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Article

Numerical Simulation and Experimental Study of a Small-Scale Vacuum Fish Pump

1
Fishery Machinery and Instrument Research Institute, Chinese Academy of Fishery Sciences, Shanghai 200092, China
2
Key Laboratory of Fishery Equipment and Engineering, Ministry of Agriculture, Shanghai 200092, China
3
Kunshan Shifuda Automation Technology Co., Ltd., Kunshan 215321, China
*
Author to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2024, 12(12), 2296; https://doi.org/10.3390/jmse12122296
Submission received: 11 November 2024 / Revised: 5 December 2024 / Accepted: 9 December 2024 / Published: 13 December 2024
(This article belongs to the Section Marine Aquaculture)

Abstract

:
The existing vacuum fish pump is too large and difficult to move, which is difficult to apply to small fishing vessels. However, the development of a small vacuum fish pump is not a single scaling of the existing vacuum fish pump but requires the support of relevant experiments and simulation theories. In this study, a vacuum fish pump suitable for small fishing vessels was developed. Firstly, a numerical model of the internal flow field during the vacuum fish pump’s working process was established using computational fluid dynamics (CFDs) and verified its effectiveness by physical experiments. It is found that the VOF model can well predict the variation of the volume fraction of the liquid phase in the whole calculation area with time during the suction or drainage process of the vacuum fish pump. Then, the internal flow field characteristics of the fish pump under different working conditions were simulated, and the rationality of the design of the fish pump was evaluated according to the numerical results. Finally, a separate physical experiment was carried out on grass carp, carp, crucian carp, silver carp, and bighead carp, respectively, and the capture efficiency and corresponding fish damage rate for different fish were analyzed. The experimental and numerical results show that the vacuum suction fish pump can achieve efficient and automatic suction and transport of live fish.

1. Introduction

Fish catch and transportation is an important link in fishery production. The fish suction pump has become a key mechanized equipment in the process of fish catch and transportation because of its advantages of high automation, high work efficiency, low labor intensity, and few operators [1]. China is the world’s largest mariculture country. According to statistics, the output value of mariculture in 2023 will reach 488.548 billion, accounting for 47.0% of the total fishery output value [2]. The demand for aquaculture equipment is becoming more and more urgent with the rapid development of marine aquaculture industry. Among them, the demand for fishing equipment accounts for the largest proportion, accounting for 30.48%. However, fishing equipment is mainly used in some large fishing vessels or fishing farms, and small fishing vessels or fishing farms are still dominated by traditional fishing. Traditional fishing mainly adopts artificial methods, which are time-consuming, labor-intensive, and easily damage the fish body, and it is difficult to adapt to the needs of catching and transporting live fish [3]. A fish pump is a kind of advanced equipment specially used in the process of live fish unloading [4]. It first appeared in the 1950s and is generally used in trawling and purse seine fishery activities, such as fishing at sea and port transfer and unloading operations [5]. It can not only greatly reduce labor intensity and labor cost but also has the advantages of high unloading efficiency and low fish damage rate [6]. The main forms of fish pumps are centrifugal, jet, and vacuum. The centrifugal fish pump uses the high-speed rotation of the blades in the hydraulic drive pump to form a negative pressure fish suction, and its moving parts will cause obvious damage to the fish body, which is less used in the transportation of live fish. The fish suction pump uses the negative pressure generated by the high-speed flowing water to suck fish. Due to the action of the low-pressure zone and the shear layer inside, it has caused certain damage to the liver of the fish [7]. The vacuum fish pump is based on the principle of negative pressure to suck fish. Because there are no moving parts, the damage to live fish is relatively small, but the negative pressure environment in the pump may lead to hypoxia and damage to fish. Considering the friendliness of vacuum pumps to live fish transportation and the actual demand of aquaculture enterprises, at the same time, the common vacuum fish pump on the market, such as the vacuum fish pump produced by ‘ETI’ company in the United States, is widely used, but the price is high. And it is generally used in large-cage aquaculture enterprises, which is difficult to apply to pond aquaculture enterprises. Therefore, the design of a vacuum pump suitable for aquaculture ponds has important application prospects. The vacuum pump is composed of a vacuumed-fish-collecting cylinder, water ring vacuum pump, control box, valve, instrument, and pipeline. The specific working principle is that the water ring vacuum pump pulls out part of the air in the vacuumed-fish-collecting cylinder to form a negative pressure, and the fish and water are inhaled from the upper inlet of the vacuumed-fish-collecting cylinder under the action of the pressure difference between the inside and outside of the vacuumed-fish-collecting cylinder. After reaching the set liquid level, the water ring vacuum pump stops pumping air from the inside of the vacuumed-fish-collecting cylinder; the upper vent of the vacuumed-fish-collecting tube is connected with the atmosphere or the outlet of the water ring vacuum pump. Under the action of gravity or pressure, fish and water are discharged from the lower part of the vacuumed-fish-collecting tube, thus completing a suction or discharge process.
Some scholars have conducted some research on vacuum pumps, such as Tian et al., who designed a vacuum pump for single tank in aquaculture pond and verified the rationality of the structure through a live-fish-catching experiment [8]. Later, based on the research results of single-tank vacuum fish pump in freshwater aquaculture pond, the Institute of Oceanology, Chinese Academy of Sciences, proposed an improved double-connected vacuum fish suction device. Xu proposed the design method of the valve for the vacuum fish pump and pointed out that the advanced degree of the fish pump depends on the stability and reliability of the valve control [9]. In addition, previous studies have found that the complex hydrodynamic factors and shear layer inside the fish pump are the main causes of scale shedding. The impact and low-pressure environment in the fish pump are the main causes of gill cover damage, swim bladder rupture, liver and kidney damage, and fish swimming balance damage. The research on the hydrodynamic performance of the fish pump, such as Hong’s multi-objective optimization of the hydrodynamic performance of the fish pump based on the NSGA-II algorithm [10]. On the basis of ensuring the damage rate of the fish body, the structural parameters of the vacuum fish pump with optimal hydrodynamic performance under 167 sets of parameter values were obtained. For example, Liu designed and analyzed the impeller of centrifugal and jet fish pump by ANSYS CFX 15.0 [11,12]. Xu et al. studied the influence of the cross-sectional area ratio of the throat fish nozzle on the transport performance and fish damage of the jet fish pump [13]. Xu et al. numerically calculated the influence of fish movement on the internal flow of the jet fish pump [14]. In general, there are few studies on the application of fish vacuum pumps in pond aquaculture, and there is almost no research on the hydrodynamics of fish vacuum pumps [15,16,17]. The existing research on vacuum fish pumps mostly focuses on the structural design, intelligent control design and experiment of vacuum fish pump. The design and calculation of vacuum fish pumps mainly depends on engineering experience, which has great randomness and uncertainty. If there is no rich design experience accumulation, the designed vacuum fish pump is likely to be unable to meet the actual engineering requirements.
In this study, a small vacuum pump suitable for small fishing vessels or fishing farms was designed based on previous research experience. Then, the numerical model of the internal flow field in the working process of the vacuum fish pump was established by using the computational fluid dynamics method, and the validity of the model was verified by physical experiments. Then, the numerical simulation of the internal flow field characteristics of the fish pump under different working conditions was carried out, and the flow velocity distribution and pressure distribution inside the fish pump were analyzed. Finally, separate physical experiments were carried out on grass carp, carp, crucian carp, silver carp, and bighead carp, and the capture efficiency and corresponding fish damage rate of different fish were analyzed. The experimental and numerical results show that the developed vacuum fish pump can achieve efficient and automatic suction of live fish.

2. Geometric Model Description

2.1. Parameters of Physical Model

The vacuum fish pump takes the experimental cultured fish species as the object of capture. The average weight of these fish is about 2.5 kg/tail, the average density is 1033 kg/m3, and the average diameter of the largest section of the fish is about 90 mm. As shown in Figure 1, the vacuum fish pump is mainly composed of a frame, a water ring vacuum pump, a vacuum valve, a control system, a fish inlet pipe, and a fish outlet pipe. The water ring vacuum pump is used as the main equipment to form a negative pressure, and the vacuum negative pressure is used to generate suction force on the fish water to achieve efficient automatic suction and capture of live fish.
The water ring vacuum pump is used as the main equipment to form a negative pressure, and the vacuum negative pressure is used to generate suction force on the fish water to achieve efficient automatic suction and capture of live fish. The single-tank fish pump used in the experiment is made of stainless steel, and the volume of the tank is 0.6 m3. The specific design parameters are shown in Table 1.

2.2. Design Criteria

In order to ensure that the fish do not gather in the inlet pipe, the diameter of the inlet pipe is generally two times the maximum cross-sectional diameter of the fishing object. Considering other influencing factors, the inlet pipe diameter is selected as 200 mm.
v = Q S
where S is the cross-sectional area of the fish inlet pipe, which is 0.0314 m2; v is the water injection speed of the fish pipeline. Q is the flow rate of the fish inlet pipe. According to Equation (1), which is 0.0314 m3/s.
The working time of the fish pump suction process is 20 s, and the working time of the fish discharge process is 10 s. The inlet flow of the fish pump suction process can be calculated according to Equation (2).
Q = M ρ m t
where Q is the flow rate of the suction pump inlet pipe, which is 0.0314 m3/s; the average density of fish–water mixture is 1020 kg/m3. Substituting the data, M is the total weight of the fish and water suction, which is about 77 t/h. Considering the fish–water ratio of 1:1, the total amount of fish sucked per hour should be no less than 38.5 t/h.
In addition, it is necessary to select the appropriate vacuum pump according to the requirements of live fish capture [9]. The calculation formula of the pumping rate of the vacuum pump is as follows:
S e = 2.3 V 1 + V 2 n t lg H H H g Δ h
Se is the pumping rate in the vacuum pump, which is 1.83 m3/min; v1 is the volume of vacuum fish pump, about 0.6 m3; v2 is the volume of the pipeline, which is about 0.3 m3; t is the time of single pumping, which is 20 s; n is the number of vacuum pumps, which is 1; H is the height of the water column at atmospheric pressure, which is 10 m; Hg is the water absorption height value of the fish pump, which is 4.5 m; Δh is the distance between the center of the pump body and the ground, which is 0.1 m; k is the reserve coefficient, generally take 1.1. According to the parameters such as pump pressure, pumping rate, and safety factor, SLK vacuum pump is selected, and its pumping rate is 4 m3/min, which meets the design requirements.
The efficiency of vacuum fish pump is also an important parameter that designers pay close attention to.
η f i s h   p u m p = M f i s h E f i s h   p u m p
where ηfish pump is efficiency of the vacuum fish pump. Mfish is total mass of live fish caught per unit time. Efish pump is electricity consumed by the vacuum pump per unit time.
In addition, many studies have shown that the efficiency of the vacuum fish pump is mainly affected by the vacuum pump pumping time and the emptying time of the fish tank. The pumping time of the vacuum pump is calculated as follows:
t 1 = 2.3 V 1 S p lg p i p
where t1 is the pumping time of vacuum pump. V1 is the volume of fish pump. Sp is the pumping speed of vacuum pump. pi is the pressure in the fish collecting tank at the beginning of pumping. p is the pressure in the fish collecting tank after pumping in t1 time.
The emptying time of the fish tank is calculated as follows:
t 2 = 2 V 2 0.82 × π 4 × d 2 × 2 g H 1
where t2 is the emptying time of the fish collecting tank. V2 is the volume of water. d is the diameter of the drainage pipe. Hl is the height of water level.
According to Formula (3), the suction of the suction pump during fishing is inversely proportional to the pumping time of the vacuum pump when the pumping speed is constant. The suction of the suction pump during fishing determines the speed of absorbing the fish–water mixture. According to the Formula (4), the emptying time of the fish tank also increases with the increase in the water volume in the fish tank or the decrease in the fish–water mixing ratio. In addition, in order to prevent the collision damage between fish bodies caused by excessive fish density in the fish collection tank, it is required that the fish–water ratio in the fish collection tank should not be greater than 1.

3. Numerical Simulation of Vacuum Fish Pump

In this Section, the numerical simulation results of variable suction of vacuum suction pump are first discussed, and then the numerical simulation results of constant suction of vacuum suction pump are carried out. Finally, the differences between the two are compared, and some guidance suggestions are provided for the vacuum suction pump experiment in Section 3.2 below.

3.1. Numerical Method Description

In this study, the fish–water mixtures are regarded as a viscous incompressible continuous liquid phase, and the air is regarded as a viscous compressible continuous gas phase. The three-dimensional unsteady Reynolds-averaged Navier–Stokes equations are as follows:
ρ t + ρ u i x i = 0
ρ t + ρ u i u j x j = ρ x i + ρ x j μ + μ t u i x j + u j x i
where t is time. p is fluid pressure. ui and uj are velocity components. xi and xj are displacement components. μ is the dynamic viscosity coefficient. ρ is the fluid density. μt is the turbulent viscosity coefficient.
Due to the complex flow problems, such as large strain rate, swirling flow, and liquid–solid separation in the process of fish suction, the turbulence model adopts SST k-ω turbulence model [18,19]. The k-equation and ω-equation of SST k-ω are as follows:
t ( ρ k ) + x i ( ρ k u i ) = x j ( Γ k k x j ) + G k Y k + S k , t ( ρ ω ) + x i ( ρ ω u i ) = x j ( Γ ω ω x j ) + G ω Y ω + D ω + S ω .
where k is the turbulent kinetic energy, ω is the turbulent frequency, Γk and Γω are the turbulent diffusion coefficients, Gk and Gω are the turbulent generation terms, Yk and Yω are the turbulent kinetic energy dissipation terms, and Sk and Sω are the custom source terms, respectively.
The VOF model is suitable for stratified or free surface flow, and the mixed model or Euler model is suitable for the case where there is phase mixing or separation in the flow, or the volume fraction of the dispersed phase exceeds 10%. For the gas–liquid two-phase flow contact inside the fish pump, the difficulty lies in the tracking of the free liquid surface. The VOF model constructs and tracks the free surface by introducing the volume fraction of each phase fluid in the grid element at each time α [20]. The reconstruction of the water–air free interface is realized by solving the following form of continuity equation:
α g t + u i α g x i = 0 .
For the gas–liquid two-phase flow field inside the fish pump, the volume fraction of air in the unit is ag, and the volume fraction of water is 1 − ag. There are three possibilities for ag in the calculation unit: ag = 0, indicating that the unit is full of water. 0 < ag < 1, indicating that there is both air and water in the unit; ag = 1, indicating that the free surface unit is filled with air.
As shown in Figure 2, the suction pump inlet, the pressure inlet boundary is selected, the initial pressure value is set according to the parameters of the vacuum pump, the fish outlet and the suction pump cavity are set as the fixed non-slip wall boundary, and the suction pump outlet is set as the pressure outlet boundary.
The CFD numerical of this study was realized based on a computational fluid dynamics commercial software FLUENT 2022R2. For the solution of unsteady flow field, the above partial differential equation is transformed into the solution of linear equations. Firstly, the finite volume method (FVM) of unstructured grids is used to discretize these partial differential equations, as shown in Figure 2. For pressure–velocity coupling, the semi-implicit method of pressure–velocity coupling is used to discretize the convection term. Then, the pressure and momentum are spatially discretized by the second-order upwind difference scheme. The second-order implicit difference scheme is used for time discretization. Finally, the Gauss–Seidel iteration method is used to solve the algebraic equation by using an algebraic multigrid (AMG) solver. For the discretized continuous equation, momentum equation, and energy equation, the convergence criterion of the inner iteration is set to 10−6.
To ensure the stability and convergence of the computation, a Courant–Friedrichs–Lewy number (C0) should control to be less than 1, which can be defined as follows:
C 0 = U M a x Δ t Δ x M i n
where Umax is the maximum flow velocity, Umax = 13 m·s−1. The ΔxMin is the minimum grid size, ΔxMin = 7.5 × 10−3 m. Δt is time step, Δt = 5 × 10−4 s.
In order to verify the effectiveness of the above numerical method, the experiment device is shown in Figure 3a, including a set of PIV systems (light source system, synchronization system, image capture system, image analysis system) and a physical model of vacuum fish pump made of transparent plexiglass. The specific arrangement of the experiment is shown in Figure 3b, an optical camera is placed at the front end of the transparent fish pump, and a laser emitter is placed at the bottom to measure the cross-sectional velocity information of the measured flow field. There are four monitoring points (point A, point B, point C, point D) were set up on the middle section of the suction pump. The flow velocity values of the monitoring points under the pump pressure of 0.4 bar and constant pump pressure (equivalent inlet flow velocity v = 1 m/s) were measured, respectively, by an optical image analysis system. However, the tracer particles in the image will be misaligned when the fluid PIV experiment is carried out in a circular tube, and the measurement results deviate from the real flow velocity. In this regard, a linear image correction method is used to linearly scale the deformed image in the radial direction according to a certain proportion to correct the distorted image [21]. The image correction uses a bilinear interpolation algorithm. When the distorted image is linearly corrected, it is scaled n31 times in the radial direction, and n31 can be calculated by the following formula:
n 31 = n 3 n 1
where n3 is the refractive index of air, n3 = 1.0. n1 is the refractive index of water, n1 = 1.33. The slope of the initial section of the distortion curve has nothing to do with the material of the pipe but only with the refractive index of the medium in the pipe.
The main difference between the numerical simulation method of variable pressure and constant pressure vacuum fish pump is whether the pressure value of the initial pressure boundary is a constant value, and the other settings are completely consistent. Table 2 is a comparative analysis of the water flow velocity results measured at monitoring points 1, 2, 3, and 4 under the numerical simulation and experimental methods of the constant pressure vacuum fish pump. It can be seen that the error between the experimental values and the numerical simulation results at the four monitoring points is small. At the same time, three kinds of mesh sizes (10−3, 5 × 10−4, 10−4) corresponding to mesh 1, mesh 2, and mesh 3 are selected for mesh independence verification analysis. The results show that the selected mesh size achieves numerical convergence. In order to ensure the computational efficiency, the mesh size of 5 × 10−4 m is selected in the subsequent numerical calculation. Figure 4 is a comparative analysis of the numerical simulation of the variable pressure vacuum fish pump and the experiment results at the monitoring point A. It can be seen that the fish pump has the maximum speed at the monitoring point A when it starts to catch; that is, the fish pump has the maximum suction. With the increase in time, the speed at the monitoring point A gradually decreases, and the final speed decays to 0. At the same time, the experimental results are basically consistent with the numerical results, indicating the effectiveness of the numerical method of the variable pressure vacuum fish pump.

3.1.1. Numerical Simulation of Variable Suction of Fish Suction Pump

The internal liquid phase cloud diagram of the pump at 0–4 s when the vacuum fish pump is working, as shown in Figure 5. It can be found that when the internal pressure of the fish pump is balanced with the external atmospheric pressure, the fish pump stops catching, and the internal water level of the pump reaches the highest value. With the increase in the pressure difference between the working pressure and the external pressure, the higher the internal water level of the pump, the higher the catching efficiency. Through quantitative analysis, it can be concluded that the total water body inside the fish pump is 1.947 m3 when the working pressure is 0.1 bar, accounting for 92.7% of the pump; when the working pressure is 0.2 bar, the total water inside the fish pump is 1.476 m3, accounting for 70.3% of the pump. When the working pressure is 0.3 bar, the total water inside the fish pump is 1.236 m3, accounting for 58.8% of the pump. When the working pressure is 0.4 bar, the total water inside the fish pump is 0.846 m3, accounting for 40.3% of the pump.
Figure 6a shows the velocity duration curve at the inlet of the vacuum pump. On the whole, with the increase in time, the fishing speed of the suction pump becomes lower and lower, and the speed is zero at about 3–3.5 s (the internal and external pressure difference is equal at this time). Because the water has inertial speed, it will increase again and then gradually decay to zero. At the same time, the attenuation of velocity shows obvious nonlinearity, and the larger the internal and external pressure difference is, the more obvious the nonlinearity is. With the increase in the internal and external pressure difference, the start-up speed and the average speed of the arrest are greater, and the time to reach the equilibrium time is longer. This also explains the reason why the higher the pressure difference between the external pressure and the working pressure in Figure 5, the higher the internal water level of the pump.
The pressure duration curves at the inlet and outlet of the fish pump are shown in Figure 7; it can be seen that the pressure at the inlet gradually increases with time and finally rises to the equilibrium value (external atmospheric pressure value). According to Figure 6, the inlet velocity is 10–13 m/s at the initial time. If the live fish enters the suction pump at this high speed, there will be a great probability of damage or even death. In other words, it is difficult to control the suction by using this variable suction method. If the initial suction is too large, it will cause damage to the live fish, and if the suction is too small, the capture efficiency is too low. For the above problems, we think that we can reduce the initial suction of the fish pump and control the continuous pumping of the fish pump to maintain a constant pressure difference inside and outside the fish pump. This is an effective measure to reduce the damage rate of the fish body without affecting the fishing efficiency.

3.1.2. Numerical Simulation of Constant Suction of Fish Suction Pump

In order to verify the feasibility of the above ideas, the variation in the internal flow field of fish suction pump under constant pressure difference was simulated using the CFD method. The distribution of liquid volume fraction at different water injection speeds is shown in Figure 7; it can be seen that the water level in the suction pump rises significantly more slowly than that in the suction pump with variable suction. When the pumping speed is 1 m/s, the total water inside the fish pump is 0.945 m3, accounting for 45% of the pump at 15 s. When the pumping speed is 1.5 m/s, the total water inside the fish pump is 1.428 m3, accounting for 68% of the pump at 15 s. When the pumping speed is 2.0 m/s, the total water inside the fish pump is 1.89 m3, accounting for 90% of the pump at 15 s. As shown in Figure 8, the main part of the impact of the fish body is the bottom of the suction pump, and the impact of the bottom of the suction pump is smaller with the increase in the water level. As the velocity increases, the impact point gradually moves away from the velocity inlet.
Obviously, using the control method of constant pressure difference can reduce the impact of the small fish body on the suction pump chamber, but the catching efficiency will be significantly reduced. From the perspective of live fish transportation, this suction method is more suitable.

3.2. Physical Experiment of Vacuum Fish Pump

The vacuum fish pump prototype was used to carry out the capture experiment of pond fish, as shown in Figure 9. The experimental pond is mainly the grass carp, mixed carp, crucian carp, silver carp, and bighead carp. The vacuum fish pump is fixed horizontally on the bank of the pond. The center of gravity of the fish pump is about 1.5 m away from the shore and about 2.5 m from the water surface of the pond. The suction port of the fish suction pump is placed in the fish cage of the fish pool. The inner wall of the suction pipe is smooth and soft, and will not cause damage to the fish body. The vacuum fish suction pump in the aquaculture pond has initially realized the function of sucking fish with water. The fish suction time is 20 s, the fish release time is 10 s, and the fish suction and release operations are carried out in turn. Based on the numerical results, the experiment uses continuous pumping to keep the pressure difference between the inside and outside of the fish pump unchanged, which means that the fishing speed of the fish pump remains unchanged. In this experiment, the suction speed is 1, 2, and 3 m/s, respectively. Taking the suction speed of 1 m/s as an example, the average suction amount of the actual fish-water mixture was measured to be about 73 t after 1 h of operation and 120 cycles.
The vacuum fish pump experiment mainly performs performance experiments, including fish intake, cycle times, and fish body damage experiments. The fish body damage experiment mainly observes whether the fish body surface has bleeding, lack of scales, and scars. In order to further quantify the degree of fish damage, we propose a preliminary formula for calculating the fish damage rate applicable to vacuum suction pumps, as follows:
F i s h d a m a g e = S d a m a g e S F i s h
where Fishdamage is fish damage due to vacuum suction pumps. Sdamage is the sum of the areas on the fish with missing scales or scars. SFish is surface area of the fish.
The single-tank vacuum suction pump experiment was carried out on grass carp, carp, crucian carp, silver carp, and bighead carp, and the detailed parameters of the experiment fish are shown in Table 3. The results showed that the average single suction amount of crucian carp and carp was larger, and the average single suction amount of grass carp and bighead carp was smaller. The fish–water ratio was in the range of 1:1.5~1.68, and the dephosphorization of fish surface was observed. The damage rate of fish body was quantitatively analyzed from the dephosphorization of fish body surface. As shown in Figure 10, the damage rates of grass carp, silver carp, and bighead carp were 2%, 0.3%, and 0.1%, respectively, under the condition of suction speed of 1 m/s. Carp and crucian carp were not damaged, and the average suction volume of the suction pump was about 23 t/h. Under the condition of suction speed of 1.5 m/s, the damage rates of grass carp, carp, silver carp, and bighead carp were 3%, 0.3%, 0.7%, and 0.3%, respectively. Crucian carp was not damaged, and the average suction volume of suction pump was about 28 t/h. Under the condition of suction speed of 2 m/s, the damage rates of grass carp, carp, crucian carp, silver carp, and bighead carp were 5%, 1%, 0.5%, 1%, and 0.5%, respectively, and the average suction volume of the suction pump was about 36 t/h. The actual total amount of fish suction was low in the experiment due to the performance of vacuum pump, the influence of sealing performance of fish suction pump, and the fish–water ratio.
The suction capacity of the fish pump corresponding to the three suction speeds is about 23 t/h, 28 t/h, and 36 t/h, respectively, which has a certain gap with the design value. This is mainly due to the performance of the vacuum pump, the sealing performance of the fish pump, and the small fish–water ratio. With the increase in suction speed, the average weight of single suction increases gradually, and this increase does not show a geometric multiple growth relationship according to the increase in suction speed. In addition, the damage rate of the fish body also increases with the increase in suction speed. It can be seen from Table 4 that the damage rate of fish with relatively small size is relatively low. For example, crucian carp did not appear damaged until the suction speed reached 2 m/s, and the damage rate was only 0.5%. Common carp did not appear damaged until the suction speed reached 1 m/s, and the damage rate was only 0.3%. Grass carp is longer and easier to scale than common carp, crucian carp, silver carp, and bighead carp. The damage rate was 5% at a high suction speed (v = 2 m/s). This shows that the degree of damage to the fish body depends largely on the hardness of its own scales and the size of the fish body. The larger the size of the fish body, the more likely it is to be damaged during high-speed transport. The damage rate of fish body at the suction speed of 1 m/s meets the actual demand, so it is the best suction speed. The working efficiency of the fish suction pump mainly depends on the flow velocity of the suction pipe and the fish–water ratio of the fish collection system. The greater the flow velocity, the greater the suction volume per unit time of the suction pump, and the higher the working efficiency; the working efficiency of the vacuum fish pump is not only related to the flow velocity but also closely related to the fish–water ratio. Studies have shown that when the fish–water ratio is 1:1, the best fish absorption effect can be achieved. The fish–water ratio of the suction pump in this study is small, in the range of 1:1.5~1.68. When the suction pump begins to pump, the fish–water ratio is large, but as the suction progresses, the fish–water ratio in the fish collection system will gradually decrease, affecting the efficiency of the suction pump. In order to improve the working efficiency of the fish pump, it is necessary to develop an efficient fish collection device in the subsequent research, which can maintain the fish–water ratio at a certain level. In addition, it is also a meaningful measure to carry out research on the double-tank vacuum fish pump.
In order to obtain the efficiency of the vacuum suction pump, the electrical energy consumed and the total mass of live fish caught by the vacuum suction pump in one hour of continuous operation were measured. The efficiency of the vacuum fish suction pump can be calculated according to Equation (4). The calculation results of the efficiency of the vacuum fish pump under three suction speeds and fish–water ratios are shown in Table 4. It can be clearly seen that under the condition of a certain fish–water ratio, its efficiency gradually decreases with the increase in the suction speed of the suction pump. As the fish suction pump fish–water ratio increases, the efficiency of the vacuum fish suction pump also gradually increases when the suction speed is 1 m/s. This is because as the fish–water ratio increases, the fish discharge time (t2) will increase, while the fish suction time (t1) will be shortened. As a result, the actual pumping operation time of the vacuum pump becomes shorter, i.e., the power consumption becomes less. Although the total mass of live fish pumped also decreases as the fish–water ratio increases, the reduction is not as great as the reduction in electrical energy consumption. Therefore, the efficiency of the vacuum suction pump increases as the fish–water ratio increases.

4. Conclusions

In this study, a vacuum fish pump used in small fishing vessels or fishing farms was designed according to the design theory of fish pumps. Then, the computational fluid dynamics method was used to simulate the internal flow field during the operation of variable suction and constant suction vacuum fish pump. Finally, five kinds of cultured fish were selected for separate physical experiments. The main conclusions are as follows:
(1)
The VOF model can well predict the change in liquid volume fraction with time in the whole calculation area during the suction process of vacuum fish pump.
(2)
The fish suction method with constant pressure difference can reduce the impact of fish body on the cavity of suction pump, which is suitable for the transportation of live fish.
(3)
The degree of damage to the fish body depends largely on the hardness of its own scales and the size of the fish body. The larger the size of the fish body, the more likely it is to be damaged during high-speed transport.
(4)
As the suction speed of the suction pump increases, the efficiency of the vacuum suction pump gradually decreases. The efficiency of the vacuum suction pump gradually increases with the increase in the fish–water ratio of the suction pump.
(5)
The suction speed of 1 m/s is an appropriate pumping speed to ensure both the damage rate of fish and the efficiency of vacuum pump.

Author Contributions

C.T., conceptualization of the study, simulation and design of experiments. Z.Q., data collection, data analysis. X.C., manuscript writing, manuscript revision. M.H., manuscript writing, graph creation. Y.Z., literature search, data collection. F.W., main writing of the manuscript, main revision of the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

The research work supported by the earmarked fund for China Agriculture Research Systenm (CARS-45-26) and the National Key Research and Development Program of China (2023YFD2401702).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

All authors of this study have been informed and consented to publication.

Data Availability Statement

Data is contained within the article.

Conflicts of Interest

Author Qu Zhi was employed by the company Kunshan Shifuda Automation Technology Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential con-flict of interest.

References

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Figure 1. The structure diagram of vacuum fish pump. 1. Rack. 2. Tank body. 3. Inlet gate. 4. Inlet pipe. 5. Outlet pipe. 6. Outlet gate. 7. Tee pipe. 8. Vent. 9. Vacuum valve. 10. Vacuum pump. 11. Tank.
Figure 1. The structure diagram of vacuum fish pump. 1. Rack. 2. Tank body. 3. Inlet gate. 4. Inlet pipe. 5. Outlet pipe. 6. Outlet gate. 7. Tee pipe. 8. Vent. 9. Vacuum valve. 10. Vacuum pump. 11. Tank.
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Figure 2. Boundary condition setting and mesh division.
Figure 2. Boundary condition setting and mesh division.
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Figure 3. Experimental layout diagram: (a) experimental model; (b) experimental monitoring layout.
Figure 3. Experimental layout diagram: (a) experimental model; (b) experimental monitoring layout.
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Figure 4. Numerical simulation and experiment values of fish suction pump with variable suction.
Figure 4. Numerical simulation and experiment values of fish suction pump with variable suction.
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Figure 5. The gas–liquid distribution at the inlet of the vacuum fish pump.
Figure 5. The gas–liquid distribution at the inlet of the vacuum fish pump.
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Figure 6. (a) The velocity duration curve. (b) Pressure duration curve.
Figure 6. (a) The velocity duration curve. (b) Pressure duration curve.
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Figure 7. The gas–liquid distribution at the inlet of the vacuum fish pump.
Figure 7. The gas–liquid distribution at the inlet of the vacuum fish pump.
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Figure 8. The pressure distribution at the inlet of the vacuum fish pump.
Figure 8. The pressure distribution at the inlet of the vacuum fish pump.
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Figure 9. The experiment of fish pump.
Figure 9. The experiment of fish pump.
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Figure 10. (a) Grass carp. (b) Carp. (c) Crucian carp. (d) Silver carp. (e) Bighead carp.
Figure 10. (a) Grass carp. (b) Carp. (c) Crucian carp. (d) Silver carp. (e) Bighead carp.
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Table 1. Parameter of fish pump design.
Table 1. Parameter of fish pump design.
Sizes of Vacuum Fish PumpValues
Tank volume0.6 m3
Inner diameter of suction pipe200 mm
Single suction time of fish20 s
Single fishing time10 s
Weight of the arrested object≤2.5 kg
Maximum cross-sectional diameter of the fishing object≤0.005 m2
Table 2. Numerical simulation and experiment values of water flow velocity at point A to point D.
Table 2. Numerical simulation and experiment values of water flow velocity at point A to point D.
Monitoring PointMesh 1
(m/s)
Mesh 2
(m/s)
Mesh 3
(m/s)
Experimental Value (m/s)
Point A0.1370.1420.1350.15
Point B0.3250.3210.3120.35
Point C0.1920.2130.2110.21
Point D0.3840.4010.3780.42
Table 3. Data of fish pump experiment.
Table 3. Data of fish pump experiment.
Type of Experiment ObjectLength × Maximum Diameter/mmAverage Quality/kgAverage Suction Velocity/m·s−1
Grass carp700 × 13031
1.5
2
Carp260 × 1001.51
1.5
2
Crucian carp240 × 900.51
1.5
2
Silver carp470 × 1201.51
1.5
2
Bighead carp500 × 13021
1.5
2
Table 4. Efficiency of vacuum fish pump.
Table 4. Efficiency of vacuum fish pump.
Efficiency of Vacuum Fish Pump/t (kW h)−1Suction Speeds/m−1Fish-Water RatioElectric Power Consumption/kW hTotal Mass of Live Fish/t
5.9921:13.6722
9.881.51:11.6716.5
11.0011:1111
11.7311:1.50.758.8
14.0411:20.527.3
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MDPI and ACS Style

Tian, C.; Qu, Z.; Che, X.; Han, M.; Zhou, Y.; Wu, F. Numerical Simulation and Experimental Study of a Small-Scale Vacuum Fish Pump. J. Mar. Sci. Eng. 2024, 12, 2296. https://doi.org/10.3390/jmse12122296

AMA Style

Tian C, Qu Z, Che X, Han M, Zhou Y, Wu F. Numerical Simulation and Experimental Study of a Small-Scale Vacuum Fish Pump. Journal of Marine Science and Engineering. 2024; 12(12):2296. https://doi.org/10.3390/jmse12122296

Chicago/Turabian Style

Tian, Changfeng, Zhi Qu, Xuan Che, Mengxia Han, Yin Zhou, and Fan Wu. 2024. "Numerical Simulation and Experimental Study of a Small-Scale Vacuum Fish Pump" Journal of Marine Science and Engineering 12, no. 12: 2296. https://doi.org/10.3390/jmse12122296

APA Style

Tian, C., Qu, Z., Che, X., Han, M., Zhou, Y., & Wu, F. (2024). Numerical Simulation and Experimental Study of a Small-Scale Vacuum Fish Pump. Journal of Marine Science and Engineering, 12(12), 2296. https://doi.org/10.3390/jmse12122296

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