Thermal Non-equilibrium Phenomena Research Articles

Comparison of subcooled flow boiling in concentric annuli with bare and finned surfaces

In this study, the hydrodynamic and thermal characteristics of subcooled flow boiling in concentric annuli with inner heating wires, featuring bare and finned surfaces designed for heat transfer enhancement, were experimentally investigate, with a focus on cooling technology for EV charging cables. The bare wire test module consisted of a 300 mm long wire, with a 290 mm long inner heating surface and a diameter of 10 mm. The outer tube was constructed from transparent polycarbonate with a 22 mm diameter, allowing optical access to observe the flow. The finned wire test module has an identical construction, except for the addition of six longitudinally extruded fins with cross-sectional dimensions of 2 × 5 mm2. Experiments and flow visualizations, aided by high speed imaging, were conducted using HFE-7100 as the working fluid at mass velocities, G, 463.7 to 1,309.6 kg/m2s and heat fluxes from 0.90 to 9.93 W/cm2, under inlet pressures between 120 and 240 kPa. The study investigated thermal non-equilibrium phenomena, driven by asymmetric bubble accumulation, and their effects on circumferential temperature distribution, which showed opposing trends between the bare and finned surfaces. An evaluation of previous empirical correlations for predicting subcooled flow boiling heat transfer coefficients was performed, followed by an assessment of fin performance in enhancing heat transfer. A comparative analysis with the bare surface module of the same dimensions confirmed that the longitudinal fins provided superior thermal management, maintaining the surface temperature below the safety threshold, Ts,safe. The longitudinal fins demonstrated a 54.5 % reduction in charging time and a 183.7 % increase in charging current for charging up to 80 % state of charge (SOC) of a 77.4 kWh battery, compared to the bare wire.

A hybrid CFD-RMD multiscale coupling framework for interfacial heat and mass simulation under hyperthermal ablative conditions

Under hypersonic flow conditions, the heat and mass transfer at the gas-solid interface of thermal protection material becomes highly complicated due to the strong chemical and thermal non-equilibrium phenomena, associated with the real gas effect, the high temperature effect and various heterogeneous reactions occurring at the interface or inside the TPM. Interfacial heat and mass transfer is strongly coupled with non-linear reactions at different spatiotemporal scales. Based on the traditional macroscopic Computational Fluid Dynamics (CFD) and microscopic Reactive Molecular Dynamics (RMD) methods, a hybrid CFD-RMD multiscale simulation framework is established in this work to improve the accuracy of aerothermal prediction by considering the heat and mass transfer at the gas-solid interface under hyperthermal non-equilibrium conditions. To validate the proposed multiscale framework, phenolic resin composite is investigated in this work with particular interest in the pyrolysis gas injection effects. Benchmarking ablation studies are conducted under three representative altitudes of 40, 60 and 80 km, respectively, and the recession rates of 7.47 × 10−4, 1.53 × 10−3 and 1.08 × 10−3 mm/s are obtained which are comparable with the experimental results. Much higher computational efficiency of the hybrid CFD-RMD multiscale framework is achieved comparing to a full RMD simulation method at the same spatial and temporal level. Not only to the pyrolysis study, the established framework can also potentially be extended to other gas-solid interfacial reaction situations, which is of great help in improving the mechanism understanding of various physics-derived microscopic models for macroscopic predication of hypersonic aerothermal characteristics.

Numerical investigation on the flow characteristics of a reverse-polarity plasma torch by two-temperature thermal non-equilibrium modelling

A two-temperature (2T) thermal non-equilibrium model is developed to address the thermal non-equilibrium phenomenon that inevitably exists in the reverse-polarity plasma torch (RPT) and applied to numerically investigate the plasma flow characteristics inside and outside the RPT. Then, a detailed comparison of the results of the 2T model with those of the local thermal equilibrium (LTE) model is presented. Furthermore, the temperature of the plasma jet generated by a RPT and the RPT’s voltage are experimentally measured to compare and validate the result obtained by different models. The differences of the measured excitation temperature and the arc voltage between the 2T model and experimental measurement are less than 13% and 8%, respectively, in all operating cases, validating the effectiveness of the 2T model. The LTE model overestimates the velocity and temperature distribution of the RPT and its plasma jet, showing that thermal non-equilibrium phenomena cannot be neglected in the numerical modelling of the RPT. Unlike other common hot cathode plasma torches, the thermal non-equilibrium phenomenon is found even in the arc core of the RPT, due to the strong cooling effect caused by the big gas flow rate.

3D numerical investigation of a free-burning argon arc with metal electrodes using a novel sheath coupling procedure

A novel coupling procedure describing the complicated plasma-electrode interaction process has been developed and applied into the 3D finite volume simulation of a direct current tungsten inert gas welding system. This is achieved by making the space charge layer (sheath) incorporated into the computation domain and interact with both bulk plasma and cathode through the effective electrical conductivity. Both chemical and thermal nonequilibrium phenomena as well as the self-induced magnetic fields have been taken into consideration by the model to ensure a realistic numerical description of a non-thermal arc. The applicability of this coupling procedure is further improved by calculating the real electric potential, which is capable of accounting for the effects of the complicated drift and diffusion processes. Numerical results of both 100 and 200 A discharge currents are presented, field reversal is obtained at near-anode regions in both cases, which is followed by the negative anode sheath potential drop. The region of the strongest electron overpopulation appears at the intersection of plasma fringes and electrode surface. Numerical results of plasma temperature and voltage show good agreement with experimental measurements.

Model of two-temperature convective transfer in porous media

In this paper, we study the asymptotic behaviour of the solution of a convective heat transfer boundary problem in an $$\varepsilon $$ -periodic domain which consists of two interwoven phases, solid and fluid, separated by an interface. The fluid flow and its dependence with respect to the temperature are governed by the Boussinesq approximation of the Stokes equations. The tensors of thermal diffusion of both phases are $$\varepsilon $$ -periodic, as well as the heat transfer coefficient which is used to describe the first-order jump condition on the interface. We find by homogenization that the two-scale limits of the solutions verify the most common system used to describe local thermal non-equilibrium phenomena in porous media (see Nield and Bejan in Convection in porous media, Springer, New York, 1999; Rees and Pop in Transport phenomena in porous media III, Elsevier, Oxford, 2005). Since now, this system was justified only by volume averaging arguments.

Natural Convection Heat Transfer About a Vertical Cone Embedded in a Tridisperse Porous Medium

This work studies the natural convection heat transfer about a vertical cone embedded in a tridisperse porous medium with constant wall temperature. The three-velocity three-temperature formulation is used to derive the governing partial differential equations. The coordinate transformation and the order-of-magnitude analysis are then used to obtain the nonsimilar boundary layer partial differential equations. The cubic spline collocation method is then used to solve the nonsimilar boundary layer partial differential equations. The effects of two inter-phase heat transfer parameters, three modified thermal conductivity ratios, and two permeability ratios on the heat transfer and flow characteristics of the tridisperse porous medium are studied. Results show that increasing the three modified thermal conductivity ratios or the two permeability ratios tends to increase the natural convection heat transfer rate of the vertical cone in a tridisperse porous medium. Moreover, the thermal non-equilibrium phenomena between the three phases of the tridisperse porous medium are affected by the streamwise coordinate and the two inter-phase heat transfer parameters. The thermal non-equilibrium effects between the three phases of the tridisperse porous medium become significant as the two inter-phase heat transfer parameters or the streamwise coordinates are small.

Coupled radiation and flow modeling in ceramic foam volumetric solar air receivers

► A model for the performances of volumetric solar air receiver was developed. ► Sensitivity studies show the thermal non-equilibrium phenomena are distinct locally. ► The mean cell size has a dominant effect on the performances of solar air receiver. ► The solid thermal conductivity of absorber is not important to the air receiver. ► The desired temperature distribution of the absorber is realizable. Ceramic foams are promising materials for the absorber of volumetric solar air receivers in concentrated solar thermal power (CSP) receivers. The macroscopic temperature distribution in the volumetric solar air receiver is crucial to guarantee that volumetric solar air receivers work steadily, safely and above all, efficiently. This study analyzes the temperature distribution of the fluid and solid phases in volumetric solar air receivers. The pressure drop in the ceramic foams and the interfacial heat transfer between the flowing fluid and solid are included in the model. The radiative heat transfers due to concentrated solar radiation absorption by the ceramic foam and the radiation transport in the media were modeled with the P 1 approximation. The energy fields of the fluid and solid phases were obtained using the local thermal non-equilibrium model (LTNE). Comparison of the macroscopic model with experimental results shows that the macroscopic model can be used to predict the performance of solar air receivers. Sensitivity studies were conducted to analyze the effects of velocity, porosity, mean cell size and the thermal conductivity of the solid phase on the temperature fields. The results illustrate that the thermal non-equilibrium phenomena are locally important, and the mean cell size has a dominant effect on the temperature field.

Multiple-temperature kinetic model for continuum and near continuum flows

A gas-kinetic model with multiple translational temperature for the continuum and near continuum flow simulations is proposed. The main purpose for this work is to derive the generalized Navier-Stokes equations with multiple temperature. It is well recognized that for increasingly rarefied flowfields, the predictions from continuum formulation, such as the Navier-Stokes equations lose accuracy. These inaccuracies may be partially due to the single temperature assumption in the standard Navier-Stokes equations. Here, based on an extended Bhatnagar-Gross-Krook (BGK) model with multiple translational temperature, the numerical scheme for its corresponding Navier-Stokes equations is also constructed. In the current approach, the energy exchange between x, y, and z directions is modeled through the particle collision, and individual energy equation in different direction is obtained. The kinetic model, newly constructed is an enlarged system in comparison with Holway’s ellipsoid statistical BGK model (ES-BGK). The detailed difference is presented in this paper. In the newly derived “Navier-Stokes” equations from the current model, all viscous terms are replaced by the temperature relaxation terms. The relation between the stress and strain in the standard Navier-Stokes equations is recovered only in the limiting case when the flow is close to the equilibrium, such as small temperature differences in different directions. In order to validate the generalized Navier-Stokes equations, we apply them to the study of Couette and Poiseuille flows with a wide range of Knudsen numbers. In the continuum flow regime, the standard Navier-Stokes solutions are precisely recovered. In the near continuum flow regime, the simulation results are compared with the direct simulation Monte Carlo solutions. The anomalous phenomena in the pressure and temperature distributions from the standard Navier-Stokes equations in the Poiseuille flow case at Kn=0.1 are well resolved by the generalized Navier-Stokes equations. This paper clearly shows that many thermal nonequilibrium phenomena in the near continuum flow regime can be well captured by modifying some assumptions in the standard Navier-Stokes equations.

Experimental study of local thermal non-equilibrium phenomena during phase change in porous media

This study presents a systematic experimental method of estimating the extent of the phase front under the local thermal non-equilibrium condition in porous media saturated with phase change materials (PCM). It describes a comparison of measured temperature of the solid matrix at pre-selected sites with the average pore temperature in order to substantiate the existence of the local thermal non-equilibrium condition. Also, the measured data are compared with the theoretical predictions reported in the literature. The agreement between the experimental and theoretical prediction is highly satisfactory. The results clearly show that, early during the phase change process, the Sparrow number is relatively small and the solid matrix is not at the local thermal equilibrium with the pore materials. At larger time, the Sparrow number rapidly increases and the system approaches the local thermal equilibrium condition.

New Aspects in Physics on Gel'fand Triplets

New observations in Gel'fand triplets are studied. An interesting one is vortex phenomena that stem from zero energy solutions of two-dimensional Schrodinger equations with central potentials V(ρ) ∝ ρr(ρ2 = x2 + y2 and r ≠ −2), which are eigenstates of conjugate spaces of Gel'fand triplets. The zero energy solutions for all the potentials V(ρ) are shown to have the same structure with infinite degeneracy by making use of the conformal transformation ξα = zα with z = x + iy. The infinite degeneracy is observed as variety of vortex patterns in real physical phenomena. Some simple vortex patterns such as vortex lines and vortex lattices are presented. Such a new freedom on the Gel'fand triplets can be treated in a statistical mechanics. In the theory a new entropy being different from the so-called Boltzmann entropy appears. Transitions between the two entropies occur in thermal nonequilibrium phenomena, where energy emissions are observed.