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280 || AWSAR Awarded Popular Science Stories - 2019
surfacequicklyhencereducefluctuationswith better heat transfer efficiency. So, keeping this concept in mind we have conducted experiments and studied the performance of two new innovative designs of fourteen parallel microchannel heat exchangers.
In the first design, we have combined two techniques i.e. fabricate inlet restrictor at the inlet of each microchannel and grown nanostructure by the new and easy method of chemical oxidation which converts plain copper channel surface into grass-like copper oxide superhydrophilic nanostructured surface of microchannel in 10 minutess of reaction time. The inlet restrictors are nozzle type short length passage having small cross- section area as compared to the cross-section area of the microchannel. This increases the resistance to the flow of vapor in the upstream direction, however, increases pressure drop across it. The fluctuations are quantified by the standard deviation of
fluctuating data.
It is found that for low
flow rate the inlet restrictor is
more effective at low heat flux
as compared to high heat flux
in terms of reducing surface
temperature. It also reduces
the fluctuations in temperature,
pressure and mass flux during
the presence of flow pattern
fluctuations and annular flow
pattern inside the channel.
At a high flow rate, the inlet
velocity of fluid inside inlet
restrictor constriction increases
(as in a nozzle) which flushes vapor and inhibits upstream movement of vapor hence reduce fluctuations which are also observed through the videos captured using high speed camera. The nanostructured surface observed to be more effective at high heat flux where its superhydrophilic nature of surface sustains
thin liquid film at channel wall during the annular flow pattern. The heat transfer during the presence of an annular flow pattern is more effective because of the presence of thin liquid film present at the wall, therefore, it is advantageous to operate microchannel heat sink maintaining stable annular flow pattern at high heat dissipation range. But danger lies in drying out of this thin liquid film at high heat flux which can cause a sudden rise in surface temperature eventually cause the failure of the device. This nanostructure proves to be efficient in the wetting wall of the channel during high heat fluxes as compared to plain wall microchannel.
By combining these two different techniques we can tap the advantage of both the technique. This combination is proved to be efficient (lesser surface temperature) at low as well as high heat fluxes with lesser fluctuations in temperature pressure and mass flux. This
provides a more stable annular regime as compared to plain microchannel which increases the heat dissipating capacity of the heat exchanger. During the flow regime cycle also the low frequency and high amplitude fluctuations are transformed into high frequency and low amplitude fluctuations by virtue of inlet restrictor and nanostructure. (The observation of this research is under review in “International Journal of Thermal Science”)
Motivated from previous results, it has been seen that if the vapor generated in the channel can be efficiently flush away to outlet plenum then the efficiency of the heat exchanger will improve. Also, the design of other components affects the working of a microchannel. Keeping this in mind we fabricated and started doing experiments on
   . At high heat flux in microchannel as vapor bubble generates, the vapor size increases and quickly confines between the small microchannel wall (width-wise) and then
it starts expanding in both upstream and downstream direction along the length.
  









































































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