(578f) Numerical Investigation for Heat Transfer of Supercritical Methane Heated in a Horizontal Circular Tube

Authors: 
Ruan, B., Shanghai Jiao Tong University
Lin, W., Shanghai Jiao Tong University
In the receiving terminal, LNG needs to go through the gasifier to absorb heat and transfer to natural gas in the normal temperature. The gasification pressure is generally between 5-9MPA. In this process, CH4, the main component of LNG, is in supercritical state. Its physical properties will change greatly, especially when they are near the pseudo critical point. Unusual as its heat transfer law may be, there are few studies about supercritical methane at present. Most of the studies on other supercritical fluids show that the heat transfer becomes worse when the heat flux is very large in the vertical tube. However, in the LNG gasifier, the heat flux is not large, and there are a lot of horizontal tubes. In this case, will the heat transfer law be similar to what mentioned in literature? So the heat transfer of supercritical methane in a horizontal tube is simulated. Compared to the experiment, numerical simulation helps us to see more details of the flow and heat transfer process.

The simulation conditions are similar to the actual IFV (Intermediate Fluid Vaporizer) heat transfer conditions: the diameter of the tube is 12mm, the flow rate 0.03-0.07kg/s, the pressure 6-9MPA, and the temperature range 150-300K. Research findings:

In the low temperature region, with higher flux density, the influence of the buoyancy force is more obvious, and the temperature difference between the top and bottom of the tube is greater. Along the flow direction, the heat transfer is enhanced at the top of the tube, but at the bottom, the heat transfer deterioration occurs at first, and then the heat transfer coefficient increases. The greater the heat flux is, the more obvious the phenomenon of heat transfer enhancement and deterioration will be. Observe the internal flow and heat transfer of the tube, as the supercritical methane density varies greatly with temperature, the fluid in the middle which is far from the wall of the tube is in low temperature with big density. It sinks down gradually along the flow direction; The fluid near the wall is in higher temperature with lower density, it rises up to the top. The buoyancy leads to the great temperature difference and different heat transfer coefficients between top and bottom. In some literatures, the buoyancy is equivalent to the natural convection. However, further study shows that there is no strong natural convection in the circular tube, and the buoyancy force is only responsible for the uneven distribution of density. In addition, under the same heat flux, when the mass flow rate is higher, the heat transfer coefficient will become larger and the influence of the buoyancy force smaller. The influence of pressure can be ignored.

Near the pseudo critical point, the influence of the buoyancy force is more obvious, and the temperature difference is greater between the top and bottom of the tube than that in the low temperature region. Different from the low temperature region, the heat transfer coefficient at the bottom is very close under different heat flux, but it decreases greatly at the top along the flow direction, and it will not rise again. This phenomenon will be more obvious with greater heat flux. And we also studied the effects of density, heat capacity, thermal conductivity, viscosity change on the heat transfer. It is found that the heat transfer coefficient is positively related to the specific heat and thermal conductivity, and negatively related to the viscosity. The change of density will lead to the heat transfer deterioration at the top and heat transfer strengthen at the bottom. The heat transfer of supercritical methane is influenced not only by the turbulent kinetic energy mentioned in literature, but also by the changes of all the physical properties. Near the pseudo critical point, the specific heat and thermal conductivity change greatly. Their influence is very severe, which makes the heat transfer law different from that in the low temperature zone. In addition, under the same heat flux, if the mass flow rate gets higher, the heat transfer coefficient will be larger, and the influence of the buoyancy force smaller. The heat transfer law under different pressures is very close, but the temperature difference between the top and bottom becomes larger when the pressure is smaller, for the physical property changes more greatly under smaller pressure.

In the high temperature region, the influence of the buoyancy force is very small with more uniform density distribution, and the temperature and heat transfer coefficients of the top and bottom are very close, for the density does not change much in the temperature far from the pseudo critical point. Under the same mass flow rate, if the heat flow becomes higher, the heat transfer coefficient will be smaller, and it decreases along the flow direction. The overall heat transfer coefficient becomes larger with the increase of pressure, which should be attributed to the different physical properties under different pressures.

In addition, the criterion of floating force Gr/Re2<10-3in literature is compared with the simulation results, and the criterion is proven accurate.