Giovanni Cortella

Natural convection from a horizontal cylinder in a rectangular cavity

Abstract A numerical and experimental analysis is performed for natural convection heat transfer from a horizontal cylinder enclosed in a rectangular cavity. The temperature distribution in the air and the heat transfer coefficients are measured by a holographic interferometer and compared with numerical predictions obtained by a finite-element procedure based on the streamfunction-vorticity formulation of the momentum equations. The influence of the Rayleigh number and the geometry of the cavity on the heat transfer are investigated.

CFD simulation of refrigerated display cabinets

Abstract The finite element method is employed for the analysis of velocity and temperature distributions in refrigerated open display cabinets. The CFD code is based on the streamfunction-vorticity formulation, and incorporates a LES turbulence model. As an example of application, a vertical multi-deck cabinet is investigated under different operating conditions. The numerical results have been validated by comparison with experimental tests performed in accordance with the EN441 Standard. The influence of various design parameters has been investigated.

CFD-aided retail cabinets design

Abstract Food requires strict temperature control throughout the so-called ‘cold-chain’, in which a particular storage temperature is needed for each link. Display cabinets are known to be the weakest link from this point of view and, therefore, particular attention is paid to their design. In this paper, the usefulness of Computational Fluid Dynamics ( cfd ) is illustrated, as a tool for the prediction of the air flow pattern and of food temperature inside the cabinet. The numerical method is described: the airflow pattern and the load temperature are solved in sequence, in order to save computation time. The results of simulations performed both on a vertical and a horizontal cabinet are reported, as an example of the capabilities of the method. The influence of warm air infiltration on the energy balance of the cabinet is investigated and the results are validated by comparison with experimental tests. The applications reported in this paper showed the model to be reliable, and of valuable help to the designer.

Beneficial effects of silicon on hydroponically grown corn salad (Valerianella locusta (L.) Laterr) plants.

Soil-less cultivation of horticultural crops represents a fairly recent innovation to traditional agriculture which has several advantages including higher water-use efficiency. When plants are grown with this system, their roots come in contact with nutrients solely via the hydroponic solution. Although its beneficial effects have been widely demonstrated, silicon (Si) is mostly omitted from the composition of nutrient solutions. Therefore, the objective of this study was to assess the beneficial effect of Si addition to hydroponic solution on quali-quantitative aspects of edible production of two cultivars of corn salad (Valerianella locusta (L.) Laterr.) grown in soil-less floating system. Impacts on shelf life of this food were also studied. Results show that the supply of Si increased the edible yield and the quality level reducing the nitrate concentration in edible tissues. This result might be attributed to changes either in the metabolism (such as the nitrate assimilation process) or to the functionality of root mechanisms involved in the nutrient acquisition from the outer medium. In fact, our results show for the first time the ability of Si to modulate the root activity of nitrate and Fe uptake through, at least in part, a regulation of gene expression levels of the proteins involved in this phenomenon. In addition, the presence of Si decreased the levels of polyphenoloxidase gene expression at harvest and, in post-harvest, slowed down the chlorophyll degradation delaying leaf senescence and thus prolonging the shelf life of these edible tissues. In conclusion, data showed that the addition of Si to the nutrient solution can be a useful tool for improving quali-quantitatively the yield of baby leaf vegetable corn salad as well as its shelf life. Since the amelioration due to the Si has been achieved only with one cultivar, the recommendation of its inclusion in the nutrient solution does not exclude the identification of cultivars suitable for this cultivation system and the comprehension of agronomical and environmental factors which could limit the Si benefits.

Influence of hydroponic and soil cultivation on quality and shelf life of ready‐to‐eat lamb's lettuce (Valerianella locusta L. Laterr)

BACKGROUND Nowadays, there is an increasing interest in the hydroponic floating system to cultivate leafy vegetables for ready-to-eat salads. It is reasonable that different growing systems could affect the quality and shelf life of these salads. RESULTS The quality and shelf life of ready-to-eat lambs lettuce grown in protected environment in soil plot or in soil-less system over hydroponic solution with or without the addition of 30 µmol L⁻¹ silicon were evaluated. Minimum effects were observed on colour, firmness and microbial counts. Hydroponic cultivation largely affected plant tissue hydration, leading to weight loss and structural modifications during refrigerated storage. The shelf life of lambs lettuce was limited by the development of visually detectable unpleasant sensory properties. Shelf life, calculated by survival analysis of consumer acceptability data, resulted about 7 days for soil-cultivated salad and 2 days for the hydroponically grown ones. The addition of silicon to the hydroponic solution resulted in an interesting strategy to increase plant tissue yield and reduce nitrate accumulation. CONCLUSIONS Although hydroponic cultivation may have critical consequences on product quality and shelf life, these disadvantages could be largely counterbalance by increased yield and a reduction of nitrate accumulation when cultivation is performed on nutritive solutions with supplemental addition of silicon.

Experimental evaluation of refrigerant mixtures as substitutes for CFC12 and R502

Abstract Several selected refrigerant mixtures are tested as potential short- and mid-term substitutes for CFC12 and R502. HCFC22 and some hydrocarbons are considered as components of retrofit mixtures. Their influence on the solubility of various lubricant oils is investigated by measuring critical solubility temperatures. The performance of the CFC12 and R502 refrigerants and of their proposed alternatives is compared by testing two different refrigerating units.

A STREAMFUNCTION-VORTICITY-BASED FINITE-ELEMENT FORMULATION FOR LAMINAR-CONVECTION PROBLEMS

Abstract The finite-element method is used to solve mixed-, farced-, and natural-convection problems in two-dimensional incompressible laminar flows. The streamfunction-vorticity equations are uncoupled and solved in sequence with the energy equation. The wall vorticity is evaluated in the framework of the streamfunction equation, and particular care is taken to specify inflow and outflow boundary conditions properly. The resulting scheme achieves convergence without the traditional need for upwinding, even for very high values of the Reynolds and the Rayleigh numbers. Stability and accuracy of the approach are demonstrated by the solution of three well-known test problems concerning mixed and forced convection downstream of a backward-facing step, and natural convection in a heated square cavity.

Corn salad (Valerianella locusta (L.) Laterr.) growth in a water‐saving floating system as affected by iron and sulfate availability

BACKGROUND Unbalanced nutrient availability causes disequilibrated plant growth, which can result in a worsening of harvested product quality, such as high nitrate content in edible tissues. To cope with this problem, improved knowledge of the mechanisms involved in nutrient acquisition and regulation is necessary. For this purpose the responses of acquisition mechanisms of N, Fe and S were studied as a function of Fe and S availability using two corn salad cultivars grown hydroponically, considering also aspects related to N metabolism. RESULTS The results showed that an increase in Fe or S availability enhanced nitrate uptake and assimilation, which in turn increased biomass production of leaves with lower nitrate content. In particular, high S availability exerted a positive effect (gene expression and functionality) both on the uptake and metabolism of N and on Fe acquisition mechanisms. CONCLUSION The data presented here show close interactions between N, S and Fe, highlighting that relevant improvements in yield and quality from soilless culture might also be obtained through appropriate adjustments of nutrient availability. In this respect, concerning the role of S in the acquisition mechanisms of N and Fe and in N metabolism, its level of availability should be taken into high consideration for equilibrated plant growth.

OPEN BOUNDARY CONDITIONS FOR THE STREAMFUNCTION -VORTICITY FORMULATION OF UNSTEADY LAMINAR CONVECTION

Abstract The streamfunction-vorticity formulation is used to analyze unsteady laminar-convection problems in two-dimensional incompressible flows. The Bubnov-Galerkin finite-element method and a sequential procedure are employed to discretize and solve the governing differential equations. Very accurate results are obtained by employing “advective derivative conditions” at the outflow for all the variables involved. The boundary conditions for the streamfUnction at internal walls are imposed during the assembly process, and the vorticity at inflow and wall boundaries is evaluated in the framework of the stream/unction equation. The accuracy of the approach is demonstrated by the solution of two well-known benchmark problems concerning forced convection over a circular cylinder in cross flow and mixed convection in a plane channel heated from below.

Finite element analysis of coupled conduction and convection in refrigerated transport

Abstract As an alternative to the continuous analysis of coupled velocity and temperature fields, the average fluid velocities and the convective heat transfer coefficients are estimated first by standard engineering procedures. Afterwards, these values are specified as part of the input data in a finite element code that calculates the detailed temperature distributions in the solid and the bulk temperature variations in the fluid. In the examples, the finite element formulation is validated by comparison with a typical benchmark solution, and the capabilities of the finite element method are demonstrated by the analysis of coupled conduction and convection problems in refrigerated transport.