Behaviour of Thermal Density Currents in Cold Receiving Water Bodies

Abstract

<p>In cold climates, temperatures higher than the ambient have been observed near the bottom of water lakes in the vicinity of thermal discharges. Concern has been expressed about the adverse effects of such abnormally warm water on the winter ecology of lake bottoms. It is expected that the existence of a density extremum in water at 4° C and the resulting nonlinear relation between density and temperature gives rise to densimetric flows which are markedly different from those in the linear range.</p> <p>This thesis presents experimental and numerical investigations that give some insight into the phenomenon of the thermal bar and the manner in which it may infIuence nearshore transport processes in the vicinity of a thermal outfalI in a cold climate. The investigations are restricted to an idealized model where the lock exchange mechanism is selected due to the fact that its behaviour is close to that expected in the prototype situation.</p> <p>The experimental investigation provides dramatic proof that the existence of an extremum in the density-temperature relation has a profound influence on the behaviour of densimetric flows in general and lock exchange behaviour in particular. Three zones in the vicinity of a thermal bar are clearly demonstrated viz. (i) the thermal overflow region. (ii) the thermal bar, and (iii) the thermal underflow region. The experiments provide data on the horizontal scale at which sinking takes place.</p> <p>A numerical model has been constructed to develop a means of modelling the behaviour of a thermal bar at the outfall of a steam electric generating station cooling water system. The numerical model employs a finite-difference scheme where the resulting algebraic finite difference equations are soIved using an alternating direction implicit method and a sparse-matrix package. The numericaI modeI has been verified by comparing it to numerical solutions of four different cases of the idealized problem of steady laminar flow in an enclosed rectangular cavity with differentially heated end walIs. Moreover, additional acceleration techniques are introduced to improve the numerical solution procedure. The numerical model is employed to solve the actual problem of simulating lock exchange flows created between two water bodies having different temperatures around the temperature of maximum density (i .e. having temperatures above and below 4° C). The general behaviour which has been observed experimentally is also confirmed numerically. The sensitivity of the associated parameters is examined. The relative extension of the thermal bar is correlated with relevant system parameters. Difficulty was experienced in obtaining numerical results for the same (high) Rayleigh numbers as were used for the physical experiments. Despite this, an encouraging degree of consistency was observed between simulated and observed behaviour.</p> <p>The important aspect of the study is to draw attention to the adverse effects of the sinking phenomenon (thermal bar), which may occur in the vicinity of man-made warm effluents as well as in natural bodies of water during the spring warming period. The study is significant in terms of the horizontaI scaIe at which sinking takes place, for the design of power station once-through-cooling water systems that must operate in cold climate winter conditions.</p>