Mini-channels application in laser mirrors and chips cooling systems Yu.I. Shanin


Аuthors

Shanin Y. I.

NII NPO "LUCH", Podolsk, Russia

e-mail: ShaninYuI@sialuch.ru

Abstract

In practice, the large heat flows withdrawal in a restricted volume by heat exchangers with mini- and micro-channels is required. According to the generally accepted gradation, the chan- nels of real cooling systems of laser mirrors relate to mini-channels, their hydraulic diameter lies within the range of 0.2 < dg <3 mm. In the existing cooling systems of laser mirrors all three modes of the coolant flow, such as laminar, transient, and developed turbulent, can be realized. In the chips’ cooling systems the laminar and transitional flow modes are generally implemented. Numerous studies have established that for laminar flow in smooth mini-channels hydraulic re- sistance submits to the classical friction law — the Poiseuille’s law. At the same time, an increase in resistance and an earlier transition to a turbulent flow were observed with a relative roughness of the channel walls exceeding 3–4%. Heat transfer in a minichannel with one-sided heat flow is characterized by the non-uniform thermal boundary conditions over the channel perimeter. At pre- sent, there is no consistent uniform definition of the heat-transfer factor in mini- and micro- channel structures, and, thus, a wide scatter of experimental data on heat transfer is observed. The main intent of the laser mirror cooling systems is to minimize the thermal displacements of the opti-



cal surface and the heat sink to the base of the mirror. The assign of cooling systems of chips con- sists in eliminating the body parts overheating by ensuring efficient heat exchange with the entire finned heat exchange surface while minimizing the coolant rate and energy for coolant pumping. The article presents a solution for a one-dimensional temperature field in a multilayer cooling system, and performs its analysis. It analyses the main difference when cooling laser mirrors and chips, consisting in the need of a heat sink reducing into the mirror base, and achieving identical temperature over the channel perimeter for the cooling systems of chips. Simple equa- tions for the effectiveness evaluation of multilayer cooling systems of chips were obtained. The previously obtained experimental data on hydraulic resistance and heat transfer were generali- zed by power-law dependencies with account for the surface roughness of the channels, and their comparison with the known relations was performed. There is a strong dependence of hy- draulic resistance on roughness for the laminar fluid flow mode. A simple linear approximation of the dependence of coefficient of hydraulic resistance on the relative roughness of the channel walls was obtained. For all studied sections, the earlier transition to a turbulent flow was observed (the Reynolds number of the transition was Re = 500–2000). In the area of the developed turbulent flow (Re > 4·103), the results on the hydraulic resistance were generalized employing well- known relations for rough pipes. The heat transfer of the studied rough mini-channels is more conservative to the roughness of the walls and, as comparison with the known relations re- vealed, differs insignificantly from the heat transfer in the channels with smooth walls. The ob- tained experimental data adequately fit into the proposed mathematical model, with which de- tailed analysis of temperature field performing and achievable intensification of heat exchange estimating in mini-channels with rough walls is possible.

Keywords:

laser mirror, chip, cooling system, mini- and micro-channel, roughness, hydrau- lic resistance, heat transfer.

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