resulted in shorter stationary times and higher temperature gradients in FG nanocomposite cylinders. Moreover, the use of graphene in nanocomposites had stronger impact on thermal response than CNT.

The model development and case study for transient thermal response of the human body interacting with the coupled fabric-environment system

A one-dimensional transient thermal response model for the human body subjected to the influence of coupled fabric-environment system was developed in this study. To account for the human body's thermophysiological response, the bio-heat equation was used to predict the temperature distribution within each body tissue. The fabric composed of multiple pores full of wet air was treated as the porous media. The movement, heat transfer, fiber absorption, and evaporation of sweat within the porous media were considered in the developed model and it was solved by the CFD package software-ANSYS FLUENT to simulate the transient behavior of heat and moisture transfer in the coupled human body and fabric-environment system. Two typical settings of human activities, including human acclimatization without sweating and human activity with sweating, were considered in the numerical study. To investigate the effect of ambient temperature on the transient response of the body's skin temperature, we explored three different ambient temperatures. The numerical results showed that sweating caused skin temperature to drop gradually and that the magnitude to which the skin temperature changed was nearly insensitive to the change of environmental temperatures. Moreover, the sweating rate increased with the increase of ambient temperature.