The ␣-chemokine stromal cell-derived factor (SDF)-1␣ binds to the seven transmembrane G-protein-coupled CXCR-4 receptor and acts to modulate cell migration and proliferation. The signaling pathways that mediate the effects of SDF-1␣ are not well characterized. We studied events following SDF-1␣ binding to CXCR-4 in a model murine pre-B cell line transfected with human CXCR-4. There was enhanced tyrosine phosphorylation and association of components of focal adhesion complexes such as the related adhesion focal tyrosine kinase, paxillin, and Crk. We also observed activation of phosphatidylinositol 3-kinase. Wortmannin, a selective inhibitor of phosphatidylinositol 3-kinase, partially inhibited the SDF-1␣-induced migration and tyrosine phosphorylation of paxillin. SDF-1␣ treatment selectively activated p44/42 mitogen-activated protein kinase (Erk 1 and Erk 2) and its upstream kinase mitogenactivated protein kinase kinase but not p38 mitogenactivated protein kinase, c-Jun amino-terminal kinase or mitogen activated protein kinase kinase. We also observed that SDF-1␣ treatment increased NF-B activity in nuclear extracts from the CXCR-4 transfectants. Taken together, these studies revealed that SDF-1␣ activates distinct signaling pathways that may mediate cell growth, migration, and transcriptional activation.

An experimental study of convective condensation of R134a in an 8.38 mm inner diameter smooth tube in inclined orientations is presented. This article, being the first of a two-part paper (the second part concentrates on the pressure drops and void fractions), presents flow patterns and heat transfer coefficients during condensation for different mass fluxes and vapour qualities for the whole range of inclination angles (from vertical downwards to vertical upwards). The results were compared with three flow pattern maps available in literature. It was found that for low mass fluxes and/or low vapour qualities, the flow pattern is strongly dependent on the inclination angle whereas it remains annular for high mass fluxes and high vapour qualities, whatever the tube orientation. The models of flow pattern maps available in the literature did not predict the experimental data well. In the inclination-dependent zone, experiments showed that there is an optimum inclination angle that leads to the highest heat transfer coefficient for downward flow. The heat transfer coefficient is strongly affected by the liquid and vapour distributions and especially by the liquid thickness at the bottom of the tube for stratified flows. Thus developing a mechanistic model of flow pattern maps is the first step in achieving a predictive tool for the heat transfer coefficient in convective condensation in inclined tubes.

The first law and second law efficiencies are determined for a stainless steel closed-tube open rectangular cavity solar receiver. It is to be used in a small-scale solar thermal Brayton cycle using a micro-turbine with low compressor pressure ratios. There are many different variables at play to model the air temperature increase of the air running through such a receiver. These variables include concentrator shape, concentrator diameter, concentrator rim angle, concentrator reflectivity, concentrator optical error, solar tracking error, receiver aperture area, receiver material, effect of wind, receiver tube diameter, inlet temperature and mass flow rate through the receiver. All these variables are considered in this paper. The Brayton cycle requires very high receiver surface temperatures in order to be successful.These high temperatures, however, have many disadvantages in terms of heat loss from the receiver, especially radiation heat loss. With the help of ray-tracing software, SolTrace, and receiver modelling techniques, an optimum receiver-to-concentrator-area ratio of A' ≈ 0.0035 was found for a concentrator with 45° rim angle, 10 mrad optical error and 1° tracking error. A method to determine the temperature profile and net heat transfer rate along the length of the receiver tube is presented. Receiver efficiencies are shown in terms of mass flow rate, receiver tube diameter, pressure drop, maximum receiver surface temperature and inlet temperature of the working fluid. For a 4.8 m diameter parabolic dish, the larger the receiver tube diameter and the smaller the mass flow rate through the receiver, the higher the receiver surface temperature and the less efficient the collector becomes. However, the smaller the receiver tube diameter, the higher the pressure drop through the tube and the smaller the second law efficiency. It was found that the 2 receiver with larger tube diameter would perform better in a solar thermal Brayton cycle. An overall solar-to-heat efficiency of between 45% and 70% is attainable for the solar collector using the open-cavity receiver.Keywords: solar, receiver, cavity, tracking, Brayton, efficiency 1.Introduction and background The solar thermal Brayton cycleThe closed Brayton cycle was developed in the 1930s for power applications [ The open Brayton cycle uses air as working fluid, which makes this cycle very attractive for use in water-scarcecountries. The open and direct solar thermal Brayton cycle is shown in Fig. 1 [2]. The parabolic dish concentrator is used to reflect and concentrate the sun's rays onto the receiver aperture so that the solar heat can be absorbed by the inner walls of the receiver. The heat is then transferred to the working fluid (air). The compressor increases the air pressure before the air is heated in the receiver. The compressed and heated air expands in the turbine, which produces rotational power for the compressor and the electric load.In the recuperator, hot exhaust air preheats the colder air before it enters the receiver. For the so...

In this paper, the convective heat transfer enhancement of aqueous suspensions of multiwalled carbon nanotubes flowing through a straight horizontal tube was investigated experimentally for a Reynolds number range of 1 000 -8 000, which included the transitional flow regime. The tube was made out of copper with an internal diameter of 5.16 mm.Experiments were conducted at a constant heat flux of 13 kW/m 2 with 0.33%, 0.75% and 1.0% volume concentrations of multi-walled carbon nanotubes. The nanotubes had an outside diameter of 10 -20 nm, an inside diameter of 3 -5 nm and a length of 10 -30 µm.Temperature and pressure drop measurements were taken, from which the heat transfer coefficients and friction factors were determined as a function of Reynolds number. It was found that heat transfer was enhanced when comparing the data on a Reynolds-Nusselt graph but when comparing the data at the same velocity, it was shown that heat transfer was not enhanced. Performance evaluation of the nanofluids showed that the increase in viscosity was four times the increase in the thermal conductivity, which resulted in an inefficient nanofluid.

The wind characteristics of seven locations in Jubail, Saudi Arabia were analysed by using five years of wind data of six sites and three years data of one site at 10 m above ground level (AGL). The highest annual mean wind speed of 4.52 m/s was observed at

A wind-pv-diesel hybrid power system has been designed fro a village in Saudi Arabia which is presently powered by a diesel power plant consisting of eight diesel generating sets of 1,120kW each. The study found a wind-pv-diesel hybrid power system with 35% renewable energy penetration (26% wind and 9% solar PV) to be the feasible system with cost of energy of 0.212US$/kWh. The proposed system was comprised of 3 wind turbines each of 600kW, 1000kW of PV panels, and four diesel generating sets each of 1120kW rated power. The system was able to meet the energy requirements (AC primary load of 17,043.4MWh/y) of the village with 4.1% energy in excess. The annual contributions of wind, solar pv and the diesel generating sets were 4,713.7, 1,653.5, and 11,542.6MWh, respectively. The proposed hybrid power system resulted in avoiding addition of 4,976.8 tons of GHG equivalent of CO 2 gas in to the local atmosphere of the village and conservation of 10,824 barrels of fossil fuel annually.

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SUMMARYThe Brayton cycle's heat source does not need to be from combustion but can be extracted from solar energy. When a black cavity receiver is mounted at the focus of a parabolic dish concentrator, the reflected light is absorbed and converted into a heat source. The second law of thermodynamics and entropy generation minimisation are applied to optimise the geometries of the recuperator and receiver. The irreversibilities in the recuperative solar thermal Brayton cycle are mainly due to heat transfer across a finite temperature difference and fluid friction. In a small-scale open and direct solar thermal Brayton cycle with a micro-turbine operating at its highest compressor efficiency, the geometries of a cavity receiver and counterflow-plated recuperator can be optimised in such a way that the system produces maximum net power output. A modified cavity receiver is used in the analysis, and parabolic dish concentrator diameters of six to 18 metres are considered. Two cavity construction methods are compared. Results show that the maximum thermal efficiency of the system is a function of the solar concentrator diameter and choice of micro-turbine. The optimum receiver tube diameter is relatively large when compared with the receiver size. The optimum recuperator channel aspect ratio for the highest maximum net power output of a micro-turbine is a linear function of the system mass flow rate for a constant recuperator height. For a system operating at a relatively small mass flow rate, with a specific concentrator size, the optimum recuperator length is small. For the systems with the highest maximum net power output, the irreversibilities are spread throughout the system in such a way that the internal irreversibility rate is almost three times the external irreversibility rate.