Heat Transfer Correlations for compressible flow in Micro Heat Exchangers

The paper discusses the definition of dimensionless parameters useful to define a local correlation for convective heat transfer in compressible, micro scale gaseous flows. A combination of static and stagnation temperatures is chosen, as it allows to weight the temperature change related to the heat transfer and that induced by conversion of internal energy into kinetic one. The correlation offers a purely convective local Nusselt number, i.e. correlating the heat flow rate with the local flow parameters and wall surface temperature. The correlation is validated through a series of numerical computations in both counter-current and co-current micro heat exchanger configurations. The numerical computations take into account rarefaction and conjugate heat transfer effects

A new transfer function model for the estimation of non-point-source solute travel times

The scope of this work is to present a new fast and reliable transfer function model, which simulates the spatio-temporal distribution of non-point-source solutes along the unsaturated zone, suitable to be used at large scales within a web-based Decision Support System. With the assumptions of a) a gravity induced water flow, b) a non-reactive solute and c) a purely convective flow, the model uses the transfer functions, i.e., the travel time (TT) probability density functions, derived from the unsaturated hydraulic conductivity curve k(θ). The output concentration of a solute is simply the convolution of the transfer functions with the input concentrations to the system. A model sensitivity analysis, based on Monte Carlo simulations, was carried out, showing that saturated water content and the tortuosity parameter τ were the parameters that affected the mean TT more. The model was validated against concentration experiments carried out on four large soil columns. Results were really good for all soils, with the best agreement with R2 = 0.97, RMSE = 0.11 and ME = −0.01. Moreover, the outputs obtained applying the model to 46 soil profiles sampled in the Valle Telesina, in Southern Italy, completely characterised from the hydrological point of view, were compared with those obtained from the Richard-based model Hydrus 1D. The result of the comparisons gave a very high correlation coefficient (above 0.8), a mean absolute error between the two models of around 40 days and a percent bias of −16%. Finally, the application of transfer function model to a large spatial extent is presented, to show its possible use for the groundwater vulnerability assessment

A new transfer function model for the estimation of non-point-source solute travel times

The scope of this work is to present a new fast and reliable transfer function model, which simulates the spatio-temporal distribution of non-point-source solutes along the unsaturated zone, suitable to be used at large scales within a web-based Decision Support System. With the assumptions of a) a gravity induced water flow, b) a non-reactive solute and c) a purely convective flow, the model uses the transfer functions, i.e., the travel time (TT) probability density functions, derived from the unsaturated hydraulic conductivity curve k(θ). The output concentration of a solute is simply the convolution of the transfer functions with the input concentrations to the system. A model sensitivity analysis, based on Monte Carlo simulations, was carried out, showing that saturated water content and the tortuosity parameter τ were the parameters that affected the mean TT more. The model was validated against concentration experiments carried out on four large soil columns. Results were really good for all soils, with the best agreement with R2 = 0.97, RMSE = 0.11 and ME = −0.01. Moreover, the outputs obtained applying the model to 46 soil profiles sampled in the Valle Telesina, in Southern Italy, completely characterised from the hydrological point of view, were compared with those obtained from the Richard-based model Hydrus 1D. The result of the comparisons gave a very high correlation coefficient (above 0.8), a mean absolute error between the two models of around 40 days and a percent bias of −16%. Finally, the application of transfer function model to a large spatial extent is presented, to show its possible use for the groundwater vulnerability assessment

Direct Dry Carbonation of Mining and Industrial Wastes in a Fluidized Bed for Offsetting Carbon Emissions

The direct dry mineral carbonation of selected mining and industrial wastes, using carbon dioxide derived from combustion flue gas, was evaluated. Specifically, coal fly ash from two Australian brown coal-fired power plants, red mud from the refinement of bauxite into alumina, and diamond tailings were considered, due to their relevant residual alkali content. These materials were tested in a laboratory-scale fluidized bed reactor at different temperatures (300–450° C), in a reactive environment that simulated the typical CO2 concentration in a combustion flue gas. The experimental results showed a low, but still appreciable, CO2 capture capacity for three of the tested materials, which appears to be more favorable in the lower temperature range and with relatively fast kinetics, indicating the practical relevance of the process. One of the fly ashes exhibited a different behavior; starting at 350° C, the sorbent began to release CO2, rather than absorb it. This suggested that the sorbent was already extensively carbonated by weathering before the tests. This study provides some evidence for the possible viability of recycling mining waste and for the circular economy in offsetting carbon emissions in the mining industry

Transcritical CO2 commercial refrigeration plant with adiabatic gas cooler and subcooling via HVAC: field tests and modelling

Subcooling methods at the exit of the gas cooler in transcritical CO2 commercial refrigeration systems have been studied in the recent years showing that overall remarkable improvements can be obtained. Another strategy that results efficient is the use of evaporative systems at the gas cooler (adiabatic cooling) as it allows to significantly reduce the refrigerant quality at the liquid receiver and to lower the heat rejection pressure. In this work, a fully instrumented CO2 transcritical booster system with parallel compression, in operation in a small size supermarket in northern Italy, made available measured data of its performance when subcooling and/or adiabatic cooling are active. The plant operates in a mild climate, where it suffers operation at transcritical conditions for most of the year. Subcooling in this plant is performed by coupling the refrigeration system with the HVAC system. Taking advantage of experimental measurements, a model in the TRNSYS environment is validated and allows the prediction of the annual plant performance when these strategies are adopted. The adiabatic cooling showed to allow a significant reduction (about 10%) in the energy use, and makes unnecessary the use of a parallel compressor. Subcooling by the HVAC gives rise to a reduced saving (2.9 %) due to the absence of a dedicated mechanical subcooler, however it is almost comparable to parallel compression. These trends are confirmed in two other hot and humid climates