In the present study, a numerical investigation of natural convection and volumetric radiation interactions has been analysed in an annulus between two isothermal concentric square ducts filled with a semi-transparent medium. Two-dimensional solution was obtained using a discrete ordinates method based on a combined finite volume-immersed boundary method. The fluid is assumed to be gray absorbing-emitting and non-scattering and all walls are gray, diffuse and opaque. The Rayleigh number is fixed to 106, and a study of the effect of three optical thicknesses τ=0.2-1-5 is performed on the thermal and dynamic fields and consequently on the heat transfer rate has been examined. A pure natural convection case has been shown for a comparative study. It is found that the radiation has a large influence on the flow and surface averaged Nusselt number and especially for high Rayleigh numbers.

Alternative fuels used in diesel buses are becoming the subjects of interest today. LPG, one of these fuels, has some economical and ecological benefits compared to diesel. Converting diesel engine into gaseous fueling is accompanied with performances degradation. Therefore, combustion must be improved to remedy this problem. Combustion Improvement is directly related to the enhancing of intake aerodynamic mixing movements which is influenced by the air-gas mixing system. This paper investigates the air-LPG engine mixer geometry effect on performance of a six-cylinder, heavy duty, IVECO engine, which is used to power urban buses in Tunisia. This engine was retrofitted from its diesel version into bi-fuel gasoline/LPG technology. The gaseous fueling technique used is the carburetion due to its benefits against the conversion cost for a wide range converted buses. Investigations were performed by numerical and experimental methods. A 3D modeling of the turbulent flow through engine intake system (with multiple-holes “MH” mixer and without mixer) was undertaken. The model is based on solving Navier-Stokes and energy equations in conjunction with the standard k-ε turbulence model. This model allowed the identity of the mixer effect. Results indicated that the addition of MH mixer can produce a better homogenous mixture. Experimental measurements are also carried out to validate the mixer influence by measuring the important engine performances. Comparative analysis of the experimental results showed the improvement of BP, BT, BTE and BSFC by 6.25 %, 3 %, 9 % and 6.6 % respectively using MHM mixer in LPG engine operation.

The main goal of this research work is to propose a convenient model that describes the evolution of the latent heat during a full transformation cycle and defines the thermal-induced phase transformation behavior of the NiTi shape memory alloy under zero stress conditions. The proposed model is constructed based on experimental data obtained from Differential Scanning Calorimetry (DSC) analyzes, carried out at various cooling/heating rates. The NiTi thermal behavior was studied and modeled across the different temperature ranges of the thermal transformation cycle. The developed constitutive equations were implemented in the Matlab software. The expression of the enthalpy change during both forward and reverse phase transformations was predicted as function of the cooling/heating rate. Moreover, the transformations temperatures were theoretically determined from the proposed model and compared to the experimental ones. A good agreement between experimental and theoretical findings proves that the proposed model provides a precise estimation. The efficiency of the adopted hypotheses is well confirmed.