Fundamental Studies Of Crossflow Heat Exchangers For Laminar And Turbulent Flows

Abstract

The focus of this thesis research is heat transfer and fluid flow in heat exchangers where a fluid flows across heated cylindrical elements. Three unique devices are considered, and highly detailed and accurate solutions are obtained for each by making use of advanced numerical simulation techniques. As prerequisite to the implementation of these solutions, validation of the numerical procedure was obtained by comparing highly accurate and complete experimental data to the numerical predictions for a relevant test case. The first considered situation is a two-dimensional, in-line tube bank where the number of rows and the Reynolds number serve as parameters. New methods were devised to determine the prevailing flow regime in the tube bank, one based on the calculation of the turbulent viscosity and the other utilizing a comparison of heat transfer coefficients respectively determined from laminar and turbulent models. Array-based average heat transfer coefficients showed that shorter arrays gave rise to higher values of the transfer coefficient, in contrast to certain literature predictions. The second studied case is the simultaneous treatment of heat transfer in a pin-fin array and the fluid flow created by a conventional rotating fan which is delivered to the inlet of the array. The basic issue is the nature of the delivered flow. Even when a blower curve is used, it is assumed that the delivered flow is uniformly distributed across the heat exchanger. In reality, when blade rotation of the fluid mover are taken into account, the uniformity disappears. In fact, the delivered flow includes a swirl component superimposed on the main axial flow. The velocity of the delivered flow may be larger adjacent to the walls than it is in the core of the flow. In many cases, backflow occurs, driven by the rotation of the hub of the fan. The outcome of the work is that correct results require simultaneous treatment of the fluid mover and the heat exchanger. The final dealt-with case is the cylinder in crossflow and provides the most complete set of transient heat transfer results ever.