Experimental Investigation of Unsteady Pressure Pulsation in New Type Dishwasher Pump with Special Double-Tongue Volute

A pressure pulsation experiment of a dishwasher pump with a passive rotation double-tongue volute was carried out and compared with the pressure pulsation of a single-tongue volute and a static double-tongue volute. The pressure pulsation of the three volute models was compared and analyzed from two aspects of different impeller speeds and different monitoring points. The frequency domain and time–frequency domain of pressure pulsation were obtained by a Fourier transform and short-time Fourier transform, respectively. The results showed that the average pressure of each monitoring point on the rotating double-tongue volute was the smallest and that on the single-tongue volute was the largest. When the impeller rotates at 3000 rpm, there were eight peaks and valleys in the pressure pulsation time domain curve of the single-tongue volute, while the double-tongue volute was twice that of the single-tongue volute. Under different impeller speeds, the changing trends of pressure pulsation time and frequency domain curves of static and rotating double-tongue volutes at monitoring point p1 are basically the same. Therefore, a volute reference scheme with passive rotation speed is proposed in this study, which can effectively improve the flow pattern and reduce pressure inside the dishwasher pump, and also provide a new idea for rotor–rotor interference to guide the innovation of dishwashers.


Introduction
Today, dishwashers are used by more and more families. Compared with manual dishwashing, a dishwasher can not only shorten the cleaning time but also save water and labor, making dishwashing simple. Therefore, domestic and foreign companies have launched various models of dishwasher to meet the market, for example, the DWA5-1513 dishwasher from TOSHIBA, the SC73M12TI dishwasher from SIMENS, the NP-8LZK5RX dishwasher from Panasonic, and the RX600 dishwasher from Midea, etc. What these dishwashers have in common is that they perform the cleaning function through the passive rotation of the built-in spray arm. The dishwasher pump system is an important part of the dishwasher system, which can complete the water transport and spraying function. This paper is based on FOTILE company dishwasher pump system research. The dishwasher of this company is accepted by many people because it can achieve open cleaning without pipelines. The pipeline-free cleaning replaces the pipeline with a doubletongue volute flow channel that can be passively rotated. The symmetrical design of the volute is conducive to its rotation, and the jet hole is set on the volute spray arm to realize the function of water spraying. This avoids the problem of fouling and is more convenient for daily cleaning and maintenance. Of course, the nozzle structure, injection angle, and jet velocity will affect the cleaning effect [1,2]. However, the pressure pulsation during the operation of the dishwasher pump will cause the vibration of the dishwasher, which interference mechanism between the high-speed rotation of the impeller and the low-speed passive rotation of the volute, this paper studies the pressure pulsation characteristics of the double-tongue volute under passive rotation at different impeller speeds through an innovative rotating pressure test device. In addition, the pressure pulsation characteristics of the single-tongue volute and the double-tongue volute at rest are compared and studied, which provides a reference for the design and optimization of the open pump.

Open Test Rig Setup for Dishwasher Pump
As shown in Figure 1, the open test setup of the dishwasher pump was set at the National Research Center of Pumps, Jiangsu University. The test rig was inside a transparent water tank in order to observe the working state of the dishwasher more conveniently, and the tank is 400 × 440 × 400 mm. The shaft of the motor at the bottom of the water tank connects the impeller to control the operation of the dishwasher pump. tion be larger than those without the floor-attached vortex.
At present, the research on the pressure pulsation in the pump is mostly based o the static condition of the volute, and the research on the pressure pulsation under th passive rotation of the volute is still rare. Therefore, in order to reveal the new 'ro tor-rotor ' interference mechanism between the high-speed rotation of the impeller an the low-speed passive rotation of the volute, this paper studies the pressure pulsatio characteristics of the double-tongue volute under passive rotation at different impelle speeds through an innovative rotating pressure test device. In addition, the pressur pulsation characteristics of the single-tongue volute and the double-tongue volute at res are compared and studied, which provides a reference for the design and optimization o the open pump.

Open Test Rig Setup for Dishwasher Pump
As shown in Figure 1, the open test setup of the dishwasher pump was set at th National Research Center of Pumps, Jiangsu University. The test rig was inside a trans parent water tank in order to observe the working state of the dishwasher more conven iently, and the tank is 400 × 440 × 400 mm. The shaft of the motor at the bottom of th water tank connects the impeller to control the operation of the dishwasher pump. The dishwasher pump consists of a compound impeller with eight blades and spray arm of the volute type. The bottom end of the compound impeller is a for ward-curved axial flow blade, the top is a radial centrifugal blade, and there is no obv ous front shroud and back shroud. The volute had two symmetrical flow channels tha helped the spray arm rotate. In particular, an unsealed connection between the impelle and the volute is also designed to enhance the cleaning effect of the dishwasher by pas sively rotating the spray arm. The specific design parameters of the dishwasher pump ar shown in Figure 2 and listed in Table 1. The flow rate Q and head H under design poin were 55 L/min and 2 m, respectively. The impeller speed n was designed as 3000 rpm.  The dishwasher pump consists of a compound impeller with eight blades and a spray arm of the volute type. The bottom end of the compound impeller is a forward-curved axial flow blade, the top is a radial centrifugal blade, and there is no obvious front shroud and back shroud. The volute had two symmetrical flow channels that helped the spray arm rotate. In particular, an unsealed connection between the impeller and the volute is also designed to enhance the cleaning effect of the dishwasher by passively rotating the spray arm. The specific design parameters of the dishwasher pump are shown in Figure 2 and listed in Table 1. The flow rate Q and head H under design point were 55 L/min and 2 m, respectively. The impeller speed n was designed as 3000 rpm.  Figure 3 shows a schematic diagram of data acquisition and the transient pressure test setting of the dishwasher pump. The whole test system consists of two parts. The first system was to change the speed of the impeller. By adjusting the speed control button on   Figure 3 shows a schematic diagram of data acquisition and the transient pressure test setting of the dishwasher pump. The whole test system consists of two parts. The first system was to change the speed of the impeller. By adjusting the speed control button on the panel, the panel will feedback a number. One hundred times, the number is the rotating speed of the impeller, and the unit is rpm. The second system was to measure the transient pressures of the dishwasher pump. The pressure pulsation sensors (SCYG314) produced by Senno Sci-tech Co., Ltd. were supported by a 15 V DC power source. The accuracy of each pressure sensor was 0.2%, and the range of measurement was 0-20 kpa. The sensors were connected to the data acquisition unit by a converter, and the computer accepted the converted current signal by a data line. The test software of pressure pulsation is smart and integrates functions that can change the sampling time and frequency. In order to ensure the accuracy of the test, there was zero calibration of the sensor before the formal test. The sampling frequency fs was set at 10,000 Hz, and the data were acquired for 2 s when the dishwasher pump reached a stable condition. The sampling frequency is much larger than blade frequency and satisfies Shannon's sampling law [26][27][28][29]. In addition, in order to solve the problem of the pressure pulsation test in the process of volute rotation, the rotating pressure pulsation test device was designed. The top of the device is bolted to the cover of the water tank, and the sensor probe extending from the bottom of the device is placed in a preset monitoring hole on the volute. The part of the device near the sensor probe can be rotated by the electricity slip ring, which can not only ensure the normal electricity consumption of the sensor but also effectively avoid the wire winding problem in the rotation test process. The whole device was waterproofed to ensure the normal operation of the sensor.

Experimental Tests
In order to verify that the test repeatability could be reliable, repeated multiple measurements for the pressure pulsation of monitoring point p1 were carried out. According to the obtained data, the time domain curves of the pressure pulsation of the three tests are drawn, as shown in Figure 4a, and they are periodic distribution in the time of the impeller rotating for two cycles, and the curve trend is basically the same. It is clearly shown that the maximum deviation of average pressure is 1.9% from Figure 4b.
The positions of pressure pulsation monitoring points of the volute of the dishwasher pump are shown in Figure 5. The pressure monitoring point p1 is on the side of the volute, and it is positive in the impeller outlet direction. The pressure monitoring points p2, p3, p4, p5, and p6 were distributed in the top of the volute, and in turn, it extends from the tongue position of the volute to the outlet of the volute.
To investigate the influence of the impeller speeds on the pressure pulsation in the dishwasher pump, five different speeds of the impeller were modified by the impeller speed control system, which was previously mentioned in Figure 3. As shown in Figure 6, in order to further study the influence of the volute form and the passive rotation of the volute on the pressure pulsation, the pressure pulsation tests were carried out on the singletongue volute and the double-tongue volute at rest and compared with the double-tongue volute rotation model. In order to make the comparative test more scientific and rigorous, each pressure pulsation monitoring point position of three different volute models was consistent during the test.  In order to verify that the test repeatability could be reliable, repeated multiple measurements for the pressure pulsation of monitoring point p1 were carried out. According to the obtained data, the time domain curves of the pressure pulsation of the three tests are drawn, as shown in Figure 4a, and they are periodic distribution in the time of the impeller rotating for two cycles, and the curve trend is basically the same. It is clearly shown that the maximum deviation of average pressure is 1.9% from Figure 4b.  In order to verify that the test repeatability could be reliable, repeated multiple measurements for the pressure pulsation of monitoring point p1 were carried out. According to the obtained data, the time domain curves of the pressure pulsation of the three tests are drawn, as shown in Figure 4a, and they are periodic distribution in the time of the impeller rotating for two cycles, and the curve trend is basically the same. It is clearly shown that the maximum deviation of average pressure is 1.9% from Figure 4b.  The positions of pressure pulsation monitoring points of the volute of the dishwasher pump are shown in Figure 5. The pressure monitoring point p1 is on the side of the volute, and it is positive in the impeller outlet direction. The pressure monitoring points p2, p3, p4, p5, and p6 were distributed in the top of the volute, and in turn, it extends from the tongue position of the volute to the outlet of the volute. The positions of pressure pulsation monitoring points of the volute of t washer pump are shown in Figure 5. The pressure monitoring point p1 is on th the volute, and it is positive in the impeller outlet direction. The pressure mo points p2, p3, p4, p5, and p6 were distributed in the top of the volute, and in tu tends from the tongue position of the volute to the outlet of the volute. To investigate the influence of the impeller speeds on the pressure pulsatio dishwasher pump, five different speeds of the impeller were modified by the speed control system, which was previously mentioned in Figure 3. As shown i 6, in order to further study the influence of the volute form and the passive rotati volute on the pressure pulsation, the pressure pulsation tests were carried ou single-tongue volute and the double-tongue volute at rest and compared with ble-tongue volute rotation model. In order to make the comparative test more s and rigorous, each pressure pulsation monitoring point position of three differen models was consistent during the test.   To investigate the influence of the impeller speeds on the pressure pulsation in the dishwasher pump, five different speeds of the impeller were modified by the impeller speed control system, which was previously mentioned in Figure 3. As shown in Figure  6, in order to further study the influence of the volute form and the passive rotation of the volute on the pressure pulsation, the pressure pulsation tests were carried out on the single-tongue volute and the double-tongue volute at rest and compared with the double-tongue volute rotation model. In order to make the comparative test more scientific and rigorous, each pressure pulsation monitoring point position of three different volute models was consistent during the test.

Result and Discussion
To facilitate the normalization of pressure pulsation data, the time-frequency analysis method was introduced to describe the pressure pulsation. The method of time-frequency can not only give the frequency of the pressure pulsation but also show the information about the frequency domain representation changes with time. This method is using a short-time Fourier transform (STFT) to transform the time domain signal of pressure pulsation [30][31][32]. The source signal is divided into several small signal segments by the window function. Firstly, the signal of each segment is converted to one-dimensional by Fourier transform, and then the two-dimensional time-frequency diagram is obtained by the translation of the window function.
where x(t) is the source signal, and w(t − t ) is the window function. The window function is Hanning.  Figure 7 shows the time-frequency domain of pressure pulsation for three volutes at six different monitoring positions. It shows the dynamic pressure pulsation is unsteady and dependent on time. The main frequency of pressure pulsation of the single-tongue volute is 1 times blade frequency, while the main frequency of pressure pulsation of the double-tongue volute is 2 times blade frequency, and the secondary frequency is 1 times blade frequency, indicating that the static and dynamic interference is the root cause of pressure pulsation. The amplitude pulsation of pressure pulsation of the double-tongue volute at 1000 Hz is also strong at monitoring points p1, p2, and p6. Compared with the stationary double-tongue volute, the amplitude of the double-blade frequency of the rotating double-tongue volute will reach a maximum between 400 and 1200 ms, which is higher than the amplitude of the main frequency at other times (Figure 7i,l). This is probably caused by the new dynamic interference between the volute and the rotating impeller, which is also used as a rotating component. Figure 8 shows a comparison of the average pressure at different pressure pulsation monitoring points when the dishwasher pump with three different types of volute. It can be seen that the average pressure of the single-tongue volute is the largest at the monitoring point p4, while that of the double-tongue volute reaches the maximum at the monitoring point p3. In three different volute models, the average pressure of each monitoring point is the largest at the same impeller speed due to the minimum flow passage of the singletongue volute. Similarly, in the process of passive rotation, the double-tongue volute can effectively alleviate the squeezing effect of water flow on the volute, making the average pressure minimum. The variation trend of average pressure of three volute models from p1 to p3 at the monitoring point is the same. However, the change of the rotating doubletongue volute from p3 to p6 at the monitoring points is more smooth, and the maximum deviation of the average pressure is 1.4%. The maximum deviations of average pressure of the single-tongue volute and static double-tongue volute are 4.5% and 4.9%, respectively. This is because when the double-tongue volute rotates, the flow pattern in the volute channel is improved, and the pressure distribution is uniform.

Dominant Frequency Amplitudes of the Pressure Pulsation
The dominant frequency amplitudes of the pressure pulsation for three volutes at different measuring points are shown in Figure 9. On the double-tongue volute, the amplitude of dominant frequency of the pressure pulsation increases first and then decreases from p1 to p6 at the monitoring point and reaches the maximum at the monitoring point p2. The amplitude of dominant frequency of pressure pulsation at different monitoring points of the single-tongue volute has no obvious regularity, and the amplitude of p1 reaches the maximum at the monitoring point. This is because the experiment is carried out at the impeller speed of 3000 rpm, which is much higher than the design speed of the single-tongue volute. The leakage vortex at the outlet of the impeller disturbs the flow field around the monitoring point p1, changes the stable structure of the flow field at the outlet of the impeller, and leads to severe pressure pulsation in the flow field. The larger impeller speed increases the flow velocity in the vortex chamber and deteriorates the flow pattern, which is the reason for the irregular amplitude of the dominant frequency of the pressure pulsation in the single-tongue volute. 8 Figure 8 shows a comparison of the average pressure at different pressure pulsation monitoring points when the dishwasher pump with three different types of volute. It can be seen that the average pressure of the single-tongue volute is the largest at the monitoring point p4, while that of the double-tongue volute reaches the maximum at the monitoring point p3. In three different volute models, the average pressure of each monitoring point is the largest at the same impeller speed due to the minimum flow passage of the single-tongue volute. Similarly, in the process of passive rotation, the double-tongue volute can effectively alleviate the squeezing effect of water flow on the volute, making the average pressure minimum. The variation trend of average pressure of three volute models from p1 to p3 at the monitoring point is the same. However, the change of the rotating double-tongue volute from p3 to p6 at the monitoring points is more smooth, and the maximum deviation of the average pressure is 1.4%. The maximum deviations of average pressure of the single-tongue volute and static double-tongue volute are 4.5% and 4.9%, respectively. This is because when the double-tongue volute rotates, the flow pattern in the volute channel is improved, and the pressure distribution is uniform.  Machines 2021, 9, 288 9 of monitoring point p3. In three different volute models, the average pressure of each mo itoring point is the largest at the same impeller speed due to the minimum flow passa of the single-tongue volute. Similarly, in the process of passive rotation, the do ble-tongue volute can effectively alleviate the squeezing effect of water flow on the v ute, making the average pressure minimum. The variation trend of average pressure three volute models from p1 to p3 at the monitoring point is the same. However, change of the rotating double-tongue volute from p3 to p6 at the monitoring points more smooth, and the maximum deviation of the average pressure is 1.4%. The ma mum deviations of average pressure of the single-tongue volute and static double-tong volute are 4.5% and 4.9%, respectively. This is because when the double-tongue volu rotates, the flow pattern in the volute channel is improved, and the pressure distributi is uniform.

Dominant Frequency Amplitudes of the Pressure Pulsation
The dominant frequency amplitudes of the pressure pulsation for three volutes different measuring points are shown in Figure 9. On the double-tongue volute, the a plitude of dominant frequency of the pressure pulsation increases first and then d creases from p1 to p6 at the monitoring point and reaches the maximum at the monit ing point p2. The amplitude of dominant frequency of pressure pulsation at differe monitoring points of the single-tongue volute has no obvious regularity, and the amp tude of p1 reaches the maximum at the monitoring point. This is because the experime is carried out at the impeller speed of 3000 rpm, which is much higher than the desi speed of the single-tongue volute. The leakage vortex at the outlet of the impeller d turbs the flow field around the monitoring point p1, changes the stable structure of flow field at the outlet of the impeller, and leads to severe pressure pulsation in the flo field. The larger impeller speed increases the flow velocity in the vortex chamber a deteriorates the flow pattern, which is the reason for the irregular amplitude of t dominant frequency of the pressure pulsation in the single-tongue volute.

Time-Domain Analysis of Pressure Fluctuation
Pressure pulsation appears when the rotating impeller blades sweep through the tongue of the volute. Apparently, the number of blades skimming over the volute tongue and the structure of the volute play an important role in the change of pressure pulsation. Figure 10 shows the time domain variation of pressure pulsation at p1 for three volutes at different impeller speeds. With the decrease of impeller speeds, the pressure peak value and the amplitudes of pressure pulsation decrease. When the impeller rotates at 3000, 2500, and 2000 rpm, the variations of pressure pulsation with different volutes all have certain periodicity. When the impeller rotates at 1500 and 1000 rpm, the variations of pressure pulsation with different volutes all have certain periodicity, the pressure fluctuates violently, and the pressure distribution is uneven. This is because, at low rotational speeds, the pressure and flow rate of the blade acting on the flow will decrease, which keeps air in the pump chamber. The coupling of water and air makes the flow field in the dishwasher pump more complex, and the flow pattern becomes unstable, which lets the time domain curve of pressure pulsation be uneven.

Time-Domain Analysis of Pressure Fluctuation
Pressure pulsation appears when the rotating impeller blades sweep through the tongue of the volute. Apparently, the number of blades skimming over the volute tongue and the structure of the volute play an important role in the change of pressure pulsation. Figure 10 shows the time domain variation of pressure pulsation at p1 for three volutes at different impeller speeds. With the decrease of impeller speeds, the pressure peak value and the amplitudes of pressure pulsation decrease. When the impeller rotates at 3000, 2500, and 2000 rpm, the variations of pressure pulsation with different volutes all have certain periodicity. When the impeller rotates at 1500 and 1000 rpm, the variations of pressure pulsation with different volutes all have certain periodicity, the pressure fluctuates violently, and the pressure distribution is uneven. This is because, at low rotational speeds, the pressure and flow rate of the blade acting on the flow will decrease, which keeps air in the pump chamber. The coupling of water and air makes the flow field in the dishwasher pump more complex, and the flow pattern becomes unstable, which lets the time domain curve of pressure pulsation be uneven.
As shown in Figure 10a, the curves of pressure pulsation have eight peaks and troughs when the impeller rotates at 3000, 2500, and 2000 rpm, and the dishwasher pump has a single-tongue volute. The number of cycles corresponds to the number of blades [33][34][35]. In addition, from Figure 10b,c, it is apparent that the curves of pressure pulsation have 16 peaks and troughs when the pump has a double-tongue volute and the impeller is 3000 and 2500 rpm. It is the same as the number of volute tongues multiplied by the number of blades. It is explained that the rotor-stator interference between the impeller and volute is the fundamental cause of pressure pulsation. Compared with the pump double-tongue volute under static and rotating conditions, the amplitudes of pressure pulsation of the pump with static volute are significantly higher than those of the pump with a rotating volute at different impeller speeds.

Frequency-Domain Analysis of Pressure Fluctuation
The shaft frequency (fn) and blade frequency (fBPF) of the pump are determined the rotational speeds of the impeller (n), and blade frequency depends on the axial f quency and blade number. Equation (2) lists the relationship between the fn (kHz), f (kHz) and n (rpm) [36][37][38].  As shown in Figure 10a, the curves of pressure pulsation have eight peaks and troughs when the impeller rotates at 3000, 2500, and 2000 rpm, and the dishwasher pump has a single-tongue volute. The number of cycles corresponds to the number of blades [33][34][35]. In addition, from Figure 10b,c, it is apparent that the curves of pressure pulsation have 16 peaks and troughs when the pump has a double-tongue volute and the impeller is 3000 and 2500 rpm. It is the same as the number of volute tongues multiplied by the number of blades. It is explained that the rotor-stator interference between the impeller and volute is the fundamental cause of pressure pulsation. Compared with the pump double-tongue volute under static and rotating conditions, the amplitudes of pressure pulsation of the pump with static volute are significantly higher than those of the pump with a rotating volute at different impeller speeds.

Frequency-Domain Analysis of Pressure Fluctuation
The shaft frequency (f n ) and blade frequency (f BPF ) of the pump are determined by the rotational speeds of the impeller (n), and blade frequency depends on the axial frequency and blade number. Equation (2) lists the relationship between the f n (kHz), f BPF (kHz) and n (rpm) [36][37][38].
f n = 1/ 60 The calculated parameters of the pump at different impeller speeds are given in Table 2. To obtain frequency domain curves of pressure pulsation at different impeller speeds, pressure pulsation data of monitoring point p1 were transformed by FFT [39][40][41]. Table 3 shows the dominant frequency of pressure pulsation obtained at different impeller speeds. It can be seen that the dominant frequencies at each impeller speed are distributed in the shaft frequency and multiple blade frequency. For the pump with a single-tongue volute, the dominant frequency of pressure pulsation is blade frequency when the impeller rotates at 3000, 2500, 2000, and 1500 rpm. When the impeller speed is 1000 rpm, the dominant frequency turns to shaft frequency, and the dominant frequency decreases with the decrease of impeller speed. For the pump with the double-tongue volute, the dominant frequency is concentrated at twice the blade frequency when the impeller rotates at 3000 and 2500 rpm. When the impeller speeds are 2000, 1500, and 1000 rpm, the dominant frequency is doubleblade frequency, triple-blade frequency, and shaft frequency, respectively.  Figure 11 shows the frequency domain variation of pressure pulsation at p1 for three volutes at different impellers speeds. By comparing (a) with (b) in Figure 11, it is clear that the dominant frequency of the double-tongue volute at different impeller speeds is higher than that of the single-tongue volute. In addition, the amplitude range of the double-tongue volute is obviously larger than that of the single-tongue volute. This is because, compared with the single-tongue volute, the double-tongue volute has two symmetrical chambers, and the flow pattern becomes more complex, which causes the pressure pulsation of the flow field more violent. As shown in Figure 11b,c, we can find that the frequency domain variation of the rotating double-tongue volute under the same impeller speeds has little difference to that of the static double-tongue volute.
because, compared with the single-tongue volute, the double-tongue volute has symmetrical chambers, and the flow pattern becomes more complex, which causes pressure pulsation of the flow field more violent. As shown in Figure 11b,c, we can that the frequency domain variation of the rotating double-tongue volute under the s impeller speeds has little difference to that of the static double-tongue volute.

Dominant Frequency Amplitudes of the Pressure Pulsation
The dominant frequency amplitudes of the pressure pulsation are carried ou evaluate the pressure pulsation test to explore the influence of the impeller speeds on pressure pulsation in the dishwasher pump. Figure 12 shows the dominant frequ amplitudes of the pressure pulsation at different speeds in the pump with three diffe volutes' conditions. The amplitude of dominant frequency of pressure pulsation of t different volutes decreases with the decrease of impeller speeds. This is because redu the speed of the impeller makes the pressure of the impeller on the flow decrease and energy of the pressure pulsation decrease. Compared with the double-tongue vo when the impeller speed is greater than 2500 rpm, the amplitude of the dominant quency of the pressure pulsation is not obvious. When the impeller speed is 3000 the amplitude of the single-tongue volute increases by only 3%, and the amplitude o double-tongue volute increases by 57% and 53%, respectively, when it is static and tating. This is because the design flow rate of the double-tongue volute is higher than

Dominant Frequency Amplitudes of the Pressure Pulsation
The dominant frequency amplitudes of the pressure pulsation are carried out to evaluate the pressure pulsation test to explore the influence of the impeller speeds on the pressure pulsation in the dishwasher pump. Figure 12 shows the dominant frequency amplitudes of the pressure pulsation at different speeds in the pump with three different volutes' conditions. The amplitude of dominant frequency of pressure pulsation of three different volutes decreases with the decrease of impeller speeds. This is because reducing the speed of the impeller makes the pressure of the impeller on the flow decrease and the energy of the pressure pulsation decrease. Compared with the double-tongue volute, when the impeller speed is greater than 2500 rpm, the amplitude of the dominant frequency of the pressure pulsation is not obvious. When the impeller speed is 3000 rpm, the amplitude of the single-tongue volute increases by only 3%, and the amplitude of the double-tongue volute increases by 57% and 53%, respectively, when it is static and rotating. This is because the design flow rate of the double-tongue volute is higher than that of the single-tongue volute at the same impeller speed, and the dishwasher uses an open pump. When the impeller speed is greater than the critical speed, the water will leak out from the impeller outlet and the volute connection. The dishwasher uses an open pump. When the impeller speed is greater than the critical speed, the water flow will leak out from the impeller outlet and the volute connection, which makes the pressure in the pump not change greatly.
of the single-tongue volute at the same impeller speed, and the dishwasher uses an open pump. When the impeller speed is greater than the critical speed, the water will leak out from the impeller outlet and the volute connection. The dishwasher uses an open pump. When the impeller speed is greater than the critical speed, the water flow will leak out from the impeller outlet and the volute connection, which makes the pressure in the pump not change greatly.

Conclusions
Experimental tests were carried out to analyze the effects of the single-tongue volute, the static double-tongue volute, and the rotating double-tongue volute on the pressure pulsation characteristics of the dishwasher pump. The pressure pulsation characteristics caused by rotor-stator interference and rotor-rotor interference of the dishwasher pump were analyzed by time domain analysis, frequency domain analysis, and time-frequency domain analysis.
(1) Compared with the single-tongue volute and the static double-tongue volute, the average pressure of each monitoring point in the dishwasher pump with a rotating double-tongue volute is the smallest, and from the pressure monitoring point from p3 to p6, the average pressure changes more gently, and the maximum deviation value is only 1.4%. The amplitude of the main frequency of the pressure pulsation of the double-tongue volute increases first and then decreases from the monitoring point p1 to p6 and reaches the maximum at the monitoring point p2. However, the amplitude of the main frequency of the single-tongue volute changes irregularly, and the maximum amplitude appears at the monitoring point p1. Therefore, for the dishwasher, the volute with the passive rotation has the best results, which can not only reduce the pressure and reduce vibration noise but also perform all-around cleaning, improving dishwasher efficiency. (2) When the impeller rotates at 3000 rpm, the number of peaks and valleys of the pressure pulsation time domain curve of the single-tongue volute is eight, while that of the double-tongue volute is two. The main frequency of the single-tongue volute is 0.396 kHz, which is concentrated near the blade frequency. The main frequencies of the static double-tongue volute and the rotating double-tongue volute are 0.791 kHz and 0.786, respectively, which are concentrated near the double-blade frequency.

Conclusions
Experimental tests were carried out to analyze the effects of the single-tongue volute, the static double-tongue volute, and the rotating double-tongue volute on the pressure pulsation characteristics of the dishwasher pump. The pressure pulsation characteristics caused by rotor-stator interference and rotor-rotor interference of the dishwasher pump were analyzed by time domain analysis, frequency domain analysis, and time-frequency domain analysis.
(1) Compared with the single-tongue volute and the static double-tongue volute, the average pressure of each monitoring point in the dishwasher pump with a rotating double-tongue volute is the smallest, and from the pressure monitoring point from p3 to p6, the average pressure changes more gently, and the maximum deviation value is only 1.4%. The amplitude of the main frequency of the pressure pulsation of the double-tongue volute increases first and then decreases from the monitoring point p1 to p6 and reaches the maximum at the monitoring point p2. However, the amplitude of the main frequency of the single-tongue volute changes irregularly, and the maximum amplitude appears at the monitoring point p1. Therefore, for the dishwasher, the volute with the passive rotation has the best results, which can not only reduce the pressure and reduce vibration noise but also perform all-around cleaning, improving dishwasher efficiency. (2) When the impeller rotates at 3000 rpm, the number of peaks and valleys of the pressure pulsation time domain curve of the single-tongue volute is eight, while that of the double-tongue volute is two. The main frequency of the single-tongue volute is 0.396 kHz, which is concentrated near the blade frequency. The main frequencies of the static double-tongue volute and the rotating double-tongue volute are 0.791 kHz and 0.786, respectively, which are concentrated near the double-blade frequency. (3) Under the high impeller speeds of 2000, 2500, and 3000 rpm, the pressure pulsation time domain curves present periodicity, and the main frequencies are given priority with the blade frequency and integer times of the blade frequency. At the low impeller speeds of 1000 and 1500 rpm, the pressure pulsation becomes disordered, the periodicity of the time-domain curves disappears, and the main frequency is mainly axial frequency.