Assessing the Performance of a Cascaded Composite Phase Change Material Roadway Cooling System Against Heat Hazard from Sustainable Mine Geothermal Energy

Abstract

Sustainable mine geothermal energy causes high-temperature hazards in mine roadways, severely endangering miners’ lives. There is an urgent need to enhance research on the performance of composite phase change material (CPCM) roadway cooling systems, as they can effectively control ambient temperatures. However, existing research on CPCM roadway cooling system performance remains limited. This study innovatively establishes a numerical model for a novel cascade CPCM roadway cooling system and employs the control variable method to investigate the influence of multi-parameter regulation on system performance. The study reveals that the ring pipe radius ratio significantly impacts the system’s heat exchange efficiency and temperature distribution. The optimal comprehensive system performance is achieved at an annular tube radius ratio of 2:3, where the CPCM solid phase percentage for 89.03% and the average temperature of the monitoring surface decreases by 9.54 °C. Increasing the cascaded tube spacing enhances the overall cooling effect, but cooling efficiency diminishes when the spacing exceeds 0.5 m. The CPCM phase change temperature must align with the mine’s geothermal conditions, with CPCM utilization and cooling efficiency peaking at 25 °C. The air deflector structure effectively mitigates cooling lag in the lower roadway section. At an installation angle of 30°, the expansion distance of the lower low-temperature zone increased by up to 48.89% without compromising cooling efficiency in the upper roadway section, while also delaying the recovery rate of heat damage.

1. Introduction

Deep geothermal resources in mines represent a renewable energy source with substantial reserves and significant development potential [1,2,3]. Statistics indicate that geothermal resources within 3 km of coal mining areas nationwide amount to 379.539 billion tons of standard coal equivalent, equivalent to one-third of China’s proven coal reserves [4]. However, in current deep mining practices, mine geothermal energy remains underutilized. Instead, it contributes to elevated underground temperatures, creating severe thermal hazards that endanger miners’ lives and impede safe coal production [5,6,7]. Consequently, developing novel technical equipment capable of simultaneously achieving efficient cooling and thermal energy recovery has become an urgent need in deep coal mining. Addressing deep mine heat hazard mitigation, existing research primarily follows two technical pathways. The first involves artificial cooling technologies dominated by mechanical refrigeration. While effective at controlling temperatures, their high energy consumption costs and “heat transfer rather than utilization” approach struggle to meet green mining development requirements [8,9,10,11,12]. The second involves geothermal resource utilization technologies, such as ground-source heat pump systems utilizing mine drainage water. However, their application is constrained by hydrogeological conditions and the systems’ large scale, making efficient coordination with dynamically advancing mining operations difficult [13,14,15,16,17]. In recent years, phase change materials (PCMs) have demonstrated significant potential in mine heat hazard control due to their outstanding thermal management capabilities [18,19,20,21,22].

Scholars worldwide have conducted extensive research on the application of PCMs in mitigating mine heat hazards. Wang et al. [23] performed field experiments on a paraffin-based CPCM used in cooling uniforms for miners. Their results demonstrated that the CPCM exhibits excellent thermal performance in high-temperature mining environments, effectively reducing miners’ body temperature and improving thermal comfort. Mlynarczyk et al. [24] evaluated the cooling effect of rescue underwear containing PCM using heat flux measurements and thermal imaging. They found that wearing a PCM-equipped T-shirt increased heat flux by an average of 15 W/m² within the first 20–30 min, significantly lowering skin temperature. Mika et al. [25] conducted a field trial on using ice slurry as a PCM in central air conditioning systems for mines. Replacing chilled water with ice slurry in the existing pipeline system at the LW Bogdanka mine increased cooling capacity by 50% and noticeably reduced air temperature at the working face. Saleem et al. [26] numerically investigated a solar thermal panel with nanofluid and PCM for a mine office building, showing that larger tube diameter reduced panel temperature and PCM melting, providing 56% and 18% of hot water in hot and cold seasons, respectively. Wu et al. [27] explored the use of PCM in coal mine refuge chambers. Their study showed that PCM could maintain the indoor temperature at around 30 °C for 76 h, after which it gradually rose to 33 °C over 96 h. The material exhibited high heat absorption efficiency initially, which decreased over time. Choure et al. [28] reviewed heat transfer enhancement techniques for PCM-based thermal storage, showing that structural optimizations and high-conductivity additives like graphene significantly improve thermal performance, with graphene increasing conductivity by up to 245.7% at low loadings. Yuan et al. [29] proposed a novel hybrid cooling method combining latent heat thermal energy storage with pre-cooling of enclosures, analyzed using semi-analytical and numerical approaches. This method proved safe, stable, and reliable in special environments without power supply, while also reducing PCM usage and increasing the operational temperature difference. Wang et al. [30] investigated the heat transfer and phase change behavior of cementitious backfill mixed with PCM. The cooling release process displayed distinct temporal stages and unique spatial distribution patterns. Lower initial liquid fractions resulted in longer melting times and a slower average temperature rise. Martin et al. [31] investigated a method to enhance the thermal conductivity of phase change materials (PCMs) by compounding sodium acetate trihydrate with copper-based metal foam, developing a thermal energy storage device for waste heat recovery from internal combustion engines in mining machinery. They successfully achieved uniform heat storage throughout the entire volume of the storage unit, validating the feasibility of this composite structure. Roshani et al. [32] studied the enhancement of geothermal heat exchanger performance by incorporating microencapsulated PCM into the backfill material. Their results demonstrated a significant reduction in the outlet fluid temperature of the heat exchanger and an approximately 7.5% improvement in system efficiency.

In summary, although PCM technology has been applied in coal mine cooling, existing research has primarily focused on individual protection or heat extraction from goaf backfilling, lacking innovative solutions for managing thermal hazards in complex roadways. Particularly, the performance characteristics and operational parameters of CPCM roadway cooling systems, as well as their compatibility with mine geothermal conditions, remain poorly understood. Addressing this research gap, the authors propose a novel cascaded CPCM roadway cooling system. This system employs a multi-stage gradient heat exchange mechanism to simultaneously reduce roadway thermal environment temperatures and efficiently harness mine geothermal resources. This paper establishes a numerical model of the novel cascaded CPCM roadway cooling system to analyze how installation and operational parameters influence its cooling performance. Key parameters—including ring pipe radius ratio, cascade spacing, phase transition temperature, and air guide plate angle—are optimized to explore CPCM’s integrated potential in mine heat hazard control and thermal energy recovery. These findings provide novel insights for advancing the green development of deep coal and geothermal resources. They hold promise for transforming mine geothermal energy from a “thermal hazard” into a valuable “thermal resource,” thereby promoting the sustainable development of mine energy systems.

2. Materials and Methods

2.1. Overview of the Typical High-Temperature Mine

The Xinhu Coal Mine, situated in Guoyang County, Anhui Province, has a designed annual production capacity of 3.00 million tons. It employs a shaft-and-district development approach and is divided into three mining sections: the central, eastern, and northern areas. The first mining operation is conducted in the 81st mining district. Total coal resources at the mine amount to 818.66 million tons, mainly composed of coking coal, 1/3 coking coal, and fat coal. The mine is equipped with comprehensive systems including ventilation, drainage, cooling, and mechanical transportation. Among these, the cooling system was installed and commissioned during the mine’s construction phase to mitigate thermal hazards caused by deep geothermal heat. Geologically, the mine area is characterized by a predominantly monoclinal structure, accompanied by numerous faults and folds, resulting in moderately complex geological conditions. Hydrogeological conditions are classified as moderate. With an estimated service life of approximately 63 years, the Xinhu Coal Mine plays a significant role in supporting the regional energy supply.

This paper systematically collected temperature measurement data from exploration boreholes in the Xinhu Coal Mine. A total of 27 temperature measurement boreholes were gathered across the study area, including 5 approximate steady-state temperature measurement boreholes and 22 simple temperature measurement boreholes. Among these methods, simple temperature measurement offers a rapid and convenient approach for transient temperature assessment. This method involves recording a temperature curve before and after logging parameters, with an interval of 12 to 24 h between the two measurements, and is calibrated using the “three-point method.” For temperature measurements in quasi-steady-state boreholes, readings are taken at intervals of 12, 12, 24, and 24 h sequentially, continuing until the total measurement duration reaches 72 h. The geographical location of the Xinhu Coal Mine, along with its geothermal monitoring data, is presented in Figure 1.

The temperature measurement depth of each borehole in the Xinhu Coal Mine ranges from 640 to 1480 m, with bottom-hole temperature values ranging from 34.05 to 57.57 °C. The temperature correction data from each borehole were used to plot the temperature curves, as shown in Figure 1b. The study found that temperature increases with depth, showing a good linear trend and characteristics of conductive heating. Within the shallow range of 300 m, the temperature at the same depth is highest in Borehole 3-1 and relatively low in Borehole 1-1. Beyond the 300 m depth range, the temperature at the same depth is highest in Borehole 24-5, while the lowest temperatures are generally found in Boreholes 14-1 and 16-5. Based on the geothermal monitoring data, the depth and temperature of the constant temperature zone in the Xinhu Coal Mine were determined to be 30 m and 17.1 °C, respectively. The geothermal gradient at depths below 200 m was calculated to be between 1.7 and 3.5 °C/hm, which falls within the normal geothermal gradient range. In contrast, within the shallow depth range above 200 m, the geothermal gradient distribution is more scattered and generally higher than that below 200 m. Additionally, according to the cloud map of temperature changes at the −700 m level, the temperature variation ranges from 33.00 to 39.27 °C, with an average ground temperature of 35.57 °C. The maximum ground temperature is 39.27 °C, located near Borehole 24-5. High-temperature areas are mainly concentrated around the perimeter of the mining area, while the central and northeastern parts are low-temperature zones.

In summary, due to heat release from surrounding rocks and mine water, the Xinhu Coal Mine faces severe thermal hazards. There is an urgent need to develop and optimize the performance of a new cascaded composite phase change material roadway cooling system to address the thermal challenges posed by this high-temperature environment in the Xinhu Coal Mine.

2.2. Preparation and Performance Analysis of CPCM

The CPCM in this study was prepared using PCM and expanded graphite (EG). The DSC (Differential Scanning Calorimetry) curves and meso-structure of all materials are shown in Figure 2. The paraffin adopted in this article is semi-refined paraffin wax with the quality index of 58#, the oil content is 1.8% (mass fraction), and the implementation standard is GB/T 254–2022 [33], purchased from Henan Baihuali Chemical Products Co., Ltd. (Zhengzhou, China). During the heating process, the melting temperature of the PCM was 31.93 °C, with a latent heat of fusion of 195.80 J/g. Two distinct peaks appeared at 40.33 °C and 57.24 °C. During the cooling process, the solidification temperature was 59.21 °C, with a latent heat of solidification of 194.50 J/g and a peak temperature of 55.00 °C. Furthermore, the thermal conductivity of the PCM was 0.29 W/(m·K), necessitating thermal conductivity enhancement using EG. EG is 80 mesh before expansion, carbon content is 90–99%, and the standard implemented is GB/T 10698–2023 [34], purchased from the Qing Dao Teng Sheng Da Tan Su Ji Xie Co., Ltd. (Qingdao, China). The preformed CPCM prepared by vacuum adsorption was tested for its physicochemical properties, such as thermal conductivity and phase change latent heat, with these parameters simultaneously incorporated into the Phase Change Modules in COMSOL Multiphysics 6.2 software for numerical simulation of the phase change behavior of the CPCM. Measurements of its thermal conductivity and specific heat capacity yielded values of 5.30 W/(m·K) and 3.74 kJ/(kg·K), respectively. These values are 18.23 times and 1.12 times those of the pure PCM, indicating a significant enhancement in thermal conductivity. Additionally, the DSC curve of the CPCM showed a melting temperature of 33.58 °C during heating, with a latent heat of fusion of 148.37 J/g, which is 75.78% of the latent heat of the pure PCM. During cooling, the solidification temperature was 58.69 °C, with a latent heat of solidification of 149.08 J/g, corresponding to 76.65% of the pure PCM’s value. Thus, the CPCM exhibits favorable latent heat properties. SEM (Scanning Electron Microscope) images of the CPCM reveal that it maintains a worm-like morphology with clear outlines and abundant surface wrinkles, indicating that the PCM was fully adsorbed into the porous structure of the EG, demonstrating effective adsorption.

2.3. CPCM Roadway Cooling System and Numerical Model Construction

Although the non-cascaded PCM system benefits from a simple structure and low initial cost, its thermal management efficiency is constrained by a narrow operating temperature range—stemming from a single phase-change point—and poor thermal resistance matching with the coolant. These limitations result in a low overall heat storage density, which makes the system inadequate for handling the wide-ranging and intense thermal loads typical of downhole environments. To address these shortcomings, the cascaded PCM system has been introduced. By incorporating multiple phase-change temperatures, it enables efficient, stepwise cooling of the coolant across a broad temperature span, thereby markedly enhancing the system’s overall thermal utilization and environmental adaptability. The novel cascaded CPCM roadway cooling system primarily consists of a multi-stage double-orifice cooling tube module and a cooling circulation module. The multi-stage double-orifice cooling tube module is arranged along the airflow direction in the roadway and comprises multiple cooling units (Unit 1#~Unit n#) with progressively lower phase change temperatures. Similarly, within each cooling unit, double-orifice cooling tubes (filled with CPCM) are arranged along the airflow direction, also featuring progressively lower phase change temperatures. When the CPCM inside a cooling unit completes melting and heat absorption, the cooling circulation module is activated. The cooling medium begins to flow within the inner cooling tube, absorbing the latent heat from the CPCM. This heat is then carried to a heat recovery unit, enabling waste heat utilization. By utilizing multi-stage gradient heat exchange, this roadway cooling system reduces the temperature of the thermal environment in the roadway. It addresses issues associated with traditional cooling technologies, such as low heat exchange efficiency and poor spatial adaptability. The working mechanism of the system is illustrated in Figure 3.

A three-dimensional heat transfer model of the novel cascaded composite phase change material roadway cooling system was developed using the Phase Change, Laminar Flow, and Heat Transfer in Porous Media modules in COMSOL Multiphysics 6.2 software. This study selected one cooling unit for simulation, constructing a four-stage double-orifice cooling tube array. The main components of the double-orifice cooling tubes consist of multi-stage annular tubes and horizontal ring tubes, connected to bottom multi-stage columns and left-right connecting pipelines. Additionally, the roadway and surrounding rock were modeled. The roadway cross-section was designed as a straight-wall semi-circular arch with a base width of 5 m, wall height of 1.5 m, and semi-circular arch diameter of 5 m. The surrounding rock domain was set to 5.5 times the roadway height to minimize boundary condition effects, resulting in a 22 m high surrounding rock zone. Consequently, the model dimensions were set to 24 m (length) × 14.5 m (width) × 22 m (height), with a total of 628,671 mesh elements. The simulation time step and total duration were set to 1 min and 120 h, respectively. The numerical model configuration is shown in Figure 3.

2.4. Control Equations and Model Parameter Settings

When constructing the numerical model of the novel cascade composite phase change material roadway cooling system, the following assumptions are made to simplify the model: ignore the natural convection effect caused by the phase change in the PCM; ignore the shape change caused by temperature variation and the contact thermal resistance between different materials; assume that the surrounding rock is isotropic, and the heat-carrying medium is an incompressible fluid. The control equations of the model are as follows:

$$\left\{\begin{array}{l}(\alpha {\rho }_s{C}_s+\epsilon {\rho }_f{C}_f){\displaystyle \frac{\partial T}{\partial t}}+{\rho }_f{C}_{f}u·\nabla T+\nabla ·[-(\alpha k_s+\epsilon k_f)\nabla T]=Q\\ {\rho }_s=\theta {\rho }_s\prime +(1-\theta ){\rho }_l\prime \\ H={\displaystyle \frac{\theta {\rho }_s\prime H_s+(1-\theta ){\rho }_l\prime H_l}{\theta {\rho }_s\prime +(1-\theta ){\rho }_l\prime }}\\ k_s=\theta k_s\prime +(1-\theta )k_l\prime \\ C_s={\displaystyle \frac{\theta {\rho }_s\prime C_s\prime +(1-\theta ){\rho }_l\prime C_l\prime }{{\rho }_s}}+L{\displaystyle \frac{d{\varphi }_s}{dT}}\\ (\rho A{C}_p){\displaystyle \frac{\partial T}{\partial t}}+\rho A{C}_{P}v·\nabla T+\nabla ·(-\mathit{Ak}\nabla T)={\displaystyle \frac{1}{2}}{f}_D{\displaystyle \frac{\rho A}{2d_h}}\left|v\right|v^2+Q+Q_{\mathit{wall}}\partial \\ {\rho }_f{C}_f({\displaystyle \frac{\partial T}{\partial t}}+u_f·\nabla T)=\nabla ·(k_f\nabla T)+\mu [\nabla u_f+{(\nabla u_f)}^T]+Q\end{array}\right.$$

(1)

where ρ_s is the density of the porous medium, kg/m³; ρ_f is the fluid density, kg/m³; C_s is the constant pressure heat capacity of the porous medium, J/(kg·°C); u is the Darcy velocity, m/s; k_s is the thermal conductivity of the porous medium, W/(m·K); k_f is the thermal conductivity of the fluid, W/(m·K); ε is the porosity of the porous medium, %; Q is the heat source, W/m³; α is the solid volume fraction, %; ρ_s′ is the density before phase change, kg/m³; ρ_l′ is the density after phase change, kg/m³; k_s′ is the thermal conductivity before phase change, W/(m·K); k_l′ is the thermal conductivity after phase change, W/(m·K); L is the latent heat of phase change, J/kg; ϕ_s is the mass fraction, %; ρ is the density of the heat-transfer medium, kg/m³; C_P is the heat capacity of the heat-transfer medium, J/(kg·°C); k is the thermal conductivity of the heat-transfer medium, W/(m·K); v is the tangential velocity of the heat-transfer medium, m/s; f_D is the viscosity coefficient of the heat-transfer medium, Pa·s; d_h is the mean hydraulic diameter, m; Q_wall is the heat exchange source at the pipe wall, W/m³; u_f is the fluid velocity, m/s; μ is the dynamic viscosity, Pa·s.

The settings of model parameters are directly related to the accuracy of the simulation results. The boundary temperature of the surrounding rock serves as a heat source for the roadway, and its value directly affects the cooling effect of the roadway. If the boundary temperature of the surrounding rock is high, the temperature difference between the cooled roadway and the surrounding rock is significant, leading to rapid heat replenishment. As a result, the temperature of the cooled roadway recovers quickly, resulting in a poor cooling effect. Similarly, the temperature of the cooling medium also directly influences the roadway’s cooling effect. If the cooling medium temperature is low, the temperature difference between the cooling medium and the interior of the roadway is greater, allowing the cooling medium to absorb more heat and thereby improving the roadway’s cooling effect. However, a lower cooling medium temperature increases the cost of roadway cooling. In order to accurately obtain the boundary temperature T_BR and initial temperature T_SR of the surrounding rock, this paper determines the temperature measurement point below the Xinhu Colliery mine based on the geothermal monitoring data, which is the south wing track main gallery (at a depth of 41.3 m), with a measurement height of −956 m, and a temperature of 40 °C. That is, the boundary temperature and initial temperature of the surrounding rock are set at 40 °C. The temperature of the cooling medium is determined to be 10 °C based on the previous research of the author’s team. In addition, all other data are obtained from laboratory tests. The performance parameters of the CPCM are shown in Section 2.2, and the other parameters are set as shown in

Table 1. Model parameter settings.

Parameter	Value	Parameter	Value	Parameter	Value	Parameter	Value
TBR/TSR	40 °C	kSR	2.33 W/(m·K)	ρSR	2400 kg/m3	Cp-SR	1.07 kJ/(kg·K)
kair	0.02 W/(m·K)	ρair	1290 kg/m3	Cp-air	0.72 kJ/(kg·K)

.

2.5. System Parameters and Research Protocol Design

This paper primarily investigates installation and operational parameters such as the fourth-stage phase change temperature T_CPCM-4, the cascaded tube spacing L₁, the annular tube radius ratio R_r of the inner pipe radius to the outer pipe radius, and the intermittent ratio n (the ratio of cooling pump stop time to operation time).

Based on previous research findings from the author’s team, the optimal installation parameters for the graded CPCM roadway cooling system have been determined as follows: the composite phase change temperature T_CPCM-4 of 25 °C, the cascaded tube spacing L₁ of 0.5 m, the annular tube radius ratio R_r of 2:3, the intermittent ratio n of 1:3, and the gradient temp. difference ΔT_CPCM-4 of 2 °C. On this basis, a control variable method was employed to conduct the study. Air deflectors were installed with reference to the height of the horizontal loop pipes at the bottom of the second and fourth stages, and the influence of the installation angle of the air deflectors on the cooling performance of the system was analyzed. A total of 13 single-factor study schemes were designed, as detailed in

Table 2. Single factor research program.

Schemes	Rr	L1/m	TCPCM-4/°C	n	Air Deflector Angle
C1	1:3	0.5	25	1:3	/
C2	1:2	0.5	25	1:3	/
C3	3:5	0.5	25	1:3	/
C4	2:3	0.3	25	1:3	/
C5	2:3	0.4	25	1:3	/
C6	2:3	0.6	25	1:3	/
C7	2:3	0.5	27	1:3	/
C8	2:3	0.5	29	1:3	/
C9	2:3	0.5	31	1:3	/
C10	2:3	0.5	25	1:3	/
C11	2:3	0.5	25	1:3	30°
C12	2:3	0.5	25	1:3	60°
C13	2:3	0.5	25	1:3	90°

.

2.6. Evaluation Indicators and Evaluation Method Design

This study investigates the regulatory patterns of roadway temperature fields under varying operating conditions, focusing on the influence of annular tube radius ratio, cascaded tube spacing, the fourth-stage composite phase change temperature, and air deflector angle. Evaluation metrics include the average outlet temperature T_ave of the cooling cycle module, the solid phase percentage φ of the CPCM, and the average temperature of monitoring surface T_EP. The monitoring surface is positioned 0.9 m downstream of the fourth cascade phase transition module. Additionally, since the air deflector angle primarily addresses cooling lag effects in the lower roadway section, the aforementioned evaluation metrics are not fully applicable. Therefore, an additional monitoring line was introduced with endpoint coordinates (0, −5.5, 2.6) and (0, 9, 2.45). The monitoring line temperature average T_EPL (representing the average temperature within the space behind the guide plate), the cooling zone extension distance S_ED in the lower roadway section, and T_EP were adopted as evaluation metrics to assess the influence of air deflector angle on the cooling system’s performance. The layout plan for impact indicator parameters on monitoring surfaces and monitoring lines is shown in Figure 4 below. The operational performance evaluation metrics and methodology for the cascaded CPCM roadway cooling system are detailed in

Table 3. Evaluation index and evaluation method of operation performance of cooling synergistic heat recovery device.

Evaluating Indicator	Unit	Evaluating Method
Average outlet temperature Tave	°C	Higher Tave values indicate stronger cooling capability against the thermal environment and more significant heat recovery effect.
CPCM solid phase percentage φ	%	A greater range of φ values indicates a higher utilization rate of CPCM phase change.
Average temperature of monitoring surface TEP	°C	Lower TEP values indicate a larger cooling range within the roadway environment, suggesting better overall temperature control performance.
Lower low-temp zone expansion distance SED	m	Larger SED values indicate more significant cooling improvement effects of the air deflector on the lower part of the roadway environment.
Average temperature of monitoring line TEPL	°C	Smaller TEPL values indicate a greater impact of the air deflector on the roadway temperature field behind the air deflector.

.

3. Results and Analysis

3.1. Annular Tube Radius Ratio

The influence of different annular tube radius ratio on the average outlet temperature (T_ave), the solid phase percentage of CPCM (φ), and the average monitoring surface temperature (T_EP) are shown in Figure 5.

The study found that the average outlet temperature (T_ave) increases with the annular tube radius ratio, but the rate of increase gradually diminishes. When the radius ratio increases from 1:3 to 2:3, T_ave at the end of the operation period rises from 10.67 °C to 10.85 °C. During the cooling phase, T_ave decreases as the pump operation time increases. The initial T_ave value increases with a larger radius ratio. The initial T_ave values for schemes C1 (1:3), C2 (1:2), C3 (3:5), and C10 (2:3) are 10.77 °C, 11.01 °C, 11.11 °C, and 11.14 °C, respectively. For a radius ratio of 1:2, T_ave during the pump operation phase on the second day decreases from an initial 11.01 °C to a stable value of approximately 10.80 °C, representing a change of about 0.21 °C. For a radius ratio of 3:5, the initial T_ave value on the second day is closest to that of the 2:3 ratio scheme. Although its cooling rate is slightly slower than the 2:3 scheme, both achieve similar stable temperatures, which are superior to the C1 and C2 schemes. Both the solid phase percentage of the CPCM (φ) and its utilization rate improve as the annular tube radius ratio increases. A smaller radius ratio is less conducive to the solid-state recovery of the CPCM, thereby hindering the cooling effect. Conversely, a larger radius ratio results in a more stable phase change cycle and higher phase change utilization. On the first day of operation, with a radius ratio of 1:3, the CPCM continuously absorbs ambient heat, and its φ stabilizes after reaching 67.42% at the end of the day. As the radius ratio increases from 1:2 to 2:3, the recovery rate of φ for the corresponding material also increases, rising from 80.25% to 89.03%. With a radius ratio of 2:3, the phase change cycle becomes more stable over time, and the rate of solid-phase change accelerates during the system operation period. The average temperature of the monitoring surface (T_EP) decreases as the annular tube radius ratio increases. When the ratio is 1:3, the temperature difference at the monitoring surface is the smallest, indicating the poorest cooling effect. As the ratio increases, both the maximum and minimum T_EP values across the different schemes decrease. The improvement in cooling effectiveness becomes less significant when the radius ratio increases from 3:5 to 2:3. For a ratio of 3:5, TEP after 5 days of operation is 31.06 °C, representing a temperature reduction of 8.94 °C. For a ratio of 2:3, T_EP increases gradually over the operation time, reaching 30.46 °C at the end of the period. This scheme achieves the largest temperature reduction at the monitoring surface, amounting to 9.54 °C.

To further analyze the influence of the annular tube radius ratio on the temperature field within the roadway space, spatial distribution nephograms of the roadway temperature field under different annular tube radius ratios were generated based on numerical simulation results, as shown in Figure 6.

The study found that during the cooling phase, the high-temperature thermal hazard was alleviated to varying degrees under all schemes compared to the initial condition. At 102 h of system operation, as the annular tube radius ratio increased, the average temperature within the roadway decreased progressively. The cooling zone was primarily concentrated in the upper part of the roadway. The scheme with a radius ratio of 2:3 resulted in the most extensive cooling coverage, indicating that a larger radius ratio favors the diffusion of low-temperature air. By 110 h, the temperature in the lower part of the roadway decreased with increasing radius ratio. The average temperature in the upper extended zone was 33.57 °C, while it was 37.82 °C in the lower extended zone. At 120 h, the extent of the low-temp zone in the upper roadway expanded further with larger radius ratios. For the 2:3 ratio scheme, the average temperature in the upper zone was 31.90 °C, representing the greatest temperature reduction and the most far-reaching cooling extent among all schemes. By comparing the temperature distribution nephograms of the different schemes at 102 h, 110 h, and 120 h, the following patterns were identified: In the initial stage of pump operation, high-temperature areas were predominantly concentrated in the lower roadway, while the thermal hazard in the upper part had been preliminarily mitigated. As pump operation time increased, the average temperature of the temperature field in the lower roadway decreased, and the low-temperature zone expanded further, eventually reaching about half the extent of the upper roadway’s expansion. Subsequently, until the pump stopped, the diffusion rate continuously decreased, with the primary direction of diffusion being from the upper to the lower roadway. A larger annular pipe radius ratio resulted in a faster diffusion rate from the upper to the lower roadway. In summary, the influence of the annular tube radius ratio on the cooling effect can be concluded as follows: A larger radius ratio leads to faster cooling in the upper roadway and a quicker final migration rate of the low-temperature zone toward the lower roadway. The cooling hierarchy in the upper section is superior to that in the lower section, with the lower expansion range being approximately half that of the upper expansion range.

In summary, the influence of the annular tube radius ratio on the performance of the roadway cooling system is as follows: the average outlet temperature increases with a larger radius ratio; the solid phase percentage and utilization rate of the CPCM improve as the radius ratio increases, with a larger ratio resulting in a more stable phase change cycle; the average temperature of the monitoring surface decreases as the radius ratio increases; within the roadway space temperature field, a larger radius ratio leads to a lower average roadway temperature and a wider cooling range, but beyond a certain point, the enhancement of the cooling effect diminishes.

3.2. Cascaded Tube Spacing

The influence of different cascaded tube spacing on the average outlet temperature (T_ave), the solid phase percentage of CPCM (φ), and the average monitoring surface temperature (T_EP) are shown in Figure 7.

The study found that the average outlet temperature (T_ave) increases with larger cascaded tube spacing. While a greater cascaded tube spacing accelerates the rate of change in T_ave, it also intensifies cooling challenges and degrades the heat exchange efficiency of the inner pipe. As system operation time extends, T_ave gradually decreases by the end of the pump operation phase. At the conclusion of the pump operation on the third day, when the cascaded tube spacing increases from 0.3 m to 0.6 m, T_ave rises correspondingly from 10.80 °C to 10.95 °C. The final T_ave values at the end of the operation period for cascaded tube spacing of 0.3 m, 0.4 m, 0.5 m, and 0.6 m are 10.79 °C, 10.84 °C, 10.85 °C, and 10.94 °C, respectively. The T_ave curve demonstrates relative stability when the cascaded tube spacing is set at 0.4 m or 0.5 m. The utilization rate of the CPCM improves with increased cascaded tube distance. When the pump is idle, a smaller cascaded tube distance reduces the system’s capacity to absorb ambient heat, thereby diminishing the cooling effect. During the pump idle period on the first day, the CPCM achieves its highest utilization rate at a cascaded tube distance of 0.6 m, with a range of φ 36.42%. Conversely, the phase change utilization is lowest at a 0.3 m distance, where the range of φ is only 27.64%. When the pump operates on the first day, the heat exchange rate within the pipe is fastest at a 0.6 m cascaded tube distance, allowing the φ of the CPCM to recover to approximately 89.9% after pump operation ceases. However, at a 0.3 m cascaded tube distance, the final recovery degree of the CPCM at the end of days 1–5 surpasses that of other schemes, with φ reaching a maximum of about 90.42%. The average monitoring surface temperature (T_EP) initially increases and then decreases as the cascaded tube distance enlarges, peaking at a distance of 0.5 m. During the initial system operation phase, the influence of underground airflow is significant, and a smaller cascaded tube distance enhances natural ventilation effectiveness. At a 0.3 m cascaded tube distance, T_EP measures 31.82 °C at the start of pump operation on the first day. As pump operation continues, T_EP drops to 29.74 °C when the pump stops. The ultimate T_EP after 5 days of operation is lowest at 29.69 °C for this configuration, indicating a marginally superior long-term cooling performance compared to other schemes. Consequently, the cooling effect weakens with increased cascaded tube distance. The average T_EP reductions for cascaded tube distances of 0.3 m, 0.4 m, 0.5 m, and 0.6 m are 10.31 °C, 9.85 °C, 9.55 °C, and 9.71 °C, respectively.

To further analyze the influence of the cascaded tube distance on the temperature field within the roadway space, spatial distribution nephograms of the roadway temperature field under different cascaded tube distance were generated based on numerical simulation results, as shown in Figure 8.

The study found that as the cascaded tube distance increases, the temperature diffusion pattern within the roadway at different time intervals demonstrates a characteristic where the upper section cools preferentially. The cooling initiation in the lower roadway section lags behind the upper section, and the cooling rate is slower in the lower part. At 102 h of operation, the spatial temperature distribution patterns in the roadway are nearly consistent across different schemes. For cascaded tube distances of 0.3 m, 0.4 m, 0.5 m, and 0.6 m, the average temperatures in the upper extended zone are 33.25 °C, 32.83 °C, 31.90 °C, and 32.25 °C, respectively, with a minimum temperature difference of only 0.35 °C. By 110 h, the low-temperature zone in the upper roadway section further expands, and the extent of the lower expansion zone increases with larger cascaded tube distances. The maximum temperature differences between various schemes are primarily concentrated near the cooling inner pipes in the lower roadway section. As the cascaded tube distance increases, the average temperature recorded at the 8th hour of pump operation on the fifth day progressively decreases. At a cascaded tube distance of 0.6 m, the average temperature near the inner pipe of the fourth stage is approximately 10 °C. In comparison, when the cascaded tube distance is increased by 0.1 m from 0.5 m, the extent of the lower expansion zone does not show a positive correlation with the increase, and the average temperatures within the lower expansion zones are nearly identical. At 120 h, the temperature variation patterns in the roadway space across different schemes align with those observed at the 8th hour of pump operation on the fifth day. As the cascaded tube distance increases, the low-temperature zone continues to expand. A limiting diffusion range appears at a cascaded tube distance of 0.5 m; beyond this point, further increases in the row distance result in negligible changes in temperature differential. In conclusion, the influence of cascaded tube distance on the cooling effect can be summarized as follows: As the cascaded tube distance increases, the upper roadway section experiences prioritized and continuous cooling. For the lower roadway section, a limiting diffusion range exists as the cascaded tube distance increases from 0.3 m to 0.5 m. The optimal cooling performance is achieved when the cascaded tube distance is 0.5 m.

In summary, the influence of cascaded tube distance on the cooling system performance is as follows: the average outlet temperature increases with larger row distances; the solid-phase utilization rate of CPCM improves with increased row distance; the average monitoring surface temperature initially rises and then decreases with greater row distance, while the cooling effect weakens; and within the roadway temperature field, the upper section cools preferentially, reaching its maximum diffusion range at a row distance of 0.5 m. Although increasing the cascaded tube distance generally enhances the overall cooling effect of the roadway, it should not be excessively large to avoid impeding heat exchange in the cooling system.

3.3. Phase Change Temperature

The influence of different T_CPCM-4 on the average outlet temperature (T_ave), the solid phase percentage of CPCM(φ), and the average monitoring surface temperature (T_EP) are shown in Figure 9.

The study found that the average outlet temperature (T_ave) shows little variation with increasing CPCM phase change temperature. In the initial stage of system operation, heat exchange within the tube is significant, and T_ave changes rapidly. When the CPCM phase change temperature is 25 °C, T_ave decreases from 11.14 °C to 10.85 °C during the pump operation phase on the second day, a reduction of approximately 0.29 °C. When the CPCM phase change temperature increases from 25 °C to 31 °C, T_ave exhibits minor fluctuations before heat exchange, rising from 11.14 °C to 11.2 °C before stabilizing. After heat exchange occurs, T_ave remains stable, ultimately settling between 10.85 °C and 10.87 °C for all cases. The solid phase percentage of the CPCM (φ) increases with higher CPCM phase change temperatures, while the phase change utilization rate decreases. The cooling effect is most significant when the CPCM phase change temperature is relatively low, as the phase change occurs most fully under these conditions. At a CPCM phase change temperature of 25 °C, the range of φ is largest, with a maximum of 33.65% and a minimum of 30.78%, indicating the highest overall phase change utilization rate. Within the 5-day operation period, φ can recover up to 89.23%. In contrast, at a CPCM phase change temperature of 31 °C, the range of φ is significantly smaller, with a maximum of 12% and a minimum of 11.09%, and φ can recover up to 99.64%. However, compared to other schemes, the comprehensive utilization rate of the CPCM is lowest at this temperature. As the CPCM phase change temperature increases, the range of the average monitoring surface temperature (T_EP) becomes larger. Lower CPCM phase change temperatures lead to faster T_EP reduction rates, but the final stabilized T_EP after cooling is higher. The maximum temperature differences of T_EP within the fifth day for CPCM phase change temperatures of 25 °C, 27 °C, 29 °C, and 31 °C are 2.69 °C, 3.38 °C, 3.85 °C, and 4.46 °C, respectively. A lower CPCM phase change temperature results in a more balanced T_EP distribution. At a CPCM phase change temperature of 25 °C, T_EP after 5 days of operation is 30.46 °C, with a cumulative temperature reduction of 9.54 °C. The difference in the final T_EP compared to the case with a CPCM phase change temperature of 31 °C is only 0.18 °C.

To further analyze the influence of the CPCM phase change temperatures on the temperature field within the roadway space, spatial distribution nephograms of the roadway temperature field under different CPCM phase change temperatures were generated based on numerical simulation results, as shown in Figure 10.

The study found that during the cooling phase, the average temperature in the upper expansion zone of the roadway increases further as the CPCM phase change temperature rises. At 102 h of operation, the average temperatures in the upper expansion zone for CPCM phase change temperatures of 25 °C, 27 °C, 29 °C, and 31 °C were 32.61 °C, 32.81 °C, 33.09 °C, and 33.38 °C, respectively. The maximum temperature difference in the lower expansion zone among the different schemes was only 0.01 °C. Combined with the solid phase percentage variation curve, under identical geothermal geological conditions, a higher CPCM phase change temperature corresponds to a lower utilization rate of the composite phase change material. By 110 h of operation, the average temperature at the inner pipe of the fourth stage continuously decreased as the CPCM phase change temperature increased, reaching a trough of 10.39 °C at a CPCM phase change temperature of 29 °C. The temperature diffusion effect within the upper expansion zone was superior to other schemes at this temperature. At 120 h of operation, the spatial temperature variation patterns in the roadway were consistent across the different groups. The average temperature in the upper expansion zone was 33.16 °C, while it was 37.76 °C in the lower expansion zone. The solid phase percentage was highest at a CPCM phase change temperature of 31 °C and lowest at 25 °C. However, the phase change utilization rate was highest at 25 °C, indicating the most complete phase change occurrence. Excessively high CPCM phase change temperatures are not well-suited to the mine’s geothermal geological conditions, resulting in extremely low phase change utilization. Therefore, the influence of different CPCM phase change temperatures on the cooling effect can be summarized as follows: The selection of the CPCM phase change temperature should be based on the geothermal geological conditions of the specific mine. A CPCM phase change temperature of 25 °C provides the best cooling effect and the most complete phase change. As the CPCM phase change temperature increases, the utilization rate of the composite phase change material decreases.

In summary, the impact of CPCM phase change temperature on the CPCM roadway cooling system performance is as follows: A higher CPCM phase change temperature leads to a lower stabilized value of the average outlet temperature. The solid phase percentage increases with rising CPCM phase change temperature, while the phase change utilization rate decreases. The range of the average monitoring surface temperature increases with higher CPCM phase change temperatures, with the best cooling effect and maximum cumulative temperature reduction achieved at 25 °C. Within the roadway spatial temperature field, the selection of CPCM phase change temperature must consider the mine’s geothermal conditions. A temperature of 25 °C yields the optimal cooling effect, whereas higher temperatures reduce the utilization rate of the composite phase change material and deteriorate compatibility with the geothermal conditions.

3.4. Air Deflector Angle

The influence of different air deflector angle on the lower low-temp zone expansion distance (S_ED), the average temperature of the monitoring line (T_EPL), and the average monitoring surface temperature T_EP are shown in Figure 11.

The study found that the growth rate of the lower low-temperature zone expansion distance (S_ED) initially increases and then decreases as the air deflector angle increases, peaking at a deflector angle of 30°. When no deflector was installed (C10), the roadway remained in a high-temperature environment at the 102 h cooling mark. As pump operation time extended, the S_ED eventually expanded to 4.52 m. After installing deflectors (C11, C12, C13), the cold airflow path was altered. In the initial cooling stage, the low-temperature zone, obstructed by the deflector structure, first concentrated in the space beneath the deflector, significantly reducing the local temperature and improving the high-temperature environment. Subsequently, the low-temperature zone began to expand, with an earlier expansion onset correlating to larger deflector angles. At a 90° deflector angle, the lower roadway’s low-temperature zone started expanding at 105 h, with an initial S_ED of 3.20 m. The maximum expansion distances for the lower low-temperature zone in groups C11, C12, and C13 were 6.73 m, 5.27 m, and 5.11 m, respectively. After deflector installation, the cooling effect in the space behind the deflector significantly enhanced. When deflector angles were 30°, 60°, and 90°, the T_EPL values were all lower than those of the C10 scheme (no deflector). While improving the delayed cooling effect in the lower roadway, the cooling amplitude in the upper roadway further increased. The T_EPL decreased first and then increased with larger deflector angles, reaching the maximum cooling at a 30° deflector angle. At the end of pump operation on the first day, the T_EPL values for schemes C10 (no deflector), C11 (30°), C12 (60°), and C13 (90°) were 32.16 °C, 29.99 °C, 31.01 °C, and 31.63 °C, respectively. By the end of pump operation on the third day, the cooling rate of T_EPL continuously decreased with increasing deflector angles, with the C13 group showing only a 0.45 °C improvement compared to the C10 group. At a 30° deflector angle, after 120 h of system operation, the T_EPL was 29.84 °C, indicating a significantly enhanced cooling effect compared to the setup without a deflector, with a total reduction increase of 2.35 °C by the end of the operation period. The cooling effect at the monitoring surface location improved with larger deflector angles. After deflector installation, the flow path of the low-temperature zone changed, leading to a further reduction in the average monitoring surface temperature (T_EP). At the end of pump operation on the first day, the T_EP values at the locations for groups C10, C11, C12, and C13 were 34.12 °C, 33.70 °C, 32.93 °C, and 32.37 °C, respectively. When the pump stopped on the fourth day, the temperature recovery was highest for the C10 group (no deflector) at 36.02 °C. By the end of the system operation period, the T_EP reductions for the different groups were 5.95 °C, 6.37 °C, 7.13 °C, and 7.73 °C, respectively, demonstrating that the cooling effect strengthens with increasing deflector angles.

To further analyze the influence of the air deflector angle on the temperature field within the roadway space, spatial distribution nephograms of the roadway temperature field and bar charts showing the expansion of the lower low-temperature zone under different air deflector angle were generated based on numerical simulation results, as shown in Figure 12.

During the fifth day of system operation, the roadway temperature gradually recovered after the pump stopped. The inhibitory effect on temperature recovery initially increased and then decreased with larger deflector angles. At a deflector angle of 90°, the cross-sectional distance of the low-temperature zone in the upper roadway was only 5.18 m. As pump operation time increased, the enhancement effect of the deflector structure on the expansion of the lower low-temperature zone first increased and then decreased with larger deflector angles, achieving optimal performance at 30°. The maximum expansion distance during the operation period was 6.73 m, representing a 48.89% increase compared to the configuration without a deflector (C10). Combined with Figure 11, the deflector structure improved the delayed cooling effect in the lower section without weakening the cooling effect in the upper section, while further reducing the temperature in the space behind the deflector. At 108 h of operation, the expansion distances of the lower low-temperature zone for the different deflector schemes (C11, C12, C13) increased by 33.81%, 26.93%, and 30.09%, respectively. By 114 h, the expansion rate of the C12 group compared to the C13 group increased by 0.03 m per hour relative to the rate at 108 h. The expansion distance of the lower low-temperature zone for the C11 group was 6.14 m. At 119 h, the expansion distances of the lower low-temperature zone for all schemes reached their optimal values: 4.52 m, 6.73 m, 5.27 m, and 5.11 m, respectively. The enhancement effect of the deflector schemes on the expansion of the lower low-temperature zone was significantly superior to the configuration without a deflector. At 120 h, after the pump stopped, the roadway temperature partially recovered, but both the cooling effect and the ability to delay high-temperature recovery were better in the deflector-equipped groups than in the configuration without a deflector. Therefore, the influence of different deflector angles on the cooling effect can be summarized as follows: The cooling effect in the lower roadway section initially improves and then diminishes with increasing deflector angles but remains superior to the configuration without a deflector. The deflector structure not only effectively enhances cooling in the lower section and mitigates its delayed effect but also does not weaken the cooling effect in the upper section. Additionally, it significantly slows down the temperature recovery rate in the roadway after the pump stops.

In summary, the impact of the air deflector on the cooling system performance is as follows: The expansion effect of the lower low-temperature zone initially increases and then decreases with larger deflector angles. The average temperature on the monitoring line (T_EPL) first decreases and then rises with increasing deflector angles, achieving optimal performance at 30°, with a 48.89% increase in the lower low-temperature zone expansion distance (S_ED) and a maximum temperature reduction of 2.35 °C. The overall cooling effect on the monitoring surface continuously improves with larger angles, reaching its optimum at 90° with a 30.3% enhancement in cooling value. The installation of a deflector effectively alters the airflow path, improves the delayed cooling effect in the lower section without compromising the upper section’s cooling performance, and significantly delays the overall temperature recovery rate in the roadway after the pump stops. Considering the comprehensive underground operating conditions, a 60° angle for the air deflector is determined to be optimal.

Based on the previous analysis of the impact of key installation and operation parameters—such as the annular tube radius ratio, the cascaded tube spacing, the composite phase change temperature, and the air deflector angle—on the cooling performance of the roadway cooling system, the optimal combination of key installation and operation parameters has been determined as follows: an annular tube radius ratio of 2:3, a cascaded tube spacing of 0.5 m, a composite phase change temperature of 25 °C, and an air deflector angle of 60°.

4. Conclusions

Based on the issue of high geothermal hazards in deep coal mine roadways, this study proposes a novel cascaded CPCM roadway cooling system. Using numerical simulations, the influence of key installation and operational parameters—such as the annular tube radius ratio, cascaded tube spacing, phase change temperature at the 4th stage of CPCM, and air deflector angle—on the roadway cooling performance of the system was analyzed. The main conclusions are as follows:

(1)

The annular tube radius ratio significantly influences heat transfer efficiency and temperature distribution. Increasing the radius ratio improves the utilization rate of the CPCM, expands the cooling coverage within the roadway, and raises the average outlet temperature of the cooling circulation modules. The system performs optimally at a radius ratio of 2:3, where the solid-phase percentage of CPCM reaches 89.03%, the temperature at the monitoring surface decreases by 9.54 °C, and a stable low-temperature zone forms in the upper part of the roadway.

(2)

The cascaded tube spacing affects the balance between heat exchange efficiency and economic feasibility. Increasing the cascaded tube spacing raises the average outlet temperature and enhances CPCM utilization. However, the average temperature at the monitoring surface first increases and then decreases as spacing increases, while the overall cooling effect weakens. Although a larger cascaded tube spacing is beneficial for improving cooling performance, it should not be excessively large; the effective diffusion limit is reached at 0.5 m.

(3)

The CPCM phase change temperature must match the mine’s geothermal conditions to maximize cooling effectiveness. A higher CPCM phase change temperature has a minor impact on the average outlet temperature but increases the solid-phase percentage of CPCM while reducing its utilization rate. The extreme difference in the average monitoring surface temperature increases with higher CPCM phase change temperatures. At a CPCM phase change temperature of 25 °C, CPCM utilization is maximized, yielding the best cooling effect and a maximum cumulative temperature reduction of 9.54 °C at the monitoring surface. Further increases in CPCM phase change temperature reduce utilization and compromise compatibility with geothermal conditions.

(4)

The air deflector structure plays a critical role in mitigating the delayed cooling effect in the lower part of the roadway. The expansion distance of the low-temperature zone, the average temperature along the monitoring line, and the average temperature at the monitoring surface all exhibit an initial improvement followed by degradation as the air deflector angle changes. The maximum expansion of the lower low-temperature zone—an increase of 48.89%—occurs at an air deflector angle of 30°. This structure not only enhances cooling in the lower region and delays temperature recovery but also maintains cooling performance in the upper roadway.

(5)

Based on the previous analysis of the impact of key installation and operation parameters on the cooling performance of the roadway cooling system, the optimal combination of key installation and operation parameters has been determined as follows: an annular tube radius ratio of 2:3, a cascaded tube spacing of 0.5 m, a composite phase change temperature of 25 °C, and an air deflector angle of 60°.