Saad A. El-Sayed

Investigation of turbulent heat transfer and fluid flow in longitudinal rectangular-fin arrays of different geometries and shrouded fin array

Abstract The effects of the fin arrays geometries and also the fin tip-to-shroud clearance on the heat transfer, the fluid flow and the pressure drop characteristics of longitudinal rectangular-fin arrays have been investigated. During the experiments, different geometrical parameters were varied such as the fin height (H), the fin thickness (t), the inter-fin space (W), the fins number and the fin tip-to-shroud clearance (C) was varied parametrically; starting with the no-clearance case. Two shroud types were used (one plain and the other is equipped with wires coil as modified shroud). The heat transfer coefficient corresponding to the presence and absence of clearance were compared under the condition of equal airflow rate. Air is the working fluid, the flow regime is turbulent, and the flow pattern around the tested models was visualized. It was found that the axial pressure drop along the tested model is increased as the flow travels in the inter-fin region deeply in the stream-wise (X) direction, with increasing the fin height, the Reynolds number, and with decreasing the inter-fin space and the fin thickness. The tested model-mean Nusselt number (Num) increases with increasing the Reynolds number, the inter-fin space, and the fin thickness and with decreasing the fin height. Increasing the clearance between the fin tip-to-shroud decreases the tested model-mean Nusselt number and at certain clearance to fin height ratio (C/H)=1.25, the effect of the fin tips plain shroud goes off. It was also found that the tested model with the modified shroud (equipped with wire coil) gives higher mean Nusselt number than that with the plain shroud, and at certain clearance to fin height ratio (C/H)=1, the effect of the wire coil goes off. Eleven empirical equations are derived to correlate the mean Nusselt number as a function of the Reynolds number and other experimental variable parameters individual, fin thickness, fin height, inter-fin space and shroud clearance to fin height ratio. Finally the present work general empirical formula is given in the form Nu m =5.734146(Re L ) 0.42422 W L 0.214171 H W −0.36263 t W 0.15250885 where 24,649⩽ReL⩽189,462, 3 mm ⩽t⩽9 mm, 23 mm ⩽H⩽92 mm, 10 mm ⩽W⩽42 mm and L=150 mm.

Experimental study of turbulent flow inside a circular tube with longitudinal interrupted fins in the streamwise direction

Abstract Experiments were performed to determine the detailed module-by-module pressure drop characteristics of turbulent flow inside circular finned tubes. The tubes were equipped with longitudinal fins interrupted in the streamwise direction by their arrangement in both a staggered and an inline manner. Experiments were carried out for two different fin geometries with hydraulic diameters D h = 14.89 and 13.46 mm, two different numbers of fins ( N = 6 and 12), and two relative fin heights ( H/r 0 = 0.5 and 0.3). Air is the working fluid, with Prandtl number 0.7 and Reynolds number ranging from 5000 to 50,000, based on the hydraulic diameter and velocity via minimum cross-sectional area of the tested finned tubes. The velocity profiles for all the tested finned tubes in the direction at the symmetry boundary in the periodic fully developed region were presented. The module-by-module friction factor was found to vary only in the initial modules and then to attain a constant periodic fully developed value after four to six modules. The results showed that, in the periodic fully developed region, the tube pressure drop with continuous fins is higher than that of the inline arrangement of fins and lower than that of the staggered arrangement of fins.

Smoldering combustion of dust layer on hot surface

Abstract The critical temperature as well as the critical flux for ignition of a dust layer of cornflour and a mixture of wheatflour and cornflour (80% wheatflour+20% cornflour) on a hot plate have been determined. The moulded sample was cylindrical in shape and of different heights and diameters. The particle size of dusts ranged between 63 μm to 150 μm. The temperature–time histories for self-heating without ignition and with ignition are offered, showing the critical boundaries between them. Also the times to ignition for each dust, showing the effect of sample size on their values, are determined. Certain experimental correlations which relate to times to ignition, as well as the critical temperature for ignition and thermal and geometrical dimensions of sample are presented.

Critical conditions in uniform temperature systems with variable heat transfer coefficient in thermal explosion theory

Abstract It is common in thermal explosion theory to neglect the temperature dependence of thermophysical properties except in the reaction rate. In practice, there are certain circumstances in which this is not suitable. This paper examines how the criterion for criticality for a system with uniform temperature (Semenov case) is affected when the heat transfer coefficient h is not constant, but depends on temperature. The lower boundary at which criticality disappears (transition) is determined.

Effect of heating rate on the chemical kinetics of different biomass pyrolysis materials

The present work is attempting to focus on thermal degradation properties and the kinetics of pyrolysis for biomass samples like wheat straw, wheat dust, and corn cob using thermogravimetric analysis in a nitrogen atmosphere at heating rates of 10, 15, and 20 K/min. It appears that as the heating rate is increased, the thermal degradation process is delayed and the main step of mass loss takes place between 250 °C and 400 °C, with about 75% loss of the initial mass of the sample. The ignition and burnout temperature values of biomass samples are determined at different heating rates. It was found that corn cob has the highest values. Proximate and ultimate analysis of the three biomass fuels was investigated as corn cob has the highest heating and volatile matter values (3971.86 Cal/gm and 84%, respectively). Kinetics parameters such as activation energy, frequency factor, and reaction order are determined using integral and sequential methods.

Ignition characteristics of metal particles in thermal explosion theory

The thermal theory of explosion has been applied to metal particle ignition. Critical conditions in the θ - η plane for ignition are investigated. The analysis covers two kinetic laws of interaction between metals and gases, and uses the exact form of the Arrhenius expression for the rate of reaction law, as well as the Frank-Kamenetskii approximation. The relationship between the critical temperature and the gas temperature has been obtained from the definition d 2 θ /d η 2 = 0 and d 3 θ /d η 3 =0. On the other hand, the relationship between the critical parameters can be determined from the definition d ψ m /d θ m =0 or d ψ m /d η m = 0.

Critical and transition conditions of gaseous explosion

Abstract The critical and transition conditions of a gaseous explosion at constant pressure and varying pressure is investigated. The study covered the solutions of the problem in pressure-time plane, temperature-time plane, as well as pressure-temperature plane. The explosion characteristics of combustible gases using the definition of criticality as an inflection point in critical trajectory. as well as other definitions of criticality are presented. The analytical study showed that the critical parameters for constant pressure explosion are lower than those for constant volumes where θ ∗ tr is lowered from 0.5 to 0.29 and θ atr is also lowered from 0.25 to 0.17. The sensitivity of the critical pressure to the ambient temperature and the boundary between ignition and non-ignition regions are also offered. It was also found that the critical parameters in the temperature–pressure plane were different from those obtained in the temperature-time plane or the pressure–time plane.

Thermal explosion of autocatalytic reaction

Abstract Analytical and numerical solutions are used to determine the critical conditions for thermal explosion of autocatalytic reaction. The solutions covers both the reaction governed by the Arrhenius kinetics equation and the Frank-Kamenetskii approximation for that equation. The definition of criticality as the point at which d 2 θ / dβ 2 =0, d 3 θ / dβ 3 =0 and dθ / dβ 〉0 is used here. The study is dealt with low and high exothermicity ( B ) of the reaction and their effects on the critical parameters. The numerical solutions cover the whole reaction from start at β = 0 up to the end at β = 1.0. All trajectories from subcritical, critical to supercritical are offered. The effects of different parameters such as B , ψ and θ a (ambient temperature) on the critical conditions are presented. The results showed that the lower the autocatalytic factor ( β 0 ) is, the pronounced autocatalytic reaction explosion. The analytical solution offered analytical expressions for the critical condition and the different limits of the solutions are clarified. It was found that the numerical results confirm the analytical solution.

Organic Dust Ignition in the High Temperature Flow Behind a Shock Wave

A theoretical model was developed to determine the ignition characteristics of organic dust. In this model, an unsteady particle heat transfer equation with an Arrhenius type of reactive source term was coupled with the flow conservation equations. Theoretical results show the influence of dust concentration on the ignition delay time of wheat flour dust. The flammability limits of wheat flour dust have also been estimated. Results also show the effect of particle size on the ignition delay time. The theoretical results were in reasonable agreement with most of the experimental data.

Experimental study of organic dust ignition behind shock waves

Abstract In the present work, an experimental study was conducted to determine the ignition characteristics of wheat flour and provender (mainly cotton seed residual) dusts. Experiments were conducted in a horizontal shock tube, with a new technique for distributing the dusts. Samples with different shapes and the same volumes are suspended in the centre of the shock tube, and the shock wave is responsible for disperson of the dusts. The ignition delay time of the aforementioned dusts, based on the pressure rise criterion, has been measured. Experimental results show the influence of dust concentration on the ignition pressure difference, and the ignition delay time. In addition, the effect of the sample shapes on ignition delay time is shown. Kinetic constants of grain dusts are also estimated by using the experimental results. The flammability limits and concentration at optimum conditions for these dusts are determined.