Proposed liquid CO 2 cycle refrigeration system for heat hazard control

. Liquid CO 2 can absorb heat via phase change and generates cryogenic CO 2 which can effectively solve the problem of thermal damage in the deep coal mining process. The CO 2 cycle refrigeration device system is designed to effectively cool down the working surface of the mine in which CO 2 is cyclically utilized. COMSOL Multiphysics simulation software is used to characterize the CO 2 cycle refrigeration system in mine of the heat transfers process between CO 2 and the air flow in tunnel. The results show that the reduction of steady air flow temperature reach at 8 o C in the tunnel by CO 2 cycle refrigeration system before the air flow into work face. we analyzed the influence of main parameters on refrigeration system and gets the results:1) The refrigeration system get higher cooling efficiency of cryogenic CO 2 when the ventilation velocity of local fan is increased, and the temperature of outlet CO 2 and steady air flow in tunnel has increased; 2) Increasing the CO 2 flow, the refrigeration effect of the system is enhanced obviously, but CO 2 refrigeration capacity utilization ratio is reduced; 3) Increasing the length of helical tube would led to use CO 2 refrigerating capacity more efficiently; 4) The cooling effect of the cooling system can be improved obviously by lowering the the CO 2 cooling temperature.


Introduction
With the increases in the depths of mines, hightemperature mines have been more and more now [1]. A high-temperature environment directly affects mineworkers in work condition and work efficiency, and is easy to cause insecure behavior and lack of enthusiasm [2]. In addition, it will seriously endanger the health of workers that working in a high temperature environment for a long time [3]. Therefore the heat harmful problem of high temperature mine needs to be solved urgently.
Various cooling technologies have been used to prevent heat hazards and ensure safe and efficient mining. Based on the principle of vortex tube energy separation, Ma Li et al. studied the portable cooling technology and method in high temperature mine for individual cooling [4]. Chen W et al. proposed a splittype vapor compression refrigerator [5], using air as the refrigerant, pneumatic refrigerator was cooled down in a local freezer and it combined with cooling clothing to cool the workers in high temperature working environments. Deng bao developed an air-to-air conditioning system with compressors, condensers, evaporators, etc., to cool down the working face [6]. However, due to the small heat capacity of the air, the thermal damage of the working face was not effectively controlled. SUN Xikui et al. studied the influencing factors of radiation cooling technology of ice water cooling, and established a refrigeration system that can produce 37.5t of ice and 62.7t cold water at 2 o C per hour [7]. Guo Pingye et al. has designed a refrigeration systems which used the cold water as the cold source, and established evaluation method for mine refrigeration system to optimizes the refrigeration system and improve the cooling effect of the working face [8,9]. However, this system needs a lot of cold water from the ground as refrigeration. Furthermore, there are impurities in the water, and the cooling system pipes form scales in the process of transporting water after a long run time, which increases the resistance of the water transportation and increase the system operation burden. The heat exchange efficiency is reduced because of the decrease in the heat conductivity of the pipes caused by the scale of water, which leads to a distinct reduction in the cooling effect of the refrigeration system. Due to its low price, accessibility, large heat capacity and being environmental friendly, the use of CO2 as a refrigerant has attracted widely attention in recent years [10]. Previous studies have focused on the transcritical cycle and the relationship between the storage environment of carbon dioxide and refrigeration capacity is conducted [11][12][13][14]. However, the processes of heat exchange between the CO2 and air flow or thermal diffusion in mine are less reported. These are the critical processes for the refrigeration system which ultimately determine the cooling effect for the work face, refrigeration efficiency and working cost of the refrigeration system. In this paper, a model of mine CO2 cycle cooling system is developed, and the parameters of 2 Cooling method of CO2 cycle refrigeration in mine The CO2 cycle refrigeration system is designed to cool down the working face of a high temperature mine. The system includes compressor, gas cooler, throttle device, monitoring device, evaporator, pipeline, heat-exchanger and local fan. The low temperature CO2 gas is performed as cold source and cyclically utilized. If the liquid CO2 is cooled in the heat exchange system directly, it will easy to the tube jam in pipelines for the liquid CO2 phase change produce dry ice. So the liquid CO2 phase transition is carried out in a special evaporator. In the evaporator device, as the pressure reduce, the liquid CO2 absorb heat by change of state and forms low temperature CO2, then the low temperature CO2 enters the helical tube. The helical tube is located in the local air duct and contact with the wind flow provided by the local fan in the tunnel, which makes the low temperature CO2 exchange heat with the wind flow to form the cold air flow. The cold air flow is sent to the working face in the last by the duct to create comfortable working conditions for workers. Fig.1 shows a schematic of CO2 cycle refrigeration system.
In the Fig. 2, after the heat exchange, the CO2 gas returns to the ground through the pipeline and forms liquid CO2 by compression system and gas cooler. Then the liquid CO2 through the pipeline into vaporization system which are installed on the underground and vaporizes into low-temperature CO2 gas. In order to ensure the safety of the system, a safety valve is installed on the ground of the conveying pipe. When the CO2 leaks occur in the pipe, the safety valve will be opened and the CO2 in the pipeline wiil be released into the air on the ground. The CO2 is circularly used in the CO2 cycle refrigeration system for sustainable cooling the work face in mine, which greatly reduces the cost of materials required for refrigeration. At the same time, the CO2 has more larger heat capacity than water and air resulting it has a better cooling effect at the same mass of water or air. The heat exchange between CO2 and air is the key for the refrigeration system. To analyze the process of heat exchange between CO2 and the air flow as well as the influence of main parameters on the cooling effect, a cooling model of CO2 refrigeration in mine is established in this paper.
where p is the pressure, Pa; ρ is the density, kg/m 3 ; A is the cross-sectional area, m 2 ; u is the tangential fluid velocity, m/s; dh is the pipe diameter, m; and F is the volume force, N/m 3 .
In Eq.(1), fD is the Darcy friction factor, which is related to the Reynolds number, pipe surface roughness and pipe diameter. It can be expressed as: where the B is defined as: C is defined as: where Re is the Reynolds number; e is the surface roughness; d is the pipe diameter, m.

Energy conservation equation
The energy equation of pipeline flow is used to describe the heat exchange and thermal diffusion between the fresh air flow and the CO2 pipeline.
where Cp is the heat capacity at constant pressure, J/(kg·K); T is the temperature, K; and k is the thermal conductivities, W/(m·K). Qwall is the a source of heat exchange between the wall and the surrounding wall, W/m; which can be can be expressed as: where Z is the circumference of the CO2 transportation pipeline, m; h is the overall coefficient of heat exchange, W/(m 2 ·K); Text is the external temperature of the CO2 conveying steel tube, K; and T is the temperature of CO2, K.

Physical model and numerical simulated
Based on the Section 2.1and 2.2, as a background of CO2 cycle refrigeration system in a mine, a simulated model of heat-exchange system in tunnel was constructed at a ratio of 1:1, as depicted in Fig. 3. The physical model was a cuboid tunnel with width of 4 m, height of 4 m and length of 30 m. The tunnel of inlet wind speed is set to 1m/s, and the temperature set as 313.15 K. There is a duct with a radius of 0.6 m in the tunnel, which is supplied by a local fan with wind air of 5 m/s. A helical steel tubes for conveying low temperature CO2 gas are installed inside the wind tubing for heat transfer between air flow and CO2, which is 3.6 m long, radius of the outer ring with 0.4 m, radius of the inner ring with 0. 05 m. The CO2 flow of transmission pipeline inlet is set to 4 m/s and the inlet temperature is set to 233.15 k. The physical model of heat-exchange system in tunnel mesh illustration consisting of 10,000 elements. To achieve the coupled process, all the governing equations can be implemented into and numerical solved by COMSOL multiphysics based on the finite element method, as determination of wind speed and temperature distribution under steady state, and the transient solver was properly selected. Fig. 3 The model of heat-exchange system in tunnel.

Simulated result
In the Fig. 4, we can see that a high speed wind flow is gradually mixed with the air flow after it comes out of the local wind duct, and the air speed is gradually distributed evenly in the tunntl. In the process of wind flow movement, the wind flow in the duct exchanges heat with the helical pipeline which used to carried CO2 , forming a large amount of cold air flow. Then, the high speed cold wind flow enters the tunnel gradually mixes with the fresh air flow and the wind flow tend to stable gradually, at the same time the temperature of wind flow decrease. Finally, the stable air flow temperatures in the tunnel are reduced obviously. We can see the temperature distribution in the tunnel in Fig. 5. Fig. 4 Velocity distribution in the model. With the increase of ventilation speed, the average heat exchange capacity of per cubic flow decreases, so the outlet wind temperature in the duct increases obviously. However, due to the increase of the total cold air flow, the temperature of cold air flow dilution by the hot air in tunnel is weakened, and the steady air flow temperature in the tunnel change little compared to low ventilation speed.
In Fig. 7(b) shows that with the increase of ventilation speed, the heat exchange between CO2 and air increase, and the temperature of CO2 at the outlet pipeline increases. However, the temperature of steady air flow increases because of the increase of cold air volume, but the air flow temperature in the tunnel is very small after dilution.
To obtain the functional relationship between the cooling effect in tunnel and the ventilation speed. The formula of steady air flow temperature and ventilation speed are fitted by using ORIGIN software. The expression of function relation between steady air flow temperature    Fig. 8 illustrates the effect of the CO2 flow l on the CO2 cycle refrigeration system. The local ventilation speed is set at 5 m/s, and the CO2 velocity in the pipeline is set at 2, 3, 4, 5 and 6 m/s, respectively. It can be seen from Fig. 8(a) that with the increase of CO2 velocity, the cooling capacity of the system increases, and the temperature of air flow decrease more obviously. When the CO2 velocity is 6 m/s, the temperature of cold wind in the duct reaches to 20.39 o C, and the steady air flow temperature in the tunnel decreases to 30.39 o C.

The effect of the CO2 flow
Compared with the increase of ventilation speed, the temperature descends more obvious by increases CO2 velocity. According to figure. 8(b), with the increase of CO2 flow velocity, the CO2 temperature in outlet pipeline decreases after heat transfer, so the cooling utilization ratio of CO2 decreases, but the air flow temperature in tunnel drop more obvious. The formula of steady air flow temperature and CO2 flow velocity are fitted by ORIGIN software. Then, the expressions of steady air flow temperature   Therefore, when refrigeration effect of the system improved by increase of the CO2 flow velocity, the length of heat transfer pipe or the air speed of ventilation should be increased in order to increase the cooling utilization ratio of cryogenic CO2 and the cost be saved. Local fan air speed (m/s) Air outlet temperature Air duct temperature Export CO 2 temperature F(X) (b) Fig. 8. The effect of CO2 flow l on the CO2 cycle refrigeration system, where the subfigure (a) correspond to the CO2 velocity in pipeline at l = 2, 3, 4, 5 and 6 m/s with the temperature curve of the central axis of the duct, respectively.

The effect of the helical tube length
The helical pipes are used to heat exchange with wind flow directly and transport CO2. It can be observed in the Fig. 9 that with the increase of the helical tube length, the heat exchange between CO2 and air flow increase results in the increase of CO2 temperature after heat exchange and more efficient use of CO2 refrigerating capacity. When the length of helical tube was increased from 1.8 m to 4.2 m, the temperature of CO2 in outlet pipeline increased from 22.01 to 23.25 o C due to the sufficiently heat exchange between CO2 and air flow. Meanwhile the air flow temperature at outlet of tuyere from 27.76 o C reduced to 25.50 o C and air temperature in the laneway is also reduced by 1.34 o C degrees centigrade than that of the original. Air outlet temperature Export CO 2 temperature Air duct temperature (b) Fig. 9. The effect of CO2 helical tube length on the CO2 cycle refrigeration system, where the subfigure (a) correspond to the CO2 helical tube length at L= 1.8, 2.4, 3.6 and 4.2 m with the temperature curve of the central axis of the duct, respectively.

The effect of CO2 the cooling temperature
In the evaporator, the heat is absorbed by liquid CO2 phase change which forms low temperature CO2, and then the low temperature CO2 enters the heat exchanger and exchange heat with the air flow to form the cold air flow. Before CO2 entering the heat exchanger, The CO2 temperature have an important impact on the CO2 cycle refrigeration system. We can be seen from the Fig.10, as the CO2 cooling temperature raise, the temperature of wind flow in duct, CO2 in outlet pipeline and steady air in tunnel are all increased because of CO2 refrigerating capacity decreases. When the CO2 cooling temperature is -45 o C, the temperature of steady air flow in tunnel drop 8 o C after passing through the CO2 refrigeration system. However, when CO2 cooling temperature is at the -30 o C, the steady air flow temperature is only reduced 6.72 o C. Thus, the temperature of CO2 formed by the phase transition of the liquid CO2 in the evaporator is an important parameter affecting the refrigeration system. Before CO2 enters the heat exchanger, the heat preservation measures should be taken on the CO2 pipeline to prevent cooling loss to improve the cooling effect of the cooling system on the working surface in the mine.

Conclusion
In this work, a fully coupled steady model of compositional gas flow in tunnel and the nonequilibrium heat transfer between the CO2 and the gas is developed to characterise the CO2 cycle refrigeration system in mine of the heat transfer process between CO2 and the air flow in the tunnel. Based on the results of this study, the following conclusions can be drawn: 1) The wind flow provided by the local fan is cooled by CO2 heat exchange system and then the high speed wind flow is gradually mixed with the air flow form low temperature steady cold air flow in tunnel. 2) When the the CO2 flow in pipeline is constant, the refrigeration efficiency would be improved with the increase of ventilation speed of local fan, and the temperature of steady air flow in tunnel and CO2 in outlet pipeline also has increased.
3) Under the condition of wind speed of local fan is constant, the increase of CO2 velocity is beneficial to enhance the refrigeration capacity of the system, but the efficiency of CO2 refrigeration will be reduce. Therefore, the length of the heat exchanger tube and air speed of the local fan should be increased when the CO2 flow velocity is increased. 4) With the increase of the helical tube length, the heat exchange recovery between CO2 and the air flow increase results in the temperature of CO2 in outlet pipeline increases and the use of CO2 refrigerating capacity more efficiently. 5) The temperature of cooling CO2 have an important impact on the cycle refrigeration system. The air outlet temperature, export CO2 temperature and air duct temperature would increased with the increase of the CO2 cooling temperature. To improve the cooling effect of the cooling system on the working surface in the mine, the heat preservation measures should be taken on the CO2 pipeline to prevent cooling capacity loss.