Flow and heat transfer of supercritical LNG in spiral microchannel

Liquefied natural gas (LNG) is stable and safe, which is why the natural gas is usually liquefied before transported. The heat exchanger is widely used as the key component of vaporizing LNG, and it is composed of a large number of microchannels. This paper mainly analyzes the flow of supercritical LNG in a spiral microchannel, and compares the flow and heat transfer characteristic of spiral microchannel with different pitch. The result was indicative that with the lessen of pitch, the heat transfer is improved, but the flow characteristic is decreased. Compared with the straight channel, the spiral channel with appropriate pitch value can markedly improve the heat transfer properties, but has less effect on the flow characteristic. The discussion also includes the flow and heat transfer of microchannel with different mass flux and heat flux.


Introduction
With the increasing global energy consumption, natural gas, due to its low carbon dioxide emission and high calorific value, has become the best choice to replace coal, oil and other traditional fossil energy [1]. The industrial chain related to liquefied natural gas (LNG) continues to develop, and the optimization of LNG storage, transportation, and utilization is of great significance. The heat exchanger is a key component in the vaporization process of LNG, and its optimization research is of high value.
In practical application, LNG needs to be pressurized above the critical pressure to reach the supercritical state, and then vaporized through the receiving station [2]. The interior of the heat exchanger is composed of many microchannels. It is found that the flow and heat transfer characteristics of fluid in the microchannel are obviously better than those in the conventional scale conditions, but the miniaturization of the scale will also cause large pressure drop and resistance [3]. Supercritical fluids have the advantages of high density, high heat conduction coefficient, low viscosity and fast diffusion speed [4]. The pressure drop and resistance generated by the flow in the micro-flow channel are greatly reduced, and better flow and heat transfer performance can be obtained [5]. The microchannel with the structure shape of staggered S-shaped fins can achieve a lower resistance pressure drop while ensuring good heat transfer performance. Literature have shown that curved flow channels of Z-type and S-type can ameliorate the heat transfer performance of heat exchanger, and better flow heat transfer performance can be obtained when the bending angle is 15° [6]. The microchannel with the structure shape of staggered S-shaped fins can achieve a lower resistance pressure drop while ensuring good heat transfer performance. Improving heat transfer performance and reducing pressure drop by changing the shape and structure of the runner is one of the main research directions for heat exchangers in recent years [7][8][9][10][11][12][13]. In this paper, the spiral microchannel is designed, and the flow and heat transfer performance of supercritical LNG in a single spiral microchannel with different shapes, different velocities or different heat flux are mainly simulated and discussed.

Microchannel for supercritical LNG flow
The basic microchannel model is a straight rectangular runner with a cross section of 2.0mm×1.85mm and a longitudinal length of 50mm. The rectangular section is divided into two domains: the inner supercritical LNG runner and the outer thermal conductive solid wall. The cross section of the inner domain is a semicircle with a radius of 0.75mm, as displayed in Fig. 1. In our present work, a spiral micro-flow channel is designed based on the straight microchannel model. The longitudinal distance advanced by a spiral rotation is the pitch p. The fluid domain models of basic straight channel, of p=50mm, p=25mm and p=16.67mm (p=50 mm, rotating 3 times) respectively are shown in Fig. 2.

Turbulence model and boundary condition
The flow heat transfer of supercritical LNG in spiral microchannel is numerically simulated with steady-state calculation. The turbulence model is K-ω model, because of it can get better accuracy in internal flow, and the open energy equation. The boundary conditions are mass inlet and pressure outlet, and constant heat flux is applied on the upper and lower thermal conductive solid walls and its right and left walls are no slip wall.

Thermal physical properties
After fitting the thermophysical property curves used in reference [14], the parameter polynomials of thermophysical property are obtained as follows table1: Table 1. The parameter polynomials of thermophysical property.
where ρ, density; T, temperature, (K); cp, specific heat capacity; λ, thermal conductivity; μ, flow velocity. According to table 1, the curves of cp and thermal conductivity change with temperature after polynomial fitting are displayed in Fig. 3.
h is the convective heat transfer coefficient. It can be obtained by formula: where q w , wall heat flux density, (W/m 2 ); T wall , wall temperature; T b , average of inlet temperature and outlet temperature; T in , inlet temperature; T out , outlet temperature.
The pressure loss expressed by Euler number is formulated as follows: where ΔP, pressure difference; ρ, denotes the density; and u, flow velocity.

Grid convergence
For the spiral model of p=25mm, three kinds of fluid computing grids are discussed respectively, including rough grid, normal grid and fine grid. The total numbers of grids corresponding to each grid scheme are 10 million, 20 million and 40 million units.
The straight channel grid used in simulation is shown in Fig. 4. In reference [14], when the mass flux of 260mm straight channel is 325kg/m 2 s, the Eu number is 8.1401. The simulated Eu number is 8.2604, and the error is less than 2%.
The y+ values of three grids are mainly distributed between 0.1 and 1, and the grid at the entrance of different grids in Fig. 5.  In comparison of Nu with different grids numbers is shown that the heat transfer performance is very closely. As shown in comparison of Eu with different grids numbers, the pressure drop with the normal grid. So, the normal grid can meet the accuracy requirements.   It can be seen from the contours that the temperature and speed of LNG increase with the flow direction. The LNG temperature in the center of flow channel is lower and the flow speed is faster. The flow in straight channel, the channel of p=50mm and of p=25mm are more stable, while the flow in channel of p=16.67mm is worse.

Comparison of heat transfer flows in different shapes
Comparison of Nu and Eu numbers are shown in Fig. 9. According to the Fig. 9, with the grow in rotation angle, the heat transfer effect will also be enhanced. Among them, the heat transfer effect of the microchannel with pitch of 16.67mm is significantly better than other shapes, because of it have the higher Nu, while the pressure loss of the channel with pitch of 16.67mm is the worst with the higher Eu. In addition, the heat transfer effect of the microchannel with pitch of 50mm and 25mm are also significantly better than that of the straight channel. And that the pressure loss of straight channel, the channel with pitch of 50mm and of 25mm have little difference. In Fig. 12, with the increase of mass flux, Nu number increases while Eu number decreases, and both heat transfer and flow characteristic are improved. However, when mass flux exceeds 450kg/m 2 s, flow characteristic has little room for improvement.

Comparison of heat transfer flow conditions under different heat flux
In the case of different heat flux on the above and below the thermal conductive solid wall. The temperature contours under different heat flux have similar variation tendency along the flow direction (Fig. 13). As displayed in Fig. 14, the flow in different heat flux is stable. CFD simulation is carried out on the microchannel with 25mm pitch under the condition that mass flux at the inlet was 325kg/m 2 s. The pressure is 10.5MPa, and heat flux of 15000W/m 2 , 30000W/m 2 , and 45000W/m 2 are applied on the above and below the thermal conductive solid wall respectively. The comparison figure of Nu and Eu was obtained, as displayed in Fig. 15.

Conclusion
The flow and heat transfer of supercritical LNG flowing in a single spiral microchannel is simulated. According to the section 4, the channel with more turns shows better heat transfer characteristic. In the light of both flow and heat transfer performance, when the channel radius is 0.75mm, the channel with the pitch of 50mm obtain better heat transfer than straight channel, and has inapparent effect on the flow characteristic.

flow direction
In the spiral microchannel, the heat transfer performance is improved with the acceleration of flow velocity. With the increase of the heat flux of the upper and lower thermal conductive solid walls, the heat transfer performance changes little, and the flow characteristic in the heat flux of 30000W/m 2 have the lowest Eu number.