Exergy Variation Research Articles

The effect of fouling on thermodynamic performance of forced convective heat transfer through a duct

Based on the first and second thermodynamic laws, a new systematic approach to study in detail the effect of fouling on the thermodynamic performance of forced convective heat transfer through a duct with constant wall temperature and constant wall heat flux for thermally and hydrodynamically fully developed turbulent flow is investigated. When considering fouling exists inside the duct, the local and mean exergy variation coefficient, exergy variation flux, dimensionless exergy variation number and the equation of exergy variation rate of working fluids, etc. have been put forward and their generalized expressions derived. A criterion evaluating the effect of fouling on the exergy variation of working fluids of the forced convective heat transfer process, which is defined as the exergy variation degradation rate, has been put forward. By reference to a duct, the numerical results of the exergy variation of working fluids are obtained (considering fouling or not), the effects of Reynolds number, the thickness of the fouling layer, dimensionless inlet temperature difference and wall heat flux on the exergy variation of working fluids are discussed. The results show that the exergy variation degradation rate increases with the increase of Reynolds number and decreases with the increase of dimensionless inlet temperature and wall heat flux. The exergy variation caused by the heat conduction of fouling plays an important role in the total exergy variation of working fluids.

Flow exergy as a Lagrangian for the Navier-Stokes equations for incompressible flow

A novel variational derivation of the Navier-Stokes equations for incompressible flows is presented and discussed. The Lagrangian density is obtained from the exergy balance equation written for both the (Lagrangian) steady and quasi-stationary isothermal flows of an incompressible fluid. The exergy of a fluid mass (composed of a kinetic, a pressure-work, a diffusive, and a dissipative portion, the latter being the result of viscous irreversibility) is derived first, and it is then shown that a formal minimisation of the exergy variation (i.e. destruction) generates, without recurring to “local potentials”, the Navier-Stokes equations of motion under the given assumptions. The acceleration being held constant, the proposed variational method can be classified as a “restricted” principle. The problem is also briefly discussed both in its historical perspective and in its possible formal and practical implications.

태양열을 동력원으로 한 물펌프 연구개발 - 에너지변환실험과 성능해석 -

In this study, energy conversion from thermal energy to mechanical power by using n-pentane was tested and exergy variation, cycle number, water quantity pumped and thermal efficiency were analyzed. The energy conversion was done and the water head could be ten meters on the experimental conditions. The operating temperature range of cycle was recommended to be around the liquid-vapour saturation temperature of the working fluid on the viewpoint of the maximum work. The cycle diagram was analyzed by the exergy analysis. For the constant water head, the cycle number was decreased and the water quantity per day was increased and thermal efficiency become higher when the water quantity per cycle become increasing. For the constant pumping water quantity per cycle, cycle number and the water quantity per day was decreased and the thermal efficiency become higher because the saturation temperature become higher when the water head become higher.

Energy and Exergy Calculations of Latent Heat Energy Storage Systems

An experimental study and a theoretical study have been conducted to evaluate the performance for a closed latent heat energy storage system using energy and exergy analyses. The energy storage tank is neither fully mixed nor fully stratified. It may be considered as semistratified. Experiments were performed on sunny winter days in 1996. In this study a complete storing cycle and charging and discharging periods are considered. Energy and exergy efficiencies, total energy and exergy variations, and mean energy and exergy efficiencies are also calculated by using experimental data.

Exergy analysis of solar assisted double effect absorption refrigeration system

Exergy or the available energy is based on the second law of thermodynamics and goes back to Maxwell and Gibbs. It is the exergy content and not the energy content, that truly represents the potential of the substance to cause change. Exergy is the only rational basis for evaluating the system performance. The aim of this project is to study in detail the exergy variation in the solar assisted absorption system. The influence of the cycle parameters are analysed on the basis of first law and second law effectiveness and the results indicated various ways of improving system performance by better design. Also a better quality of the evaporator has more effect on the system performance than the better quality of other components. It was shown that second law analysis quantitatively visualizes losses within a system and gives clear trends for optimization.