Validation Methods for Oscillating Motion in Large-Scale Oscillating Heat Pipes for Solar Water Heaters

2.1 Fabrication

2.1.1 System components and functioning

The solar water heater was designed and fabricated by dividing it into four main components: the solar collector, heat exchanger, water tank, and supporting structure. A schematic illustration of the system is presented in Fig. 1. The heater operates as follows:

Fig. 1.

Schematic of the solar water heater

• a. Solar panels absorb sunlight, heating up in the process.
• b. Heat from the solar panels is transferred to the heat sinks within the heat exchanger via the oscillating motion of the working fluid in the oscillating heat pipe (OHP) tubes.
• c. Water is pumped from the tank through a hose into the heat exchanger.
• d. The water circulates through the heat exchanger, passing over the heat sinks to absorb heat.
• e. The heated water is then returned to the tank via another hose.

2.1.2 Solar collector

The solar collector consisted of three sets of solar panels, each equipped with back insulation plates, double-paned glass covers, and insulated edge walls. Each solar panel incorporated a six-turn OHP made from copper tubes sandwiched between aluminum straps. The decision to fabricate six-turn OHPs was based on a reference OHP for which neutron imaging and synchronized temperature data were available.^[27,28] Acetone was selected as the working fluid because it initiated oscillatory motion at a relatively lower heat input and exhibited more vigorous oscillations, resulting in better thermal performance compared to water under the same heat input.^[28]

The dimensions of each tube segment were 237 mm × 1,145 mm, with inner and outer diameters of 1.65 mm and 3.18 mm, respectively. The aluminum straps were painted with selective black paint to maximize absorption in the visible and infrared light spectrum. Detailed dimensions of the OHP are shown in Fig. 2. Solar irradiance was measured using an SP-110-SS pyranometer (Apogee Instruments, USA), with a sensitivity of 0.2 mV per W·m^-2 and a calibration uncertainty of less than 3% at 1,000 W·m^-2. Measurements were recorded at one-second intervals.

Fig. 2.

Dimensions of OHPs and positions of thermocouples attached to the surfaces of OHPs. Each thermocouple is represented by a numbered red “x”

2.1.3 Heat exchanger

The OHP sets were inserted into a heat exchanger made up of six aluminum heat sinks attached to aluminum plates. The ends of the OHP tube segments were positioned between these aluminum plates. The outer casing of the heat exchanger was constructed from acrylic, providing transparency for visual inspection and ease of machining. Accessibility was prioritized in the design, allowing the front wall to open via draw latches, enabling easy access for maintenance or modifications. Fiberglass insulation wrapped around the heat exchanger improved thermal performance, as shown in Fig. 3.

Fig. 3.

The fabricated edge walls and heat exchanger wrapped with fiberglass insulation

2.2 Experimental setup

The experimental setup, including the heat exchanger and three solar panels, is schematically illustrated in Fig. 1. Selective black paint was applied to the aluminum straps of the solar panels to absorb the visible and infrared wavelengths that carry most of the radiant energy. The fabricated solar panels and heat exchanger, prior to the installation of double-paned glass and double edge walls, are shown in Fig. 4.

Fig. 4.

Solar panels and heat exchanger. Selective black paint was applied to the surface of the panels. Thermocouples were attached to the positions indicated in Fig. 2. The glass and edge walls were not installed at this point.

2.2.5 Construction of space–time diagrams from neutron images

The detailed experimental setup and analysis methods for the small-scale reference OHP, which provide previously validated data for this study, are available.^[27] To visualize the dynamic behavior of two-phase flow in the oscillating heat pipe (OHP), a spatiotemporal image processing technique was applied to neutron radiography data.^[30] Each frame was processed to extract a one-dimensional brightness profile along the curved flow path. This was achieved by digitally straightening the bent tube geometry, beginning from the middle position of the long connecting leg at the condenser side, which is the connector located between thermocouple 23 and 24 in Fig. 1 of a reference work,^[27] and continuing through the entire loop. The resulting profiles, which reflect neutron attenuation (and thus phase distribution along the tube), were stacked horizontally in chronological order to generate a two-dimensional space-time diagram. Fig. 5 (b) presents a representative space-time diagram. In this diagram, the vertical axis represents position along the tube, while the horizontal axis denotes time, expressed as frame index. Darker regions correspond to liquid slugs (high neutron attenuation), while brighter areas indicate vapor plugs. The darker regions on the left appear as bands, indicating static liquid slugs. Over time, these transit into streaks, representing active motion of the slugs. In this way, the space-time diagrams reveal oscillatory motion, flow regime transitions, and slug–plug dynamics within the OHP.^[30]

Fig. 5.

(a) Temperature responses of the evaporator and condenser sections in the small-scale OHP. (b) Corresponding spatiotemporal diagram constructed from neutron images of the small-scale reference OHP. Dark regions represent liquid slugs, while bright regions correspond to vapor plugs. (c) Temperature changes in the OHP used for the solar water heater.

3.1 Sudden temperature drops on the evaporator

An OHP is a heat transfer device that transports heat from the evaporator (heating section) to the condenser (cooling section) through the oscillating motion of a working fluid within a vacuumed tube. This oscillation is driven by pressure differences between vapor plugs in the evaporator and condenser. When heat is applied to the evaporator, the pressure difference increases, causing the nearly static liquid slugs to suddenly begin oscillating. This sudden movement results in an abrupt and enhanced transfer of heat from the evaporator to the condenser, leading to a sudden temperature drop at the evaporator.

In an OHP used in a solar water heating system, as solar intensity increases, heat is absorbed in the evaporator, causing a rise in temperature and evaporation of the working fluid. This leads to pressure differences between the evaporator and condenser. When a sufficient pressure difference develops, the liquid slugs suddenly begin to oscillate actively and continue their motion. As a result of this active oscillation, the heat accumulated in the evaporator is suddenly transferred to the condenser, leading to a sudden temperature drop in the evaporator.

Fig. 5 (a) shows that there are sudden temperature drops in the evaporator and condenser of the small-scale OHP at around 120 seconds, while the temperatures were previously increasing. From observations of image data, liquid slugs in the OHP begin to oscillate actively at this point. This means that temperatures increase steadily before active oscillation begins and then decrease steeply once oscillation starts. This suggests that active oscillation triggers sudden temperature drops. The OHP in the solar water heater shows similar sudden temperature drops at the evaporator at around 1:40 PM for panel B (TC 5 and TC 6) and at 2:00 PM and 2:30 PM for panel C (TC 9 and TC 11). This indirectly indicates that liquid slugs begin to actively oscillate at around these times in panels B and C, even though oscillatory motion cannot be observed directly. Therefore, this temperature behavior can be used to validate oscillatory motion in an OHP used for solar water heaters.

On the other hand, the evaporator temperature of panel A does not show any sudden temperature drop, although it exhibits gentle temperature fluctuations between 1:00 PM and 2:30 PM. This suggests that either oscillatory motion did not occur or it was very weak in panel A. A possible cause is an undetected leak in the tube, which would have led to a lower vacuum level. This was confirmed during dismantling: when the tube was cut, it produced only a very weak hissing sound, in contrast to the clear hissing sounds observed when cutting the tubes of panels B and C. While the condenser temperature also shows a sudden decrease in the small-scale OHP, the OHPs in the solar water heater instead show sudden increases in temperature at the same time the evaporator temperature suddenly decreases. This is the opposite behavior compared to the condenser temperature changes in the small-scale OHP. This difference is likely due to the rate of heat transfer from the condenser to the cold reservoir. When the liquid slugs oscillate actively, a large amount of heat is transferred from the evaporator to the condenser. From there, the heat is dissipated to the cold reservoir via water convection on the surface of the condenser. As shown in Fig. 6, the relative size of the condenser in the solar water heater OHP is much smaller than that in the small-scale OHP. This implies that a relatively large amount of heat is transferred into the condenser from the evaporator, while only a relatively small amount is removed from the condenser to the cold reservoir. This imbalance causes a temporary accumulation of heat in the condenser, resulting in a sudden temperature rise in the condenser of the large-scale OHP, which is similar to a bottleneck effect. In contrast, the condenser in the small-scale OHP is able to effectively and immediately dissipate heat to the cold reservoir due to its relatively large size, which causes a sudden temperature drop.

Fig. 6.

Relative size comparison between the small-scale reference OHP and the OHP used for the solar water heater. (a) The small-scale reference OHP with available neutron imaging and synchronized temperature data. (b) The large-scale OHP fabricated for the solar water heater system in this study.

Studies conducted by other research groups also report sudden temperature changes at the onset of oscillatory motion. For example, the study by Wang et al._[4] shows that temperature increases steadily until oscillatory motion begins. In Figure 8 of Wang et al.,^[4] the temperature suddenly stops increasing and begins to fluctuate when oscillation starts which is shown in the green dotted box. The study by Winarta et al.^[7] also reports a steady temperature increase followed by a sudden transition to fluctuations or drop, with no further increase as long as the heat input remains constant, as illustrated in Figures 2 and 3 of their study for T11. The study by Xu et al.^[10] also presents similar temperature behavior, as illustrated in Fig. 4 of their study for T4. Therefore, a sudden temperature change—whether a sudden drop or a sudden stop in temperature increase—is synchronized with the onset of oscillatory motion. This can serve as a validation method for detecting oscillatory motion in OHPs used in solar water heaters. One might wonder what conditions cause a sudden temperature drop and what conditions cause a stop in temperature increase. Although this was not considered in this study because it is beyond the scope, it is a topic worth investigating in future research.

3.2 Sudden efficiency increases

A sudden change in the efficiency of a solar water heater utilizing OHPs can serve as an additional method to validate the occurrence and continuation of oscillatory motion. The efficiency of the system used in this study is determined using the following equation,^[31]

where q is determined by the following equation,^[31]

The efficiency graph in Fig. 7 (a) shows sudden increases, which are synchronized with sudden temperature changes. At around 13:40, efficiency suddenly increases as the evaporator temperature of panel B suddenly decreases. At around 14:10, another sudden efficiency increase is observed as the evaporator temperature of panel C drops. Since sudden temperature drops in the evaporator are caused by the onset of active oscillatory motion, the sudden increase in efficiency is also presumably caused by the onset of oscillatory motion. Therefore, a sudden increase in efficiency is another indicator of the onset of oscillatory motion in OHPs used in solar water heaters. One important point is that oscillatory motion is a key factor in the performance of solar water heaters with OHPs. The data show that efficiency increases significantly once liquid slugs begin oscillating actively. This highlights the importance of ensuring oscillatory motion for better performance in solar water heaters with OHPs. Thus, it is critical to initiate oscillatory motion and maintain it for as long as possible. It is suggested that future studies focus on identifying conditions that facilitate the initiation and maintenance of oscillatory motion.

Fig. 7.

(a) Sudden increases in thermal efficiency are observed, particularly around 13:30-14:40. (b) The corresponding solar irradiance during the same period is shown for reference.

3.3 Periodical temperature fluctuation synchronized with oscillatory motion

Fig. 8 shows the temperature changes of the small scale OHP recorded by thermocouples 4 and 5, along with the position changes of a liquid slug as it oscillates near these thermocouple locations. This demonstrates the thermal coupling between internal two-phase oscillations and external wall temperature response. It shows that temperatures fluctuate periodically when the liquid slug passes the points where the thermocouples are attached to the outer surface of the tube. The data clearly indicate that these temperature fluctuations are synchronized with the oscillatory motion of the liquid slug. Additionally, the frequency of temperature fluctuations matches the frequency of the oscillatory motion of the liquid slug. From the observation of image data, temperature fluctuations at a thermocouple do not always occur when a liquid slug passes the point where the thermocouple is attached. However, when periodic temperature fluctuations are present, they are always associated with oscillatory motion. In other words, the presence of periodic temperature fluctuations confirms the existence of oscillatory motion. Therefore, temperature fluctuations can be used to validate the continuation of oscillatory motions in OHPs used in solar water heaters.

Fig. 8.

Correlation between the oscillatory motion of a liquid slug and temperature fluctuations measured by thermocouples TC 4 and TC 5 of the small-scale reference OHP

It is worth noting that not all thermocouples display noticeable temperature fluctuations, even when liquid slugs are clearly oscillating near the locations where the thermocouples are attached. This can be explained by the difference in time scales between fluid motion and thermal response. The thermocouples are mounted on the outer surface of the OHP tube, while the fluid oscillates inside the tube. When a liquid slug passes by, it can alter the internal wall temperature, but it takes a finite amount of time for this heat to conduct through the tube wall and reach the outer surface where the thermocouple senses it.

If the oscillatory motion is very fast, the liquid slug may move away before enough heat is transferred to the outer wall. In such cases, the thermocouple cannot respond quickly enough to capture each individual temperature change caused by a passing slug. Instead, it may register only a smoothed average or appear as if no fluctuation is occurring. This mismatch between the fluid dynamics time scale and the thermal response time of the system explains why some thermocouples do not reflect the internal oscillation behavior, even though it is physically taking place. Therefore, in this study, thermocouples TC 4 and TC 5 were selected for analysis because they exhibited clear, periodic thermal responses that correlate well with the observed slug oscillation.

Fig. 9 (a) shows the time-series temperature data recorded by thermocouples TC 5, TC 6, TC 9, and TC 11. Two specific time intervals are highlighted with red and blue rectangles. The red rectangle marks a period before oscillatory motion begins, while the blue rectangle indicates a period during which oscillatory motion is present. Fig. 9 (b) presents a closer look at these two intervals. The plots on the left correspond to the red rectangle in Fig. 9 (a) and represent the temperature behavior during non-oscillatory conditions. During this period, the temperatures show a gradual increase with no clear periodic pattern—only random fluctuations are observed. In contrast, the plots on the right in Fig. 9 (b) correspond to the blue rectangle in Fig. 9 (a), where oscillatory motion is active. These graphs show clear periodic temperature fluctuations, indicating that oscillatory heat transfer mechanisms are occurring and sustained during this phase.

Fig. 9.

(a) Time-series temperature data. (b) Enlarged views of the two time intervals marked by the red and blue rectangles in (a). The plots in the left column correspond to the red-highlighted interval (around 11:00), while those in the right column correspond to the blue-highlighted interval (around 15:30).

This suggests that periodic temperature fluctuations observed in small-scale OHPs can also be detected in the OHPs used in solar water heaters. Therefore, periodic temperature fluctuations can be used to validate the continuation of oscillatory motions in OHPs of solar water heaters. However, although OHPs in solar water heaters exhibit periodic temperature fluctuations, their frequencies and amplitudes differ from those observed in small-scale OHPs. Visual observation suggests that the frequency of periodic temperature fluctuations in small-scale OHPs is higher than in solar water heater OHPs, while the amplitudes of small-scale OHPs are smaller compared to those in solar water heater OHPs. This may be due to differences in liquid slug sizes—since liquid slugs in the solar water heater OHPs are larger than those in small-scale OHPs, they may oscillate more slowly. This finding is important for OHP design in solar water heaters. A larger evaporator area provides an advantage by collecting more solar energy, but it may also result in slower oscillatory motion, which could negatively impact heat transfer efficiency from the evaporator to the condenser. Therefore, future studies should focus on determining the optimal size of OHPs to ensure frequent oscillatory motion of liquid slugs and enhance heat transfer efficiency.

3.4 Thermal resistance

Fig. 10 shows thermal resistance of OHPs for panel B and C for four selected days, which was determined using the following equation,

Fig. 10.

Thermal resistance of the OHPs in the solar water heater for selected days

The heat transfer rate, q, used in this equation was determined using Equation (2) under the assumption that the heat absorbed in the evaporator flows to the condenser, and then to the heat exchanger without loss. This assumption is considered reasonable due to the insulation of the solar collector, which minimizes heat losses to the surroundings. The four days were chosen from a dataset because they exhibited clear and distinct patterns of evaporator temperature drop, which supports the idea that sudden temperature reductions in the evaporator can be used to identify the onset of oscillating motion. On all selected days, a distinct pattern is observed where thermal resistance decreases suddenly during the midday to early afternoon hours, generally between 11:00 and 14:00, and increases again later in the afternoon. This trend is explained by the onset of oscillating motion within the OHP. When oscillation begins, the oscillatory motion of liquid slugs enhances heat transport efficiency. As a result, the evaporator temperature drops suddenly, leading to a smaller temperature difference across the system. Since thermal resistance is defined as the ratio of temperature difference to heat input, this drop in temperature difference leads to a corresponding sudden decrease in thermal resistance, indicating improved thermal performance. From the observation of the thermal resistance, a sudden drop of thermal resistance can be used as a method to validate the onset and continuation of oscillatory motion in an OHP.

The primary aim of this study is not to evaluate the frequency or statistical occurrence of oscillation, but rather to propose a validation method for detecting the onset and continuation of oscillating motion in large-scale OHPs used in solar water heaters. Based on observations of the relationship between temperature changes and neutron imaging data, a sudden drop in evaporator temperature consistently appears when oscillating motion begins and continues. Therefore, days that showed this clear temperature response were chosen to illustrate how temperature behavior can be used as an indicator of oscillation. Data without such behavior were excluded, not to disregard contrary results, but because they did not involve the onset of oscillation and were therefore outside the scope of the validation method being proposed in this study.

3.5 Effect of operating and environmental parameters on system performance

Fig. 11 and Fig. 12 show the efficiency distributions with respect to solar irradiance and ambient temperature over three testing days. Each data point represents an individual efficiency value, categorized into four conditions:

Fig. 11.

Efficiency distributions with respect to solar irradiance

Fig. 12.

Efficiency distributions with respect to ambient temperature

(1) no oscillatory motion,
(2) oscillatory motion in panel B only,
(3) oscillatory motion in panel C only, and
(4) oscillatory motion in both panels B and C.

“No oscillatory motion” refers to the absence of oscillation in the OHPs in both panels B and C. “Oscillatory motion in panel B only” and “oscillatory motion in panel C only” indicate that oscillation occurred exclusively in the OHP of panel B or C, respectively. “Oscillatory motion in both panels B and C” indicates that oscillatory motion occurred in the OHPs of both panels simultaneously. Data from panel A were excluded due to a malfunction of its OHP. While the solar irradiance data shown in Fig. 11 was collected every second, the ambient temperature was measured every 5 minutes, which make the plotted data points look less dense compared to the ones shown in Fig. 11.

The data in Fig. 11 clearly show that system efficiencies differ significantly depending on the presence of oscillatory motion. When no oscillatory motion is observed, the efficiency remains below 20% regardless of solar irradiance. In contrast, when oscillatory motion occurs in both panels B and C, efficiency is overall higher than in cases where motion is present only in panel B or C. This indicates that the initiation and maintenance of oscillatory motion are critical for achieving desirable performance in this system. Therefore, future studies are recommended to identify the factors that initiate and sustain oscillatory motion in OHPs when applied to solar water heating systems.

Notably, some outliers in Fig. 11 reach up to 38% efficiency when oscillatory motion occurs in panel B only. These high values coincide with sudden drops in solar irradiance, likely due to passing clouds, and are therefore considered temporary anomalies rather than representative system behavior. Interestingly, at a solar irradiance of approximately 1.1 W/m², the efficiency during oscillatory motion in both panels B and C is lower than that in panel B only, which appears counterintuitive. This can be explained using the data in Fig. 12. It shows that the ambient temperature under dual-panel motion was around 21-22°C, while it was significantly higher (28-30°C) during B-only motion. The lower ambient temperature leads to greater heat loss from the system to the environment, reducing thermal efficiency further. This indicates that, in addition to initiating and sustaining oscillatory motion, improving insulation is critical to maximizing the efficiency of this system.

Fig. 13 shows the efficiency distributions with respect to wind speed over three testing days. Wind speed was recorded every 5 minutes. When oscillatory motion occurs only in panel B, the wind speed ranges from approximately 1.5 m/s to 4.2 m/s. Although higher wind speeds are generally expected to reduce efficiency due to increased convective heat loss, the data in this figure show no significant variation in efficiency with wind speed in this range. A similar trend is observed for data points with oscillatory motion in both panels B and C, where the wind speed ranges from 0 to 1.4 m/s. There is no clear relationship between efficiency and wind speed. These results suggest that, within the observed wind speed range of 0 to 4.2 m/s, wind speed does not significantly influence the efficiency of the current system.

Fig. 13.

Efficiency distributions with respect to wind speed

3.6 Onset and continuation of oscillatory motion vs. environmental parameters

This study demonstrates that the onset and continuation of oscillatory motion in oscillating heat pipes (OHPs) are critical for achieving optimal system performance. Therefore, it is important to study the factors that influence this behavior. Oscillatory motion is affected by various factors, including but not limited to vapor pressure inside the OHP tubing, filling ratio, and OHP geometry. However, these aspects are beyond the scope of this study, as their mechanisms are complex and not yet fully understood. As a result, only environmental parameters were considered. Specifically, solar irradiance, ambient temperature, and wind speed were examined to identify potential correlations with the occurrence of oscillatory motion in this system.

Fig. 14 presents the average values of these parameters on days when oscillatory motion either did or did not occur. While it is generally expected that higher solar irradiance would promote oscillatory motion, the data do not consistently support this. In particular, Group B, where oscillatory motion did not occur, exhibits higher average irradiance than Group C, where it did occur—contradicting this expectation. A likely explanation involves ambient temperature. As observed in this study, higher ambient temperatures improve system efficiency by reducing heat loss. Group C shows consistently higher ambient temperatures than Group B, which may have made it easier for oscillatory motion to start and persist. Group D supports this pattern as well: although it had the highest solar irradiance of all cases, no oscillatory motion was observed, possibly due to its exceptionally low ambient temperature. Wind speed, on the other hand, appears to have little influence.

Fig. 14.

Onset and continuation of oscillatory motion Vs. environmental parameters

Group C exhibits higher average wind speeds than Group B, yet still experienced oscillatory motion. This aligns with the earlier finding that wind speed does not significantly impact system efficiency. It is important to emphasize that these findings are specific to the data and configuration of this particular system. They should not be generalized to other OHP designs or operational contexts without further validation.

3.7 Efficiency comparison with a flat plate collector and an evacuated tube collector

Fig. 15 shows the instantaneous efficiency of the solar water heater on a testing day when the OHPs in both panel B and panel C exhibited oscillatory motion and environmental parameter data were available. The data shown corresponds to the efficiency values in Fig. 7 (a), but is limited to the time range from 14:40 to 15:40. This specific time window was selected because both OHP panel B and panel C exhibited oscillatory behavior during this period. It may be argued that the selected time span highlights only the best performance, as it captures a period when the OHP panels were fully active. However, the purpose of Fig. 15 is not to show peak efficiency, but to illustrate the relationship between efficiency (y-axis) and reduced temperature difference (x-axis) in a system that fundamentally relies on OHP-induced oscillatory heat transfer. Therefore, including data from periods without oscillatory motion would misrepresent the actual operating behavior of the OHP-based system.

Fig. 15.

Instantaneous efficiency of the solar collector utilizing OHPs

A study reports the instantaneous efficiencies of flat plate and evacuated tube collectors for both summer and winter conditions.^[32] Although a direct, side-by-side comparison of their efficiencies with one of the system used in this study is not possible, a brief comparison is summarized in Table 2. The OHP-based collector's efficiency range (30-40%) falls within the overall ranges reported for both flat plate (20-60%) and evacuated tube collectors (20-54%). However, its operating range is low compared to the others. This suggests that the temperature difference between the fluid and ambient air was relatively small, or the system was tested under relatively high solar irradiance. Due to the limited availability of detailed data from the referenced study, the underlying cause cannot be identified in the present study. Further experimental and comparative analysis is recommended to clarify this issue.

Table 2.

A brief comparison of efficiencies[32]

3.8 Repeatability

To evaluate the repeatability of the sudden temperature drops, multiple experiments were conducted across different days. Fig. 16 shows the temperature changes of the evaporators for panel B and panel C on four selected days during which temperature drops were observed. panel B exhibits a sudden temperature drop only on Day 1, whereas panel C shows such drops on all selected days. This suggests that the temperature drops caused by the onset of oscillatory motion are repeatable events, rather than isolated occurrences.

Fig. 16.

Average temperatures of the evaporators for panel B in (a) and panel C in (b) on four selected days

The reason why panel B does not exhibit sudden temperature drops on Days 2, 3, and 4, while panel C does, remains unclear. This implies that panel B did not experience the onset or sustained oscillatory motion on those days. The initiation and continuation of oscillatory motion in an OHP are complex phenomena, and the underlying mechanisms are not yet fully understood.^[3] Therefore, this issue is beyond the scope of the present study, though it is considered a valuable topic for future research.

3.9 Future studies

As oscillatory motions are the key factor in achieving high performance in solar water heaters with OHPs, it is important to initiate oscillatory motions and maintain them. The onset of oscillatory motion depends on pressure differences between vapor plugs or temperature differences between an evaporator and a condenser. In experimental environments for testing, conditions such as ambient temperature, heat input, and light intensity can be controlled. Under such conditions, the onset of oscillatory motion can be readily achieved by supplying higher heat input to the evaporator and improving cooling at the condenser. However, solar water heaters in actual field applications cannot have controlled conditions. The working conditions of OHPs in solar water heaters vary depending on weather conditions such as solar intensity, ambient temperature, and wind speed. This means that initiating and maintaining oscillatory motions can be challenging due to uncontrollable weather conditions. Therefore, it is important to study under what weather conditions, such as solar intensity, ambient temperature, and wind speed, oscillatory motions are readily initiated and maintained in future research.

Another suggestion for future study is to visualize oscillatory motions in the OHPs of the solar water heaters used in this study. Although the three validation methods proposed in this study can successfully indicate the onset and continuation of oscillatory motions in OHPs by comparing neutron image data of small-scale OHPs, the oscillatory motions in the OHPs of the solar water heater in this study have not been visually confirmed. Therefore, it is important to visualize the oscillatory motions of liquid slugs using non-destructive techniques such as neutron imaging in future studies.