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ABSTRACT
In this study, saturated flow boiling characteristics of deionized water in single rectangular minichannels are investigated experimentally. A special attention is paid to the effect of aspect ratio (channel width to depth, Wch/Hch) on the heat transfer and total pressure drop. Experiments are conducted for various values of the mass flux and the wall heat flux. Flow visualization is used as a complementary technique for a deeper physical understanding of flow phenomena. The results show that the channel aspect ratio has a significant effect on both the local two-phase heat transfer coefficient and the total pressure drop. In general manner, the aspect ratio of 1 presents the highest heat transfer coefficients, while the aspect ratio of 0.25 demonstrates the lowest ones. On the other hand, the lowest values of the pressure drop are obtained at the extreme values of the aspect ratio (0.25 and 4).
Nomenclature
| = | cross-sectional area (m2) | |
| = | base area of heat sink (m2) | |
| = | total heated area of minichannels (m2) | |
| = | aspect ratio (channel width to depth) | |
| = | specific heat (kJ kg−1 K−1) | |
| = | diameter of circular duct (m) | |
| Dh | = | hydraulic diameter (m) |
| = | Fanning friction factor | |
| = | mass flux (kg m−2 s−1) | |
| = | Graetz number | |
| = | heat transfer coefficient (kW m−2 K) | |
| H | = | height (m) |
| = | latent heat of vaporization (kJ kg−1) | |
| = | incremental pressure drop number | |
| = | thermal conductivity (W m−1 K−1) | |
| = | length of minichannel (m) | |
| = | dimensionless duct length | |
| = | distance between the bottom wall surface of the channel and base of the channel piece (m) | |
| = | distance between thermocouple and top of the copper block (m) | |
| = | mass flow rate (kg s−1) | |
| = | Nusselt number, | |
| = | Prandtl number | |
| = | Reynolds number, | |
| = | thermal contact resistance (m2 K W−1) | |
| = | effective heat flux (kW m−2) | |
| = | wall heat flux (kW m−2) | |
| = | thermal power applied by heaters | |
| = | heat loss (kW) | |
| = | temperature (K) | |
| = | inlet temperature (shallow plenum) (K) | |
| = | outlet temperature (shallow plenum) (K) | |
| = | saturation temperature (K) | |
| = | specific volume (m3 kg−1) | |
| = | volumetric flow rate (m3 s−1) | |
| W | = | width (m) |
| = | total width of the heat sink (m) | |
| = | local vapor quality | |
| = | exit vapor quality | |
| = | dimensionless length |
Greek symbols
| = | channel depth to channel width or width to depth ratio, | |
| = | pressure drop (kPa) | |
| = | contraction pressure loss (deep-shallow plenums) (Pa) | |
| = | contraction pressure loss (shallow plenum-minichannel) (Pa) | |
| = | expansion pressure recovery (shallow-deep plenums) (Pa) | |
| = | expansion pressure recovery (minichannel-shallow plenum) (Pa) | |
| = | difference between thermocouple and ambient temperatures | |
| = | logarithmic mean temperature difference (K) | |
| = | wall super heat temperature (K) | |
| = | density (kg m−3) | |
| = | dynamic viscosity (N s m−2) |
Subscripts
| = | based on square root of flow area | |
| b | = | bulk |
| c | = | copper |
| ch | = | channel |
| d | = | developing |
| f | = | fluid |
| fd | = | fully developed |
| L | = | liquid |
| m | = | mean |
| sp | = | single phase |
| tot | = | total |
| tp | = | two phase |
| w | = | wall |