Undersökning av förstärkt värmetransport i fallfilmsindunstare

The aim with this master`s thesis project was to examine what effect structured heating surfaces have on the mass and heat transfer properties of falling film evaporation. Based on a literature study, some alternative surface structures that could be of interest for falling film evaporation of black liquor were selected for further investigation. A new structure was also selected to be tested. Tubes of copper and PVC were utilized as a basis for the experimental investigation. Five different copper test tubes were studied: one smooth, two equipped with dimples located in a row or diagonal towards each other, one spirally threaded and one spirally winded with a copper thread. Two test tubes of PVC were studied: one smooth and one equipped with the new structure. The tests were carried out by attaching the different test tubes one by one to the lower edge of a vertically placed falling film tube. Ordinary tap water was utilized as the testing fluid. A divider placed at the top of the falling film tube allowed an evenly distributed film to be formed that flowed therafter down over the falling film tube wall and the test tube. Since the test tubes were not provided with a heating element, the mass transport was measured from the test tubes to the falling film. This result was then used to estimate the heat transfer through calculations. As mass transport medium benzoic acid was chosen which was applied on the test tubes dissolved in a small amount of acetone. The mixture of dissolved benzoic aid in acetone was sprayed with a spurt bottle on the test tube surface, where the acetone was evaporated and left behind a relatively even layer of benzoic acid. For each test run, four samples of the falling film were taken out at fixed time points after that the falling film had passed over the benzoic acid covered test tube. The mass concentration of benzoic acid was then determined with the help of spectrophotometry. The different test tubes mass flux were determined from the measured mass concentrations and utilized film flow after that the benzoic acid covered surfaces had been calculated for each trial run. These mass fluxes were then divided by the solubility of benzoic acid in water to estimate the mass transport coefficients which were in turn used to estimate the heat transport coefficients (heat transition number) based on the Chilton-Colburn analogy. The results indicate that all the tested surface modifications should be able to enhance the mass and heat transport in a falling film of water comperad to a smooth surface. The new structure and the spirally threaded structure achived the highest enhancements (113 and 114%) of the estimated heat transport coefficients compared to the reference smooth tube structure. The heat transport coefficient enhancements for the other structures tested were 69% (structure with dimples located diagonally towards each other), 65% (structure with a spirally winded copper thread), and 44% for the structure with dimples located in a row towards each other