SOIL SOLAR-WARMING WITH DIFFERENT TYPES OF MULCH

temperature was studied during two successive seasons of 2002 and 2003 at Kaha experimental station, Agricultural Research Center, Cairo, Egypt (Latitude, longitude, and altitude are 30.8, 31.15, and 16.9 m respectively). Cucumber seedlings (Cucumis sativus L. F1 local hybrid Sinai 1) were grown in clay soil under unheated two plastic houses. One of them was covered with one year old cladding material (PE 200μm) while the other was covered with new cladding. Soil surface was covered with different plastic mulch colors, i.e., black, silver, and transparent, in addition to a new suggested mulch that was craft-paper saturated with paraffin wax. Soil temperatures were measured at 5 cm depth. Global solar radiation and air temperature were measured inside and outside the greenhouses. The vegetative growth , i.e., plant height, leaf number, leaf fresh and dry weight, early and total yield were recorded. The results showed that new cladding cover promoted plant growth and yield. Craft-paper saturated with paraffin wax gave the highest soil temperature during the sunny days in comparison with other mulches and best results concerning vegetative growth and yield were obtained by both black mulch and craft-paper saturated with paraffi

Melting of PCM in a thermal energy storage unit: Numerical investigation and effect of nanoparticle enhancement

The present paper describes the analysis of the melting process in a single vertical shell-and-tube latent heat thermal energy storage (LHTES), unit and it is directed at understanding the thermal performance of the system. The study is realized using a computational fluid-dynamic (CFD) model that takes into account of the phase-change phenomenon by means of the enthalpy method. Fluid flow is fully resolved in the liquid phase-change material (PCM) in order to elucidate the role of natural convection. The unsteady evolution of the melting front and the velocity and temperature fields is detailed. Temperature profiles are analyzed and compared with experimental data available in the literature. Other relevant quantities are also monitored, including energy stored and heat flux exchanged between PCM and HTF. The results demonstrate that natural convection within PCM and inlet HTF temperature significantly affects the phase-change process. Thermal enhancement through the dispersion of highly conductive nanoparticles in the base PCM is considered in the second part of the paper. Thermal behavior of the LHTES unit charged with nano-enhanced PCM is numerically analyzed and compared with the original system configuration. Due to increase of thermal conductivity, augmented thermal performance is observed: melting time is reduced of 15% when nano-enhanced PCM with particle volume fraction of 4% is adopted. Similar improvements of the heat transfer rate are also detecte

A numerical investigation of heat transfer in phase change materials (PCMs) embedded in porous metals

The effects of metal foams on heat transfer enhancement in Phase Change Materials (PCMs) are investigated. The numerical investigation is based on the two-equation non-equilibrium heat transfer model, in which the coupled heat conduction and natural convection are considered at phase transition and liquid zones. The numerical results are validated by experimental data. The main findings of the investigation are that heat conduction rate is increased significantly by using metal foams, due to their high thermal conductivities, and that natural convection is suppressed owing to the large flow resistance in metal foams. In spite of this suppression caused by metal foams, the overall heat transfer performance is improved when metal foams are embedded into PCM; this implies that the enhancement of heat conduction offsets or exceeds the natural convection loss. The results indicate that for different metal foam samples, heat transfer rate can be further increased by using metal foams with smaller porosities and bigger pore densities