WP2: Cooling liquids with optimal physical and transport properties

This work package includes crucial activities of the project in terms of methodologies and materials development.

  1. In a prototype device, sample linear vortex generators (LVG), for example, winglets, corrugated/staggered channels or pin-fins, will be identified and fabricated. The most attractive design will be designated through micro-PIV measurements.
  2. CFD models comprising the Navier-Stoke
  3. s and energy equations will be solved in an Eulerian framework for the liquid matrix, which will include sub-grid scale models for the nanoparticle motion. Source terms will be added in the momentum equations of the liquid/solid (pseudo-liquid) phases in the Eulerian-Eulerian approach and in the force balance of the Lagrangian formulation for particle motion, corresponding to drag, lift, virtual mass, wall lubrication and particle-particle interaction forces in the macroscale, as well as thermophoretic and Brownian forces at the nanoscale. The separate velocity fields for the two phases will allow the occurrence of drift velocities, which can contribute to micro-convection mechanisms.
  4. Family of Phan-Thien-Tanner models will be employed to describe the contribution of viscoelastic stresses in the momentum equation, where also a term will be incorporated to account for the effect of the liquid second normal-stress difference. Shear thinning effects will be described by Carreau-Yasuda type of modelling approaches. The numerical approaches will be validated in distinct simplified cases of vortical motion (Rankine, Goertler and Bénard vortices/cells).
  5. The developed CFD methodology with the sub-grid-scale (SGS) models will be utilized in engineering-scale geometries corresponding to the geometrical configurations developed in WP1 for particle image velocimetry and heat-flux experiments.
  6. The previously synthesized fluids will be measured on the optically accessible parts by means of PIV experiments. The data is expected to estimate the vortex structures in the flow. These measurements should illustrate the effect of different viscoelastic additives on the magnitude and temporal evolution of coherent vortices emanating in the benchmark geometries. A matrix of tests will be conducted for Reynolds numbers in the range 500-2000. The subsequent parametric analysis will cover a range of nanoparticle sizes, materials concentrations, viscosity to shear rate relation, polymer viscosity and relaxation time, as well as normal stress differences coefficients to identify optimal values.
  7. The heat-flux experiments will be performed that will refer to macroscopic heat-transfer coefficient measurements. The flow to be investigated will be set to the same Reynolds numbers as for the PIV measurements and for values of the volumetric heat-rate up to 1000 kW/m3. The local temperature field and heat flux in various locations will be measured by a grid of pin-sized thermocouples and ATL sensors housed within the metallic body of the test pieces, in order to estimate the local heat transfer coefficients and the degree of temperature uniformity. The local distribution of the heat transfer coefficient is closely correlated to the presence of vortical motion and local disruptions of the thermal boundary layer.
  8. The previously mentioned numerical models will be validated by the experimental data, which is obtained on the benchmark configurations.
  9. By the parametric analysis from experimental studies with input from the MD simulations, the optimal rheological properties of the non-Newtonian medium (viscosity, extent of shear-thinning, elasticity, normal stress differences) and thermo-physical properties (density, specific heat and thermal conductivity), as well as nanoparticle concentration and shape will be designated. Based on that, new blends of viscoelastic-based nanofluids will be made. The tailor-made liquids will be capable of enhancing heat transfer both due to the influence of their properties but also through sustaining coherent vortical motion, a passive heat-transfer enhancement mechanism [1], and minimizing pressure losses. Small Angle Neutron Scattering (SANS) measurements will be conducted at ISIS Neutron and Muon Source to elucidate the viscoelastic polymer-chain topology and dynamics (elongation and coiling).

[1] Karathanassis, I. K. et al. Int. J. Heat Mass Transf. 84, (2015).