Matthias Hase

Effect of viscosity on droplet-droplet collision outcome: Experimental study and numerical comparison

The influence of viscosity on droplet-droplet collision behavior at ambient conditions was studied experimentally and numerically. N-decane, monoethyleneglycol (MEG), diethyleneglycol (DEG), and triethyleneglycol were used as liquid phase providing viscosities in the range from 0.9to48mPas. Collision Weber numbers ranged approximately from 10 to 420. A direct numerical simulation code, based on the volume-of-fluid concept, was used for the simulations. Experimentally, observations of two droplet streams using a modified stroboscopic technique (aliasing method) were used to investigate the whole range of impact parameters during one experimental run. The experimental method has previously been verified for the water/air system [C. Gotaas et al., Phys. Fluids 19, 102105 (2007)]. In the present work, it was tested and validated for the n-decane/air system. Measured data agree well with those published in the literature. Well-defined regions of stretching separation and coalescence were identified, while refl...

Transient heat transfer of deforming droplets at high Reynolds numbers

A numerical study of heat transfer enhancement due to the deformation of droplets at high Reynolds numbers is described. The two phase‐flow has been computed with a 3D DNS program using the volume‐of‐fluid method. The droplets are deformed because of the surrounding gas stream especially due to a sudden rise of flow velocity from zero to Ui. As the governing non‐dimensional parameter the Weber number of the droplets has been varied between 1.3 and 10.8 by assuming different surface tensions at Reynolds numbers between 360 and 853. The dynamical behavior of the droplets as a function of the Weber and the Ohnsorge number are in good agreement with experimental results from the literature. At the highest Reynolds number Re=853, a significant dependency of Nu on We has been found. The comparison of a Nusselt number computed with the real surface area with a Nusselt number computed with the spherical surface area shows that the heat transfer increases not only due to the droplet motion but also due to the larger surface area of the deformed droplet.

CFD Heat Transfer Predictions of a Single Circular Jet Impinging with Crossflow

Jet impingement cooling is used today in a large number of applications ranging from electronic cooling to cooling applications in gas turbines. However, the numerical prediction of impingement is difficult and numerical methods lack inaccuracy for the prediction of heat transfer rates. The aim of the current investigation is to determine the degree of accuracy to which the heat transfer rates of a single circular jet impinging through a well-defined crossflow can be predicted. A commercial CFD package (CFX-5.7.1) with a 3D RANS approach is used for the numerical analysis of the highly turbulent flow field. Computational results are compared to experimental data available from the open literature. Different turbulence models are tested for the configuration of a single jet without crossflow to evaluate a model suited best for the impingement cases. For the crossflow cases, two different nozzle-plate spacings (Z/D = 6 and Z/D = 12) are investigated. Jet Reynolds numbers range from 33,400 to 121,300 while the crossflow velocities investigated correspond to Reynolds numbers ranging from 40,900 to 155,900. The numerical results show a very good agreement with the experimental reference data. The deflection of the jet due to the presence of the crossflow and the corresponding shift of the point of maximum heat transfer rate are captured correctly. Some deviations between the numerical and the experimental results can be observed in the regions around the stagnation point but the effects on averaged heat transfer rates are negligible.

Parallel Computation of the Time Dependent Velocity Evolution for Strongly Deformed Droplets

A fully three-dimensional numerical procedure has been used to predict the behavior of spherical and deformed droplets in a gas flow. The computational grid is moving with the droplet to minimize grid size and computation time. Numerical results of drag coefficients for spherical droplets show good agreement with literature data. The behavior of droplets with initially cylindrical or disk shapes has been compared with corresponding spherical droplets for different viscosities of the droplet liquid. For low viscosities the droplets are oscillating. For higher viscosities the initially strongly deformed droplets approach a spherical shape asymptotically. The influence of the strong initial deformation is shown. The simulation has been run on the Cray T3E/512-900 at the HLRS.

Numerical Simulation of 3D Unsteady Heat Transfer at Strongly Deformed Droplets at High Reynolds Numbers

The dependency of the heat transfer on an initial deformation of droplets has been investigated at high droplet Reynolds numbers. The two-phase flow has been computed with an inhouse 3D DNS program (FS3D) using the Volume-of-Fluid method. For the droplets initial prolate and oblate shapes with an axial approaching flow has been studied. In addition, a spherical shape has been used as reference. The initial droplet Reynolds number for the present study has been Re 0 = 660 for all investigated cases. Due to the fact that the steady droplet velocity for the considered droplets has been much lower than the initial velocity of the droplets, the droplet velocity is decreased during the simulation. To gain more knowledge about the influence of deformation on the heat transfer, the time dependent, spatial averaged Nusselt number Nu t and the time and spatial averaged Nusselt number Nu m has been matched by the temperature and velocity field around a deformed droplet. By this comparison the oscillation phase with the largest heat transfer has been observed. The simulations have been performed on the Cray T3E/512-900 at the HLRS with 32 processors. The parallel performance in dependency of the number of processors has been investigated.

Predictions of the 3D Unsteady Heat Transfer at Moving Droplets

A 3D numerical program for the transient simulation of the dynamic behavior of incompressible two-phase flows has been extended to the computation of heat transfer. In the program the VOF-method with interface reconstruction has been used for the calculation of the disperse phase. The governing equations and the implemented numerical model are described. Numerical results for a transient heat conduction problem of a rigid sphere show good agreement with analytical solutions. The predicted averaged Nusselt numbers for this problem from numerical simulations match well with experimental data from the literature. On the basis of two examples the difference between intermediate and high Reynolds number flow and heat transfer is pointed out. Finally, the influence of different initial droplet velocities on the time dependent temperature evolution is shown. The simulation has been performed on the Cray T3E/512-900 at the HLRS with up to 128 processors.