Thermal and hydraulic characteristics of cooled deformable laser mirrors


Аuthors

Shanin Y. I.

NII NPO "LUCH", Podolsk, Russia

e-mail: syi@luch.podolsk.ru

Abstract

The article designates some hydrodynamics and heat transfer specifics with reference to the flow in cooling systems of active (deformable) laser mirrors optical blocks:

  • Three-dimensional flow in the channels of complex shape;

  • Cylindricity and non-uniformity of flow with radial distribution of the heat carrier in round mirrors; 

  • The need to increase the reduced heat transfer coefficient in the area of moderate Reynolds numbers (Re≤2∙104);

  • The need to develop the heat-exchanging surface with simultaneous limiting of the heat, in‑ coming to the mirror base.

The experimental data on resistance and heat transfer were obtained with the cooling systems models for laminar, transient and turbulent regimes of water flow at room temperature. The studies were performed for the two cooling systems, the channel system with a discontinuous wall (cuts) and the wafer cooling system. Both systems are most suitable for cooling the optical blocks of the active mirror. In the cooling systems with a discontinuous wall (totally 6 models were studied) the step and a slope angle of the additional transverse channels were being varied. In the wafer cooling systems models (totally 24 models were studied) the angle of the relative crossing of channels and the attack angle on the wafer structure were varied, following from the corridor to the chess scheme of the low. During the experiment the pressure drop, flow rate and temperature fields (the one-dimensional heat flow in the model was provided with a “thermal wedge”) over the thickness of the cooling system model were measured. The result of data processing allowed obtain hydraulic resistance coefficient and reduced coefficient of heat transfer. The last one allowed the surface heat transfer coefficient restoring. The obtained experimental data was generalized by criterion equations by the method of least squares. It was found that the channel cooling system with discontinuous wall was energy favorable (the heat transfer increase exceeds the resistance growth) in the Reynolds number range of Re = (1–4) ∙ 103. A common feature for wafer structures is the in‑ crease in resistance and heat transfer with the increase in the angle of attack. While transition from a flow in corridor structure to a flow in chess structure the prevailing growth of the hydraulic resistance (at 14–16 times) compared to the heat transfer (by 1.6–2.2 times) is characteristic. While comparative analysis of the cooling system impact on the temperature field along the thickness of the optical mirror block, three components were accounted for in the thermal displacement of the optical surface of mirror. They are thermal expansion, shear thermal deformation, and bending thermal deformation of the optical block. A comparative analysis of the cooling systems efficiency revealed the preferable material for the optical mirror block design (the materials are ranged in the following sequence: copper, silicon carbide, molybdenum, tungsten).It was found that the sur‑ face heat transfer in the wafer and channel cooling systems with cuts is higher than that in the simple channel systems.

Keywords:

laser mirror, deformable mirror, cooling system, hydraulic resistance, heat transfer

References

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