# Ultrasonic degassing of total dissolved gas supersaturated, flowing water in a laboratory flume

W. Ludwig Kuhn, Bjørn Winther Solemslie, Jean-Yves Hihn, Ole Gunnar Dahlhaug

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W. Ludwig Kuhn, Bjørn Winther Solemslie, Jean-Yves Hihn, Ole Gunnar Dahlhaug. Ultrasonic degassing of total dissolved gas supersaturated, flowing water in a laboratory flume. IOP Conference Series: Earth and Environmental Science, 2024, 1385 (1), pp.012018. 10.1088/1755-1315/1385/1/012018. [hal-04701264](https://hal.science/hal-04701264v1)

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Ultrasonic degassing of total dissolved gas supersaturated, flowing water in a laboratory flume

To cite this article: W. Ludwig Kuhn et al 2024 IOP Conf. Ser.: Earth Environ. Sci. 1385 012018

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# Ultrasonic degassing of total dissolved gas supersaturated, flowing water in a laboratory flume

W. Ludwig Kuhn¹, Bjørn Winther Solemslie¹,², Jean-Yves Hihn³ and Ole Gunnar Dahlhaug²*

¹ Norwegian Institute for Nature Research, Trondheim, Norway

² Department of Energy and Process Engineering, Norwegian University of Science and Technology, Trondheim, Norway

³ Université de Franche-Comté, CNRS UMR 6213 UTINAM, Besançon, France

E-mail: *ole.g.dahlhaug@ntnu.no

**Abstract.** Artificial total dissolved gas (TDG) supersaturation is a potentially lethal threat to the aquatic ecology downstream of hydropower plants. The severity of this threat dependents on the actual saturation level as well as on the duration of the event. Natural degassing is highly influenced by the river's morphology and characteristics, and is in most cases insufficiently slow. To protect fish and aquatic invertebrates from gas bubble disease, a direct consequence of TDG supersaturation, the use of ultrasonic degassing to mitigate TDG supersaturation is investigated. In this process, a high-power ultrasonic field is introduced into the water, leading to an immediate reduction of the TDG saturation level. Experiments are conducted at the Waterpower Laboratory at NTNU, Norway. Within an open flume, water is channeled past an ultrasonic transducer, which introduces an acoustic field with a frequency of 20 kHz. The flow rate in the flume is adjustable, and two different flow rates are tested. In addition, the dependency of the degassing process on the acoustic power is investigated. Results indicate a strong dependency on the acoustic power, with the highest tested powers resulting in the largest reduction in TDG saturation level. The flow velocity appears to have a positive effect on the degassing efficiency as well, even though this effect is minor compared to the effect of the acoustic power. A continuous effect of the acoustic degassing in form of a gas bubble cloud leading to increased liquid-gas mass transfer is observed.

## 1. Introduction

The ongoing global change of climate has increased the need for very low to zero-carbon emission energy sources, such as renewable energy sources, nuclear power, or fossil fuels with carbon capture and storage (CCS) [1]. Hydropower, as a well-established renewable energy source, plays a significant role in facilitating the energy transition and provides flexibility to energy grids that are increasingly incorporating intermittent energy sources, e.g. wind or photovoltaic solar energy [2, 3]. Nevertheless, the construction and operation of hydropower plants under certain circumstances lead to social or environmental problems [4].

One such environmental impact on the aquatic ecology downstream of hydropower plants is artificial total dissolved gas (TDG) supersaturation. This phenomenon occurs due to air entrainment into the power plant's or downstream waterways, which typically takes place during flood situations and subsequent spilling operations, or at under-dimensioned brook intakes [5, 6]. Despite being a known problem since the 1970s, official threshold values exist only in parts of

<figure><img src="image_3.png" /></figure>
the USA and Canada [7–9]. Nonetheless, research on the prevention and mitigation of TDG supersaturation, as well as its effects on local aquatic species, has been increasing in recent years [5, 10, 11].

TDG supersaturation in water refers to cases where the saturation level of all gases in solution exceeds the equilibrium value, which is 100%TDG in case of atmospheric pressure. The saturation is linked to the gas concentration and therefore directly proportional to the ambient pressure, as stated in Henry's law. An overall consensus exists on the degree of TDG supersaturation being tolerable to fish, with a threshold being set around 110%TDG. Beyond this level, the mortality rates of fish and aquatic invertebrates increase, depending on factors such as the degree of TDG supersaturation, the exposure time, the local river depth, the species, and the life-stage of the individual [6, 12, 13]. A major consequence of TDG supersaturation is gas bubble disease (GBD). This condition occurs in fish and aquatic invertebrates exposed to TDG supersaturated water and leads to gas bubbles forming within in their tissues, particularly in the fins and eyes, but potentially in organs and blood vessels as well. Ultimately, GBD can cause the death of the affected organism due to gas embolism. Secondary effects of GBD include the formation of scare tissue or increased proneness to diseases after healing, in addition to loss of submergence and subsequent habitat loss or vulnerability to predators [6, 14]. Consequences of TDG supersaturation and response behavior of local fish species and other aquatic lifeforms are ongoing research topics (see e.g. [10, 11, 15–19]).

Current research primarily focuses on preventing air entrainment in the stilling basin below large dams by adapting the spillway [20–22]. Additional research is found on the use of microbubbles to mitigate TDG supersaturation [23–25], or on adjusting the operation of hydropower plants to prevent the occurrence or diminish the distribution of TDG supersaturation [26–28]. Nevertheless, there is still a lack of highly effective mitigation measures that can rapidly decrease TDG saturation levels in flowing water.

In Norway, the problem of artificial TDG supersaturation in the waterways downstream hydropower plants was discovered about 50 years ago, and multiple committees provided guidelines and recommendations on countermeasures through studies and reports (see e.g. [6, 29–31]). Most instances of TDG supersaturation in Norway occur when air is entrained at brook intakes during flooding, but clogged intake trash racks or turbine aeration are causes as well [6, 32]. Due to expected increases in flooding due to changing precipitation patterns as a cause of climate change [33–39], the occurrence probability of artificial TDG supersaturation is rising as well. Due to the slightly different emergence of TDG supersaturation in power plants utilizing medium- to high-head Francis turbines, the dissolution rate of air in water is much higher than in a stilling basin downstream a dam, by such resulting in much higher TDG saturation levels. Reports from a river in southern Norway show measured values of more than 180%TDG at the power plant outlet, with elevated saturation levels persisting up to 30 km downstream [40].

This issue led to the formation of the DeGas project, which focuses on studying the applicability of technical methods to mitigate TDG supersaturation downstream hydropower plants. One promising approach is the use of ultrasonic degassing. To evaluate its effect and efficiency, experiments are conducted in a laboratory flume at the Waterpower Laboratory (Norwegian University of Science and Technology, NTNU). Unlike other laboratory studies that treat a static volume of water, this research implements a flow past the degassing device, by such decreasing residence time and creating a better representation of an actual application in the field. The preparation of TDG supersaturated water to feed the channel will be described next to the degassing channel in the methods part. Moreover, the derivation of the liquid-gas mass transfer coefficient, allowing for comparison with other degassing methods, will be presented.
## 2. Methods

The procedure implemented at the Waterpower Laboratory at NTNU to study the mitigation of TDG supersaturation by technical methods consists of three steps: increasing the TDG saturation level in a large volume of water, transferring the water to the degassing channel, and performing the degassing within the channel. An in-house pressure system was utilized for the first step, while the degassing experiments took place in a custom-made test rig. All steps are described in this section.

### 2.1. *Reaching a higher TDG saturation level in water*

To increase the TDG saturation level in water, two principles were applied: increasing the system pressure and expanding the interfacial area between air and water. The first was implemented using a pump to pressurize 15 m³ of water in an 18 m³ pressure tank to 2 bar in the head-space tank. The system pressure was monitored using a GE PTX 610 pressure sensor at the top and a SITRANS P Z series pressure sensor at the bottom of the tank. The filled tank was isolated from the laboratory environment. In that state, the system allowed only for air dissolution through the water surface at the top of the water column. To enhance the dissolution efficiency, additional air was bubbled in through a porous diffuser placed at the bottom of the tank. The diffuser, featuring a pore size of 0.5 µm and a surface area of 11.2 cm², introduced small-sized bubbles into the system. Thereby, the interfacial area was increased and the dissolution of gas in the water was promoted. Air was supplied from an in-house compression system at 1.5 bar overpressure at the bottom of the pressure tank. The ambient pressure was maintained constant at 2 bar due to an adjustable BREWTOOLS pressure relief valve placed at the top of the tank. The system was operated for 2 h to reach a high TDG saturation level. Throughout this time, the TDG level was monitored using an INWATERTECH PT4 TDG sensor positioned at the bottom of the tank. A method described by [41] was used to calibrate the sensor prior to the experiments. To flush the sensor and provide mixing of water within the system, a GRUNDFOS TP 40-50/2 pump was utilized below the pressure tank.

Due to the simultaneous mixing effects of the pump and the air being flushed in into the system, the TDG saturation level was assumed to be constant across the water column. Nevertheless, after stopping the air input into the system and decreasing the ambient pressure, a gradient in the TDG saturation level can develop. Therefore, the degassing experiments were initiated immediately after completing the TDG supersaturation process.

### 2.2. *Water transfer to the test rig*

For practical reasons, the outlet size at the bottom of the pressure tank was small compared to the horizontal cross-sectional area of the entire tank. Pressure hoses were connected to two valves at the bottom of the tank to control the outflow of water. Between the pressure exerted by the hydrostatic head at the bottom of the pressure tank and the atmospheric pressure at the inlet of the degassing channel, a pressure drop occurred, which is increased by the sudden reduction in area at the outlet. This imposed the danger of natural degassing occurring during the water transfer from the pressure tank to the degassing test rig. To smoothen the pressure drop, the two hoses were dimensioned to a length of 15 m each. Such, the pressure drop was distributed in a way that effectively prevented natural degassing during the transfer process.

### 2.3. *Degassing test rig*

The degassing test rig, designed and built at the Waterpower Laboratory at NTNU, is an open channel measuring 17.8 m in length with a cross-section 300 mm by 400 mm. It features an inlet section with an additional length of about 2 m to ease the transition of water between the hoses and the channel. The inlet consists of two diffusers separated by a flow straightener. Degassing devices can be placed at a chosen point within the test rig channel, or in a module between the
two inlet diffusers. At the outlet, a V-notch weir allows in combination with a low pressure sensor (VEGA Vegabar 14, 0.0 bar to 0.1 bar) for measuring the channel flow rate following the ISO 1438 standard. For detailed design information of the V-notch weir, refer to [42]. Its design determines the maximum flow rate within the channel, which is 4 dm³/s. In Figure 1, the degassing test rig is displayed using a technical drawing.

<figure><img src="image_4.png" /><figcaption>Figure 1: Technical drawing of the degassing test rig, including a detailed side view of the modular inlet featuring two diffusers and a flow straightener (top), and a side view (center) as well as an isometric view of the full test rig (bottom). Flow through the channel from left to right. Units in mm.</figcaption></figure>

The TDG saturation level was monitored throughout the channel using INWATERTECH PT4 TDG sensors. They were placed near the hose connection within the first inlet-diffuser and at four positions within the channel: at inlet and at 4.25 m, 9.25 m and 14.25 m downstream, respectively. Windows at each of the sensor positions downstream in the channel and within the second inlet diffuser provide visual access to the water column.

The maximum flow rate and the channel cross-section result in a maximum velocity within the channel of 3.7 cm/s. Flushing water with TDG supersaturation led to the formation of gas bubbles on all surfaces due to natural degassing. This could not be prevented, as the water depth did not create sufficient pressure [41], nor was the water velocity sufficient to dislodge the bubbles. Such gas bubbles forming up on the silastic membrane of the TDG sensors lead to erroneous readings. A solution to this was found in a timed, local acceleration of the water at the
TDG sensor positions. This was provided by EHEIM compactON 5000 pumps placed underneath the channel, as displayed in Figure 1. They drew water from 100 mm upstream of the sensors and increased the flow velocity locally to 1.97 m/s. Thus, bubbles attached to the TDG sensor membrane were successfully flushed away. The pumps were operated in a way that they flush water for 3 s every 30 s to minimize flow disruption in the channel.

## 2.4. Calculating the liquid-gas mass transfer coefficient

The TDG saturation measurements within the channel are used to calculate of the liquid-gas mass transfer coefficient, $k_L a$. This coefficient is determined by multiplying the partial transfer conductance, $k_L$, which represents the rate of molecular diffusion in the liquid, by the interfacial area, $a$, which describes the size of the area between the liquid and gas phases. Through its determination, the efficiency of the ultrasonic degassing process can be evaluated. Following mass balance equations, a general equation for liquid-gas mass transfer was developed:

$$ \Phi = V k_L a (S_{\text{TDG,avg}} - S_{\text{TDG}}^*), \qquad (1) $$

where $\Phi$ describes the loss of dissolved gas between two measurement locations, $V$ is the volume of that channel section, $S_{\text{TDG,avg}}$ is the average TDG saturation within the channel section, and $S_{\text{TDG}}^*$ is the baseline saturation, which is the TDG saturation at equilibrium and atmospheric pressure (100%TDG). The loss of dissolved gas between two measurement points, A and B, was determined from the difference in the measured TDG saturation at these locations multiplied by the flow rate. The average TDG saturation within a channel section was calculated using the logarithmic mean:

$$ \Phi = (S_{\text{TDG,A}} - S_{\text{TDG,B}}) Q, \qquad (2) $$

$$ S_{\text{TDG,avg}} = \frac{S_{\text{TDG,A}} - S_{\text{TDG,B}}}{\ln S_{\text{TDG,A}} - \ln S_{\text{TDG,B}}}. \qquad (3) $$

Taking the two Equations 2 and 3 and inserting them into Equation 1 yields the following derivation for $k_L a$:

$$ Q(S_{\text{TDG,A}} - S_{\text{TDG,B}}) = V k_L a \left( \frac{S_{\text{TDG,A}} - S_{\text{TDG,B}}}{\ln S_{\text{TDG,A}} - \ln S_{\text{TDG,B}}} - S_{\text{TDG}}^* \right) \qquad (4) $$

$$ \Rightarrow k_L a = \frac{Q(S_{\text{TDG,A}} - S_{\text{TDG,B}})}{V \left( \frac{S_{\text{TDG,A}} - S_{\text{TDG,B}}}{\ln S_{\text{TDG,A}} - \ln S_{\text{TDG,B}}} - S_{\text{TDG}}^* \right)} \qquad (5) $$

## 2.5. Ultrasonic degassing

The process of ultrasonic degassing is mostly known from metallurgic applications [43]. It is based upon the introduction of a low-frequency, high-power ultrasonic field into a liquid body. The acoustic cause bubbles to form due to the spontaneous creation of a single-bubble at a nucleus within the liquid, specific sources at walls or in the liquid, or through fragmentation of already existing bubbles [44]. These bubbles then grow within the acoustic field as a reaction to coalescence or accumulation at antinodes of the acoustic field. At a certain threshold, they start an inertial oscillation and grow through a process called rectified diffusion. Once they reach the fragmentation threshold, the bubbles break up and multiply [45]. Shape-stable bubbles at
resonance size contribute to the ultrasonic degassing process when they become buoyant and move to the liquid surface. They are therefore referred to as degas bubbles [46].

## 2.6. *Experimental procedure*

A 20 kHz rod transducer produced by SINAPTEC was placed in the first section of the degassing test rig, i.e. between the inlet and the TDG sensor placed at 4.25 m, with its tip facing downstream. It was powered by a SINAPTEC INSIDE generator, which supplies energy to provide a maximum acoustic power of 1.2 $kW_{RMS}$. The transducer was operated using the SINAPTEC NexTgen software and was set to continuous sonication for 20 min at a given acoustic amplitude. Three acoustic powers were tested: 556 W, 318 W and 130 W, which in the following will be labeled as high, medium, and low acoustic power, respectively. Higher acoustic powers were not tested, due to the insufficient rigidity of the transducer attachment within the channel, which dampened the transmitted power.

The flow rate within the channel was determined by the height of the water column within the pressure tank used to store the TDG supersaturated water. Two different flow rates were tested against the acoustic powers: a high flow rate of approximately 3.5 $dm^3.s^{-1}$ and a low flow rate of about $2dm^3.s^{-1}$. As the TDG sensor's response time $t_{95}$, i.e. the time to reach 95% of the actual measurement value, was very long (7 min, [47]), and the tests reach substantially high TDG saturation values, each test was initiated by flushing TDG supersaturated water for 15 min, i.e. double the response time, to allow adjustment of the TDG sensors to the new saturation level. This was followed by the 20 min sonication period. NI DAQ modules and an in-house written NI LabVIEW script were used for data collection. Data processing was conducted using MATLAB R2022b.

The liquid-gas mass transfer coefficient was calculated using the data recorded from the TDG sensors at the different measurement positions in the channel. Following Equation 5, the values measured at the channel inlet were always used for measurement position A, while the downstream positions were designated as B. Thus, the channel was divided into three sections, extending from the channel inlet to the respective measurement position downstream.

## 3. Results and Discussion

Looking at how the liquid-gas mass transfer is influenced by acoustic power and flow rate, different things stand out. In Figure 2, $k_{La}$ is plotted over the flow rate for each section individually, as well as for high, medium, and low acoustic power. The highest values for the liquid-gas mass transfer are apparent in section 1, where the ultrasonic transducer is placed. In the downstream sections, $k_{La}$ is not as high, meaning that the decline in saturation is lower compared to the first section. This is as expected since most of the degassing takes place within the acoustic field in the first section. In the direct vicinity of the acoustic transducer, large gas bubbles accumulate within the acoustic field and rise directly up to the water surface. The highest drop in $k_{La}$ values takes place between the second and the third section, which is due to a visibly observable gas bubble cloud appearing downstream of the acoustic transducer. Due to the increased interfacial area, this cloud contributes to the liquid-gas mass transfer during its travel downstream and upwards in the channel. Therefore, the acoustic degassing has a continuous effect within the second section. All bubbles rise to the water surface, and the bubble cloud disappears from the water column before reaching the third section of the channel.

Regarding the flow rate, higher values lead to higher liquid-gas mass transfer coefficients. This takes place independently from acoustic power and channel section. However, the low maximum flow rate within the degassing test rig does not allow for studying potential maxima or plateau effects. Nevertheless, the decreased residence time of water passing through the acoustic field does not lead to a decrease in the liquid-gas mass transfer. This can be explained by the approach of an optimal renewal rate within the acoustic field, i.e. the water replenishing the TDG saturation
within the acoustic field at the rate that is beneficial to the liquid-gas mass transfer achieved through acoustic cavitation.

<figure><img src="image_5.png" /><figcaption>Figure 2: Resulting $k_L a$ values from measurements in the degassing test rig over flow rate for each section and different acoustic powers.</figcaption></figure>

The influence of the flow rate is even more perceptible when plotting liquid-gas mass transfer over acoustic power for the two flow rates, as displayed in Figure 3. The high flow rate increases the liquid-gas mass transfer coefficient on average over all sections by a factor of 1.65. In addition, this figure shows the great influence the acoustic power has on $k_L a$. Towards higher acoustic powers, a large increase in $k_L a$ is detected in section one, but also the other sections show the same increase. Throughout all sections, the increase in $k_L a$ between low and medium power is on average over all sections by a factor of 1.68, while the increase from low to high power is by a factor of 2.13. This is coherent with literature, which shows an almost linear increase in mass
transfer with the acoustic power transferred to the liquid, e.g. [48–50]. Yet, the experiments conducted in these cases are mostly using still water. In the present case, linearity is only apparent in the $k_L a$ values in the third section, while in section 1 the high power values are slightly increased. Opposite to that, the values at high power in the second section are reduced. That can be explained due to the high efficiency taking place in the first section at high powers, by such reducing the effect of the gas bubble cloud towards the second section as the saturation gradient at the interfacial area is lower. Therefore, the continuous effect is less distinct at higher powers.

<figure><img src="image_6.png" /><figcaption>Figure 3: Resulting $k_L a$ values from measurements in the degassing test rig over acoustic power for different flow rates in Section 1 (left), 2 (middle) and 3 (right).</figcaption></figure>

Overall, a combination of high flow and high acoustic power is beneficial to the liquid-gas mass transfer. The limitations discussed earlier regarding the maximum flow rate within the channel do not allow for the investigation of potential maxima or plateau effects. Also, the insufficient rigidity of the transducer mount limits the range of transferred acoustic power of the transducer. The continuous effect introduced by the gas bubble cloud may decrease with further increasing acoustic power.

## 4. Conclusion

Ultrasonic degassing was applied to flowing water with TDG supersaturation. A parameter study was conducted in the degassing test rig at the Waterpower Laboratory at NTNU to investigate the influence of flow rate and acoustic power on the degassing efficiency. Studying liquid-gas mass transfer on continuously replenished, TDG supersaturated water in a laboratory setting is unique. The results show a step-like reduction in the TDG saturation introduced by the acoustic degassing. This leads to high values of the liquid-gas mass transfer coefficient within the section of the test rig channel where the ultrasonication takes place, i.e. the first section. A gas bubble cloud generated within the acoustic field leads to a prolonged effect and further degassing, by such increasing the values in the second section compared to the third. This effect decreases with higher acoustic powers, as the TDG saturation gradient is lowered due to increased degassing in the direct vicinity of the transducer. The full range of the ultrasonic equipment was not exploited due to insufficient rigidity of the mount. Tests to prove the decrease of the continuation effect at higher acoustic powers need to be conducted using a re-designed transducer mount.

A higher flow rate is beneficial to the liquid-gas mass transfer. Yet, the degassing test rig's flow rate limitations make it challenging to study potential maxima or plateau effects. This may be resolved by adjusting the test rig and the outflow from the pressure system.

The test rig allows for testing and combining other technical methods as well. The combined effect of gas bubbling may enhance the overall liquid-gas mass transfer in this case, as that
would provide numerous nuclei that can be utilized as starting points for the degassing within the acoustic field. In addition, the transducer has different modes, such as frequency sweeps or pulsed modes, that remain to be studied regarding their effect on the liquid-gas mass transfer.

Acknowledgments

The presented study is part of the DeGas project, which was funded by the Research Council of Norway. The grant agreement number is 308747.

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