RICERCA ITALIANA     

Research program

Innovative techniques for the enhancement of forced convection

University Co-ordinator

Università Politecnica delle MARCHE - ENERGETICA - ()

Research Unit Leader

Gianni Cesini

Description

This research is aimed to increase the convective heat transfer coefficient of systems operating with liquid in forced laminar flows. The basic idea is to locally promote the turbulence and so to increase the heat transfer; a local increase may be obtained by forcing a turbulent boundary layer only in the areas of interest and maintaining a laminar boundary layer in the rest of the circuit. The presence of a laminar boundary layer may reduce the overall pressure drops.
The cooling of electronic components is one of the possible application fields for this research: in this applicative sector there are steady and/or unsteady high specific heat fluxes to dissipate and frequently there are narrow spaces available for the thermal control systems. In these cases a slow circulating hydraulic circuit may be used to supply the cooling liquid in all the electronic apparatus for the thermal dissipation. This circuit should have a laminar flow for all the length except for the areas where are placed the electronic components to be cooled. A previous PRIN was aimed to analyze the laminar boundary layer destabilization by means of a mechanical vibration localized in a confined area of the flow domain. A miniaturized piezoelectric actuator (MEMS) glued on a elastic membrane was used with good results. The outcomes becoming from this previous research may be used as starting point for this PRIN research; the same technique could be used also for hydraulic closed loops upon a careful choosing of materials and mechanical actuation systems.
The turbulence boundary layer may be used to increase the heat transfer coefficient of the flat channel walls or also to increase the overall heat transfer coefficient of finned heat sinks placed along the channel. In this second case the research involves also the sector of the heat exchangers.
So the research reveals two main aspects: a basic research aimed to the phenomenon understanding and an applied research for technological applications.
As far as the phenomenological aspect is concerned, measurements of temperature, velocity and heat transfer are needed [a]. They are challenging due to complex materials, configurations, and non-isothermal flows in electronic systems. For instance, accurate surface temperature measurements require the development of adequate models of two-layer transient heat conduction (thermocouple – solid) with contact resistance [b] to reduce the so-called ‘installation errors’.
Heat sinks of short pin fins could be an interesting practical application to use the proposed method; the greater heat transfer coefficient of the turbulent flow may induce an higher thermal exchange for the first heat sink rows, that some time are the only present due to the reduced available spaces.
The calculation of the temperature distribution in these fins and, hence, of the heat flux exchanged needs to outline a 2D model, whose solution may be approached through analytically and numerically based methods [b].
This research unit should realize an hydraulic circuit having a modular test channel; this will have to contain both the system able to promote the turbulence and the finned heat sink to test. This apparatus will be designed basing on the knowing obtained from the PRIN 2005 results. Once the test channel will be realized, the know how acquired on the boundary layer destabilization in external flow should be redirected for this sector related to the internal flow. The electromechanical systems more useful to promote the boundary layer disturbances should be analyzed and choose. The frequencies and the amplitudes for the disturbances should be characterized with respect to the boundary layer receptivity. A finned heat sink to be introduced in the test channel will be defined and its thermal behaviour will be monitored by using a not intrusive test technique like the IR thermography.