Executive Summary : | The AGNI missiles face loss of communication during reentry due to plasma formation on the outside of the missile. One of the possible ways to overcome this reentry blackout is to use higher frequencies and power of communication signals, which results in increased heating of the Monolithic Microwave Integrated Circuits (MMIC). The space inside missiles is premium and there are no external fluids available for cooling other than the ambient air. In view of these issues, DARPA has developed high heat flux flat heat pipes, which have effective thermal conductivity even upto 50 times that of copper, the technology of which is highly restricted. In India, there are currently no companies that have the know-how to develop flat, compact, high heat flux heat pipes. Considering that such unavailability would be a restricting factor for national security, it is proposed to develop high heat flux flat heat pipes for use with high power MMICs. The proposal hence aims to develop a low-cost, flat, heat spreader to be used with MMICs that would not need external fluid supply, will be compact and will remove high heat fluxes. The initial target heat flux would be a minimum of 20W from 5mm x 5mm, with the thickness of the device being a few mm and length a few cm so that the surface of the chip could be maintained below 150 C with the ambient air at 70 C. From a literature survey, a lumped modelling of the flat heat pipe would be done for the expected heat loads so as to obtain the design parameters of the heat pipe, namely, the mean pore size and porosity of the wick, the size of the heat pipe and the type and the amount of the fluid to be used. Once the overall design parameters are obtained, CFD simulations of the boiling, condensation and percolation in various porous media will be conducted to optimize the pore geometry. Investigators expect a directed porous media with continuous pore size variation, as would be obtained by a collection of micro tubes would help in fast transport of gases as well as fast film spreading through the interstices (see enclosed schematic). Such a structure is expected to work well for a range of heat fluxes, which can be tuned by tuning the tube radii. The wick will then be fabricated, tested to find out the heat fluxes that it can transport and will then be encased, the case evacuated and the working fluid filled to fabricate the prototype. The prototype will then be tested using the MMIC of AMPL, Hyderabad and later modified if needed. The technology will then be transferred to AMPL. The deliverables are expected to be (a) A design methodology and software for development of flat heat pipes for various heat fluxes. (b) A prototype flat heat pipe that can remove at least 20 W from 5mm x 5mm and maintain the surface temperature at less than 150 C. (c) Basic knowledge generation of boiling, condensation and percolation in porous media. |