NUCLEATE POOL BOILING HEAT TRANSFER FROM TWO HORIZONTAL PLAIN AND WIRE SCREEN WRAPPED TUBES IN A VERTICAL GRID

Abstract

The thesis deals with the experimental investigation related to nucleate pool boiling heat transfer from a horizontal plain tubes- grid and also when the tubes of this grid are wrapped with wire screen. The grid in both cases is held in vertical plane. The distance between tubes is 75 mm. The liquids investigated are pure satui;ated distilled water and benzene at atmspheric pressure, Ahereas the heat flux ranges from 22,231 to 46,483 W/mz. The experimental setup essentially consists of a stainless steel test vessel to hold a pool of boiling liquid, the heating tubes- grid on which boiling takes place, and a .vertical condenser to condense the vapours generated from liquid pool. ,,,,The present investigation involves the measurement of call--- and liquid- temperatures. The wall- temperature of the heating tubes is measured at three places along the circumference at the top-, the side-, and the bottomL positions with the help of copper constantan thermocouples placed inside the holes drilled in the thickness of the heating tubes. The bulk liquid-temperature of the pool is also measured at the corresponding top-, side-, and bottom- positions around the heating tubes with the help of copper constantan thermocouples. Before conducting the experimental data, the tubes of the grid are thermally stabilized and aged for about 72 hours of submergence and boiling. The dissolved air from the system is removed completely. The experimental data have been obtained for boiling of liquids on tubes of the grid when the tubes are heated separately and also simultaneously. The latter case helps in determining the effect of vapour bubbles emerging out from lower ,tube on heat transfer coefficient of upper tube of the grid. Similar experiments have been carried out when these plain heating tubes of the grid are wrapped with wire screen of different mesh number and layers. Based on data of plain tubes obtained as above, it has been found that each tube of the grid has its own surface characteristics inspite of the fact that they have been fabricated from a given rod by using the same technique for machining of outer surfaces of tubes and boring of their central holes. A relationship of the form, h a 0.7 is established for both tubes of the grid when they are heated separately. When lower and upper tubes of a grid are heated simultaneously, heat transfer coefficient of upper tube As enhanced; whereas that of lower tube does not. For the PT- grid enhancement factor is more at lower heat flux and decreases with increase in heat flux. It is important to note that enhancement in heat transfer coefficient of upper tube results owing to vapour bubbles emerging out from lower tube and entering the superheated liquid layer around upper tube. A correlation relating enhancement factor to heat flux of lower and upper tube has been obtained. Correlation for prediction of heat transfer coefficient of upper tube of the PT- grid has also been obtained as a function of heat flux of lower and upper tubes. From experimental data on wire screen wrapped tubes, it ii is noted that wrapping of wire screens of 80, 100, & 200 mesh has been found to augment heat transfer coefficient of tubes conside- rably. Augmentation in heat transfer coefficient depends upon layers of screen, heat flux, and boiling liquid. Amongst the three bronze wire screens of 80, 100, and 200 mesh investigated, augmentation is the greatest when screen is of 100 mesh. As regards heat transfer coefficient of a wire screen wrapped tube, it changes with heat flux raised to power of 0.23, which in excellent agreement with some of the earlier intern t~ions. When tubes of the grid having one layer of 100 mesh wire screen are heated simultaneously, el lancement in heat transfer coefficient of upper tube is not as much as that of this tube in the FT- grid. For upper tube in the WSWT- grid, enhancement does not change with heat flux; whereas it changes with heat flux for the PT- grid. A aorrelitttioh fo trAnnroz' 000ttiolont 6$1 WooAr tube of the WSWT- grid as a function of heat flux of lower and upper tubes has also been obtained.