Research on the Formation Mechanism of Hot-Water Heat Damage in High-Temperature Roadway and the Cooling Effect of Spraying a Thermal Insulation Layer

Abstract

This study establishes a numerical simulation model based on heat and mass transfer theory to reflect the variations in temperature and humidity conditions within a tunnel. It analyzes the impact of high-temperature fissure water, humid porous media, and drainage methods on the temperature and humidity distribution in a tunnel. The results indicate the following: (1) When the area of the humid porous media increases from 150 m² to 300 m², the relative humidity (RH) of the air in the tunnel rises from 52.7% to 55.8%, but the impact on air temperature (T_a) is minimal. (2) The heating and humidification effects of hot water in a drainage ditch on the airflow cannot be overlooked. Meanwhile, the hot water transfers heat to the surrounding rock, with heat transfer predominantly driven by the surrounding rock convection. Compared to a drainage pipe, the heat transfer amount increases by 44.9%, and RH rises by 9.3%. (3) For every increase of 5 °C in water temperature (water volume of 90 m³/h), the ventilation outlet T_a linearly increases by 0.15 °C, and the rate of increase in RH accelerates with rising water temperature. (4) Covering a drainage ditch with a cover plate can reduce RH by 12.3%, while spraying a 10 cm insulation layer on the tunnel walls can significantly lower T_a by 0.66 °C. These findings provide a potential solution for the application of insulation materials in reducing the thermal hazards of deep high temperatures.

1. Introduction

The increasing demand for mineral resources and advancements in mining technology have led to a shift towards deeper mining levels globally [1,2]. As mining depths continue to increase, high mine temperatures have emerged as a critical issue in deep mining research [3,4]. The discharge of large volumes of hot water has resulted in elevated levels of thermal damage in mines [5,6]. Seeping hot water acts as a “reinforcing agent”, regulating the temperature and humidity of the tunnel airflow [7,8]. Excessive temperatures not only reduce productivity but also heighten the risk of accidents [9,10,11]. Consequently, high-temperature heat damage has become a major constraint on the safe operation of mines.

Several scholars have analyzed the distribution of temperature and humidity fields in tunnels affected by high-temperature fissure water theoretically. Qu et al. demonstrated that the complexity of the fissure network influences the temperature distribution in the surrounding rock and plays a role in the formation of thermal hazards in mines [12]. Gong et al. [13] explored the coupling characteristics of seepage and temperature fields, as well as the heat transfer and exchange mechanisms related to fissure extension. Chen et al. [14] examined the heat transfer mechanisms of groundwater flow under coupled seepage-temperature conditions. Zeng et al. [15] investigated the thermal regulation of tunnels and the surrounding ambient temperatures and analyzed the impact of surrounding rock and ventilation on the tunnel temperature field. However, these studies assumed dry surrounding rock and did not account for the effects of humidity. Li et al. [16] explored the influence of various parameter conditions on the temperature and humidity of tunnel airflow based on field measurements, derived a model for heat exchange between the airflow and tunnel enclosure considering the reverse evaporation of water from the shaft, and corrected the heat exchange coefficient between the enclosure rock and airflow. Wei et al. [17,18] comprehensively analyzed the factors affecting tunnel airflow temperature and humidity through orthogonal experiments. Li et al. [19] highlighted that neglecting latent heat could lead to significant errors in predicting tunnel exit temperatures. Moreover, dehumidifying the initial airflow or increasing the ventilation rate can facilitate latent heat release. The aforementioned studies primarily focus on heat and mass transfer between the wetted tunnel and airflow, without addressing the scenario of high-temperature fissure water entering the tunnel. Wang et al. [20] explored the variations in the tunnel airflow temperature and humidity fields under high-temperature fissure water using similar test methods. Xin et al. [21] demonstrated that the Reynolds number significantly affects the convective heat transfer coefficient and wall temperature, while the channel shape has the greatest influence on the length of the longitudinal heat transfer zone. Xiao et al. [22] addressed these knowledge gaps by conducting water uptake experiments on fractionally wet artificial porous media, varying in water–wet ratios, pore sizes, and wettability heterogeneities. The application of spray-on thermal insulation materials for cooling in tunnels is a widely used active cooling method, and numerous scholars have conducted extensive research on thermal insulation materials for mining. For example, Paustian et al. [23] examined the relationship between capillary pressure, saturation, and interfacial area in a three-phase flow system within wet porous media. Yu et al. [24] prepared a lightweight porous insulation concrete using tailings, perlite particles, EPS particles, and cement, and verified its excellent performance through experimental testing. Zhang et al. [25] developed a thermal insulation material for spray application on tunnel surfaces using materials such as bamboo branches, gravel, expanded clay, and glass hollow beads. Yang et al. [26] used dolomite tailings and granite waste as raw materials, and produced a lightweight, porous insulation material through silicon carbide foaming treatment. Cheng et al. [27] prepared a high-performance thermal insulation material using modified polyurethane, bentonite, cement, and expanded clay as raw materials. At the same time, there has been extensive research on the application of thermal insulation materials. Qin et al. [28] examined the temperature field and heat dissipation dynamics of the tunnel envelope after adding an insulation layer. Yao et al. [29] numerically simulated the tunnel temperature field changes post-insulation, concluding that the low thermal conductivity of the slurry material is a key factor influencing the temperature field distribution. Li et al. [30] conducted experiments and numerical simulations to investigate the effects of various factors on the thermal insulation performance of wall-mounted insulation linings, exploring the impact of natural convection on the heat transfer mechanisms of wall insulation linings. Despite the comprehensive analysis of insulation layers, the complex heat and humidity environment in tunnels still a need a comprehensive means of treatment that combines multiple methods.

In summary, scholars have conducted some research on the temperature and humidity fields in high-temperature underground engineering; however, studies on the humidity distribution within tunnels under the influence of high-temperature fissure water are limited, and there are few reports on the impact of humid porous media on the thermal and humidity distribution in tunnels. This paper innovatively integrates the interaction between humid porous media and high-temperature fissure water into a thermal–humidity coupling model, advancing previous research. It quantitatively reveals for the first time the coupling mechanisms of surrounding rock thermal conduction, hot water evaporation, and airflow convection. By breaking through the traditional limitations that focus solely on dry surrounding rocks, it proposes a multi-phase heat and humidity transfer theoretical framework, quantifying the effects of drainage methods and insulation layers on the tunnel environment, and fills the gap in predicting and managing thermal hazards in tunnels under high-humidity fissure water conditions.

Table 1. Comparison between this study and similar works.

Previous Research and Shortcomings

Highlights of This Article

Qu et al. [12], Gong et al. [13], Chen et al. [14], and Zeng et al. [15] studied the temperature variation patterns in tunnels under the influence of fracture water, but did not consider changes in humidity. Li et al. [16] derived a model for the impact of water evaporation on airflow and thermal exchange between the tunnel surrounding rock. Wei et al. [17] and Wang et al. [20] studied the variation trends of temperature and humidity fields in tunnels under the influence of fissure water, without considering the presence of moisture-containing porous media. Qin et al. [28], Yao et al. [29], and Li et al. [30] conducted numerical simulations to study the effect of the method of using sprayed thermal insulation materials on the temperature field inside tunnels.

The impact of humid porous media on the thermal and humidity environment in tunnels is considered for the first time, quantitatively revealing the coupling mechanisms of surrounding rock thermal conduction, hot water evaporation, and airflow convection. The effects of drainage methods and insulation layers on the thermal and humidity environment in tunnels are also quantified.

presents a comparison between this study and previous research.

2. Engineering Background

2.1. Status of Works

A coal mine situated in the central region of China’s Ju Ye Coal Field has a burial depth ranging from 700 to 1200 m, with an average ground temperature gradient of 2.20 °C/100 m. The original rock temperature in most sections of the mine is 37–42 °C, placing it within the second level of thermal hazardous areas [31]. The temperature of the hot water can reach up to 60 °C, representing a typical geothermal anomaly within the tunnels. The 1# drainage tunnel, located in the middle of the mine, was selected as the research object. This channel passes through faults 1, 2, and 3, and is adjacent to fault 4, as Figure 1a illustrates. Hot water discharge is managed through both drainage ditches and pipes. To mitigate the large volume of water in the shower area, a tarpaulin is placed over the top of the tunnel, as shown in Figure 1b, with hot water being diverted through the tarpaulin and channeled into the drainage ditch to minimize direct contact between the hot water and air. Figure 1c depicts the drainage pipe arrangement, with a 40 cm deep and 30 cm wide drainage ditch, and a 30 cm diameter metal drainage pipe. Ventilation is provided by a press-in ventilation system at one end of the tunnel, with a specified capacity of 14.8 m³/s.

To analyze the temperature and humidity field distribution within the tunnel under the influence of high-temperature fissure water, a 900 m section of the drainage tunnel was selected for on-site measurements. As shown in Figure 2, measurement points were set every 100 m, with separate points before and after the areas of water accumulation and the shower zone. All measurement points were positioned 1.8 m above the ground. A YWSD 100 temperature and humidity meter was used to assess the thermal environment in the tunnel, with a temperature accuracy of 0.1 °C and humidity accuracy of 0.1% RH. The hot water parameters at multiple drainage ditches and areas of water accumulation were measured, and the thermodynamic parameters were tested by sampling the wet porous medium of the tunnel floor.

2.2. Analysis of the Measured Results of Ambient Temperature and Humidity in the Tunnel

The data from each monitoring point within the tunnel were compiled and analyzed, as shown in Figure 3. The airflow temperature in the tunnel is generally higher than 30 °C, and the RH is generally higher than 80%. At the initial measurement point, the T_a was 27.4 °C, and the relative humidity was 58.5%. As the distance into the tunnel increased, both the temperature and humidity rose to varying extents due to the influence of high-temperature surrounding rock and high-temperature fissure water. A detailed analysis of these variations follows below.

Within the first 200 m of the tunnel, both T_a and RH increased rapidly. The rise in T_a in this section was primarily attributed to heat transfer from the surrounding rock to the airflow in the tunnel. While no significant water accumulation or drenching was observed in this area, the large area of moisture-containing porous media on the ground led to an increase in RH. A reduction in both T_a and RH at the 300 m measurement point was observed, which was attributed to air leakage from broken ducts between the 200 m and 300 m points. The decrease in RH between 450 m and 570 m was due to the reduced area of moisture-containing porous media. The fluctuation in RH at the 600 m measurement point was caused by a small area of water accumulation in that section. From 800 m to 900 m, a large high-temperature shower area was observed, with water temperatures reaching as high as 62.3 °C. Despite the use of a tarp for shielding and drainage, the tarpaulin showed varying degrees of damage, allowing water to directly humidify and heat the air. This resulted in a rapid increase in both T_a and RH values.

The heat and moisture exchange process in the tunnel involves the interaction between the airflow, high-temperature fissure water, and the surrounding rock, as Figure 4 illustrates. The areas with moist porous media, water accumulation, and significant inflow of water all influence the longitudinal distribution of Ta and RH, specifically manifested as follows:

(1)

In the region of moisture-containing porous media, heat and moisture exchange occurs between the gas flow and wet porous media, and RH rises continuously. Due to the small amount of fracture water in this area, the heat carried has little effect on the temperature of the moisture-containing porous media, and the temperature of the moisture-containing porous media is mainly affected by the heating of the surrounding rock. The vertical distribution trend of T_a is primarily controlled by the high-temperature surrounding rock and the drainpipe, and the moisture-containing porous medium mainly plays the role of humidifying the airflow.

(2)

In the water accumulation area, the continuous evaporation of standing water leads to an increase in the temperature and humidity of the airflow. However, due to the relatively small amount of water being added, the heat carried is also minimal, and the standing water primarily serves to humidify the airflow.

(3)

In regions where large quantities of fracture water enter the tunnel, both T_a and RH increase rapidly. The airflow is heated and humidified by the high-temperature shower water and hot water in the drainage ditch. Simultaneously, the hot water transfers heat to the surrounding rock, initiating convection between the hot water and surrounding rock, followed by heat transfer from the surrounding rock to the airflow. This process leads to a significant increase in both the temperature and humidity of the airflow.

Figure 4. Heat and moisture coupling transfer of air-hot water–rock in the tunnel.

Figure 4. Heat and moisture coupling transfer of air-hot water–rock in the tunnel.

3. Numerical Simulations

3.1. Basic Assumptions

The thermal environment and humidity within a tunnel are inherently complex and influenced by numerous factors, making it impossible to develop a mathematical model that fully captures the tunnel conditions. To create a computational model that accurately reflects the temperature and humidity dynamics, the following assumptions are made:

(1)

Homogeneous medium assumption: the surrounding rock and insulation layer are treated as homogeneous, continuous media with constant physical properties.

(2)

Incompressible fluid assumption: since the tunnel environment is at normal temperature and pressure, it is assumed that both air and water are incompressible fluids, and the effects of internal thermal radiation are neglected.

(3)

Smooth surface assumption: the inner surfaces of the tunnel, drainage ditch, and drainage pipe are assumed to be smooth.

(4)

Impermeable rock mass assumption: The surrounding rock mass is considered impermeable, restricting the movement of hot water to drains and pipes. The water flow is assumed to be uniform and unidirectional.

3.2. Mathematical Modeling

The numerical model developed in this study employs a set of control equations divided into flow control equations and temperature field control equations. For the flow field analysis, the Navier–Stokes equations are utilized. Based on the maximum water influx through the fissures, the Reynolds number for the water flow within the ditch indicates a laminar flow regime. The mass conservation and momentum conservation equations for the hot water flow are expressed as follows:

$${\displaystyle \frac{\partial {\rho }_w}{\partial t}}+\nabla \left({\rho }_w{v}_w\right)=0$$

(1)

$${\rho }_w({\displaystyle \frac{\partial v_w}{\partial t}}+v_w·\nabla ·v_w)=\nabla [-p_{w}I+u_w(\nabla v_w+{(\nabla v_w)}^{T_w}\left)\right]+F$$

(2)

where ${\rho }_w$ is the density of water (kg/m³), t is the time (s), $v_w$ is the water flow velocity field (m/s), $p_w$ is water pressure (Pa), N is the identity tensor, $u_w$ is the dynamic viscosity of water (Pa·s), and $T_w$ is water temperature (K).

$$F=\rho X-{\displaystyle \frac{\partial }{\partial x}}(u^{\prime }\nabla ·v)$$

(3)

where $\rho$ is the fluid density (kg/m³), $v$ is the fluid velocity (m/s), X is the body force and the direction of the coordinate axis, $u^{\prime }={\displaystyle \frac{2}{3}}u(\nabla v)$ is the second viscosity coefficient of the fluid, and $u$ is the viscosity coefficient of the fluid (Pa·s).

For the airflow in the ducts and lanes, the Reynolds numbers exceed 10⁶, indicating a turbulent state. The Reynolds-averaged Navier–Stokes (RANS) method is applied to manage the control equations of the mean turbulent field [32]. The quasi-k-epsilon two-equation model, which is as follows, is widely applied to address turbulence issues [33]:

$${\displaystyle \frac{\partial {\rho }_a}{\partial t}}+\nabla \left({\rho }_a{v}_a\right)=0$$

(4)

$${\rho }_a({\displaystyle \frac{\partial v_a}{\partial t}}+v_a·\nabla ·v_a)=div(-p_{a}I+K)+F$$

(5)

$$K=(u_a+u_T)(\nabla v_a+{(\nabla v_a)}^{T_a})-{\displaystyle \frac{2}{3}}(u_a+u_T)(\nabla v_a)I-{\displaystyle \frac{2}{3}}{\rho }_{a}qI$$

(6)

$${\rho }_a({\displaystyle \frac{\partial k}{\partial t}}+v_a·\nabla ·q)=\nabla \left[\right(u_a+{\displaystyle \frac{u_T}{{\sigma }_q}})\nabla ·q]+p_q-{\rho }_a\epsilon$$

(7)

$${\rho }_a({\displaystyle \frac{\partial \epsilon }{\partial t}}+v_a·\nabla ·\epsilon )=\nabla \left[\right(u_a+{\displaystyle \frac{u_T}{{\sigma }_{\epsilon }}})\nabla ·\epsilon ]+C_{\epsilon 1}{\displaystyle \frac{\epsilon }{q}}{p}_q-C_{\epsilon }{\rho }_a{\displaystyle \frac{{\epsilon }^2}{q}}$$

(8)

$$p_q=u_T[\nabla v_a:(\nabla v_a+{(\nabla v_a)}^{T_a})-{\displaystyle \frac{2}{3}}{(\nabla v_a)}^2]-{\displaystyle \frac{2}{3}}{\rho }_{a}q·\nabla v_a$$

(9)

$$u_T={\displaystyle \frac{{\rho }_a{C}_{\mu }{q}^2}{\epsilon }}$$

(10)

where ${\rho }_a$ is the density of moist air (kg/m³), $v_a$ is the airflow velocity field (m/s), $p_a$ is air pressure (Pa), $u_a$ is the dynamic viscosity of moist air (Pa·s), $u_T$ is the eddy viscosity coefficient (Pa·s), $T_a$ is air temperature (K), q is turbulent kinetic energy (J), $\epsilon$ is the turbulent kinetic energy dissipation rate, and ${\sigma }_k$, ${\sigma }_{\epsilon }$, $C_{\epsilon 1}$, $C_{\epsilon 2}$, $C_{\mu }$ are constants with values 0.09, 1.44, 1.92, 1.0, and 1.3, respectively.

The temperature field is controlled by the energy conservation equations, distinguishing between conduction in the surrounding rock and structures (Equation (11)) and convection in the air and water (Equation (12)), with the equations being as follows:

$${\rho }_s{C}_s{\displaystyle \frac{\partial T_s}{\partial t}}=\nabla ({\lambda }_s\nabla T_s)+Q_s$$

(11)

$${\rho }_f{C}_f{\displaystyle \frac{\partial T_f}{\partial t}}+{\rho }_f{·C}_f{·v}_f·\nabla T_f=\nabla ({\lambda }_f\nabla T_f)+Q_f$$

(12)

where ${\rho }_s$ and ${\rho }_f$ are the solid and fluid densities (kg/m³), $C_s$ and $C_f$ are specific heat capacities (J/(kg·K)), $T_s$ and $T_f$ are temperatures (K), ${\lambda }_s$ and ${\lambda }_f$ are thermal conductivities (W/(m·K)), $Q_s$ and $Q_f$ are heat sources (W), and $v_f$ is the fluid velocity field (m/s).

The humidity field is governed by component transport equations, one of which follows:

$$M_m{\displaystyle \frac{\delta C_m}{\delta t}}+M_m{v}_a·\nabla C_m=\nabla \left(M_m{D}_m\nabla C_m\right)+G$$

(13)

where $M_m$ is the molar mass of water vapor (kg/mol), $C_m$ is water vapor concentration (mol/m³), $D_m$ is the water vapor diffusion coefficient (m²/s), and $G$ is the mass source (kg/s) [34].

The coupling between these physical fields is achieved through variable boundary conditions and material properties. Air and water viscosity, density, and thermal conductivity are temperature-dependent, while air thermophysical parameters are also humidity-dependent. Specific parameter values are detailed in Section 3.3.

3.3. Model Geometry and Material Parameters

The model’s geometry, shown in Figure 5, focuses on a 200 m section of the tunnel. The drainage ditch, drainage pipe, thermal cover plate, and insulation layer span the entire length of the tunnel. The underground space is modeled as a semicircular arch, with surrounding rock dimensions of 200 m × 50 m × 50 m. The drainage ditch measures 30 cm in width and 40 cm in depth. The thermal insulation cover plate is 5 cm thick and 70 cm wide, while the insulation layer has a thickness of 20 cm, covering both the top and side walls. The model uses a fully structured mesh composed of tetrahedral cells with high mesh quality; most cells exhibit a skewness greater than 0.3, and the minimum mesh quality is 0.1205. The specific parameter values for each material are shown in

Table 2. Material calculation parameters.

Materials	Density kg/m3	Thermal Conductivity Coefficient W/(m⋅K)	Specific Heat Capacity J/(kg⋅K)	Permeability (m2)	Porosity
Rock	2400	2.3	880	/	/
insulation layer	750	0.08	2	/	/
Cover plate	2300	2.9	850	/	/
Moisture-containing porous media	1300	2	900	1 × 10−9 m2	0.45

. Air and water are set using COMSOL 6.1 built-in materials, and the thermophysical parameters are a function of temperature change.

3.4. Boundary Conditions and Solution Settings

The air intake temperature of the tunnel is maintained at 20 °C with a relative humidity of 50%, and the air intake rate is 14.8 m³/s. The surrounding rock at the thermal boundary is set to a temperature of 30 °C. The parameter settings for different calculation conditions are shown in

Table 3. Calculation conditions.

Parameters	Water Volume (m3/h)	Water Temperature Tw (°C)	Humidity-Containing Porous Media Areas (m2)	Area of Water Accumulations (m)
Values	2.5 m3/h, 90 m3/h	40 °C, 45 °C, 50 °C, 55 °C, 60 °C, 65 °C	150 m2, 200 m2, 250 m2, 300 m2	150 m2, 200 m2, 250 m2, 300 m2

. COMSOL 6.1 was employed to solve the numerical model, with flow field accuracy set to 10⁻³ and temperature and humidity field accuracy set to 10⁻⁵. The solution for the flow field is steady-state, while other physics are modeled transiently. The total simulation time was set to 300 days.

3.5. Grid Independence Test

The mesh size and quality in the finite element numerical simulation are critical for determining both accuracy and computational efficiency. To investigate mesh independence, three grid sizes—Grid A, Grid B, and Grid C—were analyzed using hot water drainage from a gutter at 65 °C. Grid A consisted of 1,968,923 elements, Grid B of 8,607,815 elements, and Grid C of 30,685,711 elements.

As shown in Figure 6, a comparison of the calculation results for three mesh configurations was conducted. When comparing Mesh A and Mesh B, the maximum temperature and humidity errors were 0.35 °C and 4.3%, respectively, with a reduction in computation time of 76% (Mesh B took 7.6 h, while Mesh A took 1.9 h). The maximum temperature and humidity errors for Mesh B and Mesh C were much smaller, at 0.04 °C and 1.3%, respectively, with a reduction in computation time of 74% (Mesh C required 29.5 h, while Mesh B took only 7.9 h). Therefore, we believe that Mesh A has a higher error and is unable to accurately capture the temperature and humidity distribution within the tunnel. Although Mesh C has the highest computational accuracy, its computational efficiency is too low. Mesh B has only a small error and maintains a relatively high computational accuracy. Thus, to balance computational accuracy and efficiency, this study chose Mesh B for the calculations.

The numerical model’s accuracy in predicting temperature and humidity distributions was validated by comparing simulation results with field monitoring data. A finite element model of the tunnel, 500 m in length, was created based on site conditions, and the results were compared with on-site measurements, as shown in Figure 7. Due to the inability to achieve the idealized rock uniformity and boundary conditions assumed in the COMSOL simulation software in real-world environments, the simulated temperature and humidity results are higher than the field measurements. In most areas, the simulated temperature and humidity results exceed the field measurement values. The errors in temperature and humidity for the two methods are shown in

Table 4. Comparison between simulation results and actual measurement results.

	Point 1	Point 2	Point 3	Point 4	Point 5	Point 6
Ta (%)	0	1.4	1.68	2.2	3.5	4.7
RH (%)	0	1.6	7	3.1	7	5.9

, with maximum errors of 7% and 4.7%, respectively. Despite these absolute errors, both the numerical and field data exhibited similar spatial distribution trends for temperature and humidity. This alignment indicates that the model accurately predicts the temperature and humidity trends in the tunnel, confirming that the numerical model is both reasonable and reliable.

4. Distribution Characteristics of Ambient Temperature and Humidity in the Tunnel Under Different Factors

4.1. Area of Moisture-Containing Porous Media

The area of the moist porous medium varies from 150 m² to 300 m², and its initial temperature is 35 °C, and heat exchange occurs between it and the surrounding rock. The other variables are consistent with the basic conditions, and Figure 8 illustrates the results. The effect of different moist porous media areas on the spatial distribution of RH follows a similar pattern. The longitudinal RH distribution in the tunnel exhibits a slow decrease from the ventilation entrance to the moisture-containing porous medium region, followed by a rapid increase within the moisture-containing porous area, and a gradual decrease as it exits the region towards the ventilation exit. Upon entering the moisture-containing porous medium region, RH increases as the area of the medium expands. When the area of the moisture-containing porous medium increases from 150 m² to 300 m², the maximum RH increases from 52.7% to 55.8%, demonstrating the significant effect of the moisture-containing porous medium on the tunnel’s humidity. The spatial distribution of temperature in the tunnel with varying areas of moisture-containing porous media shows similar trends. Since the flow of fissure water is minimal and carries relatively little heat, its influence on the temperature field is not significant.

4.2. Area of Water Accumulations

To investigate the effect of the contact area between the airflow and water accumulation on the temperature and humidity in the tunnel, the water accumulation area ranges from 150 m² to 300 m², with the gushing water temperature set to 35 °C. Thermal convection occurs between the water accumulation area and the surrounding rock. Other variables are kept consistent with the basic conditions, and Figure 9 presents the results.

As shown in Figure 9a, the longitudinal RH distribution in the tunnel exhibits a similar trend for different water accumulation areas. With a 150 m² water accumulation area, the RH of the airflow increases by 9.8%. With the increase in the distance of the area of water accumulation, the growth rate of the RH slows down gradually. Therefore, the larger the area of water accumulation, the higher the RH, but as the area of water accumulation increases, the growth rate of RH at the ventilation outlet gradually decreases. Figure 9b shows that the longitudinal T_a distribution follows a similar trend across different water accumulation areas. From the ventilation inlet to the ventilation outlet, T_a increases slowly. However, with an increasing water accumulation area, a slight increase in T_a at the ventilation outlet is observed, though the change is relatively small.

4.3. Water Temperature

As the fissure water volume increases, the disparity in heat carried by the fissure water at varying temperatures also becomes more pronounced. To investigate the impact of hot water temperature on the temperature and humidity of the tunnel environment, the water temperature was varied between 40 °C and 65 °C, while maintaining a fissure water volume of 90 m³/h and other primary conditions constant.

Figure 10 presents the temperature field of the surrounding rock in the model’s cross-section, illustrating that when hot water is drained through the ditch, it not only heats and humidifies the air directly but also raises the temperature of the surrounding rock in contact with the drainage ditch. This process enhances the heat exchange between the tunnel’s surrounding rock and the airflow. As the water temperature (T_w) increases, the extent of the high-temperature perimeter rock expands, resulting in a corresponding rise in the rock’s temperature.

Figure 11a shows that the longitudinal T_a distribution in the tunnel follows a consistent upward trend across different water temperatures. The T_a value at the ventilation outlet increases linearly, with T_w, rising by 0.15 °C for every 5 °C increase in water temperature. From Figure 11b, it can be seen that the spatial distribution trend of RH is different from the difference of T_w, and when the water temperature is higher, the RH shows a trend of decreasing and then increasing. With the increase of T_w, the RH at the ventilation outlet also increased, and the rate of RH growth also increased. When the water temperature was 40 °C, 45 °C, 50 °C, 55 °C, 60 °C, and 65 °C, the values of RH at the ventilation outlet were 53%, 53.8%, 54.4%, 55%, and 56% in that order.

4.4. Comparison of the Effect of Drainage Ditch and Drainpipe on the Hot and Humid Environment

A comparative analysis was conducted to assess the impact of two drainage methods—drainage ditch and drainpipe—on the heat and humidity environment in the tunnel, with a water temperature of 65 °C and a flow rate of 90 m³/h, while other parameters remained consistent.

Figure 12 illustrates that the use of a drainage ditch for hot water drainage exacerbates heat damage in two ways. First, the high-temperature fissure water heats a portion of the tunnel’s perimeter rock, raising its temperature and increasing the heat exchange between the surrounding rock and the airflow. Second, the hot water directly heats and humidifies the air. The area of the surrounding rock affected by heat from the drainage ditch is much larger than the area of direct contact between the hot water and the air, making heat transfer between the surrounding rock and the air the dominant process. This is confirmed by the results in Section 4.5, where the addition of a cover plate above the drainage ditch was shown to limit heat transfer. In contrast, when a drainpipe is used, the hot water does not directly contact the surrounding rock, leading to less heat transfer. While the drainpipe heats the air, the limited contact area results in a less significant impact on the temperature field within the tunnel.

Figure 13a reveals a marked difference between the two drainage methods in terms of their effect on the thermal environment. As the distance into the tunnel increases, the difference in T_a between the two methods becomes more pronounced. At the ventilation exit, the T_a in the drainage ditch tunnel is 0.89 °C higher than in the drainpipe tunnel, with a 45% greater heat transfer. This demonstrates that using a drainage ditch for hot water drainage has a far greater impact on the tunnel’s thermal environment compared to using a drainpipe. Figure 13b shows the longitudinal distribution of RH in the two drainage methods. When a drainage ditch is used, the RH exhibits a sharp increase along the tunnel’s length. In contrast, with a drainpipe, the RH gradually decreases. Ultimately, the RH at the ventilation exit in the drainage ditch tunnel is 9.3% higher than in the drainpipe tunnel.

4.5. Optimization of Drainage Methods for Drains

The use of a drainage pipe for hot water drainage offers better heat damage control; however, the drainage pipes are prone to blockage due to mineral deposition (with an annual blockage rate exceeding 15%), and their installation cost is 30% higher than that of drainage ditches. Regular cleaning is required to maintain flow capacity, which may increase operational and maintenance complexity. To address the limitations of drainage ditches, this study proposes the addition of a thermal insulation cover plate above the drainage ditch and the application of thermal insulation material on the inner wall of the tunnel to mitigate heat damage. Three simulation scenarios were designed based on the CO₂ foam concrete thermal insulation material parameters developed by Ta et al. [35], as shown in

Table 5. Simulation schemes.

Simulation Group	Water Temperature (°C)	Surge Volume (m3/s)	Cover Thickness (cm)	Insulation Thickness (cm)
Working Condition ①	65	90	0	0
Working Condition ②			5	0
Working condition ③			5	10

.

As can be seen from Figure 14a, with the increase in the distance from the ventilation inlet, the T_a value of working condition ② decreases very little compared with working condition ①, and simply adding a drainage ditch cover plate has a small contribution on the thermal insulation. It can be concluded that the thermal impact of the drainage ditch on the tunnel environment is primarily due to the heating of the surrounding rock and the enhanced heat exchange between the surrounding rock and the airflow. In contrast, the difference in T_a between conditions ③ and ① increases progressively. In condition ③, the T_a value is 0.66 °C higher than in condition ①, which highlights the significant cooling effect of the insulation layer.

As shown in Figure 14b, adding a cover plate above the drainage ditch results in a 12.3% decrease in RH at the ventilation outlet, indicating a significant moisture insulation effect. Due to the lower tunnel temperature under condition ③ compared to condition ②, the RH in condition ③ is slightly higher than in condition ②.

5. Conclusions

A numerical simulation model was established based on heat and mass transfer theory to analyze the impact of varying fissure water conditions on the temperature and humidity distribution within the tunnel, combining finite element calculations with field monitoring data. The cooling effect of increasing the gutter cover and thermal insulation coating was studied.

(1)

Field measurements indicated that fissure water entering the tunnel forms moisture-containing porous media and water accumulation areas. For an area of 150 m², the moisture-containing porous media increased the RH of the airflow by 5.1%, while the water accumulation area raised the RH by 9.8%. The degree of influence was positively correlated with the area covered. Due to the small amount of water inflow, the longitudinal distribution of T_a is mainly controlled by the high-temperature surrounding rock and drainage facilities.

(2)

The hot water in the drainage ditch heats and humidifies the airflow. At the same time, the hot water also transfers heat to the surrounding rock, experiencing convection between the hot water and the surrounding rock→heat conduction in the surrounding rock→convection between the airflow and the surrounding rock, leading to a substantial increase in T_a and RH, with heat transfer through the surrounding rock being the dominant mechanism. Under the same fissure water conditions, the air temperature (T_a) values at the ventilation outlets of the drainage ditch and drainage pipe tunnels are 21.97 °C and 20.08 °C, respectively. In comparison, the use of the drainage pipe results in a decrease in Ta by 0.89 °C (a reduction of 4.2%), with heat transfer reduced by 44.9% and relative humidity (RH) decreased by 9.3%.

(3)

When a large amount of fissure water enters the tunnel space, both T_a and RH increase rapidly. The water temperature directly influences the distribution of T_a and RH. Specifically, the T_a value at the ventilation outlet exhibited a linear growth trend with increasing T_w. When the fissure water volume was 90 m³/h, the T_a value at the ventilation outlet rose by 0.15 °C for every 5 °C increase in water temperature, and the RH growth rate accelerated as T_w increased.

(4)

The addition of a cover plate above the drainage ditch resulted in a 12.3% decrease in RH, with minimal impact on T_a. Spraying an insulation layer on the inner wall of the tunnel caused a significant reduction in T_a, with a 0.66 °C decrease observed when the insulation layer thickness reached 10 cm.

In this study, although it was confirmed that the heat transfer of the drainage ditch is 1.45 times higher than that of the drainage pipe, the use of drainage ditches remains widespread. Therefore, based on the existing foundation, methods to reduce the heat dissipation of drainage ditches are proposed, such as lining the inner walls of the drainage ditch with cement-based insulation materials and optimizing the structure and materials of the drainage ditch cover. This will also be an important focus of our future research. Additionally, future work will explore the effects of increasing ventilation rates, lowering inlet temperatures, and incorporating active cooling devices. Furthermore, the model will be extended to other tunnel geometries, with attention paid to other insulation materials.