Sarit Kumar Das

A second law analysis is presented for thermally dispersive flow through a plate heat exchanger. It is well known that in plate or plate fin type heat exchangers the backmixing and other deviations from plug flow contribute significantly to the inefficiency of the heat exchanger, which is of importance to heat exchangers working in the cryogenic regime. The conventional axial heat dispersion model which is used so far is found to be better than 'plug flow' model but still unsatisfactory where the timescale related to heat transfer is comparable with the thermal relaxation time for the propagation of dispersion. The present work therefore considers dispersion as a wave phenomenon propagating with a finite velocity. The study discusses the nature of variation of different contributions to total exergy loss in the heat exchanger with respect to dispersion parameters of the Peclet number and propagation velocity of the dispersive wave. The practical example of the single-pass plate heat exchanger demonstrates how a second law optimization can be carried out for heat transfer equipment under such conditions. © 1998 Elsevier Science Ltd. All rights reserved.

The present analysis gives an account of exergy destruction in regenerative heat exchangers used in cryogenic applications from the view point of Second Law of Thermodynamics. Unlike the previous studies, the present work considers the solid storage matrix with temperature variation along both spatial and temporal co-ordinates. The present analysis also considers the effect of matrix longitudinal heat conduction on the Second Law behaviour. Finite longitudinal conductivity, which results from the distribution of temperature in the matrix, acts as a major non-ideality associated with the analyses done so far. The present analysis shows that the introduction of longitudinal conduction ensures the optimization of charging period for the regenerator. This makes it possible to optimize the regenerator performance globally to produce optimum combination of Ntu and charging time. It is also observed that a decrease in longitudinal conductivity reduces the optimum charging time. Thus the non-existence of thermodynamic optima at the absence of longitudinal conduction is explained adequately. © Springer-Verlag 1999.

A comprehensive account of the emerging concept of dispersion of heat along the axial direction as a fluid flows through a passage bounded by solid wall has been presented with its most recent and remarkable advancement. This new proposition takes axial dispersion as a disturbance which propagates as a wave with a finite velocity. It has been proposed that this sound like propagation be named as the "third sound wave in flowing fluid". The fundamental analysis of this theory has been presented with particular emphasis on the boundary condition which plays a key role in the propagation of the wave. A general flux formulation has been used for this purpose. Analysis has also been presented for a two fluid situation. It has been found that the 'subsonic' and 'super sonic' flow with respect to third sound wave behave differently particularly at entry and exit. The theoretical background developed has been substantiated by three examples-one purely theoretical condition, one comparison with numerical analysis and finally application to a complete apparatus. © 1998 Elsevier Science. All rights reserved.

An efficient strategy for the solution of N-S Equations using collocated, non-orthogonal grids is presented. The governing equations have been discretized in the physical plane itself without co-ordinate transformation, thereby retaining the lucidity of the basic finite volume method. The non-orthogonal terms and QUICK type corrections for the convective terms in the momentum equations are treated explicity, while the other terms are taken in implicit form. In the pressure correction equation, the non-orthogonal terms have been dropped altogether. The discretized equations have been solved by the preconditioned conjugate gradient square method. The specific combination of above steps has resulted in better convergence properties as compared to those of existing algorithms, even for highly skewed grids. The scheme has been validated against benchmark solutions such as lid-driven flow in square and skewed cavities and experimental results of flow over a single cylinder. Its applicability has also been illustrated for flow through a bank of staggered cylinders, with anti-symmetric inlet and outlet boundary conditions.