A Convenient Synthesis of Alkyl and Aryl Substituted Bis-1,3,4-oxadiazoles

Substituted bis-1,3,4-oxadiazoles 3a-1 have been synthesized from the cyclization of N,N'-diaroyl or N,N'-dialkanoyl diacyl hydrazides, in presence of phosphorus oxychloride. A11 new compounds have been identified by ¹H-NMR ¹³C-NMR and elemental analysis.     

On heat transfer effects of a viscous fluid squeezed and extruded between two parallel plates

The unsteady squeezing and extrusion of a viscous fluid between two parallel plates of constant temperature is examined. The dimensionless extrusion parameter,=U/V, is introduced to represent the effects of the extrusion on the squeezing velocities. The squeezing parameter=VH/, represents the effect of the inertial forces on heat and fluid flow characteristics. It is found that increasing the extrusion parameter will increase both the velocity and the heat transfer rates to the viscous fluid. Increasing the squeezing parameter had also decreased the fluid velocity and enhanced heat transfer rates. Increasing the viscous effects or the Eckert number E=U²/c_p (T_E – T_s) heated the fluid and consequently decreased the heat transfer rates. Different velocity profiles, temperature profiles, and Nusselt numbers against various dimensionless groups are drawn.     

No slip boundary effects in non-Darcian mixed convection from a vertical wall in saturated porous media. Analytical solution

An analytical study is made for wall effects in non-Darcy mixed convection from vertical impermeable surfaces embedded in a saturated porous medium. The governing equations are transformed into a dimensionless form by non-similar transformation to cover both forced and natural convection dominated regimes. Two different dimensionless parameters that measure the strength of mixed convection were found in both regimes. The parameters of forced convection dominated regime can be related to those of natural convection dominated regime. An approximate analytical solution for the governing equations was obtained. Temperature and velocity profiles for both regimes are presented. Received on 9 September 1997     

Compositional dependence of the electrical conductivity of calcium vanadate glassy semiconductors

Scanning electron microscopy (SEM), X- ray diffraction (XRD), density (d), oxygen molar volume (V_m) and dc conductivity of different compositions of calcium vanadate glasses are reported. SEM exhibits a surface without any presence of a microstructure which is a characteristic of the amorphous phase. The overall features of these XRD curves confirm the amorphous nature of the present glasses. Density was observed to decrease with an increase in V₂O₅ content. The experimental results were analyzed with reference to theoretical models existing in the literature. It has been observed that the high-temperature conductivity data are consistent with Mott's nearest-neighbor hopping model. However, both Mott variable-range hopping (VRH) and Greaves intermediate range hopping models are found to be applicable. The hopping at high temperatures in the calcium vanadate glasses occurs by non-adiabatic process in contrast to the vanadate glasses formed with conventional network formers. The hopping model of Schnakenberg can predict the temperature dependence of the conductivity data. The percolation model of Triberis and Friedman applied to the small polaron hopping (SPH) regime is also consistent with data. The various model parameters such as density of states, hopping energy, etc., obtained from the best fits were found to be consistent with the glass compositions.     

Magnetic field effect on heat transfer and fluid flow characteristics of blood flow in multi-stenosis arteries

Heat and fluid flow characteristics of blood flow in multi-stenosis arteries in the presence of magnetic field is considered. A mathematical model of the multi-stenosis inside the arteries is introduced. A finite difference scheme is used to solve the governing equations in terms of vorticity-stream function along with their boundary conditions. The effect of magnetic field and the degree of stenosis on wall shear stress and Nusselt number is investigated. It was found that magnetic field modifies the flow patterns and increases the heat transfer rate. The severity of the stenosis affects the wall shear stress characteristics significantly. The magnetic field torque will increase the thermal boundary layer thickness and the temperature gradient in the streaming blood, and hence increasing the local Nusselt number     

Heat-and-mass transfer analysis from vegetable and fruit products stored in cold conditions

Heat and mass transfer process taking place during fruit and vegetable products in cold storage are studied. A mathematical model describing these processes is presented and the resulting governing equations are solved for different storing conditions. The relative humidity of the ventilating air and the temperature of the stored product bulk are found for different initial air relative humidity and airflow rates. As the product bulk depth increased up to 4.2 m, the relative humidity of the ventilating air approaches the steady state value. When the relative humidity is larger than the equilibrium relative humidity value, an increase in the ventilating air rate reduces the losses of the product during the period of its storage, while larger losses occur when the relative humidity values are lower than the equilibrium ones.