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Highlights • Flow regime nomenclature. • Developing and fully developed transitional flow experimental data. • The transitional flow regime of developing flow decreased with axial position. • Free convection effects decreased the width of the transitional flow regime. • Correlations to predict the start and end of the transitional flow regime.

The harmful effects of traditional methods of power generation on the environment has created a need to strategically plan and develop renewable and sustainable energy generation systems. This paper presents the wind farm site suitability analysis using multi-criteria decision making (MCDM) approach based on geographic information system (GIS) modeling. This analysis is based on different climatic, economic, aesthetic and environmental criteria like wind resource, accessibility by roads, proximity to the electrical grid, and optimum/safe distance from various settlements and airports. Using information from published literature, criteria constraints like buffer zones, exclusion zones and suitability scores for each criteria is developed. An analytical hierarchy process, AHP is employed to assign appropriate weights to the criteria according to their relative importance. The developed model is then applied to the entire Kingdom of Saudi Arabia. The most suitable sites are found to be (i) near Ras Tanura on the coast in the Eastern Province; (ii) Turaif in Al-Jawf region at northern borders and (iii) Al-Wajh on the coast in the western region. The central and southeastern region is found to be unsuitable mainly due to scarce wind resource, few settlements and less connectivity by roads and electrical grid.

The enhanced thermal characteristics of nanofluids have made it one of the fastest-growing research areas in the last decade. Numerous researches have shown the merits of nanofluids in heat transfer equipment. However, one of the problems is the increase in viscosity due to the suspension of nanoparticles. This viscosity increase is not desirable in the industry, especially when it involves flow, such as in heat exchanger or micro-channel applications INTRODUCTIONColloidal suspension dates back to Maxwell's study in 1873 [1]. Though the idea behind his study was vivid, the imposed problems were too enormous for profitable engineering solutions [2]. In 1995, Choi [3] came up with a pioneering idea based on Maxwell's study and suspended ultrafine particles (nanoparticles) in conventional heat transfer fluids. His invention has opened up a myriad of opportunities in research and development. Nanofluids descriptively are colloidal suspensions containing metallic (Ag, Au, Al, Cu, Ni, etc.), non-metallic (single-and multi-wall carbon nanotube (SWCNT and MWCNT), Si, Graphene, etc.), metallic oxide (Al2O3, CuO, NiO2,TiO2, etc.) and oxides of non-metals (SiO2, SiC, MgO, CaCO3, etc.) nanoparticles suspended in conventional heat transfer fluids such as water, engine oil, ethylene glycol, transformer oil, gear oil or mixture of two or more heat transfer fluids [2,3]. When compared with previous microparticle suspensions in conventional heat transfer fluids, it is a special type of fluid with numerous applications potentials because of its enhanced thermal conductivity, stability and homogeneity [3,4]. Microscale particles in suspensions lead to abrasion, clogging of flow paths, pressure drop and high pumping power requirements, therefore, its sustainability was impossible. Besides, nanofluids can reduce the pumping power in engineering equipment significantly and do not pose the problem of clogging and abrasion of equipment flow paths [5][6][7][8]. Therefore, the design and engineering of physical systems are now being tailored towards using nanofluid as working fluid.The impact of colloidal suspension cuts across the fields of science, biological science, medical, pharmaceutical and engineering. In the context of sustainable energy development and thermal management, nanofluids are becoming more and more significant as the need for efficient thermal management is of paramount importance. Moreover, the level of miniaturisation of devices today as technology advances is overwhelming. Devices such as microprocessors, microelectromechanical systems (MEMS), nanoelectromechanical systems (NEMS), microchannels and lab-on-chips come with high-density heat flux that needs quick heat removal for 3 efficient performance, stability and durability, which, from all indications, could be provided by nanofluids for now [9][10][11][12]. The following are also emerging areas of applications of nanoparticles and nanofluids: (i) they could be useful in medicine in the targeted treatment of malignant cells without damaging healthy tis...

Highlights• Three laminar regions: Forcedand Mixed Convection Developing and Fully Developed. • Thermal entrance length correlations for forced and mixed convection laminar flow. • Local and average laminar Nusselt number correlations for mixed convection flow. • Influence of free convection on the laminar-turbulent transition. ABSTRACT Correlations to calculate the local and average heat transfer coefficients for single-phase laminar flow through horizontal circular tubes with a constant heat flux boundary condition are usually restricted to fully developed flow, high Prandtl numbers or constant fluid properties. What further complicates the heat transfer characteristics of developing flow, is the local transition from laminar to turbulent flow along the tube length, above the critical Reynolds number. The purpose of this study was to investigate the effects of free convection on the development of the local heat transfer characteristics in smooth horizontal circular tubes heated with a constant heat flux. An experimental set-up was designed, built, and results were validated against literature. Two smooth circular test sections with innerdiameters of 4 mm and 11.5 mm were used, and the maximum length-to-diameter ratios were 1 373 and 872, respectively. Heat transfer measurements were taken at Reynolds numbers between 500 and 10 000 at different heat fluxes. A total of 1 046 mass flow rate measurements and 89 459 temperature measurements were taken. Water was used as the test fluid and the Prandtl number ranged between 3 and 7. Three different regions were identified for developing laminar flow and were qualitatively and quantitatively defined. Correlations were developed to determine the thermal entrance lengths, as well as local and average Nusselt numbers for developing and fully developed laminar flow in mixed convection conditions with a constant heat flux boundary condition. In the transitional flow regime, the laminar-turbulent transition along the tube length was divided into four regions, and it was