Jorge Facão

Thermal behaviour of closed wet cooling towers for use with chilled ceilings

A new closed wet cooling tower, adapted for use with chilled ceilings in buildings, was tested. Experimental correlations were obtained for mass and heat transfer coefficients. Existing thermal models for this type of cooling tower were found to predict well thermal performance, if the above correlations are used.

Evaluation of the use of artificial neural networks for the simulation of hybrid solar collectors

Abstract In the last decade, Artificial Neural Networks (ANNs) have been receiving an increasing attention for simulating engineering systems due to some interesting characteristics such as learning capability, fault tolerance, speed and nonlinearity. This article describes an alternative approach to assess two types of hybrid solar collector/heat pipe systems (plate heat pipe type and tube heat pipe type) using ANNs. Multiple Layer Perceptrons (MLPs) and Radial Basis Networks (RBFs) were considered. The networks were trained using results from mathematical models generated by Monte Carlo simulation. The mathematical models were based on energy balances and resulted in a system of nonlinear equations. The solution of the models was very sensitive to initial estimates, and convergence was not obtained under certain conditions. Between the two neural models, MLPs performed slightly better than RBFs. It can be concluded that similar configurations were adequate for both collector systems. It was found that ANNs simulated both collector efficiency and heat output with high accuracy when “unseen” data were presented to the networks. An important advantage of a trained ANN over the mathematical models is that convergence is not an issue and the result is obtained almost instantaneously. #Contributed by the Organizing Committee for the First International Exergy, Energy and Environment Symposium (IEEES-1). Paper presented at IEEES-1, Izmir, Turkey, 13-17 July 2003. Manuscript received by IJGE on 2003-12-04; final revision received on 2004-02-17. Corresponding guest editors: I. Dincer and A. Hepbasli.

MODELING LAMINAR HEAT TRANSFER IN A CURVED RECTANGULAR DUCT WITH A COMPUTATIONAL FLUID DYNAMICS CODE

ABSTRACT Flow and convective heat transfer in a curved rectangular channel was modeled and the heat transfer coefficient assessed, using the FLUENT code. The conditions considered are representative of an application to solar collectors with heat pipes. A wide range of Reynolds and Dean numbers was considered, under laminar flow conditions. Results show that the Nusselt number for the curved duct is up to 10 times higher than for a straight duct, due to counterrotating secondary vortices formation. The flow pattern changes from one pair of vortices to three pairs, for a Dean number of 131. Above a Dean number of 1,200, the contours of secondary flow streamlines are similar and the variation in Nusselt number is small.

Heat and Mass Transfer in an Indirect Contact Cooling Tower: CFD Simulation and Experiment

The present work is focused on the computational analysis of heat and mass transfer in an indirect contact cooling tower. The main objectives of the study are to contribute to the understanding of heat and mass transfer mechanisms involved in the problem and to check the possibility of making use of a commercial computational fluid dynamics (CFD) code for simulating mass and heat transfer phenomena occurring in indirect cooling towers. The CFD model uses as boundary conditions the temperatures of the tubes obtained by a correlation model developed by Mizushina. The available mass transfer correlations for indirect cooling towers are presented and compared with a correlation obtained from CFD simulations. The assumption of analogy between heat and mass transfer is also discussed.

Expérimental uncertainty analysis in solar collectors

SYNOPSIS Experimental test standards for solar collectors do not define any procedure for estimating the uncertainty of the measured efficiency. A new method is presented in this paper and applied to a plate heat pipe solar collector. The detailed uncertainty analysis was performed by separating the contribution of systematic and random uncertainties for each variable. The uncertainty associated with linear regression (efficiency curve) has been calculated, when variables present systematic, random and correlated systematic uncertainties. The results show that efficiency uncertainty is higher for smaller values of the parameter .