MAGNETOTHERMAL CONVECTION OF POLYATOMIC GASES IN HOMOGENEOUS MAGNETIC FIELDS Part I: Theory

The theory of magnetothermal convection of gases in homogeneous magnetic fields is developed from magnetogas-dynamic theory. Application is made to nonionized paramagnetic and diamagnetic gases between parallel vertical plates. An additive contribution to the natural thermal convection heat transfer results from the temperature dependence of the magnetic permeability of the gas. The result may be expressed parametrically in a dimensionless magnctoconvection number, which characterizes the magnetothermal contribution to the total heat transfer just as the Grashof number characterizes the natural thermal contribution. For oxygen gas at low pressure the magnetothermal contribution is shown to be proportional to p³H²(ΔT)² /ηT⁵ where p is the gas pressure, H is the magnetic field strength, ΔT is the temperature difference between plates, η is the viscosity coefficient and T is the absolute temperature.     

梯度磁场作用下自然对流换热强化

为验证梯度磁场作用下自然对流换热的变化规律,揭示热磁对流现象的机理,利用钕铁硼永磁体构建了楔形梯度磁场空间,并对位于该空间的封闭腔内的氧气自然对流换热进行了实验研究.实验获得了氧气自然对流的激光散斑干涉图像,通过数据处理得到了氧气自然对流的温度场,进而获得了壁面局部Nu分布.结果表明,磁加速度近似与重力加速度方向相同的永磁梯度磁场布置可使氧气自然对流换热过程得到强化.     

Numerical simulation of natural convection in a square enclosure filled with nanofluid using the two-phase Lattice Boltzmann method

Considering interaction forces (gravity and buoyancy force, drag force, interaction potential force, and Brownian force) between nanoparticles and a base fluid, a two-phase Lattice Boltzmann model for natural convection of nanofluid is developed in this work. It is applied to investigate the natural convection in a square enclosure (the left wall is kept at a high constant temperature (T_H), and the top wall is kept at a low constant temperature (T_C)) filled with Al₂O₃/H₂O nanofluid. This model is validated by comparing numerical results with published results, and a satisfactory agreement is shown between them. The effects of different nanoparticle fractions and Rayleigh numbers on natural convection heat transfer of nanofluid are investigated. It is found that the average Nusselt number of the enclosure increases with increasing nanoparticle volume fraction and increases more rapidly at a high Rayleigh number. Also, the effects of forces on nanoparticle volume fraction distribution in the square enclosure are studied in this paper. It is found that the driving force of the temperature difference has the biggest effect on nanoparticle volume fraction distribution. In addition, the effects of interaction forces on flow and heat transfer are investigated. It is found that Brownian force, interaction potential force, and gravity-buoyancy force have positive effects on the enhancement of natural convective heat transfer, while drag force has a negative effect.     

A heat transfer study for a partially immersed refractory rod in a plasma

The thermal behavior of a tungsten rod partially immersed in a plasma flame is analyzed. The longitudinal temperature profiles are obtained by numerical solution of the heat balance equation. Results show that a heat transfer model consisting of pure natural convection up to the plasma outer edge and forced convection inside the plasma, fails to predict temperatures at the hot tip of the rod. In order to make the experimental and calculated profiles to converge, one has to consider modifications in the mechanical and thermal conditions of the surrounding medium due to the presence of the rod in the plasma. This leads to the use, in the “cold” gas zone, of particular values of heat transfer coefficient h and of a parameter T_x which is defined only at the immediate boundary of the rod. The latter are given in the form of variations along the axis of the rod. This perturbation effect decreases with the rod diameter. Allowance is made for the variation of the thermal properties of the solid.     

Experimental investigation of flow resistance and convection heat transfer of CO2 at supercritical pressures in a vertical porous tube

Experiments were conducted to measure the convection heat transfer in a vertical porous tube with particle diameters of 0.2–0.28 mm at supercritical pressures. The local heat transfer coefficients, fluid bulk temperatures and wall temperatures were measured to investigate the influence of the inlet fluid temperature, pressure, heat flux and flow direction on the convection heat transfer in the porous tube. The measured friction factors in a heated tube are much larger than those predicted using the Aerov-Tojec correlation for both upward and downward flows. Therefore, two new correlations are presented for upward and for downward flows to predict the friction factors of supercritical pressure CO₂ in heated porous tubes. The experimental results also show that the inlet temperature, pressure and heat flux all significantly influence the convection heat transfer. When the inlet temperature (T₀) is higher than the pseudocritical temperature (T_pc), the local heat transfer coefficients are much less than those when the inlet temperature is lower than the pseudocritical temperature. The convection heat transfer coefficients are found to vary nonlinearly with heat flux. For T₀ < T_pc, the local heat transfer coefficients along the porous tube have a maximum for upward flow and have a peak value for downward flow when the local fluid bulk temperatures are near T_pc and the wall temperatures are slightly higher than T_pc. The different variations of the local heat transfer coefficients along the porous tube for upward and downward flows are attributed to the effect of buoyancy. However, when the wall temperatures are much higher than T_pc, the local heat transfer coefficients along the porous tube decrease continuously for both upward and downward flows.     

Experimental study of natural convection heat transfer from a vertical plate in a non-newtonian fluid

An experimental investigation of natural convection heat transfer to non-Newtonian fluids from isothermal vertical plates was carried out. Three aqueous solutions of Carbopol 934 (1.25%, 1.5% and 1.75%) were selected as pseudoplastic fluids providing a flow behavior index range from 0.2 to 0.67. All the experimental data lay in the laminar flow range (N_GrN_Pr < 10⁹). A successful correlation for predicting natural convective heat transfer coefficients for non-Newtonian fluids from flow behavior index 0.2 to 1.0 inclusive was developed.     

Polymer–CO2 systems exhibiting retrograde behavior and formation of nanofoams

The sorption of compressed gases in polymers causing a reduction in the glass transition temperature (T_g) is well established. There is, however, limited information on polymer–gas systems with favorable interactions, producing a unique retrograde behavior. This paper reports on using a combination of established techniques of in situ gravimetric and stepwise heat capacity (C_p) measurements using high‐pressure differential scanning calorimetry (DSC) to demonstrate the occurrence of this behavior in acrylonitrile–butadiene–styrene copolymer (ABS)–CO₂ and syndiotactic poly(methyl methacrylate) (sPMMA)–CO₂ systems. The solubility and diffusion coefficient of CO₂ in the range 0 to 65 °C and pressures up to 5.5 MPa were determined, which resulted in a heat of sorption of ? 15.5 and ? 15 kJ mol^?1, and an activation energy for diffusion of 28.3 and 32.1 kJ mol^?1 in the two systems, respectively. The fundamental kinetic data and the changes in C_p of the polymer–gas systems were used to determine the plasticization glass transition temperature profile, its relationship to the amount of gas dissolved in the polymer, and hence the formation of nano‐morphologies. Copyright © 2006 Society of Chemical Industry     

Thermal strengthening of low-expansion glasses and thin-walled glass products by ultra-fast heat extraction

Thermal strengthening remains the primary method for enhancing the practical strength of commodity glass products, however, the process is limited in terms of applicable glass thickness and coefficient of thermal expansion. The primary reasons for this limitation are the achievable heat transfer coefficient when using conventional gas cooling, and the occurrence of transient surface tension in the early stages of rapid quenching. We revisit this problem for the case of thin borosilicate glass sheet. Using liquid gallium as the cooling medium, ultra-fast heat extraction is achieved, with a heat transfer coefficient exceeding 5000 Wm⁻² K⁻¹. The low vapor pressure of gallium even at high temperatures enables preheating to a wide range of sheet entrant temperatures. We demonstrate thermal strengthening of low-expansion borosilicate glass with persistent surface compression of up to 85 MPa, and quenching to a fictive temperature of ~190 K above the glass transition temperature. Glass sheet obtained in this way exhibits notably enhanced surface defect resistance to sharp indentation. In addition to thermal strengthening, the extraordinarily high heat extraction rates achieved by liquid metal immersion enable exploitation of high-T_f glass properties beyond small and thin sample geometry.     

Effect of dissolved gases on subcooled flow boiling heat transfer

The influence of various dissolved gases (He, N₂, Ar, CO₂, C₃H₈) on subcooled boiling heat transfer was investigated for flow of water and of heptane in an annulus with a heated core. Flow velocity, liquid bulk temperature, system pressure, gas partial pressure and heat flux were all varied over a wide range.In comparing the measured heat transfer coefficients with those for subcooled boiling of the corresponding degassed liquids, it was found that the coefficients were always increased owing to the desorption of the dissolved gases. The extent of the increase depended on the solubility of the given gas in the given liquid and could be as much as several hundred per cent. In addition, the solid surface temperature required for the inception of bubble formation was reduced considerably, in some cases far below the saturation temperature of the pure liquid.Attempts were made to extend the prediction of incipient boiling temperatures to cases where gases are dissolved in the liquid.     

Non‐Isothermal Decomposition Kinetics,Heat Capacity,and Thermal Safety of 2‐Nitroimino‐5‐Nitro‐Hexahydro‐1,3,5‐Triazine (NNHT)

The kinetic equation describing the thermal decomposition reaction of NNHT obtained by TG‐DTG data, integral isoconversional non‐linear method and integral method of treating TG‐DTG curves is . The specific heat capacity (C_p) of NNHT was determined with the continuous C_p mode of the microcalorimeter. The equation of C_p (T) was obtained. The standard molar heat capacity of NNHT was 218.41 J mol⁻¹ K⁻¹ at 298.15 K. With the help of the onset temperature (T_e) and maximum peak temperature (T_p) from the non‐isothermal DTG curves of NNHT at different heating rates (β), the apparent activation energy (E_K and E_O), and the pre‐exponential constant (A_K) of the thermal decomposition reaction obtained by Kissinger’s method and Ozawa’s method, C_p obtained by microcalorimetry, density (ρ) and thermal conductivity (λ), the decomposition heat (Q_d, taking half‐explosion heat), Zhang‐Hu‐Xie‐Li’s formula, Smith’s equation, Friedman’s formula, Bruckman‐Guillet’s formula, and Wang‐Du’s formulas, the values (T_e0 and T_p0) of T_e and T_p corresponding to β→0, thermal explosion temperature (T_be and T_bp), adiabatic time‐to‐explosion (t_TIad), 50 % drop height (H₅₀) of impact sensitivity, critical temperature of hot‐spot initiation (T_cr), thermal sensitivity probability density function [S(T)] versus temperature (T) relation curves for spheroidic NNHT with radius of 1 m surrounded with ambient temperature of 300 K, peak temperature corresponding to the maximum value of S(T) versus T relation curve ( ), safety degree (SD), and critical ambient temperature(T_acr) of thermal explosion of NNHT are calculated. The following results of evaluating the thermal safety of NNHT are obtained: T_SADT=T_e0=453.34 K, T_SADT=T_p0=454.86 K, T_be=462.68 K, T_bp=467.22 K, t_TIad=1.03 s, H₅₀=17.69 cm, T_α=461.4 K. SD=72.74 %, P_TE=27.26 %, and T_acr=321.96 K.     

PEMFC Performance in a Magnetic Field

A polymer electrolyte fuel cell is operated in a magnetic field gradient (B × dB/dx = ±3 T²�m^–1) and the power output is investigated at lower partial pressure of oxygen gas and lower temperature than customarily used in fuel cells. The effects of the magnetic field are not confirmed at the cell current of i < 12 mA cm^–2. On the other hand, the cell performance is improved or deteriorated depending on the direction of the magnetic field gradient at higher current densities at which the mass transfer of oxygen gas is limited by diffusion. The results suggest that the magnetic field influences the diffusion process of the oxygen molecules rather than the catalysis.     

Thermodynamic functions for 3-fluoropropene

Thermodynamic functions C_p°, S°, (H°-H₀°)/T, —(F°-H0°)/T have been calculated for 3-fluoropropene in the ideal gas state from 298.15° to 800°k. at 1 atm. pressure. The restricted rotational contribution has been treated by the conventional method of Pitzer and Gwinn.     

MHD‐Buoyancy Aiding and Opposing Flows with Viscous Dissipation Effects from Radiate Vertical Surfaces

The combined effects of forced and natural convection heat transfer in the presence of transverse magnetic field form a vertical surfaces with radiation heat transfer is studied. The buoyancy aided flow and the buoyancy opposing flows are investigated with viscous dissipation effects included in the governing equations. It is found that four parameters can describe the problem under consideration, the mixed parameter, χ, the radiation‐conduction parameter, R_d, the magnetic field parameter, Ha²_x/Re_x, and the Eckert number, Ec. The local wall shear stresses and the local Nusselt number variations are drawn for different dimensionless groups.     

Natural convection heat transfer enhancement through latent heat transport in vertical parallel plate channel flows

A numerical analysis is carried out to investigate the effects of latent heat transfer, in connection with the vaporization of a liquid film, on natural convection heat transfer in a vertical parallel plate channel. Major nondimensional groups identified are Gr_T, Gr_M, Pr and Sc. Results for Nusselt and Sherwood numbers are specifically presented for the air-water and air-ethanol systems under various heating conditions to illustrate the heat transfer enhancement through latent heat transfer during the evaporation processes. Considerable enhancement in heat transfer due to the exchange of latent heat was clearly demonstrated.     

An analysis of the steady state thermal behaviour of electrolysers with recycle

A mathematical model, based on thermal balances and heat transport equations for the computation of steady state values of key temperatures and heat loss rates, is employed to analyse electrolyser performance. In particular, the effect of recycling rate and thermal insulation thickness are investigated from the viewpoint of cost-optimal operation.Notation A _E heat transfer loss area (electrolyser) - A _T heat transfer loss area (recycle line) - C _H specific cost of thermal energy - C _i specific cost of insulation (including labour and amortisation) - C specific heat capacity of electrolyte - d _i thickness of lagging on recycle line - F Faraday constant - f _R recycle ratio (Q _R/Q) - H height of electrolyser - H _E rate of heat loss from electrolyser - H _T rate of heat loss from recycle line - h ₀ outer-surface heat transfer coefficient - I electric current - k _i thermal conductivity of lagging - L _E length of electrolyser - L _R length of the recycle line - MU symbol of an arbitrary monetary unit - Q volumetric flow rate of inlet electrolyte - Q _R volumetric flow rate of recycle stream - R _E electrolyte resistance in electrolyser - r _i inner radius of recycle line - T _A ambient temperature - T _m temperature of electrolyte entering the electrolyser - T _R temperature of electrolyte leaving the recycle line - T ₁ temperature of fresh intake electrolyte - T ₂ temperature of electrolyte leaving the electrolyser - t _i thickness of insulation on electrolyser - U _E overall heat transfer coefficient (electrolyser) - U _T overall heat transfer coefficient (recycle line) - V _i volume of lagging - W width of electrolyser - z valency - H _R heat of reaction - T _LM log mean temperature (electrolyser) - _LM log mean temperature (recycle line) - symbol for time - density of electrolyte     

Computer calculation of heat capacity of natural gases over a wide range of pressure and temperature

This paper presents a method whereby specific heats or heat capacities of natural gases, both sweet and sour, at elevated pressures and temperatures may be made suitable to modern day machine calculation. The method involves developing (1) a correlation for ideal isobaric heat capacity as a function of gas gravity and pseudo reduced temperature over the temperature range of 300 to 1500 K and (2) a mathematical equation for the isobaric heat capacity departure based on accepted thermodynamic principles applied to an equation of state that adequately describes the behavior of gases to which the Standing and Katz Z factor correlation applies. The heat capacity departure equation is applicable over the range of 0.2 ≤ P_r ≤ 15 and 1.05 ≤ T_r ≤ 3. The significance of the method presented here lies in its utility and adaptability to computer applications.     

THERMAL CONVECTIVE INSTABILITY IN TRANSLUCENT POROUS MEDIA WITH RADIATIVE HEAT TRANSFER

The onset of thermal convection in a translucent porous layer is considered. Attention is focused on the effect of radiative heat transfer on the critical Rayleigh-Darcy number and the convection cell shape. If we consider the contribution of radiative heat transfer, the basic temperature profile is non-linear and the thermal convective instability is influenced by the ratio of conduction to radiation heat flux, the temperatures at the boundary surfaces, and radiative parameters such as wall emissivity, scattering albedo and extinction coefficient as well as the usual Rayleigh-Darcy number. Effects of these parameters on the onset of convective instability are investigated with the help of linear stability theory employing the Darcy's law and the radiative transport equation simplified by the P₁ approximation. The increased effective thermal conductivity due lo the radiation inhibits the onset of convection and causes increased critical Rayleigh-Darcy number and decreased convection cell size. The results of the present work may be exploited to find out the optimal diameter of aerogel pellets and the air pressure in the double pane window filled with the translucent silica aerogel granules to suppress natural convection.     

应用CaSO4载氧体燃烧技术的CaO再生过程

引言近年来CaO作为CO2吸收剂在近零排放煤直接制氢、生物质气化、吸收增强型天然气重整制氢、焦炉煤气重整制氢、燃煤电站CO2捕集等过程的应用受到国内外的持续关注和大量研究。在这些过程中CO2以CaCO3的形式固化下来,为循环利用CaO吸收剂并收集CO2,需要将产物CaCO3煅烧分解,这一过程称为CaO再生过程。CaO再生是强吸热过程,为该过程提供热量的一     

盐浴螺旋盘管式焦炉上升管余热回收装置传热性能试验

对盐浴螺旋盘管式结构的焦炉上升管高温荒煤气余热回收装置进行了以烟气代替荒煤气的传热性能模拟试验,得到了上升管余热回收装置螺旋盘管环形套筒的外壁温度分布、上升管内烟气侧的传热系数、环形套筒内螺旋盘管外盐浴的自然对流传热系数等试验研究结果。结果表明,装置螺旋盘管环形套筒的外壁温度分布并非均匀,随上升管内烟气温度的升高而波动增大;烟气侧对流传热系数在Re数高于2900后随Re数的增大而明显上升;螺旋盘管外盐浴的自然对流传热系数随熔盐温度的升高几乎不变。根据试验结果拟合出环形套筒内螺旋盘管外盐浴的自然对流传热关联式,为实际的工程应用提供参考。