The Effect of Sloshing on a Tank Pressure Build-up Unit

Abstract

This thesis work has aimed to identify how sloshing will affect a liquefied natural gas (LNG) fuel tank. The physical nature of LNG means it needs to be kept cooled and pressurized in order to remain in a liquid state. By implementing a pressure build-up unit (PBU) it is possible to pressurize the tank vaporizing the tank’s contents, for the vapour then to return to tank in a loop, building pressure in the process. A tank pressure build-up unit has been built in the laboratory at the university, which will be used for conducting experiments on sloshing. To be able to perform calculations on the system, the efficiency of the electrical heater is found through a simple experiment, measuring the mass of water the heater is able to vaporize per minute, as well asmeasuring the electrical power used. Four different power settings are tested, showing that the relation between flow and power are close to linear, meaning that it is possible to interpolate the vaporized flow should one want to use a different power setting than the ones already tested. The electrical power tested ranges between 570-2019W, with the efficiency ranging from 83.8-95.2%. The difference in efficiency is likely due to the water level rising when subject to higher levels of power, thus utilizing more of the heating coil, which is not fully submerged initially. The heat loss of the system, or more specifically the heat transfer coefficient between the interior of the tank and its environment is found through logging of the system’s temperature as the tank cools down, having initially been heated up to temperature of more than 110°C. Once the liquid and vapour temperature are equal, one can assume that no heat transfer is happening inside the tank, thus the following drop in temperature is due to heat loss to the surroundings. By recording the drop in temperature over a given time, the heat transfer coefficient is then calculated to beU Æ 0.31W/m2K. To find howsloshing affects this specific system, several tests are performed at different frequencies in order to observe how the severity of the sloshing affects the thermodynamical properties within the tank. The tank is heated to predetermined parameters, before the heater is turned off for the sloshing to be initiated, this in order to be in control over the amount of energy present in the systemwhen the sloshing is performed. It is found that sloshing definitely increases heat and mass transfer over the liquid-vapour interface when compared to test conducted where no iv tank motion is initiated. There is a clear distinction in heat and mass transfer between the different sloshing frequencies as well, where the general perception is that a higher frequency of sloshing results in a higher rate of energy transfer. It is attempted to conduct a series of experiments where the heater is on during the tests, in order to see if compensation of the pressure drop during sloshing was possible. It is found that utilizing the heater does result in reduced drop in pressure, in addition to showing that pressure compensation is possible in lower frequencies with the electrical power at disposal for these experiments, albeit not entirely stable. With control regulation, compensation should work well. The effect sloshing would have on heat and mass transfer was calculated from results gathered from the experiments. Calculating for a selection of tests with sloshing period ranging from T=2.50 seconds to T=4.00 seconds, as well as for a reference test with no tank excitation, it is found that sloshing at the lowest frequency tested, T=4.00 seconds, increases heat and mass transfer by 75% compared to the case with no tank excitation. The rate of heat and mass transfer increases as the sloshing frequency increases, calculations showing a 500%increase for sloshing period T=2.50 seconds.