nitriding is a common method for improving the hardness , mechanical properties, wear and corrosion resistance of metals. Laser nitriding of metals is an efficient process, where the irradiation of surfaces in nitrogen atmosphere leads to the fast take-up of nitrogen in the irradiated surfaces. In this paper laser surface nitriding of carbon steel carried out by melting the surface of substrate using high power CW CO2 laser with nitrogen as shrouding environment. Laser tracks were arranged as single tracks with the use of various laser powers ranging from 0.8kW to 2.0 KW , and laser scanning rates from 120mm/min to 200 mm/min. The influence of laser beam power and laser scanning rates on the microstructure and hardness profiles analysed. The laser nitride layer studied using optical microscope, and scanning electron microscope. The laser surface nitriding of carbon steel led to refinement in the microstructure and the diffusion of nitrogen caused the formation of iron nitrides on the surface. The micro hardness of the nitride surface improved to a maximum of 400HV in the present set of laser processing conditions as compared to 150HV of as received substrate as a result of the appearance of marten site in the melted pool and heat effect zon

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Abstract

Although listening skill plays a significant role in daily communication and

education process in teaching English , listening subject has long been neglected

in second language teaching. This study aims to explore problems, difficulties

and barriers in teaching listening to Libyan students in our case study Alassaba

area, 3rd year secondary science school, Mosab Ben Omer Secondary School

and Omer Ben Alkhtab Secondary school. The participants of our study were 5

teachers. Classroom observation and teacher’s interview are used for data

collection. It is found that all teachers were using grammar translation method

in teaching listening. Most classes were uncomfortable learning atmosphere,

beside the absence of technology which used and facilitate teaching listening.

The findings based on data analysis and discussion may be of a great

significance to find solutions for these problems and improving the way of

teaching listening

Prosopis farcta (Banks & Sol.) J. F. Macbr. has been reported for the first time from Libya. This species is native in Asia. Flowering specimens of Prosopis farcta collected from Sokna Region about 650 Km south-east Tripoli. It is presented with updated nomenclature, taxonomic description, geographical distribution, place of occurrence in Libya, and field colored photographs are also provided. The voucher specimens were deposited in the Herbarium of Botany Department (ULT), University of , Tripoli, Libya.

Experimental and numerical investigations have been performed to study the natural convection heat transfer from a vertical rectangular fin arrays at different orientation angles.An experimental setup was constructed and calibrated to test different fin configurations. It basically consists of base plate, an array of parallel longitudinal fins, heating unit and layers of thermal insulation. Fin length (L) and fin thickness (t) were kept fixed at 187 and 6.5 mm respectively, while fin spacing (S) was varied from 3 to 16 mm and fin height (H) was varied from 15 to 45 mm. The orientation angle (β) was changed from 0° to 60°, and temperature difference between fin and surrounding (∆T) from 30 to 95 o C.Base-to-ambient temperature difference was also varied through a calibrated wattmeter ranging from 10 to 180W. To understand the general flow patterns dominating flows from the heat sink, the three-dimensionless elliptic governing equations were solved using finite volume computational fluid dynamics (CFD) code. A comparative study between the experimental and numerical results was performed to verify the numerical code. It was found for the configuration tested that the heat transfer rate per unit base area increases with the increase in the fin spacing and reaches a maximum value then decreases with farther increase in the fin spacing. The maximum heat dissipation occurs at optimal spacing S opt =7 mm. Empirical correlations between Nussult number, Rayleigh number, fin spacing, fin height, orientation angle, temperature difference between the fin and surroundings were derived. Finally the present work general empirical formula is given in the form =. .. .. Where , 15 mm ≤ H ≤ 45 mm, 3mm ≤ S ≤ 16 mm, °0 ≤ β ≤°60, t = 6.5 mm, L = 187 mm.

ABSTRACT :

Transport modes are very important to Libyan’s Gharian travellers for their

activities and daily trips. However, the increase of total number of private car and

private transport namely (taxi and micro buses) on the road increases and causes

many road problems such as traffic congestion, accidents and noise pollution. The

aim of this research is to study the two important factors to shifting to public

transportation. Analyses about relation between factors as travel time and cost

have been made to answer research questions. A questionnaire survey in Gharian

city was carried out to study the potential reduction of private cars on road through

optimal use of suitable public transportation system (PT) in order to reduce traffic

congestion problems such as travel time. Logistic regression model has been used

to analyse the factors that influence users to shift their trips to public transportation

alternatives.

Key words: travel time, travel cost, private car, public transport (PT), traffic

congestion and Logistic regression.