J.M. Gurgel

Thermal Diffusivity and Adsorption Kinetics of Silica-Gel/Water

There have been performed experimental measurements of effective thermal conductivity of silica-gel for a stagnant cylindrical fixed bed under transient and steady state conditions in the presence of dry air at different pressures and for different amounts of adsorbed water. The Bauer-Schlünder and Kunii-Smith models have been used to identify the thermal solid conductivity of silica gel pellets from measurements of the conductivity in an adsorbent bed. Sorption rates of water vapor in silica gel were measured using a single-step thermal method by monitoring the sample surface temperature by infrared detection. In order to obtain the mass diffusivity it is necessary to match the numerical solution of the mathematical model to the experimental data.

Thermal conductivity of hydrated silica-gel

The temperature of points inside a cylindrical packed bed of silica-gel grains is measured under steady-state and transient radial heat flow conditions, for different pressures and water contents of the silica. The stationary results are used to calculate the thermal conductivity of the silica packed beds. The transient results are used to find the thermal diffusivity of the beds using a numerical solution of the heat conduction equation. The conductivity of the particle material can be calculated using the Bauer and Schluender model for the heat transfer inside the granular bed. The thermal conductivity of the packed bed is found to increase with pressure at pressures below 10 kPa. The thermal conductivity of the bed, and of the solid particles, is found to vary linearly with the water content of the silica-gel.

NUMERICAL ANALYSIS OF NATURAL CONVECTION IN CAVITIES WITH VARIABLE POROSITY

This work analyzes the effects of variable porosity on the heat transfer by natural convection, in a cavity with isothermal vertical walls and adiabatic horizontal ones and a porous medium inside. The hydrodynamic field in the porous medium is modelled according to the general model obtained by Brinkmans and Forchheimers terms. An exponential variation of the porosity near the walls was considered. The equations in terms of the real variables were numerically solved by the finite-volume method, with a staggered variables arrangement. The pressure-velocity coupling was treated with the PRIME algorithm. The simulations involved both Darcian (Da = 10 -7 ) and non-Darcian flows (Da = 10 -6 ). In each case, the modified Rayleigh range (Ra m = Ra 2 Da) was from 10 to 1,000, and from 100 to 5,000, respectively. For the numerical simulation we considered Pr = 1, Rk = 1, A = 1, and k X = 0.36. The results are shown through streamlines and isotherms and velocity and temperatures profiles. This shows that the generalized method with variable porosity means an increase in the average Nusselt number. For physical validation, some experimental sets were simulated in which glass was the porous medium, and water, alcohol, and transformer oil were used as the fluids. The simulated results indicate that the adopted model reduces the discrepancy in the experimental results obtained with water and alcohol. We proposed a correlation to evaluate the average Nusselt number as a function of six parameters: Rayleigh number, Prandtl number, the dimensionless particle diameter, thermal conductivity ratio between solid and fluid phase, porosity, and the aspect ratio of the cavity.This work analyzes the effects of variable porosity on the heat transfer by natural convection, in a cavity with isothermal vertical walls and adiabatic horizontal ones and a porous medium inside. The hydrodynamic field in the porous medium is modelled according to the general model obtained by Brinkmans and Forchheimers terms. An exponential variation of the porosity near the walls was considered. The equations in terms of the real variables were numerically solved by the finite-volume method, with a staggered variables arrangement. The pressure-velocity coupling was treated with the PRIME algorithm. The simulations involved both Darcian (Da = 10 -7 ) and non-Darcian flows (Da = 10 -6 ). In each case, the modified Rayleigh range (Ra m = Ra 2 Da) was from 10 to 1,000, and from 100 to 5,000, respectively. For the numerical simulation we considered Pr = 1, Rk = 1, A = 1, and k X = 0.36. The results are shown through streamlines and isotherms and velocity and temperatures profiles. This shows that the gene...

Numerical analysis of natural convection in parallel, convergent, and divergent open‐ended channels

Purpose – To investigate the natural convection in open‐ended parallel, convergent, and divergent channels using a fully elliptic procedure without extending the domain outside the channel for the application of the boundary conditions at the inlet and outlet of the channels.Design/methodology/approach – The model is two‐dimensional and fully elliptic in x and y directions, and the equations are solved only inside the channel by the finite volume method using a co‐located arrangement with a segregated procedure and boundary fitted coordinates. The pressure‐velocity coupling is solved by the PRIME algorithm.Findings – The results are shown in terms of velocity vectors, streamlines, isotherms, and the local and the average Nusselt number for all fluids and configurations investigated. For high values of the Rayleigh number, a recirculation region in the outlet of all investigated configurations and Prandtl numbers was observed. Based on the results, a single correlation is proposed to evaluate the average N...

Estudo teórico e experimental da tecnologia dessecante aplicada ao condicionamento de ar em João Pessoa

The desiccant rotor is rotating heat and mass exchangers air-to-air, impregnated with adsorbent material used for dehumidification of moist air. This work is carried out an experimental theoretical study on the behavior / performance of a desiccant rotor, used for desiccant air conditioning system operating under the climatic conditions of the city of João Pessoa. The problem consists of a rotating cylindrical rotor, honeycomb type impregnated with desiccant material through which pass two opposite air flows: a process (adsorption) and other regeneration (desorption). It has been tested and validated a mathematical one-dimensional transient model, involving heat and mass transfer in the fluid stream and adsorptive matrix, consisting of mass conservation equations and energy, adsorption isotherm and psychometric relations. This system of equations was discretized by finite volume method, fully implicit formulation, with a staggered arrangement. The numerical solution procedure used for the iterative Gauss-Seidel technique with under-relaxation and Thomas algorithm (TDMA). The results indicate that the mathematical model require enhancement to represent more accurately the experimental results, as presented maximum relative error of 10% temperature and 14% for moisture ratio.