Considering the Diffusive Effects of Cavitation in a Homogeneous Mixture Model

: Homogeneous mixture models are widely used to predict the hydrodynamic cavitation. In this study, the constant ‐ transfer coefficient model is implemented into a homogeneous cavitation model to predict the heat and mass diffusion. Modifications are made to the average bubble temperature and the Peclet number for thermal diffusivity in the constant ‐ transfer coefficient model. The evolutions of a spherical bubble triggered by negative pressure pulse are simulated to evaluate the prediction of heat and mass diffusion by the homogeneous model. The evolutions of three bubbles inside a rectangular tube are simulated, which show good accuracy of the homogeneous model for multibubbles in stationary liquid.


Introduction
Cavitation is usually caused by pressure decrease, leading to the growth of bubbles and followed by their rapid collapse. During the bubble growth, the temperature inside the bubble decreases and heat transfer occurs. Meanwhile, evaporation occurs at the bubble wall and vapor diffuses into the bubble. The heat and mass transfer make it complex to accurately predict the bubble dynamics [1][2][3]. Interface capturing methods, such as VOF and level set, can accurately predict the dynamics of pure gas bubbles [4]. However, they are inapplicable when there are huge number of bubbles. Instead, homogeneous models are widely used to simulate the hydrodynamic cavitation.
Homogeneous models usually use the Rayleigh-Plesset-type equations to predict the bubble dynamics [5][6][7][8][9][10][11][12][13]. Usually, only the local pressure but not the far-field pressure is known for numerical simulations. Ye et al. [14] proposed a homogeneous mixture model based on the bounded Rayleigh-Plesset equation [15]. This model solves the bubble dynamics using the local pressure, and can accurately predict the bubble dynamics of an isothermal gas bubble before bubble rebound. As some other homogeneous models [16][17][18], the heat transfer is simply treated by setting the polytropic index to be a constant and the mass transfer is simply treated by assuming the vapor pressure to be constant. Different working conditions need different constants. It is almost impossible to accurately predict the heat and mass transfer by estimating proper constants. These treatments of heat and mass transfer need to be improved.
Preston et al. [19] proposed a theoretical model named constant-transfer coefficient model, which can capture the effect of heat and mass transfer efficiently for a spherical bubble. In this model, two transport equations are used to, respectively, record the pressure (pGV) and the mass of vapor (mV) inside the bubble, and two constant-transfer coefficients are used to estimate the heat and mass flux at the bubble wall. In this study, this model is implemented into the homogeneous mixture model proposed by Ye et al. [14], and the calculations of the average bubble temperature and the Peclet number for thermal diffusivity in the constant-transfer model are modified. In the validation part, the evolutions of a spherical bubble triggered by pressure pulse are simulated and the evolutions of three bubbles in a regular arrangement are simulated to evaluate the accuracy for multibubbles. Comparisons with the predictions by the VOF method and the full computation (uses six equations to predict the heat and mass transfer) are made.

Homogeneous Mixture Cavitation Model
The homogeneous mixture model proposed by Ye et al. [14] is based on the bounded Rayleigh-Plesset equation [15] as follows: where R is the bubble radius, the over dot denotes the derivative in time, pb is the liquid pressure at the bubble surface, pe is the pressure at r = Re, and ρL is the liquid density. R  of a 3D spherical bubble is predicted by [14]: where p is the local pressure and α is the volume fraction of gas-vapor mixture. In order to improve the numerical stability, a minimum collapse rate min R  = −30 m/s is given, below which the collapse rate will not decrease any more, by the modification of R  as: when max , 0 when (3) pb can be determined by: where the subscripts V and 0, respectively, denote the vapor and the initial value, p0 is the initial pressure outside the bubble, ps0 is the initial vapor saturation pressure, R0 is the initial bubble radius, γ is the polytropic index, S denotes the surface tension coefficient, and μL is the dynamic viscosity of liquid. γ = 1 for an isothermal process while γ =  (adiabatic exponent) for an adiabatic process. Usually, pV is simply supposed to be equal to ps0, and γ is set to be a constant between 1 and  . In this study, the pressure inside the bubble pGV (the subscript G denotes the noncondensable gas) will be predicted by the constant-transfer coefficient model [19].

Constant-Transfer Coefficient Model
The constant-transfer coefficient model [19] is a theoretical model for spherical bubble dynamics that considers the effects of heat and mass transfer. Several assumptions were made: 1) the gas-vapor mixture was a perfect gas, 2) constant transport properties and surface tension coefficient, 3) thermal equilibrium and vapor-pressure equilibrium at the gas-liquid interface, 4) pGV was spatially uniform, 5) the liquid temperature was uniform, 6) no diffusion of noncondensable gas in the liquid, and 7) the liquid was "cold." Two transport equations are used to, respectively, predict pGV and mV. All variables were nondimensionalized in Ref. [19], which are converted into dimensional form in this study. The initial pGV is determined as: The source term of the transport equation of pGV is determined by: where T is the temperature, C is the mass fraction of vapor, the subscript w denotes the value at the bubble wall,  are, respectively, the adiabatic exponent, thermal conductivity, and perfect gas constant of the gas-vapor mixture at the bubble wall, and cp0 and D0 are, respectively, the specific heat and the diffusivity of initial gas-vapor mixture. Since the vapor pressure at the bubble wall is assumed to be in equilibrium, Cw can be obtained by: where M is the molecular weight. The gradients of C and T at the bubble wall are modeled using constant-transfer coefficients βC and βT as [19]: is the average vapor mass fraction, T0 is the initial temperature, and T is the average bubble temperature estimated by Preston et al. [19] as: (10) According to the perfect gas law, it is modified to: βC and βT are obtained by linear analysis as [19]: where PeG and PeT are, respectively, the Peclet numbers for mass and thermal diffusivity, ρGV0 is the initial density of gas-vapor mixture, and Ω0 is the bubble natural frequency determined by: mV is predicted by another transport equation, whose source term is as follows: The initial mV and mG are determined as: When pGV is obtained, pb is determined as follows instead of Equation (4):

Validation
The validation cases are similar to those in [19]. A spherical bubble is triggered by a negative pressure pulse. The gravity is neglected and the flows are assumed to be laminar. T0 is 298 K and the negative pressure pulse takes the following form: Table 1 shows the parameters of these validation cases. Cases A to H simulate the evolutions of a pure gas bubble to evaluate the prediction of heat transfer. The negative pressure pulse is employed at the distance of Re = 5 mm away from the bubble (Figure 1). Comparisons are made with the predictions by the VOF method since it can accurately predict the heat transfer. For better comparison, the surface tension is neglected. Before the comparison, all these cases are simulated under the isothermal assumption to validate the accuracy of the VOF method and homogeneous model without heat transfer, by comparing with the theoretical results predicted by Equation (1). Results show that the relative differences between the predicted maximum bubble radius by the VOF method and Equation (1) are less than 0.1%, and those between the homogeneous model and Equation (1) are less than 0.22%. After the comparisons of one bubble, the validation case of three bubbles [14] is simulated to evaluate the homogeneous model for multibubbles.
Cases I and J simulate the evolution of a gas-vapor bubble to include the mass transfer. Since it is complex to consider the phase change for the VOF method, comparisons are made with the theoretical results by the full computation [19] and constant-transfer coefficient model. It should be clarified that Re = ∞ in the prediction by the full computation, while Re = 1 m in the prediction by the constant-transfer coefficient model and homogeneous model. In order to prove the independency of Re, the dynamics of a pure gas bubble with Re = 1 m and ∞ are simulated by Equation (1) under the isothermal assumption; the relative difference of Rmax between them is only 0.001%.  Figure 1 shows the 2D computational grid for the homogeneous mixture model, which contains 1718 quadrilateral meshes. Rmax decreases by 0.11% for case D when the grid number is increased to 5087. The bubble is located at the lower-left corner of the computational domain. Since there is only one bubble, a virtual initial bubble number density (n0) of 10 9 m −3 is given inside the region with the radius of   1/3 0 4 3 n   (marked in red in Figure 1). Additionally, the initial gas volume fraction inside this region is    also increases with p0, and it reaches 4.1% at 800 kPa, which means the heat transfer is obviously overpredicted. In order to reduce the heat transfer at large p0, the calculation of PeT (Equation (15)) is modified by fixing p0 to the atmospheric pressure. After this modification, max R  at 800 kPa decreases to 1.1%, much smaller than the original value. It can be seen from Figure 3c    Then the homogeneous model is used to simulate three gas bubbles trigged by the pressure pulse in Equation (22), which was simulated by Ye et al. [14] under the isothermal assumption. The parameters are as follows: R0 = 50 μm, T0 = 298 K, S = 0, n0 = 10 9 m −3 , p0 = 101325 Pa, βT = 6.82, and Ap = 1.4; two cases are simulated with tw = 20 and 200 μs. The computational domain and boundary conditions are shown in Figure 4. The size of the computational domain is 0.5 mm × 0.5 mm × 50 mm. The pressure pulse is specified at the right face, whereas p0 is specified at the left face, and the rest four are symmetry planes. Three bubbles are placed inside the computational domain in a regular arrangement at the interval of 1 mm. The computational grids are the same as that in Ref. [14]. Briefly, 1.23 million elements are employed for the VOF method while 500 elements are employed for the homogeneous model. Figure 5 compares the total bubble volume predicted by the VOF method and homogeneous model. It can be seen that the bubble volume is well predicted by the homogeneous model with the consideration of heat transfer. The maximum bubble volume of the two cases predicted by the homogeneous model are, respectively, 1.5% (tw = 20 μs) and 0.63% smaller than the corresponding values predicted by the VOF method. The underestimate of the bubble volume is more obvious at smaller tw, since the bubble radius is underestimated by the constant-transfer model at small tw, as shown in Figure 3c.

Gas-Vapor Bubbles
The computational grid for cases I and J is similar to Figure 1, with Re be increased to 1 m and the grid number be increased to 34,000. Figure 6 compares R, T , C , and Cw of case I predicted by the full computation [19], constant-transfer model, and homogeneous model. R is a little overpredicted and T is underestimated by the homogeneous model and constant-transfer model, which might be due to the overprediction of evaporation. Figure 7 compares R of case J predicted by the full computation, constant-transfer model, and homogeneous model. The prediction by the homogeneous model shows good agreement with that by the constant-transfer model; the relative difference of Rmax between these two methods is below 0.26%. It can be seen from Figure 3c that the bubble radius can be well predicted if the evaporation of case J is neglected, thus the overprediction of Rmax of case J is mainly due to the overprediction of evaporation.

Conclusions and Prospects
The constant-transfer coefficient model is implemented into a homogeneous cavitation model to consider the heat and mass transfer inside the bubble. Two transport equations are added to, respectively, record the pressure and the mass of vapor inside the bubble. The calculations of the average bubble temperature and the Peclet number for thermal diffusivity in the constant-transfer coefficient model are modified. According to the validation cases of pure gas and gas-vapor bubbles, the numerical predictions by the homogeneous model match well with the theoretical predictions by the constant-transfer coefficient model: the relative difference of Rmax between these two methods is below 0.26%; the prediction of heat transfer at large initial pressure is obviously improved after the modification to the Peclet number for thermal diffusivity: the relative error of Rmax at p0 = 800 kPa decreases from 4.1% to 1.1%. According to the validation cases of three gas bubbles, the cavitation of multibubbles in stationary liquid can be well predicted by the homogeneous model. This paper focuses on the bubble dynamics in stationary liquid; the influences of nucleation and turbulence-cavitation interaction need to be considered in future, which are important for hydrodynamic cavitation.