Hydrodynamics and Heat Transfer in Vertical Falling Films with Smooth and Modified Heat-Transfer Surfaces – An Experimental and Numerical Investigation
Sammanfattning: High energy utilization efficiency is important to achieve a sustainable society. By having highly efficient heat exchangers, more energy can be recovered and reused at a lower cost. One commonly used type of heat exchangers is a vertical falling film unit, which is a device where a thin liquid film is flowing down a vertical wall in the presence of a gas layer. Such units can operate with small temperature differences and have short residence times and small pressure losses. Those qualities are favourable in the food industry for the concentration of heat-sensitive fluids such as fruit juice and dairy products, or, in the pulp and paper industry for concentration of black liquor in the chemical recovery process. Since evaporation is inherently very energy-intensive and these industrial units are typically large, relatively small efficiency improvements can lead to significant reduction of operational costs. In this thesis, the hydrodynamics and heat transfer in vertical falling films are investigated. Novel measuring procedures are developed to study the local time-varying properties of the film, such as film thickness profiles. The procedures are used to distinguish between entrance effects and statistically steady conditions, which is of importance for an industrial unit. It is shown that these steady conditions are typically achieved 1-2 meters downstream from the inlet. Film thickness profiles, along with local heat transfer coefficients, are measured to categorize and understand the variation between different operating conditions for industrially relevant fluids. The measurements are also used to introduce and validate a numerical framework for falling films for industrially relevant conditions. The framework solves the Navier-stokes equations in two dimensions using the Volume of Fluid method. The numerical framework is used to study complex flow inside the thin liquid film and to differentiate between the convective and conductive mechanisms that govern the rate of the heat transfer. It is shown that both mechanisms are of importance for the heat transfer. The knowledge acquired about the film flow behaviour on smooth heat-transfer surfaces used nowadays is utilized to propose a novel design of heat transfer surfaces, with millimetre-high structures, for industrial use. Both the experimental measurements and the numerical framework show that it is possible to enhance the rate of heat transfer by up to 150 %, dependent upon how sharp the new modified surfaces are constructed. The modified surfaces enhance the rate of heat transfer by initializing more bulk mixing by utilizing the time-varying behaviour of the falling film. The irregularities in the flow lead to the occurrence of time-fluctuating recirculation zones behind the surface modifications, which results in a significant increase in the convective heat transfer.
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