The Influence of the Discharge Port Structure on the Infrared Characteristics of Underwater Vehicle Thermal Jets

Abstract

The circulating cooling water of underwater vehicle power systems is discharged through the discharge port and is mixed with the environmental water to form a thermal jet, which diffuses, floats up in the environmental water and displays infrared characteristics on the surface of the water body. In order to explore the influence of the discharge port structure on the infrared characteristics of underwater vehicle thermal jets, this paper adopts the methods of simulation analysis and experimental verification, and it establishes an underwater vehicle motion model based on the CFD computing software platform to design elliptic discharge port structures with different radius ratios and to compare their different thermal jet infrared characteristics. The influence of the oval discharge port radius ratio on the infrared characteristics of thermal jets is verified by the reduced-scale water tank experiment, and the authenticity of the simulation calculation method and design parameters is verified. On the basis of oval discharge ports, the number and distribution position of discharge ports are further designed to suppress the infrared characteristics of thermal jets and to improve the thermal stealth performance of underwater vehicles. According to the simulation calculation and experimental results, under the condition of the same discharge flow, as the radius ratio becomes smaller, the mixing heat transfer effect of the thermal jet becomes better, and the infrared characteristics become less obvious. Moreover, increasing the number of discharge ports and adopting the symmetrical arrangement of discharge ports can further strengthen the temperature attenuation of thermal jets and reduce the surface maximum temperature.

1. Introduction

In order to ensure the normal operation of the equipment, the vehicle uses cooling water to absorb the heat generated by the operation of the equipment in the process of underwater movement, and the cooling water is discharged to form a thermal jet [1,2,3,4], which is mixed with the environmental fluid for heat exchange. The thermal jet floats up to form infrared characteristics on the water surface, which can be easily detected by infrared detection equipment.

Domestic and foreign scholars have conducted numerous experimental and simulation studies on the characteristics of thermal jets. In [5], the authors used the similarity model of underwater vehicles in the thermal jet experiment to measure parameters under certain constraints (the Reynold number of fluid is lower than the normal value) and observed the turbulent flow field of the thermal jet through PIV technology. In [6,7], the authors experimentally studied the thermal characteristics of turbulent jet impingement on water surfaces at different Reynold numbers. An infrared thermal image of the water surface was obtained by a high-resolution infrared detector. In [8], the authors studied the vertical temperature distribution of seawater in the actual marine environment and carried out numerical calculations on the thermal jets caused by the appendages and propellers of underwater vehicles in the temperature-stratified environment. In [9], Yang established and gradually improved the laboratory underwater vehicle thermal jet discharge simulation experimental platform and carried out thermal jet discharge simulation experiments in the laboratory in water with a uniform and stratified temperature. The formation mechanism, the means of detection and key influencing factors of thermal jets were explored. Alfaifi et al. compared the flow characteristics of thermal jets and non-thermal jets under the condition of a Froude number with the same density through the discharge experiment. The experimental results show that the thermal jet flow track was farther, and the flow field range was wider in the lentic environment [10]. According to the similarity principle, the authors carried out the infrared detection experiment with a scaled underwater vehicle model under the condition of realizing geometric similarity, heat flux similarity and temperature field similarity, and the authors studied the influence of different experimental conditions on the infrared detection of thermal jets in [11]. Mohammadshahi et al. [12] analyzed the characteristics of different types of thermal jets through experiments, focusing on the difference of thermal jet temperature changes. Hassanzadeh et al. [13] experimentally observed the mixing process of thermal jets and environmental fluid through a high-speed camera, laser imaging and ultrasonic velocimetry. The mixing processes of jets can be divided into fully turbulent regime and semi-turbulence regime, and the semi-turbulence regime can be further divided into the momentum-dominated regime and the buoyancy-dominated regime. It was concluded that the fluid flow is controlled by the interactions among inertia, viscosity and the buoyancy of fluids. Khosravi et al. [14] analyzed the flow state of a unilateral thermal jet in a cross-flow environment under an unsteady N–S equation, and the results show that the ratio of the jet velocity to the environmental water flow velocity affected the initial temperature of the jet. Ishigaki et al. [15] studied the influence of the non-orthogonality of grids on numerical calculation solutions of thermal jet flow and used a hexahedral grid element and a prism grid element to divide and compare. The results show that the prismatic grid element increased the computational instability compared with the hexahedral grid element. Although the non-orthogonal correction method can improve calculation stability, the convergence time increased. Kumar et al. [16] proposed a unified formula for thermal jet flow based on grid difference, derived the pressure correction equation from the energy equation and simplified the Poisson equation as a continuity constraint. This method can better simulate the fluid flow process with a large density change. Bloutsos et al. [17] discussed the mixing characteristics of the vertical round thermal jet mean flow in a weak cross-flow by an integral method and established Reynolds mean partial differential equation with continuous tracer mass and momentum conservation in a curvilinear–cylindrical coordinate system to describe the phenomenon of thermal jet flow. Based on the above research, the study of thermal jet characteristics and its changing process is relatively mature, but the influence of the discharge port structure on the characteristics of thermal jets has not been deeply studied and discussed.

The thermal jet discharge port of an underwater vehicle generally has a circular structure. However, it is not sufficient to study whether different discharge port structures affect the thermal jet properties. In addition, compared with the single discharge port, the multi-discharge port structure can change the single thermal jet into multiple thermal jets and can strengthen the heat transfer effect of mixing thermal jets with environmental fluids. Liu et al. [18] conducted numerical simulations on the discharge process of cross-flow thermal jets through circular and square ports of equal area based on an RSM model. The calculation results show that the velocity loss near the discharge port was large. Compared with the square port jet, the round port jet had a stronger impact, stronger mixing with environmental fluid and faster temperature attenuation. Chen et al. [19] studied the flow field and temperature field of dual jets. The analysis shows that, when the distance between the discharge ports is larger, the interaction between sub-jets is smaller, but the mixing degree between the rear sub-jets and environmental water is greater in the flow direction of the environmental water. Kannan et al. [20] calculated the thermal jet flow field from one discharge port to five discharge ports by using the open-source computing platform and compared it with relevant experimental data. The calculations show that the near field flow characteristics of porous jet are nonlinear. The author of [21] carried out relevant studies on the pressure fields, velocity fields and temperature fields of different numbers of porous jets. The biggest difference between porous jets and single-hole jets was that there was an adsorption effect between adjacent thermal jets. At the same section distance from the discharge port, as the number of ports increased, the correlation characteristic value became greater. Shah Sanil conducted a numerical analysis on the heat transfer process between multiple jets and wall surfaces [22]. When the ratio of jet velocity to environmental velocity was large, the Nussel number increased sharply. Singh.p et al. [23] conducted relevant research on porous turbulent jets in the CFD-FLUENT software (Fluent 2020R1, Wuhan, China) and analyzed variations in the flow-related heat transfer coefficients, temperature and turbulent kinetic energy under different discharge port structures and distributions. However, there are still some deficiencies in the study of discharge port structures. This paper mainly focuses on the structure of discharge ports, taking into account oval discharge ports with different radius ratios and the number and distribution of discharge ports, to obtain the discharge port structure which is most beneficial for restraining the infrared characteristics of the thermal jet, which can improve the thermal stealth performance of underwater vehicles, can reduce the risk of underwater vehicles being detected by space-based or air-based infrared equipment and can provide theoretical research ideas for industrial design and manufacturing.

2. Physical Equations and Models

The complexity of fluid motion is determined by the characteristics of the fluid itself. The inertia, compressibility and viscosity of a fluid make it difficult to describe and measure fluid motion. In addition, fluid motion can be steady or unsteady, and laminar or turbulent, and these flow characteristics also lead to the complexity of fluid motion. According to the physical characteristics of fluid motion, mathematicians and physicists have established the related equations of fluid motion in the form of mathematical expressions to describe fluid motion [24].

• Mass Conservation Equation

$$\frac{\partial u_i}{\partial x_i}=0$$

(1)

2.

Momentum Conservation Equation

$$f_i-\frac{1}{\rho }\frac{\partial p}{\partial x_i}+v{\nabla }^2{u}_i=\frac{\partial u_i}{\partial t}+u_j\frac{\partial u_i}{\partial x_j}$$

(2)

where $\rho$ is fluid density, $p$ is pressure, $u_i$ is the velocity components, $x_i$ is the coordinate component, $v$ is the kinematic viscosity and $f_i$ is the unit mass force.

3.

Energy Conservation Equation

$$\rho c_p\frac{\partial T}{\partial t}+\rho c_p{u}_j\frac{\partial T}{\partial x_j}=u_j\frac{\partial p}{\partial x_j}+\frac{\partial }{\partial x_j}\left(\lambda \frac{\partial T}{\partial x_j}\right)$$

(3)

where $c_p$ is the specific heat capacity at a constant pressure, $T$ is the temperature and $\lambda$ is the coefficient of thermal conductivity.

The Realizable $k-\epsilon$ model is selected to relate the additional term of the turbulence pulsation value to the time mean [24].

Turbulent kinetic energy equation:

$$\frac{\partial }{\partial t}(\rho k)+\frac{\partial }{\partial x_i}\left(\rho k{u}_i\right)=\frac{\partial }{\partial x_j}\left({\alpha }_k{\mu }_{eff}\frac{\partial k}{\partial x_j}\right)+G_k+G_b-\rho \epsilon -Y_M+S_k$$

(4)

Epsilon turbulent dissipation rate equation:

$$\frac{\partial }{\partial t}(\rho \epsilon )+\frac{\partial }{\partial x_i}\left(\rho \epsilon u_i\right)=\frac{\partial }{\partial x_j}\left({\alpha }_{\epsilon }{\mu }_{eff}\frac{\partial \epsilon }{\partial x_j}\right)+G_{1\epsilon }\frac{\epsilon }{k}\left(G_k+G_{3\epsilon }{G}_b\right)-G_{2\epsilon }\rho \frac{{\epsilon }^2}{k}-R_{\epsilon }+S_{\epsilon }$$

(5)

For incompressible fluids, $G_b=0$ and $Y_M=0$. The source term does not need to be considered in the calculation in this paper, so $S_k=0$ $S_{\epsilon }=0$. Therefore, Equations (4) and (5) become:

$$\frac{\partial }{\partial t}(\rho k)+\frac{\partial }{\partial x_i}\left(\rho k{u}_i\right)=\frac{\partial }{\partial x_j}\left({\alpha }_k{\mu }_{eff}\frac{\partial k}{\partial x_j}\right)+G_k-\rho \epsilon$$

(6)

$$\frac{\partial }{\partial t}(\rho \epsilon )+\frac{\partial }{\partial x_i}\left(\rho \epsilon u_i\right)=\frac{\partial }{\partial x_j}\left({\alpha }_{\epsilon }{\mu }_{eff}\frac{\partial \epsilon }{\partial x_j}\right)+G_{1\epsilon }\frac{\epsilon }{k}{G}_k-G_{2\epsilon }\rho \frac{{\epsilon }^2}{k}-R_{\epsilon }$$

(7)

${\mu }_{eff}=\mu +{\mu }_t$, ${\mu }_t=\rho C_{\mu }\frac{k^2}{\epsilon }$, $G_k=-\rho \overline{u_i^{\prime }{u}_j^{\prime }}\frac{\partial u_j}{\partial x_i}$, $R_{\epsilon }=\frac{C_{\mu }\rho {\eta }^3\left(1-\eta /{\eta }_0\right)}{1+\beta {\eta }^3}\frac{{\epsilon }^2}{k}$, $\eta =S\frac{k}{\epsilon }$, $S=\sqrt{2S_{ij}{S}_{ij}}$, $S_{ij}=\frac{1}{2}\left(\frac{\partial u_i}{\partial x_j}+\frac{\partial u_j}{\partial x_i}\right)$.

Turbulence model constant [24]:

$G_{1\epsilon }=1.42$, $G_{2\epsilon }=1.68$, $C_{\mu }=0.0845$, ${\sigma }_k=1.0$, ${\sigma }_{\epsilon }=1.3$, ${\eta }_0=4.38$, $\beta =0.012$.

3. Calculation Condition

The fluid computing domain is 8 m × 1.1 m × 0.5 m. The simulation model of the underwater vehicle is 100 mm away from the left inflow surface, 7000 mm away from the right outflow surface, 100 mm away from the bottom, 300 mm away from the top (200 mm away from the water surface), 300 mm away from the back of the fluid computing domain and 700 mm away from the front of the fluid computing domain. The fluid computing domain is shown in Figure 1.

In the setting of boundary conditions, the discharge port is the velocity inlet. The inflow surface is the velocity inlet. The outflow surface is the pressure outlet. The top of the fluid computing domain is the pressure outlet. The inflow surface temperature is 293 K, and the inflow velocity is 0.1 m/s. The thermal jet temperature is 333 K, and the speed is 0.3 m/s [25].

In this paper, the Fluent software (Fluent 2020R1, Wuhan, China) is adopted for simulation calculations. In the solution setting, considering the water surface infrared characteristics of an underground vehicle thermal jet, the air layer is divided out in the fluid computing domain, and the VOF fluid volume function is used to track the gas–liquid interface layer. The thickness of the air layer is 100 mm on the water surface. The solving method is the SIMPLE algorithm based on a finite volume method. The solver type is pressure-based, and Transient is adopted for the time attribute. Moreover, the influence of gravity on fluid movement is reflected in the solution process. The reliability requirements of the calculations and solution are reflected in the limitation of the convergence residual of each parameter. The convergence order of the energy residual is less than 10⁻⁷, and the convergence order of the residual of other parameters is less than 10⁻⁴. The results can be considered reliable if the residual convergence is achieved [25].

In order to eliminate the influence of the number of grids on the calculation results, the central maximum temperature values at the horizontal distances of 0.1 m, 0.3 m and 0.5 m between the thermal jet and the center of the discharge port are selected as verification data, and the central maximum temperature of the underwater vehicle thermal jet at the same position and under different grid numbers is compared and verified. The verification results are shown in Figure 2. The data in the figure shows that, when the number of grids exceeds 1 million, the maximum temperature at the center of the thermal jet at different distances from the center of the discharge port is relatively consistent, with a small error within 0.01 K. Therefore, it is appropriate to control the number of grids at about 1.5 million when setting the local size and global size of meshes.

4. The Simulation Comparison of the Oval Discharge Port

In the spatial dimension, a three-dimensional cartesian coordinate system is established with the center of the discharge port as the origin, the flow direction of the environmental fluid as the negative direction of the X-Axis, the discharge direction of the thermal jet as the positive direction of the Y-Axis, and the gravity direction as the negative direction of the Z-Axis. The radius of the oval discharge port is distributed along the X-Axis and Z-Axis. The radius ratio is defined as the ratio of the oval discharge port radius in the Z-Axis to the oval discharge port radius in the X-Axis.

Table 1. Structure parameters of different radius ratios of oval discharge ports.

Discharge Port	X-Axis Radius/m	Z-Axis Radius/m	Radius Ratio
Str 1	0.004	0.025	4:25
Str 2	0.005	0.02	1:4
Str 3	0.008	0.0125	16:25
Str 4	0.01	0.01	1:1
Str 5	0.0125	0.008	25:16
Str 6	0.02	0.005	4:1
Str 7	0.025	0.004	25:4

shows the structural parameters of the oval discharge port with different radius ratios. The structure of the oval discharge port is shown in Figure 3.

Figure 4 shows the flow track of a thermal jet discharging from a circle discharge port. Under the influence of environmental water movement, the cooling water displays the flow separation phenomenon after discharging from the discharge port and forms two thermal jets. One thermal jet floats faster in the Z-Axis direction but has a smaller movement distance in the Y-Axis direction. The other thermal jets have a lower buoyant height but a longer discharge distance.

Figure 5 shows the fluid movement near the discharge port. When the inflow is mixed with the cooling water, a vortex movement is formed behind the discharge port. Due to the high temperature and fast speed of the thermal jet, there is a pressure difference between the thermal jet and the environmental water behind the discharge port. Thus, fluid movement is formed behind the discharge port, and it is completely opposite to the inflow direction. Therefore, the cooling water forms two thermal jets with different motion states after discharging.

The dimensionless temperature difference $\theta$ is defined as $\theta =\left(T-T_0\right)/\left(T_1-T_0\right)$, where $T$ is the maximum temperature at the center of the thermal jet, $T_1$ is the initial temperature of the thermal jet and $T_0$ is the temperature of the environmental water. Figure 6 shows the variation trend of $\theta$ of the thermal jet at the circle discharge port along the X-Axis and Z-Axis.

The thermal jet diffuses in the negative direction along the X-Axis under the influence of the motion of environmental water. In the X-Axis direction, the temperature of the thermal jet decreases the fastest, especially in the initial section. Within 0.06 m from the center of the discharge port, the temperature of the thermal jet decreases 22 K, and the total temperature attenuation distance of the thermal jet in the X-Axis is 2.855 m. The temperature attenuation distance increases by 0.12 m for every 1 K of temperature attenuation in the middle section of the thermal jet and by 0.71 m for every 1 K of temperature attenuation in the rear section of the thermal jet. With the increase in the thermal jet temperature attenuation distance along the X-Axis, $\theta$ gradually tends towards 0.

The temperature attenuation distance of the thermal jet in the Z-Axis direction is called the floating height of the thermal jet, and the floating height of the thermal jet is far less than the temperature attenuation distance in the X-Axis direction. The initial buoyancy of the thermal jet is not obvious. When the temperature of the thermal jet drops to 300 K, the height of the thermal jet is 0.02 m. For every 1 K of temperature drop in the middle section of the thermal jet, the floating height increases by 0.0187 m. In addition, the floating height in the rear section of the thermal jet increases rapidly, and the maximum floating height of the thermal jet is 0.228 m.

In order to reflect the difference in the thermal characteristics of the oval discharge ports with different radius ratios, the temperature attenuation distance in the X-Axis direction and the floating height of the thermal jet from the circle discharge port (Str 4) are taken as standard values. More specifically, the temperature attenuation distance in the X-Axis direction is 0.270 m, and the floating height is 0.020 m in the initial section of the thermal jet. The temperature attenuation distance in the X-Axis direction in the middle section of the thermal jet is 0.460 m, and the floating height is 0.048 m. The temperature attenuation distance in the X-Axis direction in the rear section of the thermal jet is 2.140 m, and the floating height is 0.160 m. The thermal jet temperature attenuation parameters with other radius ratios are regarded as deviation values. The thermal jet temperature attenuation deviation characteristics with different radius ratios are shown in Figure 7, and the deviation degree $\alpha$ is defined as (deviation value − standard value)/standard value.

As can be seen from Figure 7a, in the X-Axis direction, the temperature attenuation deviation of the initial section of the thermal jet from the Str 2 and Str 3 oval discharge port are negative, whereas that of the initial section of the thermal jet at other structures oval discharge ports are positive. In the middle section of the thermal jet, only the Str 1 and Str 3 oval discharge ports have negative deviation, and in the rear section of the thermal jet, the oval discharge ports of each structure have small deviations. In general, the Str 1 and Str 2 oval discharge ports accelerate the temperature attenuation process of the initial section of the thermal jet and shorten the temperature attenuation distance along the X-Axis direction of the initial section of the thermal jet. The Str 2 and Str 3 oval discharge ports accelerate the temperature attenuation process of the middle section of the thermal jet and shorten the temperature attenuation distance of the middle section. The maximum temperature attenuation distance of the thermal jet in the X-Axis direction for different radius ratios of oval discharge ports deviates from the standard value within 0.15 m, and the maximum temperature attenuation distance of the thermal jet in the X-Axis direction for different radius ratios of oval discharge ports is higher than the standard distance.

As shown in Figure 7b, when the radius ratio of the discharge port is less than one, except for Str 3, the deviation degree is negative, and the floating height of the initial section of the thermal jet decreases. When the radius ratio is greater than one, the deviation degree is positive, and the floating height of the initial section of thermal jet increases obviously. In the middle section, except for Str 1 and 3, the deviation degree is positive, and the thermal jet floating height increases. Except for Str 2 and 3, the deviation degree of the rear section is negative, and the thermal jet floating height decreases. In general, except for Str 2, the floating height of the thermal jet is relatively consistent. The floating height of the thermal jet from the Str 2 discharge port is 0.164 m, and those of the thermal jets from other structure discharge ports are 0.228 m.

When the underwater vehicle thermal jet diffuses and floats to a certain height, it no longer rises, but the process of temperature transfer and attenuation still exists, which means that the thermal jet influences the temperature field of the free surface of the environmental water to a certain extent, thus leading to the temperature rise phenomenon of the free surface of the environmental water. Figure 8 shows the characteristics of the temperature changes caused by the thermal jet from the circle discharge port on the free surface of the environmental water.

In Figure 8, the water surface forms part of the rising temperature area affected by the temperature of the thermal jet. The rising temperature area spreads around a certain point as the center. Due to the phenomenon of diverging thermal jets, there are two temperature centers in the rising temperature area of the water surface. The temperature diffusion area of the water surface is dominated by diffusion along the X-Axis.

Table 2. Surface temperature values of thermal jets for different radius ratios of oval discharge ports.

Discharge Port	Maximum Temperature/K	Diffusion Distance on theX-Axis/m	Diffusion Distance on the Y-Axis/m
Str 1	293.071	2.152	0.521
Str 2	293.109	2.140	0.580
Str 3	293.101	2.119	0.539
Str 4	293.148	2.138	0.794
Str 5	293.100	2.129	0.541
Str 6	293.152	2.213	0.586
Str 7	293.196	2.333	0.589

lists the values of the relevant characteristics of the temperature diffusion area formed on the water surface by different radius ratios of oval discharge ports.

The characteristic value of the circle (Str 4) discharge port is taken as the standard value. When the radius ratio of the discharge port is less than one, the maximum temperature of the water surface decreases, and the maximum decrease is 52.03%. Compared with the standard value, when the discharge port radius ratio changes, the maximum diffusion distance of the surface temperature diffusion region on the X-Axis changes little, and the maximum diffusion distance decreases by 25.82–34.38% on the Y-Axis. By changing the radius ratio of the discharge port, the maximum distance of the temperature diffusion on the Y-Axis in the rising temperature region formed by the thermal jet on the water surface is reduced, and the temperature diffusion trend of the thermal jet on the water surface is inhibited on the Y-Axis.

Figure 9 shows the maximum temperature difference in the rising temperature region formed by the thermal jet on the water surface for different radius ratios of oval discharge ports. As can be seen, except for the Str 6 and 7 discharge ports, the maximum temperature difference is less than the standard value, effectively reducing the thermal jet surface temperature characteristics. The maximum temperature difference for the Str 1 discharge port is 0.071 K. With respect to the perimeter of the discharge port, the perimeter of the circle discharge port is 62.832 mm, whereas the perimeter of the Str 1 oval discharge port is 241.080 mm, which is about 3.5 times the length of the perimeter of the circle discharge port. In the case of the same discharge flow, changing the discharge radius ratio can effectively increase the mixing heat exchange interface between the thermal jet and the environmental water and can improve the mixing heat exchange effect.

Although the areas of different radius ratios of oval discharge ports are the same, the perimeters of the oval discharge ports are different. The maximum perimeter of the oval discharge port is for Str 1 and Str 7, reaching 241.080 mm, whereas the minimum perimeter of the oval discharge port is for Str 4, which is about 62.832 mm. As the perimeter of the discharge port becomes larger, the contact surface between the thermal jet and the environmental water at the discharge port becomes larger, and the mixing heat transfer effect becomes better.

Figure 10 shows the different flow tracks of the thermal jet between Str 1 and Str 7. It can be seen from the thermal jet flow track at the oval discharge port with a radius ratio of 4:25 and 25:4 that, as the radius ratio becomes smaller, the width of the thermal jet at the discharge port becomes smaller, and the flow rate of the thermal jet becomes smaller. The influence of the thermal jet on the rising surface temperature diffusion area of the discharge port with a radius ratio of 4:25 is less than that of the discharge port with a radius ratio of 25:4.

5. Experimental Study on Oval Discharge Ports

The experimental equipment prepared for this experiment can be roughly divided into five four categories: the experimental pool, the underwater vehicle model, the electric drive track trailer, the infrared thermal imager and the photographic equipment.

Figure 11 is the schematic diagram of this experiment. Figure 12 shows that the experimental pool is 7.5 m long, 1 m wide and 1 m high. At the bottom of the pool, there are two water inlets and one water outlet. The water inlet flow is 2084 cm³/s, and the water outlet drainage flow is 69 cm³/s. The controller of the trailer platform is shown in Figure 13.

The geometric parameters are the same as the simulation model. The underwater vehicle model is shown in Figure 14. The model length of the underwater vehicle is 0.591 m, the maximum diameter of the underwater vehicle is 0.061 m and the discharge port is located 0.150 m behind the center of the underwater vehicle. The geometric dimensions of the model are obtained by scaling 200:1 according to the structural parameters of the actual underwater vehicle [25,26].

The electric drive track trailer is located above the pool. The trailer adjusts the speed through the control box to drive the underwater vehicle to go straight in the pool. The speed of the trailer movement is the sailing speed of the underwater vehicle. Above the trailer, it is provided with a water tank, booster pump, a flow control valve, etc. The velocity of the water flow into the underwater vehicle model is adjusted by controlling the opening of the valve to adjust the velocity of the thermal jet discharge.

The underwater vehicle model is placed 0.02 m below the water surface in the pool and is connected with the water tank through the iron pipe and the pipeline above the trailer, and the hot water in the water tank is discharged through the discharge port and is mixed with the room-temperature environmental water in the pool for mixing heat transfer. The temperature of the hot water is kept 40 °C higher than that of the environmental water. A black tracer is added into the water tank to reflect the motion state of the thermal jet. Moreover, the trailer is started by the control box to drive the underwater vehicle straight on the track to simulate the navigation state of the underwater vehicle. The infrared thermal imager is used to measure the temperature field of the water surface, and the infrared characteristics of the thermal jet water surface are measured.

As shown in Figure 15, the thermal jet continuously diffuses in the environmental water over time. At the initial stage, the upward floating of the thermal jet is not obvious. After a period of movement, the thermal jet rises upward and reaches the water surface. The temperature difference between the thermal jet and the environmental water leads to different densities between the two types of fluids. The density of the thermal jet is small and rises upward under the action of floating.

Under the influence of the thermal jet, a part of the rising temperature area is formed on the water surface, as shown in Figure 16. Different from the simulation results, the water surface temperature diffusion is uneven and discontinuous. However, the diffusion is mainly in the direction of motion, and the maximum temperature difference of the water surface is also low.

Under the same experimental conditions, Figure 17 shows the maximum surface temperature difference of the thermal jet affected by oval discharge ports with different radius ratios. Compared with the circle (Str 4) discharge port, when the radius ratio of the discharge port is less than one, the maximum water surface temperature difference decreases by 12.84%~35.80%. When the radius ratio of the oval discharge port is greater than one, the maximum temperature difference of the water surface increases.

Under the condition of a certain discharge flow rate, by adjusting the discharge port radius ratio, the perimeter of discharge port changes obviously, which enlarges the mixing heat exchange contact surface between the thermal jet and the environmental fluid and strengthens the mixing heat exchange effect. Moreover, as the discharge radius ratio becomes smaller, the outlet height of the thermal jet becomes lower, which weakens the upward temperature diffusion of the thermal jet to some extent and reduces the maximum surface temperature difference of the thermal jet.

The experimental results show that changing the radius ratio of the oval discharge port has an obvious influence on the temperature diffusion zone formed by the thermal jet on the water surface. The experimental results are consistent with the simulation results. Reducing the radius ratio of the oval discharge port is beneficial to strengthen the mixing heat transfer between the thermal jet and the environmental fluid, and it suppresses the infrared characteristics formed by the thermal jet on the water surface.

6. Simulation Analysis of Discharge Multiport

According to the simulation and experimental results, when the radius ratio of the oval discharge port is 4:25, the perimeter of the discharge port is the largest, the radius in Z-Axis is the smallest, the width of the thermal jet is the smallest and the mixing effect of the thermal jet and the environmental fluid is the best. Therefore, on the basis of the oval discharge port of Str 1, the total area of the discharge port is kept unchanged, the number of the discharge ports is increased, the distribution position of the discharge port is adjusted and the radius ratio of each discharge port should be kept unchanged. The specific structure of the discharge port is shown in Figure 18.

As can be seen from Figure 19, when the discharge port is located on the side of the underwater vehicle, the coiling and mixing phenomenon occur between the discharge ports, which is not conducive to the rapid decline of the thermal jet temperature.

When the two discharge ports are distributed up and down, the momentum loss of the thermal jet is rapid, the velocity loss is large and the mixing effect with the environmental water is poor. The gap between the Str A discharge port is long and narrow, and the thermal jet entrainment phenomenon between the two ports is obvious. Therefore, the environmental water cannot enter the gap of the discharge port well and mix with the thermal jet for heat exchange, and it can only contact the thermal jet at the edge. The actual perimeter of the interface for the mixing heat transfer between the thermal jet at the Str A discharge port and the environmental water is 188.268 mm, which is smaller than the perimeter of 241.080 mm at the Str 1 discharge port. The interface between the thermal jet and the environmental water is reduced, and the mixing effect of heat transfer is weakened.

When the two discharge ports are distributed in the front and the back, the distance of the thermal jet increases in the initial motion direction, the temperature drops rapidly and the mixing effect with the environmental water is better. The gap between the Str B discharge port is small, and the heat transfer is sufficient when the thermal jet is mixed with the environmental water. The perimeter of the interface for the mixing heat transfer between the thermal jet and the environmental water from the Str B discharge port is 340.973 mm, which is much larger than the perimeter of the interface from the Str A and Str 1 discharge ports. The discharge port area is the same, the discharge velocity is the same and the flow rate is the same, but the perimeter of the discharge port is greatly increased, which improves the mixing heat transfer effect of the thermal jet and the environmental water.

The temperature of the thermal jet drops the fastest when the two discharge ports are located on either side of the underwater vehicle. There is no coiling and mixing phenomenon between the discharge ports, which is beneficial for improving the heat transfer effect of mixing the thermal jet with environmental water. The perimeter of the interface for the mixing heat transfer between the thermal jet and the environmental water from the Str C discharge port is the same as that of the interface from the Str B discharge port, and the perimeter of the discharge port increases significantly. In this structure, the entrainment phenomenon of the thermal jet disappears, and only the shunt phenomenon exists. The mixing heat transfer effect is the best, and the thermal jet temperature decays the fastest. However, as the Str C discharge port is distributed on both sides of the vehicle, the thermal jet temperature diffusion region increases, and there are temperature diffusion regions on both sides of the vehicle.

Figure 20 shows the variation curves of the thermal jet floating height at each discharge multiport. It can be seen from the figure that the thermal jet rising at the Str A discharge port is faster, and its temperature drops slowly, whereas the thermal jet temperature at the Str B and Str C discharge ports is faster, and their rising height is lower.

On the one hand, as the perimeter of the contact surface of the mixing heat transfer between the thermal jet and the environmental water from the Str B and Str C discharge ports is longer than the perimeter of the contact surface from the Str A discharge port, the mixing heat transfer effect is better, and the temperature of the thermal jet decays rapidly. On the other hand, due to the smaller discharge widths of the Str B and Str C discharge ports, the outlet height of the thermal jet is lower, and the thermal jet floating height from the Str B and Str C discharge ports is significantly lower than that from the Str A discharge port.

Compared with the single oval discharge port with a radius ratio of 4:25, the effect of the thermal jet discharged from the Str A discharge port is stronger, and the maximum temperature difference of the water surface is 0.128 K. The maximum temperature difference in the temperature center formed by the thermal jet at the discharge port of Str C is 0.052 K, and the infrared characteristics of the thermal jet are weakened.

7. Conclusions

This paper explores the influence of the underwater vehicle discharge port structure on thermal jet infrared characteristics by means of simulations and experiments. The oval discharge port is designed, the radius ratio of the discharge port is changed and the perimeter of the discharge port contact surface is increased. Moreover, the discharge multiport is further designed, the number of discharge ports is increased and the distribution of discharge ports is adjusted. According to the research results, the following conclusions are drawn:

(1)

As the radius ratio of the discharge port becomes smaller, the thermal jet mixing contact surface becomes larger, the thermal jet mixing heat transfer effect becomes better and the infrared characteristics of the thermal jet becomes less obvious.

(2)

As the radius on the Z-Axis of the discharge port becomes smaller, the width of the thermal jet at the outlet becomes narrower, and the floating height of the thermal jet becomes lower.

(3)

Increasing the number of discharge ports and reducing the gap between the discharge ports are conducive to accelerating the thermal jet temperature attenuation process and reducing the maximum temperature difference in the temperature center formed by the thermal jet on the water surface.

(4)

When the discharge ports are distributed on both sides of the underwater vehicle, the coiling and mixing phenomenon between the discharge ports disappears, the thermal jet temperature attenuation process speeds up and the characteristics of the temperature diffusion region formed on the water surface are less obvious.

(5)

The structure of the discharge port has a significant influence on the infrared characteristics of the thermal jet. Changing the shape, number and distribution location of the discharge port is beneficial for improving the thermal stealth performance of the underwater vehicle.

Through this study, the idea and basis for an actual underwater vehicle discharge port design are provided.