Laboratory convection experiments with internal, non-contact, microwave generated heating, applied to Earth’s mantle dynamics – TERRA-MWH
The fluid is heated from within by a microwave device that delivers a uniform volumetric heating from 10 to 70 kW/m3; the upper boundary of the fluid is kept at constant temperature, whereas the lower boundary is adiabatic. A laser sheet illuminates a cross-section of the tank and a CCD camera registers the scattered light. The laser sheet can scan half of the tank length. The use of thermochromic liquid crystals (TLC) allows temperature mapping on a 2D-plane in the fluid flow without perturbing it .The experimental cell was seeded with several types of TLCs. Each type of TLC produces one bright line, which represents an isotherm. The fluid was also seeded with small hollow glass spheres that can be considered as passive tracers.
The velocity field was calculated through cross-correlations between images, using PIV package DaVis from LaVision. The same camera was used for both the temperature and velocity fields. The thermal structure and the velocity field enable us to characterize the geometry of the convective regime as well as its bulk thermal evolution.
Numerical simulations, conducted using Stag-3D in 3D Cartesian geometry, reproduce the experimental setup (i.e., boundary conditions, box aspect ratio, temperature dependence of physical parameters and internal heating rate).
1) Microwave induced homogeneous heating
We have designed and tested a micro-wave field homogeniser that insures a uniform internal heating (for details see Proceeding 2)
2) Optimization and power stability of the prototype
After the first preliminary convection experiments we realised that the heating deposited power derived in time for time lapses larger that 30-45minutes. The magnetron power generator was re-designed, and the microwave adapting circuit was optimised in order to reduce the reflected wave. The entire microwave system was temperature controlled by a thermostatic bath. The stability was tested and validated over 6 hours experiment time. (for details see Proceeding 3)
3) Agreement between experimental and numerical results
The modified prototype (see results 1 and 2) was used in a systematic study of internal heating convection function of fluid properties and heating power. We compared the experimental and numerical results obtained in the same conditions: (i.e., boundary conditions, box aspect ratio, temperature dependence of physical parameters and internal heating rate). The attached figure shows such an example. This is the first time that such a comparison is possible. The agreement between the experimental and numerical results for both thermal structure and velocity field validates our approach of microwave-induce heating convection experiments for mantle convection study. (for details see Proceeding 4)
The successful comparison between experimental and numerical results shows that we have been able to overcome challenging technological issues inherent with the generation of a uniformly internally heated fluid and with the ability to accurately quantify the temperature and velocity fields during the experiment.
We consider this as a first and necessary step before introducing further complexities, such as bottom heating (to mimic heat flux from the core into the mantle) and compositional heterogeneities (to mimic portions of the mantle variably enriched/depleted in heat producing elements).
Thermal evolution of planets is modeled using scaling laws relating the surface heat flux to the Rayleigh number.These scaling laws, determined numerically, show a large variability. Here we detain for the first time a combination of experimental and numerical methods. Our approach will help us to better constrain the thermal history of the Earth and related planets.
Proceedings
1. E. Surducan, V. Surducan, C. Neamtu, A. Limare, D. Dabala, “Characterization of the microwaves levels in the proximity of one scientific microwaves power experimental setup for the user biological protection purpose” , the 8 th INTERNATIONAL WORKSHOP OF ELECTROMAGNETIC COMPATIBILITY, Abstracts - CEM 2012, Sibiu, Romania, 2012, pp. 29-30
2. E.Surducan, A.Limare, V.Surducan, C.Neamtu, E. Di Giuseppe, “Microwaves Power Distribution Map Revealed by Liquid Crystals”, International Conference on Electromagnetics in Advanced Applications, Proceedings pag 287-288, ISBN 978-1-4673-5707-4/13, ICEAA-IEEE, APWC-EMS '13, September 9-13, Turin, Italy, 2013
3. E. Surducan, C. Neamtu, V. Surducan, A. Limare and E. Di Giuseppe,” Microwaves Heating in a Specific Experimental Configuration”, Book of Abstracts, p.49, the International Conference Processes in Isotopes and Molecules (PIM ’13), September 25-27, Cluj-Napoca, Romania, 2013
4. A. Limare, E. Surducan , V. Surducan, C. Neamtu , E. di Giuseppe, K. Vilella, C. G. Farnetani, E. Kaminski and C. Jaupart, “Microwave-based laboratory experiments for internally-heated mantle convection” Book of Abstracts, p8, the International Conference Processes in Isotopes and Molecules (PIM ’13), September 25-27, Cluj-Napoca, Romania, 2013
Articles
1. V. Surducan, E. Surducan, «Low-Cost Microwave Power Generator for Scientific and Medical Use [Application Notes]« Microwave Magazine, IEEE , vol.14, no.4, pp.124,130, June 2013 dx.doi.org/10.1109/MMM.2013.2248651
2. E. Kaminski and M. Javoy, « A two-stage scenario for the formation of the Earth's mantle and core» Earth Planet. Sci. Lett., 365, 97-107, doi:10.1016/j.epsl.2013.01.025, 2013.
Patent application
1. V. Surducan, E. Surducan, A. Limare, ”Stabilization and control block for the current filament supply of the magnetrons”, Patent pending (RO) A00573/31.07.2013
Performing laboratory convection experiments with internal, non-contact, microwave generated heating will help us to better understand Earth’s convecting mantle.
Solid-state thermal convection of planetary mantles is generated both by volumetric heating and heating from below. Volumetric heating is due to radioactive decay of uranium, thorium and potassium distributed throughout the mantle. In steady–state approximation, secular cooling can also be approximated as a volumetric heat source. For terrestrial planets, cooling and solidification of the iron-rich core is the primary source of heating from below, and the heat flux out of the core, transported by thermal convection, controls the strength of the magnetic field toghether with the solidification rate of the core.
Although internal volumetric radioactive heating represents the major source of heat in the Earth, we still lack a quantitative understanding of its role on the thermal evolution of the Earth. The studies are sparse, and provide only a first order insight on the dynamics of the system, expressed typically as an average cooling rate function of the relative contribution of internal and base heating. Intrinsic chemical heterogeneities in planetary bodies imply large heterogeneities in the distribution of internal sources that shall significantly affect thermal convection, which in turn controls the rate of creation and destruction of chemical heterogeneities in the system. Since the flow of convective planetary mantles is chaotic, it is likely that such a coupling significantly affects the bulk dynamics of the system and yields results unexpected in uniform systems.
Thermal convection in a heterogeneous system heated from within has not been treated yet due to many technical difficulties of the problem. Numerically, an accurate modelling of chemical mixing in chaotic systems is still beyond our reach, especially over the large time scales of geological processes. Experimentally, specific techniques have to be developed to obtain heterogeneous heating in an evolving system. This last point is the main objective of the present project.
Here we propose to perform convection studies in the general case of heating from below and cooling from above, in presence of volumetric heating. We propose several geometries, each one meant to address particular questions relative to Earth and more general to the convection in other planets. The novelty of our approach relies on the fact that the internal heating is produced by microwave (MW) absorption. Microwave heating (MWH) provides an original way of producing non-contact, very localised, or yet extended heat sources by a convenient choice of microwave antenna, excitation sequence and selective absorption of fluids. Therefore, our secondary goal is to design, prototype and test a specific MWH device for these particular laboratory convection experiments. Such device includes a power MW generator, an embedded system able to drive the MW generator, a circuit and an applicator system (antenna) on which the MW radiation is propagating into the probe volume. Due to the complexity of the volumetric heating process, MW radiation power and excitation pattern must be finely tuned. To our knowledge, the advantage offered by MWH to control the deposited heat into a large volume of liquid has never been exploited. Known applications, are mainly related to medical MW diathermia or hyperthermia procedures, where the main goal is rising homogenously the temperature of a tissue to a higher temperature (and maintaining it during the treatment period) while keeping the external boundary of the heated volume to a lower temperature. The essential difference between MW medical applications and the experiments here proposed is that volumetric heating is related to a convecting volume phase incorporated into a larger volume. MW is the most suitable candidate for such a procedure.
Project coordination
Angela Limare (INSTITUT DE PHYSIQUE DU GLOBE DE PARIS) – limare@ipgp.fr
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
Partner
IPGP INSTITUT DE PHYSIQUE DU GLOBE DE PARIS
Help of the ANR 233,844 euros
Beginning and duration of the scientific project:
December 2011
- 36 Months
