Heat and Mass Transfer

The performance of two-bed, closed-cycle solid-sorption heat pumps are affected by the switching frequencies at which the sorbent beds are cycled between the regeneration and adsorption processes. However, there always exists a switching frequency that optimizes the system performance (i.e., thermal COP) for given design and operating conditions. In this paper, the performance of two-bed, closed-cycle solidsorption heat pumps are optimized based on the switching frequency and the effect of the design and operating parameters on the optimum performance are investigated. The effect of the sorbent bedNTU, the thermal resistance within the sorbent, the contact resistance between the sorbent and the tube wall of the heat transfer fluid, the fraction of inert mass within the sorbent bed heat exchanger, and amount of fluid resident within the heat exchanger, on the optimum system performance is documented.

The freezing of a binary solution saturating a porous matrix is investigated experimentally and numerically. An aqueous solution of sodium chloride constituted the binary solution. Spherical glass beads constituted the porous matrix. The freezing was initiated at one of the vertical walls of a rectangular cavity. The temperature distribution in the solid, liquid and mush regions were recorded. A mathematical model that considered conduction as the only mode of heat transfer was developed. The predictions of the model were compared with experimental data. In general, as the model were compared with experimental data. In general, as the model ignored natural convection, the predicted temperatures agreed well with the data during early times in the freezing when conduction was the dominant mode of heat transfer. But as freezing progressed the predictions departed from the data and this departure was larger when the initial superheat was high i.e., when the natural convective flow was vigorous and its influence strong on the freezing process.

This article aims to study the mass transient diffusion in solids with an arbitrary shape, highlighting boundary condition of the third kind. To this end, the numerical formalism to discretize the transient 3D diffusion equation written in generalized coordinates is presented. For the discretization, it was used the finite volume method with a fully implicit formulation. An application to drying of roof tiles has been done. Three models were used to describe the drying process: (1) the volume V and the effective mass diffusivity D are considered constant for the boundary condition of the first kind; (2) V and D are considered constant for the boundary condition of the third kind and (3) V and D are considered variable for the boundary condition of the third kind. For all models, the convective mass transfer coefficient h was considered constant. The analyses of the results obtained make it possible to affirm that the model 3 describes the drying process better than the other models.

Discoloration and browning are caused primarily by various reactions, including Maillard condensation of hexoses and amino components, phenol polymerization and pigment destruction. Convective drying can be combined with various pretreatments to help reduce undesired color changes and improve color parameters of dried products. In this study, effects of ultrasound-assisted osmotic dehydration as a pretreatment before convective drying on color parameters of Mirabelle plum were investigated. Variations of L* (lightness), a* (redness/greenness), b* (yellowness/blueness), total color change (ΔE), chroma, hue angle and browning index values were presented versus drying time during convective drying of control and pretreated Mirabelle plums as influenced by ultrasonication time, osmotic solution concentration and immersion time in osmotic solution. Samples pretreated with ultrasound for 30 min and osmotic solution concentration of 70% had a more desirable color among all other pretreated samples, with the closest L*, a* and b* values to the fresh one, showing that ultrasound and osmotic dehydration are beneficial to the color of final products after drying.

The development of a new emulsified fuel is described, from the conceptual idea to the semi-industrial tests of the final product. The starting point was the necessity to lower the particulate matter (PM) emissions produced by the combustion of more than 200 MBD of heavy fuel oil (HFO) used for electric power conversion. The major component of HFO is a vacuum residue of the oil refining process mixed with light cycle oils to make it pumpable. An alternative to handle and burn the high viscosity residue (solid at room temperature) is by converting it in an oil-in-water emulsion. The best emulsions resulted of 70% residue in 30% water, Sauter Mean Diameter of 10–20 μm and a stability of more than 90 days. Spray burning tests of the emulsion against HFO in a semi-industrial 500 kW furnace showed a reduction in PM emissions of 24–36%.

The technique of conformal transformations of the metric, widely used in general relativity and in cosmology, is applied to the analysis of heat conduction in an anisotropic medium, in which the thermal conductivity is described by a tensor instead of a scalar. The anisotropic medium is reduced to an effective equivalent one, which is isotropic. The simplification is achieved for a particular conformal factor of the transformation, uniquely determined under physically reasonable assumptions on the thermal conductivity, density, and specific heat of the medium. Another application consists in the formal elimination of source or sink terms from the heat equation by using a suitable conformal transformation.