Reducing honeycomb-generated turbulence with a passive grid

Honeycombs are widely used to laminarize fluid streams by inhibiting the lateral components of the fluctuating velocity. However, they also produce additional turbulence by themselves due to the formation of large-scale instabilities and the breakup of the individual velocity profiles stemming from the honeycomb cells. In the present research, we use 2D-planar particle image velocimetry to study how honeycomb-generated turbulence is affected by a downstream grid. It is found that placing a grid near the honeycomb discharge drastically enhances flow uniformity by separating the strong jets stemming from the individual honeycomb cells into many smaller jets that are much more rapidly dissipated. The results show that using a grid reduces the integral length scale by up to a factor 10, and the axial and lateral energy spectra reveal that the grid primarily limits the energy contained in eddies with lower wave numbers. Furthermore, the grid can reduce the magnitude of peak turbulence intensity by as much as 95% and leads to a large reduction of the correlation length, as long as it is positioned upstream of the onset of the large-scale honeycomb-induced instabilities. A downstream grid is highly beneficial for both a laminar and turbulent honeycomb discharge and is most effective when there is a slight offset between the grid and honeycomb. Even though longer honeycombs generally produce more turbulence than short ones due to the larger length-scale of the shear layers, these effects are almost entirely decoupled when using a honeycomb-grid combination. Finally, a honeycomb-grid combination effectively inhibits both axial and lateral turbulence

Nitrate Intake Does Not Influence Bladder Cancer Risk: The Netherlands Cohort Study

OBJECTIVES: N-nitroso compounds, endogenously formed from nitrate-derived nitrite, are suspected to be important bladder carcinogens. However, the association between nitrate exposure from food or drinking water and bladder cancer has not been substantially investigated in epidemiologic studies. METHODS: We evaluated the associations between nitrate exposure and bladder cancer in the Netherlands Cohort Study, conducted among 120,852 men and women, 55–69 years of age at entry. Information on nitrate from diet was collected via a food frequency questionnaire in 1986 and a database on nitrate content of foods. Individual nitrate exposures from beverages prepared with tap water were calculated by linking the postal code of individual residence at baseline to water company data. After 9.3 years of follow-up and after excluding subjects with incomplete or inconsistent dietary data, 889 cases and 4,441 subcohort members were available for multivariate analyses. We calculated incidence rate ratios (RR) and corresponding 95% confidence intervals (CIs) using Cox regression analyses. We also evaluated possible effect modification of dietary intake of vitamins C and E (low/high) and cigarette smoking (never/ever). RESULTS: The multivariate RRs for nitrate exposure from food, drinking water, and estimated total nitrate exposure were 1.06 (95% CI, 0.81–1.31), 1.06 (95% CI, 0.82–1.37), and 1.09 (95% CI, 0.84–1.42), respectively, comparing the highest to the lowest quintiles of intake. Dietary intake of vitamins C and E (low/high) and cigarette smoking (never/ever) had no significant impact on these results. CONCLUSION: Although the association between nitrate exposure and bladder cancer risk is biologically plausible, our results in this study do not support an association between nitrate exposure and bladder cancer risk

Infrared thermography of sorptive heating of thin porous media: experiments and continuum simulations

We have studied the imbibition of water from a stationary nozzle into thin, moving porous media that are suspended in air, as well as the accompanying evaporation and condensation processes. Due to sorptive heating and the latent heat associated with the phase change processes, the temperature of the porous medium becomes non-uniform. We have measured the temperature distributions using infrared thermography as a function of substrate speed. Moreover, we developed a numerical model coupling Darcy flow and heat transfer in the thin porous medium with gas flow, heat and water vapor transport in the surrounding gas phase. The numerical simulations reproduce the measurements very well and point at an intricate buoyancy-induced gas-phase convection pattern

Counterflow pulse-tube refrigerators

The regenerators in standard pulse-tube refrigerators are large, heavy, expensive, and are a source of losses. In this contribution we avoid using regenerators by combining two pulse-tube refrigerators which operate in opposite phase. The regenerators are replaced by a counterflow heat exchanger. We treat the basic thermodynamic equations, make some design considerations, and report the results of some preliminary experiments. ©2002 American Institute of Physics

Infrared thermography of sorptive heating of thin porous media:experiments and continuum simulations

\u3cp\u3eWe have studied the imbibition of water from a stationary nozzle into thin, moving porous media that are suspended in air, as well as the accompanying evaporation and condensation processes. Due to sorptive heating and the latent heat associated with the phase change processes, the temperature of the porous medium becomes non-uniform. We have measured the temperature distributions using infrared thermography as a function of substrate speed. Moreover, we developed a numerical model coupling Darcy flow and heat transfer in the thin porous medium with gas flow, heat and water vapor transport in the surrounding gas phase. The numerical simulations reproduce the measurements very well and point at an intricate buoyancy-induced gas-phase convection pattern.\u3c/p\u3

Deformation and dewetting of thin liquid films induced by moving gas jets

We study the deformation of thin liquid films subjected to impinging air-jets that are moving with respect to the substrate. The height profile and shape of the deformed liquid film is evaluated experimentally and numerically for different jet Reynolds numbers and translation speeds, for different liquids and substrate materials. Experiments and numerical results are in good agreement. On partially wetting substrates film rupture occurs. We imaged the appearance of dry spots and emergence of droplet patterns by high-speed, dual-wavelength interference microscopy. We systematically evaluated the resulting average droplet size and droplet density as a function of the experimental conditions. We show that within experimental accuracy the distribution of dry spots is dependent only on the residual film thickness and is not directly influenced by the shear stress and pressure gradients of the air-jet, nor by the speed of the substrate

The optimal stack spacing for thermoacoustic refrigeration

The characteristic pore dimension in the stack is an important parameter in the design of thermoacoustic refrigerators. A quantitative experimental investigation into the effect of the pore dimensions on the performance of thermoacoustic devices is reported. Parallel-plate stacks with a plate spacing varying between 0.15 and 0.7 mm are manufactured and measured. The performance measurements show that a plate spacing in the stack of 0.25 mm (2.5k) is optimum for the cooling power. A spacing of 0.4 mm (4k) leads to the lowest temperature. The optimum spacing for the performance is about 0.3 mm (3k). It is concluded that a plate spacing in the stack of about three times the penetration depth should be optimal (3k) for thermoacoustic refrigeration