Experimental Study of Vapor Supercavitation Suppression of Capillary Outlet Jet Noise

,e degree of bubble bursting at the inlet of an evaporator is the key factor to determine the size of the injection noise at the capillary outlet. In this study, by using the theory of cavitation dynamics, the transition tube between the capillary and evaporator is improved to suppress the bubble bursting at the entrance of the evaporator, so as to reduce the jet noise at the outlet of the capillary. ,e influence of aeration structure on noise reduction of a refrigerator (Haier BCD-520) was studied by numerical simulation, and experiments were carried out. ,e results show that the admixture structure significantly inhibits the bubble bursting and reduces the injection noise by 1.5 dB(A).


Introduction
Noise is one of the most important quality indicators of refrigerators. To solve this problem, pioneers invest a lot of budget and manpower [1][2][3][4][5][6][7]. With the continuous improvement and optimization of the refrigerator compressor, the compressor noise has been greatly reduced. However, the refrigerant flow noise became easier to be heard, especially the jet noise at the capillary outlet. Jet noise affects not only people's quality of life but also the hygiene of food in refrigerators [8]. Since the hydrodynamic characteristics of two-phase flow change with the change of bubble characteristics, bubbles are considered to be one of the main sources of noise of two-phase flow. In recent years, many scholars began to conduct scientific experimental research on noise characteristics and also proposed noise suppression methods. For example, Jascha Ruebeling and Steffen Grohmann analyzed the mechanism of fluid-induced noise at the capillary outlet of the refrigeration system [9]. Min Seong Kim et al. proposed using the noise model diagram to predict the noise caused by refrigerants in refrigerators [10]. Hyung Suk Han et al. studied the refrigerant flow pattern and bubble characteristics at the inlet of the evaporator and discussed the relationship between refrigerant noise and bubble characteristics [11][12][13]. Yubo Xia et al. established a transition tube with different structures between the capillary and the evaporator and experimentally studied the influence of refrigerant flow state in the transition tube on the refrigerator noise [14]. When the refrigerant is in twophase flow state, it usually produces refrigerant flow noise. At present, the effect of the proposed scheme to suppress the jet noise from the capillary outlet of refrigerator is not very ideal. e study based on the theoretical and experimental research, the refrigerant in the outlet of capillary jet noise, considering the pipe fluid dynamics and bubble dynamics, aimed at curbing the evaporator inlet air bubbles burst and thus effectively controls the refrigerant in capillary outlet jet noise, puts forward the feasible solution, and achieves the goal of curing.

Frequency Distribution of Refrigerator Noise
Refrigerator noise is mainly the audible sound between 20 Hz and 20 kHz that humans can hear. e system noise is mainly distributed between 20 Hz and 10 kHz, including the following: (1) Mechanical noise (0 Hz-300 Hz) (2) Bubble bursting noise (300 Hz-2500 Hz) (3) Electromagnetic noise (above 2500 Hz) (4) Cross noise of three kinds of noise (resonance) e typical spectrum distribution is shown in Figure 1. Figure 1 shows that the refrigerator noise varied with direction. e noise at the side of the compressor chamber and at the left side was slightly louder than the noise at the front and right sides. A peak was observed with the range of 125Hz-2000 Hz.
Since the size and shape of the bubble vary with the flow pattern in the pipeline, the acoustical characteristic should be varied according to the flow pattern in the pipeline.
e acoustical characteristic can be estimated according to equation (1) [15], by assuming that a unit bubble can be modeled as a one-degree of freedom springmass system. where We assumed that the volume of the bubble oscillates with an amplitude of α as follows: en, the natural frequency of the oscillating bubble can be obtained as follows [16]: where R 0 is the equivalent bubble radius.
Since the burst frequency of bubbles is close to the natural frequency of bubbles, it can be concluded that the frequency of bubble burst noise is mainly distributed between 125 Hz and 2000 Hz.
When the equivalent bubble radius is larger than the inner tube radius, the bubble shape cannot be spherical. Since the bubble radius is larger than the pipe radius, it should be irregularly deformed along the axial direction of the pipe and become slug flow. In this paper, the shape of long projectile was assumed to be cylindrical for the convenience of explaining its frequency characteristics [17].

Numerical Simulation of Jet Noise at the Capillary Outlet
For small refrigeration systems, the capillary has been widely used because the capillary has numerous advantages such as simple structure, low cost, convenient installation, and stable quality. Previous studies showed that a metastable phenomenon occurs in the refrigerant flow through the capillary, which affects the flow rate, pressure, and injection state at the capillary outlet.
According to the reference, the jet noise at the capillary outlet is mainly caused by the refrigerant bubble growing violently and bursting suddenly at the inlet of the evaporator. Usually, there is more than one evaporator in the refrigeration system of the refrigerator. For example, the refrigerator selected in this experiment is divided into the fresh-keeping chamber, the cold chamber, and the freezing chamber. ere are three evaporators, which exchange heat with the inside of the refrigerator through natural convection and radiation heat exchange. In this experiment, the structure of the capillary outlet of the fresh-keeping chamber is improved, which directly affects the burst of refrigerant bubbles at the capillary outlet and then affects the injection noise level at the capillary outlet. By comparing the numerical simulation results before and after optimization, the preliminary analysis was carried out.

Capillary Numerical Simulation.
In order to describe the flow of refrigerant in the capillary outlet and the transition tube accurately, the capillary numerical simulation model was firstly established. e capillary numerical simulation was divided into three parts.

Contraction Section of Capillary Inlet.
e singlephase refrigerant flows from the drying filter into the capillary. As the pipe size suddenly decreases, the refrigerant flow rate will be increased and the pressure will be decreased. e pressure drop of this part can be expressed as where ζ is local resistance coefficient of import which is given by where A c is the cross-sectional area of capillary, A d is the cross-sectional area of filter drier, and

Subcooled Liquid Single-Phase Model.
e liquid refrigerant flows at a high speed in the capillary tube, causing strong friction with the inner surface of the capillary tube and then resulting in the decrease of refrigerant pressure. e control equation is as follows.
Momentum equation: where f sp is the friction drag coefficient and f sp , using the Churchill equation, gives Energy equation in the capillary: Energy equation in compressor suction pipe: Ignoring the contact thermal resistance and axial thermal conductivity between the capillary tube and the compressor return tube, the energy equation is e convective heat transfer coefficient of capillary and compressor return pipe is given by Pata [18]:

Gas-Liquid Two-Phase Flow Model.
As the refrigerant flows in the capillary, the refrigerant pressure begins to drop to the saturation pressure corresponding to the supercooled temperature, and the refrigerant is in a two-phase flow state. e control equation is as follows. Mass conservation equation: Momentum equation: where shear stress is given as follows:

Vapor-Liquid Two-Phase Numerical Simulation of the Transition Tube.
According to the state parameters of refrigerant R600a at the capillary outlet, the gas-liquid two-phase flow model and the Schnerr-Sauer cavitation model of the transition tube were established by using CFD commercial software. Unstructured mesh was used to build a 3D model. e boundary conditions are velocity inlet and pressure outlet. e reliable turbulence model and the mixture model were used for the two-phase flow model. e cavitation model was selected. e capillary inner diameter is D1 � 0.7 mm, and the outer diameter is D2 � 1.8 mm. e length of the transition tube is 70 mm. e inner diameter of the transition tube is D3 � 6 mm, and the outer diameter is D4 � 8 mm. e capillary was inserted into the transition tube at a depth of 19.65 mm. e inner and outer diameters of bypass tubes are the same as those of capillary tubes. e insertion position of the bypass pipe is 20 mm away from both ends of the transition pipe. e structure before and after improvement is shown in Figures 2 and 3. e design conditions of Haier BCD-520 were selected for simulation, and the calculated parameters are shown in Table 1.
e simulation results are shown in Figure 4. e local pressure and gas-phase volume fraction at the axis of the transition pipe are shown in Figure 4. e starting point of the horizontal coordinate in Figure 4 represents the position of the end of the capillary outlet, and the positive direction of the abscissa is consistent with the refrigerant flow direction. Figure 4(a) represents the partial pressure distribution of the refrigerant in the transition pipe, which shows that the vapor admixture structure can obviously increase local pressure of the evaporator inlet by introducing the gas-phase refrigerant. Figure 4(b) represents the volume fraction distribution of the gas-phase refrigerant in the transition pipe, which shows that the gas-phase volume fraction of the refrigerant at the transition pipe is significantly decreased after optimization because the refrigerant cavitation phenomenon is suppressed, the supercavitation state is reached, and at the same time, the pressure fluctuation become smaller. According to the results, we can know that the structure can effectively inhibit the bursting of the refrigerant bubble at the entrance of the evaporator, so that the bubbles continue to flow along the pipeline and at the same time it can reduce the noise value of the refrigerant jet at the capillary outlet, solving the noise problem from the source and achieving the effect of treating both symptoms and root causes. Figure 5. Between the capillary outlet and the evaporator inlet, improved transition structure is installed, while the rest of the system remains in relative position.

Test Equipment.
e experimental testing room and experimental equipment are provided by Qingdao Haier.
e experiment was carried out in a standard noise suppression chamber. e refrigerator prototype was Haier BCD-520, and the noise spectrum analyzer was LMS Test.Lab. According to the test method specified in GB_T8059, the refrigerator is placed on the horizontal ground in the middle of the muffling room, and four microphones are placed around the refrigerator. e distance of microphones from the refrigerator is 1m, and the height is 1.5 m. e refrigerator noise evaluation system is shown in Figure 6. e ambient temperature and humidity are 20.5°C/ 73%, and the atmospheric pressure is 1.0118 × 10 5 Pa. e temperature of the fresh-keeping chamber is set at 6°C.

Result Analysis.
From the above discussion, it can be found that the burst noise of the bubble at the capillary outlet is one of the main noise sources of the refrigerator, and its main frequency range is 250-8000 Hz. en, we conducted experiments within this noise frequency range. e experimental data are shown in Figures 7 and 8.
As we can see from Figures 8(a) and 8(b), the improved noise spectrum becomes smoother, especially within 125 Hz-2000 Hz. After optimization, the level of noise signal is reduced by 1.5 dB(A), and the fluctuation of noise curve is smaller. ese results show that the optimization is feasible.
at is to say, by adjusting the vapor admixture at the evaporator entrance, the bubble burst at the capillary outlet can be inhibited, as well as the spectrum amplitude of the jet noise at the capillary outlet. e standard specifies that ambient temperature in the laboratory can be from 10 to 43°C; in our test, it was (25 ± 0.5)°C. e relative humidity in the laboratory was generally (55 ± 1)%, and the ambient airflow was less than 0.25 m/s.