Methods of calculating thermal conductivity of binary mixtures involving polyatomic gases

Summary The experimental binary thermal conductivity data of nineteen different gas pairs have been discussed and the competence of the rigorous, approximate and empirical procedures to represent them is investigated and discussed. In addition a new semi-theoretical method is suggested and tested. The suggested semi-theoretical procedure works very satisfactorily and is of good accuracy. It also compares favourably with the other methods. We also suggest a procedure for estimating thermal conductivity values at high temperature. This is an interesting and useful suggestion in view of the great practical need and their meagre availability.     

Determination of specific heat and true thermal conductivity of glass from dynamic temperature data

A method to determine the true specific heat and true thermal conductivity for glass and other semitransparent materials from dynamic temperature data is presented. A unique fabrication technique to obtain high quality dynamic temperature data from glass test plates employing thermocouples fused to the glass is described. The true thermal conductivity and specific heat of float glass has been measured using these techniques, and the results are compared with the scant data available in the literature. Sensitivity of the measured specific heat and thermal conductivity to sources of uncertainty is identified and these are discussed.     

Effective thermal conductivity of heat pipes

Several heat pipes were designed and manufactured to study the effect of the working fluids, container materials, and the wick structures on the heat transfer mechanism of the heat pipes. Also, the effect of the number of wick layers on the effective thermal conductivity and the heat transfer characteristics of the heat pipes have been investigated. It was found that the flow behavior of the working fluid depends on the wicking structures and the number of wick layers. The heat transfer characteristics and the effective thermal conductivity are related directly to the flow behavior. Increasing the number of wick layers (up to 16 layers) increases the heat flux with smaller temperature differences. The flattening phenomena of the thermal resistance was observed after 16 wicks layers due to the entrainment limit.     

Indirect measurement of the heat conductivity and specific heat coefficients of homogeneous fluid

The coefficients of heat conductivityk and of specific heatc _p have been determined as parameters of a system of differential equations of balance and boundary conditions. The values of the coefficientsk andc _p are obtained by solving the inverse convection problem. The inverse convection problem includes a system of differential equations resulting from the mass, momentum and energy balances, the boundary conditions and an additional condition consisting of satisfying the postulate of the minimum squares of differences of the temperatures computed numerically and those measured at given points in the measurement tube.     

Measurements of the thermal conductivity of gases and gas mixtures,methods and results

Two methods for measuring the thermal conductivity have been employed:

1. a cylindrical cell type of apparatus for absolute measurements, with an accuracy of 0.5%.
2. a thermistor katharometer bridge for differential measurements, allowing an accuracy of 0.1%. The thermistor bridge is described in some detail and its performance is analysed.

Results are presented for a number of pure gases and binary mixtures. A special study was made of mixtures with water vapour or heavy water vapour as a component. Measurements were performed at room temperature and 333 K. In general our results compare well with those of other investigators, although differences are noted that surpass the degree of accuracy we believe to hold for our values. For a number of the binary mixtures investigated no results have -to our knowledge-been reported previously. The results for six of the binary mixtures containing water as a component are represented by a Wassiljewa-type formula. No agreement was obtained with values calculated from four different theories based on this formula.     

燃气射流冲击传热特性的数值模拟

针对射流传热问题,利用基于RNGk-ε湍流模型的数值方法模拟了射流垂直冲击平板的流动过程,并与实验数据比较,验证了模型的可行性。在此基础上,以火箭喷管入口参数为入口条件,建立了超音速燃气射流垂直冲击平板和冲击浸没平板的计算模型,分析了不同冲击条件下努塞尔数分布规律和温度分布规律, 论述了超音速射流传热的特性及影响传热特性的因素。得到了冲击距离为(14~18)D的努塞尔数取值范围,并表明冲击距离和射流温度是影响传热效率的关键因素;冲击距离增加,传热效率降低,冲击平板表面的射流温度越高,传热效率越高。     

The role of several heat transfer mechanisms on the enhancement of thermal conductivity in nanofluids

A modelling of the thermal conductivity of nanofluids based on extended irreversible thermodynamics is proposed with emphasis on the role of several coupled heat transfer mechanisms: liquid interfacial layering between nanoparticles and base fluid, particles agglomeration and Brownian motion. The relative importance of each specific mechanism on the enhancement of the effective thermal conductivity is examined. It is shown that the size of the nanoparticles and the liquid boundary layer around the particles play a determining role. For nanoparticles close to molecular range, the Brownian effect is important. At nanoparticles of the order of 1–100 nm, both agglomeration and liquid layering are influent. Agglomeration becomes the most important mechanism at nanoparticle sizes of the order of 100 nm and higher. The theoretical considerations are illustrated by three case studies: suspensions of alumina rigid spherical nanoparticles in water, ethylene glycol and a 50/50w% water/ethylene glycol mixture, respectively, good agreement with experimental data is observed.     

On the conduction of heat in rarefied gases and its manometric application. I

Summary Formulae for the conduction of heat in pure gases and in binary mixtures relevant to a tube with a thin, axial heated wire are developed. They cover the region of pressure between that for which the heat conduction is independent of pressure and that for which M. Knudsen's formula is valid. A theory for precision manometry is given and the manometric constants for twenty-five gases have been measured.     

Theory of the relaxation pressure in a dissociating gas

The influence of a slow dissociation reaction (the reaction rate is much lower than the rate of energy exchange between the translational and internal degrees of freedom of the gas particles) on the transfer coefficients is studied using the Chapman—Enskog method. It is shown that changes occur only in the diagonal part of the pressure tensor, where an additional coefficient, the so-called relaxation pressure, appears.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 101–107, March–April, 1978.The author thanks A. S. Pleshanov for useful discussions and valuable comments.     

Nonequilibrium laminar boundary layer of dissociating air on axisymmetric bodies

Results are presented of numerical calculations of a nonequilibrium laminar boundary layer on axisymmetric bodies whose surface has arbitrary catalytic activity using a proposed technique. In the published studies devoted to the exact numerical methods for calculating the boundary layer with chemical reactions, it is assumed that the surface of the body is either noncatalytic or has infinite catalytic activity [1], that thermochemical equilibrium exists at the surface [2], or that the temperature and composition of the gas at the surface are given [3, 4]. This problem has been examined in the approximate formulation in several papers, specifically [5].The authors wish to thank V. V. Lunev and I. N. Murzinov for their counsel and comments.     

基于数据驱动模型求解热传导反问题

热传导反问题求解在工程领域具有重要的应用价值.本文发展数据驱动模型识别了管道内壁几何形状和皮肤肿瘤生长参数等热传导反问题.在管道内壁几何形状识别问题中,首先采用随机生成模型结合有限元法求解热传导正问题,并采用有效导热系数转化的思想,建立机器学习模型,求解了测点温度与有效导热系数之间的抽象映射关系,进而实现管道内壁几何形...     

Determination of heat fluxes in the neighborhood of a double curvature stagnation point in a flow of dissociating gas of arbitrary chemical composition

Numerical solutions are obtained for the equations of a uniform compressible boundary layer with variable physical properties in the vicinity of a stagnation point with different principal curvatures in the presence of an injected gas with the same properties as the incident flow. The results of the numerical solutions are approximated for the heat flux in the form of a relation that depends on the variation of the product of viscosity and density across the boundary layer and on the ratio of the principal radii of curvature.Using the concepts of effective diffusion coefficients in a multicomponent boundary layer, previously introduced by the author in [1], and the generalized analogy between heat and mass transfer in the presence of injection, together with the numerical solutions obtained, it is always possible, even without additional solutions of the boundary-layer equations, to derive final formulas for the heat fluxes in a flow of dissociating gas of arbitrary chemical composition, provided that we make the fundamental assumption that all recombination reactions take place at the surface.By way of example, formulas are given for the heat transfer to the surface of a body from dissociating air, regarded as a five-component mixture of the gases O, N, NO, O₂, N₂, and from a dissociating mixture of carbon dioxide and molecular nitrogen of arbitrary composition, regarded as an eleven-component mixture of the gases O, N, C, NO, C₂, O₂, N₂, CO, CN, C₃, CO₂.In the process of obtaining and analyzing these solutions it was found that, in computing the heat flux, a multicomponent mixture can be replaced with an effective binary mixture with a single diffusion coefficient only when the former can be divided into two groups of components with different (but similar) diffusion properties. In this case the concentrations of one group at the surface must be zero, while the diffusion flows of the second group at the surface are expressible, using the laws of mass conservation of the chemical elements, in terms of the diffusion flows of the first. Then the single effective diffusion coefficient is the binary diffusion coefficient D(A,M), where A relates to one group of components and M to the other.In view of the small amount of NO(c(NO) < 0.05), the diffusion transport of energy in dissociated air maybe described with the aid of a single binary diffusion coefficient D(A, M)(A=O, N, M=O₂, N₂, NO). However even in the case of complete dissociation into O and C atoms at the outer edge of the boundary layer, the diffusion transport of energy in dissociated carbon dioxide can not be described accurately enough by means of a model of a binary mixture with a single diffusion coefficient, since the diffusion properties of the O and C atoms are distinctly different.     

Experimental investigation of plastic finned-tube heat exchangers,with emphasis on material thermal conductivity

In this paper, two modified types of polypropylene (PP) with high thermal conductivity up to 2.3 W/m K and 16.5 W/m K are used to manufacture the finned-tube heat exchangers, which are prospected to be used in liquid desiccant air conditioning, heat recovery, water source heat pump, sea water desalination, etc. A third plastic heat exchanger is also manufactured with ordinary PP for validation and comparison. Experiments are carried out to determine the thermal performance of the plastic heat exchangers. It is found that the plastic finned-tube heat exchanger with thermal conductivity of 16.5 W/m K can achieve overall heat transfer coefficient of 34 W/m² K. The experimental results are compared with calculation and they agree well with each other. Finally, the effect of material thermal conductivity on heat exchanger thermal performance is studied in detail. The results show that there is a threshold value of material thermal conductivity. Below this value improving thermal conductivity can considerably improve the heat exchanger performance while over this value improving thermal conductivity contributes very little to performance enhancement. For the finned-tube heat exchanger designed in this paper, when the plastic thermal conductivity can reach over 15 W/m K, it can achieve more than 95% of the titanium heat exchanger performance and 84% of the aluminum or copper heat exchanger performance with the same dimension.     

Lattice Boltzmann simulation of the effects of cavity structures and heater thermal conductivity on nucleate boiling heat transfer

The boiling heat transfer technology with cavity surfaces can provide higher heat flux under lower wall superheat, which is of great significance for the cooling of electronic chips and microelectromechanical devices. In this paper, the boiling characteristics of the cavity surfaces are investigated based on the lattice Boltzmann(LB) method, focusing on the effects of cavity shapes, sizes, and heater thermal conductivity on the heat transfer performance. The results show that the triangular cavi...     

The mean condensate heat resistance of dropwise condensation with flowing,inert gases

The quantification of the condensate heat resistance is studied for dropwise condensation from flowing air-steam mixtures. Flows are essentially laminar and stable with gas Reynolds numbers around 900 and 2000. The condensate shaping up as hemispheres on a plastic plane wall and the presence of inert gases make it possible that thermocapillary convection occurs making the resistance less than the mean condensate thickness (ca. 0.185 mm) divided by the heat conduction coefficient. The analysis of experiments shows that the effective mean condensate resistance might indeed be less, by a factor of 0.8±0.2. The analysis takes account of the sensible heat transfer which may be as large as 35% of the total heat transfer if inlet vapor concentration,c _in, is low (ca. 0.07). A method is presented to determine the gas-condensate interface temperature,t _i , that is needed in the analysis of the heat resistance. The highest temperature differences(t _i –t _w ),t _w being the mean temperature of the condenser plate at the gas side, have been found to occur for relatively high values ofc _in (ca. 0.3).Es wird der thermische Kondensationswiderstand bei Tropfenkondensation aus einem strömenden Luft-Dampf-Gemisch untersucht. Die Strömung ist laminar und stationär mit Gas Reynolds-Zahlen zwischen 900 und 2000. In Anwesenheit von Intertgas kann thermokapillare Konvektion am halbkugelförmigen Kondensattropfen auftreten, wobei der thermische Widerstand kleiner ist, als der Quotient aus der mittleren Kondensatdicke (ca. 0,185 mm) und dem Wärmeübergangskoeffizienten. Die Analyse der Experimente zeigt, daß der effektive mittlere Kondensatwiderstand tatsächlich um einen Faktor 0,8±0,2 kleiner sein kann. Der Energieanteil infolge Kondensatunterkühlung wird berücksichtigt, er kann bis zu 35% der gesamten Wärmeübertragung ausmachen, falls die Dampfkonzentrationc _in am Eintritt klein ist (ca. 0,07). Zur Bestimmung der in die Berechnung des thermischen Widerstandes eingehende Temperatur an der Grenzfläche zwischen Gas und Kondensatt _i , wird eine spezielle Methode entwickelt. Die größte Temperaturdifferenz (t _i –t _w ) ergibt sich bei relativ hohenc _in-Werten (ca. 0,3), wobeit _w die mittlere Temperatur der Kondensatorplatte auf der Gasseite ist.     

Darcy-Forchheimer flow of variable conductivity Jeffrey liquid with Cattaneo-Christov heat flux theory

The role of the Cattaneo-Christov heat flux theory in the two-dimensional laminar flow of the Jeffrey liquid is discussed with a vertical sheet. The salient feature in the energy equation is accounted due to the implementation of the Cattaneo-Christov heat flux. A liquid with variable thermal conductivity is considered in the Darcy-Forchheimer porous space. The mathematical expressions of momentum and energy are coupled due to the presence of mixed convection. A highly nonlinear coupled system of equations is tackled with the homotopic algorithm. The convergence of the homotopy expressions is calculated graphically and numerically. The solutions of the velocity and temperature are expressed for various values of the Deborah number, the ratio of the relaxation time to the retardation time, the porosity parameter, the mixed convective parameter, the Darcy-Forchheimer parameter, and the conductivity parameter. The results show that the velocity and temperature are higher in Fourier's law of heat conduction cases in comparison with the Cattaneo-Christov heat flux model.