The present study is a step in developing efficient helical coil heat exchangers for small scale applications. However much research is required to cover the following aspects:
• It is desirable to experimentally test more coil geometry for accurately defining
the optimum designs. Additionally, more accurate prediction methods could be developed using the generated data by applying different test conditions.
• It is desirable to experimentally test more fluids in helical coils for better
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some of the current refrigerant will be banned such as R134a, R22. Some
refrigerant such as Propane (R290) and CO2 (R744) are currently under
extensive research in flow boiling through straight tubes due to their negligible global warming potential. Therefore, testing these environmental friendly refrigerants is recommended.
• The effects of helical cross-sections such as elliptical, square or rectangular on
the heat transfer augmentation need to be investigated. Researchers reported that flow boiling in straight rectangular channels is better than circular ones due to the hold up of liquid at the corners of the channel and thinning the liquid film causing better heat transfer coefficient (Thome, 2004). Such effects should be considered for designing efficient helical coil evaporators.
• The thermal model of miniature cooling system was performed incorporating the
empirical correlation developed using the dimensional analysis. As the artificial neural network method predicted the boiling heat transfer coefficient more accurately, it is recommended to incorporate the ANN method in the thermal model to produce more accurate prediction of the system performance with the various helical coil evaporators.
• Different nanoadditive materials such as Cu, CNT, CuO need to be investigated
to assess the effect of nanoparticle material on improving heat transfer. It is desirable to engineer the nanoparticles material that could produce high thermal performance with insignificant pressure drop.
• Comparing between different nanofluids modelling approaches such as mixture
model, particle dispersion model, Eulerian-Eulerian model and single phase approach is also needed. Although multiphase modelling approaches requires much computational effort, the multiphase modelling approaches does not
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requires the measurement of effective thermophysical properties of the nanofluids. This will be beneficial for investigating the new particle materials by specifying more general modelling approach.
• The effect of nanoadditives on the flow boiling process. The research of
nanoparticles on the flow boiling process is in its initial stage. More experimental and numerical investigation is required for understanding the boiling process using nanoadditives as some researchers proved it improves the critical heat flux of the heated surface.
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9
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