Gasless combustion wave with extremely high excess enthalpy under heat loss conditions

A numerical experiment on investigation of a gasless combustion wave propagating at the limit due to a high excess enthalpy in the heating zone is performed. Under heat loss conditions, waves emerge on the surface of the combustion front, causing changes in the rate of chemical reactions similar to those realized on the burning surface of propellants and explosives.     

低温工况下翅片管换热器的设计计算

采用分段模型将气化压力高于介质临界压力的翅片管换热器内低温介质的气化过程分为液相、气相两个传热区。同时考虑气化过程中翅片管表面结霜情况,对低温介质在翅片管换热器内的吸热气化过程进行传热分析,给出了适合各分区传热特性的计算关联式,为工程设计提供参考。     

Influence of nucleation on the flow boiling heat transfer coefficient of a refrigerant mixture under varied heat flux conditions

The influence of nucleation on the flow boiling heat transfer coefficient of R-134a/R-290/R-600a refrigerant mixture is experimentally studied in a smooth horizontal tube of 12.7 mm diameter. The heat transfer coefficients are experimentally measured for stratified flow patterns under a varied heat flux condition; a condition found in the evaporator of refrigerators and deep freezers. The experiments are conducted in a counter-current heat exchanger test section. By regulating the flow rate and inlet temperature of acetone, which is the heating fluid flowing in the outer tube, a varied heat flux is provided to the refrigerant flowing in the inner tube. The refrigerant mass flow rate is fixed between 3 and 5 g s⁻¹ and its inlet temperature between −8.59 and 5.33°C, which corresponds to a pressure of 3.2 to 5 bar. The significance of nucleate boiling prevailing in the above-mentioned evaporators is highlighted. The experimental heat transfer coefficients are also compared with well known heat transfer correlations.     

Oscillatory thermal-diffusive instability of combustion waves in a model with chain-branching reaction and heat loss

In this paper we investigate the properties and the linear stability of premixed combustion waves in a non-adiabatic thermal-diffusive model with a two-step chain-branching reaction mechanism. Here we focus only on the emergence of the pulsating instabilities, and the stability analysis is carried out for Lewis numbers for fuel greater than one, and various values of Lewis number for radicals. We consider the problem in two spatial dimensions to allow perturbations of a multidimensional nature. It is demonstrated that the flame speed as a function of the parameters is a double-valued C-shaped function, i.e. for a given set of parameter values there are either two solutions, fast and slow solution branches, propagating with different speed, or the combustion wave does not exist. The extinction of combustion waves occurs at finite values of the parameters and non-zero flame speed. The flame structure demonstrates a slow recombination regime behaviour with negligible fuel leakage for the fast solution branch away from the extinction condition. For parameter values close to the extinction condition and on the slow solution branch, the fuel leakage is significant and a fast recombination regime is observed. It is demonstrated that two types of instabilities emerge in the model: the uniform planar and the travelling instability. The slow solution branch is always unstable due to the uniform perturbations. The fast solution branch is either stable or loses stability due to the travelling or uniform perturbations. The switching between the onset of various regimes of instability is due to the bifurcation of co-dimension two. In the adiabatic limit this bifurcation is found for Lewis number for fuel equal to one, whereas in the non-adiabatic case it moves towards values above unity. The properties of the travelling instability are studied in detail.     

Effect of combustion chamber insulation on the performance of a low heat rejection diesel engine with exhaust heat recovery

A computer simulation of the turbocharged turbocompound diesel engine system is used to study the effect of combustion chamber insulation on the performance of low heat rejection system configurations with exhaust heat recovery. The analysis is carried out for zirconia coatings of various thickness applied on the cylinder head and piston. It is found that an intercooled turbocompound engine derives a modest thermal efficiency benefit from insulation, e.g. 4.3% improvement at a 60% reduction in heat loss. The addition of Rankine compounding can improve the thermal efficiency of the turbocompound engine by 10–14%, depending on the level of insulation and the system configuration. Furthermore, Rankine compounding can make the otherwise inferior performance of a non-intercooled engine match the performance of an intercooled engine. Finally, use of an insulating material of low conductivity and low heat capacity can increase the thermal efficiency benefits, but at the expense of increased component thermal loading.     

Formation of oriented cadmium telluride films on an amorphous substrate under extremely nonequilibrium conditions

The results of the first experiments related to oriented CdTe film growth on a nonorienting substrate (glass) cooled to negative Celsius temperatures under extremely nonequilibrium conditions are reported. Technological, electron-diffraction, and X-ray investigation results are presented. A condensation diagram characterized by two regions within which the growth rate of films is anomalously low is obtained. The films grown at these rates are shown to possess a nearly perfect crystalline texture. The formation processes of the oriented films on an amorphous substrate under the above conditions are adequately interpreted in the context of a heteroepitaxy soliton model.     

Separation of a molecular mixture in a thermodiffusion column under supercritical conditions

The process of separation of carbon dioxide and hexafluorethane mixture in a thermodiffusion column under supercritical conditions is studied. It is shown experimentally that the separation efficiency of thermodiffusion column increases when passing from the gas phase to supercritical conditions.     

Sound radiation from an aperture in a rectangular enclosure under low modal conditions

This paper investigates both theoretically and experimentally the sound radiation from an aperture placed in an enclosure wall for the particular case of low modal sound field. The incidence field is composed of the enclosed sound field, which is calculated using the theoretical modal model presented. The transmitted sound is calculated by the Rayleigh radiation equation after continuity conditions have been applied in the aperture plane, assuming the condition of a thin wall. The model is experimentally validated by measuring the directivity and sound pressure radiated from an aperture in the side of a rectangular box. Because the walls of the enclosure are not rigid, an experimental procedure to determine its admittance is also presented. The experiments have been carried out for the first four modes of the enclosed sound field, and good agreement is found between the theoretical and experimental results. These results indicate that the admittance of the aperture, its radiation efficiency, and its directivity are all functions of the predominant mode shape, and the frequency, as well as the location and shape, of the aperture relative to the predominant enclosed mode shape.     

Limiting conditions for the combustion of a rich gas mixture on the surface of a permeable matrix

The limit of the surface combustion of rich methane-air mixtures on a planar and a volumetric permeable matrix is theoretically and experimentally determined. The influence of a radiation screen on the expansion of the region of stable surface combustion at a heightened specific thermal load on the matrix is examined. The possibility of a substantial expansion of the lower limit of surface combustion in passing from a planar to a volumetric matrix is demonstrated. It is shown that heat recovery allows stable surface combustion of natural gas-air mixtures at air-to-fuel ratios of up to α = 0.35.     

Effect of structural conduction and heat loss on combustion in micro-channels

This paper presents a simple analytical model for the effects of heat exchange within the structure of a micro-channel combustor, and heat loss from the structure to the environment. This is accomplished by extending reasoning similar to that employed by Mallard and Le Chatelier in their thermal theory for flame propagation. The model is used to identify some of the basic parameters that must be considered when designing an efficient micro-combustor and its predictions are compared with the results of a numerical simulation of stoichiometric premixed combustion of a hydrogen–air mixture stabilized between two parallel plates. The simulation incorporates a one-dimensional continuity/energy equation solver with full chemistry coupled with a model for thermal exchange in the structure. The results show that heat exchange through the structure of the micro-combustor can lead to a broadening of the reaction zone. Heat loss to the environment decreases the broadening effect and eventually results in flame quenching. This behaviour, which arises from the thermal coupling between the gas and the structure, influences the maximum achievable power density of microscale combustors.     

Thermal explosion of a reactive mixture under conditions of oscillating ambient temperature

The problem of the thermal explosion of a finite volume of a reactive material in a medium with harmonically oscillating temperature is solved in the classical formulation. A kind of resonance is shown to arise when the oscillation period is commensurate with the adiabatic induction period of thermal explosion at the mean ambient temperature. At both high and very low oscillation frequencies, the critical condition parameter and induction period are only slightly affected by ambient temperature oscillations. By contrast, at moderately low frequencies, even small-amplitude oscillations of ambient temperature can strongly influence the critical condition and, especially, induction period of thermal explosion.     

Chromatic polarization in solid parahydrogen with an argon impurity at an extremely low concentration

The effect of chromatic polarization observed in samples prepared for studying thermal conductivity of weak solid solutions (p-H₂)_{1 − x} Ar _x has been described. Bulk cylindrical samples have been grown in a molybdenum glass ampule through desublimation followed by control of their quality in polarized light. In contrast to weak solid solutions (p-H₂)_{1 − x} Ne _x with a heavy quasi-isotopic neon impurity, the samples with argon at the same concentrations (several parts per million) exhibit inclined chromatic bands due to elastic mechanical stresses induced after cooling to liquid-helium temperature.     

The effect of heat loss on the propagation of strongly exothermic combustion waves

We study the propagation in one spatial dimension of combustion waves in a strongly exothermic, premixed fuel. After using numerical methods to determine the form of the combustion waves, we use the method of matched asymptotic expansions to obtain asymptotic approximations for the permanent form travelling wave solutions. This allows us to determine the leading-order behaviour of the maximum temperature, the residual concentration of fuel behind the wave, the wave speed and the maximum heat transfer coefficient that allows the propagation of waves, along with the qualitative form of the wave. In all cases, good agreement is found between numerical and asymptotic results.     

Measurement-based entanglement under conditions of extreme photon loss

The act of measuring optical emissions from two remote qubits can entangle them. By demanding that a photon from each qubit reaches the detectors, one can ensure that no photon was lost. But retaining both photons is rare when loss rates are high, as in Moehring et al. where 30 successes occurred per 10(9) attempts. We describe a means to exploit the low grade entanglement heralded by the detection of a lone photon: A subsequent perfect operation is quickly achieved by consuming this noisy resource. We require only two qubits per node, and can tolerate both path length variation and loss asymmetry. The impact of photon loss upon the failure rate is then linear; realistic high-loss devices can gain orders of magnitude in performance and thus support quantum computing.     

High performance thermoelectric Tl9BiTe6 with an extremely low thermal conductivity

Tl 9BiTe (6) exhibits a thermoelectric figure of merit of ZT approximately 1.2 around 500 K, which significantly exceeds the state-of-the-art materials in this temperature range. The extraordinary thermoelectric performance is mainly due to the extremely low thermal conductivity of Tl 9BiTe (6) [ 0.39 W/(m times K) at 300 K]. In fact, the minimum lifetime of the phonons has to be taken into account to describe the thermal conductivity data.     

Characteristics of auto-ignition in a stratified iso-octane mixture with exhaust gases under homogeneous charge compression ignition conditions

Ignition and propagation of a reaction front in a counterflow system of an iso-octane/air stream mixing with an exhaust gas stream is computationally investigated to understand the fundamental characteristics of homogeneous charge compression ignition (HCCI) auto-ignition. Various mixing rates are imposed on the system and the effects of dissipation rates on auto-ignition are studied. Ignition delay and front propagation speed across the mixing layer are determined as a function of a local mixture fraction variable. The results show that mixture inhomogeneity and dissipation rate have a significant influence on ignition. Diffusive transport is found to either hamper or advance ignition depending on the initial reactivity of the mixture. Based on the relative importance of diffusion on ignition front propagation, two distinct ignition regimes are identified: the spontaneous ignition regime and the diffusion-controlled regime. The transition between these two regimes is identified using a criterion based on the ratio of the timescales of auto-ignition and diffusion. The results show that ignition in the spontaneous regime is more likely under typical HCCI operating conditions with iso-octane due to its high reactivity. The present analysis provides a means to develop an improved modelling strategy for large-scale engine simulations.     

Conjugate convective heat transfer of a crystal with an ambient medium under the conditions of thermal gravitation convection in Czochralski method

Results on conjugate convective heat transfer in a system geometrically similar to the system “crystal-gas-growing vessel shell” of a growing vessel obtained by the Czochralski method are presented. Equations of thermal gravitation convection in Boussinesq approximation are solved by the finite element method in the following variables: temperature, stream function, and vorticity in cylindrical coordinates. Heat transfer from the crystal was studied within the range of Grashof number 100 ≤ Gr ≤ 10⁴ for the fixed configuration of calculation domain. Dependence of the temperature field in the silicon crystal on convection intensity in gas (argon) was examined.