Niccolo Giannetti

Development and validation of an analytical formulation of the Nusselt and Sherwood numbers on a partially wetted absorber tube

With reference to a simplified analytical solution of the governing differential equation for falling film absorption, the corresponding formulation of the average Nusselt and Sherwood numbers obtained around a smooth horizontal tube is validated through a dedicated experimental investigation and data from previous literature. The assumption of a linear temperature profile together with the average interfacial mass fraction gradient and the hypotheses of small penetration for physical absorption are applied at the film interface to reach closed analytical expressions of average Nusselt and Sherwood numbers within falling film absorbers and generators. The inclusion of partial-wetting effects brings the results closer to the measurements and widens the applicability of these expressions.

Thermodynamic analysis of irreversible desiccant systems

A new general thermodynamic mapping of desiccant systems’ performance is conducted to estimate the potentiality and determine the proper application field of the technology. This targets certain room conditions and given outdoor temperature and humidity prior to the selection of the specific desiccant material and technical details of the system configuration. This allows the choice of the operative state of the system to be independent from the limitations of the specific design and working fluid. An expression of the entropy balance suitable for describing the operability of a desiccant system at steady state is obtained by applying a control volume approach, defining sensible and latent effectiveness parameters, and assuming ideal gas behaviour of the air-vapour mixture. This formulation, together with mass and energy balances, is used to conduct a general screening of the system performance. The theoretical advantage and limitation of desiccant dehumidification air conditioning, maximum efficiency for given conditions constraints, least irreversible configuration for a given operative target, and characteristics of the system for a target efficiency can be obtained from this thermodynamic mapping. Once the thermo-physical properties and the thermodynamic equilibrium relationship of the liquid desiccant mixture or solid coating material are known, this method can be applied to a specific technical case to select the most appropriate working medium and guide the specific system design to achieve the target performance.