M. Jelinek, A. Levy & I. Borde
Mechanical Engineering Department, Ben-Gurion University of the Negev,
P.O. Box 653, Beer-Sheva 84105, Israel

In the developed triple-pressure level single stage cycle a jet ejector of a special design is added at the absorber inlet. The device serves two major functions : it facilitates pressure recovery and improves the mixing process between the weak solution and the refrigerant vapor coming from the evaporator. These effects enhance the absorption process of the refrigerant vapor into the solution drops. To facilitate the design of a jet ejector for absorption machines, a numerical model of simultaneous heat and mass transfer between the liquid and the gas phases in the ejector was developed. The refrigerant R125 and the absorbent DMETEG were used in these calculations.
Based on the computerized simulation program a parametric study of the jet-ejector was carried out. In addition to the study of the design parameters of the jet-ejector (pressure recovery, temperature and concentration as functions of the length and cone angle of the diffuser), the influence of the jet-ejector on the performance of the absorption cycle and the actual size of the unit were studied.

R. Boukhanouf ¹, S.B. Riffat¹ and R. Shuttleworth^2
1 School of the Built Environment, The University of Nottingham, University Park, Nottingham NG7 2RD, UK
² School of Engineering, The University of Manchester, Manchester M13 9PL, UK

This paper describes the principal design features and the dynamic and thermodynamic analysis of a prototype Stirling engine intended for use as a low cost and simple prime mover in natural gas-fired small-scale power generator system. The engine is of the diaphragm type with a displacer separated from the cylinder by a narrow gas-bearing gap. The combination of the rubber diaphragm and the absence of sliding seals lead to very quiet operation and lower frictional losses than for engines with piston rings. The engine operates using air as working fluid which places less stringent mechanical constraints on containing the working fluid than using a lighter gas such as helium. A detailed thermal analysis was carried out using a second order computer model which computes temperature and pressure variation throughout the engine working space, heat supplied to and rejected from the engine, work output as calculated from the PV diagram, and efficiency of the cycle.
Finally a proof-of-concept prototype has been built and tested to demonstrate the feasibility of the system.

A.C. Bwalya, H. Sabir and M. Raine
Kingston University, Department of Mechanical, Aeronautical and Production Engineering
Friars Ave., London SW15 3DW, U.K.

A conceptual design of a tubular water evaporator for lithium bromide – water absorption refrigeration systems which circumvents the requirement for two heat exchangers by exchanging heat directly with the fluid to be cooled is described. The technique employed to enhance heat transfer entails a capillary induced thin ‘annular’ refrigerant film inside the evaporator tube. The pertinent heat transfer characteristics of the proposed evaporator design are demonstrated as part of an investigative program having experimental and mathematical components.
The analytical results, obtained using existing single phase and two phase boiling heat transfer correlations from the literature, show that a heat transfer rates of up to 470 W and overall heat transfer coefficients of up to 55 W/m²-K can be achieved for a unfinned evaporator with an external heat transfer area of 0.08m², giving a maximum heat flux of about 6 kW/m², for air velocities up to 4 m/s.

Celestino Ruivo¹, A. Rui Figueiredo², José J. Costa^2
1Área Departamental de Engenharia Mecânica, Escola Superior de Tecnologia, Universidade do Algarve - Campus da Penha, 8000 Faro, Portugal (cruivo@ualg.pt)
² Departamento de Engenharia Mecânica, Universidade de Coimbra - Pólo II, 3030 Coimbra, Portugal (rui.figueiredo@mail.dem.uc.pt), (jose.costa@mail.dem.uc.pt)

This paper describes a simplified numerical simulation of the heat and mass transfer phenomena associated to both the humidification and dehumidification of the desiccant solid media used in air-conditioning installations. The algorithm is based on mass and energy balance equations, that are applied to the desiccant medium and to the airflows, and include the equilibrium moisture relationship for the desiccant. The convective mass and heat transfer coefficients are characterised by suitable correlations.
The developed model can be used to simulate a large set of working conditions, providing an efficient tool for studies of parametric sensibility, and for control and optimisation strategies.

Résumé :
Cet article concerne une étude de simulation numérique des phénomènes de transfert de chaleur et de masse dans les roues dessicantes utilisées dans les centrales de traitement d’air avec refroidissement évaporatif. L’algorithme du modèle simplifié est basé sur les équations de conservation d’énergie et de masse, appliquées à la matrice solide hygroscopique et aux écoulements d’air, utilisant les courbes d'équilibre matrice solide-air humide et des coefficients d’échange de chaleur et de masse estimés par des expressions empiriques.
Le modèle est un outil efficient pour faire des études de sensibilité paramétrique et définir le système de contrôle, ainsi que pour l’optimisation des stratégies de fonctionnement.

Keywords : Air-conditioning, rotary desiccant systems, numerical modelling