K. Gasljevic

On two distinct types of drag-reducing fluids, diameter scaling, and turbulent profiles

Two distinct scaling procedures were found to predict the diameter effect for different types of drag-reducing fluids. The first one, which correlates the relative drag reduction (DR) with flow bulk velocity ( V), appears applicable to fluids that comply with the 3-layers velocity profile model. This model has been applied to many polymer solutions; but the drag reduction versus V scaling procedure was successfully tested here for some surfactant solutions as well. This feature, together with our temperature profile measurements, suggest that these surfactant solutions may also show this type of 3-layers velocity profiles (3L-type fluids). The second scaling procedure is based on a correlation of w versus V, which is found to be applicable to some surfactant solutions but appears to be applicable to some polymer solutions as well. The distinction between the two procedures is therefore not simply one between polymer and surfactants. It was also seen that the w versus V correlation applies to fluids which show a stronger diameter effect than those scaling with the other procedure. Moreover, for fluids that scale according to the w versus V procedure, the drag-reducing effects extend throughout the whole pipe cross section even at conditions close to the onset of drag reduction, in contrast to the behavior of 3L fluids. This was shown by our measurements of temperature profiles which exhibit a fan-type pattern for the w versus V fluids (F-type), unlike the 3-layers profile for the fluids well correlated by drag reduction versus V. Finally, mechanically-degraded polymer solutions appeared to behave in a manner intermediate between the 3L and F fluids. Furthermore, we also showed that a given fluid in a given pipe may transition from a Type A drag reduction at low Reynolds number to a Type B at high Reynolds number, the two types apparently being more representative of different levels of fluid/flow interactions than of fundamentally different phenomena of drag reduction. After transition to the non-asymptotic Type B regime, our results suggest that, without degradation, the friction becomes independent of pipe diameter and that the drag reduction level becomes also approximately independent of the Reynolds number, in a strong analogy to Newtonian flow.

Asymptotes of maximum friction and heat transfer reductions for drag-reducing surfactant solutions

Abstract A new maximum drag reduction asymptote (MDRA) for surfactant solutions is presented. Various concentrations including cationic and non-ionic surfactant solutions were used to experimentally determine this asymptote. It is shown that if solvent viscosity is used to compute Reynolds and Prandtl numbers for viscous solutions, it leads to underestimations of the friction coefficient. To avoid uncertainties in the selection of the fluids viscosity, most solutions used were intentionally conditioned so their shear viscosity was water-like in the ranges covered. Using the same solutions, a maximum heat transfer reduction asymptote (MHTRA) was also determined – a correlation that did not exist for surfactants until now. Finally, by using slightly modified definitions to quantify the heat transfer and drag reductions (TRH and TRD), it is possible to express the ratio between the MHTRA and MDRA with a constant value of 1.06, independent of Reynolds number. This relationship can be used as an auxiliary criterion to determine whether or not a solution is asymptotic when there is an uncertainty about the shear viscosity.

Experimental Investigation of Thermal and Hydrodynamic Development Regions for Drag-Reducing Surfactant Solutions

The reductions in friction and heat transfer exhibited by a surfactant solution in the entry region of a circular pipe were measured and analyzed, with special attention paid to the relationship between the local heat transfer and friction. Two entrance configurations were used, a cone contraction and wire mesh plugs used as a device for velocity profile flattening. Both the simultaneous development of temperature and velocity profiles and the development of temperature profile with hydrodynamically predeveloped flow were studied. Interestingly, the local heat transfer measurements for surfactant solutions matched very well a correlation developed for polymer solutions, but for surfactants the development of the heat transfer and velocity profiles appear coupled, unlike what is thought to happen for polymer solutions. The development patterns appear to be independent of velocity and entrance type at low disturbance levels. At high disturbance levels, however, some striking changes in the fluid itself, likely due to temporary micellar structure degradation by high local shear stress in the inlet region, were observed as well, and quantified.

Coupling Between Heat and Momentum Transfer Mechanisms for Drag-Reducing Polymer and Surfactant Solutions

Drag-reducing solutions exhibit simultaneous friction and heat transfer reductions, yet it has been widely believed that there is no direct coupling between the two. In this work, we have conducted a study to re-examine this issue, using measurements of friction and heat transfer over a wide range of flow conditions from onset to asymptotic, various pipe diameters, and several polymer and surfactant solutions. Contrary to some earlier suggestions, out tests show that no decoupling of the momentum and heat transfer mechanisms was seen at the onset of drag reduction, nor upon departure from the asymptotes, but rather that the friction and heat transfer reductions change simultaneously in those regions. For asymptotic surfactant and polymer solutions, the ratio of heat transfer and drag reductions was seen to be constant over a large range of Reynolds numbers, if modified definitions of the reduction parameters are used. In the nonasymptotic region, however, the ratio of heat transfer to drag reductions is higher and is a function of the reduction level, but is approximately the same for polymer and surfactant solutions

An improved diameter scaling correlation for turbulent flow of drag-reducing polymer solutions

The friction coefficient was measured for developed flow of drag-reducing polymer solutions in tubes of 2, 5, 10, 20 and 52 mm i.d. Our results were processed along with other authors’ data in terms of different parameters in order to investigate the possibility of developing a simple empirical method for the prediction of the pipe diameter effect on friction. We found that the drag-reduction coefficient (DR), if expressed as a function of the fluid bulk velocity (V), becomes independent of the tube diameter in the subcritical region (i.e. without fluid degradation), with the deviations being smaller than about 5% for all the diameters and velocities covered. This correlation proved to be not only better than similar procedures based on friction velocity, but also more convenient and physically more meaningful. It was also found that the logarithmic layer shift in the 3layers velocity profile is also better correlated with the bulk velocity than with the friction velocity. Finally, existing models for drag-reduction involving non-dimensional correlations between the integral flow parameters and the fluid properties were also reevaluated in light of these findings. # 1999 Elsevier Science B.V. All rights reserved.

Measurement of temperature profiles in turbulent pipe flow of polymer and surfactant drag-reducing solutions

A device was built to measure temperature profiles of turbulent pipe flows of various drag-reducing fluids. It is easy to use and reliable. We measured temperature profiles over a range of conditions leading to accurate measurements down to y+≈10, for tests carried over Reynolds numbers (Re) between 10 000 and 90 000. The effects of high heat fluxes and buoyancy, in particular, were quantified to ascertain the parameter range for accurate measurements. Temperature profiles measured for type-A polymer solution and for cationic surfactant solutions allowed us to see strong similarity between velocity and temperature profiles for drag-reducing surfactant solutions. A comparison between the slopes of the thermal and velocity buffer layers resulted in calculated turbulent Prandtl numbers between 6 and 9 for those drag-reducing solutions. We also used this tool to investigate drag reduction for a nonionic surfactant solution, which showed a significantly different fan-type profile, and also for a type-B drag-re...

Buoyancy effects on heat transfer and temperature profiles in horizontal pipe flow of drag-reducing fluids

Abstract We have studied the extent to which buoyancy effects in horizontal pipe flows of drag-reducing viscoelastic fluids cause distortions to both laminar and turbulent temperature profiles. In the case of laminar flows, these distortions may lead to variations in Nusselt numbers that are larger than those seen for Newtonian pipe flows under similar conditions. In the case of turbulent drag-reducing flows, the effects of buoyancy can also be large and may in turn result in large errors in estimated Nusselt numbers if not properly accounted for. These errors are quantified and recommendations are made on how to reduce them.

Increased production of extracellular polysaccharide by Porphyridium cruentum immobilized in foam sheets

Large‐size plate bioreactors were used to compare the production of extracellular polysaccharide by the red microalga Porphyridium cruentum when grown in suspension and in a foam sheet. A well‐defined illuminated area and unidirectional light propagation allowed us to generate information that is better quantified when expressed in terms of illuminated area. This is essential for meaningful comparison of data, especially considering that for a well‐designed and managed bioreactor, the culture production rates are believed to be light limited. At the same level of illumination, the culture immobilized in foam showed double the production rate of extracellular polysaccharide compared with the culture in suspension. The saturation level of biomass density per unit of illuminated area was eight times higher for the immobilized culture compared with the culture in suspension. Despite the increased biomass density for the immobilized culture, an increase in the light level above the optimum found for the culture in suspension reduced the extracellular polysaccharide production, suggesting that the photoinhibition light level was surpassed.