Solar air heating (SAH) is a low-cost method for air heating using solar energy. An energy and exergy performance is performed experimentally on new designed tubular SAH having tubular absorber of adjacent tubes forming flat pack. Each tube of the absorber contains three adjacent internal tubes forming nabla shape (∇). The nabla tubular SAH (∇TSAH) performance is studied compared to ordinary tubular SAH (OTSAH) having tubular absorber only for single pass (SP) and double pass (DP) flow conditions and different mass flow rates of air (MFRA). The study is investigated under upper Egypt hot climate conditions and 0.018–0.081 kg/s MFRA. The results demonstrate that ∇TSAH has higher outlet air temperature, energy gain, higher energy, exergy, and thermohydraulic efficiencies, and lower top losses than the OTSAH. The new design boosts the exit air temperature by about 13.5 and 5 °C for SP and DP …

In this article, the spectrum trading problem between primary users and secondary networks is
investigated. The secondary network requests multiple channels with the targeted availability to satisfy its
users' demands. Due to the uncertainty about the channels availability, stochastic optimization techniques
are adopted to nd the optimal set of channels for each secondary network for the lowest cost. Two different
constraints on the secondary demand are dened. The rst one is when the throughput has to be fully satised
for a certain percentage of time, and the second one is when the expected value of the throughput has
to exceed a certain percentage of the requested one. Also, the possibility for channel subleasing among
the secondary networks is investigated to reduce the demand shortage. The results show that demanding
simultaneous channels increases the cost as it reaches up to 20% higher than if the same resources were
requested individually. Also, channels subleasing reduces the demand shortage probability and increases
the achieved throughput, especially at low value of requested demand. In this case, the demand satisfaction
probability increases by around 30% while the achieved throughput increases up to 40% compared to the
scenario where channels subleasing is not allowed.

Since more than one-third of dam failures have been attributed to uncontrolled seepage, it is of great importance to investigate the behaviour of this phenomenon in order to achieve the maximum degree of safety for such dams. The present work investigated the influence of the permeability coefficient of the different materials used in zoned earth dams on different seepage parameters. For the modelling and analysis processes, the Seep/w and Seep2D software were employed. The numerical results prove that the optimum relative hydraulic conductivity between the inner and transition shells is about 0.001, and it is better to use filling materials with less hydraulic conductivity in the upstream transition and outer shells than in the downstream ones. A good agreement was noted between the obtained results from Seep/w and those from Seep2D. Reducing the hydraulic conductivity of both the upstream and the downstream shells, or of the downstream shells only, causes the pore water pressure in the dam body to increase significantly, and causes a remarkable reduction in the seeped water quantity and velocity. A moderate reduction in the different seepage parameters is achieved by reducing the hydraulic conductivity of the upstream transition shell, and a small reduction is noticed by reducing the hydraulic conductivity of the upstream outer shell.