Guoyi Peng

Progress in numerical simulation of cavitating water jets

This paper reviews recent progress made toward modeling of cavitation and numerical simulation of cavitating water jets. Properties of existing cavitation models are discussed and a compressible mixture flow method for the numerical simulation of highspeed water jets accompanied by intensive cavitation is introduced. Two-phase fluids media of cavitating flow are treated as a homogeneous bubbly mixture and the mean flow is computed by solving Reynolds-Averaged Navier-Stokes (RANS) equations for compressible fluid. The intensity of cavitation is evaluated by the gas volume fraction, which is governed by the compressibility of bubble-liquid mixture corresponding to the status of mean flow field. Numerical results of cavitating water jet issuing from an orifice nozzle are presented and its applicability to intensively cavitating jets is demonstrated. However, the effect of impact pressure caused by collapsing of bubbles is neglected, and effectively coupling of the present compressible mixture flow method with the dynamics of bubbles remains to be a challenge.

Numerical analysis of cavitation cloud shedding in a submerged water jet

Focused on the unsteady behavior of high-speed water jets with intensive cavitation a numerical analysis is performed by applying a practical compressible mixture flow bubble cavitation model with a simplified estimation of bubble radius. The mean flow of two-phase mixture is calculated by unsteady Reynolds averaged Navier-Stokes (URANS) for compressible flow and the intensity of cavitation in a local field is evaluated by the volume fraction of gas bubbles whose radius is estimated with a simplified Rayleigh- Plesset equation according to pressure variation of the mean flow field. High-speed submerged water jet issuing from a sheathed sharp-edge orifice nozzle is treated. The periodically shedding of cavitation clouds is captured in a certain reliability compared to experiment data of visualization observation and the capability to capture the unsteadily shedding of cavitation clouds is demon- strated. The results demonstrate that cavitation takes place near the entrance of nozzle throat and cavitation cloud expands conseque- ntially while flowing downstream. Developed bubble clouds break up near the nozzle exit and shed downstream periodically along the shear layer. Under the effect of cavitation bubbles the decay of core velocity is delayed compared to the case of no-cavitation jet.

A Practical Combined Computation Method of Mean Through-Flow for 3D Inverse Design of Hydraulic Turbomachinery Blades

A practical combined computation method of the circumferentially averaged mean through-flow is presented for 3D inverse computations of hydraulic turbomachinery blades to consider the influence of interrelated hydraulic components. A comprehensive computation domain including the runner blades and related components is adopted and the mean flow is calculated altogether by solving a set of rotational flow governing equations simultaneously. The method has been applied to the case of Kaplan turbine. Computational results were compared to experimental data and their agreement was confirmed. Numerical investigation indicates that the mean flow is dependent on the configuration of guide vanes and the effect of runner blades reaches to the far upstream. The importance of properly taking account of the effect of blade geometry and the influence of interrelated hydraulic components is demonstrated.

Study on the natural air cooling of electronic equipment casings: effects of the outlet vent and the power heater locations on the cooling capability

Abstract This paper is concerned with the natural cooling of electronic equipment casings. Effects of the size and the location of the outlet vent as well as the relative distance from the outlet vent location to the power heater position on the flow resistance have been investigated experimentally by using a simple model casing simulated for practical natural air cooled electronic equipment casings. The result shows that the mean temperature rise inside the casing increases with increase in the heater input power, but decreases linearly with increase in the vent porosity coefficient in logarithmic coordinates. As the heater approaches the outlet vent, the temperature rise increases linearly in the logarithmic scale. By defining a new dimensionless parameter called the equivalent Reynolds number, involving the Reynolds number and the porosity coefficient, the flow resistance coefficient is found to have a close logarithmic linear correlation, K = kxχ−1.5, with the equivalent Reynolds number. Its validation has been proved by the good agreement with experimental results. The result indicates that the relative distance from the outlet vent to the heat dissipation unit can be considered as a chimney height in the practical engineering design. Although further more detailed validations are needed, a useful correlation between the flow resistance coefficient, the Reynolds number and the porosity coefficient has been presented for the design application of natural cooling electronic casings.

Numerical analysis of heat transfer in a compact plastic ball grid array package air cooling model

The thermal performance of a temple package air cooling model composed of a 672-pin plastic ball grid array (PBGA) package mounted on a printed circuit board (PCB) and a compact system box has been investigated by numerical simulations of heat transfer. A geometry model resembling the PBGA/PCB package with directional homogeneous solid blocks was constructed, and a three-dimensional computational approach of thermal flow simulation was developed considering conduction and convection modes of heat transfer. Having been verified by experimental results, the approach was applied to the analysis of heat transfer in the package air cooling system. Computational results show that the thermal resistance of PGBA/PCB package model under the condition of natural air cooling is about 27.0 K/W and closes to 25.0 K/W gradually with the increase of heat spreading. Under the condition of forced air cooling, its thermal resistance decreases with the increase of airflow velocity, and the reasonable velocity of air cooling is revealed to be about 0.8 m/s.

Numerical Simulation of Unsteady Cavitation in a High-speed Water Jet

Abstract Concerning the numerical simulation of high-speed water jet with intensive cavitation this paper presents a practical compressible mixture flow method by coupling a simplified estimation of bubble cavitation and a compressible mixture flow computation. The mean flow of two-phase mixture is calculated by RANS for compressible fluidU he intensit. T y of cavitation in a local field is evaluated by the volume fraction of gas phase varying with the mean flow, and the effect of cavitation on the flow turbulence is considered by applying a ion to density correctthe evaluation of eddy viscosity. High-speed submerged water jets issuing from a sheathed sharp-edge orifice nozzle are treated when the cavitation number, σ = 0.1, and the computation result is compared with experimental data The result reveals that cavitation occurs initially at the entrance of orifice and bubble cloud develops gradually while flowing downstream along the shear layer. Developed bubble cloudbreaks up and then sheds downstream periodically near the sheath exit. The pattern of cavitation cloud shedding evaluated by simulation agrees experimental one, and the possibility to capture the unsteadily shedding of cavitation clouds is demonstrated. The decay of core velocity in cavitating jetis delayed greatly compared to that in no-activation jet, and the effect of the nozzle sheath is demonstrated.

Numerical simulation of heat dissipation from a plastic ball grid array package in a thin flat box: Effects of some operative and geometric parameters

This article concerns the numerical simulation method of heat transfer in a compact system box housing the assembly of a 672-pin plastic ball grid array (PBGA) package and a printed circuit board (PCB). A computational model for the PBGA/PCB assembly was developed to account for complex phenomena of three-dimensional heat and fluid flow, and conjugate heat transfer in tiny package components. Simulation results for a sample parameter set were compared with experimental data, and their agreement was confirmed. Then, the modeling approach was applied to a wide range of the parameter domain and effects of some operative and geometric parameters were investigated

Fundamentals of cavitation and bubble dynamics with engineering applications

Cavitation and bubble dynamics are important topics of fluid mechanics. Generally, cavitation phenomenon plays a negative role in the hydraulic machineries (e.g. hydroturbines, pumps, space engines, and marine propellers) through various kinds of mechanisms. First, the cavitation could cause serious damage on the surfaces of the materials (e.g. patterned erosions with pits) during its final collapse near the boundary. Second, flow inside/outside the machines will be further deteriorated by the vortices generated by the cavitating flows. Third, significant unstable flow could persist for a long time due to the generation of cavitation. As a result, both the efficiency and stable operation of the machines will be challenged by the cavitation.

Visualization Observation of Cavitation Cloud Shedding in a Submerged Water Jet

An experiment investigation on the behaviour of cavitation cloud caused in submerged water jets issuing from a sheathed sharp edge orifice nozzle was carried out by high-speed camera visualizing observation. It is demonstrated that cavitation bubble cloud appears when the cavitation number σ decreases to the level of 0.6–0.7. The dominant frequencies of bubble cloud expanding and contracting in the axial direction is closely related to the pressure pulsation of plunger pump, which is often employed in industry application of water jets. However, the dominant frequencies of jet width oscillation in the radial direction mainly depend on the shedding of shear vortexes as well as the collapsing of cavitation bubble clouds.

Two-dimensional direct numerical simulation of bubble cloud cavitation by front-tracking method

Unsteady bubble cloud cavitation phenomenon caused by negative pressure pulse has been treated numerically by applying a front tracking method. The behaviour of bubble cloud expanding and contracting is evaluated by tracking the motion of all bubble interfaces. Numerical investigation demonstrates that: (1) In the collapsing of bubble cloud micro liquid jets toward the inner bubbles are formed while the outer layer bubbles contract extremely, and then a high impact pressure is released when the inner central bubble contacts to its minimum. (2) The oscillation of bubble cloud depends upon the void fraction greatly. In the case of high void fraction, the frequency of cloud oscillation is lower than that of individual bubble and the decay of the oscillation becomes much slowly also.