Th.H. van der Meer

Numerical study of laminar and turbulent natural convection in an inclined square cavity

Two-dimensional numerical simulations of the natural convection flow of air in a differentially heated, inclined square cavity were performed for both laminar and turbulent flows. The angle of inclination of the cavity was varied from 0° (heated from below) to 180° (heated from above). For Rayleigh numbers between 104 and 1011 the natural convection flow has been calculated. A detailed analysis was made for Rayleigh numbers of 106 and 1010. The standard k−e model for turbulence was used in the prediction of turbulent flows. Numerical predictions of the heat flux at the hot wall and the influence of the angle of inclination on the Nusselt number are presented. The Nusselt number shows strong dependence on the orientation of the cavity and the power law dependence on the Rayleigh number of the flow. Flow patterns and isotherms are shown to give greater understanding of the local heat transfer. For the high Rayleigh number calculations hysteresis of the solution was found at a transition of flow patterns.

Convective heat transfer on a plate in an impinging round hot gas jet of low Reynolds number

Abstract An experimental study has been made of the local heat transfer on the plane isothermal surface in the normal impinging round hot jet of combustion products produced by a rapid heating tunnel burner. A conductivity heat plug, impact tubes and fine wire thermocouples were used to measure heat flux, mean velocity and temperature distributions. Some centerline relative turbulence intensity measurements were done with a Laser Doppler Anemometer. All measurements were obtained at two efflux Reynolds numbers 1860 and 1050; the density ratio between hot combustion products and ambient air was 7.6. Heat transfer was measured at distances between 2 and 20 D. The stagnation point heat transfer within the distances x D ⩽ 5 is in good agreement with Sibulkins laminar boundary layer theory. In the developed region x D ⩾ 8 strong free jet turbulence effects augmenting the convective heat transfer were observed. The radial heat transfer distributions are qualitatively consistent with those known in the impinging cold jet investigations at low Reynolds numbers.

Double-diffusive natural convection in trapezoidal enclosures

This paper presents some results of numerical and experimental study of turbulent double-diffusive natural convection of a mixture of two gases in a trapezoidal enclosure with imposed unstable thermal stratification. The geometry and boundary conditions are selected to mimic an idealised situation in underground coal gasification. In the computations turbulent fluxes of momentum, heat and mass were modelled by standard and low-Re-number (Launder-Sharma) κ-e eddy diffusivity models with inclusion of thermal and mass buoyancy. The computed mean velocity, temperature and concentration, show satisfactory agreement with measurements, indicating that despite some deficiencies, the numerical model can reproduce most of the essential features of the process.

Application of infrared thermography to the evaluation of local convective heat transfer on arrays of cubical protrusions

Abstract This paper reports on the experimental technique and measurement method used for the investigation of the local convective heat transfer from cubical protrusions placed on a vertical wall of a rectangular-sectioned wind tunnel. This experimental configuration simulates a simplified air-cooled vertical electronic circuit board, resembling a situation in an electronic cabinet. A controlled heat flux was generated by internal heating of the elements. An advanced infrared system was used to measure the surface temperatures of the cubical elements, which is crucial for determining the local convective heat transfer. The difficulties and uncertainties encountered in the course of measurements are discussed together with possible solutions of the problems. A selection of results, validated by means of an uncertainty analysis, is presented, showing a strong variation of the local convective heat transfer around the individual elements. Qualitative interpretation of the local heat transfer coefficients was facilitated by smoke visualization of the mean flow pattern around the protrusions.

Hydrodynamic properties and mass transfer characteristics of electrochemical flow-through cells of the confined wall—jet type

Abstract The hydrodynamic properties and mass transfer characteristics of confined wall—jet electrochemical flow-through cells have been investigated for an electrochemically and chemically reversible, uncomplicated electrode reaction. Numerical as well as experimental results are presented. The approximate analytical solution for the stationary Sherwood number of a cell with a disk working electrode with radius R is: 〈Shj〉anal=0.924Re 1 3 d(d/b) 2 3 (d/R) 2 3 Sc 1 2 This relation is valid only when the diffusion boundary layer is small, creeping flow is assumed and the contribution of the impingement area to the signal can be neglected. Numerical correlations give the following solution for the mass transfer at a disk working electrode for Red values between 10 and 180; 1000 ⩽ Sc ⩽ 2000; 2 ⩽ R/d ⩽ 16.7 and 0.083 ⩽ b/d ⩽ 0.67 (reproduction of the solute at the auxiliary electrode is accounted for): 〈Shd〉num = 0.82Re0.36d(d/b)0.63(d/R) 2 3 Sc 1 3 For experimental verification a hydrodynamically optimized cell has been developed with a gold working electrode and a glassy carbon auxiliary electrode. For 10−3M potassium hexacyanoferrate(II) in 1 M potassium nitrate, the experimental results deviated less than 15% from the numerical solution when 8 ⩽ Red ⩽ 78; 0.083 ⩽ b/d ⩽ 0.67; Sc=1400 and R/d = 16.7.

Turbulent natural convection flow due to combined buoyancy forces during underground gasification of thin coal layers

Abstract During the process of gasifying thin coal layers underground, an open channel structure might exist. In this channel, chemical reactions will have an effect on both the temperature and the concentration distribution. The resulting temperature and concentration gradients give rise to opposing buoyant forces. The resulting flow field determines the heat and mass transfer and the product gas composition. Results for the two-dimensional numerical prediction of the flow field and the product gas composition are given for a number of situations. The amount of air injected as well as the radiation extinction coefficient of the gas are varied. The predictions show that the process will produce useful gases for certain air inlets. The solution of the equations is time dependent for some situations: its periodicity is presented and discussed in this paper.

Heat transfer coefficients for viscous fluids in a static mixer

Abstract An experimental study has been done on heat transfer at constant heat flux as well as at constant wall temperature of a pipe filled with Sulzer mixing elements. A semi-empirical model is developed with which the experimental results agree. The influence on heat transfer of free sections between mixing sections has been studied.

Effects of Small- and Large-Scale Structures in a Piloted Jet Diffusion Flame

Laser Doppler Anemometry (LDA) and Planar Laser-Induced Fluorescence (PLIF) measurements have been performed in a turbulent nonpremixed jet flame. One of the features of this configuration is a central co-axial fuel jet surrounded by a turbulent annular air flow. The whole is placed within a low-speed coflowing air stream. This three-flow system with turbulent primary air differs from flow systems used for nonpremixed jet flames reported in the literature and is very useful for obtaining information on the mixing process between fuel and primary air. Next to the characterization of the velocity field, special attention has been paid to the conditional seeding of the central fuel jet and of the annular air flow. Together with visualizations of the OH radical, an important combustion intermediate which is formed during combustion, and the NO radical, which is seeded to the central jet flow, the resulting statistics reveal the properties of small- and large-scale structures in the flame.

OH concentration fluctuations in turbulent natural gas jet flames

Abstract OH radical concentrations in a turbulent non-premixed natural gas flame were measured using laser-induced fluorescence. Instantaneous concentration profiles along a line were obtained using a diode array camera. Investigation of the molecular transitions during laser excitation shows that concentrations, where calibrated with a one-dimensional laminar premixed flame, are biased with a factor of about two. This bias is similar for different flames, so that results of different flames can be compared with an accuracy of about 20%. Three different flames were studied, with fuel jet Reynolds numbers of 9.7 × 103, 6.8 × 103 and 4.9 × 103. Average concentrations and probability density functions show that concentrations close to the nozzle in the flame with highest turbulence are low, which may indicate local extinction. Integral length scales and Taylor micro scales, derived from spatial correlation, exhibit minima at radical locations where OH fluctuations exhibit maxima.

The implementation of a 3D Navier-Stokes algorithm on an algorithm oriented processor

Abstract The multiprocessor system ATOMS has been used to solve 3D Navier-Stokes problems. ATOMS was originally designed at AT&T Bell Laboratories for Molecular Dynamics calculations. However, certain hardware features were included in the design to permit data transfer between processor boards as closely coupled linear array of processors. In this mode we refer to the multiprocessor system as DNSP (AT&Ts/Delft Navier-Stokes Processor). An algorithm for calculating the buoyancy-driven laminar/turbulent flow in a 3D cavity has been implemented on the DNSP. For this algorithm an efficiency of 35% (which amounts to 14 Mflops) is obtained. This high efficiency can be reached thanks to the strong coupling between the algorithm and the architecture of the multiprocessor system. The speed is obtained at very low cost, resulting in a cost/performance ratio for the DNSP which is at least an order of magnitude lower than (mini-)supercomputers.