Active oxygen transport in tissue by interstitial flow

Oxygen (O2) transport through diffusion from capillary to tissue has long been established by Krogh. However, the interstitial fluid in the interspace between tissue and capillary has a high Prandtl number around 103 and hence its convective mass transport is more efficient than its diffusive transport. The interstitial flow drained by the initial lymphatics contributes to the convective transport of O2 through tissue, which can be modeled as aligned blood capillaries in parallel and the initial lymphatics. It is found that both the O2 concentration distribution and the total O2 flux are sensitive to the flow rate of interstitial fluid. The convection contribution has been evaluated based on the Peclet number, feature flow rate, and convection-diffusion boundary. At the same interstitial flow rate, convection delivers more O2 to type I muscle fibers with a higher concentration of myoglobin than to type IIX muscle fibers. Even with a small external force, tissue with a higher specific hydraulic conductance (permeability) has a larger interstitial flow rate and a higher O2 transport rate than those in healthy tissue. Hence, the overall O2 transport from capillary to tissue includes two components, i.e., active convection transport by interstitial flow due to pressure gradient and passive diffusion transport due to concentration gradient. The active convective O2 transport is crucial for the recovery of damaged tissue where the contribution from passive diffusion transport is constrained by regulation of capillary opening. The convection facilitated O2 transport can be the basis for cell differentiation, morphogenesis, and therapeutic effects of massage and acupuncture.Key pointsInterstitial flow plays a key role in active O2 transport in tissue due to its high Prandtl number v/D~103;O2 transport in tissue is balanced by both active convection and passive diffusion transport.Interstitial flow in form of active convective transport can pump more than hundred times of O2 into tissue than those by passive diffusion transport due to the concentration gradient.Active convection transport can be triggered by external pressure, which is crucial for damage tissue recovery.

Effects of Reaction Mechanisms and Differential Diffusion in Oxy-Fuel Combustion Including Liquid Water Dilution

The influence of chemistry and differential diffusion transport modeling on methane oxy-fuel combustion is analyzed considering different diluent characteristics. Analyses are conducted in terms of numerical simulations using a detailed description of the chemistry. Herein, different reaction mechanisms are employed to represent the combustion of methane. Simulations were performed with the computational fluid dynamics (CFD) code CHEM1D following different numerical setups, freely propagating flame, counter flow flame, and propagating flame in droplet mist reactors. The employed method is validated against experimental data and simulation results available in the literature. While the counter-flow flame reactor is exclusively used in the validation stage, different scenarios have been established for propagating flame simulations, as in single- or two-phase flow configuration. These comprehend variations in diluent compositions, reaction mechanisms, and different models to account for diffusion transport. Conducted investigations show that the choice for a specific reaction mechanism can interfere with computed flame speed values, which may agree or deviate from experimental observations. The achieved outcomes from these investigations indicate that the so-called GRI 3.0 mechanism is the best option for general application purposes, as a good balance is found between accuracy and computational efforts. However, in cases where more detailed information and accuracy are required, the CRECK C1-C3 mechanism demonstrated to be the best choice from the evaluated mechanisms. Additionally, the results clearly indicate that commonly applied simplifications to general flame modeling as the unitary Lewis number and mixture averaged approach strongly interfere with the computation of flame propagation speed values for single- and two-phase flows. While the application of unitary Lewis number approach is limited to certain conditions, the mixture averaged approach demonstrated a good agreement with the complex model for flame speed computations in the various tested scenarios. Such an outcome is not limited to oxy-fuel applications, but are straightly extensible to oxy-steam and air-blown combustion.