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ABSTRACT
Novel types of double delta winglets (DDWTs) and double delta winglet with holes (DDWTHs) have been developed. The influence of parameters, including the angle of DDWTs and the distance between DDWTs, on thermal enhancement and flow resistance characteristics in a rectangular channel is examined. The results reveal that the larger angle and increasing distance could make an active role in turbulent heat transfer enhancement, and slight influence on laminar flow. According to the field profile, the cross-flow and encircling flow are generated by DDWTs, which increases the mixing efficiency and reduces the thermal boundary layer thickness as well. Vortex generators, such as delta wings, rectangular wings, DDWTs, and DDWTHs, are compared. The delta wings and rectangular wings perform better heat transfer enhancement and higher resistance, while the DDWTs and DDWTHs have better overall heat transfer performance, especially for large Reynolds numbers.
Nomenclature
| A | = | heat transfer surface area (m−2) |
| a | = | distance from the channel inlet (m) |
| Ap | = | sum area of heat elements |
| cp | = | specific heat (J/(kg · K)) |
| de | = | hydraulic diameter (m) |
| f | = | friction factor |
| g | = | distance between DDWTs |
| H | = | height of the channel (m) |
| hv | = | convective heat transfer coefficient (W/(m2 · K)) |
| j | = | Colburn factor |
| L | = | length of the channel (m) |
| l | = | wing length (m) |
| m | = | mass flow rate (kg/s) |
| Nu | = | Nusselt number |
| Pr | = | Prandtl number |
| Q | = | heat transfer rate (W) |
| Re | = | Reynolds number |
| S | = | distance from the channel bottom (m) |
| T | = | temperature (°C) |
| u | = | average velocity (m/s) |
| W | = | width of the channel (m) |
| Δp | = | pressure drop (Pa) |
| α | = | angle between the two combined delta wings (°) |
| v | = | kinematic viscosity (N/m2) |
| ρ | = | fluid density (kg/m3) |
| λ | = | thermal conductivity of the plate (W/(m · K)) |
| μ | = | dynamic viscosity (kg/(m · s)) |
| Subscripts | = | |
| air | = | air side |
| gas | = | gas side |
| i | = | heat element |
| in | = | inlet |
| out | = | outlet |
| w | = | wall surface |
Nomenclature
| A | = | heat transfer surface area (m−2) |
| a | = | distance from the channel inlet (m) |
| Ap | = | sum area of heat elements |
| cp | = | specific heat (J/(kg · K)) |
| de | = | hydraulic diameter (m) |
| f | = | friction factor |
| g | = | distance between DDWTs |
| H | = | height of the channel (m) |
| hv | = | convective heat transfer coefficient (W/(m2 · K)) |
| j | = | Colburn factor |
| L | = | length of the channel (m) |
| l | = | wing length (m) |
| m | = | mass flow rate (kg/s) |
| Nu | = | Nusselt number |
| Pr | = | Prandtl number |
| Q | = | heat transfer rate (W) |
| Re | = | Reynolds number |
| S | = | distance from the channel bottom (m) |
| T | = | temperature (°C) |
| u | = | average velocity (m/s) |
| W | = | width of the channel (m) |
| Δp | = | pressure drop (Pa) |
| α | = | angle between the two combined delta wings (°) |
| v | = | kinematic viscosity (N/m2) |
| ρ | = | fluid density (kg/m3) |
| λ | = | thermal conductivity of the plate (W/(m · K)) |
| μ | = | dynamic viscosity (kg/(m · s)) |
| Subscripts | = | |
| air | = | air side |
| gas | = | gas side |
| i | = | heat element |
| in | = | inlet |
| out | = | outlet |
| w | = | wall surface |