A Transport-Phenomena Approach to Model Hydrodynamic Cavitation of Organic Pollutants

Detergent Dissolution Intensification via Energy-Efficient Hydrodynamic Cavitation Reactors

In this study, we explored the potential of hydrodynamic cavitation (HC) for use in dissolution of liquid and powder detergents. For this, microfluidic and polyether ether ketone (PEEK) tube HC reactors with different configurations were employed, and the results from the reactors were compared with a magnetic stirrer, as well as a tergotometer. According to our results PEEK tube HC reactors present the best performance for dissolution of liquid and powder detergents. In the case of liquid detergent, for the same level of initial concentration and comparable final dissolution, the PEEK tube consumed 16.7 and 70% of the energy and time of a tergotometer and 16.7 and 14.8% of that of a magnetic stirrer, respectively. In the case of powder detergent, the PEEK tube used 12% less power than a tergotometer and 81.2% less power than a magnetic stirrer. Additionally, the time required to dissolve the detergent was reduced significantly from 1200 s in the tergotometer and 1800 s in the magnetic stirrer to just 50 s in the PEEK tube. These results suggest that HC could significantly improve the dissolution rate of liquid and powder detergents and energy consumption in washing machines.

Research on Noise-Induced Characteristics of Unsteady Cavitation of a Jet Pump

The dynamic cavitation characteristics of normal-temperature water flowing through a transparent jet pump under different cavitation conditions were experimentally studied by adjusting the pressure ratio. The common results are presented at different pressure ratios, including the temporal and spatial changes of the pressure and noise, together with the visual observation of the cavitation unsteady behaviors using a high-speed camera. The analyses on the measured data and images reveal that the cavitation cloud is generated by periodic oscillations of the jet traveling pressure wave and the bubble traveling pressure wave. The oscillation of the two kinds of interface waves is caused by the collapse of the bubbles, which is the main mechanism of the bubble cloud shedding. As the pressure ratio increases, the maximum length of the jet cloud and bubble cloud linearly decreases, while their oscillation frequency increases gradually. Combined with the cavitation-cloud visualization data and noise frequency analysis, it is proposed that the strong impact between the jet traveling pressure wave and the bubble traveling pressure wave is the main cause of noise. Specially, the acoustic pressure reaches the maximum when the oscillation frequency of the jet traveling pressure wave is the same as that of the bubble traveling pressure wave. Also, the jet traveling pressure wave has a great influence on the migration of bubbles in the cavity. The results can provide guidance for the optimal operating condition in cavitation applications such as jet aerator and quantitative addition.

Novel strategies to enhance hydrodynamic cavitation in a circular venturi using RANS numerical simulations

Hydrodynamic cavitation is a popular advanced oxidation technique and it has received wide range of applications from waste water treatment to the nanoparticles synthesis in recent years. The enhancement of the intensity of the hydrodynamic cavitation is always been an emerging field of research. Within this framework, we have proposed and investigated three distinct strategies to enhance the intensity of cavitation in a circular venturi, namely, (1) by introducing the surface roughness on the wall (2) single or multiple circular hurdles in the diverging section (3) By modifying the diverging section from planer to the trumpet shape. RANS (Reynolds Averaged Navier-Stokes) based numerical simulations are carried out the over wide range of conditions: 2≤PR≤6 (pressure ratio), 6.2^∘≤β≤10^∘ (half divergent angle), 15^∘≤α≤20^∘ (half convergent angle), and 1≤l/d≤3 (throat length). An extensive numerical and experimental validation with the literature have been presented to ensure the reliability and accuracy of present work. Detailed results on velocity fields, local and average volume fraction, pressure loss coefficients, cavitation number, discharge coefficient and pressure distribution are reported as function of dimensionless parameters. Five designs of various combinations of surface roughness, circular hurdles, and trumpet diverging section have been compared. The effect of surface roughness on trumpet diverging wall has been observed to be more pronounced than the other designs. Trumpet diverging wall with surface roughness is found to be optimum for the practical applications.

Estimation of chemical and physical effects of cavitation by analysis of cavitating single bubble dynamics

Cavitation events create extreme conditions in a localized 'bubble collapse' region, leading to the formation of hydroxyl radicals, shockwaves and microscopic high-speed jets, which are useful for many chemical and physical transformation processes. Single bubble dynamics equations have been used previously to investigate the chemical and physical effects of cavitation. In the present study, the state of the art of the single bubble dynamics equations was reviewed and certain noteworthy modifications were implemented. Simulations reaffirmed previously reported collapse temperatures of the order ~5,000 K and collapse pressures well over ~1,000 bar under varying operating conditions. The chemical effects were assessed in terms of the hydroxyl radical generation rate (OHG), calculated by applying the minimization of the Gibb's Free Energy method using simulated collapse conditions. OHG values as high as 1x1012OH molecules per collapse event were found under certain operating conditions. A new equation was proposed to assess the physical effects, in terms of the impact pressure of the water jet - termed as the jet hammer pressure (JHP), formed due to the asymmetrical collapse of a bubble near a wall. The predicted JHP were found to be within a range of ~100 to 1000 bar under varying operating conditions. Important issues such as the onset of cavitation and chaotic solutions, for a cavitating single bubble dynamics were discussed. The Blake threshold pressure was found to be a sufficient criterion to capture the onset of cavitation. The impact of key operating parameters on the chemical and physical effects of cavitation were investigated exhaustively through simulations, over the parameter ranges relevant to acoustic and hydrodynamic cavitation processes. Presented methodology and results will be useful for optimisation and further investigations of a broad range of acoustic and hydrodynamic cavitation-based applications.

ANN based modelling of hydrodynamic cavitation processes: Biomass pre-treatment and wastewater treatment

We have developed artificial neural network (ANN) based models for simulating two application examples of hydrodynamic cavitation (HC) namely, biomass pre-treatment to enhance biogas and degradation of organic pollutants in water. The first case reports data on influence of number of passes through HC reactor on bio-methane generation from bagasse. The second case reports data on influence of HC reactor scale on degradation of dichloroaniline (DCA). Similar to most of the HC based applications, the availability of experimental data for these two applications is rather limited. In this work a systematic methodology for developing ANN model is presented. The models were shown to describe the experimental data very well. The ANN models were then evaluated for their ability to interpolate and extrapolate. Despite the limited data, the ANN models were able to simulate and interpolate the data for two very different and complex HC applications very well. The extrapolated results of biomethane generation in terms of number of passes were consistent with the intuitive understanding. The extrapolated results in terms of elapsed time were however not consistent with the intuitive understanding. The ANN model was able to generate intuitively consistent extrapolated results for degradation of DCA in terms of number of passes as well as scale of HC reactor. The results will be useful for developing quantitative models of complex HC applications.