In order to determine average transfer coefficients for plate fin and elliptic tube exchangers, mass transfer experiments were performed using the naphthalene sublimation technique. By means of the heat and mass transfer analogy, the average coefficients of mass transfer can be easily converted to heat transfer results. In accordance with the analogy, the experimental conditions, with naphthalene plates forming the rectangular channel of the exchanger, simulated isothermal fins. Since in practical situations the fins are not isothermal, one can rationalize such a fact by using the concept of fin efficiency, which is a common procedure in heat transfer calculations. The transfer coefficients of the present research were compared with those for circular tube exchangers, found in the pertinent literature, and the comparison revealed no major differences. This is a positive outcome, since the use of elliptic tubes, as it will be shown, reduces substantially the pressure drop in the rectangular channel and increases the fin efficiency improving the transfer characteristics. The mass transfer measurements were performed by weighing the naphthalene pieces before and after a data run characterized by the flow Reynolds number. The Reynolds number range was 150–1300. The present results are directly applicable to air conditioning devices.

In this study, a two-dimensional (2-D) heat transfer analysis was performed in circular and elliptic tube heat exchangers. The finite element method was used to discretize the fluid flow and heat transfer governing equations and a 2-D isoparametric, four-noded, linear element was implemented for the finite element analysis program, FEAP (O.C. Zienkiewicz, R.L. Taylor, The Finite Element Method, vol. 1, McGraw-Hill, London, 1989, Chapter 15). The numerical results for the equilateral triangle staggering configuration, obtained with the new element were then validated qualitatively by means of direct comparison to previously published experimental results for circular tubes heat exchangers (G. Stanescu, A.J. Fowler, A. Bejan, Int. J. Heat Mass Transfer 39 (2) (1996) 311-317). Next, a numerical geometric optimization was conducted to maximize the total heat transfer rate between the given volume and the given external flow both for circular and elliptic arrangements, for general staggering configurations. The results are reported for air in the range 300 less than or equal to Re-L less than or equal to 800, where L is the swept length of the fixed volume. Circular and elliptical arrangements with the same flow obstruction cross-sectional area were compared on the basis of maximum total heat transfer. The effect of ellipses eccentricity was also investigated. A relative heat transfer gain of up to 13% is observed in the optimal elliptical arrangement, as compared to the optimal circular one. The heat transfer gain, combined with the relative pressure drop reduction of up to 25% observed in previous studies (H. Brauer, Chem. Process Eng., August (1964) 451-460; S.N. Bordalo, F.E.M. Saboya, Determinacao experimental dos coeficientes de perda de carga em trocadores de calor de tubos circulares e elipticos aletados, in: Proceedings of the 13th COBEM, Congresso Brasileiro de Engenharia Mecanica (in Portuguese), Belo Horizonte, Brasil, 1995) show that the elliptical arrangement has the potential for a considerably better overall performance than the traditional circular one. (C) 2001 Published by Elsevier Science Ltd.

Experiments were performed to determine average heat transfer coefficients and friction factors for turbulent flow through annular ducts with continuous longitudinal rectangular fins. In addition, the fin efficiency was also determined by means of a numerical two-dimensional heat transfer analysis. The total number of fins attached to the inner wall of the annular region was 20. The measurements were made by using a double-pipe heat exchanger. The fluids were, air flowing in the annular section, and water through the inner tube. The average heat transfer coefficients were obtained from the experimental determination of the overall heat transfer coefficients of the heat exchanger. To attain fully developed conditions at the entrance and exit, the heat exchanger was built with additional lengths before and after the test section (30 hydraulic diameters). The double-pipe heat exchanger and fins were made of brass. Due to the high thermal conductivity of the brass and the small water temperature variation, the surface of the inner tube was practically isothermal. The external tube was well insulated and can be considered adiabatic. The results are presented in dimensionless forms, in terms of the average Nusselt number, friction factor and fin efficiency, as functions of the flow Reynolds number. A comparison of the present results with those for smooth sections (without fins) is also presented. The purpose of such comparison is to study the influence of the presence of the fins on the pressure drop and heat transfer rate.

In this work, a two-dimensional (2-D) heat transfer analysis is performed in one- and two-row tubes and plate fin heat exchangers (circular and elliptical sections), using experimentally determined heat transfer coefficients from a heat and mass transfer analogy. The temperature distribution on the fin and air free stream, and the fin efficiency are determined for heat exchangers, with eccentricity 0.5 and 0.65, as a function of the Reynolds number. For tubes and plate fin heat exchangers, new numerical results of fin efficiency for elliptical tubes are compared with published results for circular tubes. A relative fin efficiency gain of up to 18% is observed in the elliptical arrangement, as compared to the circular one. The efficiency gain, combined with the relative pressure drop reduction of up to 25% observed in previous studies (Brauer 1964; Bordalo and Saboya 1995) show the elliptical arrangement has the potential for a considerably better overall performance than the conventional circular arrangement.

Experiments were carried out to determine average heat transfer coefficients and friction factors for turbulent flow through equilateral triangular ducts with pin fins. The measurements were made by using a triangular duct heat exchanger. The fluids in the heat exchanger were air and water, and the average heat transfer coefficients were determined by measuring the overall heat transfer coefficients of the heat exchanger. To attain fully developed conditions at the entrance and exit, the heat exchanger was built with additional lengths before and after the test section (30 hydraulic diameters). The triangular ducts of the heat exchanger consisted of two metal walls (brass) and a wall of a less conductive material. The metal walls of the air side were finned with an array of short pin fins. Due to their high thermal conductivity and the small temperature variation of the water, these two walls were practically isothermal. The third wall of the duct was well insulated and can be considered adiabatic. The results are presented in dimensionless form, in terms of Nusselt numbers and friction factors as functions of the Reynolds number. The pin fin efficiency, which depends on the heat transfer coefficient, is also determined as a function of dimensionless parameters. It was found that the fins increase both the heat transfer and the pressure drop compared with an unfinned (smooth) duct. A better comparison between finned and smooth ducts was made for the same pumping power and heat transfer area. The results show that the Nusselt number for the finned duct, in the present work, is always greater than the one for the smooth duct.

Abstract A model was developed for calculating the operating conditions of a non-adiabatic liquid dehumidifier used in solar air conditioning systems. In the experimental facility used for obtaining the data, air and triethylene glycol circulate countercurrently outside staggered copper tubes which are the filling of an absorption tower. Water flows inside the copper tubes, thus cooling the whole system and increasing the mass transfer potential for drying air. The methodology for calculating the mass transfer coefficient is based on the Merkel integral approach, taking into account the lowering of the water vapor pressure in equilibrium with the water glycol solution.

Heat exchangers consisting of finned tubes are commonly employed in air conditioning systems, air heaters, radiators, etc. Local measurements of mass transfer coefficients on fins, obtained by Saboya and Sparrow, are very nonuniform. In the present work, an experimental apparatus was set up to measure overall heat transfer coefficients for two-row tube and plate fin heat exchangers. The obtained results, together with Shepherd’s results for one-row exchangers, are used to transform the local mass transfer coefficients into local heat transfer coefficients. A numerical two-dimensional heat transfer analysis has been performed in order to obtain the temperature distribution and fin efficiency. The influences of the Reynolds number and fin material are also analyzed.

The growing demand for renewable energy resources gave rise to practically rediscovering the Darrieus wind turbine in the sixties. The economic feasibility of its utilization however is dependend upon the local wind energy and the knowledge of the performance of the turbine under different operating conditions. Several turbines have been tested in this research for different values of the solidity, which is related to the mass of the turbine blade. A method has been developed to determine the performance of small wind rotors, which can be done in small wind tunnels. This methodology significantly reduces testing time and research costs, besides providing a continuous function for the power coefficient, that is, the fraction of the wind energy that can be extracted by the turbine. By using power coefficient and wind velocity data, a numerical simulation of the performance of the turbines is carried out for one year period, under a constant speed operating condition; for two cities in Brazil. The equipment operates in this condition with simple controls and generates electric energy at constant frequency. The experimental performance data were found to closely fit those of Blackwell, Sheldahl and Feltz.

A brief addendum is presented to a previously reported experimental study of air-side transfer characteristics of one- and two-row plate fin and tube heat exchanger configurations. The experiments were performed as mass transfer studies using the naphthalene sublimation technique, with corresponding heat transfer results being obtainable via the analogy between heat and mass transfer. Quantitative results were given for the local and average transfer coefficients. Various transfer mechanisms were identified, the most remarkable being the vortices which develop in front of the tubes and are swept around the sides. An extension of the original research program to include configurations with three staggered rows of tubes has recently been carried out, and the results are reported.

The heat-mass transfer analogy, in conjunction with the naphthalene sublimation technique, was used to investigate the transfer capabilities and transfer mechanisms in two-row plate fin and tube heat exchanger configurations. Local and average transfer coefficients were determined from measurements of the mass transferred in an analogical system consisting of a pair of naphthalene plates and an array of spacer disks. The analogical system modeled a typical heat exchanger flow passage. The results were presented in dimensionless form to facilitate their conversion to Nusselt numbers.

The analogy between heat and mass transfer has been used to obtain local and average transfer coefficients for a one-row plate fin and tube heat exchanger configuration. The mass transfer experiments were performed using the naphthalene sublimation technique. A heat exchanger passage was modeled using naphthalene plates; spacer disks simulated the tubes. Detailed measurements of the surface elevation of the naphthalene plates were made before and after a data run, and this enabled evaluation of the local transfer coefficients. Average transfer coefficients were determined both by surface integration of the local mass transfer or by weighing the test plate with a precision balance; the two methods gave results that agree within one or two percent. The local measurements revealed high values of the transfer coefficient on the forward part of the fin due to the presence of developing boundary layers. In addition, owing to a natural augmentation effect caused by a vortex system which develops in front of the tube, there are very high fin transfer coefficients in a U-shaped band that rings the tube. The effect of the vortex system is more pronounced at higher Reynolds numbers. Relatively low transfer coefficients are encountered on the portion of the fin that lies downstream of the minimum flow cross section. The coefficients are especially low in the region behind the tube.