Non-dimensional Thermal Conductivity Research Articles

Revisiting effective Lewis number of combustible mixtures

Lewis number (Le) is of importance in the field of premixed combustion. In this study, the effective Lewis numbers of combustible mixtures containing a single fuel (CH4/air, C3H8/air, H2/air, C2H4/O2/N2), two fuels (CH4/C2H4/air, H2/NH3/air, H2/CH4/N2O) and two oxidizers (CH4/N2O/O2) over a wide range of equivalence ratios were evaluated via various methodologies. The Markstein length was obtained from previous studies, or extracted experimentally from the outwardly propagating flames. Firstly, the mass diffusivity and Lewis number of a reactant in mixture were evaluated using different models. Secondly, the effective Lewis number (Leeff) was calculated based on various theories, which was compared with the experimental ones. The results indicate that depending on the components, the heat-release, volumetric and diffusion models can be optimal for the depiction of Leeff. For the studied mixtures, the measured Le based on λ(x)=x (Bechtold & Matalon, Combust. Flame, 2001, 127(1–2): 1906–1913) was found to be more universal, where λ is the non-dimensional thermal conductivity. Additionally, the experimental Lewis numbers derived from different models can be very close, and the key parameters were compared and analyzed.

Fracture Analysis for a Crack in Orthotropic Material Subjected to Combined 2i-Order Symmetrical Thermal Flux and 2j-Order Symmetrical Mechanical Loading

The problems of crack formation in orthotropic materials under 2i order polynomial function heat flow and 2j order polynomial function mechanical loading are considered. An extended local insulation crack model is proposed, and fracture analysis is carried out for the above problems. Utilizing Fourier transform technique (FTT) and principle of superposition, the jumps of temperature, elastic displacements on the crack, and so on are obtained. The advantage of this analysis is that the explicit closed form solutions of main parameters in classical fracture mechanics, i.e., the stress intensity factor, the energy release rate, and the energy density have been presented. A simple example is used to demonstrate the method proposed in this paper. The analysis results show that the non-dimensional thermal conductivity and the combined ratio of the heat flux per thickness perpendicular to the crack surface to the mechanical load have a great influence on the calculation of fracture parameters. Only when they meet certain conditions can the correct fracture parameter calculation results be obtained.

Measurements of laminar burning velocity and Markstein length in outwardly-propagating spherical SNG-air premixed flames at elevated pressures

A study has been conducted to measure unstretched laminar burning velocities and burnt Markstein lengths for outwardly-propagating spherical SNG-air premixed flames at elevated pressures. The range of flame radius and extrapolation models (LM, NM I and NM II) were first optimized for CH4-air premixed flames at normal temperature. The presently measured unstretched laminar burning velocities yielded the minimal errors when Aramco 2.0 and NM II were adopted for the optimized range of flame radius. The capability of predicting Markstein length with a composite model was investigated by using different definition of flame thickness and non-dimensional thermal conductivity for four iso-temperature surfaces. The theoretical Markstein lengths with appropriately defined flame thickness and non-dimensional thermal conductivity best trace the presently measured ones for the iso-temperature surface of 600 K.Unstretched laminar burning velocities against equivalence ratio were measured and compared with the predicted ones with Aramco 2.0 for SNG-air premixed flames. Except 0.1 MPa, the predicted ones were deviated appreciably from measured data when chamber pressure increases. The capability of predicting unstretched laminar burning velocities against equivalence ratio was improved via modifying several kinetic data based on sensitivity analysis to Aramco 2.0 Mechanism. The capability of predicting Markstein lengths was also tested for SNG-air premixed flames at elevated pressures up to 0.7 MPa. Choosing the iso-temperature surface of 600 K excellently traced the presently measured data when flame thickness and non-dimensional thermal conductivity are appropriately defined, while several issues are raised up as future works.

MHD Convection of an Electrically Conductive Fluid with Variable Thermal Conductivity

The effect of temperature-dependent thermal conductivity on the magneto-convection in a low Prandtl number liquid is investigated numerically. The liquid is contained in a closed square cavity with isothermal vertical walls kept at different temperatures. The top and bottom walls are assumed to be insulated. To solve the governing non-linear differential equations (mass, momentum and energy) a finite volume code based on SIMPLER algorithm is utilized. The results for different Rayleigh numbers (Ra), Hartmann numbers (Ha) and temperature coefficient of thermal conductivity (η) are presented in form of streamlines, isotherms and Nusselt number. It is found that the heat transfer decreases appreciably across the cavity with a decrease in thermal conductivity. It is observed that at low Hartmann number (Ha=30) the thermal boundary layer is formed near the side walls and as η increases the thickness of these boundaries decreases. Also it is found that as non-dimensional thermal conductivity increases the peak of velocity profile increases; however, this phenomenon is very weak at high Hartmann number.