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ABSTRACT
In the present study, flow boiling in a single minichannel with a longitudinal step on its each corner is studied experimentally. Bubble/flow dynamics, and thus, underlying physical phenomenon is investigated through high-speed flow visualization experiments. The results are also compared with those of the conventional single rectangular minichannel (plane channel) in terms of heat transfer and total pressure drop. Experiments are performed for different values of the mass flux and wall heat flux at a constant inlet temperature. It is obtained that the stepped channel improves heat transfer performance up to 13.2% compared to the plane channel. During the visualization studies, five flow patterns are observed: bubbly flow, slug flow, churn flow, annular flow, and inverted annular flow.
Nomenclature
| = | cross sectional area (m2) | |
| = | base area of heat sink (m2) | |
| = | total heated area of minichannels (m2) | |
| = | specific heat (kJ kg−1 K−1) | |
| = | mass flux (kg m−2 s−1) | |
| = | heat transfer coefficient (kW m−2 K) | |
| H | = | channel height (m) |
| = | latent heat of vaporization (kJ kg−1) | |
| = | thermal conductivity (W m−1 K−1) | |
| = | length of minichannel (m) | |
| = | distance between the thermocouple and the channel bottom surface (m) | |
| = | mass flow rate (kg s−1) | |
| = | heat flux (kW m−2) | |
| = | thermal power applied by heaters (kW) | |
| = | heat loss (kW) | |
| = | temperature (K) | |
| = | volumetric flow rate (m3 s−1) | |
| W | = | channel width (m) |
| = | total width of the heat sink (m) | |
| = | vapor quality |
Greek symbols
| ∆P | = | 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) | |
| ∆Thl | = | difference between thermocouple and ambient temperature |
| ∆Tsat | = | wall super heat temperature (K) |
| = | density (kg m−3) |
Subscripts
| c | = | copper |
| ch | = | channel |
| e | = | local |
| eo | = | exit |
| eff | = | effective |
| i | = | inlet |
| L | = | liquid |
| o | = | outlet |
| pc | = | plane channel |
| sat | = | saturation |
| sc | = | stepped channel |
| tot | = | total |
| tp | = | two phase |
| w | = | wall |